应用数学和力学,第 39 卷 第 4 期 2018 年 4 月 1 日出版 Applied Mathematics and Mechanics Vol.39,No.4,Apr.1,2018 文章编号:1000⁃ 0887(2018)04⁃ 0371⁃ 24 ⓒ 应用数学和力学编委会,ISSN 1000⁃ 0887 Transverse Harmonic Oscillation of a Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach∗ HABTE Mussie A, WU Chuijie ( State Key Laboratory of Structural Analysis for Industrial Equipment ( Dalian University of Technology) ; School of Aeronautics and Astronautics, Dalian University of Technology, Dalian, Liaoning 116024, P.R.China) ( Contributed by WU Chuijie, M. AMM Editorial Board) Abstract: We combined the 3D lattice Boltzmann method ( LBM) with the immersed boundary method ( IBM) to study the flow physics induced by an elastic rectangular container undergoing harmonic oscillations surrounding a viscous fluid. We proposed a semi⁃microscopic expression for the drag force to compute the hydrodynamic forces at the boundary nodes. An analytical de⁃ formation solution was used based on a thin plate elastic deformation theory to calculate the displacement experienced by the boundary. The numerical simulation results① based on the pro⁃ posed method agreed with the theoretical predictions for channel flow with stationary boundary. The oscillating boundary simulation exhibits the expected flow pattern in line with theory. Key words: lattice Boltzmann method; immersed boundary method; harmonic oscillation CLC number: O35 Document code: Introduction A DOI: 10.21656 / 1000⁃ 0887.390040 There has been a focus on flow physics within structures undergoing rotational or os⁃ cillatory motion. The most recent studies include flows in an oscillating deformable con⁃ tainer [1] , liquid sloshing damping in an elastic vessel [2] , flows in a vibrating cylinder [3⁃4] , and particle⁃laden flows in an oscillating cuboid [5] . Most of the studies consider flows in a circular cross section container. Under these geometric assumptions, the boundary is set to vibrate with deformation or rotate around the axis of the geometry. Although there has been a long⁃standing interest in flow through rectangular ducts, and numerical simulation of flows through elastically deforming container walls, an investigation is required on the ∗ Received 2018⁃ 01⁃ 24; Revised 2018⁃ 03⁃ 17 Project supported by the National Natural Science Foundation of China (11372068) and the Nation⁃ al Basic Research Program of China(973 Program) (2014CB744104) First author, HABTE Mussie A, Doctoral Student, E⁃mail: afeworkimussie@ yahoo.com Corresponding author, WU Chuijie, Professor, E⁃mail: cjwudut@ dlut.edu.cn ① All the results on figure axes, in this article, are displayed in lattice units. 371 372 HABTE Mussie A WU Chuijie potential influence of harmonic oscillations on the secondary flows. Drag effect within a boundary layer causes a secondary flow perpendicular to the axial direction of flow. A straight duct with a rectangular cross section fundamentally exhibits secondary flow. Flow moves towards and away the wall corners along the wall and the an⁃ gle bisector, respectively, as a phenomenon known as “ secondary flow of the second kind” [6] . Axial mean velocity contours protrude near the corners to manifest the presence of secondary flows. A secondary flow of the “ third kind” , also known as “ streaming” or “ acoustic streaming ” , and appearing in a stationary fluid as the boundary layer at the body, induces a reciprocating and small⁃amplitude harmonic oscillation. Viscosity in the boundary layer adjacent to the body causes characteristic steady secondary flow, which is imparted to the entire fluid close to the oscillating body despite periodic nature of the oscil⁃ lation [7] . The induced secondary flow moves in the direction of a decreasing amplitude. For a fluid in which the fluid elements move with a velocity, a secondary flow is formed at a considerable distance far from the wall with a magnitude independent of viscosity. Recently, Habte et al. [5] combined LBM and IBM to investigate the influence of wall motion on a particle⁃laden flow within a rectangular container made up of 6 rigid laminae oscillating without deformation. The fluid inertia effects on the boundary were negligible, and the boundary motion was mainly based on a pre⁃defined boundary velocity. Similarly, Buxton et al. [8] integrated the LBM and the lattice spring model ( LSM) to capture the dy⁃ namic coupling between an elastic shell and the hydrodynamic response of an enclosed vis⁃ cous fluid. The focus was on a Newtonian fluid where the no⁃slip boundary conditions in⁃ duced the interaction. Wu et al. [9] conducted a numerical simulation of a clamped micro⁃ plate with different boundary conditions and fluids with different viscosities. Their results reveal that the dynamics of the clamped⁃edge microplate is less influenced by the highly viscous dissipation of the fluid compared with other types of boundary conditions. They ob⁃ tained an analytical expression for the fluid loading impedance using a double Fourier transform approach. Complex enclosed flow phenomena, however, lack analytical solu⁃ tions, therefore, require a numerical simulation or an experiment. Kozlov et al. [1] recently proved the presence of a 2D steady flow in an experiment con⁃ ducted with a circular cylinder containing a stationary fluid under oscillation. Therein, the effect of the dimensionless frequency and amplitude of oscillation was studied to show the existence of 2D vortices along the axial direction. The study includes a range of oscillation frequencies with 2 dimensionless parameters governing the flow, namely, the pulsation Reynolds number and the dimensionless frequency. The frequency parameter governs the e⁃ volution of the vorticity field similar to the study of low⁃frequency and large⁃amplitude os⁃ cillations of cantilevers in viscous fluids [10] . There have been attempts to conduct a numerical simulation of flow within an elastic structure using the LBM where the deformation prediction was based on a given pressure⁃ radius relationship [11⁃12] . A recent study [13] applied the LBM in the analysis of flow through Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 373 axially symmetrical elastic system using an equation of motion. The LBM is a numerical method based on the fluid distribution functions, which propagate and collide among neighboring fluid nodes to produce locally averaged system properties that obey the Navier⁃ Stokes equations. The LBM is particularly useful in simulating fluid flows involving compli⁃ cated boundary geometries. However, it becomes difficult to achieve accurate geometrical descriptions of the structure included, especially when geometries are non⁃axis aligned. Consequently, the flow solution and the geometrical description require decoupling. There⁃ fore, the IBM adopts the LBM to describe the underlying fluid mesh independent of the ge⁃ ometrical treatment and to enforce the no⁃slip boundary condition. The most recent stud⁃ y [14] combined the 2 lattice⁃based methods in a study of a particle suspension system. How⁃ ever, a simplified form of coupling between the IBM and the LBM is still missing to solve the fluid structure interaction ( FSI) of a viscous fluid within an oscillating boundary. We aim to address the FSI effects with a simplified yet advanced numerical modeling technique by coupling the structural mechanics with the fluid dynamics. Section 1 intro⁃ duces the mathematical formation of the wall deformation through the linear theory of elas⁃ ticity for thin plates. In section 2, we describe the flow and the proposed boundary condi⁃ tion solvers. Section 3 contains a description of the simulation setup and the details of the parameters. Section 4 presents the simulation results and discusses the outcomes. Section 5 conclusively summarizes the implications of the findings. 