Les Bevel Nozzle

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Euromech Colloquium 467: Turbulent Flow and Noise Generation July 18-20, 2005 – Marseille, France

Numerical Prediction of Noise from Round and Beveled Nozzles

K. Viswanathan1 Mikhail Shur

2

[email protected] [email protected]

Mikhail Strelets2 Philippe R. Spalart

1

[email protected] [email protected]

1 The Boeing Company, PO Box 3707, Seattle, WA 98124, USA 2 St.-Petersburg State Polytechnic Univ., 29, Polytechnicheskaya str., St.-Petersburg 195251, Russia

Abstract

Numerical simulations of the flow field and the noise generated by round and beveled nozzles are carried out.The objective of this study is to gain insights into the flow characteristics that yield a noise reduction for the

beveled nozzle. For aircraft applications, the geometry of the nozzles must be optimized both for

aerodynamic and acoustic performance. Results from both RANS and LES computations are presented. The

aerodynamic predictions from RANS are in very good agreement with experimental measurements. The

noise predictions from LES agree with the trends observed in the measurements. Given the complexity of the

problem and the extreme grid requirements, good spectral predictions are obtained, albeit with a strict limit

on the maximum Strouhal number. For the subsonic jets, the noise is consistently under-predicted close to the

jet direction. The results are encouraging and this study is a part of on-going efforts to better understand the

flow physics, and possibly derive fresh ideas from a broad visibility of the turbulence.

Keywords: Jet Noise, Computational Aeroacoustics, Large-Eddy-Simulation, Beveled Nozzle

1 Introduction

Jet noise continues to be the dominant noise component during takeoff, even for modern commercial aircraft.

Despite significant research carried out over the last fifty years, there is no accepted complete theory for the

generation and radiation of jet noise, and no methodology capable of predicting the spectra at all angles and

over the wide frequency range of interest to the aerospace industry. Therefore, there is a heavy reliance on

experimental measurements, which tend to be very expensive and limited in the quantities that are measured.

Detailed knowledge of the entire turbulent flowfield is necessary in order to predict noise; there are

significant challenges in accomplishing even this first step. Only in recent years have non-intrusive optical

techniques been developed that permit the measurement of the turbulent fluctuations, and their accuracy is

limited. Even if all the requisite flow information were known, there would remain the twin challenges of identifying the noise sources and actually predicting farfield noise to the required accuracy. The problem of

relating subtle changes in the flow field, say due to modifications to the nozzle geometry, to the radiated

noise is formidable. Significant gaps remain in our understanding of turbulence and noise.

Numerous recent studies have addressed the issue of turbulence-generated noise with the goals of obtaining

better insight into the flow and improving our ability to predict noise. With the advances in computing

capability, the use of Large Eddy Simulation (LES) for this purpose is becoming attractive. Many researchers

have adopted this approach; for example, see Bogey and Bailly [1], Bodony and Lele [2], Paliath and Morris

[3], Shur et al [4], and Uzun et al [5]. The other approach of using the steady-state solution from a Reynolds

Averaged Navier-Stokes (RANS) as input to a noise prediction methodology suffers from severe limitations,

see [4]. In most of the past LES studies, the nozzle is not included. Instead, a simple inflow profile is

specified. This practice is not satisfactory, especially for the geometries considered here.

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vorticity field immediately downstream of the nozzle, shown along with the grid in Figure 2, indicates that thegrid used this far is somewhat too coarse in the axial direction, and is not aligned with the shear layer. This could

well be the reason for the observed smooth roll-up and vortex pairing and more rapid damping of the turbulence inthe upper shear layer. This example highlights some of the issues with grid and numerics that come with beveled

nozzles. This deficiency will be remedied, and should be kept in mind when examining the noise results.

