# Result of Backward Facing step

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 October 11, 2023, 09:29 Result of Backward Facing step #1 New Member   seohee Jang Join Date: Oct 2022 Posts: 8 Rep Power: 3 Dear community I am doing the simulation of Backward Facing step with different Reynolds number. However, no matter what I change the value of Re number by changing the velocity (same step size, only different is inlet velocity), the simulation result are same except the magnitude of velocity. For example, suppose inlet U is 0.5, the max U becomes 0.5, but if inlet U is 5, max U becomes 5 like this I thought the reattachment point should be different depending on the Re number I use the k-epsilon RAS model with steady state condition test with Re 5600 to 56000, Do you have any idea what is the reason for it?? I mean are there any limitation point for Reynolds number that fixed the length of Reattachment point? Best Seohee

 October 11, 2023, 09:48 #2 Senior Member   Join Date: Apr 2020 Location: UK Posts: 640 Rep Power: 13 It's probably because you are using the standard (high Re) k-epsilon model with the standard (hi-Re) wall function ... these models are not Re-number dependent, so it makes sense that your solution does not change. pickfirst likes this.

 October 11, 2023, 10:37 #3 New Member   seohee Jang Join Date: Oct 2022 Posts: 8 Rep Power: 3 Dear Tobermory Thank you for fast replying but I don't know why the model is independent for Re number since it has the Re term in the k and epsilon equation in terms of kinematic viscosity. Best Seohee

 October 11, 2023, 11:21 #4 Senior Member   Join Date: Apr 2020 Location: UK Posts: 640 Rep Power: 13 The only terms with viscosity in the k and eps equations are the diffusion terms, and for these the turbulent diffusion is probably orders of magnitude higher than the molecular diffusion - try calculating it and comparing. This means that the model is behaving in a high-Re asymptotic limit, as is its purpose/design. If you want to explore Re number effects, then you need to start using models that attempt to represent the low Re effects ... in both the closure model AND the wall function. Just using standard k-eps and standard wall function won't work. Good luck. PS - it also worth checking that there actually IS some Re# variation for your test case at the Re numbers you are simulating ... is there experimental data you can validate against? pickfirst likes this.

 October 11, 2023, 13:13 #5 New Member   seohee Jang Join Date: Oct 2022 Posts: 8 Rep Power: 3 Thank you Tobermory I understand what you said the things that I want to check can be captured when the Re number effect is included in the model. And unfortunately I don't have experimental data that can validate but some of the literature which include different Re number for Backward facing step provides different position of reattachment effect I will try the other model to check it Thank you for giving me an advice for this problem Best Seohee Tobermory likes this.

October 11, 2023, 13:35
#6
Senior Member

Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 1,267
Rep Power: 33
Can you post contour plot of velocity. Your behaviour of velocity seems very strange.

Quote:
 Originally Posted by pickfirst Dear community I am doing the simulation of Backward Facing step with different Reynolds number. However, no matter what I change the value of Re number by changing the velocity (same step size, only different is inlet velocity), the simulation result are same except the magnitude of velocity. For example, suppose inlet U is 0.5, the max U becomes 0.5, but if inlet U is 5, max U becomes 5 like this I thought the reattachment point should be different depending on the Re number I use the k-epsilon RAS model with steady state condition test with Re 5600 to 56000, Do you have any idea what is the reason for it?? I mean are there any limitation point for Reynolds number that fixed the length of Reattachment point? Best Seohee

 October 11, 2023, 14:21 #7 New Member   seohee Jang Join Date: Oct 2022 Posts: 8 Rep Power: 3 Hi arjun when I do the plot in case of Re 1000 and compare with Re 10000 or the other cases it shows that there are some difference for recirculation point or reattachment point However, once the Re becomes more than some limitation such as 5600, the further Re case shows similar flow dynamics I think that is because the turbulent diffusion is more dominant when Re becomes increase so that it cannot capture low Re phenomena Best Seohee

October 12, 2023, 02:16
#8
Senior Member

Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 1,267
Rep Power: 33
The reason why i wanted to see the contour plot is that the behaviour is similar to full slip flow. In this case you max velocity would likely to be what your inlet velocity is.

This will be the case when shear stress at wall is almost zero or zero.

so now that i can not see the plot, i leave you to judge this yourself.

Quote:
 Originally Posted by pickfirst Hi arjun when I do the plot in case of Re 1000 and compare with Re 10000 or the other cases it shows that there are some difference for recirculation point or reattachment point However, once the Re becomes more than some limitation such as 5600, the further Re case shows similar flow dynamics I think that is because the turbulent diffusion is more dominant when Re becomes increase so that it cannot capture low Re phenomena Best Seohee

 Tags backward-facing step, k-epsilon model