1 Problem Formulation and Numerical Methods We consider a rectangular duct with a square cross section whose walls are made up of oscillating elastic laminae encompassing a quiescent viscous fluid. Description of flow in⁃ teraction with deforming laminae involves an elastic network immersed in a viscous fluid. In the 2D framework, the lamina is modeled as a thin plate with characteristic length L and width W. The lamina represents a moving boundary for the encompassed incompressible Newtonian fluid. The body forces from the lamina are added to the Navier⁃Stokes equations governing velocity field u. The solution process requires 2 distinct force balance equations for the 2 phases where the solid phase involves constitutive laws. However, we choose to apply a steady⁃state analytical solution to the description of wall deformation. Hence, we introduce the wave equation for transverse vibration of an isotropic, undamped rectangular thin lamina that represents a container wall subjected to a harmonic point load. 1.1 Forced Vibration of a Clamped Lamina Simulation of the dynamically deforming container requires an external energy supply on its boundary. An externally applied sinusoidal force oscillates a rectangular cuboid with sides made up of clamped rectangular plates. A piezoelectric actuator supplies the necessa⁃ ry external force to cause a small deformation that obeys the linear elasticity theory [15⁃16] . The expected deformation exhibits the classical equation of forced vibrations of a thin plate [17⁃18] , eq. ( 1) . For a thin elastic rectangular plate, the wave equation for transverse 374 HABTE Mussie A WU Chuijie forced vibration of an isotropic undamped plate, irrespective of the rotational inertia and transverse shear deformation effects, is given by the following equation: D Ñ4 ζ + ρ s h ∂2 ζ = F( x,y,t) , ∂t 2 (1) where the dynamic forcing function F( x,y,t) may constitute the external loading applied on the plate, which includes the excitation force and the surrounding fluid hydrodynamic force given by F ext( x,y,t) = F o( x,y) e iωt and F D = C d( u f - u w ) , respectively. F o( x,y,t) is the dynamic amplitude of the external force referring to a unit surface area of the lamina, C d represents the drag coefficient of the hydrodynamic force at the fluid⁃lamina interface with velocity u w , and u f refers to the fluid velocity at the lamina location. The transverse lamina displacement, its stiffness in bending, the Poisson’ s ratio, and the modulus of elasticity are represented by ζ, D = Eh 3 / (12(1 - ν 2 ) ) , ν, and E, respectively. The concentrated ex⁃ ternal load at arbitrary point ( x′,y′) is pinned to the lamina through delta function δ(·) . For the fluid⁃lamina interaction model, the aggregate external force is equal to 1.2 F( x,y,t) = F ext( x,y,t) δ( x - x′) δ( y - y′) + F D . The Fluid⁃Lamina Interaction Model (2) Suppose a harmonic external point force F ext is applied at frequency ω and acts at point ( x′,y′) along the positive z⁃ axis. Expressing the transverse displacement of the lamina as a harmonic motion ζ( x,y,t) = P( x,y) e iωt , and conducting the Fourier transform of eq. (1) , give the following simplified governing equation. é æ ρ f L ö F D( x,y,ω) ùú - = F o( x,y) , D Ñ4 P( x,y) + m ω 2 êê P( x,y) + ç (3) ÷ ρ f ω 2 L úû èρshø ë - = ρ s h and h represent the area mass density and the lamina thickness, respectively. where m Term ρ f L / ( ρ s h) is the non⁃dimensional fluid to lamina mass ratio that describes the extent of fluid loading. On the balance of lamina force density, the drag force density on the network of lami⁃ na nodes is equal to the sum of the externally applied and network elastic forces( eq.(4) ) . F ext + Ñ·σ e + C d( u f - u w ) = 0, (4) where σ e denotes the elastic stress tensor. An appropriate constitutive law gives the elastic stress tensor. The fluid exchanges momentum with the surrounding Lagrangian solid nodes through the link bounce⁃back [19] to generate a hydrodynamic loading. However, for a low fluid to lamina mass ratio, the lamina remains stationary unless externally excited, which implies that a one⁃way fluid⁃wall interaction takes place as ( F ext + Ñ·σ e ) / ( C d( u f - u w ) ) ≫ 1. Applying the Galerkin method to eq.( 3) after the simplification, the following model for rectangular plates gives a steady⁃state transverse lamina deformation: ζ o( x,y) = ∞ ∞ F o ϑ mn( x,y) ϑ mn( x′,y′) ∑ ∑ D( I I = = m 1 n 1 1 2 - + 2I 3 I 4 + I 5 I 6 ) - m ω 2 I2 I6 , where the shape functions are composed in the form of the product (5) Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 375 ϑ mn( x,y) = X m( x) Y n( y) . See appendix A for the model details. 1.3 The Lattice Boltzmann Model The lattice Boltzmann model simulation consists of fluid particles that propagate to neighboring lattice sites and collide at the site. The hydrodynamics is described by a 2nd⁃ order accurate spatio⁃temporal evolution of the fluid density distribution function, f α( r,t) , at time t and position r. f α ( r + ξ α Δt,t + Δt) = f α ( r,t) + Δα ( f α ( r,t) ) + F b , (6) where ξ α denotes the discrete velocity. The extra term, F b ( see appendix B) , on the RHS of eq.(6) represents a lamina in the flow field. Δt stands for the time increment, and Δα is the post⁃collision change in f α ( r,t) . This collision operator, Δα , is constructed through lin⁃ earization about the local equilibrium f αeq( r,t) : [20] Δα ( f α ) = Δα ( f αeq ) + ∑ ℓ αj f jneq , j (7) where ℓ αj are the matrix elements of the collision operator and f jneq = f j - f jeq is the non⁃equi⁃ librium distribution. The equilibrium distribution is conserved locally during collision, = f α ( r,t) + Δα ( f α ( r,t) ) and f α ( r + Δα ( f αeq ) = 0. With term F b excluded for simplicity, f ∗ α ( r) ξ α Δt,t + Δt) = f ∗ α ( r) describe the collision process and propagation of the post⁃collision dis⁃ tribution functions, respectively. Fig. 1 Schematic diagram of the point⁃particle IBM, and the Eulerian grid which hosts f α . The arrows indi⁃ cate the D3Q19 lattice microscopic velocity directions A uniform lattice spacing uniquely characterizes the LBM discretization of the flow do⁃ main. The present study utilizes the D3Q19 lattice scheme [21] to simulate the incompressi⁃ ble Newtonian fluid motion. The 19 lattice velocities, see fig. 1, define the characteristic line of fluid particle evolution across the neighboring lattice nodes, α = 0, ìï(0,0,0) , ï ξ α = c × í( ± 1,0,0) (0, ± 1,0) (0, 0, ± 1) , α = 1,2,…,6, (8) ïï α = 7,8,…,18, î( ± 1, ± 1,0) ( ± 1, 0, ± 1) (0, ± 1, ± 1) , 376 HABTE Mussie A WU Chuijie where c = Δx / Δt represents a unit particle speed. The equilibrium density distribution func⁃ tion, f αeq , is written based on the flow macroscopic velocity: ( ξ α·u) ( ξ α·u) 2 é u2 ù + - 3 2ú , f αeq = ρA σ êê 1 + 3 (9) 9 2 4 c 2c 2c úû ë where the values of weights are A0 = 1 / 3, A1 = 1 / 18 and A2 = 1 / 36. The 1st 3 moments of the distribution function describe the hydrodynamic quantities. Therefore, fluid density ρ, mo⁃ mentum j = ρu and momentum flux Π are defined as follows: ρ = ∑ f αeq( u) , ρu = ∑ ξ α f αeq( u) , Π = ∑ ξ α ξ α f αeq( u) . (10) The equilibrium momentum flux is obtained from Πeq = ρRTδ αβ + ρuu. The post⁃collision distribution is written as a series of the moments in the local