12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022

WAKE FLOW – ACOUSTIC RESONANCE IN A GENERIC AIR OUTLET

Asvath RavichandranMichael Manhart
Professorship of Hydromechanics
Professorship of Hydromechanics
Technical University of Munich
Technical University of Munich
Arccistr. 21, 80333 Munich, Germany Arccistr. 21, 80333 Munich, Germany
asvath.ravichandran@tum.demichael.manhart@tum.de

Florian Schwertfirm
Kreuzinger+ Manhart Turbulenz GmbH
Kirchenstr. 34, 81675 Munich, Germany
f.schwertfirm@km-turbulenz.de

ABSTRACT

Nikolaus Peller
HVAC Simulation
AUDI AG
Ingolstadt, Germany
nikolaus.peller@audi.detions of flat plate, tandem plate and cascade plates (Honget al., 2020; Stoneman et al., 1988; Parker, 1967; Koch, 2009;
Yokoyama et al., 2013) an insight into the noise generation
mechanisms of individual elements could be gained, but theinteraction of those elements is still quite complex to interpret.

We aim to investigate the noise generation mechanism in a
generic geometry representing a typical air outlet of an HVAC
system of a car. We use an Aero-Acoustic simulation based onLarge-Eddy Simulation and an experiment to study the interaction between a throttle plate (used to control the flow rate)
and fins (used to guide the flow). We can identify several soundgeneration mechanisms, of which the so-called Parker β -mode
is the most prominent in the sensible frequency range. Our focus is on the interaction and resonance of this acoustic modewith the vortex shedding modes in the wake of the throttle
valve, which seems to play a major role in determining theamplitude of the acoustic mode.

Previously, Kreuzinger et al. (2013) did a pure Computational Aero-Acoustic(CAA) simulation in an AUDI TT air
outlet. It was concluded that the anti-symmetrical Parker’s
β -mode was not excited by the flow from the comparison
with hybrid flow acoustic simulations and experiment. Also,
Ravichandran et al. (2022) did investigations with different inflow conditions in an automotive series air outlet and a generic
air outlet. The different inflow conditions had a slight effect on
the increase in the amplitude of the resonance modes – particularly the β -mode – due to upstream turbulent intensity. The
Parker-β -mode denotes the fundamental acoustic mode aroundthe throttle valve with pressure oscillation in anti-phase aboveand below the center of the plate as proposed by Parker (1967).
The pressure nodes and therefore anti-nodes of acoustic velocity are located at the leading edge and trailing edge of thethrottle valve as in Ewert & Kreuzinger (2021) and Stoneman
et al. (1988). The current paper considers the numerical results
obtained to date, which are still a work in progress.

INTRODUCTION
The noise generated by the air outlet of the HVAC system of a car has attracted increasing research interest. The
sources of noise generation in a HVAC unit can be classified
into low frequency and high-frequency noises. The hummingand buzzing noises at low frequency are mostly due to the components, namely blower and condenser. The tonal noises athigh frequency are mostly from the air outlets, channels anddiscontinuities in the system such as air channel junctions.
These are associated to the characteristic dimensions of thesystem namely duct height, section length and dimension of
the flow guides (Perot et al., 2011). Guerin et al. (2005) haveinvestigated the aero acoustics of an automotive air outlet witha generic model followed by different configurations namely,
Throttle Valve (TV) parallel and inclined to the mean flow direction along with fins. When the TV was parallel to the flow
direction only weak noise was observed with fins being themain noise source. But with the inclined TV, mutual interaction between the TV and fins resulted in a significant increasein broadband noise levels. It was seen that increasing the distance between TV and fins reduced the noise by several decibels. But this was observed only for the inclined TV configuration. The mutual interaction between TV and fins at different
gap settings was not investigated for TV parallel to the mean
flow direction.
From previous numerical and experimental investiga-

In this study, we investigate the reason for the strong excitation of the β -mode as the TV is moved closer to the fins,where a strong whistle noise is heard at a certain gap setting.

METHODOLOGY
We investigated a setup consisting of a test rig with amass flow regulator, precursor channel and generic air outlet
with microphone arrays in the far field as shown in Figure 1.
The experimental setup of the generic air outlet is based on
the work of Biermeier et al. (2012). The generic air outlet
and the precursor channel are made of an acrylic glass (Figure2). It also has a detachable as well as adjustable throttle valve
and five fins for the investigation of aeroacoustic interaction asshown in the Figure 3. This setup has been exactly represented
in the simulation. Its main parameters are given in Table 1.

