Need some advice or feedback on CFD? (Computation Fluid Dynamics)

[JArcuri]

I am looking to aerodynamic test a new car design , right after being digitized from clay. We have an option for an actual wind tunnel test on a scale model but would it be more practical to work out any major faults in the design through a CFD package first?

-Has CFD evolved to the point where an actual wind tunnel test can be negated from the process all together?

-Are there any companies that can do this work for us, in or near the Carolinas?

-if anybody has gone though this process, your feedback would be very much appreciated.

Thank you,
Joe

 

[GregLocock]

It's cheaper to do a 1/5 scale model test than CFD. We use CFD because our cooling/HVAC requirements are a significant part of the design even at an early stage in the program.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rob45]

JArcuri:
I know of a company doing CFD work, "Exa Corp." in Burlington, Mass.

They do work for us at present, quite successfully.
One interesting thing about them that distinguish them from most others doing CFD is that their software is NOT based on the Navier-Stokes equations, but instead use what I think they described as the Lattice-Boltzmann Method, producing more accurate results.
They specialize in whole-vehicle simulations as well as underhood airflow modelling.

 

[Greenlight]

When testing a scale model in a wind tunnel, make sure the concepts of similitude are taken into consideration.

Similitude is a concept used in the testing of engineering models. A model is said to have similitude with the real application if the two share geometric similarity, kinematic similarity and dynamic similarity.

A 1/10 scale model may require wind tunnel velocities 5 times that of the actual application. So, if the actual vehicle force analysis is targeted for 70 mph, the wind tunnel will need to move air at 350 mph.

It may be hard to find a wind tunnel to meet your needs.

CFD has progressed significantly in the last decade and it is my opinion that you can get accurate results (although it's quite expensive) if the CFD operator is competent.

 

[CentralCoaster]

For the sake of similitude and dimensionless modeling, isn't water sometimes used to simulate aerodynamics? I'm rusty on this, but it basically consists of matching up reynolds numbers right?

 

[GregLocock]

Yes, racing teams have used water troughs to develop aero.

These days I think it is unlikely, the level of sophistication is such that the simple scaling laws are no longer reliable, so they modify the geometry of the scale models to allow for this.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cdx]

I had read somewhere that when performing a water trough test, the model is pulled along at a very slow speed - around 4 miles per hour.
If someone is testing the wake pattern on a street car that rarely exceeds 75 MPH, at what speed should a 1/5 scale model be pulled along at in the water trough ?

Also,how accurate is this sort of test when compared to using a model in a wind tunnel ?

Thanks

 

[modesign]

One advantage of the water tank was with the use of a glass bottom it allowed flow under the car to be looked at.A track under the tank allowed a camera to move at the same speed as the model.
Regards,
Sandy