introduction - oss.jishulink.comoss.jishulink.com/caenet/forums/upload/2014/01/23/388/... ·...

Tutorial: Aeroacoustic for a Helmholtz Resonator With the

Direct Method (CAA)

Introduction

The purpose of this tutorial is to provide guidelines and recommendations for the basicsetup and solution procedure for a typical aeroacoustic application using computationalaeroacoustic (CAA) method.

In this tutorial you will learn how to:

• Model a Helmholtz resonator.

• Use the transient k-epsilon model and the large eddy simulation (LES) model foraeroacoustic application.

• Set up, run, and perform postprocessing in FLUENT.

Prerequisites

This tutorial assumes that you are familiar with the user interface, basic setup and solutionprocedures in FLUENT. This tutorial does not cover mechanics of using acoustics model, butfocuses on setting up the problem for Helmholtz-Resonator and solving it. It also assumesthat you have basic understanding of aeroacoustic physics.

If you have not used FLUENT before, it would be helpful to first review FLUENT 6.2 User’sGuide and FLUENT 6.2 Tutorial Guide.

Problem Description

A Helmholtz resonator consists of a cavity in a rigid structure that communicates through anarrow neck or slit to the outside air. The frequency of resonance is determined by the massof air in the neck resonating in conjunction with the compliance of the air in the cavity.The physics behind the Helmholtz resonator is similar to wind noise applications like sunroof buffeting.

The Helmholtz-Resonator considered is shown in Figure 1. We assume that out of the twocavities that are present, smaller one is the resonator. The motion of the fluid takes placebecause of the inlet velocity of 27.78 m/s (100 km/h). The flow separates into a highlyunsteady motion from the opening to the small cavity. This unsteady motion leads to apressure fluctuations. Two monitor points (Point-1 and Point-2) act as microphone pointsto record the generated sound. The acoustic signal is calculated within FLUENT. The flowexits the domain through the pressure outlet.

c© Fluent Inc. March 2, 2005 1

http://www-internal.fluent.com/Depts/Products/userdoc/fluent62/html/ug/main_pre.htm

http://www-internal.fluent.com/Depts/Products/userdoc/fluent62/html/ug/main_pre.htm

http://www-internal.fluent.com/Depts/Products/userdoc/fluent62/html/tg/main_pre.htm

shifajia

高亮

shifajia

高亮

shifajia

高亮

shifajia

高亮

shifajia

高亮

Aeroacoustic for a Helmholtz Resonator With the Direct Method (CAA)

Preparation

1. Copy the files steady.cas.gz and steady.dat.gz into your working directory.

2. Start the 2D double precision (2ddp) version of FLUENT.

Setup and Solution

Step 1: Grid

1. Read the initial case and data files for steady-state (steady.cas.gz and steady.dat.gz).

File −→ Read −→Case & Data...

Ignore the warning that is displayed in the FLUENT console while reading these files.

2. Keep default scale for the grid.

Grid −→Scale...

3. Display the grid and observe the locations of the two monitor points, Point-1 andPoint-2 (Figure 1).

Figure 1: Graphics Display of the Grid

4. Display and observe the contours of static pressure (Figure 2) and velocity magnitude(Figure 3) for the initial steady-state solution.

Display −→Contours..

2 c© Fluent Inc. March 2, 2005

shifajia

高亮

shifajia

高亮


Contours of Static Pressure (pascal)FLUENT 6.2 (2d, dp, segregated, rke)

6.09e+025.29e+024.49e+023.69e+022.89e+022.09e+021.29e+024.87e+01-3.14e+01-1.11e+02-1.91e+02-2.72e+02-3.52e+02-4.32e+02-5.12e+02-5.92e+02-6.72e+02-7.52e+02-8.32e+02-9.12e+02-9.92e+02

Figure 2: Contours of Static Pressure (Steady State)

Contours of Velocity Magnitude (m/s)FLUENT 6.2 (2d, dp, segregated, rke)

3.92e+013.72e+013.53e+013.33e+013.14e+012.94e+012.74e+012.55e+012.35e+012.16e+011.96e+011.76e+011.57e+011.37e+011.18e+019.80e+007.84e+005.88e+003.92e+001.96e+000.00e+00

Figure 3: Contours of Velocity Magnitude (Steady State)



Step 2: Models

1. Select unsteady solver.

Define −→ Models −→Solver...

(a) Under Time, select Unsteady.

(b) Keep default settings for other parameters.

2. Ensure that Non-Equilibrium Wall Functions is selected under Near-Wall Treatment inthe Viscous Model panel.

Define −→ Models −→Viscous...

Near-Wall Treatment predicts good separation and re-attachment points.

Step 3: Materials

Define −→Materials...

Ensure that under Properties, ideal-gas is selected in the Density drop-down list forair in the Materials panel.Ideal gas law is good in predicting the small changes in the pressure.

Step 4: Solution

1.1. Monitor the static pressure on point-1 and point-2.

Solve −→ Monitors −→Surface...

(a) Increase the Surface Monitors to a value of 2.

(b) Enable Plot and Print options for monitor-1 and monitor-2.