Figures 3 and 4, respectively, show contours of the pressure time-derivative for the beveled nozzle and

comparisons of the predicted directivities of the overall sound pressure levels (OASPL) with data. We notice inthese figures that the predicted noise radiated to the sides (φ=±90º) by bevel45 is slightly higher than that for theround nozzle, a trend seen in the experiments. The predicted noise levels radiated to the azimuthal angle of 0º is

lower than those at both 90º and 180º, again matching the experimental trends. Furthermore, the shift in the peak

radiation angle by ~25º between φ=0º and φ=180º is captured well by the simulations. However, the peak angles

themselves remain too low, as they were in simpler cases [4]. There is good qualitative agreement between the

predictions and data. The reason for the ~3 dB over-prediction at φ=180º (at least partly) is suspected to be due to

the problem with the grid in the vicinity of the upper shear layer noted in Figure 2. Too smooth a transition andvortex pairing could lead to increased noise levels; an examination of the spectra (not shown) indicates a false

peak at ~7 kHz due to the vortex pairing, which is not correct.

Sample illustrations of the results of the simulations for the under-expanded jets are given in Figures 5 and 6.

“Numerical schlierens” in Figure 5 reveal clearly a system of shock cells interacting with turbulence. Figure 6demonstrates good spectral predictions of the noise caused by this interaction (broad-band shock-associated noise)for both the round and beveled nozzles. There is very good agreement up to a frequency of ~20 kHz (St=1.57),

which is the upper limit for the grid used. Absolute predictions of the shock-peak locations and levels, without any

empirical adjustments, attest to the validity of the approach and indicate that the right physics is captured in thesimulations.

3 Discussion

This study represents our initial efforts at the prediction of the flow features and noise of beveled nozzles.

The sample results included here demonstrate good agreement between the predictions and measurements for

the effect of the bevel. New issues with grid and numerics are being addressed. Detailed results from

simulations at different jet conditions and predicted spectra will be presented in the full paper. Given thecomplexity of the problem, these preliminary results are encouraging and it is hoped that a viable

computational tool for the reliable assessment and optimization of this noise reduction concept is feasible.

References

[1] Bogey, C., Bailly, C., Investigation of subsonic jet noise using LES: Mach and Reynolds number effects,

AIAA Paper 2004-3023, 2004.

[2] Bodony, D. J., Lele, S. K., Jet noise prediction of cold and hot subsonic jet using large-eddy simulation,

AIAA Paper 2004-3022, 2004.

[3] Paliath, U., Morris, P. J., Prediction of noise form jets with different nozzle geometries, AIAA Paper 2004-3026, 2004.

[4] Shur, M. L., Spalart, P. S., Strelets, M., Noise prediction for increasingly complex jets, Part I: Methods andtests; Part II: Applications, accepted for publication in the International J. of Aeroacoustics, 2005.

[5] Uzun, A., Lyrintzis, A. S., Blaisdell, G. A., Coupling of integral acoustics methods with LES for jet noise

prediction, AIAA Paper 2004-0517, 2004.

[6] Viswanathan, K., Nozzle shaping for reduction of jet noise from single jets, AIAA Paper 2004-2974, 2004.

[7] Viswanathan, K., An elegant concept for reduction of jet noise from turbofan engines, AIAA Paper 2004-2975, 2004.



Figure 1: Snapshots of vorticity magnitude in the Figure 2: Computational grid and vorticity near theXY- and XZ-planes. M=1.0, bevel45. nozzle exit in XY-plane. M=1.0, bevel45.

Figure 3. Snapshots of time-derivative of pressure in Figure 4. Comparisons of polar directivities at

the XY- and XZ-planes. M=1.0, bevel45. various azimuthal angles. M=1.0, bevel45. Data [6].

Figure 5. Snapshots of magnitude of density gradient Figure 6. Comparisons of narrow-band spectra at a

in the XY-planes (“numerical schlierens”) for the polar angle of 50

o

for the round and bevel45 jets atround and bevel45 jets at M=1.56. M=1.56 with data [6].

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