STUDY OF THE EFFECT OF PROTUSIONS ON THE SLAT NOISE
USING BEAMFORMING TECHNIQUES AND
LATTICE-BOLTZMANN METHOD
Filipe Ramos do Amaral
Fernando Henrique Tadashi Himeno
Daniel Sampaio Souza
framaral@usp.br
fernando.himeno@gmail.com
dss em@yahoo.com.br
Carlos do Carmo Pagani Jr
paganni@sc.usp.br
Juan Carlos Serrano Rico
jcserrano@unipamplona.edu.co
Marcello Augusto Faraco de Medeiros
marcello@sc.usp.br
Sao Carlos Engineering School, University of Sao Paulo
Trabalhador Sao Carlense Avenue, 400. Zip-Code 13566-590. Sao Carlos, Sao Paulo, Brazil.
Abstract. The present work refers to an aeroacoustic experimental study of the high-lift device
called slat, containing a protrusion installed at the interior of its cove. This protrusion, which is
representative of a sealing device, was positioned at 4 different positions. The effect of the seal
was assessed in what concerns the frequency content of radiated noise and noise source field
using beamforming techniques, such as conventional beamforming and DAMAS. This study was
based in experimental data obtained at a wind tunnel of closed test section, using an array with
SECRETARIAT
62 microphones. Different anglesIkone
of attack
and flow velocities were tested in the wind tunnel.
Eventos | +55 (85) 3261-1111
Additionally one configuration
with
the
seal
and
the| Duets
original
clean
were simulated
Rua Dr. Gilberto Studart, 55 | Sala 1616
Office
Towerconfiguration
| Torre Sul
Cocóon
| 60192-105
Fortaleza | CEmethod
| Brasil (LBM). The main results eviwith a computational code based
Lattice | Boltzmann
denced that the presence of theE-mail:
seal cilamce2014@eventosikone.com.br
in the slat cove has a huge impact at its acoustic frequency
spectrum and, depending on its position, it could attenuate or intensify the low frequency tonal
peaks, which are characteristics of slat models. Simulated far-field spectrum agrees within 3
dB/Hz with wind-tunnel measurements for the clean configuration and the effect of the seal is
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
qualitatively captured by the simulation.
Keywords: aeroacoustic, slat, DAMAS, beamforming, Lattice-Boltzmann Method.
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
1
INTRODUCTION
For commercial aircraft, airframe noise can be dominant when the engines operate at low
power and high-lift devices and landing gears are deployed (Brooks and Humphreys, 2003).
Propulsion systems are becoming increasingly quieter, therefore more emphasis are held at
measure and modeling non-propulsive systems, as flaps, slats and landing gears, with the purpose of noise reduction.
It is interesting to note that for the same speed a high-lift wing is about 10 dB noisier
compared with the same wing in cruise configuration (Dobrzynski, 2010). However, for typical
flight operations this level difference is much lower because the typical speed of an aircraft in
cruise configuration is higher compared with the speed of an aircraft in high-lift configuration
during approach. Amongst the high-lift devices, the slat deserves special attention because it is
a line source distributed along the entire wingspan. For aircrafts focused in regional routes, slat
noise can be the main source during approach and landing phases.
Figure 1 presents a schematic representation of the slat region, including a typical flow
around the slat with the streamlines based on the time-averaged flow. At slat lower surface,
the boundary layer is forced to separate at the cusp by a strong streamwise adverse pressure
gradient. This separation originates a mixing layer limiting the accelerated flow, between
the slat and the main element, and the slower flow at the recirculation bubble inside the slat
cove. This mixing layer is unstable and amplifies small perturbations, which lead to the creation of discrete vortices that grows and pairs as they are convected towards the reattachment
point on the cove wall. Numerical simulations and experiments at wind tunnels show that
the separation bubble size at the cove decreases when the airfoil angle of attack increases
(Dobrzynski and Pott-Pollenske, 2001). After the reattachment, some mixing layer vortices are
ejected through the gap between the slat and the main element, and the rest of them enters the
separation bubble and get stuck inside it, recirculating, which introduces perturbations at the
beginning stage of the mixing layer, near the cusp.
Gap
Trailing-edge
Main Element
Cove
Mixing Layer
Slat
Cusp
(a)
(b)
Figure 1: Schematic drawing of a slat. (a) Main reference points and (b) typical flow field around of a slat,
including the streamlines for an angle of attack of 4 ◦ .
