Wind Tunnel air flow

[CHSGreenpower]

My opinion of water troughs for aero testing is... well, I don't know enough about them to have an opinion on the subject to be fair.
Will one of these be cheaper/easier for my purpose?
Would the computing power of my PC affect the outcome of a CFD run or will it simply take longer? I assume the answer is the latter but I don't know anywhere near as much as you guys
Thanks for being patient with me.

[freebeard]

I just asked to prompt a third post.

Air vs water involves a lot of design trade-offs. But at 1/24th to 1/10th scale water has the advantage for duplicating the correct Reynolds numbers. But you don't have as much problems with air leaks instead of water leaks. Having the water tunnel upside down means the load cells and other pieces don't have to be at the bottom of the water.

You have the posts by graysgarage, but he doesn't seem to come around much lately. I think generating hydrogen bubbles in the water to replace smoke trails is genius.

Here's a design I made for a water tunnel; it's rough.



I think two paddlewheels at the rear would be sufficient. The idea is to move water without touching it as much as is possible; to suppress turbulence.

If you call it a PC it's probably under-powered for the task. Computing power limits the granularity of the simulation and the volume it can encompass. And the speed can mean a big calculation is in a race with the heat death of Universe. I don't even try. But I will get an X-15 when it ships as a general replacement, with the expectation that the software will fall in place over time.

[CHSGreenpower]

Hi,
having it 'upside down' means securing the model to the roof of the test section I assume?
From what you say, readings on CAD-related CFD such as Autodesk Flow Design are not accurate enough to make design decisions - am I reading correctly?
The only reason I ask this is because I know of a lot of big teams in the Greenpower series relying solely on these programs which you say to treat with a pinch of salt.
Again, thanks for answering questions to which the answers probably seem quite obvious.

[freebeard]

More like suspended in the open top. I'd have to re-watch the graysgarage videos for more specific detail.

CFD is better than nothing. Compare the 8-bit video screen shots from the 1980s with newer games that have characters with embroidered silk sleeves fluttering in the wind. It's like that.

While generating absolute values is difficult, relative values (averaged over multiple runs) can be helpful. That's A-B testing.

[aerohead]

Originally Posted by CHSGreenpower

My opinion of water troughs for aero testing is... well, I don't know enough about them to have an opinion on the subject to be fair.
Will one of these be cheaper/easier for my purpose?
Would the computing power of my PC affect the outcome of a CFD run or will it simply take longer? I assume the answer is the latter but I don't know anywhere near as much as you guys
Thanks for being patient with me.

A water 'table' might be something to consider.
A water reservoir supplies pumped water to an outlet where the water diffuses through a porous material,then flows out onto a slanted table which drains back into the reservoir.
Here is a unit at Texas Tech which uses a back-lit section of 'milk' Plexiglass for the table,with a grid painted onto it.

2-dimensional models are placed in the water and weighted down so they don't float way.(a pickup truck model is shown).
Common food coloring is injected into the stream from a squeezed syringe for flow visualization.When the water becomes clouded,a few drops of bleach clears it back up.
It's limited to 2-D flow,but you can learn an awful lot with these.
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A tow tank requires a very sophisticated 3-axis strain gauge load cell mounted to a rolling gantry bridge,which provides the motive power for the submerged models and data acquisition feed to a computer for data reduction.
I think that these would beyond the scope of your capabilities.
Texas Tech abandoned theirs and filled it in.They tested 3/8-scale models which cost them $68,000 (US) a piece.
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As to the CFD.Consider contacting the mechanical engineering department at Cambridge University and see if a grad student wouldn't help you get a feel for the software and hardware required to run a professional-quality CFD program.They know quite a bit about streamlining