1

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
Table 1.

cl*Computational and experimental parameters.

cl*

lw

2

Mp

0.008 m3

Uo ( m s−1 )ReChannel

Cl1∗ /Ct1∗

Cl2∗ /Ct2∗

25

8.49 x 104

25

10

c
U

h

Mp1

ct*
2

s

z
x

Experiment

1.5

We performed a combined experimental/numerical study
to investigate the effect of placing a throttle valve in frontof the fins. The measurements were performed in a semianechoic chamber at AUDI. The setup has a precursor channel to achieve a fully developed flow. The precursor channel
has a height h = 0.05 m, breadth b = 0.05 m and a length ofl = 10h. The length of the TV was Cl1∗ = h as sketched in the
Figure 3. In this work, we investigate only at a gap setting of
lw =5C, where C = Ct1∗ = Ct2∗ .
Air is supplied by a blower unit integrated in a ValeoAcoustic test rig (Figure 1). The noise generated by the blower
is isolated with acoustic absorbing materials inside the settlingchamber, which guarantees a low-noise inflow condition. The
mass flow is supplied by a blower unit integrated in the test rig.
The mass flow rate is calculated according to ISO 5801 via thepressure in the inflow nozzle.

10h+0.140 m

placed at a distance of x=0.35 m from the air outlet centre atan angle of 45 degrees at -z and +z directions.

Numerical simulations
We used a hybrid approach to simulate the flow and theacoustic fields. The flow solver MGLET(Multi-Grid Large
Eddy Turbulence) performs a wall-modelled Large Eddy Simulation using the WALE model proposed by Nicoud & Ducros(1999) with a wall function by Werner & Wengle (1993).
MGLET is based on an Immersed Boundary Method (IBM)as described in Peller et al. (2006). The acoustic field is obtained by solving the Arbitrary Acoustic Perturbation Equation (APE) Ewert & Kreuzinger (2021) which is implemented
in MGLET. The flow and acoustic simulations are performedon the same Cartesian grid, so no interpolations are necessary
for the aero-acoustic source terms. The solver is a one way
coupled solver with no feedback from the acoustics to the flow
and has been validated by Schwertfirm et al. (2012).
In this hybrid CAA solver, the APE is solved with the following assumptions namely low mach number, an isentropic
flow, neglecting the density gradients and the mean convection
velocity. This leads to the following form of the APE

Acoustic Chamber

Y Axis in [m]0.0Generic air outletz

Uox
Mass flow
regulator

Microphones

Precursor channel-1.0

Acoustic absorbers

-2.0

0.0

-1.0

0.066 mFigure 3. Sketch of the generic air outlet with Throttle Valve(TV) and fins in 2D along with the probe points Mp1 and Mp3

Non-reflecting outer boundaries

Test rig

0.008 m2.0

X Axis in [m]Figure 1. Schematic representation of the setup with far-fieldmicrophones

∂ u0i
∂ p0
1 ∂ pic=−+∂t∂ xi ρ0 ∂ xi

(1)

∂ u0
∂ p0
= −ρ0 c2 i
∂t
∂ xi

(2)

and

in which the source term pic is the incompressible or hydrodynamic pressure, taken from the incompressible flow solution. It can be shown that this set of equation is equivalent to solving a wave equation for the acoustic pressure p0 in
which the Laplacian of the incompressible pressure appears asa source term.
At the inflow boundary, a constant mass flux with a nonreflecting boundary for the acoustic pressure is set. At the outflow boundaries, Dirichlet conditions for either pressure or total pressure depending on flow direction and a Neumann condition for the velocities are used. A non-reflective boundary
condition for the acoustic pressure and velocity is set at theouter boundaries of the domain. A global acoustic damping of
2% is set at the rigid walls.
The simulation domain has a size of (LX, LY, LZ)=(4,6m, 2,6 m, 4,6 m). The generic air outlet is placed in the centre of the simulation domain. The grid has been constructed

Figure 2. Experimental setup with the far-field microphones
in a semi-anechoic chamber

Acoustic pressure signals were measured at the far-fieldmicrophones, visible in Figures 1 and 2. The microphones are2