A schematic representation of the spectrum of slat noise is shown in Figure 2. (Imamura et al., 2009)
separates this spectrum in three components: a broadband portion at low to mid-frequencies, a
tonal peak of high-frequency and multiple peaks at the frequency range of maximum broadband
noise. Experiments and computational simulations relates the high-frequency peak to the shedding of vortices from the slat trailing-edge. On the other hand, the mechanism responsible for
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
the other two components of the spectrum is not completely understood. While the broadband
noise is presumably generated by oscillations in the mixing layer and its interaction with the
slat trailing-edge (Khorrami et al., 2004), (Dobrzynski and Pott-Pollenske, 2001), the multiple
tones may be generated by an acoustic feedback mechanism commonly reported in rectangular
cavities (Kolb et al., 2007), (Terracol et al., 2011).
PSD [dB/Hz]
Multiple Tonal
Peaks
High Tonal Peak
Broadband
Frequency [Hz]
Figure 2: Typical slat noise spectrum.
Studies aiming at aeroacoustic noise generated by the slat commonly employs clean idealized geometries. However, real airplane slats presents a series of devices which are necessary for their operation, such as deploying mechanisms, tubes from the anti-icing system and
seals. Two-dimensional Navier-Stokes simulations by (Khorrami and Lockard, 2006) shown
that a bulb seal inside the slat cove has no significant effect on the emitted noise, while threedimensional Lattice-Boltzmann results by (Bandle et al., 2012) indicated a detrimental effect of
rectangular seals at the same region.
The work reported here analyzes the effect of an excrescence on the slat cove wall, representing a sealing device, on the slat noise. Beamforming processing of wind-tunnel measurements is used to analyze the effect of the seal at different positions for five angles of attack
of the high-lift airfoil. Numerical simulations were carried out for the clean slat geometry as
well as for one configuration of seal considering an angle of attack of 4◦ . Computational and
experimental results are compared in order to cross-validate both methodologies.
2
EXPERIMENTAL METODOLOGY
The high-lift airfoil employed corresponds to the model MDA 30P30N, Fig. 3, which has
slat and flap chords, respectively, of 15 % and 30 % of the chord at cruise condition (with the
high-lift elements, flap and slat, non-deployed). The deflections of flap and slat are 30 ◦ , the
gap and overlap of the flap are, respectively, 1.27 % and 0.25 %, and the gap and overlap of the
slat are, respectively, 2.95 % and −2.50 %.
Figure 4 shows, schematically, a general configuration of the slat with the seal in the cove.
The position of the seal is defined by its distance to the slat trailing-edge, Lseal . The seals had a
square cross-section of 3 mm edge and extended through the entire model span. Four positions
of the seal were tested in the wind-tunnel, namely, Lseal = 8 mm, 17.5 mm, 30.5 mm and 45
mm, as shownin Figure 5
A subsonic wind tunnel was used, with closed section and closed circuit. This wind tunnel
is located at the Aerodynamic Laboratory (LAE) of the Sao Carlos Engineering School, University of Sao Paulo (USP-EESC). It has a test section measuring 1.30 m of height, 1.67 m of width
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
(a)
(b)
(c)
Figure 3: MDA 30P30N. (a) airfoil MDA 30P30N with high-lift devices deployed and at cruise condition
(detail for the chord line), (b) e (c) model inside the wind tunnel test section.
Main Element
Lseal
Seal
Slat
Figure 4: Detail of the high-lift airfoil MDA 30P30N containing a square seal positioned at the slat cove.
(a)
(b)
(c)
(d)
Figure 5: Model lower surface view, containing the seals at the slat cove. Positions: (a) 8 [mm], (b) 17.5
[mm], (c) 30.5 [mm] e (d) 45 [mm].
and 3.00 m of length. An axial fan of 8 blades with a 115 HP propellant, of alternated current,
was able to provide flow with velocities between 10 m/s and 45 m/s. The contraction ratio is
1 : 8 and the turbulence level is about 0.25 %. This wind tunnel was originally designed for
aerodynamics experiments and it was adapted to receive aeroacoustic experiments. The adopted
methodology decreased background noise of the wind tunnel without significant changes in its
aerodynamics characteristics, as can be seen at (Santana et al., 2010). There is a turntable at test
section, which provides the model angle of attack and have a boundary layer suction system.
