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COMPARISON OF LOW-BOOM PREDICTIONS FROM DIFFERENT SONIC BOOM PROPAGATION
CODES
Alexandra Loubeau, Gérald Carrier, Patrice Malbequi, Sriram Rallabhandi, Joel Lonzaga
To cite this version:
Alexandra Loubeau, Gérald Carrier, Patrice Malbequi, Sriram Rallabhandi, Joel Lonzaga. COM- PARISON OF LOW-BOOM PREDICTIONS FROM DIFFERENT SONIC BOOM PROPAGATION CODES. The 2nd RUMBLE Open Workshop has been performed @ e-FORUM ACUSTICUM 2020, Dec 2020, Lyon, France. �hal-03181320�
COMPARISON OF LOW-BOOM PREDICTIONS FROM DIFFERENT SONIC BOOM PROPAGATION CODES
Alexandra Loubeau1 G´erald Carrier2 Patrice Malbequi2 Sriram Rallabhandi3 Joel Lonzaga1
1Structural Acoustics Branch, NASA Langley Research Center, USA
2 Aerodynamics, Aeroelasticity and Acoustics Department, ONERA, Meudon, France
3Aeronautics Systems Analysis Branch, NASA Langley Research Center, USA
a.loubeau@nasa.gov
ABSTRACT
A comparison of sonic boom propagation codes from NASA and ONERA was conducted using low-boom test cases from the 3rd AIAA Sonic Boom Prediction Work- shop. Near-field pressures from two different shaped, low- boom demonstration aircraft concepts had been calculated with computational fluid dynamics codes and were used as starting signatures for the acoustic propagation analyses.
In addition, atmospheric profiles from NOAA’s Climate Forecast System Reanalysis database, including pressure, temperature, humidity, and winds as a function of altitude, were used to predict sonic booms propagating through re- alistic atmospheric conditions. The ground waveforms and derived loudness metrics were compared at regular inter- vals across the sonic boom carpet and at the location of the lateral cut-off rays. This code comparison exercise and in- vestigation into the possible causes of differences enables a verification of the codes and proposed recommendations for code refinement.
1. INTRODUCTION
The 3rd AIAA Sonic Boom Prediction Workshop (SBPW3) was held in January 2020 to compare state-of- the-art prediction methods for propagation of shaped low booms through the atmosphere. Test cases were provided to participants, who used their best practices and methods to predict ground sonic boom signatures and their corre- sponding loudness levels. The main goals of the com- parisons are to improve awareness of the effect of realis- tic atmospheric conditions on low-boom propagation, par- ticularly near lateral cut-off, and to understand modeling gaps that may exist across multiple codes and international teams. This workshop was a follow-on to a workshop held in 2017 [1] and included more complex test cases with lower loudness levels, in an effort to continue advancing the state-of-the-art in boom propagation modeling.
Following the workshop, the National Aeronautics and Space Administration (NASA) and the Office National d’ ´Etudes et de Recherches A´erospatiales (ONERA) con- tinued comparisons of their predictions with updated re- sults and additional parameters. NASA develops and sup- ports two different sonic boom propagation codes: PC-
Boom [2–4] and sBOOM [5, 6]. ONERA utilizes the Airbus code BANGV, developed at Sorbonne University [7–9]. All three of these codes implement geometrical acoustics ray tracing and propagation along the rays us- ing numerical solutions to the lossy Burgers equation, al- though the numerical implementations differ between the codes. The effects of nonlinearity, atmospheric absorption and dispersion, and geometrical spreading are included.
All three codes accept near field signatures, aircraft con- ditions, and atmospheric profiles as inputs, and they output ground sonic boom signals and ray landing locations.
This paper describes the test cases and prediction results undertrack, across the sonic boom carpet, and at lateral cut- off. Waveforms, sonic boom noise metrics, cut-off angles, and additional parameters are compared, and recommen- dations for future comparisons are presented.
2. TEST CASES
Two test cases with different near field signatures were pro- vided to the workshop participants. It is assumed that these pressure waveforms are appropriate as propagation code inputs, by being sufficiently far from the aircraft so that the 3D effects are fully resolved. Because low-boom experi- mental data does not exist for benchmarking, the compar- isons concentrate on describing observed differences be- tween the various predictions.
