Numerical Studies of Supersonic Jet Impingement on a Flat Plate

Transcription

1 Numerical Studies of Supersonic Jet Impingement on a Flat Plate Overset Grid Symposium Dayton, OH Michael R. Brown Principal Engineer, Kratos/Digital Fusion Solutions Inc., Huntsville, AL. October 18, 2012

2 Outline Objectives Jet impingement physics Problem setup Computational model Test data comparisons Flow physics py Feedback mechanism Conclusions 10/18/2012 2

3 Objectives of Study Validate CFD results using test data Use CFD to understand physics of jet impingement Acoustic wave propagation Feedback mechanism Effects of nozzle exit to plate separation distance Identify areas for modeling improvement Identify areas for further research 10/18/2012 3

4 Physics of Jet Impingement Jet impingement on a flat plate has interesting properties The acoustic waves from the jet impingement region radiate outward The acoustic waves interact with the jet at the nozzle exit These interactions ti create disturbances that propagate downstream These disturbances change the frequencies of the acoustic waves emanating from the plate These new frequencies propagate upward and introduce new disturbance frequencies into the jet The process repeats until the frequencies lock in This is called the feedback mechanism Shear layer disturbance Jet Impingement Plate Lift plate Acoustic wave interaction with jet shear layer Acoustic waves Radial wall jet 10/18/2012 4

5 Physics of Jet Impingement Wall jet is formed when the jet impinges on the surface The wall jet experiences pulsing from the impingement region This makes the wall jet a noise source This has been observed in jet impingement tests detailed in the literature Tests of supersonic jet impingement show three types of tones L1, L2, and L3 tones Shear layer disturbance Jet Lift plate Acoustic wave interaction with jet shear layer Acoustic waves Radial wall jet Impingement Plate 10/18/2012 5

6 Problem Setup Jet impingement tests were performed at Florida State University [10] Krothapalli, A., Rajkuperan, E., Alvi, F., and Lourenco, L., Flow field and noise characteristics i of a supersonic impinging i i jet, Journal of ffluid Mechanics, Vol. 392, pp , Test used an ideally expanded supersonic jet with an exit Mach number of 1.5 Nozzle pressure ratio of 3.7 (chamber pressure/nozzle lip pressure) Exit Reynolds number of 7x10 5 Nozzle was imbedded in a circular lift plate with a diameter approximately 10 nozzle throat diameters Tests were performed for three different nozzle to plate separation distances Nozzle height to throat diameter (h/d) ratios of 3.75, 4.0, and 4.25 The impingement i plate was a square plate 2.44 m x 2.44 m Near field acoustic data were recorded by a probe placed 25 cm from nozzle exit 10/18/2012 6

7 The CFD model of the test was constructed based on test description and using the test t nozzle geometry provided by Florida State Mesh consisted of a system of 84 overset grids Grid spacing between the impingement plate and the nozzle exit was kept uniform with x= y= z= 6x10-4 m This is approximately 80 points per wavelength at 7000 Hz Fine grids were placed on the nozzle walls and lift plate so that y+ 1 CFD Model Impinging Jet Grid 10/18/2012 7

8 CFD Model (cont.) Calculations were performed with OVERFLOW2 Spatial discretization was 3 rd order (MUSCL scheme) Temporal discretization was 2 nd order with implicit, dual time stepping Detached Eddy Simulation (DES) used for turbulence modeling DES Extension of Mentor s SST model Compressible formulation of constituent RANS models Grid sizes for each run in the study are shown below Runtimes were approximately 2-4 weeks on 288 processors Separation Distance (throat diameters) Grid Size (millions of points) 10/18/2012 8

9 Comparison to Test Data

10 Comparison to Test Data CFD results are compared to test data where possible Test data mostly consists of sound pressure level (SPL) data at near field location CFD results were obtained for a monitor point located in the near field close to the experimental location Monitor point was placed in the high resolution region to avoid degradation of frequencies due to increased grid spacing The monitor point is located slightly closer to nozzle than the experiment probe which results in higher SPL amplitudes but frequencies will not be affected CFD simulations were run until the pressure at the near field point became statistically steady before data were recorded for analysis 10/18/