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
Figure 6 shows the test section and the turntable mechanism.
Aeroacoustic measurements have been performed with an array composed of 62 flushmounted microphones. As shown in Figure 3c, the array is wall-mounted in a position from
which is possible focusing on the wing model lower surface by a proper signal processing technique. The array geometry stems from an optimized Archimedean Spiral Equation, (Fonseca et al., 2010).
The microphones used are of GRAS 40BD 41 in type. The data acquisition systems are based on
National Instruments hardware. More details about it can be seen at (PaganiJr et al., 2011).
The experiments were carried out for model angles of attack (AoA) of 2◦ , 4◦ , 6◦ , 8◦ and 10◦ ,
and free-stream velocities (U∞ ) of 24 m/s, 27 m/s, 31 m/s and 34 m/s for baseline case. For the
cases with seals all six angles of attack were tested but only two velocities were considered, 31
m/s and 34 m/s.
(a)
(b)
Figure 6: Wind Tunnel LAE-1. (a) Test section and (b) turntable inferior view.
3
3.1
NUMERICAL METHODS
Beamforming Technique
The beamforming technique applied to acoustics is widely used for the mapping of noise
sources. This work uses a beamforming algorithm that assumes spherical waves propagation in
a free field condition, from uncorrelated point sources distribution of monopole type. A scanning plan, containing a discrete mesh of N points, is defined around the assumed noise source
region. The beamforming code used here weights the cross-spectral matrix (CSM), processed
from array microphone measurements, with steering vectors stemmed from transfer functions
modeling (at each frequency) the sound propagation from each focal point to all array microphones. By successively focusing the array pattern to each target point of a scanning grid, it is
possible to achieve a spatial pressure level distribution, which indicates the most likely source
position according to a contour level map called beamforming map, (Mueller et al., 2002). Figure 7 displays a schema of the referential system used at beamforming formulation in this work,
the scanning mesh, which contains the region of interest, and the microphones array. The position n is a mesh point to be scanned in searching for noise sources, with 1 ≤ n ≤ N ; o as the
reference position, or origin (for this work, is assumed to be the array center); and m is the m-th
microphone position, where 1 ≤ m ≤ M ; with M being the total number of microphones.
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
Array of Microphones
rm,o
m
o
U
ro,n
rm,n
n
Mesh Points
Mesh
Figure 7: Schema of the referential used for beamforming formulation.
Equation 1 shows a general formulation for conventional beamforming widely found at
literature, as in (Sarradj, 2012). It is expected that a beamforming output brings about the
maximum response (peak) when a mesh focus point, n, coincides with the actual position of the
source.
D
E
b(~ro,n , ω) = h† (~rm,n , ω) p(~rm,o , ω)p† (~rm,o , ω) h(~rm,n , ω)
(1)
In this formulation, b is a scalar and denotes a spectral estimate of sound pressure level at
angular frequency ω (where ω = 2 · π · f ), on the focus point n (with ~ro,n being the position
vector), relative to a reference, or origin, given by o. p is a vector that contains the spectral
estimate of sound pressure level, written as Fourier Transforms of the signals acquired by the
microphones at position vector ~rm,o . The vector h accounts for the steering vectors, that are
normalized transfer functions used to model a propagation of a wave sound between a mesh
point (related to a reference ~ro,n ), and the arrays microphones (at ~rm,n ). The symbol † gives a
hermitian operator, and h· · ·i denotes a measurement expected value.
The transfer function, denoted by g, models the propagation of a pressure wave, accounting
the phase delays and amplitude decay due to spherical spreading of sound that travels between
the source and the microphones. It is a column vector, with its terms given by:
ro,n −jω(rm,n −ro,n )
c
e
(2)
gn (~rm,n , ω) =
rm,n
√
where the sound velocity is given by c and j denotes −1.
So, the steering vector h is:
h(~rm,n , ω) = qP
M
g(~rm,n , ω)
m=1
PM
m0 =1
(3)
|gm0 | |gm |
As a result of superposition of acoustic waves generated by N punctual monopole sources,
uncorrelated and distributed in space, the spectral estimate of sound pressure level in the m-th
microphone, pm , is:
pm (~rm,o , ω) =
N
X
gn (~rm,n , ω)qn (~ro,n , ω)
(4)
n=1
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
where qn is the assumed sound source amplitude, distant ~ro,n from origin position.
