# Cd vs Re cube (FLUENT)

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 March 31, 2016, 08:29 Cd vs Re cube (FLUENT) #1 New Member   Join Date: Mar 2016 Posts: 10 Rep Power: 6 Hi, I want to plot the Cd vs Re relation for a cube. The result has to be something like this: (for a cube, psi=0.806) I'am interested in the region 10^4

 March 31, 2016, 15:15 #2 Senior Member   Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0 If the correlation predicts 1.05, I don't think 0.95 is a very poor result. Anyway, your timestep size of 1s is very large for LES, too large I'd say (of course, it depends on the physical size of your domain, if your cube is the size of a city, the story is different than for a cube the size of a lego block) So, how did you estimate the timescale that you need to resolve? For LES, you should take a timescale that is in line with the scale of motions that you resolve.

 March 31, 2016, 15:45 #3 New Member   Join Date: Mar 2016 Posts: 10 Rep Power: 6 uh... the 1s timestep was set by default and i wanted to see how that would work out... I don't know how to setup a suitable timestep size. It depends on the expected period, but i dont understand how the Cd would have a period. I just ran a simulation with a timestep of 0.0001 s, and the Cd converges to 1.6068... while i expected it to be around 0.3

 March 31, 2016, 15:54 #4 Senior Member   Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0 in your setup it should not be periodic I think (of course, it may be unsteady in a turbulent flow, but that's why you take a long-term average). You can base the timetep size on the courant number, or on a physical parameter. For LES, the Taylor microscale may be an instructive timescale to take. In any case, for LES you can stay above the Kolmogorov scale.

 March 31, 2016, 16:10 #5 New Member   Join Date: Mar 2016 Posts: 10 Rep Power: 6 Thanks for your help, What would you suggest for a cube of 1 m^3, air at rho=1.225, mu=1.78*10^-5 and V=14.6 m/s? This gives Re=1*10^6. All the scales you named are very new for me (Taylor, Kolmogorov)+ etc). Time to do some more homework.

 March 31, 2016, 16:23 #6 Senior Member   Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0 Depends on your grid size. For LES, the finer the grid, the more scales are resolved and the fewer modeled - smaller resolved lengthscales, means you need smaller resolved timescales. For LES, there is not that much gained when going below the Taylor scale however - since you essentially resolve all energy containing motions (just not the dissipative ones). But before moving to LES, I'd check how steady state RANS simulations perform (k-e, SST), at least they should be close. And they are much less time consuming. (actually, to start, maybe an axisymmetric sphere is a good test. These can be solved quickly, and benchmarks are easily found)

 April 1, 2016, 02:45 #7 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,027 Rep Power: 48 Does the cube have a faster transition than a sphere? Is it maybe just transitioning at a much lower Re than expected for a sphere? I don't know. For example, putting like rods in a channel can suppress the critical Re from 2300 all the way down to 500. The LES results are very questionable but it would take a very lengthy discussion to pick it apart. In short, are you sure you did your LES correctly? 4 iterations per time-step is unlikely to be converged, especially with a timestep of 1s, which is too large to resolve anything. This type of simulation would give a result akin to an unsteady laminar simulation. For LES: You should expect a time-dependent drag coefficient in a turbulent flow, whether or not there is any periodic vortex shedding. Of course the time-averaged value is what you should compare. But I would play with simpler turbulence models before doing any LES. It's too much effort, with too many issues, for what you are trying to do right now.

 April 1, 2016, 03:57 #8 New Member   Join Date: Mar 2016 Posts: 10 Rep Power: 6 Thank you for your response, I'am a real beginner in turbulence simulations. Because the lack of a Cd-drop in the more simple turbulence models (k-e, k-w, SST, reynolds 7 eq) in the domain 10^3

April 1, 2016, 04:27
#9
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muhamed
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Quote:
 Originally Posted by jppilot Hi, I want to plot the Cd vs Re relation for a cube. The result has to be something like this: (for a cube, psi=0.806) I'am interested in the region 10^4
Hi
Could you please tell me how to get a drag coefficient for Laminar flows (Re<2000)? I did a simulation to get the same curve (Re versus CD) on a bridge pier into an open channel and I didn't use the turbulent models in ANSYS FLUENT I just kept it Laminar and I used a flow velocity of 0.0001 m/sec for water but the drag force I got is high compared to the velocity value. I tried several things but I got a reverse flow. Could you help me with this problem?

April 7, 2016, 03:51
#11
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muhamed
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I just want to ask if it is possible to use the turbulence models provided by ANSYS FLUENT (rather than the laminar model which is exist also in FLUENT) to do a simulation on a water channel flow with "Laminar flow" with a Reynolds number less than 100?

 April 11, 2016, 13:17 #12 Senior Member   Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0 well technically yes, but why would you want to? the results will be less accurate, as turbulence adds additional dissipation which is not physical. So I would not recommend doing so.

April 12, 2016, 05:24
#13
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muhamed
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 Originally Posted by CeesH well technically yes, but why would you want to? the results will be less accurate, as turbulence adds additional dissipation which is not physical. So I would not recommend doing so.
Well, if I want to study the flow around a cylinder with different Reynolds number, I will use the laminar model for Re< 2000 and turbulence models for higher Reynolds values. So, is it logical to compare between the results taken from different models, i.e. some results will be taken from the laminar model and the other will be taken from the turbulence models? Regards.

April 12, 2016, 09:25
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Lucky Tran
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Quote:
 Originally Posted by Mohammad80 Well, if I want to study the flow around a cylinder with different Reynolds number, I will use the laminar model for Re< 2000 and turbulence models for higher Reynolds values. So, is it logical to compare between the results taken from different models, i.e. some results will be taken from the laminar model and the other will be taken from the turbulence models? Regards.
It is absolutely logical if those models correspond to what the flow experiences in reality. i.e. at low Re you have laminar flow and high Re you have turbulent flow. Are you comparing models for the sake of comparing models or are you comparing models for the sake of comparing what occurs in reality? Heck, why don't you model the fluid as a solid or a gel? Why model something that does not occur?

There are situations when you might want to to use a turbulent model for a laminar flow and a laminar model for a turbulent flow, but there would be a specific need and specific reason for doing so.

April 13, 2016, 05:47
#15
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muhamed
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 Originally Posted by LuckyTran It is absolutely logical if those models correspond to what the flow experiences in reality. i.e. at low Re you have laminar flow and high Re you have turbulent flow. Are you comparing models for the sake of comparing models or are you comparing models for the sake of comparing what occurs in reality? Heck, why don't you model the fluid as a solid or a gel? Why model something that does not occur? There are situations when you might want to to use a turbulent model for a laminar flow and a laminar model for a turbulent flow, but there would be a specific need and specific reason for doing so.
Thank you LuckyTran for your reply. I just want to get the curve of CD and Re relationship which is shown above. I don't care about comparing between the models themselves. As you and other guys said that I have to choose Laminar model for the Re values less than 2000 and above that limit I can choose any turbulence model according to the situation of the flow, if it has large eddies or not, and if I used a turbulence model for low Re values less than 2000 that will not be logical . Am I right?