# Comparison between RANS and LES for fluidic oscillator

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 July 15, 2016, 07:27 calculation of time for CFD simulation #1 Senior Member   raunak jung pandey Join Date: Jun 2016 Posts: 102 Rep Power: 9 I am learning to use CFD using Ansys CFX. I know that The length of time required to complete a CFD simulation depends on the length of the simulation, the number of grid cells (assuming that you are doing a numerical simulation) and the time-step of calculations. The numerical integration scheme, the implementation of the software and the details of the hardware also make a difference. But is there is any way to calculate total time required to complete a simulation? Thank You

 July 15, 2016, 11:33 #3 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,793 Rep Power: 71 Just to say that for laminar steady flow problems, the time-dependent alghoritm has a characteristic steady time of the order of the Reynolds number. More complex is the case of fully unsteady flows where often one runs the code for many time units to accumulate several fields.

 July 19, 2016, 04:41 Comparison between RANS and LES for fluidic oscillator #4 Senior Member   raunak jung pandey Join Date: Jun 2016 Posts: 102 Rep Power: 9 I want to do a comparative study of flow field in fluidic oscillator using both RANS and LES. My computaitonal domain in 10 * 4 * 2.5 cm across. I am working with flow range between 25 m/s to 100 m/s or (0.075 m2/s to 0.3 m2/s) which falls in the Reynolds number range between 10000-60000. I have done URANS simulation using CFX and moving into LES simulations. What are the meshing rules for LES ? What considerations I have to make for such calculation? How to get rough estimation of computational time required between a URANS simulation and LES simulation using CFX ? Hardware I am using is intel i7 @3.40 GHZ with 32 GB RAM. Thank You

July 19, 2016, 05:02
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Filippo Maria Denaro
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Quote:
 Originally Posted by raunakjung I want to do a comparative study of flow field in fluidic oscillator using both RANS and LES. My computaitonal domain in 10 * 4 * 2.5 cm across. I am working with flow range between 25 m/s to 100 m/s or (0.075 m2/s to 0.3 m2/s) which falls in the Reynolds number range between 10000-60000. I have done URANS simulation using CFX and moving into LES simulations. What are the meshing rules for LES ? What considerations I have to make for such calculation. How to get rough estimation of computational time required between a URANS simulation and LES simulation using CFX. Hardware I am using is intel i7 @3.40 GHZ with 32 GB RAM. Thank You

The two formulations are very different, therefore you have to consider that LES requires a very fine grid for inhomogeneous flow directions (e.g. walls where you need 3-4 nodes within y+<=1). You have also to consider that after the LES solutions reached a fully developped flow condition (that is after the numerical transient has ended) you need further computation time to cumulate several fields in time for computing the statistics.

 July 19, 2016, 05:27 Meshing requirements for LES #6 Senior Member   raunak jung pandey Join Date: Jun 2016 Posts: 102 Rep Power: 9 I am using Ansys CFX for LES simulations of internal flow field of fluidic oscillator. I am working with velocity range 25m/s to 100 m/s i.e reynolds no. 10000 to 60000. My computational domain is 25 cm* 15 cm * 25 mm. What would be the meshing requirements for LES simulation ? what are the considerations for LES simulations Thank you

 July 19, 2016, 10:55 #7 Member   Join Date: Jun 2016 Posts: 66 Rep Power: 11 In general, you need to have such a mesh that is able to capture the large scale eddies while only small eddies need to be modeled. The larger Re, the smaller mesh size is required. The rule is that the mesh size should be in the inertial range of turbulence. There are some formulas that allow you to approximate the turbulence length scales based on turbulence quantities. These give you an idea about the order of the turbulence length scales (representative value). What you should do is to test how your results change when you change the mesh size - although this has two catches. One is that LES simulations are computationally expensive and so it is a pain to run several test cases. The second catch is that as you decrease the mesh size, more and more eddies should be resolved and less modeled. So one cannot talk about grid-independency like in case of the RANS simulations. A separate chapter is how to resolve the boundary layer. The turbulence length scales near the walls are extremely small and that's why you need to refine your mesh here. That is usually unfeasible. There are methods that can alleviate this issue, such as WMLES or DES. I personally use LES only for cases where I am not interested in boundary layer else it would become too expensive. LES modeling is an extensive topic. If you want to know more, you should seek for a dedicated literature.

 Tags cfx, les, meshing