# VELOCITY vs VELOCITY IN STN FRAME vs RELATIVE VELOCITY

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 July 13, 2015, 06:07 VELOCITY vs VELOCITY IN STN FRAME vs RELATIVE VELOCITY #1 New Member   everest Join Date: Mar 2014 Posts: 3 Rep Power: 12 Hi guys, In FLUENT we have option to display VELOCITY MAGNITUDE and RELATIVE VELOCITY MAGNITUDE, while in CFD-POST there is additionally VELOCITY IN STN FRAME. Here's short dscription of my test-case model.I have one moving reference frame in 2D with translational motion along y-direction and I am using MRF appraoch. For one case I assume zero velocity and for the second one some non-zero velocity of the reference frame. There are no interior wall boundaries insisde the moving domain (e.g. rotor walls), so there is just a rectangular domain with prescribed velocity in MRF panel. I would expect that, since there are no walls inside, the resultant velocity (absolute one) should be the same in both cases. This is true in CFD Post when I plot contours for VELOCITY IN STN FRAME and comapre it for the two cases being analysed. I always thought that VELOCITY in FLUENT also relates to absolute velocity viewed from stationary frame, however when the two cases are compared, the resulting velocity contours are different. Can you please explain what is the exact difference between VELOCITY MAGNITUDE (inside fluent) and VELOCITY IN STN RAME (inside cfd-post) ? In Fluent documentation regarding MFR the ABSOLUTE VELOCITY is defined as . Where is the relative velocity and is the translational velocity, note that there is no rotational motion in my case. I accept this definition however I cannot understand how Fluent understands VELOCITY MAGNITUDE in case of of MRF. Thanks in advance for any help ssmacae likes this.

July 13, 2015, 08:35
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everest
Join Date: Mar 2014
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OK! So I was actually wrong about the difference in VELOCITY IN STN FRMAE (cfd-post) and VELOCITY MAGNITUDE (fluent) - both look alike and are different between the two analysed cases.

However there is still a question: why there is a difference between velocities in both cases. See the attached figure which presents y-velocity, As we can see the behavior of the flow is different?

Once again thanks in advance for any help, and sorry for the previous confusion between VELOCITY (fluent) and VELOCITY IN STN FRAME (cfd-post).
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