D
E
The cross-spectral matrix, CSM, is the external product p(~rm,o , ω)p† (~rm,o , ω) of Eq. 1.
2
This three-dimensional matrix, of order M ×M , grouping M auto correlations and M 2−M cross
correlations at each angular frequency ω. At this work, the CSM elements are given as Power
Spectral Density, or PSD.
Beamforming results, submitted in a conventional way, generates acoustic maps from the
results obtained with arrays measurements. Due to both limited array resolution and spatial
sampling, the array output does not necessarily represent the actual source distribution. This
drawback may be greatly remedied by deconvolution between the true sound field and the array
pattern, or point spread function (psf ). At DAMAS (Deconvolution Approach for the Mapping
of Acoustic Sources) method, introduced by (Brooks and Humphreys, 2006), the goal is that
the desired quantities, such as sound pressure levels and sources distributions, are extracted
independently of the arrays characteristics. It is assumed N sources statistically independent,
each one in a different position at the mesh. It is set up a linear system with N unknowns, in a
way to connect conventional beamforming points field with its equivalent source distribution at
the same points location. The sources distribution is iteratively solved. Thus, the problem can
be written as:
b=A·X
(5)
This represents a linear equations system, relating the space points field with beamforming
given by the arrays response, b, and its equivalent sources distribution, x, at the same location of
beamforming results. A is a square matrix of order N , gathering all array point spread function.
Attempts to find a proper solution by simply inversion of matrix A did not provide acceptable
results, probably due to the system represented in Eq. 5 be highly ill-conditioned. Therefore, a
modified Gauss-Seidel iterative method, considering a positivity constrain to avoid non-physical
negative source auto-power estimate was applied, (Brooks and Humphreys, 2006).
For a good convergence of the DAMAS algorithm, many parameters deserve special attention, including the spacing between points at the mesh, mesh dimensions, etc. These parameters might be seen at (Brooks and Humphreys, 2006). More details about the beamforming and
DAMAS numerical method, as well the full development of the equations, codes performance
and validation analysis, can be found at (PaganiJr, 2014), (Sarradj, 2012) and (Mueller et al., 2002).
3.2
Lattice-Boltzmann Simulation
Simulations used the commercial code PowerFLOW 5.0a (EXA). This software, based on
Lattice-Boltzmann method, solves the discrete Boltzmann equation for the density function (f ),
using the approximation proposed by (Bhatnagar et al., 1954),
1
(6)
f (~r + ~cδt, ~c, t + δt) − f (~r, ~c, δt) = − [f (~r, ~c, t) − f eq (~r, ~c, t)]
τ
The mesh performed by PowerFLOW is composed of cubic volumes aligned with the Cartesian coordinates. The macroscopic flow properties are retrieved by the hydrodynamic moments,
ρ(~x, t) =
X
fi (~x, t)
(7)
i
ρ~u(~x, t) =
X
~ci fi (~x, t).
i
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
(8)
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
To represent the effect of the smallest turbulent scales a Renormalization Group k- turbulence model is used. The turbulent kinematic viscosity (νT ) is combined with the molecular
viscosity (ν) in the definition of the relaxation time, τ = (νT + ν)T + δt/2 (Teixeira, 1998),
(Fares, 2006).
For analyses of far-field propagated sound, the software uses a Ffowcs Williams-Hawkings
(FW-H) analogy algorithm, based on Farassat’s 1A formulation (Brès et al., 2010). Since the
Mach numbers in our analysis are low, the quadrupole sources in the flow were not taken into
account in the FW-H calculations. Therefore, only the slat surface and the aft portion of the
main element surface as integration surfaces.
Two configurations were simulated numerically, the clean slat and the slat with the seal
Lseal = 30 mm. The simulationsed were set to reproduce the experiments with free-stream
velocity of 34 m/s and 4◦ angle of attack. The simulated high-lift airfoil had stowed chord of
0.5 m and the distance between the upper and lower boundaries of the simulations was 1.7 m,
representing the width of the wind-tunnel (since the model is installed vertically in the tunnel, as
shown in figure 3) Periodicity was considered in the spanwise direction, which corresponds to a
wing of infinite span, given the domain length is long enough in this direction. Uniform velocity
was imposed in the inflow, whereas at the outflow the static pressure was prescribed as 1 atm.
