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Veebs November 21, 2001 11:34

Local coordinates usage & use of expression editor
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

Robin Steed November 21, 2001 22:39

Re: Local coordinates usage & use of expression ed
Hi Veebs,

Post your CEL expressions for Vy and Vz.


VEEBS November 21, 2001 22:55

Re: Local coordinates usage & use of expression ed
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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