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Doug February 18, 1999 11:29

computing fluctuating pressures
Hello. We are currently looking at trying to determine the Overall Sound Pressure levels experienced in a ram/scramjet engines and we need a way to calculate the rms fluctuating pressure intensity. We have some experimental correlations, but it would be useful to be able to also compute rms pressure values from CFD solutions we are doing.

Since we don't have the time or resources to do unsteady calculations (like LES), we would like to be able to extract rms pressure from the steady turbulent solutions we plan to do.

Does anyone have any ideas that could help that you have used or read about? Thanks in advance for your input.

John C. Chien February 22, 1999 12:59

Re: computing fluctuating pressures
How does a ram/scramjet engine produce the Overall Sound Pressure levels? Where does it produce the Sound? How do you compute the CFD ram/scramjet engine solutions? Is it unsteady supersonic flows? How do you know that the sound you measured can be related to the steady turbulent flow solution? Is the sound generated from the supersonic combustor or from the mixing layer? I don't have a solution to your problem, but I do like to know more about how you compute the flow field.

Doug February 25, 1999 11:13

Re: computing fluctuating pressures
The sound within a ram/scramjet engine can have several sources - turbulent boundary layer, shock interactions, heat release, etc. The OASPL's can be generated anywhere within the engine (inlet, combustor, or nozzle), but are the worst in the combustor. The flowfields in these engines are, to use a term from one of my old text books, "steady in the mean". That is, the unsteadiness in the flowfield is from the turbulent fluctuations.

The CFD solutions for these cases are/will be generated with compressible flow solvers such as GASP or WIND including the effects of finite-rate chemistry and two-equation turbulence models (k-epsilon or k-omega type). Since the mean flows are steady, it is an assumption that the pressure fluctuations ( which is what the sound levels are anyway) will be tied to the velocity fluctuations.

Hope this provides further insight and maybe gives someone ideas that might be useful

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