The flow properties resulted in a free-stream Mach number of 0.1 and A Reynolds number, based
on the stowed chord and free-stream velocity, of 1 million. The tunnel walls were simulated as
free-slip walls and a layer of non-physical high viscosity fluid was modeled at the outer portion
of the simulated domain to reduce the initial transient duration. The numerical grid, based in
previous convergency tests, had its smallest volumes of 0.2 mm edge in the slat region and flap
leading-edge.
4
4.1
RESULTS AND DISCUSSIONS
Comparison between LBM and experiments
Time average pressure coefficient over the surface, CP , is shown in Figure 8 for the central
section of the simulated span for the clean slat configuration and the case with seal at Lseal = 30
mm. The averaged numerical data comprise the last 0.15 s of the simulation, after discardin the
inicial transient of 0.1 s. For both cases analysed the numerical and experimental results agree
well. However, the suction peak of the main element is underestimated by the simulation.
Moreover, the simulation aticipate the separation of the flap boundary layer, indicated by the
flat region on CP curve, in comparison with the experiments, which may be caused by poor calculation of the boundary layer by the Lattice-Boltzmann code. It is important to emphasize the
match in the CP distribution over the slat between both methodologies, since the interest here
is on the noise generaed there. The addition of the seal in the slat cove increases the circulation
of this element. This behavior is captured similarly by the experiments and simulations.
Figure 10 shows spectra of far field noise from both numerical and experimental results.
Deconvolved beamforming maps based on the wind-tunnels measurements were integrated over
a region of interest (ROI) focused on the slat region. Schemes of the ROI is presented in Figure
9. The region where the spatial distribution of noise sources were calculated using beamforming
is also presented in Figure 9(a). The ROI dimensions are 0.8 m in the spanwise direction and
0.18 m in streamwise direction. The spatial mesh used in the calculation of beamforming maps
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
−6
−6
AoA = 4 deg
U∞ = 34 m/s
AoA = 4 deg
U∞ = 34 m/s
Seal at 30 mm TE
−5
−4
−4
−3
−3
CP
CP
−5
Simulation
Experiment
−2
−2
−1
−1
0
0
1
1
2
Simulation
Experiment
2
0
0.2
0.4
0.6
0.8
1
1.2
0
0.2
0.4
0.6
x/c
0.8
1
1.2
x/c
(a)
(b)
Figure 8: Pressure distribution coefficient, CP . Case (a) baseline and (b) with seal located at 30 [mm] from
slat trailing edge.
were contained in a rectangular domain of 1.4 m and 0.3 m in the spanwise and stremwise
directions, respectively. These dimensions were based on previous analysis by (PaganiJr, 2014).
He has verified the dominium independence criteria for different airfoil operational conditions,
such as many angles of attack and flow velocities. The FW-H calculations based on numerical
unsteady data were carried out for a listener at a positioned corresponding to the center of the
microphone array used in the experiments. Differently from the beamforming ROI, The FW-H
integration region had 0.0256 m in the spanwise, therefore, the PSD levels were normalized as
indicated in Figure 10.
(a)
(b)
Figure 9: (a) Mesh (minimum dimensions) and ROI used at this work and (b) ROI schema at the models
lower surface.
As observed in Figure 10a, the basic elements of slat noise are captured by both methodologies. The quantitative agreement between numerical and experimental results is good. This
good match can be seen on the frequency and the level of the multiple tonal peaks as well as the
level and decay rate of the broadband fluctuations between St∼ 6 and ∼ 12. On the other hand,
the high-frequency hump around St=20 is significantly overpredicted by the simulation. A grid
study indicated that the numerical solution at this range of frequency requires finer spatial resolution. However the noise level of this hump is at least 20 dB/Hz lower than the maximum level
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
of slat noise. Therefore, no further grid refinement were attempted.
The experimental results in figure 10b indicates that the seal at the position Lseal = 30 mm
increases the level of two low-frequency tones, the one around St=1.6 and the one at St∼ 3.2.
The same trend is observed qualitatively in simulation results. The increase in the level
of the peaks is seen but it is less pronounced in the simulations. Additionally, the experiment
present slightly higher amplitudes between St∼ 4 and ∼ 5. At the rest of the frequency range
of interest the same level of agreement is observed in the case with the seal and the baseline.