velocity: ( ξ α·u) ( ρuu + Πneq,∗ ∶ ξ α ξ α - c2s I) ù éê ú, = + + f∗ A ρ ρ α σ ê úû c2 2c4 ë s s (11) where the sound speed is c s = Δx / ( 3 Δt) , and Δx is the spacing between neighboring fluid nodes. The relaxation of non⁃equilibrium momentum flux Πneq is a result of a collision ex⁃ pressed as [22] - Πneq,∗ = (1 + λ υ ) Πneq + 1 (1 + λ ζ ) ( Πneq ∶ I) I, 3 (12) where Πneq = ∑ α ξ α ξ α f αneq = Π - Πeq , and Πneq = Π - Πeq . The overbar describes the traceless - - - projection. Relaxation parameters λ υ and λ ζ are the eigenvalues of the linearized collision operator related to the shear and bulk viscosity, respectively. Their inverse is defined as the dimensionless relaxation time, τ. For low⁃Reynolds⁃number flows, the most suitable values are λ υ = λ ζ = - 1 [19,23] . The Navier⁃Stokes equations are recovered through multiscale analysis [20] for a hydro⁃ dynamic limit corresponding to ε ≪ 1 where ε, the expansion parameter, is defined as the ratio of Δx to a characteristic macroscopic length. At a low Mach number, ‖V max ‖ ≪ c s , the spatial 2nd⁃order accuracy is guaranteed with relative errors proportional to Δx 2 besides the temporal 2nd⁃order accuracy owing to the definition of the shear and bulk viscosity: c2s æ 1 1 1ö 1 æ + ÷ , ζ =- 2 + ö÷ . υ = - c δt ç δt ç 2ø 2ø 3 èλ ζ èλ υ 2 s 2 (13) The Simulation Method This section explains a method for simulating a steady flow in a rectangular elastic box. A D3Q19 LB model and a point⁃based IBM are combined to simulate the flow. The thin lamina consists of a uniformly spaced array of immersed massless points that define the dynamic box boundary. Specifically, the individual points are triangulated to form a network of discretized vertexes connected with elastic links: X Lℓ / BK = [ ( L - W / 2) ,ℓh i ] and X Rℓ / F = [ ( L + W / 2) ,ℓh i ] for the left, right, back and front walls, where the wall point spac⁃ ing is h i = L i / N w and ℓ = 1, 2,…, N w . N w and L i represent the number of nodes along a specif⁃ Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 377 ic direction and the box dimensions in the 3 directions, respectively. The top and bottom walls are discretized similarly. The Eulerian Cartesian nodes inside this enclosure are trea⁃ ted as fluid nodes of concern. Unlike the pure LBM based simulations, the Lagrangian boundary points independently define the box geometry and do not require relocation, modification or creation of a fluid node. Fig. 2 Schematic diagram of the computational domain of Eulerian coordinate system Ω( x) ( thin lines) and time⁃varying Lagrangian locations X w( p,q,t) with the no⁃slip boundary condition. X w( p,q,t) re⁃ presents the temporary location of the oscillating lamina ( thick lines) at wall coordinate ( p,q) . The standing wave of the fluid motion represents longitudinal fluid displacement as a result of flex⁃ ural vibrations at half wavelengths. Both domains are discretized with a structured grids Fig. 2 shows the configuration of lattice sites and box boundary at a mid cross section of the box, where the cuboid axis is aligned in the y⁃ direction throughout the simulation. As the lamina deforms, every point on the lamina moves in the direction perpendicular to the box axis and performs an affine transformation in the normal direction after the oscilla⁃ tory force is applied at the middle of the wall. For each Lagrangian location, X w , under an affine transformation, we store information concerning the current position, the displace⁃ ment, the outward normal, n, and the velocity along n, dζ / dt. We apply a point⁃based IBM [5] , see sec.2.1, to enforce a no⁃slip boundary condition by pinning the surrounding fluid distribution functions onto the Lagrangian network of nodes. This method requires 8 fluid nodes surrounding a particular lamina node for smooth interpolation and distribution of fluid variables and Lagrangian quantities, respectively. At each Lagrangian site, values of the fluid distribution functions from the 19 directions are summed up to generate macroscopic quantities such as density, momentum, and stresses. 378 HABTE Mussie A WU Chuijie Based on these macroscopic quantities, the fluid force at the Lagrangian location is calcu⁃ lated from the difference in velocity between the Lagrangian and the Eulerian node veloci⁃ ties at the same position. For any wall node X w , the velocity is computed from the stored information at the node. During simulation, each Lagrangian location at the wall deforms according to eq.(5) and oscillates about their initial equilibrium position. Each wall node is displaced during consecutive time steps along wall normal direction n, as is shown in fig. 2. This approxima⁃ tion implies that the velocity of the individual wall node, V n( t) , is given by dζ o in the lattice Boltzmann unit. For the cuboid axis pointing along the y⁃ axis, the velocity at this point is integrated with the Störmer⁃Verlet method, V n = dζ o( ± i / ± k) + O( Δt) , at the cost of accu⁃ racy. The walls undergo many oscillations through the duration of the applied external force. 2.1 Boundary Condition and Force Density This section describes a point⁃based IBM ( PIBM) used to enforce a no⁃slip boundary condition and compute the force density at the boundary walls. We chose the PIBM for its desirable algorithm efficiency to facilitate modeling of the interaction between the fluid and the boundary walls. At each wall node, the momentum is exchanged to generate the hydro⁃ dynamic force density, g( X w ,t) [24] . To capture localized fluid⁃structure interactions, a reg⁃ ularized delta function distributes the singular hydrodynamic force density, f( r,t) , at off⁃ lattice IB nodes onto the nearest surrounding Eulerian grid points through a trilinear inter⁃ polation, eq.(14) , as was applied by Squires et al. [25] This operation replaces the solid do⁃ main with the surrounding fluid so that the LBM solves the entire simulation on a simple Cartesian grid [26] . The trilinear interpolation is carried out at a sequence of lattice cube ver⁃ tices labeled with ( ±1,±1,±1) with the volume⁃weighted averages of the Lagrangian quan⁃ tity g( X w ,t) : 3 f( σ 1 ,σ 2 ,σ 3 ) = g( X w ,t) ∏ i=1 { 1 - xi , xi , σ i = - 1, (14) σ i = 1, where g is a quantity at arbitrary wall node position X = ( x,y,z) , which is interpolated from the nearest 8 fluid nodes of a cube labeled with ( σ 1 ,σ 2 ,σ 3 ) where the subscripts denote the x, y, and z components of the Cartesian coordinate. The ∏ operator represents the product of 3 linear interpolations for each fluid node of a lattice cube hosting the Lagrang⁃ ian nodes. This projection operator has a compact support and maintains a sharp boundary. The values at each vertex will be denoted by V - 1 - 1 - 1 , V1 - 1 - 1 , V - 11 - 1 , …, V111 . During inter⁃ polation, for instance, a Lagrangian quantity at position ( x,y,z) within the cube will be de⁃ noted as V xyz and is given by V xyz = V - 1 - 1 - 1(1 - x) (1 - y) (1 - z) + V1 - 1 - 1 x(1 - y) (1 - z) + V - 11 - 1(1 - x) y(1 - z) + V - 1 - 11(1 - x) (1 - y) z + V1 - 11 x(1 - y) z + V - 111(1 - x) yz + V11 - 1 xy(1 - z) + V111 xyz. Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 379 For instance, everywhere the vertex has a - 1 in the subscript, it corresponds to a term of 1 - x, elsewhere, the subscript gets a term of x; the same is true for the other 2 compo⁃ nents. The force density, g, is calculated from an interpolated fluid density distribution func⁃ tion, f α ( X w ,t) , that further requires a correction for moving boundary with translational ve⁃ locity U w [20,22,23] . At the solid⁃fluid interface, fluid particles that propagate toward a bound⁃ ary node ( in the α direction) bounce off into the direction from where they originate with modification as below: f αnew = f β ( X w ,t) - 2A β ρ f ξ β·U w c2s , (15) where f αnew = f β ( X w ,t + δt) with β being a direction opposite to α. A α are the lattice constants defined earlier for the D3Q19 model. The fluid exerts a force on the solid⁃fluid interface be⁃ cause of the bounce⁃back rule. This force equals the momentum exchange rate that takes place as the fluid particles bounce off at the wall nodes. The contribution from a single bounce⁃back event is of the form: g( X w ,t) = ∑ ξ β( f αnew( Xw ,t) β - f α ( X w ,t) ) , (16) where g( X w ,t) is the force density on the immersed boundary at Lagrangian location X w at time t. Obviously, wall node locations do not necessarily coincide with the Eulerian coordi⁃ nates during movement. Eq. (16) , thus, entails an approximated fluid density distribution function, f α ( r,t) , onto the off⁃lattice Lagrangian wall node position, X w , involving the im⁃ mediate surrounding fluid mesh points through a trilinear interpolation at each time step u⁃ sing eq.(14) . f( X w ,t) = ∑ σ i∈ ±1 3 f α ( σ i ,t) ∏ i=1 { 1 - xi , xi , σ i = - 1, σ i = 1, (17) where X w is a Lagrangian marker coordinate ( x 1 ,x 2 ,x 3 ) , where x 1 , x 2 , and x 3 represent the corresponding x, y, and z components. Also, f α ( σ i ,t) denotes the fluid distribution function at the Eulerian coordinates ( σ 1 ,σ 2 ,σ 3 ) of a lattice cube surrounding a Lagrangian wall node. The reactive wall⁃fluid interaction force density exerted on the wall node is equal in magnitude and opposite in direction to g( X w ,t) . The off⁃lattice IB experiences forces due to the drag generated from the velocity mismatch between the fluid and the wall nodes. The model considers the velocity mismatch at the solid boundaries as its primary input without attempting to close the velocity mismatch. The no⁃slip condition is satisfied at the solid⁃flu⁃ id interface by pinning the Eulerian fluid velocity onto the Lagrangian particle position through the trilinear interpolation. The implemented solid⁃fluid interface boundary condi⁃ tion is straightforward, conserves the global fluid mass, and works regardless of the geo⁃ metrical orientation and complexity. The IBM accounts for the FSI forces in the governing flow equation through a source 380 HABTE Mussie A WU Chuijie term that incorporates fluid volume fraction ϕ l = 1 - ϕ p , where ϕ p denotes the local particle volume fraction, which can be very low for sparsely populated wall nodes. Flows at higher order flow Reynolds numbers, however, require a drag law [27] to compute the hydrody⁃ namic force associated with the ith wall node. This modifies eq.(16) to give semi⁃macro⁃ scopic expression F B that will be interpolated to Eulerian nodes and become F b as is shown in eq. (6) . ξ α·U w ù é ú γ( l ) , F B = A p C d( Re) U s·∑ ξ β êê f α ( X w ,t) - A α ρ f c2s úû β ë (18) where A p = 0.25πΔ 2 and U s = ‖U w - u‖ ( see appendix C) are a force scaling and the slip velocity at a single wall node, respectively. C d [28] and l are the drag coefficient and the lo⁃ cal porosity ( see appendix D) , respectively. The summation runs over all the 19 nearest neighbor nodes, i.e., overall [ 100] and [ 110] bonds of a cubic lattice. The exponent in the voidage function expression, l -κ , takes the form of κ = 3.7 - 0.65e - 0.5(1.5 -lg Re) 2 [27,29] . The configuration results in force F B due to the relative velocity that exists between the rhythmi⁃ cally oscillating walls and the fluid at the wall location. Flow features at scales smaller than the Kolmogorov length scale are merged within the drag correlations and the hindrance function [30] . The model considers the velocity mismatch at the solid boundaries as its pri⁃ mary input without enforcing the no⁃slip constraint. 2.2 Model Error Comparison As the flow simulation attains a steady state, the accuracy of the LBM velocity field is compared with the analytic solution. The simulation velocity errors are measured with the ℓ ∞ and ℓ 1 norms, where the summation considers all the lattice nodes involved in the simu⁃ lation domain, see table 1. ℓ∞ = ℓ1 = max | Ω( | u( x,y,z) - u a( x,y,z) | ) ∑Ω | max | Ω( | u a( x,y,z) | ) u( x,y,z) - u a( x,y,z) | ∑Ω | u a( x,y,z) | , , (19) (20) where u a is the analytical velocity. The ℓ ∞ ⁃norm error tracks the most significant simulation error and its location, while the simulation errors for each site are weighted equally in the ℓ 1 ⁃norm. Both error norms are used to get the simulation method order of convergence. Furthermore, an averaged error is also calculated to gain an insight into the overall accura⁃ cy of the scheme. To check the simulation convergence, the ℓ 1 ⁃norm of the velocity flow field is compared at 2 consecutive moments through eq. (21) : ∑Ω | u( x,y,z,t) - u( x,y,z,t - 1) | ∑Ω | u( x,y,z,t) | where T ol = 1 × 10 -8 is the tolerance. < T ol , (21) Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 3 381 Configuration and Parameter Setup This section presents numerical examples of simulations performed with the proposed LBM⁃IBM approach for the fully coupled fluid⁃structure interaction of a single phase. We consider a test problem based on a 3D laminar Poiseuille flow generated by a body force in a square⁃cross⁃section channel. First, as the current LBM solver, SUSP3D, is already de⁃ veloped and thoroughly validated ( Ladd et al. [19] ) , we validate only its capability to de⁃ scribe flows when it combines with the IBM. We compare the simulation results from the LBM and the LBM⁃IBM with analytical solutions for flow within a channel. Based on the validation problem, we conduct a separate LBM⁃IBM simulation where an externally ap⁃ plied time⁃varying sinusoidal load vector enforces the movement of elastic sidewalls. Fig. 3 3.1 Schematic diagram of the channel flow and the applied boundary conditions Flow Through a Channel Errors pertinent to Mach and Knudsen numbers combined with computational domain discretization preclude exact solutions of rectangular duct flow through LBM with the bounce⁃back boundary condition, especially for square cross sections [31⁃32] . Furthermore, the duct’ s aspect ratio influences the relative error in the peak velocity. Flow through a square duct shows the most significant error but an increasing aspect ratio reduces the er⁃ ror, and the result resembles the Poiseuille flow. Recently, however, it was possible to at⁃ tain an exact solution for a square duct flow with a non⁃equilibrium extrapolation boundary condition implementation based on 14 and 18 velocity multiple⁃relaxation⁃time lattice Boltz⁃ mann models with variable lattice speed c [33] . Hence, we would simulate a fully developed laminar channel flow to verify the LBM, with a simple bounce⁃back boundary condition, and LBM⁃IBM D3Q19 lattice schemes. Fig. 3 shows the computational domain for the simulations within the channel with a rectangular cross section at any fixed value of x. The fluid enters at the inlet of the numeri⁃ cal grid ( x = 1) and exits at the outlet ( x = N x ) . The domain sizes are set as height H ( along y⁃ axis) , infinite width W ( along z⁃ axis) , and length L = ( N x - 1) Δx corresponding to the streamwise direction, and H is the channel height. A no⁃slip boundary condition is applied 382 HABTE Mussie A WU Chuijie to the channel walls, while a periodic boundary condition is applied elsewhere. As a