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
of Cartesian grid blocks at 323 cubic cells, which are distributed among the CPU cores. A local refinement has beenachieved by inserting locally refined grid boxes (Manhart,
2004). A grid resolution study was performed to determine theoptimum compromise between accuracy and computationalcost. This led to grid spacings of ∆xi,min =0.1875x10-3 m and∆xi,max =12x10-3 m which were achieved by 6 refinement levels. In total 166002688 grid cells were used which were arranged in 5066 grid blocks. A time step of ∆t=1.98x10-6 s
kept the Courant-Friedrich-Lewy number below 0.5 and resulted in 200000 time steps for a sufficient flow statistics. This
led to a computational time 98498.8 CPU hours using 1920
CPUs. The physical run time of simulation is tend =0.5 s, sampling of statistics for flow and the acoustics field are started
from 0.1 s. The physical parameters used are, density ρ=
1.165 kg m−3 , kinematic viscosity ν=1.53x10-5 m2 s−1 .

experimental signal shown as grey band in the Figure 4 for abetter comparability with the simulation signals.
The simulation predicts the experimental data up to a wide
range of frequencies, and also most of the resonance peaks arewell predicted by the simulation. The simulated spectrum lies
within the variance of the experimental signal.

RESULTS AND DISCUSSION
In this section, the vortex shedding behind the throttle
valve and the acoustic resonance mode on the throttle valve
will be further investigated with the simulation results.
The noise generation of different elements, for a gap setting of lw =5C, C = Ct1∗ = Ct2∗ is shown in Figure 5. The configuration without elements ’5C G. Channel’ has no distinct
peak at any Strouhal number and generally a low noise level.
Inserting the throttle valve (configuration ’5C G. TV’) leads toan increased noise level with a clear peak at a Strouhal number (Str=0.1799) which can be assigned to Parker’s β -mode,
which is explained in the next sections. Further adding the fins(configuration ’5C G. TV & Fins’), the noise level close to thisfrequency is strongly enhanced while it remains low with thefins only. This lets us conclude that the interaction between
fins and TV generates this enhanced noise level due to an interaction between the elements.

VALIDATION
We performed a validation of the simulations by comparing with our experimental results. In Figure 4, we compare themeasured pressure signal averaged over the microphone arraywith the correspondingly averaged total pressure signal of p0
over the microphone array from the LES.

(0.1740)

5C G. TV & Fins
5C G TV
5C G. Channel
5C G. Fins

SPL(dB[A], ref: 20 Pa)

10dB(0.1799)(0.1740)(0.1999)

0.1

0.2

0.3

0.4

0.5

0.6 0.7 0.8Str(-)

Figure 4.

Validation of the acoustic simulation with exf .Ct ∗Figure 5. The noise generation of different elements, shown,is the Sound Pressure Level (SPL) of total pressure p0 =Pa + Pic at far-field microphones. Dimensionless Strouhal

periment. Dimensionless Strouhal number Str= U0 1 , whereU0 =25 m s−1 and Ct1∗ =0.002 m

f .Ct ∗

number(Str)= U0 1 , where U0 =25 m s−1 and Ct1∗ =0.002 m
The parameters used for the spectral calculations are as
follows, a FFT with a Hann-window with 50% overlap, a sampling rate of 213441 Hz and a window width of 0.04 s wasapplied. This results in a frequency resolution of 25 Hz. The
same FFT parameters were applied to the experimental data.
As the measured time of the experimental data was around 20s, the FFT was applied to the full signal (20 s) and to several
shorter signal lengths around 0.4 s to increase the variance of

In Figure 6 a snapshot of the instantaneous u-componentof the flow around the fins is shown for the configuration withthrottle valve and fins. The vortex shedding behind the throttle valve can be clearly seen. This flow field emits acoustic
waves, as seen in Figure 7, in which the acoustic pressure p0 is
rendered in the total computational domain.

