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Computational fluid dynamics and supersonic jet noise:


      Numerical simulation of fluid flows in industrial and scientific applications often involves large scale computations that can easily overwhelm the most advanced computers to date. This is especially true when the physical problems involve great difference in scales and when time accuracy is important. Acoustic problems in aerodynamic process is one of such class of problems. This include aircraft noise, automobile noise and many other noise problems. Our effort in this area includes both the development of fundamental methods to address the special need and the application of these methods to real world problems.

Development of fundamental computational methods for high accuracy simulation of high speed flow and aerodynamic noise:


      The key to successful high accuracy flow and acoustic simulation is to employ high order numerical methods. They are capable of maintaining low numerical dispersion and dissipation errors without using excessive number of grid points. This drastic departure from traditional CFD methods brings exciting new opportunities as well as difficult challenges. Main investigation topics include high order finite difference methods, dispersion relation preserving method, non-reflecting numerical boundary conditions, spectral element method based on orthogonal polynomials, shock capture schemes for high order methods.

Density field

      Density field of a Mach 1.13 cold jet from a convergent nozzle. It shows the shock cell structure disturbed by the instability waves in the shear layer. This interaction is the source of strong acoustic wave radiation to the outside of the jet flow.

Pressure field

      Pressure field of a Mach 1.2 heated jet from a convergent nozzle. It highlight the interaction between the down stream moving instability wave and the shock cell structure. Note the shock cells are fluctuating in strength without changing their positions.

Time averaged density field

      Time averaged density field of a Mach 1.2 jet from a convergent nozzle.

Applications in supersonic jet noise research:


      Jet noise is one of the most important components of aircraft noise. Others include engine noise and airframe noise. For exhaust jet at supersonic speed a special type of noise called screech tone is particularly harmful. It is caused by a feedback process involving the shock cell structure in the supersonic jet, the instability waves at the boundary of the jet flow and the reflecting surface at the nozzle lips. Under certain conditions the screech tone can be strong enough to cause structure damage to aircrafts. The phenomenon was discovered in the early 1950s. It has been proven challenging to both theoretical work and numerical simulations. Until our investigation no one was able to predict the screech tone intensity successfully. Using specially developed computational aeroacoustic method we are able to simulate screech tone in both the axisymmetric and non-axisymmetric supersonic jet. The intensity and frequency of the screech tone matches the experiment data in a large Mach number range. This is the first fully successful prediction of supersonic jet screech since its discovery in 1950s.

All these works are supported by NASA Glenn Research Center and Air Force Office of Scientific Research.

References:


H. Shen and C.K.W. Tam, "Three-Dimensional Numerical Simulation of the Jet Screech Phenomenon", AIAA Journal, v40, n1,pp.33-41, January 2002.

H. Shen and C.K.W. Tam, "The Effects of Jet Temperature and Nozzle Lip Thickness on Screech Tones", AIAA Journal, v38, n5, pp.762-767, February 2000.

H. Shen and C.K.W. Tam, "Numerical Simulation of the Generation of Axisymmetric Mode Jet Screech Tones", AIAA Journal, v36, n10, pp.1801-1807, October 1998.

H. Shen and C.K.W. Tam, "Numerical Simulation of the Jet Screech Phenomenon by Computational Aeroacoustics Method", Advances in DNS/LES, ed. by C. Liu and Z. Liu, pp. 581-588, 1997.

H. Shen, C.K.W. Tam and G. Raman, "Screech Tones of Supersonic Jets from Bevelled Rectangular Nozzles", AIAA Journal, v35, n7, pp.1119-1125, July 1997.

H. Shen and C.K.W. Tam, "Direct Computation of Nonlinear Acoustic Pulses Using High Order Finite Difference Schemes", AIAA Paper 93-4325, October 1993.

 
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