result, we specify a body force equivalent to the pressure drop to drive the flow. The driving⁃force density of ( G x , 0,0) is constantly applied in the entire computational domain. We consider a fully developed flow with an axial velocity, which is a function of only the lateral coordinates. The Navier⁃Stokes equations, hence, behave as a Poisson’s equation with a constant body force, G x , driving the fluid along the streamwise direction, μ Ñ2 u( y) = G x . The no⁃slip boundary condition at the entire solid boundary solely determines the flow. The center velocity of the analytical velocity has the following form: 2 H2 é æyö ù G x êê 1 - ç ÷ úú . (22) 2μ ë èHø û The accuracy of an LBM simulation depends on conditions related to flow initializa⁃ u|x = tion, boundary, and simulation parameters. The 4 quantities relevant to an accurate setup of the simulation parameters are the Reynolds number ( Re) that depends on the physical system, the Mach number ( Ma) of the simulation, dimensionless relaxation parameter τ and lattice constant Δy. The Reynolds and Mach numbers are key parameters in establishing a relationship among the 4 dimensionless quantities: Ma / Re = ( τ - 0.5) / ( 3 N y ) [31] . The Reynolds and Mach numbers are given as Re = Hu / υ and Ma = u / c s , respectively, where u is the bulk velocity at the center. The necessary body force that drives the flow along the streamwise direction is a function of the 4 simulation parameters: G = (8 / (3H) ) × ( Ma 2 / ( Re | max ) ) . Flow can never exceed 1 / 3 of the lattice sound speed; it should be much lower. We choose 0.1 as the highest relatively safe value for a practical purpose to satisfy the low Mach number requirement of the LBM. Hence, the tests run for a fixed maximum lattice velocity and a relaxation time of 0.1 and 1, respectively. The correspond⁃ ing simulation parameters are listed in table 1. lattice N x × N y × N z Table 1 Simulation parameters for flow at u max = 0.1 and Re = 30 44×11×11 84×22×22 172×44×44 352×88×88 2.424E-4 1.212E-4 6.061E-5 3.030E-5 0.61 τ Gx 0.72 0.94 1.38 The data is collected after the flow attains a steady state according to the criteria given by eq. (21) . The maximum value then scales all the velocity values at the channel center. 3.2 Stationary Fluid in an Oscillating Cube Two non⁃dimensional parameters govern the flow characteristics in an oscillating con⁃ tainer filled with incompressible Newtonian fluid according to Kozlov et al. [1] . The dimen⁃ tionless numbers are defined as frequency parameter olds number Re pl = 2π - - = ωW 2 / (2πυ) and oscillatory Reyn⁃ ζ o / W. In addition, non⁃dimensional amplitude of oscillation = ζ o / W is defined, which relates the Reynolds number to the frequency parameter according to Re pl = 2π - . A 3rd dimensionless number, = ωW 2 - m / D , is obtained from eq. ( 5) , which characterizes the lamina oscillation based on the frequency of vibrations and the lamina ge⁃ ometric parameters. Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 383 In the fluid⁃structure interaction problem, all of the side boundary parts oscillate with time, and no⁃slip velocity conditions are applied with the surrounding fluid pinned at the boundary nodes. An external sinusoidal point force enforces the sidewalls oscillation at 10 Hz, 20 Hz, and 25 Hz. The 3 frequencies of oscillation are selected at a 1 200⁃iteration wave sample rate, and the data are read every 30 iteration steps. The deformation quanti⁃ ties are acceptable as an outcome of the theory of elastic deformation. The density, the Poisson’ s ratio, and the elastic modulus of the thin elastic lamina are 7.8 × 10 3 kg·m -3 , 0.33 and 7.82 × 10 9 N·m -2 , respectively. Also, the thickness of the plate and the maximum exter⁃ nally applied load are 1 × 10 -4 m and 0.25 N, respectively. The kinematic viscosity of the flu⁃ 2 -1 id is 1.0 × 10 -6 m· s . The lamina width and height are 8.385 × 10 -3 m and 3.144 × 10 -2 m, re⁃ spectively. Given the above material properties and the geometric parameters, the analytical solu⁃ tions of the wall maximum deformation are obtained from eq.(5) as 7.773 1 × 10 -5 m, 7.774 0 × 10 -5 m, and 7.774 7 × 10 -5 m for 10 Hz, 20 Hz, and 25 Hz, respectively. Three non⁃dimen⁃ tional amplitudes of oscillation 9.275 77 × 10 -3 , 9.276 8 × 10 -3 , and 9.277 7 × 10 -3 , and - = 112, 224, and 280 are computed from the corresponding maximum deformation values and the fluid property. The corresponding pulsation Reynolds numbers are 6. 53, 13. 06, and 16 32. The 3 simulation cases have the same non⁃dimentional parameter of the wall fre⁃ quency of = 22.90. The dimensions of the computational domain are selected based on the convergence test to guarantee an adequate resolution for the fluid field. 4 4.1 Numerical Results and Discussions Flow Through Stationary Channel Walls We compare the simulation velocity profiles of 2 different simulation methods with the analytical solution, where the results are normalized for direct comparison. Figs. 4( a) and 4( b) show the LBM⁃IBM and the LBM results, respectively, for the 3D Poiseuille flow be⁃ tween 2 infinitely wide plates. In the LBM case, the velocity profiles recover the peak ve⁃ locity in all the test runs but retain a slip velocity near the channel walls that tends to re⁃ duce with an increasing resolution. The LBM⁃IBM approach gradually recovers the paraboli⁃ c profile at the center of the channel with an insignificant slip velocity at the channel walls during the entire test, but the highest deviations from the analytical solution occur one lat⁃ tice unit inwards from the boundary due to the applied force spreading kernel. The wall boundary conditions are the cause for the difference between the simulation and the analyt⁃ ical solution regardless of the resolution. Concerning the LBM⁃IBM approach to simulate channel flows, the local fluid momentum is forced to match the boundary node momentum through calculation and distribution of immersed boundary force F B based on the local ve⁃ locity mismatch. This matching always enforces a no⁃slip condition at the boundary, how⁃ ever, distributing F B to the surrounding fluid nodes influences the solution, where the accu⁃ racy increases with the resolution. 384 HABTE Mussie A ( a) LBM⁃IBM Fig. 4 The Poiesuille flow at different grid resolutions - Fig. 5 4.1.1 WU Chuijie ( a) ℓ 1 ⁃norm averaged ( b) LBM ( b) ℓ 1 ℓ ∞ ⁃norm convergence tests The grid convergence effect on average velocity errors for different grid resolutions Grid Convergence Study To demonstrate the accuracy of the proposed wall no⁃slip boundary condition, differ⁃ ent node sizes (11, 22, 44, and 88) are used to discretize the channel across the wall’ s normal direction. In fig. 4( a) , numerical results with the IBM⁃LBM solver show a better a⁃ greement with both the LBM and the analytical solution at a higher resolution. According to fig. 4( b) , the LBM exhibits a larger error at the channel walls in comparison to the IBM⁃ - LBM approach. Table 2 displays the maximum ℓ ∞ , the average ℓ 1 , and the global relative velocity ℓ 1 error norms for different lattice resolutions. According to fig. 5( a) , variation of - ℓ 1 ⁃norm with the lattice spacing shows that the boundary conditions in the LBM and the LBM⁃IBM are 1st and 2nd order convergent, respectively. Besides, the mean absolute er⁃ - ror, ℓ 1 , slopes of the line of best fit for the LBM and LBM⁃IBM schemes are directly pro⁃ portional to the resolution to the power of 2 and >2, respectively. This result justifies that the proposed point⁃based LBM⁃IBM