Modeling Supersonic Jet Screech Noise Using Direct Computational Aeroacoustics (CAA) 14.5 Release

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1 Modeling Supersonic Jet Screech Noise Using Direct Computational Aeroacoustics (CAA) 14.5 Release 2011 ANSYS, Inc. November 7, Workshop Advanced ANSYS FLUENT Acoustics

2 Introduction This tutorial demonstrates how to model screech tone noise radiated by an axisymmetric supersonic jet using direct computational aeroacoustics (CAA) in ANSYS FLUENT This tutorial demonstrates how to do the following: Perform axisymmetric simulation of a steady-state supersonic jet flow using realizable k-e turbulence model with model constants modified for free jet flows Calculate unsteady flow and acoustic near field by direct CAA using unsteady realizable k-e turbulence model Save acoustic data at nearfield microphone location for further spectral analysis Postprocess flowfield and aeroacoustic results 2011 ANSYS, Inc. November 7,

3 Prerequisites This tutorial assumes that you are familiar with the ANSYS FLUENT interface and that you have a good understanding of basic setup and solution procedures. Some steps will not be shown explicitly. In this tutorial you will use direct CAA method. If you have not used this feature before, first read: Chapter 15, Aerodynamically Generated Noise, of the ANSYS FLUENT 14.5 Theory Guide, and Chapter 23, Predicting Aerodynamically Generated Noise, of the ANSYS FLUENT 14.5 User's Guide Note: Approximately 27 hours of CPU time on a single 32-bit Windows machine is required to complete this tutorial. It will take only 2.5 hours on 8 parallel nodes! If you are interested exclusively in learning how to set up the steady-state and direct CAA models, you can reduce the computing time requirements considerably by skipping Steps 9 (converging steady-state run), and Steps 11 (time-marching unsteady simulation) and using the provided case and data files to postprocess results at Step ANSYS, Inc. November 7,

4 Problem Description Supersonic jet noise has three main components: 1. Broadband shock-associated noise 2. Turbulent mixing noise 3. Screech tones Screech tones radiate at discrete frequencies. They are generated by a feedback loop 1,2 (Figure 1): A quasi-periodic shock-cell structure is formed in the core of an imperfectly expanded jet. At the nozzle exist, jet shear layer is thin and receptive to external excitations. Acoustic disturbances impinge on the nozzle lip and excite instability waves, which then propagate downstream and grow as they extract energy from the meanflow. At the fourth or fifth shock cell, amplitude of the instability wave becomes large enough to interact with the shock-cell structure. This unsteady interaction generates acoustic waves which propagate, in part, upstream outside the jet, and excite another excitation of the jet mixing layer at the nozzle lip. This generates a new instability wave, and closes the feedback loop nozzle feedback acoustic waves shock cells instability waves Figure 1. Screech tone feedback loop 1 Powell, A., On the Mechanism of Choked Jet Noise, Proceedings of the Physical Society, London, Vol. 66, 1953, pp Shen, H., and Tam, C. K. W., Numerical Simulation of the Generation of Axisymmetric Mode Jet Screech Tones, AIAA Journal, Vol. 36, No. 10, 1998, pp ANSYS, Inc. November 7,

5 Problem Description The problem considers turbulent air flow associated with Mach 1.2 cold jet emitted from 1 diameter round nozzle with thick lip. Nozzle geometry and flow conditions are from experiments of Ponton and Seiner Microphone locations Jet flow Nozzle lip Figure 1. Nozzle geometry 3 Ponton, M. K., and Seiner, J. M., The Effect of Nozzle Exit Lip Thickness on Plume resonance, Journal of Sound and Vibration, Vol. 154, Issue 3, 1992, pp ANSYS, Inc. November 7,

6 Preparation 1. Copy the file mesh-caa-jet-screech.msh.gz to your working directory 2. Start the 2D double-precision version of ANSYS FLUENT ANSYS, Inc. November 7,

Setup and Solution Step 1: Mesh 1. Read the mesh file mesh-caa-jet-screech.msh.gz File Read Mesh As FLUENT reads the mesh file, it will report its progress in the console window. 2.

