# Local coordinates usage & use of expression editor

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 November 21, 2001, 11:34 Local coordinates usage & use of expression editor #1 Veebs Guest   Posts: n/a Hi CFX 5.4 users, I am solving a cavity problem (axisymmetic, so having one cell in theta direction) with CFX 5.4. Top bounding surface and bottom bounding surface are rotating at different rpm. So I need to model with stationary frame. Do define Wall BC, I followed two methods and having problem with both. Please advice 1. representing the tangential velocity in terms of Vy & Vz (my axis of rotation is X). While doing that, I calculated angle using atan function using expression editor. Now while postprocess I find some radial velocity at the walls. This looks like due to the angle it finds is more like cell centered rather than for the particular node. Even if I view with hybrid results, wall velocity components are not correct. Please let me know your thoughts on this ? 2. When I use a local cylindrical coordinate defined and represent velocity as Ur, U_theta, & U axial, the resulting velocity is simply Vx = 0, Vy = 0 & Vz = U_theta we have specified. Where we expect some component in the Vy also. Is there any limitations on using these options ? Thanks for your time Veebs

 November 21, 2001, 22:39 Re: Local coordinates usage & use of expression ed #2 Robin Steed Guest   Posts: n/a Hi Veebs, Post your CEL expressions for Vy and Vz. Robin

 November 21, 2001, 22:55 Re: Local coordinates usage & use of expression ed #3 VEEBS Guest   Posts: n/a Robin, Following are the equations used. bomega = 2*3.1415*10584 /60 [s] tomega = 2*3.1415*27634 /60 [s] angle = atan(z/y) radii = sqrt(z*z + y*y) vbtheta = bomega*radii vzb = vbtheta*cos(angle) vyb = vbtheta*sin(abs(angle)) vttheta = tomega*radii vzt = vttheta*cos(angle) vyt = vttheta*sin(abs(angle)) As my domain is in 2nd quad, I need to keep both components positive to get clockwise rotation.

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