D3Q19 model is of 2nd⁃order accuracy in space when Transverse Harmonic Oscillation of Rectangular Container With 385 Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach simulating steady 3D Poiseuille channel flows. Table 2 Relative errors for a body force driven 3D⁃Poiseuille flow. The displayed results are for the - LBM and IBM⁃LBM approaches. Each approach contains the ℓ 1 , ℓ ∞ , and ℓ 1 norm errors. The data in the last row, with the present method, are the maximum numerical velocity attained. The 2nd column shows the order of convergence in linear regression method Ladd’ s present order 1.064 velocity H = 11Δy H = 22Δy H = 44Δy H = 88Δy ℓ1 1.077×10 -1 4.989×10 -2 2.416 4×10 -2 1.173 7×10 -2 - 9.792×10 -3 2.268×10 -3 5.481 0×10 -4 1.333 7×10 -4 error τ = 0.61 0.961 ℓ∞ 1.977 ℓ1 1.408 7×10 -1 - 1.522 9×10 -2 2.064 1.923 2.923 ℓ1 ℓ∞ ℓ1 1.678×10 -1 1.675 2×10 -1 0.088 3 τ = 0.72 8.819×10 -2 4.464×10 -2 2.462×10 -3 5.416×10 -2 0.096 7 τ = 0.94 4.504 8×10 -2 1.091 9×10 -2 2.773 2×10 -4 1.220 2×10 -2 0.099 3 τ = 1.38 2.277 1×10 -2 2.340 3×10 -3 3.236 1×10 -3 3.677 3×10 -5 0.100 03 Table 2 shows 3 velocity error measurements for both the LBM and the LBM⁃IBM with different resolutions at a constant Reynolds number. As is shown in table 2, the LBM⁃IBM has smaller simulation errors compared to the LBM as the lattice resolution increases. The most significant simulation error, ℓ ∞ ⁃norm, is 2.7% for the LBM and occurs at the wall lo⁃ cation for small resolutions, while it is 0.07% for the LBM⁃IBM at the same position. The LBM solver attains the maximum axial velocity with an insignificant error in all the test runs, but the minimum possible slip velocity at the wall node was 4.5 × 10 -3 for 44 nodes. In the LBM⁃IBM approach, the flow attains a maximum numerical axial velocity of 9.93 × 10 -2 with a 0.7% error. The minimum slip velocity at the boundary was 3.05 × 10 -5 , 2 orders of magnitude less than the bounce⁃back method. Evidently, applying the LBM as a fluid and boundary condition solver introduces significant error compared to decoupling the solution process with the IBM. 4.2 An External Force Governed FSI A well⁃established validation case stated above tested the proposed point particle LBM⁃ IBM approach. The close agreement between the simulated result and the analytical solu⁃ tion confirms the validity of the proposed method. Next, we will apply the method to study flow within a container whose boundary motion varies according to different oscillation fre⁃ quencies. 4.2.1 Flow Dynamics in an Oscillatory Wall Boundary movement is achieved through application of either a predefined form of ve⁃ locity [34] or a force that will cause a boundary deformation [1] . Consecutive boundary node locations offer the wall displacements as the boundary deforms. A deformed thin lamina under an applied external load is shown in fig. 6. Simulations using the LBM have the high⁃ est velocity limit for which the flow stays under the assumption of incompressibility. Thus, a combination of small displacements and low⁃frequency ranges best suits the method as was applied in reference [35] , where convective effects were negligible. 386 HABTE Mussie A WU Chuijie Though a direct comparison of the present result with the experimental results of Ko⁃ zlov et al. [1] is not possible due to the dissimilarity in shape and size parameters, the com⁃ puted flow structure shows similarity in regards to the main features of the flow pattern. Fig. 7 shows the calculated flow field with the generated flow at relatively low frequencies at which the external load is applied. The projected component of the flow field apparently has 4 regions with 2 pairs of regions hosting a system of similar flow pattern, which rotate alongside the 2 main diagonal angle bisectors symmetrical about the vertical axis. It enters from 2 opposite diagonal corners of the box along the angle bisector towards the core and flows towards the adjacent corners, while performing a U⁃turn forming a saddle point as the highest deflection point at the center. The velocity gradient vanishes as the projected component gradually loses its tangency towards the middle, and the flow velocity attains its maximum value with a dominant vertical part of the velocity vector. Fig. 8 shows the dominant flow structure that prevails along the vertical axis in all of the different simula⁃ tions conducted; however, the flow intensity decreases with the frequency at which the external force is applied as shown in fig. 7. This relationship indicates the influence of the external force frequency on the flow structure. It is to be noted that the boundary velocity diminishes as the frequency of the applied external force increases, and the flow field tends to have a reduced intensity. Fig. 6 A typical deformed lamina structure representing the moment as the right side of the box is displaced inward as is shown in schematic diagram fig. 2. The deforma⁃ tion is magnified for visual purpose. Governed by 2 dimensionless constants, a steady flow oscillating vertically because of the periodic expansion and contraction of the container volume leads to a vertical standing wave as is shown in figs. 7( e) ~ ( h) . The x⁃z plane projections of this vertically reciproca⁃ ting flow field demonstrate the presence of a spanwise velocity gradient at every vertical section with a maximum tangency near the boundary to satisfy the no⁃slip condition. Figs. 7( a) ~ ( d) show the flow structure across a horizontal cross section for 3 distinct frequen⁃ cies. A rise in the flow strength with the frequency shows the dependence of the flow struc⁃ ture on the vibration frequency. The dimensionless numbers for the simulation results in fig. 7 and fig. 9 are = 22.9, Re pl ≈ 6.53, - = 122 for f R = 10 Hz; and = 22.9, Re pl ≈ 13.06, Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach - = 224 for f R = 15 Hz, respectively. Also, = 22.9, Re pl ≈ 16.32, - less numbers for f R = 25 Hz, but the figures are not reported here. Fig. 7 4.2.2 387 = 280 are the dimension⁃ Instantaneous vertical components of the flow field within a container oscillating at 10 Hz. The contour slice plots show the fluid velocity with the associated vector on a vertical plane cross⁃section( ( e) t / T = 0, ( f) t / T = 0.25, ( g) t / T = 0.5, ( h) t / T = 0.75) Structure of the Velocity Field The typical simulation outcome in fig. 7 reports 4 instants in an oscillation period of T within an interval of 0.25T. A significant change in the boundary motion occurs at t / T = 025, 0. 5, and 0. 75, where the boundary attains its maximum and minimum velocities. Boundary oscillation subjects the velocity and the density gradient fields into a periodic os⁃ cillation similar to a peristaltic flow through an elastic conduit. Initial wall displacement re⁃ duces the internal pressure down until the boundary displacement decelerates and turns its direction of motion inward. This is evident from the pressure versus time plot in fig. 10 ( b) , where a periodic change in pressure occurs at 3 locations that are described in fig. 2. Similarly, 3 temporal variations of the instantaneous velocity are plotted in fig. 10( a) re⁃ presenting the 3 locations. 388 HABTE Mussie A Fig. 8 Fig. 9 WU Chuijie The phase portrait of a projected flow field that shows the basic pattern of the flow trajectory with a saddle point at the center Instantaneous vertical component of the flow field within a container oscillating at 15 Hz. The contour slice plots show fluid velocity with the associated vector on a vertical plane cross⁃section( ( e) t / T = 0, ( f) t / T = 0.25, ( g) t / T = 0.5, ( h) t / T = 0.75) The fluid⁃structure interaction continues in both low and high⁃pressure states. These alternative moments create a flow reversal, which is evident at t / T = 0.25 and t / T = 0.75. Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 389 This periodic flow takes place in 2 regions of the container along the vertical direction sep⁃ arated by a distinct flow stagnation front, i.e., location ③, as is shown in fig. 7. The ve⁃ locities in the 2 regions are out of phase, while the pressure is in phase with a small change in amplitude. ( a) The fluid velocity Fig. 10 ( b) The fluid pressure The instantaneous fluid velocity ( a) and the fluid pressure ( b) at 3 locations within the container boundary oscillating at 10 Hz as are described in fig. 2 ( a) The flow field Fig. 11 ( b) The fluid pressure Instantaneous vertical velocity components of the flow field ( a) and the fluid pressure ( b) at location ② within the container as are described in fig. 2. The boundary oscillates at 4 different frequencies The upper subfigures in fig. 9 show a seemingly secondary flow velocity profile at a cross section of the container halfway from the displacement node towards the 2 regions a⁃ long the vertical direction. As is displayed, the instantaneous secondary flow of the 3rd kind is symmetrical about the corner bisector where the velocity field vectors indicate the presence of 2 pairs of opposite vortices, see fig. 9( d) . As is predicted, the region adjacent to the wall shows a velocity gradient where the viscosity plays a dominant role in enforcing a gradual tangency of the velocity as it approaches the region. Simultaneously, the domi⁃ nant role of the viscosity diminishes a few lattice intervals away from the boundary towards 390 HABTE Mussie A WU Chuijie the core region. 4.2.3 Effect of the Force Oscillation Frequency The effect of boundary oscillation is examined through application of an external load at different frequencies. Fig. 11 shows the characteristics of flow intensity at 4 external load fre⁃ quencies. It can be observed from fig. 11(a) that the vertical component of the flow at posi⁃ tion ②, see fig. 2, decreases with the increasing frequency. Also, the internal pressure de⁃ creases with the increasing rate of oscillation of the applied external load, see fig. 11(b). 5 Conclusions The LBM and the IBM are combined to investigate the fluid⁃structure interaction of a moving boundary encompassing a viscous fluid in detail. From the above discussion, the conclusions are as follows: 1) Implementing a point⁃particle based IBM as a boundary condition solver offers 2nd⁃ order convergence with negligible slip when compared to the LBM alone. The immersed boundary based boundary condition shall be used instead of the LBM if accuracy is more important than the computational cost. Instead, if the simulation result is more important at the boundary, the LBM offers a solution faster than the IBM. 2) The boundary movement is found to influence an initially quiescent viscous fluid within a container significantly and generate a standing wave in response to the presence of a dynamic pressure gradient, as expected. 3) The projected flow field reveals the presence of a secondary flow, of which the in⁃ tensity increases with the frequency of boundary oscillation, in line with previous results despite a direct comparison is not possible. Appendix A Forced Vibration Responses A harmonic external force, F ext( x,y,t) = F o( x,y) e iωt oscillating with frequency ω, is considered to act at location ( x′,y′) perpendicular to a clamped thin lamina with dimensions W × H. With a Fourier series ex⁃ pansion of F o( x,y) , a superposition of shape functions is introduced as ∞ ∞ ∑∑F F o( x,y) = m=1 n=1 mn ϑ mn( x,y) , ( A1) where shape function ϑ mn( x,y) = X m( x)·Y n( y) is composed of eigen functions X m( x) and Y n( y) , which sat⁃ isfy the lamina boundary condition. Eq. ( A2) express F mn as a function of shape functions X m( x) and Y n( y) based on their orthogonality: F mn ∫ ∫ F ( x,y) X ( x) Y ( y) dxdy = . ∫ ∫ X ( x) Y ( y) dxdy W H 0 0 o m W H 0 0 2 m n 2 n ( A2) Chosen shape functions X m( x) and Y n( y) for the clamped lamina are: Jλ λ x λ y Jλ λ y - æç m ö÷ H æç m ö÷ , Y n( x) = J æç n ö÷ - g æç n ö÷ H æç n ö÷ , è a ø è Hλ m ø è a ø è b ø è Hλ n ø è b ø X m( x) = J æç where λmxö ÷ ( A3) Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 391 J(·) = cosh(·) - cos(·) , H(·) = sinh(·) - sin(·) . In addition, λ m and λ n satisfy condition cosh( λ)·cos( λ) = 1. With the Galerkin method and the orthogonality of the shape functions, eq. (3) is converted to - D∑ ∑ W mn( I 1 I 2 + 2I 3 I 4 + I 5 I 6 ) - m ω 2 ∑ ∑ W mn( I 2 I 6 ) = m n ∑∑W m n m ∫ ∫ F ( x,y) X Y d d , mn H W 0 0 o m n x n y where the inputs to eq. (5) are defined as ∫ X ″″ x dx, I = ∫ Y ″ Y dy, = ∫ Y dy, I = ∫ Y ″″ Y dy, = ∫ X ″ X dx, I = ∫ X dx. I1 = I2 I3 W ( A4) H m 0 H 5 0 W m 0 n n n W 6 m m 0 H 2 n 0 4 m 2 m 0 Appendix B The Forcing Term The body force term, F b , in eq. (6) , suggested by Ladd [23] and Son et al. [36] , is chosen among the va⁃ rious formulations investigated in literatures. This forcing term can be expressed in the expanded form of the power series in particle velocity ξ α . ( C·ξ α ) D ∶ ( ξ α ξ α - c2s I) ù é + F b = A σ êê B + úú , c2s 2c4s ë û ( B1) where B, C, and D are determined as the moments of F b are required to match the corresponding hydrody⁃ namic equations. Particularly, B = 0, C = f, and D = [1 - 1 / (2τ) ] u′ f + fu′ . However, the expression for D calls for modification of the moment field added to each node [36] that gives an equilibrium fluid velocity and defined as ρu′ = ∑ξ α α Δt f. 2 fα + ( B2) Therefore, a forcing term formulation proposed by reference [36] is used in order to recover the correct Navier⁃Stokes equations. Fb = ( 1 - 2τ1 ) A éêêë3 ξ c- u + 9 ξ c·u ξ ùúúû·f( r,t) . α α α 2 4 α ( B3) As simulation is performed with considerable particle void fraction, F b serves as the sole interaction medi⁃ um between the 2 phases. Appendix C The Slip Velocity The Eulerian velocity in eq. ( B2) is a combination of intermediate and corrective components. u( r,t) = u ∗( r,t) + δu( r,t) . ( C1) This corrective term is a result of spreading the normalized Lagrangian force density onto a boundary loca⁃ tion, like the interpolation of fluid properties to the Lagrangian nodes using eq. ( 17) . The slip velocity is calculated through subtraction of the fluid velocity at the boundary point, U( X w ,t) , from the wall velocity at the same node. U s = U l( X w ,t) - U( X w ,t) , U( X w ,t) ) = ∑u( r,t) D( r ijk ijk - X w ) ΔxΔyΔz, ( C2) ( C3) where Δx, Δy, and Δz are the mesh sizes in the horizontal, vertical, and transverse directions of the com⁃ putational domain, respectively. The Eulerian fluid velocity is smoothly transferred to the boundary loca⁃ tion through the trilinear interpolation [25] . 