7 Setup and Solution Step 1: Mesh 1. Read the mesh file mesh-caa-jet-screech.msh.gz File Read Mesh As FLUENT reads the mesh file, it will report its progress in the console window. 2. The mesh was created in inches, and it needs to be rescaled Mesh Scale (a) Select Convert Units under Scaling (b) Select in under Mesh Was Created In (c) Click Scale once (d) Check Domain Extents: Xmin (m) = , Xmax (m) = 25.4 Ymin (m)= 0, Ymax (m) = (e) Click Close 3. Check the mesh Mesh Check FLUENT will perform various checks on the mesh and report the progress in the console window. Pay particular attention to the reported minimum volume. Make sure this is a positive number 2011 ANSYS, Inc. November 7,

8 Setup and Solution Steady-State Flow Step 1: Mesh (continued) 4. Display the mesh Results Graphic and Animation Mesh Set Up ANSYS, Inc. November 7,

Mesh at the nozzle exit Hybrid quad-tri mesh is used in this simulation. Structured quad cells are used to resolve the jet plume up to 20 nozzle diameters downstream.

9 Setup and Solution Steady-State Flow Step 1: Mesh (continued) (a) Display the grid with the default settings (Figure 3). Use the middle mouse button to zoom in on the image so you can see the mesh at the nozzle (Figure 4) Figure 3. Mesh Display Figure 4. Mesh at the nozzle exit Hybrid quad-tri mesh is used in this simulation. Structured quad cells are used to resolve the jet plume up to 20 nozzle diameters downstream. Tri cells are used in the far field. The cell size should be small enough to adequately resolve excitations of the jet shear layer at the nozzle lip, development of shear layer instability waves, shock cell structure in the nozzle core and nearfield propagation of screech acoustic waves ANSYS, Inc. November 7,

10 Step 2: Models Setup and Solution Steady-State Flow 1. Select the pressure-based steady-state axisymmetric solver Problem Setup General Note: pressure-based solver is used in this simulation to take advantage of higher order QUICK discretization scheme for the density, momentum, energy and turbulence equations on structured meshes 2011 ANSYS, Inc. November 7,

Treatment (d) Modify model constants as: C2-Epsilon = 2.02 TKE Prandtl Number = 0.324 TDR Prandtl Number = 0.377 Energy Prandtl Number = 0.422 This modification follows Ref.

11 Setup and Solution Steady-State Flow Step 2: Models (continued) 2. Select realizable k-e turbulence model Problem Setup Models Viscous (a) Select k-epsilon under Model (b) Select Realizable under k-epsilon Model (c) Retain Standard Wall Functions under Near Wall Treatment (d) Modify model constants as: C2-Epsilon = 2.02 TKE Prandtl Number = TDR Prandtl Number = Energy Prandtl Number = This modification follows Ref. [4] which proposed a set of new constants for the standard k-e model designed specifically for predicting turbulent jet flows (e) Click OK 4 Thies, A., and Tam, C. K. W., Computation of Turbulent Axisymmetric and Nonaxisymmetric Jet Flows Using the K- model, AIAA Journal, Vol. 34, No. 2, 1996, pp ANSYS, Inc. November 7,

The message Note: Enabling energy equation as required by material density method, will be printed in the console window 2.

