# Specify both velocity AND total pressure

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 May 27, 2010, 11:08 Specify both velocity AND total pressure #1 New Member   Ian Helms Join Date: Apr 2010 Posts: 9 Rep Power: 7 Hi all, Does anyone know how to specify both the inlet velocity and the total pressure at the boundary inlet? None of the options seem possible. I am running a compressible flow simulation so velocity inlet BC is not appropriate. Thanks

 May 27, 2010, 14:43 #2 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 8 I have the same problem. If anyone has an idea, please share ...

June 2, 2010, 05:24
#3
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Max
Join Date: Mar 2009
Posts: 133
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Quote:
 Originally Posted by OxIan Hi all, Does anyone know how to specify both the inlet velocity and the total pressure at the boundary inlet? None of the options seem possible. I am running a compressible flow simulation so velocity inlet BC is not appropriate. Thanks
Defining a total pressure at an inlet implies that you "know" something about the velocity there (see attachment). Thus, if you give correct static pressure, Ma-Number and total-temperature at the inlet, the velocity obtained from the solution should match with reality.

cheers
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 June 2, 2010, 06:21 #4 New Member   Ian Helms Join Date: Apr 2010 Posts: 9 Rep Power: 7 Hi Coglione, Thank you for your reply, I actually realized what my problem was. The original geometry has some streamtube variation in the axial direction, and since I was initially trying to utilize a 2D simulation the absence of the area ratio was essentially creating my problem. Turns out if you properly define your problem you tend to get a better solution . Jpo I'm sot sure what type of problem you are modeling, so I don't know if that information helps you at all, good luck.

 June 2, 2010, 10:01 #5 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 8 I am trying to achieve supersonic flow in a pipe. Because the flow is compressible, I cannot set supersonic velocity at inlet. Why? Because the available boundary conditions for compressible flow (pressure-inlet, mass-flow-inlet) do not accept velocity values. They accept mass flux or pressure values. I tried setting "large" total inlet pressure and "small" (skip the numbers here) static inlet pressure. This produces high velocity at inlet. Still, as soon as calculation starts to converge, the flow chokes and drops down to subsonic. Tried pressure-far-field as inlet. Same story.

 June 2, 2010, 10:10 #6 Member   Join Date: Apr 2009 Posts: 93 Rep Power: 8 coglione, Thank you for suggesting the formula p_total / p_static Same statement is made by the Fluent manual sec. 7.3.1: "If you choose to initialize the solution based on the pressure-inlet conditions, the Supersonic/Initial Gauge Pressure will be used in conjunction with the specified stagnation pressure to compute initial values according to the isentropic relations (for compressible flow) or Bernoulli's equation (for incompressible flow)" It seems that even if the flow is initialized as supesonic at inlet, it still reverts back to subsonic after a number of iterations. This is my problem - how to keep the flow supersonic in a pipe. If anyone has ideas, please share ....

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