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Vortex shedding in a two-dimensional diffuser: theory and simulation of separation control by periodic mass injection

Suzuki, Takao and Colonius, Tim and Pirozzoli, Sergio (2004) Vortex shedding in a two-dimensional diffuser: theory and simulation of separation control by periodic mass injection. Journal of Fluid Mechanics, 520 . pp. 187-213. ISSN 0022-1120. http://resolver.caltech.edu/CaltechAUTHORS:SUZjfm04

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Abstract

We develop a reduced-order model for large-scale unsteadiness (vortex shedding) in a two-dimensional diffuser and use the model to show how periodic mass injection near the separation point reduces stagnation pressure loss. The model estimates the characteristic frequency of vortex shedding and stagnation pressure loss by accounting for the accumulated circulation due to the vorticity flux into the separated region. The stagnation pressure loss consists of two parts: a steady part associated with the time-averaged static pressure distribution on the wall, and an unsteady part caused by vortex shedding. To validate the model, we perform numerical simulations of compressible unsteady laminar diffuser flows in two dimensions. The model and simulation show good agreement as we vary the Mach number and the area ratio of the diffuser. To investigate the effects of periodic mass injection near the separation point, we also perform simulations over a range of the injection frequencies. Periodic mass injection causes vortices to be pinched off with a smaller size as observed in experiments. Consequently, their convective velocity is increased, absorption of circulation from the wall is enhanced, and the reattached point is shifted upstream. Thus, in accordance with the model, the stagnation pressure loss, particularly the unsteady part, is substantially reduced even though the separation point is nearly unchanged. This study helps explain experimental results of separation control using unsteady mass injection in diffusers and on airfoils.


Item Type:Article
Additional Information:© 2004 Cambridge University Press. Reprintedd with permission. Received April 7 2003; Revised August 4 2004. Published online by Cambridge University Press 29 Nov 2004. The authors would like to thank Dr Douglas G. MacMartin for his helpful insights into many aspects of this work. We also thank Mr Jimmy Fung for his help with code development as well as Drs A. P. Dowling, T. P. Hynes and H. Nagib for fruitful discussions. We gratefully acknowledge our collaboration with Dr James D. Paduano under the support of the DARPA (Defense Advanced Research Projects Agency) program (the contract number F49620-00-C-0035).
Subject Keywords:BOUNDARY-CONDITIONS; FLOWS; UNSTEADY; JETS; GAS
Record Number:CaltechAUTHORS:SUZjfm04
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:SUZjfm04
Alternative URL:http://dx.doi.org/10.1017/S0022112004001405
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2945
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:08 May 2006
Last Modified:26 Dec 2012 08:51

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