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-   -   Momentum Source (loss coefficient) and velocity decrease (http://www.cfd-online.com/Forums/main/106538-momentum-source-loss-coefficient-velocity-decrease.html)

 alquimista August 31, 2012 09:26

Momentum Source (loss coefficient) and velocity decrease

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Hello,

I'm trying to simulate the pressure drop of a flow obstacle with a momentum source applying a directional loss model, so the term applied results:

S = −K_loss * (ρ/2) * |U| * U_z

I'm wondering why the velocity decreases when I apply it (see figure), I can't see what's happening theoretically. Even in a real application, the obstacle reduces the flow area and so velocity is higher.

I'm obtaining reasonable results of the pressure drop but the velocity is not what I expect (shown in the figure attached).

Thank you very much for your hints.

P.S: I post it here because I guess it's related with general CFD theory not with the particular code, by the way ANSYS CFX

 Jonas Holdeman August 31, 2012 09:51

I am interested in the idea, but don't understand the model. Could you explain the geometry and context please? Compressible/incompressible? Wouldn't a momentum source (force) be expected to change the velocity or somehow balance dissipative forces?

 alquimista August 31, 2012 10:20

I'm modelling an incompressible flow along a channel so there's some kind of flow straightener inside (I can't provide publicly exact characteristic of the device). I'm trying to model the pressure drop of this device without modelling it with that loss model.

It have sense that applying a negative force in the flow affects the velocity in that way, but maybe you can provide some ideas.

Regards.

 Jonas Holdeman August 31, 2012 18:04

I still don't have enough information about your problem so I will make some assumptions. The width or direction of the channel must be changing and you want to quickly restore some flow profile by inserting this "flow straightner". You want to model the effect of the physical straightner with some equivalent ficticous force or momentum source. Certainly the flow must be constant along the channel (average velocity inversely proportional to area) because of conservation of mass. Upstream and downstream of the obstruction the pressure will be decreasing in the direction of the flow, but there will be an additional decrease around the obstruction. I don't understand what velocity you are plotting in your figure.

With regard to your model, I think the functional form should be divergence-free (solenoidal) as it is a source term added to the divergence-free velocity, as can be seen from a pressure-free formulation of the momentum equation such as the streamfunction-velocity FEM.

Have others used this approach of using an equivalent momentum source? I still have to think about the idea and am a little skeptical from a consistency point of view. But the idea is interesting.

 Rami September 2, 2012 05:07

Your idea is quite common when porous media is simulated. The specific form you use is the Darcy law in z-direction. Some models also add another term (Forschheimer). Without specifying your case, I cannot see what is the expected solution, and what is wrong in the one you received.

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