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Drag Coefficient on flat plate normal to flow

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Old   October 24, 2015, 01:46
Default Drag Coefficient on flat plate normal to flow
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Bin
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Hi all,

I have simulated a 2D(3D as well) flat plate placed normally to the incoming flow. The Drag coefficient for 2D(and 3D) is well documented, which Cd is ~1.98 for 2D flat plate and ~1.3 for 3D flat plate.

However, the drag coefficient obtained from my data is way much greater than the established value (I get Cd>5 for 2D case). I use the same method that is used in the "vortexshedding" tutorial but I added k-w(SST) turbulence model.

Since, the drag is based on the pressure differences between the upstream and downstream region, the whole perimeter of the flat plate (the front, rear and 2 sides surfaces) is selected as the input parts.

Have I done anything wrong, especially on the "parts" selection? Should I select the front surface only instead of all surfaces? I got a close result by selecting only the front surface (Cd=2 for 2D case), but I'm not sure whether this is a correct approach.

I sincerely appreciate any comment and advice, thank you in advance.
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Old   October 24, 2015, 12:49
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Drag is not easy to get correct for highly separated flows, I suspect that your downstream refinement is too coarse. If you are doing 2D, I'm not sure how the drag coefficient would change since the experimental values are for 3D, but you should be certain to enter the same reference values.

Can you post a picture of your mesh/geometry? You should ensure that the area ratio of your virtual tunnel is good too.
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Old   October 25, 2015, 07:55
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Thank for the reply.

I was doing a preliminary simulation for this case, so the mesh is not in the 'finest' state. Currently, I am using polyhedral mesh in this case and the surface size I input during meshing is appr. 7.5% of the flat plate's width. I used this value when I simulated the circular cylinder case, so initially I think this mesh will be fine for flat plate case as well.

I will refine the downstream mesh as suggested and update you on the result by tmr.

Btw, there are 2 different drag coeff values of flat plate. If you search "drag coefficient flat plate", I believe you can find many lovely tables summarizing the drag coeff for different geometries. But for flat plate there are 2 values associated to different conditions, which are 2D or 3D flat plate. The difference between the 2D and 3D (for 3D case, I mean simulating a infinitely long flat plate, 2D flow field, with 3-dimensional consideration) drag coefficient is due to the fact that an additional dimension is considered in 3D case. So, I think if I have simulated a 2D case I should get the 2D drag value; and getting a 3D drag value if I simulate a 3D case. Correct me if I'm wrong

As for the area ratio, since I assigned symmetry plane boundaries for all boundaries except inlet, outlet and flat plate's surface(this is non-slip wall), is it still important to consider the area ratio?
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Old   October 25, 2015, 17:06
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Even if the outer walls are stationary, for most bodies if they are very close to the body of interest that can influence the wake region severely. You just have to ensure the wake flow is not being influenced by the pressures from the boundaries.
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Old   October 25, 2015, 23:02
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I see. I have tried refining the mesh near the 3D flat plate. The drag coefficient is still large. I have attached the mesh I used in my case.
Attached Images
File Type: jpg mesh2.JPG (124.9 KB, 75 views)
File Type: jpg mesh1.JPG (62.9 KB, 63 views)
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Old   October 27, 2015, 00:53
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Hi Bin,

1. Blockage ratio using into both the simulation i.e. 2D and 3D ?
2. Can you please post some contour picture i.e. velocity, pressure for both the cases.
3. Is Y+ appropriate in your simulation for selected SST model ?
4. Did you try with hexahedral\trimmed mesh (uniform cell size, if domain is not big enough with respect to computational time\resource) ?

Devesh
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Old   May 18, 2016, 12:12
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Hello! I know the thread is kinda old but I am interested in this simulation. I am not using star CCM but the flat plate case is interesting.

I think most of the drag is pressure drag in that case and I wonder if y+ value is really important to get good results on the boundary layer. I am new to CFD so maybe I am wrong. Maybe the wake is the most important thing to get proper pressure gradient on both sides of the plate? I am also wondering if the turbulence model is important here since the transition to turbulent flow is obvious.

I would like to have your opinion on what is important on such a case to get good drag prediction? Any advice or guidelines are welcome. Y+, turbulence model, wall function, wake mesh, boundary conditions and wind tunnel dimensions...


Thanks in advance
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Old   February 19, 2017, 02:56
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Hi Bin,

Just went through your texts. Am also having a similar issue. Did u break it?

Thank you
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Old   February 19, 2017, 07:20
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Try vortex shedding in transient state and for Lift try in sready state

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Old   February 19, 2017, 23:45
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Thanks Mehak for your response.

The transient condition has already been taken care of. Am still not able to get the Cd below 3.5.

May i have other suggestions please?

Thank you
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Old   April 25, 2017, 12:00
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Hello everyone

It seems the downstream domain length needs to be fairly large enough while simulating flow over a flat plate kept normal to the flow direction.

Thanks for your suggestions.

Has anybody come across the co-efficient of drag and lift values for a convex curved plate(similar to the alphabet C)? Preferably experimental analyses.

Thanks in advance
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Old   April 25, 2017, 12:10
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Do you want to simulate the flat plate or c shape profile

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Old   April 25, 2017, 12:10
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Both need a large fluid domain to get the result

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Old   April 25, 2017, 12:11
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For your airfoil can you upload the mesh file

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Old   April 25, 2017, 12:33
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Thanks for your response Mehak.

I need to simulate flow over a 'C' shaped plate. Any experimental Cd values to refer to in the range 1000< Re< 10000?
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Old   April 25, 2017, 12:43
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Can you mail me your profile

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Old   April 25, 2017, 12:44
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And boundary condition

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Old   June 21, 2017, 18:46
Default Cd for 2D and 3D Flat Plate
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Bin, I am doing research on Cds for various geometries and came across your thread where you referenced:

Cd is ~1.98 for 2D flat plate and ~1.3 for 3D flat plate.

What data is this referring to? Do you happen to have a link to the paper or handbook?
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Old   June 25, 2017, 11:10
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Please search for " Nominally 2D flow about a normal flat plate" by Lisoski...
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Old   June 26, 2017, 12:03
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Found it thank you for your help, my research is in 3D flow so I am interested in finding the differences between Cd values between 2D and 3D. There can be a very large range.
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