11 SPL Comparison: h/d= Test measured SPL show three impingement tones 150 present (L1, L2, and L3) 140 CFD results also show the 130 same three impingement 120 tones 110 Lowest frequency within SPL (db) 100 ~8% of test data 90 L2 Tone L1 Tone L3 Tone Test [10] CFD Highest frequency within ~3% of test data CFD results show that the L3 tone is dominant for this case CFD results compare well to the test measurements Frequency (Hz) 10/18/

12 SPL Comparison: h/d=4 CFD results also show the same three impingement tones Frequencies are within ~4% of test data L2 Tone L1 Tone L3 Tone CFD Test [10] db) SPL ( Frequency (Hz) 10/18/

13 SPL Comparison: h/d=4.25 CFD results also show the same three impingement tones Frequencies are within 140 ~6% of test data CFD results show more 130 prominent L2 and L1 tones 120 than the test data Higher harmonics of the L3 110 tone shows up in the test 100 Some of these harmonics are captured by CFD Additional higher harmonics of the L1 tone is seen (db) SPL L2 Tone L1 Tone L3 Tone L3 First Harmonic Test [10] CFD L3 Second Harmonic L3 Third Harmonic Frequency (Hz) 10/18/

14 Effects of Separation Distance Tone frequency decreases with separation distance The dominant tone changes with separation distance The L2 tone magnitude diminishes with distance and becomes broader in frequency CFD reproduces the trend observed in test data L2 Tone L1 Tone L3 Tone h/d=3.75 h/d=4 h/d= L2 Tone L1 Tone L3 Tone h/d=3.75 h/d=4 h/d=4.25 SPL (db) SPL (db) Frequency (Hz) Frequency (Hz) Note: Test Data CFD Results 10/18/2012 h/d=4 results offset by 20 db 14 h/d=4.25 results offset by 40 db

15 Convective Velocities: h/d=4 Velocities of vortical disturbances propagating downstream were measured in the tests for the h/d=4 separation case Comparisons were made between the CFD predicted velocities and the test measurements CFD results show good comparison with the test data (within 4%) c/u jet U c Test Data [10] CFD Test Data Average [10] CFD Average Shear layer disturbances Distance (mm) Convective Velocity of Vortices in Shear Layer: h/d=4 10/18/2012 Density Gradient Magnitude: h/d=4 15

16 Flow Physics

17 Impingement Tone Analysis Comparisons were made with data based on the complete time 140 history of the pressure at the near field point Analysis of the CFD results showed that the magnitudes of the tones changed with time A joint time-frequency analysis was performed on the data to determine the tone variation 120 with time A fixed width window was used to analyze the frequency composition with time Window width: 8192 samples (~0.011 sec) SPL (db) SPL (db) Test CFD Frequency (Hz) (a) Test CFD Frequency (Hz) (c) SPL (db) SPL (db) Test CFD Frequency (Hz) (b) Test CFD Frequency (Hz) (d) 10/18/

18 Joint Time-Frequency Analysis Results Results of the joint-time frequency analyses are shown for each separation distance The variation in the tone magnitudes with time is seen This shows that depending on the time sample analyzed, the results may appreciably differ L2 Tone L1 Tone L3 Tone h/d=3.75 h/d=4 h/d= /18/

19 Turbulent Kinetic Energy Turbulent kinetic energy (TKE) is an indicator of noise sources in the flow Contours for TKE are shown for all three separation distances The highest levels of TKE are in the shear layer and at the impingement point Acoustic waves are observed emanating from the peak TKE region at the impingement point (shown later) h/d=3.75 h/d=4 h/d=4.25 Resolved TKE (m 2 /sec 2 ) 10/18/

20 OASPL in Nozzle Centerline Plane h/d=3.75 h/d=4 h/d= /18/

21 Feedback Mechanism

22 Feedback Mechanism The feedback mechanism is illustrated using contours of density gradient magnitude The acoustic waves from the impingement region propagate out and toward the nozzle exit Part of the acoustic wave propagates into the jet The external part of the acoustic wave reflects off the plate surface while the internal wave reflects off the conical flow feature at the nozzle exit The acoustic wave in the jet deforms the conical feature, perturbing the structure The resulting normal reflection is weak and has little coherence The external acoustic wave perturbs the shear layer as it travels downstream Subsequent interaction with upward traveling waves amplifies the disturbance Internal wave (a) (c) Acoustic wave generation Acoustic wave reflection Internal wave interaction Original wave (b) (d) Acoustic wave component inside jet External wave Shear layer disturbance Oncoming wave Amplified disturbance (e) (f) Feedback Mechanism: h/d=4 10/18/