12 Step 3: Materials Setup and Solution Steady-State Flow You will use the default material, air, which is the working fluid in this problem. Air is modeled as ideal compressible gas Problem Setup Materials Fluid Air 1. Change the density formulation to ideal-gas Energy equation will be turned on automatically. The message Note: Enabling energy equation as required by material density method, will be printed in the console window 2. Change the viscosity formulation to sutherland, and accept Three Coefficient Method with default constants 3. Click Change/Create You can modify the fluid properties for air or copy another material from the database if needed. For details, refer to the Chapter 8, Physical Properties, in the ANSYS FLUENT 14.5 User's Guide 2011 ANSYS, Inc. November 7,

13 Setup and Solution Steady-State Flow Step 4: Cell Zone Conditions Problem Setup Cell Zone Conditions 1. Select air i. Click Edit... to open the Fluid panel ii. Retain the default selection of air as the fluid material in the Material Name drop-down list iii. Click OK 2011 ANSYS, Inc. November 7,

Setup and Solution Steady-State Flow Step 4: Cell Zone Conditions Problem Setup Cell Zone Conditions 2.

.. to open the Fluid panel, and check Laminar Zone Small fluid region just at the nozzle exit is modeled

The rest of the fluid is modeled as turbulent air. ii. Click OK 3. Click Operating Conditions.

14 Setup and Solution Steady-State Flow Step 4: Cell Zone Conditions Problem Setup Cell Zone Conditions 2. Select air-lam i. Click Edit... to open the Fluid panel, and check Laminar Zone Small fluid region just at the nozzle exit is modeled as laminar (Figure 5). This is necessary to augment excitation of mixing layer instability waves. The rest of the fluid is modeled as turbulent air. ii. Click OK 3. Click Operating Conditions... to open the Operating Conditions panel Set Operating Pressure to 0 Turbulent zone Laminar zone Figure 5. Mesh at the nozzle exit showing location of the laminar zone 2011 ANSYS, Inc. November 7,

15 Setup and Solution Steady-State Flow Step 5: Boundary Conditions Problem Setup Boundary Conditions 1. Set the boundary conditions at the inlet a) Select inlet under Boundary Conditions The Type will be reported as pressure-inlet b) Click Edit... to open the Pressure Inlet panel, and select Momentum tab i. Set the Gauge Total Pressure (pascal) to 242,496.5 ii. Set Supersonic/Initial Gauge Pressure (pascal) to 127,360 These settings are based on isentropic relationships, they correspond to fully expanded jet Mach number equal to 1.2 iii. Retain Normal to Boundary under Direction Specification Method iv. Select Intensity and Hydraulic Diameter under Turbulence Specification Method, and set Turbulent Intensity (%) = 0.1, and Hydraulic Diameter = Low level of turbulence is assumed at the nozzle exit c) Select Thermal tab in the Pressure Inlet panel i. Set Total Temperature (K) to 300 K For cold jets, total temperature is equal to the ambient static temperature 2011 ANSYS, Inc. November 7,

16 Setup and Solution Steady-State Flow Step 5: Boundary Conditions (continued) Problem Setup Boundary Conditions 2. Set the boundary conditions at the pressure outlet a) Select far-field under Boundary Conditions The Type will be reported as pressure-outlet b) Click Edit... to open the Pressure Outlet panel, and select Momentum tab i. Set the Gauge Pressure (pascal) to 100,000 ii. iii. Retain Normal to Boundary under Backflow Direction Specification Method Select Intensity and Viscosity Ratio under Turbulence Specification Method, and set Backflow Turbulent Intensity (%) = 1, and Viscosity Ratio = 2 c) Select Thermal tab in the Pressure Outlet panel i. Set Backflow Total Temperature (K) to 300 K Note: pressure-based coupled solver will be used in this simulation. Non-reflective outlet boundary condition option is not compatible with the pressure-based solver. To avoid spurious reflection of acoustic waves off the outlet boundary, the outlet is placed about 500 acoustic wavelengths away from the noise source, and a very coarse mesh with the mesh size Dx much larger than the acoustic wavelength L is applied in the far field (Dx / L = 25) to assure acoustic waves are fully dissipated by numerical viscosity before reaching the outlet boundary 2011 ANSYS, Inc. November 7,

Select Specified Shear under Shear Condition, and set Shear Stress X- and Y- components to 0 Walls in this simulation are modeled as slip walls since the primary physics of screech

