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Ann July 7, 2008 07:57

Drag coeffcient
Hi all,

Is there anyone know how to represent drag coefficent for an object (e.g. car) in Phoenics? In output from main menu, if you activate button to calculate drag coefficient, you will get the result after simulation. But if you have a fix drag coef for a car, how can you put it in Phoenics?

Thank you very much in advance for help.


Mike July 7, 2008 08:11

Re: Drag coeffcient
It is not clear as to exactly what flow situation you are trying to simulate with PHOENICS, and the objectives of the simulation. Since you wish to input the drag coefficient for a vehicle, rather than determine it as an outcome of the CFD solution, one might suppose that you wish to model the drag effect of the vehicle as part of say, for example, a CFD simulation of traffic in an urban street canyon or vehicles moving in a tunnel. Please clarify?

Ann July 7, 2008 08:56

Re: Drag coeffcient
Thanks for your response, Mike. I am trying to model a tunnel with cars inside. There are textbooks in automobiles give standard drag coefficient for vehicles (e.f. for a car Cd=0.28), and I would like to represent this coefficent in my model as a fixed value while changing velocity and such. In my case the car is stationary. would it possible to model moving cars in a tunnel using Phoenics? Thanks.

mike July 7, 2008 11:27

Re: Drag coeffcient
The traffic flow through each traffic lane of the tunnel might be represented geometrically by a rectangular BLOCKAGE object of air material, covering the length of the traffic lane with appropriate lateral dimensions to encompass the traffic flow. If the frontal area of this blockage object is A, then the pressure change for the vehicles travelling in a given direction will be given by something like:

0.5*density_air*sum { av_k*Cd_k*(U_vk-U)*abs(U_vk-U)/A)}

where U is the local air velocity, av_k = cross-sectional area of vehicle k,U_vk = velocity of vehicle k, and Cd_k = drag coefficient of vehicle k.

I will leave you to check this expression, and also to work out the details relating traffic flow,numbers and density.

I would imagine that the pressure term can be introduced as a momentum resistance force for the air BLOCKAGE object by using the quadratic option in the object dialogue box. failing that, the INFORM facility can always be used to implement any resistance force for an object.

Ann July 8, 2008 08:50

Re: Drag coeffcient
Hi Mike,

Thanks a lot for giving a hint. I will try to work it out and let you know. What is difference between the frontal area and cross-section area of vehile assuming that it is a rectangular blockage. They are just the same, aren't they?

Thanks. A.

mike July 8, 2008 09:55

Re: Drag coeffcient
I haven't really thought it through, but I was thinking generally in the sense that the traffic flow is likely to contain some vehicles that are taller than others, ie lorries and cars; and so the blockage would have to cover the tallest vehicles, and the frontal area of the cars would differ from those of the lorries.

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