# LES outflow condition for a subsonic flat plate boundary layer

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 August 24, 2018, 23:00 LES outflow condition for a subsonic flat plate boundary layer #1 New Member   Reggie Join Date: Aug 2018 Posts: 2 Rep Power: 0 Hello all! I'm trying to use a compressible LES code to simulate a subsonic flat plate turbulent boundary layer. For the inflow, RRM(rescaling and recycling method) will be used, but I'm not sure how to give outflow condition. If the computational field is truncated at the end of the flat plate and no extra domain is connected behind, farfield pressure cannot be specified here, right? Some paper used non-reflecting outflow and sponge layer was added, but it seems too complicated for me to understand the procedure(I'm a new learner ) I was wondering if the convective outflow can be applied in compressible flow? Is this outflow condition proper for my problem? Is there any reference for it？ Would you give me some advise? Thanks a lot!

August 25, 2018, 03:33
#2
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,685
Rep Power: 60
Quote:
 Originally Posted by chris x Hello all! I'm trying to use a compressible LES code to simulate a subsonic flat plate turbulent boundary layer. For the inflow, RRM(rescaling and recycling method) will be used, but I'm not sure how to give outflow condition. If the computational field is truncated at the end of the flat plate and no extra domain is connected behind, farfield pressure cannot be specified here, right? Some paper used non-reflecting outflow and sponge layer was added, but it seems too complicated for me to understand the procedure(I'm a new learner ) I was wondering if the convective outflow can be applied in compressible flow? Is this outflow condition proper for my problem? Is there any reference for it？ Would you give me some advise? Thanks a lot!

For compressible flows, you have to follow the characteristic lines, therefore one condition must be fixed at the outlet as the characteristic enters into the domain.

August 25, 2018, 04:19
#3
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Reggie
Join Date: Aug 2018
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Quote:
 Originally Posted by FMDenaro For compressible flows, you have to follow the characteristic lines, therefore one condition must be fixed at the outlet as the characteristic enters into the domain.
Thank you for your reply. As far as I know in most cases, the outflow boundary is set far away from the main region and the pressure at the infinity will be specified. For the flat plate boundary layer however, if the outflow boundary is set right at the end of the plate, how can I specify this one fixed condition?

 August 25, 2018, 05:09 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 5,685 Rep Power: 60 In the BL approximation, dp/dy->0, therefore you can set the external pressure

 Tags boundary condition, boundary layer flow, les