3

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022

1
0.9
0.8

(4,1)

(3,1)
(2,1)
(1,1)

0.7
10

4.5e+01f * = fd 0 /c

0.0e+00

U(m/s)
20
30

Figure 6. Instantaneous flow field U of ’5C G. TV & Fins’
configuration

(0,1)

0.6
0.5

(3,0)

0.4

(2,0)

(1,0)

0.3
0.2

(0,0)

0.1

Str=0.1799, 5C G. TV
Str=0.1740, 5C G. TV & Fins

0
0

1

2

3

4

l*

Figure 8. Parker mode diagram, x-axis corresponds to thenon- dimensional plate length l ∗ and y-axis corresponds to thenon-dimensional resonance frequency f ∗-1.0e+01-8

p' (Pa)-6-4-1.0e+00(0.1740)

Figure 7. Simulation: Instantaneous total pressure p0 = Pa +Pic of configuration ’5C G. TV & Fins’, showing ground reflections and also the dipole characteristics of Parker β -modep' - Mp 5C G. TV & Fins
1p' - Mp3 5C G. TV & Fins
p' - Mp1 5C G. TV(0.1700)

p' - Mp3 5C G. TV

10dB(0.1999)

Previously, Biermeier et al. (2012) investigated the Parker
β mode for different plate lengths starting from Cl1∗ =0.0208 to
0.0404 m and compared the measured values with the Parker
mode diagram. The same Parker mode diagram, with four
simplest modes is shown in Figure 8. The diagram is generated using the MATLAB script taken from the thesis work of
Tautz (2012). From the Parker mode diagram, the resonant frequency value for the β -mode for the throttle valve and channel
in our case can be calculated.
The peak (Str=0.1799) in the ’5C G. TV’ spectrum corresponds to f=2249 Hz. This frequency f is taken for the calculation of Parker’s non-dimensional resonance frequency f ∗ . The
0
non-dimensional resonance frequency of f ∗ = f .d
c = 0.3174
and with non-dimensional plate length of l ∗ =(0.1799)(0.1999)

Cl ∗
√ 1
=
d0 1−Ma2

0.1

1.046 corresponds to the β -mode as in Parker (1967) andcorresponding to Koch (2009) calculation scheme, whered0 = h − Ct1∗ , speed of sound c=343 m s−1 and Mach number
Ma=0.07288. The results are plotted in the Figure 8, wherein simulation f=2249 Hz and the theoretical value of β -mode
at f=2255 Hz with a deviation of only ∆f= 6 Hz. The shift ofthe other peak (Str=0.1740) in ’5C G. TV & Fins’ is further
discussed.
We are further analysing the probe points M p1 and M p3
to understand the frequency related to vortex shedding and theβ -mode, where M p1 is placed in the wake of the TV and M p3
is at the centre of the duct close to the wall as shown in the
Figure 3. The further analysis is performed with the two configurations, namely ’5C G.TV’ and ’5C G. TV & Fins’. The
reason for chosen configurations above is to understand how
the fins in the air outlet change the flow-acoustic interaction.
The peaks at the far-field are correlated to the peaks at

0.2

0.3

0.4

0.5

0.6 0.7 0.8

Figure 9. Sound Pressure Level (SPL) of total pressure p0 =Pa + Pic at microphone probes as shown in the sketch.

the probe points inside the generic air outlet. In the Figure
9, An interesting observation is in the ’5C G. TV’ configuration that the peak (Str=0.1799) at the far field p’ coincides
exactly with the probe point ’p’- Mp3 5C G.TV’ at the ductwall above the throttle valve. The other peak(Str=0.1999) atthe far field p’ coincides exactly with the probe point ’p’-Mp1
5C G. TV’ behind the wake of the throttle valve. The latterpeak (Str=0.1999) can be the trace of the vortex shedding in
the wake of the throttle valve.
Whereas in ’5C G. TV & Fins’, the peak (Str=0.1740)

412th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
at the far field p0 coincides exactly with the probe point ’p’Mp3 5C G.TV & Fins’ at the duct wall above the TV. The
peak (Str=0.1700) at the probe point ’p’- Mp1 5C G.TV &Fins’ is close to the far field peak, It looks like the vortex
shedding frequency shifted to low frequency due to additionof fins. For the further investigation, the incompressible flow
field and the acoustic components are treated separately. The
hydrodynamic pressure Pic , which is the result of the incompressible flow solver, and the acoustic pressure Pa , which iscomputed from the solution p0 of the APE and the solution
pic of the incompressible Navier-Stokes equations. At the microphone signal in simulation, the incompressible or hydrodynamic pressure Pic and acoustic pressure Pa can be separated.
The spectrum of respective pressure Pic and Pa at the probe
points Mp1 and Mp3 are shown in the Figures 10 and 11.

close to the Parker-β mode frequency at Str=0.1740 for the
TV. These two oscillators, oscillating close to the same frequency with a ∆Str=0.004 gets enhanced with noise levels at
this particular spacing. It is also quite not so clear why thegeometric Parker-β mode in the acoustic pressure spectrum
shifted from Str=0.1799 to Str=0.1740 after adding fins closer
to the throttle valve.

