# about first layer thickness

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 January 24, 2007, 06:47 about first layer thickness #1 Eric Guest   Posts: n/a Dear Friends, I am using CFX-mesh to make the mesh. Could you tell me how to decide the first layer thickness? The fluid in my simulation is compressible. Thank you in advance. Eric

 January 24, 2007, 10:55 Re: about first layer thickness #2 Alejandro Guest   Posts: n/a Hi Eric, What geometry are you simulating? A way to compute the thickness of the first elements near a wall is to estimate the height of the boundary layer. It depends on the geometry, but for example for airfoils, a good rule of thumb is to approximate the boundarylayer height, as the height of the BL for a flat plate, if your Re number is high enough so that you are in a turbulent regime, then the thickness can be estimated as d=0.14*Re^(-1/7) So you capture the effects of the viscous sublayer, you should fit at least 15-20 elements within the BL, so a a good guess for the thickness of the first layers would be d/20 (20 extrusion layers, with a small expansion factor). To decide if your choice is correct, you can also check the y+ values and compare with the ranges advised for the different turbulence models. I hope it helps. Best regards, AG. soheil_r7 likes this.

 January 24, 2007, 11:35 Re: about first layer thickness #3 John Guest   Posts: n/a Hi, When you define the inflation layers in cfx-mesh, you can input the Y+ value and the Reynolds number of flow. The CFX-mesgh will automatically generate a first layer which can meet your requirements. Regards, Jone

 January 24, 2007, 22:58 Re: about first layer thickness #4 Eric Guest   Posts: n/a Thank you very much. I will try it.

 February 4, 2007, 10:29 Re: about first layer thickness #5 Eric Guest   Posts: n/a Dear Mr.Alejandro Thank you for your answering on the www.cfd-online.com about the thickness of Bounday Layer. Could you tell which book included this formula and the theory? Because I need reference to finish my paper. Thank you in advance. Best Regards Eric My e-mail: qiuyifa@hotmail.com

 February 5, 2007, 06:08 Re: about first layer thickness #6 Alejandro Guest   Posts: n/a Hello Eric, you can find reference to that formula (although the factor listed there is 0.16 instead of 0.14) in "Fluid Mechanics" White, F. The chapter of interest is "Flow Past Immersed Bodies" Best Regards, Alejandro.

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