Drag coefficient for two ahmed bodies

nfeiler · August 1, 2011, 06:02

Hallo everybody,

I want to compare drag coefficients for two ahmed bodies. In this case I placed two vehicles (two ahmed bodies) in tandem. To solve this problem I have create a model in Star CCM+ 5.04.006. I have created a plot for the both drag coefficients, to analyze the data of solution. The given solution doesn’t fit. The drag coefficient of rear ahmed body is higher as of the front ahmed body.
Can someone tell me, if this could be right??? And can someone take a look on my model and give me some proposals to improve it. If some interested, I can send my model with details on e-mail.

With kind regards

nfeiler · August 4, 2011, 19:08

Is nobody here, who can help me???

CFDFreak · August 5, 2011, 12:49

That doesnt sound correct. However, it could also be influenced by the distance between the two ahmed bodies and the velocity of the air flowing past.

why not try changing the position of the 2nd body and noting how the drag varies ? It could be possible that the drag on the 2nd body is minimised at a particular distance only ?

http://www.cfd-online.com/Wiki/Ahmed_body
I would also look at the effect of varying the angle of slope phi.

If you can find a way to send over your case file, I cd take a look when i get a chance.

abdul099 · August 7, 2011, 15:52

Is it a 2-dimensional or 3-dimensional case you try to solve? I've heard about a similar case (2D) where the front body was pushed forward by the rear body. Switching to a 3-dimensional domain was giving much more reasonable results...

nfeiler · August 7, 2011, 17:28

It's a 3-dimensional case.
Where did you heard this???

abdul099 · August 8, 2011, 18:16

A friend did a simulation (with CFX) during his studies. First he tried to do a 2D simulation to keep the computational effort low, but he never got reasonable results. Then he switched to 3D and got much better results.

Is your domain big enough for the flow? There has to bee enough space beside (and above) the Ahmed bodies for the air to pass without being much influenced by the boundary. When you want, you can send me your sim-file and I will have a look on it. I will send you the adress by private message...

ping · August 11, 2011, 02:44

I have never done such an analysis, but I can see how the second Cd could be higher (remember we are comparing Cd and not absolute force which should be lower due to slip streaming in the wake). The disturbed air on the second vehicle is quite different and so the flow regime is different. Also the wake flow and mesh refinement is very important in all vehicle drag studies (many car lengths downstream) so this is also a probable cause.

abdul099 · September 5, 2011, 19:31

Even the absolute force on the second body is higher than on the first body. That's due to the wake of the first body which disturbes the flow approaching the second body.

August 1, 2011, 06:02	Drag coefficient for two ahmed bodies	#1
nfeiler New Member Join Date: Jul 2011 Posts: 3 Rep Power: 15	Hallo everybody, I want to compare drag coefficients for two ahmed bodies. In this case I placed two vehicles (two ahmed bodies) in tandem. To solve this problem I have create a model in Star CCM+ 5.04.006. I have created a plot for the both drag coefficients, to analyze the data of solution. The given solution doesn’t fit. The drag coefficient of rear ahmed body is higher as of the front ahmed body. Can someone tell me, if this could be right??? And can someone take a look on my model and give me some proposals to improve it. If some interested, I can send my model with details on e-mail. With kind regards

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August 4, 2011, 19:08		#2
nfeiler New Member Join Date: Jul 2011 Posts: 3 Rep Power: 15	Is nobody here, who can help me???

August 5, 2011, 12:49		#3
CFDFreak New Member Shreyas Ragavan Join Date: Aug 2011 Location: Leeds, United Kingdom Posts: 1 Rep Power: 0	That doesnt sound correct. However, it could also be influenced by the distance between the two ahmed bodies and the velocity of the air flowing past. why not try changing the position of the 2nd body and noting how the drag varies ? It could be possible that the drag on the 2nd body is minimised at a particular distance only ? http://www.cfd-online.com/Wiki/Ahmed_body I would also look at the effect of varying the angle of slope phi. If you can find a way to send over your case file, I cd take a look when i get a chance.

August 7, 2011, 15:52		#4
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 22	Is it a 2-dimensional or 3-dimensional case you try to solve? I've heard about a similar case (2D) where the front body was pushed forward by the rear body. Switching to a 3-dimensional domain was giving much more reasonable results...

August 7, 2011, 17:28		#5
nfeiler New Member Join Date: Jul 2011 Posts: 3 Rep Power: 15	It's a 3-dimensional case. Where did you heard this???

August 8, 2011, 18:16		#6
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 22	A friend did a simulation (with CFX) during his studies. First he tried to do a 2D simulation to keep the computational effort low, but he never got reasonable results. Then he switched to 3D and got much better results. Is your domain big enough for the flow? There has to bee enough space beside (and above) the Ahmed bodies for the air to pass without being much influenced by the boundary. When you want, you can send me your sim-file and I will have a look on it. I will send you the adress by private message...

August 11, 2011, 02:44		#7
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	I have never done such an analysis, but I can see how the second Cd could be higher (remember we are comparing Cd and not absolute force which should be lower due to slip streaming in the wake). The disturbed air on the second vehicle is quite different and so the flow regime is different. Also the wake flow and mesh refinement is very important in all vehicle drag studies (many car lengths downstream) so this is also a probable cause.

September 5, 2011, 19:31		#8
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 22	Even the absolute force on the second body is higher than on the first body. That's due to the wake of the first body which disturbes the flow approaching the second body.