Wind tunnel application 2

[WhackyWheels]

A few years ago I purchased some items from a government auction. An item that was in one of the lots was a 21", 14 blade enclosed axial fan. The fan is cast magneseum or alum. On the fan housing it indicates 18,000 cfm.
Initially, I had thought it may be a good size for testing aerodynamic drag on a full size go-kart. However, a typical go-kart with driver produces around 8.5 square feet of blockage.

I need some help and technical information.

a) How can I determine the RPM needed to attain the stated volume of air?
b) Where can I find the horsepower rating needed to drive this fan?
c) If this fan is too small for a full scale go-kart, which I beleive it is, what scale would it be appropiate for?
d) Alternatively, how do I determine the proper size fan/power, CFM and air speed for wind tunnel testing on a go-kart/driver?

I have read Joseph Katz's "Race Car Aerodynamics" and "Low-Speed Wind Tunnel Testing" by Barlow, Rae and Pope. Additionally, I've visited an open jet/rolling road wind tunnel. It's max. wind & road speeds were 50 m/s respectively. The fan diameter was 12.5' and took 320 kw to power it. I subscribe to and thourghly read "Racecar Engineering", "Race Tech" and "Autosport" magazines.

If anyone could get me headed in the right direction I would be appreciative.

Thank You

Pitot

 

[matt00000]

Why do you want to put a go-kart in a wind tunnel?
Surely everything you wish to find out can be done in just driving it??

 

[WhackyWheels]

Why does anybody put anything in a wind tunnel?
Perhaps not known to a lot of people, go-kart racing is not what it was ten years ago.
Looking for a competitive edge is what drives any form of motorsport and I don't believe go-kart racing is any different.

 

[matt00000]

I didnt think that you were allowed to use spliters wings etc... in go-kart racing.

If you just want to reduce the drag then just streamline it.
What you think looks right will be a pretty good guess to reduce drag.
if you want to do flow vis then use oil and drive down a straight road.
but frankly youd be better off just slippin some octane booster in the tank when no-ones lookin!

 

[WhackyWheels]

Octane boosters or any other oxygenated fuels are not allowed. Only spec fuels are used and policed by various testing procedures including spectrometer measurements.

Regarding spliters they are not allowed however, in some classes wings are required.
My original request for some help is not necessarily for these classes. The research I hope to do will provide empirical information for some competitive advantage that will still remain within the boundries of rules or policies.

On your advice of pouring some power into the fuel tank, I would suggest perhaps you do some info-search to see that were not talking about a 5hp fun kart out in the back yard.

The more powerfull shifter-karts are capable of going from "0"mph to 60mph (88fps) back to "0"mph in less than 5 seconds!

If you question these figures I would also suggest you ask Michael Schumaker, Memo Gidely, Paul Tracy and many of the other top racing drivers.

Take a look at

http://www.grandprix.com/ns/ns07242.html

"Red Bull's" F-1 driver search.

 

[matt00000]

Sure i am well aware that go karts should be taken very serioulsy and that the top drivers rebuild their engines after every race. but...
I thought that the point of gokarting was that the karts were equal and that driver skill was the road to success.
if you cannot alter the flow of air over the kart what is the point of putting it in a tunnel? what would you hope to find out? (I was joking about the octane booster!)if you put it in a tunnel you would be abole to find out the drag of the kart and driver... roughly. but then what would you do? knowing the drag? if you cant change it why should it matter wha it is?

 

[WhackyWheels]

In general, the rules are designed to make the performance of the karts competitive with each other, thus allowing the driver's talent to shine.
As I said earlier, the fast classes require a rear wing and full body-work.

Considering the major go-kart sanctioning bodies, there are thousands of smaller racing clubs, tracks and special events that accepts a broader viewpoint regarding some of the rules for thier own events.
Therefore, there are classes where the rules are not designed for the driver who wants to make it to the "Big Time". There are those who like to win races because they also practice the concept to think "Outside the Box".

The idea is to expand some of the boundries of amature go-kart racing on its own, for its own purpose. If experimenting with ideas are not explored in any form of motorsport, advancements of any kind will be slow and stail which effects developement in safety, competition and perhaps, closer and more exciting racing.

Anyway, the point to starting this inquiry was in hope to gain some technical information, not to debate the purpose, meaning, morals or perceived value of how a wind tunnel could provide to a go-kart.

If there is anyone who is capable of helping me with my initial request regarding the "fan" I have, please responed.

Pitot

 

[radomir]

To be frank, at the very beginning, I am not an expert, therefore do not expect much of a help, but if I try to reply to your query (questions) there will certainly be a couple of benefits for you:

1) First of all the attention shall be brought back to your initial questions and hopefully some experts will bother to have a look.