23 Feedback Mechanism: h/d=3.75 Shear Layer Structure: /18/

24 Feedback Mechanism: h/d=4 Shear Layer Structure: h/d=4 10/18/

25 Acoustic Wave Effects on the Shear Layer The acoustic waves induce disturbance in the shear layer These disturbances are vortices that propagate downstream and grow in strength The vortex structure t in the shear layer for the h/d=4 case can be seen in figure Q-criterion iso-surface (Q=3000) The vortices are helical which corresponds to the helical mode of the jet motion Interactions with acoustic waves increases the strength of these vortical disturbances Vortices in shear layer Q Criterion Iso-surface of Q=3000: h/d=4 ( Ω Ω S ) 1 Q = S 2 ij ij ij ij 10/18/

26 Vortex Visualization Q Criterion iso-surfaces of Q=3000 colored by density: h/d=4 10/18/

27 Acoustic Wave Effects on Jet Core The internal acoustic wave is in a supersonic region but does not propagate at supersonic speeds The acoustic wave propagates at an angle into the jet core at an angle greater than 48º relative to the jet axis At angles greater than this the flow normal to the wave travel direction is less than Mach 1 The external acoustic wave maintains contact with the internal wave through the shear layer The internal wave reflects from the compression cone at the nozzle exit The wave deforms the compression cone on contact with it The wave only weakly reflects and is not coherent This indicates that all the wave energy is reflected tangent to the compression cone which results in the cone s deformation The deformation in the compression cone causes the jet core to be perturbed 10/18/

28 Feedback Mechanism: h/d=3.75 Internal Jet Structure: h/d=4 10/18/

29 Feedback Mechanism: h/d=4 Internal Jet Structure: h/d=4 10/18/

30 Feedback Effects on Jet Behavior The motion of the jet is a function of the flow at the nozzle exit and the shear layer disturbances The jet motion near the nozzle exit is determined by the core flow at the nozzle exit The acoustic waves in the jet induces motion in compression cone which propagates down the jet core Vortices in the shear layer grow as they travel downstream until they disrupt the jet The shape of these vortices determine the jet behavior downstream Helical vortices create a helical jet mode Symmetric vortices create a symmetric jet mode The acoustic waves affect both of these regions 10/18/

31 Feedback Mechanism Investigation Results The feedback mechanism has been traditionally described as the coupling between the shear layer and acoustic waves from the impingement region The CFD results show that the acoustic waves also affect the jet core at the nozzle exit This has not been reported in the test observations in the literature survey Detecting the acoustic wave in the jet would be very difficult due to the three-dimensional structure of the flow The CFD results show the feedback mechanism is more complex than originally thought Test data would be needed to verify the analysis findings 10/18/

32 Conclusions The OVERFLOW code can be used to accurately model supersonic jet impingement Impingement tones measured in the tests were also captured in the CFD calculations The DES model captured the formation and propagation velocity of the vortical structures in the shear layer Combination of DES and a highly resolved mesh can be used to analyze aeroacoustic phenomena The feedback mechanism was successfully captured Acoustic wave source for the shear layer disturbances was observed Acoustic wave propagation into the jet core and direct influence on the motion of the jet core was observed 10/18/

33 Questions

34 Backup Slides

35 Jet Mode Examples

36 Helical Jet Mode Mach=0.5 Iso-Surface Colored by Density 10/18/2012 Showing Helical Jet Mode: h/d=4 36

37 Symmetric Jet Mode 10/18/ Mach=0.5 Iso-Surface Colored by Density Showing Symmetric Jet Mode: Modified Nozzle h/d=4

38 h/d=4.25 Density Gradient Magnitude Animations

39 Shear Layer Detail: h/d= /18/

40 Jet Core Detail: h/d= /18/