The acoustic spectra measured and simulated is presented in Figure 10. For baseline case,
Figure 10a, the typical spectra could be seen for both methods. Good concordance is noted comparing the experimental and simulation data. This is observed by amplitude levels of acoustic
spectra and the captation of peaks obtained. The decay rate is too evidenced for both methods
between St = 6 and St = 12. The hump in high frequency is overestimated in numerical
results. Probably a refined mesh could capture better the fenomena envolving this, this was not
done because its contribution has around 20 dB/Hz less than proeminent peaks located in lower
frequencies.
For case with seal, Figure 10b, the experimental data indicates that its presence increases
the noise levels at St ∼ 3.2 and mainly at St ∼ 1.6. This spectra shows that simulation
could indicate the effect of seal but not the correct level of this peaks, subestimated in this
case. This result was obtained before by (Bandle et al., 2012). Between St ∼ 3.5 and St ∼ 5
simulation subestimates the results again but decay rate and spectra levels seems remarkable.
For high frequencies simulation subestimates the result compared to experimental data as noted
for baseline case.
80
Simulation
Experiment
70
70
60
60
PSD [dB/Hz]
PSD [dB/Hz]
80
50
40
50
40
30
30
20
20
10
10
0
100
101
Simulation
Experiment
0
100
101
Strouhal
Strouhal
(a)
(b)
Figure 10: Comparison of Measured and Computed Acoustic Spectra for Slat (a) baseline and (b) with seal
at 30 [mm] from slat trailing edge.
4.2
Parametric Study
An ideal source distribution along slat wingspan is a line source. The sound pressure levels measured by the array are evaluated through the sum of these levels in a mesh subdomain
named region of interest, or ROI. This integration region might be chosen accounting the spatial
distribution of the sources at beamfoming maps for the frequencies to be analyzed. For a slat,
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
low-frequency sources are irregularly distributed, so it is difficult to set an appropriate ROI.
For medium and high frequencies, this sources are linearly distributed along slat wingspan. A
ROI has to perform a good sources distribution sampling along slat wingspan and, eventually,
exclude spurious sources. At this work, this region must typify a two-dimensional sources distribution, excluding three-dimensional sources such as the ones generated by the union between
the model and the turntable. Figure 9 exposes a schema of the mesh (blue rectangle) and the
ROI (grey rectangle) positions and dimensions with respect to the model and array.
The next figures shows the noise spectrum and beamforming maps for baseline case (without seals at slat cove). It is possible to see the three components, as (Imamura et al., 2009)
describes at their work: multiple tonal peaks, broadband and high-frequency tonal peak.
70
PSD [dB/Hz]: 20*log10(Pa
rms
2°
4°
6°
8°
10°
40
30
20
10
3
4
10
10
Frequency [Hz]
(a)
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
)
50
rms
60
/20 µ Pa
70
24 [m/s]
27 [m/s]
31 [m/s]
34 [m/s]
60
50
40
30
20
10
3
4
10
10
Frequency [Hz]
(b)
Figure 11: Baseline noise spectrum. (a) U∞ = 34 m/s and AoA between 2 and 10
U∞ = 34 between 24 and 34 m/s.
◦
and (b) AoA 4
◦
and
Figure 11a presents the noise spectrum for flow velocity, U∞ , of 34 m/s and angles of
attack, AoA, between 2 ◦ and 10 ◦ . It shows that the smaller the angle of attack, the greater the
noise intensity and low-frequency tonal peaks are more salient. The tonal high-frequency peak
occurs at a higher frequency the higher the angle of attack. At Fig. 11b, where the angle of
attack is 4 ◦ and flow velocity are posed between 24 m/s and 34 m/s, states that the tonal peaks
of both low and high frequency, are shifted to higher frequencies as the flow velocity increases.
(Dobrzynski and Pott-Pollenske, 2001) indicates that slat noise spectrum has a collapse with
Mach number when using the 4.5 power law, and that the amplitude peak of noise spectrum
might be at Strouhal number of 2 (with Strouhal number based at slat chord and free field flow
velocity).
Fig. 12 exhibits these collapses, with Mach and Strouhal numbers in reference with a
Mach number of M = 0.1, for an angle of attack of 8 ◦ and flow velocities between 24 m/s
and 34 m/s. These results indicates a good agreement with another works at literature, such as
(Imamura et al., 2009), (Ura et al., 2010) and (Dobrzynski and Pott-Pollenske, 2001).