2) Secondly, and most importantly, when they (experts)realise what I have said they will be so disgusted and urged to teach me and make me aware of my ignorance that you may get quite useful answers to your question(s).

Therefore Sir Pitot remember I am doing this for you and give my best regards to Sir Prandtle (Pitot-Prandtle).

c & d) The primary intention regarding wind tunnel testing (WTT) is to achieve airflow velocity equal to the velocity encountered in a real situation although tested model is usually scaled down therefore Re is not the same. Meaning that measured aerodynamic forces and resulting coefficients have to be corrected for the real (full size) thing.

To achieve V=72 km/h (20 m/s) with Q=18,000 cfm (appr. 8.5 m^3/s), the flow area should approximately be:
A = Q/V = 8.5/20 = 0.42 m^2 (4.5 sq.ft.)

Assuming that tested object (model) frontal area “blockage” could maximum be 1/4 of WT test section (maybe 1/6 max. is more correct in order to get more reliable results, low wall influence etc.) we come to a value of 1 sq.ft. That means that your model should be at least 3 times smaller than real thing because in such case model cross section is 3x3=9 times smaller (8.5/9=0.944 sq.ft.).

If you want to go to higher speeds that will require proportionally smaller test section consequently smaller model (e.g. 3.5 to 4 times smaller). Unfortunately smaller the model is, the more accuracy (perfection) is required.

CFM I do not get, it means Computational Fluid Dynamics to me. If it means cfm, for full size test you will roughly require at least the 9 times higher airflow.

a) Regarding your fan my wild guess is that 18,000 cfm is achieved at 3,600 rpm (Tesla AC motor speed in North America, no other logical substantiation). Since blade tip speed is well below sonic speed why not 3,600. Are you sure that it is not indicated somewhere on that housing, same as flow.

b) For incompressible fluids power of pump (theoretical-no losses) is:
P = (p2-p1) Q
p1 = intake pressure
p2 = delivery pressure
Q = volumetric flow
In this case we can assume that air is incompressible and that fan is giving a pressure rise of 1000 Pa (N/m^2). I am not quite sure is this pressure rise assumption close to reality (it actually means that fan pressure ratio is very small 1.01:1). Well, the critics will tell. Consequently the power is:

P = 1000 x 8.5 Nm/s = 8.5 kW

To be on a safe side, increasing this two times and rounding it to a higher nice number, required power is 20kW. This sounds quite reasonable to me.

The other problem is how are you going to measure drag and lift forces. Since you have so many books probably they are giving some solutions.

Happy New Year to all

 

[towbar]

I know you don't really want to debate the purpose of doing this, but I doubt if a wind tunnel will tell you much of value unless you fit a moving ground plane. Why not buy a data logger and do real life tests on the kart itself? Coast down tests will tell you about drag, and also about engine power. Down force is more difficult since you don't have any suspension, but strain gauges on bearing housings might do it.
I've seen items on this somewhere in the magazines you mention, and there are relevant SAE papers too.

 

[matt00000]

Pitot, I am sorry if I have upset you. I was not meaning to. But the cost of running a wind tunnel of the size required for the subject you wish to test would not be good. The tunnel would have to be huge (assuming it is a closed loop) Like towbar says because of the height which go-karts run at you would need a rolling road running at exactly the same speed as the airflow. You would need a pitot tube and a balance and some method of attaching all of this to your go-kart in such a way as to have no impact on the airflow around the kart.
I have asked you lots of questions, which you haven’t answered. How can someone help you if you don’t elaborate on what you wish to find out?

If it is primarily 'can I use this fan for a wind tunnel?' yes you can. 'Could you test your cart in it?' no and 'what would it be suitable for?' a test area less than a foot sq. or a ceiling fan!

On the fan housing it says 18000cfm. Is that a maximum? At what rpm is that if not? What is the pitch of the blades what is the diameter of the blades... in your post there is so much unknown that frankly it would be a waste of money and time to just try it. Don’t get upset please life is too short. Have a sense of humour. Most of the things you would want to know can be found from running tests. Racetracks are not straight lines. Wind tunnels are. When a vehicle turns a corner everything changes so what is right in the tunnel is not necessarily correct in a real life application.
I guess I should have written more in my first post so as not to upset you. Please accept my apologies.


Matt.

 

[WhackyWheels]

Matt00000,

I appreciate it. However, I wasn't getting upset, by nature I get impatient to quickly when it comes to questions of judgement. I too apoligize. Actually, my line of questions were confusing. The questions were grouped together requesting answers on two completely seperate issues.