(c) Under Every drop-down list, select Time Step.

(d) Click Define... to open Define Surface Monitor panel.


shifajia

高亮

shifajia

高亮

shifajia

高亮


i. Specify the parameters as shown in the Define Surface Monitor panel.

(e) Similarly, specify the surface monitor parameters for point-2.

2. Start the calculations using the following settings.

Solve −→Iterate...

(a) Specify a value of 0.001 for Time Step Size (s).

The expected time step size for this problem is of the size of about 1/10th of thetime period. The time period depends on the frequency (f) which is calculatedusing the following equation:

f =c

2π

√S

V [L + π2 .Dh

2 ]

where,

c = Speed of sound

S = Area of the orifice of the resonator

V = Volume of the resonator

L = Length of the connection between the resonator and the free flow area

Dh = Hydraulic diameter of the orifice

For this geometry, the estimated frequency is about120 Hz.

(b) Increase the Number of Time Steps to 220.

(c) Specify a value of 25 for Max Iterations per Time Step.

(d) Click Iterate to start the calculations.

The iterations will take a long time to complete. You can skip this simulation af-ter few time steps and read the files (transient.cas.gz and transient.dat.gz)provided with this tutorial. These files contain the data for the flow time of 0.22seconds.

As seen in Figures 4 and 5, no pressure fluctuations are present at this stage. Theoscillations of the static pressure at both monitor points has reached a constantvalue.


shifajia

高亮

shifajia

高亮

shifajia

高亮

shifajia

高亮


Convergence history of Static Pressure on point-1 (Time=2.2000e-01)FLUENT 6.2 (2d, dp, segregated, rke, unsteady)

Flow Time

(pascal)ValuesVertex

Surfaceof

Average

0.2250.2000.1750.1500.1250.1000.0750.0500.0250.000

40.0000

20.0000

0.0000

-20.0000

-40.0000

-60.0000

-80.0000

-100.0000

-120.0000

-140.0000

monitor-1Monitors

Figure 4: Convergence History of Static Pressure on Point-1 (Transient)

Convergence history of Static Pressure on point-2 (Time=2.2000e-01)FLUENT 6.2 (2d, dp, segregated, rke, unsteady)

Flow Time


Surfaceof

Average

0.2250.2000.1750.1500.1250.1000.0750.0500.0250.000

4.0000

3.5000

3.0000

2.5000

2.0000

1.5000

1.0000

0.5000

0.0000

-0.5000

-1.0000

monitor-2Monitors

Figure 5: Convergence History of Static Pressure on Point-2 (Transient)



Step 5: Enable Large Eddy Simulation

1. Enter the following TUI command in the FLUENT console:

(rpsetvar ’les-2d? #t)

2. Enable large eddy simulation effects.

The k-epsilon model cannot resolve very small pressure fluctuations for aeroacousticdue to its dissipative character. Use Large Eddy Simulation to overcome this problem.

Define −→ Models −→Viscous...

3. Retain default discretization schemes and under-relaxation factors.

Define −→ Controls −→Solution...

4. Enable writing of two surface monitors and specify file names as monitor-1.out andmonitor-2.out for monitor plots of point-1 and point-2 respectively.

Solve −→ Monitors −→Surface...

(a) Enable Write option for monitor-1 and monitor-2.

5. Start the iterations again for another 150 time steps.

Figures 6 and 7 shows that dynamically steady solution is obtained.


shifajia

高亮

shifajia

高亮


Convergence history of Static Pressure on point-1 (Time=3.7000e-01)FLUENT 6.2 (2d, dp, segregated, LES, unsteady)

Flow Time


Surfaceof

Average

0.3800.3600.3400.3200.3000.2800.2600.2400.220

200.0000

150.0000

100.0000

50.0000

0.0000

-50.0000

-100.0000

-150.0000

-200.0000

monitor-1Monitors

Figure 6: Convergence History of Static Pressure on Point-1 (LES Model)

Convergence history of Static Pressure on point-2 (Time=3.7000e-01)FLUENT 6.2 (2d, dp, segregated, LES, unsteady)

Flow Time


Surfaceof

Average

0.3800.3600.3400.3200.3000.2800.2600.2400.220

30.0000

20.0000

10.0000

0.0000

-10.0000

-20.0000

-30.0000

monitor-2Monitors

Figure 7: Convergence History of Static Pressure on Point-2 (LES Model)



Step 6: Postprocessing

1. Display the contours of static pressure to visualize the eddies near the orifice (Fig-ure 8).

Contours of Static Pressure (pascal) (Time=3.7000e-01)FLUENT 6.2 (2d, dp, segregated, LES, unsteady)

3.10e+022.90e+022.70e+022.50e+022.30e+022.10e+021.90e+021.70e+021.50e+021.30e+021.09e+028.94e+016.93e+014.92e+012.91e+019.03e+00-1.11e+01-3.12e+01-5.12e+01-7.13e+01-9.14e+01

Figure 8: Contours of Static Pressure (LES Model)

2. Enable the acoustics model.

Define −→ Models −→Acoustics...