Some beamforming maps, using DAMAS deconvolution method, for a flow velocity of 34
m/s and 4 ◦ of angle of attack are shown at Fig. 13. A dynamic range 12 dB/Hz were used.
These maps also exhibits the microphones array (blue circles, at center), the slat (rectangle of
black border, at left), the region of interest (or integration region, left greyish rectangle) and the
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms) − 10*log10((M/Mref)4.5)
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
70
24 [m/s]
27 [m/s]
31 [m/s]
34 [m/s]
60
50
40
30
20
10
0
0
10
1
10
Strouhal
Figure 12: Baseline noise spectrum colapse with Strouhal number and Mach number 4.5 power law. AoA
8◦ .
mesh where noise levels are in contour plots at PSD, in dB/Hz. The free field flow direction
is from left to right. The first three maps are in frequencies that are equivalent to the ones at
multiple tonal peaks, the fourth and fifth map are at frequencies in broadband range and the
sixth map are at high-frequency tonal peak. These three first tonal peaks maps does not have a
pattern such as broadband and high-frequency tonal peak maps, which presents a line source,
well defined, at slat wingspan.
The following data refers to seals installed at the slat cove, along its wingspan. Four positions were measured from slat trailing edge. Figure 14 shows the noise spectrum for cases with
seals at slat cove, with a flow velocity of 34 m/s and angles of attack of 4◦ and 8◦ . The first
tonal peak is better pronounced in the case with a seal positioned at 30.5 mm from slat trailing
edge. The second tonal peak is rather intense when a seal is placed at 45 mm from slat trailing
edge. Now, the third tonal peak occasionally is intense in the case with a seal at 30.5 mm or at 8
mm. Greater the angle of attack, the case of 8 mm is favored. The case with a seal at 8 mm has
a well-marked broadband component, different from the other cases, including baseline case,
in which this component collapses. Broadband component has the same behavior found with
the third peak for the 8 mm seal case greater the angle of attack, more intense the broadband
component.
In Fig. 15 is shown the intensity of multiple and high-frequency peaks at many angles of
attacks for each seal and baseline cases. The influence of the seals at slat cove is summarized in
this figure. It is easily verified that the seal at 17.5 mm has lower intensity, in comparison with
baseline and other seals cases, for almost all angles of attack. Interesting to notice that a seal at
8 mm does not behave as the other ones and baseline case, when analyzing the high-frequency
peak the peak increases in intensity with an increase in models angle of attack. The other
characteristics follow the same behave already discussed above, in Fig. 14.
When plot the peaks intensity versus seals distance from trailing edge for each angle of
attack studied, Fig. 16, is possible to conclude that there is an optimum seal position, where the
peaks intensity are reduced for almost the entire noise spectrum and for most angles of attack.
This position is around 17.5 mm.
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
(a)
(b)
(c)
(d)
(e)
(f)
Figure 13: DAM AS maps at U∞ = 34 [m/s] and AoA 4 [◦ ]. (a) 800 [Hz], (b) 1175 [Hz], (c) 1600 [Hz], (d)
2800 [Hz], (e) 5000 [Hz] and (f) 10400 [Hz].
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
/20 µ Pa
rms
PSD [dB/Hz]: 20*log10(Pa
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
Baseline
SQ08
SQ17
SQ30
SQ45
70
rms
)
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
60
50
40
30
20
10
Baseline
SQ08
SQ17
SQ30
SQ45
70
60
50
40
30
20
10
3
4
10
3
10
4
10
Frequency [Hz]
10
Frequency [Hz]
(a)
(b)
80
80
70
70
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
Figure 14: Comparison between baseline and cases with seals at slat cove for U∞ = 34 m/s. (a) AoA 4 ◦ and
(b) AoA 8◦ .