The first case is a NASA low-boom demonstrator air- craft concept called C25P (a powered equivalent of the C25D configuration used in the 2nd SBPW). The flight conditions are Mach 1.6, an altitude of 15,760 m, aircraft length (L) of 33.53 m, and extraction distance (R/L) of 3.0.
The near field was provided from -90 to +90 degrees in 10 degree increments. A selection of these near field signa- tures is included in Fig. 1.
The second case is a NASA-Lockheed Martin low- boom flight demonstrator design called C609. This is an earlier version of the design for the X-59 Quiet Supersonic Technology (QueSST) demonstration aircraft. The flight conditions are Mach 1.4, a cruise altitude of 16,459.2 m, L of 27.43 m, and R/L of 3.0. The near field was provided from -90 to +90 degrees in 2 degree increments. These near field signatures are included in Fig. 2.
The atmospheric conditions were chosen to incorporate
Figure 1. Selection of near field signatures for Case 1.
Figure 2. Selection of near field signatures for Case 2.
realistic atmospheric profiles, including winds. The pro- files were selected from the National Oceanic and Atmo- spheric Administration Climate Forecast System Version 2 (CFSv2) database [10]. The atmospheric profile cho- sen for Case 1 results in a wide carpet that maximizes the angles of the cut-off rays. The temperature, relative hu- midity, and wind profiles are presented and compared to a standard atmosphere [11, 12] in Fig. 3. The atmospheric profile chosen for Case 2 results in a wide carpet that max- imizes the physical carpet width on the ground. The tem- perature, relative humidity, and wind profiles are presented and compared to a standard atmosphere in Fig. 4. Follow- ing the convention of meteorological vector winds, positive x-wind is eastward and positive y-wind is northward.
3. RESULTS
Predicted ground sonic boom waveforms are compared for the three different software suites of sBOOM 2.82, BANGV, and PCBoom 6.7.2. Undertrack and off-track waveforms show the evolution of the signals across the car- pet, ending with the limiting ray waveform. Several sonic boom noise metrics were computed for these waveforms, using the same metrics code for all three sets of results to avoid any additional sources of differences. Results in terms of Perceived Level (PL) [13] and B-weighted Sound Exposure Level (BSEL) [14] are included here. Finally, comparisons of cut-off ray angles and other parameters are
-60 -40 -20 0 20 0
5 10 15 20 25
Altitude (km)
Case 1 Standard Atm
0 50 100
rh (%) 0
5 10 15 20 25
-40 -20 0 20 40 Wind (m/s) 0
5 10 15 20 25
x y
Figure 3. Temperature (T), relative humidity (rh), and wind speed profiles for Case 1 compared to standard at- mosphere [11, 12]. The aircraft altitude is shown as a hori- zontal dotted black line.
-60 -40 -20 0 20 0
5 10 15 20 25
Altitude (km)
Case 2 Standard Atm
0 50 100
rh (%) 0
5 10 15 20 25
-50 0 50
Wind (m/s) 0
5 10 15 20 25
x y
Figure 4. Temperature (T), relative humidity (rh), and wind speed profiles for Case 2 compared to standard at- mosphere [11, 12]. The aircraft altitude is shown as a hori- zontal dotted black line.
included to delve deeper into possible reasons for observed differences.
After the SBPW3, it was determined that the starting signatures used as inputs to the propagation codes did not match, due to differing assumptions regarding incorpora- tion of wind effects in transition from the near field CFD frame of reference to the geometrical acoustics frame of reference. This matching is complicated when winds are present at the flight altitude, which affects the initialization of the ray trajectories and the time-dependent pressure sig- nal at the origin of these rays. Although the best way to handle this matching in the presence of winds has not been resolved yet, using consistent starting signatures for each code improved agreement between the ground predictions, and these are the results presented here.
3.1 Case 1 Results 3.2 Case 2 Results
4. CONCLUSIONS
A comparison of three different sonic boom propagation prediction software suites has been conducted using test cases from the SBPW3. As expected, the results tend to be more similar undertrack, where the acoustic ray path from the flight altitude to the ground is generally the shortest and is least affected by the atmospheric conditions. Differ- ences across the carpet are presented in terms of two sonic boom noise metrics, PL and BSEL, and various other prop- agation parameters.
Further work is needed to determine the appropriate way to handle wind effects for starting signatures at flight altitude. Simpler test cases with analytical solutions, or with uniform atmospheric conditions as a function of alti- tude, are being considered for follow-on analyses to allow for a more detailed comparison.
5. REFERENCES
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