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Old   July 4, 2005, 13:32
Default turbulence
  #1
wilson
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hi guys

in the wind tunnel analysis i have one doubt i have a car in a box just like wind tunnel. i am giving some turbulence values in the form of turbulence and intensity scale at inlet. now the question is suppose i am reducing the scale ratio of my car model keeping the tunnel dimensions same, then should i change the value of turbulence quantities. for instance, initially i keep car in tunnel and give turbulence quantity say x, now my tunnel size is same but car shape is reduced to 1/3 scale should i change the turbulence quantity to x/3
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Old   July 4, 2005, 19:09
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  #2
Glenn Horrocks
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Hi,

Good question Wilson. Have a look in a textbook under non-dimensionalisation theory. I think you will find it is not possible to scale everything, usually you can scale a model keeping one non-dimensional number the same (eg Reynolds Number), but the other non-dimensional numbers have to change. This means you have to choose what is the most important parameter to keep constant during scaling.

But I am not sure that is what you are asking. Can you explain what you are trying to do a bit more?

Regards, Glenn Horrocks
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Old   July 5, 2005, 14:40
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wilson
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hi Glenn

i am simply focusing on turbulence at the inlet. if the dimensions of wind tunnel remain the same, should i decrease the turbulence with the same ratio as that of vehicle.

i feel that the turbulence parameters remain the same irrespective of the vehicle dimensions.
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Old   July 5, 2005, 18:55
Default Re: turbulence
  #4
Glenn Horrocks
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Hi,

So you have wind tunnel results using a scale model and you have done your CFX model on this scale model; and now you wish to model the full size thing with CFX?

In that case it appears you are not using non-dimensional scaling anyway as the Reynolds number of the full sized thing will be different to the model.

Shouldn't you use the turbulence levels typical of the real situation, rather than the wind tunnel? For instance external aerodynamics of vehicles need quite low levels of incoming turbulence in general.

Regards, Glenn Horrocks
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Old   July 7, 2005, 10:21
Default Re: turbulence
  #5
jorge sancho
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Look at the turbulence quantity dimensions, to have them adimensional. Ex, if you use lenght scale as a turbulence paramenter, you should scale it with your caracteristic dimension (lenght of the car). But others are non dimensional, like viscosity ratio.
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Old   August 9, 2005, 10:57
Default Re: turbulence
  #6
Andrea
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