# Porous jump and velocity

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 May 13, 2003, 10:20 Porous jump and velocity #1 Christian Guest   Posts: n/a When using a porous jump we add a pressure sink. But what about the velocity after the jump? How is it affected regarding magnitude and direction? I cannot find documentation in the manual. Cheers Christian

 May 13, 2003, 11:12 Re: Porous jump and velocity #2 Christian Guest   Posts: n/a Can I assume that the magnitude and direction of the fluid is the same on the adjacent porous jump cells ?

 May 13, 2003, 13:25 Re: Porous jump and velocity #3 Evan Rosenbaum Guest   Posts: n/a The porous jump is just a momentum sink, the velocity direction and magnitude are determined as if there is no solid obstruction in the flow.

 May 14, 2003, 02:45 Re: Porous jump and velocity #4 Christian Guest   Posts: n/a Thanks. But "momentum" sink. Momentum = m*u (mass*velocity) right. If any of these two parameters are reduced, the velocity will eventually decrease. Or am I missing the point. Cheers

 May 16, 2003, 15:15 Re: Porous jump and velocity #5 Evan Rosenbaum Guest   Posts: n/a You're missing the point. The velocity that is used does not account for any area contraction in the porous jump. Let's say you have a perforated plate with 40% open area. You would expect the velocity in this section to increase by 1/0.4 = 2.5 times. It won't. The velocity in a porous jump is calculated using 100% open area (FLUENT calls it the superficial velocity), you have to account for the velocity effects when defining the porous jump parameters.

 May 20, 2003, 16:13 Re: Porous jump and velocity #6 frank Guest   Posts: n/a We also made calculations with porous jump instead of the perforated plate. If the fluid flow is about perpendicular to the plate, the results seem to be ok. If the flow comes with a smaller angle to the plate, the results were bad. What is your experience with the porous jump model ?

 May 21, 2003, 14:24 Re: Porous jump and velocity #7 Evan Rosenbaum Guest   Posts: n/a You have to recognize the impact that the angle of entry has an a perforated plate. Imagine you are holding the perf plate in front of you, perpendicular to your axis of vision. If the plate has round holes, they look round. Now tilt the top edge either toward or away from you. The holes look smaller as the projection ovalizes. You have to account for this when defining the porous jump properties.

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