Fluid Mechanics CHAPTER 5: CONTROL VOLUME APPROACH & CONTINUITY PRINCIPLE Dr . Ercan Kahya Engineering Fluid Mechanics 8/E by Crowe, Elger, and Roberson Copyright © 2005 by John Wiley & Sons, Inc. All rights reserved. LAGRANGIAN & EULERIAN DESCRIPTIONS Lagrangian Approach: Describe the fluid particle’s motion with time. The path of a particle: r(t) = x(t) i + y(t) j + z(t) k i, j, k: unit vectors Velocity of a particle : V(t) = dr(t) / dt = u i + v j + w k Eulerian Approach: Imagine an array of windows in the flow field: Have information for the fluid particles passing each window for all time. In this case, the velocity is function of the window position (x, y, z) and time. u = f1 (x, y, z, t) v = f 2 (x, y, z, t) w = f 3 (x, y, z, t) Eulerian approach is generally favored CONTROL VOLUME APPROACH: “Focusing on a volume in space & Considering the flow passing through the volume” * It derives from the Eulerian description of fluid motion. * It involves transforming the governing equations for a given mass (Lagrangian form) into the corresponding equations for mass passing through a volume in space (Eulerian form) • Mathematical equation needed for this transformation: REYNOLDS TRANSPORT THEOREM RATE OF FLOW • Volumetric Flow Rate: ∆Volume in the figure: = Length x Area = (V ∆t) x A Q AV Q = discharge [m3/s] V = average velocity [m/s] A = cross sectional area [m2 • Mass Flow Rate: Mass of fluid passing a station per unit time [kg/s] m mass flow rate m/t ρ = density [kg/m3] m AV Q RATE OF FLOW: Generalized equation forms Volumetric Flow Rate Differential discharge: dQ Vn dA Using concept of dot product: Vn dA V cosdA V .dA Mass Flow Rate Q V .dA A m V .dA A Mean Velocity By definition: _ Q V A • In laminar flow, the mean velocity is half the centerline velocity. • In turbulent flow, velocity profile is nearly flat so the mean velocity is close to centerline velocity. REAL VELOCITY PROFILE: • Parabolic for laminar flow • Logarithmic for turbulent flow Control Volume Approach • FLUID SYSTEM: Continuous mass of fluid, containing always the same fluid particles – The mass of a system is constant • CONTROL VOLUME (cv): Volume in space. – It can deform with time – It can move & rotate – The mass of control volume can change with time • CONTROL SURFACE (cs): • Surface enclosing the control volume • or boundary of control volume Control Volume Approach M sys (t ) M CV (t ) M in M sys (t t ) M CV (t t ) M out By definition, the mass of the system is constant, so M CV (t ) M in M CV (t t ) M out The rate form of Continuity Principle: M CV M in M out dM cv min mout dt Example • Considering a CV as shown in the earlier figure, a tank with crosssectional area of 10 m2 has an inflow of 7kg/s and an outflow of 5 kg/s. Find the rate at which the water level in the tank is changing. The volume of CV: V = Ah The mass in the CV: M cv = V = Ah The rate of change of mass in the CV: dM cv dh A dt dt By the continuity equation, the rate of change of water elevation: dh min mout dt A = (7 - 5 ) / (1000 x10) = 0.0002 m/s B (extensive property) of a system: proportional to the mass of the system (like m, mV, E) b (intensive property) : independent of system mass and obtained by [B/mass] Reynolds Transport Theorem The most general form: (Read excellent explanations at pages 133-138) dBsys dt b b B: extensive property b: intensive property t: time ρ: density V: volume V: velocity vector A: area vector Left side is Lagrangian form & represents the rate of change of property B of the system Right side is Eulerian form & represents the rate change of property B in CV + the net outflow of property B through the CS This equation is often called “control volume equation” Reynolds Transport Theorem: Simplified form If the mass crossing the control surface occurs through a number of inlet and outlet ports, and the velocity density and intensive property b are uniformly distributed (constant) across each port; then dBsys dt bdV bV. A cv cs Please see the text book for the alternative form of the above equation Continuity Equation Derives from the conservation of mass which states the mass of the system is constant in Lagrangian form. (M sys = const) The Eulerian form is derived by applying Reynolds transport theorem. In this case, extensive property: B cv = M sys The corresponding intensive property: b = M sys / M sys = 1 Continuity Equation Since dM sys / dt = 0 The general form of continuity equation: =0 Accumulation rate Net outflow rate of mass in CV + of mass through CS If the mass crosses the control surface through a number of inlet and exit ports, the continuity equation simplifies to dM cv cs min cs mout dt EXAMPLE 5.4: Since there is only one inlet and exit port, the continuity equation simplifies to dM cv min mout dt Mass flow rate in : ρ V A = 1000 x 7 x 0.0025 = 17.5 kg/s Mass flow rate out: ρ Q = 1000 x 0.003 = 3 kg/s Continuity equation: dM cv 17.5 3 14.5kg / s dt Mass is accumulating in the tank at this rate! EXAMPLE: (Problem 5.49) Referring the figure below, find the velocity of the liquid through the inlet. At a certain time, the surface level in the tank is 1 m and rising at the rate of 0.1 cm/s. Solution Continuity Equation for Flow in a Pipe Steady Flow - CV is fixed to pipe walls - Volume of CV is const. - Mcv = const. min mout 22 A22V22 11 A11V11 Continuity Equation Q2 Q1 A2V2 A1V1 Incompressible flow valid for steady & unsteady incompressible flow Cavitation Phenomenon that occurs when the fluid pressure is reduced to the local vapor pressure and boiling occurs. Vapor bubbles form in the liquid, grow and collapse; producing shock wave, noise & dynamic effects. RESULT: lessened performance & equipment failure ! Cavitation typically occurs at locations where the velocity is high. In case b, flow rate is higher Cavitation damage examples Impeller of a centrifugal pump Spillway tunnel in a power dam Class Exercises: (Problem 5.44)