17 Setup and Solution Steady-State Flow Step 5: Boundary Conditions (continued) Problem Setup Boundary Conditions 3. Set the boundary conditions at walls a) Select wall-back under Boundary Conditions The Type will be reported as wall b) Click Edit... to open the Wall panel, and select Momentum tab i. Select Specified Shear under Shear Condition, and set Shear Stress X- and Y- components to 0 Walls in this simulation are modeled as slip walls since the primary physics of screech noise generation is driven by free-stream jet flow structures, and wall turbulence does not contribute to the generation of screech noise c) Select Thermal tab in the Wall panel, and retain default settings of zero heat flux 2011 ANSYS, Inc. November 7,

Setup and Solution Steady-State Flow Step 5: Boundary Conditions (continued) Problem Setup Boundary Conditions 4. Copy setting for the wall-back to other walls a) Click Copy.

18 Setup and Solution Steady-State Flow Step 5: Boundary Conditions (continued) Problem Setup Boundary Conditions 4. Copy setting for the wall-back to other walls a) Click Copy... below Boundary Conditions to open the Copy Conditions panel i. Select wall-back under From Boundary Zone ii. Select mic1, mic2, wall-lip and wall-lip-lam under To Boundary Zone, click Copy, and OK 2011 ANSYS, Inc. November 7,

Setup and Solution Steady-State Flow Step 6: Solution Methods Solution Solution Methods 1. Select Coupled under Pressure-Velocity Coupling Scheme This will activate pressure-based coupled solver 2.

19 Setup and Solution Steady-State Flow Step 6: Solution Methods Solution Solution Methods 1. Select Coupled under Pressure-Velocity Coupling Scheme This will activate pressure-based coupled solver 2. Select Green-Gauss Node Based under Gradient 3. Select Second Order under Pressure 4. Select QUICK for all other equations: Density, Momentum, Turbulent Kinetic Energy, Turbulent Dissipation Rate, and Energy 2011 ANSYS, Inc. November 7,

Setup and Solution Steady-State Flow Step 7: Solution Controls Solution Solution Controls 1. Set the Courant Number to 50 2. Set the Explicit Relaxation Factors for Momentum and Pressure to 0.25 3.

20 Setup and Solution Steady-State Flow Step 7: Solution Controls Solution Solution Controls 1. Set the Courant Number to Set the Explicit Relaxation Factors for Momentum and Pressure to Set Under-Relaxation Factor of 0.25 for Density 4. Retain default settings of Under-Relaxation Factors for other equations: Body Forces = 1.0 Turbulent Kinetic Energy = 0.8 Turbulent Dissipation Rate = 0.8 Turbulent Viscosity = 1.0 Energy = ANSYS, Inc. November 7,

21 Setup and Solution Steady-State Flow Step 8: Solution Initialization 1. Initialize the solution Solution Solution Initialization (a) Initialize the flow with values shown here Gauge Pressure (pascal) = 100,000 Axial Velocity (m/s) = 0 Radial Velocity (m/s) = 0 Turbulent Kinetic Energy (m2/s2) = 0.1 Turbulent Dissipation Rate (m2/s3) = 6 Temperature (K) = 300 (b) Click Initialize to initialize the solution 2011 ANSYS, Inc. November 7,

22 Setup and Solution Steady-State Flow Step 8: Solution Initialization (continued) 2. Run Full Multigrid (FMG) initialization (a) In the console window, which is also called Text User Interface (TUI), type: solve initialize set-fmg-initialization (b) Hit Enter on the keyboard to move down trough FMG settings. Change only: set FMG courant-number = 0.15 enable FMG verbose? yes (c) Run FMG initialization by typing in TUI: fmg-initialization yes This will provide an initial approximate solution (Figure 6) 3. Enable the plotting of residuals Solution Monitors Residuals Edit... (a) Select Plot and Print to Console under Options (b) Uncheck Check Convergence for all equations (c) Keep other setting at defaults, and click OK 2011 ANSYS, Inc. November 7, Figure 6. Contours of Mach number after FMG initialization 4. Write case and data files: run-screech-steady-fmg.cas.gz and run-screech-steady-fmg.dat.gz File Write Case & Data...

