Wind Tunnel testing for hull and rudder

I would like to test my 1/4 scale model with tell tales in a wind tunnel but don't know the
pressure flow measurement to simulate water flow across the surfaces of a 1/4 scale model.

I have a variable speed fan that is electronically connected to a digital pressure flow gauges that I was thinking of using for the system.

The fan I have has accurate flow measurement from 300 to 6300CFM and is can automatically maintain a constant 75 Pa. - 0 Pa. when it connected to the Pressure Flow gauge while the second read out will display the flow in CFM.

I can also use a second Presser Flow gauge to display velocity measurement using a pitot tube in feet per minute or meters per second.

Wind Tunnel Components:
Contraction con
Settiling chamber
Test section w/ viewing window
Diffuser and fan housing
Blower and presser flow gauges

 

Why would you wind tunnel test rather than tank test?The water supporting the hull is a thousand times denser than the air flowing past and is normally moving in a single direction,along the hull. The wind can come from any direction and may have vastly different resistance characteristics as it changes.Might I suggest you get hold of Weston Farmer's book and read the chapter about his simple technique for comparing resistance of different hulls?

 

Or scale weight it and tow test it beside a boat or even in a public pool.
Take video, measure drag, etc.
And you're going to need some kind of coaming around that cockpit.
That's going to mess with your aerodynamics.

 

Waste of time. Your wind tunnel model won't correctly show the water/air interface and that's what you need to determine the actual flow of water around the hull.

I am curious as to the fan you have that's a huge flow range and most blowers that can make the flow can't make much pressure at low flow rates without stalling. If you have a link to the fan I'd sure like to see that.

 

This idea will work as well as dragging a test model plane across a body of water, i.e., not at all.

 

The real flow around your yacht in water will only be similar to the flow in the wind tunnel if you restrict the speed-range of interest to low Froude-numbers.
In this case, you can use tell tales on the model to indicate flow separation and based on these results, try to avoid separation by hull form changes, to improve the viscous resistance of the hull.
Of course you get no information about the wavemaking resistance of the hull.

Lets say your boat is 7 meters long, then a 1/4 scale model would be 1.75 meters long.
If the Froude number is limited to 0.25, the full size boat would travel at a speed of 2 m/s which is equivalent to 4 knots.
The Reynold number in this case would be 1.26*10^7
In the wind tunnel you must achieve the same Reynold number to have viscous similarity.
The required wind speed for a model that is 1.75 m long would be 108 m/s which is most probably much higher than what you can achieve witch your blower (108 m/s is 1/3 of the speed of sound)
At low speeds you can achieve viscous similarity only by enforcing turbulent flow via a trip wire. The results with a trip wire can be erratic and depend on the position of the trip wire.

All in all, if you are intersted in the viscous flow around your hull, then a numerical method (e.g. RANS-CFD) is much faster, cheaper and more accurate, than a wind tunnel experiment

 

Yes, Wind tunnel testing would only work well for gases and not for a liquid. You see, I have all this testing equipment that I don't use any more and one things leads to another, then there you are with a boneheaded idea. My infrared camera is a great toy now that I have tried to employ into my new interest of sailboats design without any luck.
Yellowjacket,
Here is the blower specifications:
Minneapolis Blower Door™ System (with DG-1000) https://energyconservatory.com/products/product/dg1000blowerdoor/?categories=11
I do think it would make a good blower for a wind tunnel but I have no posteriori knowledge or empirical data to back up my claim.

My path to posteriori knowledge is littered with boneheaded ideas without empirical data.
Thanks for sharing your knowledge.

 

The University of Iowa College of Engineering have a nice research facility. Maybe Professor Fred Stern would let me use it in the evening and then run a CFD simulation.

 

I want one!

 

We are working with them on a Navy project. They have the capability to do a CFD sim on the whole system.. Lots of capability there.