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trying to simulate flow over a 3D Cylinder: getting High Cd and Cl Values 

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August 20, 2015, 10:12 
trying to simulate flow over a 3D Cylinder: getting High Cd and Cl Values

#1 
New Member
Jai
Join Date: Aug 2015
Posts: 5
Rep Power: 3 
Hi Guys
Im trying to simulate flow over a 3D Cylinder (pictures of the mesh generated using ICEM CFD are attached aswell as results). Im using the standard Komega model for Re=3900 however when I go ahead and import the mesh into Fluent and check the quality of the mesh I get the following message: The mesh contains very high aspect ratio hexahedral or polyhedral cells. The default algorithm used to compute the wall distance required by the turbulence models might produce wrong results in these cells. Please inspect the wall distance by displaying the contours of the 'Cell Wall Distance' at the boundaries. If you observe any irregularities we recommend the use of an alternative algorithm to correct the wall distance. Please select /solve/initialize/repairwalldistance using the text user interface to switch to the alternative algorithm. I discard the message and start the simulation, the results i get for cd mean value is greatly over predicted at 3.72 when it should be around 1.32. The mesh has 441680 nodes and an aspect ratio of 404! I would be grateful for any help that anyone can offer im really confused at the moment and urgently need to attain some good results. How can I can reduce the aspect ratio ? What else can I do to obtain a lower cd and cl value? 

August 20, 2015, 16:28 

#2 
Senior Member
Join Date: Mar 2015
Posts: 793
Rep Power: 9 
First of all, address the error message Fluent is providing you. If you can't fix your mesh; perhaps use a simpler Ogrid type mesh. This mesh type performs well for both URANS and LES turbulence models for cylinder in crossflow (2D and 3D). Furthermore, start with 2D simulations to tune the settings (there's plenty of results from simulations and experiments in the literature at Re = 3900).


August 29, 2015, 12:25 

#3 
New Member
Jai
Join Date: Aug 2015
Posts: 5
Rep Power: 3 
Thanks for your reply e
Ive spent the last week generating simpler meshes as shown attached but Im still getting overpredicted cd values. For 2D cases all meshes attain reasonable cd values (0.95 to 1.4). Cl values are also reasonable for both 3D and 2D URANS cases. I used the standard komega model (altered y+ to be around 30 close to the cylinder wall) as well as the transitional kklomega model (y+ around 1 clsoe to the cylinder). However im not sure what Im doing wrong so I'll quickly run through my case: Simulation type 3D Unsteady Double Precision Pressure based and implicit Re=3900 Diameter of cylinder= 1 u=1 mu=0.00025641 Density=1 Model  komega standard (other models were used but still yielded same results) Turbulence Intensity  5% Turbulent viscosity ratio = 10 Solving for 1000 timesteps, 0.233 time step size and 20 iterations per time step. Any advice would be greatly appreciated. Thanks (Screenshots are attached) 

August 30, 2015, 05:52 

#5 
New Member
Jai
Join Date: Aug 2015
Posts: 5
Rep Power: 3 
Ok so I made a silly mistake and left the area as 1 when simulating for a 3D case, now changing to its correct value of 11.5 m^2 the mean drag coefficient is around 0.36 now.
For the other reference values: Density = 1 Enthalpy = 0 Length = 1 Pressure = 0 Temperature = 288.16 Velocity=1 Viscosity=0.00025641 Ratio of specific heats = 1.4 

August 30, 2015, 11:02 

#6  
Super Moderator

Quote:
Take a look at this two part tutorial https://www.youtube.com/watch?v=anTkWfMyEPM 

August 30, 2015, 11:05 

#7 
Super Moderator


August 30, 2015, 12:59 

#8 
New Member
Jai
Join Date: Aug 2015
Posts: 5
Rep Power: 3 
Thanks for the link Far
Unfortunately no its not the correct value for Re=3900 URANS it should be around 0.98. I calculated the area of the cylinder using (2*pi*r*h + 2*pi*r^2). Where r is 0.5 and h is 3.14. Any ideas on what I could be doing wrong? 

August 30, 2015, 13:48 

#9 
Senior Member
anonymous
Join Date: Aug 2014
Posts: 198
Rep Power: 4 
Why 2*pi*r^2? In my opinion that area should not be taken into account


August 30, 2015, 23:48 

#10 
Super Moderator

It should be the plane area normal to flow direction. Since this is cylinder, so no more thoughts. Just use diameter * height


August 31, 2015, 07:24 

#11 
New Member
Jai
Join Date: Aug 2015
Posts: 5
Rep Power: 3 
Yep thank you that worked.


Tags 
aspectratio, cylinder, icem 
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