(0.1740)

Pa - Mp1 5C G. TV & Fins
Pa - Mp3 5C G. TV & Fins
Pa - Mp1 5C G. TV
Pa - Mp3 5C G. TV

10dB(0.1740)(0.1799)

P - Mp 5C G. TV & Fins
ic

1(0.1999)

Pic - Mp3 5C G. TV & Fins
Pic - Mp1 5C G. TV(0.1700)

Pic - Mp3 5C G. TV

10dB(0.1799)(0.1999)(0.1999)(0.1700)

0.1

0.2

0.3

0.4

0.5

0.6 0.7 0.8

Figure 11. Sound Pressure Level (SPL) of acoustic pressure
Pa at microphone probes as shown in the sketch.

0.1

0.2

0.3

0.4

0.5

In the simulation, a more detailed information on the observed peaks are gained by doing a Fourier Transformation(FT) of the acoustic pressure Pa and hydrodynamic pressure Pic
at selected frequencies during run time of the simulation. For
the method of In-Situ-FT used here, please refer to Kreuzinger
et al. (2013). For the two configurations with and without fins,the In-Situ-FT has been carried out for selected frequencies.
The In-Situ-FT pictures of acoustic pressure Pa and hydrodynamic pressure Pic of configurations with and without fins
along with throttle valve are shown in the Figure 12. The
acoustic and hydrodynamic pressure are scaled with same SPL
values for a fair comparison. The reason for the increase in theamplitude at Str=0.1740 in the p0 spectrum will be discussedwith the insitu-FT modes. The amplitude of the modes in the’5C G. TV’ configuration is quite low, of which the β -mode
in Figure 12(a) and vortex shedding mode in Figure 12(b) are
not oscillating close to a single frequency. Whereas in the ’5C
G. TV & Fins’ configuration, the vortex shedding mode got
intensified due to addition of fins in Figure 12(d) very close to
the Parker β -mode frequency in Figure 12(c), these two oscillators at very close frequency resulted in increase in amplitudeof the acoustic mode.

0.6 0.7 0.8

Figure 10. Sound Pressure Level (SPL) of incompressiblehydrodynamic pressure Pic at microphone probes, as shownin the sketch.

From the Figure 10, The incompressible pressure showsthat the peak(Str=0.1999) at ’Pic -Mp1 5C G. TV’ behind thewake is a hydrodynamic mode. This let us conclude that this isa vortex shedding mode. In ’Pic -Mp1 5C G. TV & Fins’ looking at the hydrodynamic pressure, the vortex shedding mode
shifts to a slightly low frequency(Str=0.1700) due to the addition of the fins.
Further, analysing from the figure 11, the acoustic spectrum at the probe points ’Mp1 ’ and ’Mp3 ’ for both configurations ’5C G.TV’ and ’5C G.TV & Fins’ are shown. The
amplitude of Sound Pressure Level is higher at the probe point’Mp3 ’ than ’Mp1 ’ in the wake for both of the configurations,
showing that the geometric Parker-β mode is dominant. The
trace of the vortex shedding noise is also noticeable in the
spectrum. It can be observed that the consecutive peaks at
Str=0.1799 and Str=0.1999 in the configuration ’5C G.TV’ hasa ∆Str=0.02. Whereas in the configuration ’5C G.TV & Fins’,
the trace of vortex shedding frequency at Str=0.1700 is very

Conclusion
The hybrid CAA method used here can predict the resonance frequencies, which is quite in good agreement with the512th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
Str=0.1799