(a) Under Model, enable Ffowcs-Williams & Hawkings.

To specify a value for the acoustic reference pressure, it is necessary to activatethe acoustic model before starting postprocessing.

(b) Retain default settings for other parameters.

(c) Click OK to accept the settings and close the panel.

A Warning dialog box appears. This is just informative panel and will not affectthe postprocessing results. Click OK to acknowledge the information and closethe panel.

3. Plot the sound pressure level (SPL).

Plot −→FFT...

(a) Click Load Input File... button and select monitor plot file for Point-1 (monitor-1.out).


shifajia

高亮


(b) Click Plot/Modify Input Signal...

i. Under Options, select Clip to Range.

ii. Under X Axis Range, specify a value of 0.25 for Min.

iii. Select Hanning in the Window drop-down list.

Hanning shows good performance in frequency resolution. It cuts the timerecord more smoothly, eliminating discontinuities that occur when data iscut off.

iv. Click Apply/Plot and close the Plot/Modify Input Signal panel.

(c) Select Sound Pressure Level (dB) in the Y Axis Function drop-down list.

(d) Select Frequency (Hz) in the X Axis Function drop-down list.

(e) Click Plot FFT to visualize the frequency distribution at Point-1 (Figure 9).

(f) Under Options, select Write FFT to File.

Note: Plot FFT button will change to Write FFT.

(g) Click Write FFT and specify the name of the FFT file in the resulting Select Filepanel (point-1-fft.xy).

(h) Similarly write the FFT file for monitor plot for point-2 (Figure 10).

In Figures 9 and 10, the sound pressure level (SPL) peak occurs at 125 Hz which isclose to the analytical estimation. Considering that this tutorial uses a slightly largetime step and a 2D geometry, the result is fine.


shifajia

高亮


Spectral Analysis of Convergence history of Static Pressure on point-1 (in SI units) (Time=3.7000e-01)FLUENT 6.2 (2d, dp, segregated, LES, unsteady)

Frequency (Hz)

(pascal)(dB)

LevelPressure

Sound

500450400350300250200150100500

130

120

110

100

90

80

70

60

50

40

30

Figure 9: Spectral Analysis of Convergence History of Static Pressure on Point-1

Spectral Analysis of Convergence history of Static Pressure on point-2 (in SI units) (Time=3.7000e-01)FLUENT 6.2 (2d, dp, segregated, LES, unsteady)

Frequency (Hz)

(pascal)(dB)

LevelPressure

Sound

500450400350300250200150100500

120

110

100

90

80

70

60

50

40

30

20

10

Figure 10: Spectral Analysis of Convergence History of Static Pressure on Point-2



4. Compare the frequency spectra at point-1 and point-2.

Plot −→File...

(a) Click Add... and select two FFT files (point-1-fft.xy and point-2-fft.xy)that you have saved in the previous step.

(b) Click Plot to visualize both spectra in the same window (Figure 11).

Monitor point-1 (in SI units) (Time=3.7000e-01)FLUENT 6.2 (2d, dp, segregated, LES, unsteady)

Frequency (Hz)

(pascal)(dB)

LevelPressure

Sound

500450400350300250200150100500

140

120

100

80

60

40

20

0

Monitor point-2 (in SI units)


Sound Pressure Level (dB)

Figure 11: Comparison of Frequency Spectra at Point-1 and Point-2

As the distance between the receiver point and the noise source increases, thedissipation of sound also increases. This is the reason why the peak value of SPLat Point-1 is higher than that of Point-2. Therefore, you should use the CAAmethod only for near field calculations.

The dissipation of sound also occurs due to the influence of the grid size. Thisapplies for high frequencies for which the wave lengths are very short. Therefore,a very coarse mesh is not capable of resolving high frequencies accurately.

In this case, the mesh is coarse in the far-field region because of which thediscrepancy between both spectra is more evident in the high frequency range.You can see this if you continue the simulation. There are two monitor-files(monitor-point-1-500.out and monitor-point-2-500.out) provided with thistutorial that contains data for 500 more time steps. When you plot the FFT andcompare these two files, you will see that for high frequencies, the monitor forPoint-1 generates much less noise than monitor for Point-2 (Figure 12).


shifajia

高亮


Monitor point-1 (in SI units) (Time=8.7000e-01)FLUENT 6.2 (2d, dp, segregated, LES, unsteady)

Frequency (Hz)

(dB)Level

PressureSound

500450400350300250200

80

70

60

50

40

30

20

10

0

-10

-20



Sound Pressure Level (dB)

Figure 12: Comparison of Frequency Spectra at Point-1 and Point-2 (High Frequency Rangeof 200-500 Hz)

Summary

Aeroacoustic simulation of Helmholtz resonator has been performed using k-epsilon modeland Large Eddy Simulation model. The advantage of using LES model has been demon-strated. You also learned how the sound dissipation occurs in the domain by monitoringsound pressure level at two different points in the domain. The importance of using CAAmethod has also been explained


introduction - oss.jishulink.comoss.jishulink.com/caenet/forums/upload/2014/01/23/388/... ·...

Documents