A question on vehicle aerodynamics

[xxChrisxx]

Ken not to bash you mate but you are wrong about the air not acting the same way.

If the people just put a scale model in a flow without changing it then, yes it wouldn't behave the same way.

In scale wind tunnel tests they use simlitude, (look up reynolds and mach numbers) to ensure that it does act in the same way as on a full sized vehichle.

This is the reason why people can use water tunnels to test (air)flow over cars.

Google doesn't give you all the answers I am afraid.

 

[famousken]

In model testing though; doesn't the air appear much more dense than it would with a full size vehicle? I know you could scale up the drag, but it seems to me that you would be missing out on quite a bit of important info by just using a model because there is a possibility that things like turbulance simply won't form on a smaller model because of the much greater apparent density of the air. That's just my reasoning though, I could be wrong I am not an expert.

 

[xxChrisxx]

Like I said, all that matters is that the nondimensional coefficients are the same. If thet is true the fluid will act in exactly the same way. All the information is there.

Now obviously sometihngs may be missed purely becuase of the size. All the phenomena are there, but if you are interested in a specific thing such as a vortex (that may be too small to capture by probes/smoke+camera) they sometimes run overscale models. So the model is X times larger than the actial thing.

 

[Cyrus]

In model testing though; doesn't the air appear much more dense than it would with a full size vehicle? I know you could scale up the drag, but it seems to me that you would be missing out on quite a bit of important info by just using a model because there is a possibility that things like turbulance simply won't form on a smaller model because of the much greater apparent density of the air. That's just my reasoning though, I could be wrong I am not an expert.

A few things need to be cleared up here.

[1] You were absolutely correct about full size testing. Some points though: the characterization of air through the engine scoops, moon roof, etc are very much final stage design items. It is WAYYYYYYYYYYYY to expensive to keep putting in new full size cars with different designs on them for something like engine intake scoops during the initial design of the car. The small models (and now CFD) are used for major body changes in the car. Once a more-or-less finalized design is reached a prototype is made. This full size prototype is VERY EXPENSIVE. That's why people use models first. Then the kinks are worked out in the full size tunnels you linked to earlier.

[2] The use of a small car is possible through similitude, which means (in the case of wind tunnels that have air as a working fluid), you need to make a scale model of the car. You can get away with this (pretty well) because the Reynolds dependency is relatively flat over a large range. It won't be 99% right, but it won't be very horrible. It will be pretty darn close. But that doesn't matter for a car! You can always put it in the full size wind tunnel when you make the actual prototype! Notice how you CANT do the same thing for an airplane though, which makes it much more of a challenge in terms of interpreting the data from a scale model. Provided the Reynolds number stays similar, the transition points and boundary layer should stay the same. Thats the entire point of Reynolds dependencies in the flow.