392 HABTE Mussie A WU Chuijie The above expression for the slip velocity is the primary input into the structure⁃fluid drag force mod⁃ el. For intermediate and high Reynolds number flow regimes, eq. ( 16) has to be redefined into a drag model, see section 2.1. Appendix D Porosity The porosity at boundary node X w is calculated through summation of the volumes of all the surround⁃ ing boundary nodes within suitably considered kernel radius h c . l = 1 - ∑ψ ( h ) V , j lj c j ( D1) where V j is the volume of boundary node j, and kernel ψ lj = ψ( | r l - r j | ,h c ) is a normalized and continuous B⁃spline interpolation kernel [37] expressed as 3 ìï15 2 - r2 + r 2 ï7 3 1 ï ψ( r,h) = í5 2 πh3 ï (2 - r) , 14 ïï î0, ( ), 0 ≤ r < 1, 1 ≤ r < 2, ( D2) r ≥ 2, where r = | r l - r j | / h and h c in the current simulation is set to 2 in order to create a small support in which it goes to zero at a distance of 2h c from its peak. References: [1] KOZLOV V, KOZLOV N, SCHIPITSYN V. Steady flows in an oscillating deformable container: [2] MIRAS T, SCHOTTE J⁃S, OHAYON R. Liquid sloshing damping in an elastic container [ J] . [3] LOPEZ D, GUAZZELLI E. Inertial effects on fibers settling in a vortical flow[ J] . Physical Re⁃ [4] [ 5] [6] effect of the dimensionless frequency[ J] . Physical Review Fluids, 2017, 2(9) : 094501. Journal of Applied Mechanics, 2012, 79(1) : 010902. view Fluids, 2017, 2(2) : 024306. SAURET A, CEBRON D, LE BARS M, et al. Fluid flows in a librating cylinder[ J] . Physics of Fluids, 2012, 24(2) : 026603. HABTE M A, WU Chuijie. Influence of wall motion on particle sedimentation using hybrid LB⁃ IBM scheme[ J] . Science China: Physics, Mechanics & Astronomy, 2017, 60(3) : 034711. J KAY J M, NEDDERMAN R M. Fluid Mechanics and Transfer Processes[ M] . Cambridge, New York: Cambridge University Press, 1985. [ 7] SCHLICHTING H, GERSTEN K, KRAUSE E, et al. Boundary⁃Layer Theory[M]. Vol 7. Spring⁃ [8] BUXTON G A, VERBERG R, JASNOW D, et al. Newtonian fluid meets an elastic solid: cou⁃ [9] er, 1955. pling lattice Boltzmann and lattice⁃spring models [ J ] . Physical Review E, 2005, 71 ( 5 ) : 056707. WU Z, MA X. Dynamic analysis of submerged microscale plates: the effects of acoustic radia⁃ tion and viscous dissipation[ J] . Proceedings: Mathematical, Physical, and Engineering Sci⁃ ences, 2016, 472(2187) : 20150728. [10] AURELI M, PORFIRI M. Low frequency and large amplitude oscillations of cantilevers in vis⁃ [ 11] FANG H, WANG Z, LIN Z, et al. Lattice Boltzmann method for simulating the viscous flow in [12] cous fluids[ J] . Applied Physics Letters, 2010, 96(16) : 164102. large distensible blood vessels[ J] . Physical Review E, 2002, 65(5) : 051925. DESCOVICH X, PONTRELLI G, MELCHIONNA S, et al. Modeling fluid flows in distensible Transverse Harmonic Oscillation of Rectangular Container With Viscous Fluid: a Lattice Boltzmann⁃Immersed Boundary Approach 393 tubes for applications in hemodynamics [ J] . International Journal of Modern Physics C, 2013, 24(5) : 1350030. [13] DOCTORS G M. Towards patient⁃specific modelling of cerebral blood flow using lattice⁃Boltz⁃ [14] MOUNTRAKIS L, LORENZ E, HOEKSTRA A. Revisiting the use of the immersed⁃boundary mann methods[ D] . PhD Thesis. University of London, 2011. lattice⁃Boltzmann method for simulations of suspended particles [ J ] . Physical Review E, 2017, 96(1) : 013302. [15] YAN G, LI T, YIN X. Lattice Boltzmann model for elastic thin plate with small deflection[ J] . [16] ARENAS J P. On the vibration analysis of rectangular clamped plates using the virtual work [ 17] GORMAN D. Free⁃vibration analysis of rectangular plates with clamped⁃simply supported edge Computers & Mathematics With Applications, 2012, 63(8) : 1305⁃1318. principle[ J] . Journal of Sound and Vibration, 2003, 266(4) : 912⁃918. conditions by the method of superposition[ J] . Journal of Applied Mechanics, 1977, 44( 4) : 743⁃749. [18] SUNG C⁃C, JAN C. Active control of structurally radiated sound from plates[ J] . The Journal [19] LADD A, VERBERG R. Lattice⁃Boltzmann simulations of particle⁃fluid suspensions[ J] . Jour⁃ [ 20] LADD A J. Numerical simulations of particulate suspensions via a discretized Boltzmann equa⁃ of the Acoustical Society of America, 1997, 102(1) : 370⁃381. nal of Statistical Physics, 2001, 104(5 / 6) : 1191⁃1251. tion, part 1: theoretical foundation[ J] . Journal of Fluid Mechanics, 1994, 271: 285⁃309. [21] QIAN Y, D’ HUMIÈRES D, LALLEMAND P. Lattice BGK models for Navier⁃Stokes equation [22] LADD A J. Lattice⁃Boltzmann methods for suspensions of solid particles[ J] . Molecular Phys⁃ [ 23] LADD A J. Numerical simulations of particulate suspensions via a discretized Boltzmann equa⁃ [ 24] [ J] . Europhysics Letters, 1992, 17(6) : 479. ics , 2015, 113(17 / 18) : 2531⁃2537. tion, part 2: numerical results[ J] . Journal of Fluid Mechanics, 1994, 271: 311⁃339. NIU X, SHU C, CHEW Y, et al. A momentum exchange⁃based immersed boundary⁃lattice Bo⁃ ltzmann method for simulating incompressible viscous flows[ J] . Physics Letters A, 2006, 354 (3) : 173⁃182. [25] SQUIRES K D, EATON J K. Particle response and turbulence modification in isotropic turbu⁃ [26] CAI S⁃G, OUAHSINE A, FAVIER J, et al. Moving immersed boundary method[ J] . Interna⁃ [27] [28] [29] lence[ J] . Physics of Fluids A: Fluid Dynamics, 1990, 2(7) : 1191⁃1203. tional Journal for Numerical Methods in Fluids, 2017, 85(5) : 288⁃323. DI FELICE R. The voidage function for fluid⁃particle interaction systems [ J] . International Journal of Multiphase Flow, 1994, 20(1) : 153⁃159. BROWN P P, LAWLER D F. Sphere drag and settling velocity revisited[ J] . Journal of Envi⁃ ronmental Engineering, 2003, 129(3) : 222⁃231. ESTEGHAMATIAN A, RAHMANI M, WACHS A. Numerical models for fluid⁃grains interac⁃ tions: opportunities and limitations[ C] / / European Physical Journal Web of Conferences. Vol 140. 2017: 09013. [30] SUNGKORN R, DERKSEN J. Simulations of dilute sedimenting suspensions at finite⁃particle [ 31] REIDER M B, STERLING J D. Accuracy of discrete⁃velocity BGK models for the simulation of reynolds numbers[ J] . Physics of Fluids, 2012, 24(12) : 123303. the incompressible Navier⁃Stokes equations[ J] . Computers & Fluids, 1995, 24(4) : 459⁃467. 394 HABTE Mussie A WU Chuijie [32] MAIER R S, BERNARD R S, GRUNAU D W. Boundary conditions for the lattice Boltzmann [33] ZHANG W, SHI B, WANG Y. 14⁃velocity and 18⁃velocity multiple⁃relaxation⁃time lattice Boltz⁃ [34] method[ J] . Physics of Fluids, 1996, 8(7) : 1788⁃1801. mann models for three⁃dimensional incompressible flows[ J] . Computers & Mathematics With Applications, 2015, 69(9) : 997⁃1019. HOFEMEIER P, SZNITMAN J. Revisiting pulmonary acinar particle transport: convection, sedimentation, diffusion and their interplay [ J] . Journal of Applied Physiology, 2015, 118 (11) : 1375⁃1385. [35] SHI Y, SADER J E. Lattice Boltzmann method for oscillatory stokes flow with applications to [ 36] SON S W, YOON H S, JEONG H K, et al. Discrete lattice effect of various forcing methods of [37] micro⁃and nanodevices[ J] . Physical Review E, 2010, 81(3) : 036706. body force on immersed boundary⁃lattice Boltzmann method[ J] . Journal of Mechanical Sci⁃ ence and Technology, 2013, 27(2) : 429⁃441. LIBERSKY L D, PETSCHEK A G, CARNEY T C, et al. High strain Lagrangian hydrodynamics: a three dimensional SPH code for dynamic material response[ J] . Journal of Computational Physics, 1993, 109(1) : 67⁃75. 矩形容器中黏性流体的横波谐振: 格子 Boltzmann 浸没边界方法 哈比特 M A, 吴锤结 ( 工业装备结构分析国家重点实验室( 大连理工大学) ; 大连理工大学 航空航天学院, 辽宁 大连 116024) ( 我刊编委吴锤结来稿) 摘要: 将三维格子 Boltzmann 法( LBM) 与浸没边界法( IBM) 相结合,研究弹性矩形容器中黏性流 体的横波谐振所引起的流动物理特性.提出了一个半微观表达式来计算边界节点处的流体受力.基 于薄板弹性变形理论,使用解析变形解法来计算边界所经历的位移.基于该方法的数值模拟结果与 固定边界的理论预测结果一致.采用振荡边界模拟展现了与理论预期相符合的流动模式. 关 键 词: 基 金 项 目: 格子 Boltzmann 法; 浸没边界法; 谐波振荡 国家自然科学基金(11372068) ; 国家重点基础研究发展计划(973 计划) (2014CB744104) 引用本文 / Cite this paper: HABTE Mussie A, WU Chuijie. Transverse harmonic oscillation of a rectangular container with vis⁃ cous fluid: a lattice Boltzmann⁃immersed boundary approach[ J] . Applied Mathematics and Mechan⁃ ics, 2018, 39(4) : 371⁃394. 哈比特 M A, 吴锤结. 矩形容器中黏性流体的横波谐振:格子 Boltzmann 浸没边界方法[ J] . 应用数学 和力学, 2018, 39(4) : 371⁃394.