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stuart23 October 8, 2012 08:24

Rotating Reference Fram Inlet Definition
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I am simulating a "curved" windtunnel to model the flow over an object experiencing a constant rotational speed about an axis far from the object. (See attached pic)

Air should enter from the surface on the left, flow round the domain and exit from the bottom. all other walls except the outlet and inlet are surfaces of revolution about the rotational axis (in the middle). I want the freestream velocity to be 17ms-1 on the centreline of the tunnel, and have worked out the rotational speed to be ~2.5 rads-1.

I am confused about what to do with the inlet boundary condition. I have already defined the rotational frame motion of the cell zone. To define the inlet boundary condition, do I need to create a profile? Or can I do it using cylindrical components. Under the cylindrical components, I am prompted for a tangential velocity as well as an angular velocity. I am not sure if I am to define the tangential velocity as zero, and just use the angular velocity?

Also, Is the axis that the inlet boundary condition coordinate system references the same as the centre of rotation of the cell zone? I have also seen the Local Coordinate System option, would this be more appropriate?

The velocity formulation is another aspect I am finding hard to understand. My velocity formulation is Absolute, is the inlet boundary condition Reference Frame Absolute or Relative to the Adjacent Cell Zone?

Thankyou in advance to anyone willing to explain these nuances to me, I very much appreciate it.


mememomo October 29, 2012 14:23

I've had success with something similar by using the following parameters:

Under Cell Zone Conditions edit the Interior Zone and select Frame Motion. Specify your Rotation-axis Origin (looks to be the center of your geometry), Rotation-Axis Direction (of the component itself, not the fluid - i.e. Eulerian vs. Lagrangian). and the Rotational Velocity (which you've already calculated).

For the perimeter of your flow I was told using Pressure-Outlet is acceptable. If you have upper and lower walls of the tunnel domain those may be symmetry or walls. Also, I've used hybrid initialization with success.

Note the results within Fluent will appear different from, for example, CFD-Post. Fluent results, if you've selected Absolute Velocity Formulation, will appear as such, and you'll only see local velocity deviations around your part geometry. CFD-Post defaults to showing the Lagrangian reference frame and so you'll see constant velocity bands representing your higher flow speeds as a function of increasing radius.

Hope this helps.

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