Pipe flow - orifice plates
 

Hi FLOW 3d users,

I recently started to approach to CFD. I would like to simulate the flow in a pipe with SINGLE-HOLE and MULTI-HOLE orifice plates, starting from experimental data (quite detailed and complete both for velocities and pressure measurements).
I am able to simulate a portion of the pipe working with polar coordinates and considering Z as the flow direction. I set pressure B.C. both at the inlet and the outlet of the pipe and an initial uniform velocity, but i tried also with velocity at the inlet and pressure at the outlet, with similar results. I activated a k-e turbulence model and a cavitation potential model.
The model converges and pressure and velocity fields are estimated well with respect to lab data.

The main problem is that I get the desired result modifying the parameter "ROUGHNESS" in surface properties. I found in the User's manual and in the forum that this parameter is proportional to the size of the roughness elements and has the dimensions of a length. Well, i am working with SI units and I have to use very high values of roughness in the model (0.08-0.1) to have a realistic simulation. Moreover, the model appears to work also because the distributed losses are much lower than the concentrated ones (due to the plate), while I would expect an opposite behavior in case of prevailing distributed losses.

Can anyone, please, explain this behavior? It is however correct?

Does anyone study the same problems?

Thank you for your attention,
Alessandro

Hi Alessandro,

I have a few questions to clarify.

Do you include both sides of the plate in the domain or is it adjacent to a mesh boundary? How big is the pipe?

Do I understand correctly that without using roughness, the flow losses are too small?

Hi Michael,

thank you for your reply.

The pipe I am studying has a diameter D=200 mm. I am simulating complexively a length of 15D (7D upstream and 8D downstream...I know that it is not the best choice but I have limitations in the number of cells I can use, and I however verified the development of a correct velocity profile in the upstream and downstream sections). I am using only one mesh block, in cylindrical coordinates, that covers from 30 to 90 degrees on Y (according to the specific symmetry that I can find for different multihole orifices) from the beginning to the end of the pipe, so the plate is inside the domain. The rendering option showed that the software recognized correctly solid fractions and holes of the plate.

According to what I found in my simulations I think that you're right, so that reducing the roughness too much (also setting a zero value) I have small pressure losses, if compared to the concentrated ones I expect. On the contrary, setting a high value of roughness, I can get really good results.

Please, ask me for any other doubt,

Regards

Alessandro