# Truck cfd analysis

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August 1, 2011, 02:28
#21
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sheth roh
Join Date: Jul 2011
Posts: 56
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Quote:
 Originally Posted by Andrew.Coughtrie These links explain it. drag coefficient How to use ANSYS CFX to get the drag coefficient?
Thanks for all your help. I will be updating my results here. Rep added to u.

Do one need a licence copy of the ansys cfx to publish the paper of analysis?

 August 1, 2011, 03:43 #22 New Member   Andrew Coughtrie Join Date: May 2011 Posts: 29 Rep Power: 7 You always need either a Research License or a Commercial license if you're going to be doing anything that will be published. An academic teaching license should only be used for learning the software.

 August 3, 2011, 11:38 #23 Member   sheth roh Join Date: Jul 2011 Posts: 56 Rep Power: 7 Ok. So I have done one experiment but it has about 25% error than the data i have to validate. I have attached pic of mesh and analysis. I have question that how do I refine mesh? As you can see in image there is no vortex seen behind the truck so I think I need to do refinement near that region. Last edited by sheth; February 24, 2012 at 12:14.

 August 3, 2011, 11:45 #24 New Member   Andrew Coughtrie Join Date: May 2011 Posts: 29 Rep Power: 7 Can you post a pic of your mesh as well? Also what setting are you using in CFX, what turbulence model, boundary conditions etc?

 August 3, 2011, 11:53 #25 Member   sheth roh Join Date: Jul 2011 Posts: 56 Rep Power: 7 Boundary conditions At Inlet: Velo x dir = 23.55 m/s Velo y & z dir = 0 m/s Turbulance: Intensity and length scale ; fractional int 0.05 m & eddy length scale 0.1 m Temp : Static temp. (288 K) At outlet: Static pr. = Relative pr. : 0 pa At the walls of domain (Including ground): Free slip wall At the surface of truck & support: No slip wall First two image show the meshing over the enclosure. Second shows meshing over truck body (Enclosure suppressed) Last edited by sheth; February 24, 2012 at 12:14.

 August 3, 2011, 12:08 #26 New Member   Andrew Coughtrie Join Date: May 2011 Posts: 29 Rep Power: 7 I sugest you try to produce a structured mesh with more refinement behind the truck where the features you're interested in are. you'll also need to do a mesh independence study. You should look up wake turbulence modelling and find out which turbulence models are most popular, you'll probably need to do a comparison between a few and your validation data.

August 3, 2011, 12:31
#27
Senior Member

Martin Hegedus
Join Date: Feb 2011
Posts: 474
Rep Power: 11
Quote:
 Originally Posted by sheth Ok. So I have done one experiment but it has about 25% error than the data i have to validate. I have attached pic of mesh and analysis. I have question that how do I refine mesh? As you can see in image there is no vortex seen behind the truck so I think I need to do refinement near that region.
But there must be something there, regardless of the mesh sizing. It can not be a vacuum and I doubt the flow would wrap around the corner at the back. Did you place stream lines at the back?

Or do you mean there is no unsteady vortex shedding?

August 3, 2011, 12:36
#28
Member

sheth roh
Join Date: Jul 2011
Posts: 56
Rep Power: 7
Quote:
 Originally Posted by Andrew.Coughtrie I sugest you try to produce a structured mesh with more refinement behind the truck where the features you're interested in are. you'll also need to do a mesh independence study. You should look up wake turbulence modelling and find out which turbulence models are most popular, you'll probably need to do a comparison between a few and your validation data.
So how exactly do I do structured meshing.

Here is how I do analysis.

1. In ansys workbench select CFX in analysis system table..
2. Import my truck design. & Made enclosure around it.
3. Did auto meshing.. (I think this is where I have to do something to make results accurate)

So in ansys meshing how do I create a proper structured grid? All I do is just do auto meshing.

4. Then i give input values.

I want to know which turbulence model did u chose?

5. Post processing.

Quote:
 Originally Posted by Martin Hegedus But there must be something there, regardless of the mesh sizing. It can not be a vacuum and I doubt the flow would wrap around the corner at the back. Did you place stream lines at the back? Or do you mean there is no unsteady vortex shedding?
I wanted to mean that flow doesnt curl around the rear bottom part.

August 3, 2011, 13:42
#29
Senior Member

Martin Hegedus
Join Date: Feb 2011
Posts: 474
Rep Power: 11
Quote:
 Originally Posted by sheth I wanted to mean that flow doesnt curl around the rear bottom part.
Not sure that I understand. Since there are sharp edges around the parameter of the base, the flow will separate there. Depending on the turbulence model, you may get steady results. And this seems to be what you are getting. If the results are steady a ring vortex will exists at the back end. Are you seeing the ring vortex?

Or, are you expecting unsteady vortex shedding? Of course in real life that is what you will get. But, a turbulence model might not give you that. For example, in general, an ogive-cylinder body will give you a steady base flow using SA or SST turbulence model.

 August 4, 2011, 10:15 #30 Member   sheth roh Join Date: Jul 2011 Posts: 56 Rep Power: 7 Can anyone please tell me how do I do structured meshing?

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