subaruturbo_18

Is there a way to calculate the speed of a fan sat at the front of the car at a vertical position. Obviously the wind speed hitting the fan would be the same speed of the car,(the natural wind speed due to weather will essentially be unsignificant compared to the speed of the car so at this stage i am dis-regarding it)

If any body knows a rough average speed that it may rotate at then let me know. If it has to be calculated then please help me with the text below


As far as i know, the speed will depend on the total surface area of the blades, length of the blades, and the angle of the blades.

I am studying mech eng but am not that far into the course. The only way i can think to work it out is buy using the formula to calculate torque, but using windspeed as the force and then blade length as area. I do have some information on how angles of force affect the torque, so i should be able to do that part. I would guess there would be some sort of link to the torque and the speed the fan will turn and this is would depend on its weight also, but would this be the right direction in how to calcualte it?

correct me if i'm wrong please

+Doc+

Sod mathematics, do a practical exam and get the true answer!

warrenm2

I assume you are talking about a windwill type device bolted to the front of the car. If so, then the rotational speed will depend on the efficiency of the blades at converting the oncoming energy into rotational energy. This will be
influenced by blade angle, profile and length, solidity, frictional losses etc etc

[-(o)-]

iTrader: (1)

If you're not far into the course, think you'd be safe to assume its perfectly efficient and calculate it using the volume of air passing through it and the pitch of the fan (assume its ducted and theres no hub etc.).

To do it properly would need some advanced thermodynamics - 4th year stuff when I did it. Turbomachinery: all different velocities , angles and enthalpy - I didn't get it then and I certainly can't remember it now!

corvid

Vast amount of variables. Good luck with this.

StickyMicky

Measure it......

r32

Are we talking an electrical fan motor here, or is it purely wind driven by the passage of the car through the air?

chocolate_o_brian

iTrader: (22)

Just curious (as I did a couple of homework questions on this last night)...

Are you looking to find the answer out in revolutions or radians? What if any other variables do you need to consider? My guess (if I understand correctly) is to convert the wind speed into kJ/sec and put that into the forumlae?

I'm a little patchy myself on this stuff and have an exam next week which I'm currently revising for so may look into it further.

subaruturbo_18

Yes i think it will come down to thermodynamics, not basic physics and mechanics.

choc - I'm looking to find it in revolutions.

I was just purely curious whilst bored at work. I was thinking about something we were doing at uni and it set me thinking

Still if anyone knows anything more please shed some light as its now annoying me and i can't find anything on google lol

dunx

iTrader: (3)

Thermofluids, the centre of pressure can be estimated for a single blade, treat the air as a fluid i.e. pressure, then approximate the angle of attack of the fan blade.... this may then be resolved into the force acting about the pivot, ( torque).

IMHO

dunx

what would scooby do

^^^^this pressure and drag have a big impact 9/10 for effort

WRX_Dazza

if your fan blade is 24" and you are doing 714 mph, the rpm would be close to 10,000.

be careful here as this is around 0.9 mach and your blade will start behaving weirdly above this.

HTH

ps: this is at 86 deg farenheit - lower temp gives higher rpm.

+Doc+

Surely there are too many variables to even consider this equation?

Jay m A

iTrader: (2)

what about the radiator stuck either in front or behind the fan, makes a difference you know

CrisPDuk

I don't know how you'd calculate it, but I do know that the engine driven fan bolted to the front of RWD Ford engins start trying to spin faster than the hubs of their fluid couplings will ever manage, at about 35mph

We worked this out in college many years ago in a (successful) attempt to disprove the myth then being perpetuated by electric fan manufacturers that engine driven fans sap horsepower

Leslie

There are so many variables that I don't know how you could succeed to cover all situations.

The speed would vary with the forward speed of the fan, the temperature and the density of the air, the power available to drive the fan naturally, the length,pitch and number of fan blades. The airflow through and past the fan. The type of aerofoil section of the fan blades. You could only calculate it for a set of standard conditions and to do it for a differing set of conditions would drive you scatty I would expect.

Les