# moving grid_boundary conditions

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 October 13, 2005, 04:59 moving grid_boundary conditions #1 George Guest   Posts: n/a Hello everybody! I am writing a code for 3D unsteady,compressible, turbulent flow in a cylinder with a moving piston using the pressure correction method. The difference in the formulation of a code with and without the moving piston is in the convention terms.Only to these terms the relative velocity is being used, whereas everywhere else the absolute velocity is being used. My question is the following: the boundary condition for the convention terms on the piston face will be: (DEN*A*(U-Ug))=0.0 or just: U=0.0 ,where U is the absolute velocity and Ug the grid velocity. Thank you for listening my problem.

 October 13, 2005, 06:39 Re: moving grid_boundary conditions #2 Scott Shaw Guest   Posts: n/a Appropriate b.c. to ensure no-slip is U = - Ug where U and Ug are vectors in the inviscid case this would only apply to the component normal to the piston face (no penetration). Scott

 October 13, 2005, 13:16 Re: moving grid_boundary conditions #3 Mani Guest   Posts: n/a It should be U = Ug, i.e. the flow velocity on the piston wall must be identical to the velocity of the wall for a no-slip condition. In other words, the relative velocity U -Ug must be zero. The convection through the boundaries is treated in the same way as convection through interior cells, i.e. using the grid-relative velocity U -Ug. So in this case, the no-slip condition means that the relative velocity is zero.

 October 14, 2005, 03:01 Re: moving grid_boundary conditions #4 Scott Shaw Guest   Posts: n/a Oops. Thats what I get for cutting and pasting. The negative sign needs to be removed. Scott

 October 14, 2005, 10:43 Re: moving grid_boundary conditions *NM* #5 George Guest   Posts: n/a

 October 14, 2005, 10:50 Re: moving grid_boundary conditions #6 George Guest   Posts: n/a Thank you for your reply. This is how I have implement it, but the only problem is that on the piston surface is:U=Ug , but on the cell,which is just to the left (I-1 line) the velocity is 3 times the Ug,which is too much and normally it should be around the same as Ug.And only when I set U=0.0 then the problem is solved.Any other tricks for the moving grid? (except for the GCL)

 October 17, 2005, 06:28 multiphase mixing problem withMRF model #7 Srinivas Guest   Posts: n/a hi all I am Srinivasa T, a postgraduate student from IITMadras, .My project is as follows: A open mixing tank fitted with internal helical coil and a stirror/impeller, with some volume fraction of solid particle of given dia(micro meters). no inlet and outlets to tank. Problem Description: My project is to study the effect of internal helical coils on solid suspension in mixing tanks/sparged vessels. I am using "Multiple refence plane"(MRF) model for impeller and "Eulerian Granular model" as multiphase model and "Standard k-epsolon for dispersed fluid model" for turbulence. Iam using "MixSim 2.o" provided by FLUENT Inc.for simulation. Problem I am facing: I am confused what are all the boundary conditions should i give for this problem when i shift to FLUENT after geometry construction and meshing in MixSim. So i need your help in this regard. Kindly suugest me how can i define boundary conditions for this problem. thanking you, your sincerely, Srinivasa T M-Tech (Chemical Engineering) IIT Madras Chennai.

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