60
50
40
30
20
Baseline
8 [mm]
17.5 [mm]
30.5 [mm]
45 [mm]
10
0
2
60
50
40
30
20
Baseline
8 [mm]
17.5 [mm]
30.5 [mm]
45 [mm]
10
4
6
8
0
10
2
4
Angle of Attack [°]
(a)
10
8
10
35
70
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
8
(b)
80
60
50
40
30
20
Baseline
8 [mm]
17.5 [mm]
30.5 [mm]
45 [mm]
10
0
6
Angle of Attack [°]
2
4
6
Angle of Attack [°]
(c)
8
10
30
25
20
15
10
Baseline
8 [mm]
17.5 [mm]
30.5 [mm]
45 [mm]
5
0
2
4
6
Angle of Attack [°]
(d)
Figure 15: Noise spectrum intensity. (a) First Peak, (b) Second Peak, (c) Third Peak and (d) High-Frequency
Peak.
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
80
80
70
70
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
60
50
40
30
20
2 [°]
4 [°]
6 [°]
8 [°]
10 [°]
10
0
10
60
50
40
30
20
2 [°]
4 [°]
6 [°]
8 [°]
10 [°]
10
15
20
25
30
35
40
0
45
10
15
Distance from Slat Trailing Edge [mm]
20
60
30
50
40
30
20
0
2 [°]
4 [°]
6 [°]
8 [°]
10 [°]
10
15
20
25
30
35
Distance from Slat Trailing Edge [mm]
(c)
30
35
40
45
40
45
(b)
35
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
PSD [dB/Hz]: 20*log10(Parms/20 µ Parms)
(a)
70
10
25
Distance from Slat Trailing Edge [mm]
40
45
25
20
15
10
2 [°]
4 [°]
6 [°]
8 [°]
10 [°]
5
0
10
15
20
25
30
35
Distance from Slat Trailing Edge [mm]
(d)
Figure 16: Noise spectrum intensity. (a) First Peak, (b) Second Peak, (c) Third Peak and (d) High-Frequency
Peak.
Such as Fig. 13, beamforming maps, using DAMAS deconvolution method, are shown in
Fig. 17. It can be seen that the maps does not significantly differs from the ones without seals
at slat cove, Fig. 13. The first tonal peak has two well-defined sources located on slat, at the
region of interest vertical borders. The second tonal peak presents three sources, at vertical
center and borders of the region of interest. At the third peak, it is not possible to find a source
pattern. Finally, the broadband and high-frequency peak components presents a line source
pattern, along the slat wingspan.
5
CONCLUSION
The effect of a seal at the slat cove on the aeroacoustic noise radiated were assessed. Results from wind tunnel experiments and Lattice-Boltzmann simulations were compared with the
original, clean configuration, of the high-lift M DA30P 30N airfoil and for one configuration
with a square seal at the slat cove. For the original, baseline configuration, numerical and experimental results match within ∼ 3 dB/Hz, which indicates the reliability of both methodologies.
Simulations and experiments indicated that the level of two tonal peaks are higher for the case
CILAMCE 2014
Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Amaral, F.R., Himeno, F.H.T., Souza, D.S., Serrano R, J.C., Medeiros, M.A.F.
(a)
(b)
(c)
(d)
(e)
Figure 17: DAMAS maps. (a) 800 Hz, seal at 8 mm, U∞ = 34 m/s and AoA 8◦ , (b) 1100 Hz, seal at 17.5 mm,
U∞ = 34 m/s and AoA 8◦ , (c) 1450 Hz, seal at 30.5 mm, U∞ = 34 m/s and AoA 4◦ , (d) 10800 Hz, seal at 45
mm, U∞ = 34 m/s and AoA 4◦ and (e) 4000 Hz, seal at 8 mm, U∞ = 31 m/s and AoA 6◦ .
CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
Study of the effect of protrusions on the slat noise using Beamforming techniques and Lattice-Boltzmann method
with the seal at Lseal = 30 mm in comparison with baseline configuration. However, the increase in the level of the peaks was more pronounced in the wind tunnel measurements than in
the simulations. Additionally, experiments were carried out for various positions of the seal at
different flow velocities and airfoil angles of attack. It was shown that the spectrum of slat noise
is very sensitive to the position of the seal. Comparing to the baseline, the seal, depending on
its position, increased the level of one or two tones, or altered the dominant peak, or reduced
the global level of the spectrum.
ACKNOWLEDGEMENTS
The authors would like to thanks CNPq and CAPES for their financial support.
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Proceedings of the XXXIV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014
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CILAMCE 2014
Proceedings of the XXXV Iberian Latin-American Congress on Computational Methods in Engineering
Evandro Parente Jr (Editor), ABMEC, Fortaleza, CE, Brazil, November 23-26, 2014