Str=0.1999

Hong, Zhiliang, Wang, Xiaoyu, Jing, Xiaodong & Sun, Xiaofeng 2020 Vortex sound interaction in acoustic resonanceof a flow duct containing a plate. Journal of Sound andVibration 483, 115482.
Koch, Werner 2009 Acoustic resonances and trapped modes
in annular plate cascades. Journal of fluid mechanics 628,
155–180.
Kreuzinger, Johannes, Schwertfirm, Florian, Hartmann,
Michael & Peller, Nikolaus 2013 Analysis of resonancephenomena caused by obstacles in hvac exhaust nozzles
using a combined cfd-caa approach. In 19th AIAA/CEAS
Aeroacoustics Conference, p. 2132.
Manhart, M. 2004 A zonal grid algorithm for DNS of turbulent
boundary layers. Computers and Fluids 33 (3), 435–461.
Nicoud, Franck & Ducros, Frédéric 1999 Subgrid-scale stressmodelling based on the square of the velocity gradient tensor. Flow, turbulence and Combustion 62 (3), 183–200.
Parker, R 1967 Resonance effects in wake shedding from parallel plates: calculation of resonant frequencies. Journal ofSound and Vibration 5 (2), 330–343.
Peller, Nikolaus, Duc, Anne Le, Tremblay, Frédéric & Manhart, Michael 2006 High-order stable interpolations for
immersed boundary methods. International Journal for
Numerical Methods in Fluids 52 (11), 1175–1193.
Perot, Franck, Meskine, Mohammed, Vergne, Sandrine &Gille, Frederic 2011 Aeroacoustics prediction of simplified and production automotive hvac ducts and registers. In
17th AIAA/CEAS Aeroacoustics Conference (32nd AIAA
Aeroacoustics Conference), p. 2935.
Ravichandran, Asvath, Logdesser, Andreas, Peller, Nikolaus& Manhart, Michael 2022 Experimentelle und numerische
untersuchung des effekts der zuströmbedingungen auf die
schallabstrahlung eines generischen ausströmers. In DAGA
2022 - 48. Jahrestagung fr Akustik, pp. 1250–1253.
Schwertfirm, Florian, Kreuzinger, Johannes, Peller, Nikolaus & Hartmann, Michael 2012 Validation of a hybrid simulation method for flow noise prediction. In
18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA
Aeroacoustics Conference), p. 2192.
Stoneman, SAT, Hourigan, K, Stokes, AN & Welsh, MC 1988
Resonant sound caused by flow past two plates in tandem ina duct. Journal of Fluid Mechanics 192, 455–484.
Tautz, Matthias 2012 Fluid-akustik interaktion bei der umströmung von platten in kan”alen. PhD thesis, Diplomarbeit
am Lehrstuhl fr Prozessmaschinen und Anlagentechnik derFriedrich-Alexander-Universität Erlangen-Nürnberg.
Werner, H & Wengle, H 1993 Large-eddy simulation of turbulent flow over and around a cube in a plate channel. In
Turbulent shear flows 8, pp. 155–168. Springer.
Yokoyama, H, Kitamiya, K & Iida, A 2013 Flows around acascade of flat plates with acoustic resonance. Physics of
Fluids 25 (10), 106104.

(b)

(a)
Str=0.1740

(c)

10 dB

Str=0.1700

(d)

SPL

Figure 12. Insitu-FT of (a) Pa -’5C G. TV’, (b) Pic -’5C G.
TV’, (c) Pa -’5C G. TV & Fins’, (d) Pic -’5C G. TV & Fins’.

experimental observations. From the numerical observations,the hydrodynamic and acoustic modes can be treated separately to differentiate the modes related to flow and acoustics,
which has given further insights on this interaction. The enhanced amplitude of the β -mode is due to the vortex shedding
at the matching frequency. This phenomenon can be captured
with one-way flow acoustic coupling, as there is no feedback
from acoustic to flow required in this particular case.

Acknowledgement
The first author wishes to acknowledge the support from
AUDI AG in cooperation with TUM under INI.TUM program.
The authors acknowledge the support of technicians of TUM’s
Hydromechanic lab for constructing the generic air outlet and
AUDI AG for providing the acoustic chamber and hot-wiremeasurement facilities.

REFERENCES
Biermeier, Thomas, Becker, Stefan & Risch, Paul 2012 Acoustic investigations of hvac systems in vehicle. Tech. Rep..
SAE Technical Paper.
Ewert, Roland & Kreuzinger, Johannes 2021 Hydrodynamic/acoustic splitting approach with flow-acoustic feedback for universal subsonic noise computation. Journal ofComputational Physics 444, 110548.
Guerin, S, Thomy, E & Wright, MCM 2005 Aeroacoustics ofautomotive vents. Journal of sound and vibration 285 (4-5),
859–875.

6