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Re: [ESPResSo-users] Discrepancy in velocity fluid flow between two plat

From: Stefan Kesselheim
Subject: Re: [ESPResSo-users] Discrepancy in velocity fluid flow between two plates
Date: Tue, 18 Mar 2014 16:32:52 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0

Hi Markus, hi Espressis,
I'm not aware of of the OIF tutorial, but you can not expect the results to be equivalent. Simple reason: With the rhomboid you define the velocity everywhere on the inlet. Across the channel cross section, the velocity is constant. In the Poisseuille case, the flux is not constant but parabolic. If you had a long channel, far away from the inlet BC, you would obtain Poisseuille-type behaviour, but towards the in- and outlet the flow field gets closer and closer to the applied BC. You can known the maximum velocity far away from the center by assuming an unknown pressure gradient in the center and setting the total flux through the center identical to the flux over the boundary. This equation you can solve for v_max.
Attached, you'll find the flow field obtained by your simulation. It clearly becomes more Poisseuille-ish at the center.

On 03/07/2014 01:48 PM, Wink, Markus wrote:

Dear Espresso-community,


I have got a question concerning the fluid flow between two plates (again). On the one hand, I have simulated it introducing a force density of 64000N/m^3. This should (in my system) result in a maximum velocity of 0.02m/s. This is what I get. Now I have tried setting up a rhomboid with a certain velocity so that the momentum is transferred to the fluid and the flow profile is achieved (the OIF-tutorial says, that way, the simulation should be faster). Surprisingly, this results in a max. velocity of the flow profile of 0.026 m/s, even though the velocity of the rhomboid is set to 0.02 m/s. Does anyone has a clue where this discrepancy comes from?


Attached you will find the simulation scripts and the relaxed velocity profiles.








Attachment: wink.pdf
Description: Adobe PDF document

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