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June 27, 2010, 13:17 
Drag force on scaled models.

#1 
New Member
Arun
Join Date: Jun 2010
Posts: 10
Rep Power: 8 
Hi,
I have this strange question in my mind. many of you would say "its silly". But i really need know the answer for this. In wind tunnels, it has been accepted a standard practice to use scaled models (like 1:5). My question is "wont the scaled model shows reduced drag force?". I mean, considering the test is carried out for a constant velocity, air density is also constant, drag coefficient and area are only remaining. drag coefficient is constant for a given vehicle and its size. so, if we scale the model 5 times (in case of 1:5 scaled model usage), the drag should reduce since the drag force is directly proportional to frontal area and all other quantities are constant (drag force= 0.5 * drag coefficient * density * frontal area * velocity squared). can some one please suggest me to read something which shows how to compensate for this? Many thanks, Arun 

June 30, 2010, 07:19 

#2 
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L J
Join Date: Jun 2010
Location: Singapore
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HI, ARUN,
you cannot fix the velocity, since the velocity needs to be scaled (scale^0.5)accordingly. 

June 30, 2010, 10:26 
Hi

#3 
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khurram
Join Date: Jan 2010
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Two thing important are geometric similarity and Reynould number.
More over when testing The coefficents like Cd are more important. 

July 1, 2010, 07:52 
hi,

#4 
New Member
Arun
Join Date: Jun 2010
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thanks people, that was quite helpful. but still, i have these things to say:
1. coming to geometric similarity, when we do a scaled model testing, it is an exact replica of the original full sized car, but smaller in size. hence, its geometrical equivalence is satisfied. 2. and about reynold's number, if we carry out the test for a same inlet velocity (say 80kmph), reynold's number also should remain constant. in this situation, how do we compensate for the drag reduction caused by the smaller size of the scaled models? many thanks, Arun 

July 1, 2010, 08:11 

#5 
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khurram
Join Date: Jan 2010
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Idealy geometric similarity is to be maintained. When you define your problem and level of accuracy required then you can decide which details can be left out.
When you know the Re No and Cd you can estimate the actual drag of the vehicle. 

July 1, 2010, 08:45 

#6 
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Arun
Join Date: Jun 2010
Posts: 10
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Yes. Geometric similarity is maintained. But what about size? Lets see the problem like this:
we have a big rock 10mx10mx10m (taking it as an actual sized car) and we measure the drag force on it taking it to a wind tunnel, with wind speed of say 80kmph. we get some value "Z". now we have a 1:5 scale model rock which makes it 2mx2mx2m (this is our scaled model of car). we take it to wind tunnel and measure drag force on it with the same wind speed. we get less drag because the scaled model's projected area is less tham that of the actual rock. because it is a simple geometry (cube), we can straight away say drag force ratio is 5:1 between the large rock and the smaller. how can we judge the change in drag because of size when it comes to a much much complex geometry like car? I hope I am more clear this time. Arun 

July 1, 2010, 08:54 
@longines

#7 
New Member
Arun
Join Date: Jun 2010
Posts: 10
Rep Power: 8 
can you tell me more about scaling velocity please?
many thanks Arun 

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