# CFX Compressor

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 May 7, 2020, 11:43 CFX Compressor #1 Member   Join Date: Mar 2020 Posts: 33 Rep Power: 5 Dear all, I simulated a centrifugal compressor and is currently using CFD post to view contours. I have a fluid domain which is only rotating in impeller section while diffuser and inlet are stationary. When I visualize the vector plot of velocity in CFD post, the vectors are rotating in all domains which is weird to me. For boundary conditions, the blades surfaces, hub and shroud are no slip walls (should I use counter rotating wall option for shroud?) Second thing is Absolute pressure, when I plot the contour of absolute pressure I see very large negative values of pressure, which is wrong I guess. When I clip it by range from 0 to negative_max value, I see whole domain showing negative value. Why is that? I have attached pictures. Thank you. Last edited by UsamaQ; May 17, 2020 at 18:10.

 May 7, 2020, 13:45 #2 Senior Member   Join Date: Jun 2009 Posts: 1,688 Rep Power: 29 The last figure does not help to visualize the problem for Absolute Pressure Have you tried clipping from below, i.e. Min = 0 [Pa]. You want to see the blue regions. That is where the negative values are. UsamaQ likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 May 7, 2020, 17:27 #3 Member   Join Date: Mar 2020 Posts: 33 Rep Power: 5 I clipped it below [0 Pa] to lowest value in negative i.e (-3.113e+06 Pa) as shown on the left side.

 May 7, 2020, 18:50 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,363 Rep Power: 139 This FAQ may help with the frame of reference: https://www.cfd-online.com/Wiki/Ansy...f_reference.3F The negative absolute pressure is typical in incompressible simulations as the pressure is unbounded. In reality these regions would cavitate or have some other effect to reduce the size of the negative absolute pressure region. As Opaque says, you should not ignore these regions by clipping the results as it is telling you something - these regions are likely to cavitate. UsamaQ likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 May 7, 2020, 19:01 #5 Member   Join Date: Mar 2020 Posts: 33 Rep Power: 5 Hi Glenn. I checked the FAQ and I set shroud wall to counter rotating w.r.t to relative frame. Now, the velocity on shroud wall is zero w.r.t stationary frame. Now the problem is that If you see the absolute pressure regions, the dark blue region are not present where the negative values lie. I used probe at different points in the light blue region in the contours and values there are positive. When I clip it below 0 Pa to lowest values, you see the whole domain becomes red which is confusing. Shouldn't the clipping only show the regions below 0 Pa. This is confusing and I couldn't locate the dark blue regions or negative pressure regions.

 May 7, 2020, 19:10 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,363 Rep Power: 139 Do an isosurface of absolute pressure, set to zero pressure. That will show you the negative region more clearly. It is likely to be only a very small region so you will probably have to zoom in lots to find it. UsamaQ likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 May 7, 2020, 19:24 #7 Senior Member   Join Date: Jun 2009 Posts: 1,688 Rep Power: 29 For curiosity, where did I miss the fluid is incompressible? and when did we start calling a machine handling an incompressible fluid a compressor? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 Tags cfd, cfd post, cfx, contour plot, turbomachinery

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