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Old   May 28, 2005, 05:45
Default Treatment of inflow boundary
William M.
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Hi all

I am doing a gas computation. The inlet boundary is specified as pressure. I wonder how to extrapolate the velocity at this boundary.

Some papers extrapolate the boundary conditions based on the characteristic theory. (ie, Journal of Fluids and Structures (1997), 11, 947-963).

What is done in that paper is integrating the characteristic equation along the line dx/dt=u-c which is one of the wave speed with negative value.

According to Thompson (JCP 68,1-24), it seems to be an outgoing wave and I am confused between the outgoing wave and inlet flow.

Could anyone explain about it? I really appreciate it.


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Old   May 31, 2005, 04:53
Default Re: Treatment of inflow boundary
Lionel Larcheveque
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Basically, the transformation of the 3D compressible Euler equations using characteristics theory results in 5 local ODE similar to wave equations. Three of them propagate at the local speed u (two vortical waves and one entropy wave), the fourth and fifth ones being pressure waves of u+c and u-c velocity respectively (c is the local speed of sound).

Now, considering an inflow condition, u is simply the velocity in the incoming normal direction. Therefore if the flow is subsonic, u-c is negative. From a physical point of view, this simply states that a pressure wave can travel upstream in a subsonic flow. From a numerical point of view, this means that piece of information coming from inside the computationnal domain has to go out of it, even at an "inflow" BC.

Hope this helps,

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Old   June 1, 2005, 02:46
Default Re: Treatment of inflow boundary
William M.
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Thanks, Lionel.
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