For me, its not a matter of "Why" I want to WTT a Go-Kart, it is simply a matter of "How". When it comes to the cost, I have visited several different facilities and spoke to several people on this subject. A few months ago I was granted a visit to "Automotive Research Center" in Indianapolis. It was a close-looped open test section tunnel, with a 12' fan driven by a 350 kw DC motor. The facility was commissioned in 1999 costing $7 million dollars. I HAVE NO INTENTIONS OF SPENDING $7 MILLION DOLLARS. However, I spent quit a bit of time talking with the Director of the facility, David Pitchforth. Upon understanding my intentions, Mr. Pitchforth generously offered a few alternitives regarding, but limited to the design and construction of the tunnel itself. He also spoke about the fan, power source, DAQ, stinger/balance beam and its mounting location, the mounted sensor unit, model criteria, rolling-road, and environmental controls.

I also visited a person with his own wind tunnel that he designed and built himself. It too had a 12' fan open jet, closed test section that was driven by a 500 hp I.C. engine through its automatice transmission. Imediately, I question the accurcy of precise and incrimental adjustments of the fan speed as well as the stability of the engine's speed. Additionly, the tunnel was open to climatic changes thus questioning the efficacy of the collected data. Other than that, I was quite impressed with his novel approach of the use of materials and equipment and its construction, the DAQ system and stinger/balance beam. The models were aviation related therefore he had no need for a rolling-road.

Some questions I still need answered.
a) Regarding a tunnel's maximum volicity, typically it is described in M/S. How can this be converted to "Cubic Feet per Minute" (CFM) and vis a versa? I am speaking only of the measurement at the fan's immediate exit, not in the test section. There must be other data needed to find the answer.
b) So far from what I understand now, the size of the tunnel determines the blockage area of the test model that can be tested. Or, is the size of the test model that determines the size tunnel needed. Either way, I've read that the size of the test model cannot be more than 1/3rd, 1/4 to 1/8 of the test section. What are the restriction? Is it the boundery layer of the model, the speed/CFM of the wind or the complexity or intricecy of the model? e.g. C/d of a F-1 car compared to the C/d of a model with multible tubing, and body work that is more for astetics and safety rather than drag reduction such as a Go-Kart and its driver.
c) Finally, how is the Renalds Number of a Go-Kart determined?

To all that have been a part of this quagmier of mine, thank you very much for your patience and technical support.

Pitot

 

[matt00000]

Glad we sorted that out!


there is an equation to determin the flow of air from a propeller but unfortunatly i dont remember it! any good book on aerodynamics should have the theory in and equations to work out the the airflow. or have a look at this and see what you think...
thread2-29477 easiest way is just to use a pitot tube to detrmine the velocity of the airflow and to calculate it from that.
ie l/h of the tunnel in feet multiplied by the velocity in f/m will give you the cfm. from the fan you could just do tests to see what it is but i guess that the reason you want to know is the 18000cfm! do you know if that is a maximum?... do you know who made the fan? cus you could ask them...
for the tests you are doing the ic engine would be fine its not the speed of the fan that is iomportant but the speed of the air in the test section. which you can determine. remember thought that density changes with temperature.

You have a subject that you wish to test and it is not a model for this reason you dont need to worry about Reynolds number. R no is needed to scale results from a test model up to full size i think it is R = x /(density * velocity * length) where x is the test sections length

so if it was a wing x would be a point along the chord say where flow became turbulent and 'l' would be the chord. But you dont need to worry about that.

it is the total frontal area of the model that you need to worry about wrt blockage. the greater the blockage the great the degree of error. having said that i test a model of a delta a/c in a tunnel and 'blocked' about 1/3 of the tunnel and there wasnt a great deal of error on the plots of L/AoA D/AoA etc at least none that I noticed!
In your case as you have a subject to test you want to make the tunnel more than three times the area of the front section of the kart.

"Is it the boundery layer of the model, the speed/CFM of the wind or the complexity or intricecy of the model? e.g. C/d of a F-1 car compared to the C/d of a model with multible tubing, and body work that is more for astetics and safety rather than drag reduction such as a Go-Kart and its driver"

I dont quite know what you are asking there...
boundary layer is associated with flow over a smoth surface the boundary layer on a tube will be well nothing really as it seperates at maximum diameter.
I think what you are saying is does it matter what the shape is of the test subject in relation to blockage. No only frontal area.

If i were you, i would get a good book on aerodynamics it will tell you a lot more about what you want to know and what you think you know but dont! that isnt meant to sound rude but it will help when you come to do your tests to help you realise what you are finding out.

If you do it, i hope you find out what you want to know. I am sure you will learn a lot either way.

good luck!!