# Reynolds number in wind tunne-sports car modelling

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 March 4, 2012, 03:04 Reynolds number in wind tunne-sports car modelling #1 Senior Member   Ke Wu Join Date: Jan 2012 Posts: 179 Rep Power: 7 I have set different wind speed Finally I wanna get a graph which drag coefficient against Reynolds number However, how do I know the value of Reynolds number in this case? Thanks

 March 5, 2012, 00:51 #2 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 494 Rep Power: 12 Here is my two cents. There isn't an easy answer. A race car can have a lot of different physics going on, and the Reynolds is a metric for those physics. For example, you could have a Reynolds number for the entire length of the car representing skin friction (drag buckets) and separation bubbles on the car. Or, you could have a Reynolds number for a wing which could be a representation of leading and trailing edge separation. Or you could have a Reynolds number for an exterior rear/side view mirror representing the type of separation from it. So, first, you need to identify the physics, and then the Reynolds number(s) can be determined. In other words, if you have a strong variation of Cd with velocity, you must identify where it is coming from. Unfortunately, understanding the physics can be very challenging for something with a lot of interactions, interferences, and complex geometries, such as some race cars. shreyasr likes this.

 March 7, 2012, 06:42 #3 Member   Join Date: Mar 2011 Posts: 50 Rep Power: 8 You can get a rough approximation by using Re = rho*V*D/mu where D is the height of the car

March 7, 2012, 07:24
#4
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Ke Wu
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Quote:
 Originally Posted by cdf_user You can get a rough approximation by using Re = rho*V*D/mu where D is the height of the car

Any reason to use height of car?

March 7, 2012, 07:27
#5
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Ke Wu
Join Date: Jan 2012
Posts: 179
Rep Power: 7
Quote:
 Originally Posted by Martin Hegedus Here is my two cents. There isn't an easy answer. A race car can have a lot of different physics going on, and the Reynolds is a metric for those physics. For example, you could have a Reynolds number for the entire length of the car representing skin friction (drag buckets) and separation bubbles on the car. Or, you could have a Reynolds number for a wing which could be a representation of leading and trailing edge separation. Or you could have a Reynolds number for an exterior rear/side view mirror representing the type of separation from it. So, first, you need to identify the physics, and then the Reynolds number(s) can be determined. In other words, if you have a strong variation of Cd with velocity, you must identify where it is coming from. Unfortunately, understanding the physics can be very challenging for something with a lot of interactions, interferences, and complex geometries, such as some race cars.

I want to have drag coefficient and lift coefficient against Reynolds number. So what characteristic length I should use?
Thanks

March 8, 2012, 07:50
#6
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Shreyas Ragavan
Join Date: Feb 2012
Location: India
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Quote:
 Originally Posted by itsme_kit I want to have drag coefficient and lift coefficient against Reynolds number. So what characteristic length I should use? Thanks
HI,
I think , since you are looking at the drag and lift coefficient values of the entire car, the Length of the car may be more appropriate.

The remaining kinds of Re numbers seem focussed on the effects of a particular Part of the car, whereas your focus is probably on the cumulative effect.

Fluid properties change with height.
The length is also, usually parallel to the velocity vector's direction and Drag is the resistance to the flow, in a simplistic way. Using the length seems to make more sense.

Besides, the frontal projected area comes into the Cd equation, and that means the main effect of the width and height are being accounted for.

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