23 Setup and Solution Steady-State Flow Step 9: Steady-State Solution 1. Iterate the solution Solution Run Calculations (a) Set Number of Iterations to 200 and Calculate After 200 iterations, residuals will drop three orders of magnitude (Figure 7). Contours of converged steady-state Mach number distribution are shown in Figure 8 Note: it will take about 16 minutes of CPU time to finish the steady-state run on a single Windows processor. You can reduce the computing time requirements by skipping to Step ANSYS, Inc. November 7,

24 Setup and Solution Steady-State Flow Step 9: Steady-State Solution (continued) Figure 7. Convergence of residuals Figure 8. Contours of Mach number 2. Write case and data files: run-screech-steady.cas.gz and run-screech-steady.dat.gz File Write Case & Data ANSYS, Inc. November 7,

gz and run-screech-steady.dat.gz File Read Case & Data... 2.

25 Setup and Solution Transient Flow Step 10: Transient Case Setup 1. Read in converged steady-state results from the previous step: run-screech-steady.cas.gz and run-screech-steady.dat.gz File Read Case & Data Change Time formulation to Transient in Problem Setup General 3. Change Transient Formulation to Second Order Implicit in Problem Setup Solution Methods 2011 ANSYS, Inc. November 7,

26 Setup and Solution Transient Flow Step 10: Transient Case Setup (continued) 4. Modify solution controls Solution Solution Controls (a) Set the Courant Number to 1e+15 (b) Set all other relaxation factors to 1.0 These are recommended Solution Controls settings when running transient pressure-based coupled solver Note: in the pressure-based coupled solver, the value 1/CFL, where CFL stands for Courant number, acts as an implicit relaxation factor for coupled continuity and momentum system of equations. Setting CFL to a very large number equals to removing implicit relaxation from the coupled equations 2011 ANSYS, Inc. November 7,

Setup and Solution Transient Flow Step 10: Transient Case Setup (continued) 5. Define Custom Field Function for pressure perturbations over its mean value: Define Custom Field Functions.

27 Setup and Solution Transient Flow Step 10: Transient Case Setup (continued) 5. Define Custom Field Function for pressure perturbations over its mean value: Define Custom Field Functions... (a) Select Pressure... and Static Pressure under Field Functions, and click Select (b) Click mouse pointer over keypad numbers to complete the function definition: p (c) Enter a new name pa under New Function Name, and click Define 2011 ANSYS, Inc. November 7,

28 Setup and Solution Transient Flow Step 10: Transient Case Setup (continued) 6. Enable the monitoring of static pressure perturbations at two microphone locations on the nozzle lip: Solution Monitors (a) Click Create... Under Surface Monitors (b) Change Name to mic1 (c) Select Area-Weighted Average under Report Type (d) Select Custom Field Functions... and pa under Field Variable (e) Select mic1 under Surface (f) Uncheck Print to Console and check Plot under Options (g) Check Write and specify File Name: mic1.out (h) Select Flow Time under X Axis (i) Select Get Data Every 1 Time Step (j) Click OK 2011 ANSYS, Inc. November 7,

29 Setup and Solution Transient Flow Step 10: Transient Case Setup (continued) (k) Repeat these steps to define the other monitor at mic2 surface: 2011 ANSYS, Inc. November 7,

30 Setup and Solution Transient Flow Step 10: Transient Case Setup (continued) 7. Set the time step parameters Solution Run Calculation (a) Set the Time Step Size (s) to 5e-6 (b) Set Number of Time Steps to 2000 (c) Set Max Iterations/Time Step to 10 Based on experimental data, expected fundamental screech tone frequency, f, is about 7 khz. Temporal discretization should provide 20 or more time steps per period of oscillation. This defines the maximum time step as Dt max = 1/(20*f) = 7.14e-06 sec 8. Write case and data files: run-screech-unsteady-0.00.cas.gz and run-screech-unsteady-0.00.dat.gz File Write Case & Data ANSYS, Inc. November 7,

