[rakadit]

Dear Friends!

I am modelling gravity flow of water and air using ANSYS FLUENT and USING EULERIAN MULTIPHASE MODEL.The goemetry of the channel involves sudden slope changes at 3 points. At certian points, where volume fraction of water is very low in the order of 0.2 or so, values of parmeters like dynamic pressure etc. is found very high. basic formula 0.5*rho*v2 is not followed.

can anybody please explain the logic behind this type of results.

with thanks and regards,

Rakesh

[vinerm]

Since dynamic pressure is a derived field and there is no conservation equation solved for it, hence, its value cannot be independent of velocity field. So, if dynamic pressure is wrong, so should be velocity field.

[rakadit]

Thanks so much for your reply!

Logically you are right.It should have happened like that. This mismatch between pressure and velocity field is there only in those regions where volume fraction of secondary fluid is very low in the order of 0.02 to 0.5.When volume frction is near to 1 of secondary fluid, this problem is not there.
kindly have a relook on the issue again.

with regards,

Rakesh

[vinerm]

Probably, you are comparing wrong parameters. For VOF and mixture models, there is only one velocity field. This also means that there is only one density field as well. And that density field is defined as . If you do not take this density into account then dynamic pressure calculation would not match with the expected values.

[rakadit]

Dear Mr. Vinerm,

Thanks again for your reply.

For your kind info, I am using Eulerian multiphase model,not mixture or pure VOF model. Can there be any effect due to sudden changes in slope of the channel in which I am modeling gravity flow.

Rakesh

[vinerm]

Changes in the slope will affect the velocity-field, but it will not cause dynamic pressure to be different from what it should be. Check for one thing - define dynamic pressure using true and superficial velocities as two difference custom field functions. It is possible that the dynamic pressure is being defined using one of these while you are comparing against other. I would expect it to be based on true velocity but it is better to check.

[rakadit]

Thanks for the reply

Iam using ANSYS FLUENT COMMERCIAL CFD software. I am not using any custom defined functions in the simulation. I have obserevd that near the region of obstacle, fluent is calculating dynamic pressure as rho*v2 in place of 0.5*rho*v2. please comment and suggest.

with regards,

Rakesh

[vinerm]

I am aware that you are using Fluent. Custom Field Functions are simple expressions that are defined in Fluent using primitive or derived variables. Look for Custom under User-Defined on top ribbon. I hope you know what are true and superficial velocities. You can use both and define two custom field functions representing dynamic pressure. In one, use superficial velocity and for the other one, use true velocity. Then compare these two with the dynamic pressure predicted by Fluent. It should match one of these.