• Dr. Mohammed Afsar
  • Imperial College London
  • Holden Auditorium (Room 112)
  • 4:00 p.m.
  • Faculty Host: Dr. Cornel Sultan

I show how a fundamental approach to aero-­‐acoustic modeling of flow generated noise and jet surface interaction noise can provide insight into the physics of sound generation as well as providing a viable prediction tool for industrial design. The basic approach involves using Goldstein’s (2003) generalized acoustic analogy equations, which is simply an appropriate re-arrangement of the Navier Stokes equations such that the acoustic field is expressed as integral of the fluctuating Reynolds stress tensor and an adjoint Green’s function propagator. The flow Reynolds number is assumed to be asymptotically large and the acoustic Mach number, unless otherwise stated, is subsonic.

The talk is organized as a historical overview of the results we have obtained in the last decade. Two essential ingredients go into jet noise and jet surface interaction noise models, that is: i). structural modeling of turbulence using its symmetries; and ii). analysis of wave propagation using asymptotic and numerical calculations. For example, Afsar (2010) showed how Professor Geoffrey Lilley’s classical ideas of shear noise and self noise can be obtained much more consistently as the leading order terms in the low frequency asymptotic expansion of propagator tensor in the acoustic spectrum formula. This idea was later adapted to consider more a realistic turbulence representation (Afsar et al. 2010, Karabasov, Afsar et al. 2010 & Afsar 2012) and expanded to take into account non-­‐parallel mean flow effects (Goldstein, Sescu & Afsar 2012). When considering the effect of heating, we showed that the enthalpy flux-­‐momentum flux coupling term introduces cancellation in the acoustic spectrum at low frequencies due to the odd power in the inverse Doppler factor (Afsar, Goldstein & Fagan 2011). Hence this mechanism could serve as a purely fluid mechanical means of jet noise control.

My work on jet surface interaction has focused on modeling low frequency trailing edge noise. Experimental data indicating low frequency trailing edge noise could be much as 10 dB greater than the jet noise itself, motivated this study.

The model we developed (Goldstein, Afsar & Leib 2013a & b; Afsar & Leib 2015) showed, among other things, that exact integral solutions of the Rapid Distortion Theory (RDT) equations can be expressed in terms of two integral curves of the Euler equations (i.e. functions of arbitrary convected scalar quantities) and the Rayleigh equation Green’s function. The complexity in this problem, however, lies in the fact that only certain spatial locations of the space-time Fourier transform of the transverse momentum field can be specified as an upstream input to the model. Moreover, a physically realizable upstream turbulence spectrum must include a finite de-correlation region in the transverse velocity correlation function. Our latest results show that an increase in “size” of this de-correlation region increases the low frequency algebraic decay of the acoustic spectrum with angular frequency. We conclude by discussing the implication this result has for trailing edge noise control.


M.Z.  Afsar  began  his  undergraduate  studies  at  the  University  of  Bristol,  UK,  in Aeronautical Engineering in October 1999. His research career began in the summer of 2002 when, still an undergraduate, he received a Research Assistantship from the Department of Applied Physics at Yale University working under Professor Marshall Long. He worked there as an experimentalist doing laser diagnostics for a laminar flame. He graduated from Bristol in summer of 2003 with a very high First Class Honors degree for which he received the Royal Aeronautical Society Award.

His interest in Aero-acoustics was sparked by lectures of the late Professor Martin Lowson at Bristol University. In January 2004, he began his doctoral work at the University of Cambridge in jet noise modeling under the supervision of Professor Dame Ann Dowling. He completed his Ph.D. in 2008 and in the autumn of that year he received the David Crighton Fellowship from the DAMTP, Cambridge, which he used to work with Dr. Marvin E. Goldstein at the NASA Glenn Research Center. In the spring of 2009 he was a Teaching Assistant/Grader at Robinson College Cambridge. He returned to NASA Glenn in the summer of 2009 as a Short-­‐term Research Scholar funded by Stanford University’s Center for Turbulence Research. Later that year, he was a visiting faculty member at the Department of Mathematics at Kashmir University in Srinagar, India. Between February 2010 and May 2013 he was a NASA Post-­‐doctoral Program (NPP) Fellow working with Drs. M.E. Goldstein, S.J. Leib and J. E. Bridges. In July 2013, he took up the Chapman Fellowship and Laminar Flow Control Research Associateship at the Department of Mathematics at Imperial College London.