Fluent udf--assigning u-velocity to uds

tas38 · August 10, 2009, 15:46

I was working with a Fluent udf that involved assigning the flow primitive, u-velocity, to a uds.

C_UDSI(c,t,0)=C_U(c,t)

However, when I plot the user-defined scalar within Fluent, the value is not close to the theoretical value for u-velocity. Additionally, the plot of of the uds looks does not match the plot of X-velocity.

Has anyone encountered this problem before?

I do not want to continue writing my udf if the assignments to user-defined scalars may not be accurate.

I hooked the udf to a simple test case of couette flow. The visous flow is between a stationary bottom plate and a top plate moving the x-direction at 1 m/s. The distance between the plates is 0.001 meters. Taking a Line Rake of points along the midline of the geometry, X-velocity ranges from 0 at the bottom stationary plate to 1 at the top plate--this is in agreement with theory. User-defined scalar 0, however, ranges from 0.4491 at the bottom plate to 0.5168 at the top plate?

coglione · August 11, 2009, 10:41

Hello Troy

be aware that if you define a UDS Fluent will solve a transport equation for this scalar. Therefore C_UDSI(c,t,0)=C_U(c,t) is only true at the moment this assignment is executed in your udf (which one do you use?) but values will be updated every iteration/timestep performed.

cheers

August 10, 2009, 15:46	Fluent udf--assigning u-velocity to uds	#1
tas38 Senior Member Troy Snyder Join Date: Jul 2009 Location: Akron, OH Posts: 207 Rep Power: 17	I was working with a Fluent udf that involved assigning the flow primitive, u-velocity, to a uds. C_UDSI(c,t,0)=C_U(c,t) However, when I plot the user-defined scalar within Fluent, the value is not close to the theoretical value for u-velocity. Additionally, the plot of of the uds looks does not match the plot of X-velocity. Has anyone encountered this problem before? I do not want to continue writing my udf if the assignments to user-defined scalars may not be accurate. I hooked the udf to a simple test case of couette flow. The visous flow is between a stationary bottom plate and a top plate moving the x-direction at 1 m/s. The distance between the plates is 0.001 meters. Taking a Line Rake of points along the midline of the geometry, X-velocity ranges from 0 at the bottom stationary plate to 1 at the top plate--this is in agreement with theory. User-defined scalar 0, however, ranges from 0.4491 at the bottom plate to 0.5168 at the top plate?

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August 11, 2009, 10:41		#2
coglione Senior Member Max Join Date: Mar 2009 Posts: 133 Rep Power: 16	Hello Troy be aware that if you define a UDS Fluent will solve a transport equation for this scalar. Therefore C_UDSI(c,t,0)=C_U(c,t) is only true at the moment this assignment is executed in your udf (which one do you use?) but values will be updated every iteration/timestep performed. cheers