# Rotating Boundary Condition

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 May 17, 2011, 12:00 Rotating Boundary Condition #1 New Member   Join Date: Nov 2009 Posts: 7 Rep Power: 10 Dear Edge Users, Has anyone ever applied a non-rotational symmetric boundary condition in simulations using a rotating reference frame? This means typically setting a translational velocity component as farfield b.c. to the (rotating) domain containing a rotor. (Refer for instance to the case of a helicopter in forward flight). In a rotating frame (i.e. non-zero omega in the input file) the translational velocity would be seen as a "rotating" vector. I'd be interested in knowing whether this can be obtained with the existing bc options or would mean adding new ones. Many thanks in advance and best regards, m righi

May 22, 2011, 14:55
New Freestream BC
#3
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Dear Peter,

Many thanks for your kind reply. I haven't yet written the new BC but I thought I drop a few lines to explain why I believe I need it. And feel free to correct me of course!

The reason is that even if the equations are solved for the absolute velocity (or momentum) we have to consider the grid motion with respect to the surrounding environment. Within the computational domain, the motion is taken into account of course by the fictitious advective fluxes representing rotation. But not on the boundary. In other words, the computational domain turns and for instance after a quarter of a period the it will have rotated of 90 degrees. Since the reference system turns with the domain, the freestream velocity will be seen rotated by -90 degrees.

Find attached a plot of the velocity on the boundary of a rotating empty cylinder after a bit less than half revolution, using the farfield bc and a freestream U (along x). As you can see the farfield bc keeps the farfield velocity oriented like x, while the outgoing velocity has been (correctly) rotated.
Does it make sense?

Best regards,
marcello
Attached Images
 farfieldBC.jpg (39.3 KB, 32 views)

 May 23, 2011, 03:02 #4 New Member   Peter Eliasson Join Date: Mar 2011 Posts: 3 Rep Power: 8 Dear Marcello, it makes sense. Seems to me that only if the free stream is parallel to the axis of rotation the free stream will be constant then? I am still not convince that you need new boundary condition. You should be able to use existing boundary conditions with a varying free stream in time. If I get it right, the free stream can be constant at each time level? Then it should be very easy to generate the file I talked above where you supply the velocity at discrete time levels. Then you could use this variation of velocity in combination with the far field boundary condition. What do you think? Kind regards, Peter

 May 23, 2011, 05:19 Freestream BC #5 New Member   Join Date: Nov 2009 Posts: 7 Rep Power: 10 Dear Peter, Yes of course, to both questions. If freestream velocity is parallel to the axis of rotation the BC is stationary. This is the case of most of the turbomachinery cases. In all other cases, a time-varying farfield velocity vector would also do the trick. I forgot to mention it yesterday. The only issue would be to include a sufficient number of values not to affect accuracy. I will test it asap. Many thanks and best regards, marcello

May 26, 2011, 08:22
Rotating Farfield BC
#6
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Dear Peter and Edge Users,

Specifying farfield velocity as a rotating vector (in the opposite direction with respect to the rotor) by means of an external file consistently represents the combination of translation and rotation.

Here attached the result of the this on the previous example (empty cylinder) after half a revolution. Initial freestream velocity was set in x, i.e. 180 degrees difference with respect to the direction after half a rev, as it should be.

Interpolation of external bc is first order (I understand) in time. This might explain the small, grid dependent oscillations in pressure. The result was obtained with 720 value per revolution.

Best regards,
marcello
Attached Images
 farfieldBC.jpg (92.9 KB, 31 views)

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