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pressure variation with distance

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Old   February 6, 2012, 09:08
Default pressure variation with distance
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I was wondering if someone could help me in interpreting my results:

I'm running a Fluent simulation of essentially a "blower" (velocity inlet) that has a inlet profile condition (so inlet velocity varies with location on the inlet - basically I can make a "contour" plot of inlet velocity). This air will be blown directly on the top face of a box with the inlet on the domain "ceiling". I'm going to change the box height while keeping the surface area of the top of the box constant to study the force and pressure changes on the box top.

So I ran these simulations. All the residuals converged as well as the force, and static pressure monitors on the top of the box, and the results for each individual case seem plausible. But when I compare results, I'm a little confused.

When I look at the "lift force" for the top face of the box, it nearly remains constant as I change the box height in increments of 6 inches. Even with a change from 18" to 36", the lift force changes by maybe 3 N.

The static pressure contours appear as you would imagine they would: high pressure near the center of the surface, with decreasing pressure as you move outward. The weird thing though is that if you "autorange" the static pressure contour, and record the max presssure it comes up with, the numbers for the max pressure are roughly constant. For some cases, the max pressure actually increases with an increase in distance from the v. inlet.

The only thing I can think of for the lift monitor, is that as you move away from the inlet, the air spreads out, so even if the velocity is slower by the time it gets there, it hits over a larger area and causes more lift (smaller pressure but over a larger area)? But this still doesn't explain the max static pressure phenomenon.

Anyone have any input what might be going on?

Thanks!
-Matt
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