31 Setup and Solution Transient Flow Step 11: Transient Run 1. Run the transient simulation Solution Run Calculation Click Calculate to time-march the solution to a time-periodic state The solution will advance to t = 0.01 sec by the end of the run 2. Write case and data files: run-screech-unsteady-0.01.cas.gz and run-screech-unsteady-0.01.dat.gz File Write Case & Data... Note: it will take about 26 hours of CPU time to finish the run on a single 32-bit Windows processor. You can reduce the computing time requirements considerably by skipping to Step 12 Parallel run on 8 compute 64-bit processors will take only 2.3 hours! 2011 ANSYS, Inc. November 7,

Display contours of Mach number: Results Graphics and Animations Contours (a) Select Contours of Velocity and Mach Number (b) Check Filled, Node

32 Setup and Solution Step 12: Aeroacoustic Postprocessing 1. Read in transient results from the previous step: run-screech-unsteady-0.01.cas.gz and run-screech-unsteady-0.01.dat.gz File Read Case & Data Display contours of Mach number: Results Graphics and Animations Contours (a) Select Contours of Velocity and Mach Number (b) Check Filled, Node Values, Global Range and Auto Range under Options (c) Click Display Display Views (a) Select axis and axis:001 under Mirror Planes (b) Click Apply (c) Zoom in to the region at the nozzle inlet (Figure 10b) 2011 ANSYS, Inc. November 7,

33 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) Figure 10a. Contours of Mach number: steady-state simulation Figure 10b. Contours of Mach number: transient simulation Note the difference between steady-state (Figure 10a) and transient (Figure 10b) Mach number distribution. Steady-state solution significantly suppresses the shock cell structure in the jet plume 2011 ANSYS, Inc. November 7,

34 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) 3. Display contours of static pressure perturbations: Results Graphics and Animations Contours (a) Select Contours of Custom Field Functions... and pa (b) Uncheck Global Range and set Min = -300 and Max = 300 (c) Uncheck Clip to Range and click Display sound waves Sound waves of the screech tone propagate predominantly in the upstream direction Figure 11. Contours of static pressure perturbations 2011 ANSYS, Inc. November 7,

Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) 4. Display the acoustic pressure signals at the two receiver locations: Results Plots File (a) Click Add.

35 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) 4. Display the acoustic pressure signals at the two receiver locations: Results Plots File (a) Click Add... in the File XY Plot panel This will open the Select File panel where you can now select mic1.out and mic2.out from the file list (b) Click OK to close the Select File panel 2011 ANSYS, Inc. November 7,

36 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) (c) Click Plot to display the microphone signals (Figure 12). Modify the line and marker styles as necessary, using the Curves panel. Modify the X axis to show the signal over the time rage of s. Modify legends using Change Legend Entry button in File XY Plot menu Figure 12. Acoustic pressure signals at two microphone locations on the nozzle lip 2011 ANSYS, Inc. November 7,

Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) 5. Activate Ffowcs-Williams & Hawkings (FWH) acoustics model : Problem Setup Models Acoustics Edit.

37 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) 5. Activate Ffowcs-Williams & Hawkings (FWH) acoustics model : Problem Setup Models Acoustics Edit... (a) Select Ffowcs-Williams & Hawkings under Model, retain default settings, and click OK This step is required only to make Sound Pressure Level available for FTT spectral analysis. However, FWH model will not be used to predict noise propagation 2011 ANSYS, Inc. November 7,

Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) 6. Perform spectral analysis of the receiver signals: Results Plots FFT (a) Click Load Input Files.

