## Fluids

#### Continuity convergence problem

• toraman
Subscriber

I have a ribbed duct 80*80*200 mm that is heated from sublayer (2000 W/m2 constant heat flux). Reynolds is between 10000 – 30000. I applied an inlet volume 1000 mm to obtain hydrodynamically fully developed flow additionally, also an outlet region 200 mm.

In meshing, I applied first layer thickness for inflation ( y+ = 1 and for different Reynolds number ) but continuity fixed on 10^-3 no reduce. For other residuals no problem. If I use y+ > 11, 30, etc. , the total inflation layer thickness is over my shortest rib so the rib keeps in inflation layer.

Skewness 0,21 and Orthogonal Quality 0,78

I’ve also tried many times in k-w SST, k-e RNG, etc. also but no change.

Does anybody suggest?

Thanks.

• Raef.Kobeissi
Subscriber
Try to run it for longer, have you used 2nd order Upwind? Also try to have a longer domain to avoid having flow reversal or use outflow as an outlet bc.
• toraman
Subscriber

I used the second upwind. I applied an inlet region approximately 10*Hydrolic Diameter and although no see the convergence of continuity able to observe the velocity profile well.

• Raef.Kobeissi
Subscriber
I would run it a little bit more and I would try to decrease the momentum under-relaxation factor to 0.5. Have you used the SIMPLE algorithm? Also try using Outflow rather than pressure outlet.
• toraman
Subscriber

Thanks for your reply. I also tried URF. Residuals reduced for a short time but then increased again. However, I think that the problem is estimating the first layer thickness. I saw your calculator and tried but the results are totally different from mines that ? used below the which is for duct flow.

• Raef.Kobeissi
Subscriber
I don’t think that’s the problem. You already mentioned that you y+ is 1so your thickness is alright. Try to initialise from inlet make sure your turbulence intensity is correct and insert correct hydraulic diameter at inlet. Use outflow..
• toraman
Subscriber

For 20 000 Reynolds  I=0,16 *Re^(-1/= %4  that is suitable for medium-turbulence ? What can I compare with the Turbulence Intensity more?

and the meaning of to insert correct hydraulic diameter at the inlet is v=(Re*dyn.visc.)/(Dh*density)?

• toraman
Subscriber

Sorry I checked intensity on fluent boundary conditions it is  %5 as default  I think that's ok.