[johny]

Hello Everybody,
I've relatively a strange question. I've a CFD model with a geometric similarity of that of a cuboid with two plates inside to create turbulence. Flow is entering from bottom right and bottom left(normal to the face, ie parallel to the X axis), which proceeds along the height of the structure through plates with holes to exit through outlets on the top (again in the left and right). Essentially the geometry is simple. I've a cold flow with mass flow rate boundary in inlet and pressure outlet at outlet in STARCCM. My flow us converged without any oscillations in the steady case. A few faces are reported with reversed flow though. But my inlet mass flow rate(kg/s) is different from that if outlet significantly. Could anyone provide some insight into this problem. It would be greatly appreciated. And the mesh us polyhedral.
Cheers,
Johny

[fluid23]

I could be wrong, but I don't think you are supposed to create a report for mass flow right on a BC. Try creating derived parts (constrained plane or something like that) that are offset a bit and see if that helps.

[ggulgulia]

Hi

Can you please be specific which one of the boundaries is not showing the expected mass flow rate? Is it the outlet boundary or both?

In any case you have to consider a tolerance of around 0.5% b/w inlet and outlet mass flow rates

[ggulgulia]

also try checking invalid cells in your mesh continua.. if you have one or two invalid cells, try including cell quality remediation model in your physics continua.

If you have too many invalid cells, maybe you need to check with your remeshed surface for proximity faces and other errors

[johny]

Thanks for the replies. Only the mass flow rate at the outlet is wrong. I changed the solver fron coupled to segregated and set it run for a while and then the results looks good. But I'm still wondering what was happening with the coupled algorithm.

[ggulgulia]

I don't think the coupled algorithm should create this issue.. I've used it several times with quite consistent results. Poor cell quality could be one issue, you should look into. Did you try the suggestion I gave earlier?

[johny]

Thanks Ggulgulia...Could you please tell how I could check for the cell quality. The only thing I could find was mesh diagnostics. Does it say much about the poor cells?

[ggulgulia]

Hi

Use the toolbar and go to "Repair Remeshed Surface" (refer image below) , then select regions one by one (don't select multiple regions for repair) and then click "Manage Theresholds"

Note, for a good quality surface mesh, all thresholds should be Zero

[johny]

Hi Gulgulia,
Thanks a lot. I'll check it out.
Cheers,

[johny]

Hi Gulgulia,
Including the cell quality remediation model did the trick. And there were a negligibly few cells of poor quality too. Thanks to all.
Cheers,

[johny]

Hi All,
I don't know whether this is the right forum for my question but I give it a try since it has to do with the same simulation I've discussed above. And its relatively a naive question and regarding thermal conductivity and heat transfer. My understanding is material with high thermal conductivity will get heated up easily and therefore have have high temperature fields compared to one with low thermal conductivity. But I find results exactly opposite when simulated under same boundary conditions where the flow of the hot air(which makes the material heated up) is exactly the same. One side of the material(its like a solid slab) is in contact with the fluid domain (hot air) and other side is not. All the solid boundaries are adiabatic.*It would be great if anyone could provide some insight :-)
Cheers,

[kguntur]

I believe that thermal conductivty would decide the uniformity of temperature within a solid. Higher thermal conductivity -> more uniform temperature.

But the actual temperature will depend on the heat capacity of the material.

Seniors, please correct me if i am wrong.

[johny]

Thanks for the reply. You're right as per my simulation. But why is it so?
Thermal conductivity is the heat required to raise the temperature through one degree for a unit area. Sp. heat also seems to be the same. Can you explain how exactly they differ and when to consider what?
Cheers,