38 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) 6. Perform spectral analysis of the receiver signals: Results Plots FFT (a) Click Load Input Files..., change Files of Type to All Files in Select File window and select mic1.out (b) Select Sound Pressure Level (db) from the Y Axis Function drop-down list (c) Select Frequency (Hz) from the X Axis Function drop-down list 2011 ANSYS, Inc. November 7,

39 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) (d) Click Plot FFT to plot the sound pressure spectrum for mic1 (Figure 13) The overall sound pressure level (OASPL) is printed to the console window: Overall Sound Pressure Level in db (reference pressure = e-005) = e+002 Note: The maximum frequency plotted is f = 1/[2Dt] = 100 khz, as expected. Figure 13. Spectral analysis of pressure signal for mic ANSYS, Inc. November 7,

40 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) (e) Click Axes... This will open the Axes - Fourier Transform panel i. Deselect Auto Range for the X Axis ii. Manually set the Maximum for Range to iii. Set Precision to 0 iv. Click Apply and Close the panel 2011 ANSYS, Inc. November 7,

Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) (f) Click Plot/Modify Input Signal... to open the Plot/Modify Input Signal panel.

41 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) (f) Click Plot/Modify Input Signal... to open the Plot/Modify Input Signal panel. It lets you modify and plot the signal before the Fourier Transform is applied i. Select Clip to Range and set the Min value for X Axis Range to Without clipping the temporal range, the complete pressure signal history would be analyzed including the initial transient state leading up to the quasiperiodic state ii. Click Apply/Plot and Close to return to the Fourier Transform panel Since the x-axis range was manually set for the spectral plot, you will not see the proper range when plotting the modified signal. You will need to temporarily reset the range if you want to plot the input signal 2011 ANSYS, Inc. November 7,

42 Setup and Solution Step 12: Aeroacoustic Postprocessing (continued) (g) Click Plot FFT to plot the sound pressure spectrum for mic1 (Figure 14a). The spectrum peaks at about 7000 Hz (h) Repeat above steps to plot the sound pressure spectrum for mic2 (Figure 14b) (a) screech tone (b) screech tone Figure 14. Spectral analysis of pressure signals at (a) mic1, and (b) mic2 microphone locations. Note: only the fundamental frequency of screech tones is resolved in this simulation, higher order harmonics are not resolved 2011 ANSYS, Inc. November 7,

43 Comparison with Test Data l/d j SPL (db) Screech tones of low supersonic jets are characterized by two axisymmetric screech modes A 1 and A 2. Mach numbers at which transition from one mode to another takes place may vary from experiment to experiment because of sensitivity of jet screech mechanism to specifics of experimental set-up. Figure 15a shows comparison of calculated screech wavelengths with experimental measurements. Numerically predicted wavelengths follow very closely the experimentally measured A 1 mode curve. Figure 15b shows favorable comparison between predicted and experimentally measured amplitudes of fundamental screech modes at two microphone locations at the nozzle lip. Note: Data points at other Mach numbers were obtained by re-running the tutorial cases with different pressure inlet settings A1 - experiment A2 - experiment A1 - CFD A M j Figure 15a. Wavelengths of fundamental screech modes 2011 ANSYS, Inc. November 7, microphone at r = 0.662" M j Figure 15b. Amplitudes of fundamental screech modes A1 - experiment A2 - experiment A1 - CFD

44 Summary This tutorial demonstrated the use of ANSY FLUENT's direct CAA capabilities to calculate near-field radiation of jet screech tones by an axisymmetric supersonic jet. You have learned how to set up the relevant parameters, record acoustic data at microphone locations, calculate, and postprocess the acoustic pressure signals. Calculated sound pressure level and frequency are in favorable agreement with experimental data. The main computational efforts are spent calculating the time dependent turbulent flow ANSYS, Inc. November 7,

Advanced ANSYS FLUENT Acoustics

Workshop Modeling Flow-Induced (Aeroacoustic) Noise 14.5 Release Advanced ANSYS FLUENT Acoustics 2011 ANSYS, Inc. November 7, 2012 1 Introduction This tutorial demonstrates how to model 2D turbulent flow