F1 in Schools help: Rugby GP's project

[Rugby GP]

Hi
I am entering the F1 in Schools competition where miniature balsa wood cars (200mm long) are fired down a 20m track. They are powered by a CO2 Cartridge.

http://www.f1inschools.co.uk/

I was wondering weather anyone could tell me anything you know about nozzles and weather you think that it would help or hinder the car.

My team got the idea after we watched "Richard Hammond's Engineering Connections":
Please look at 2:16 onwards of this clip:

[youtube]http://www.youtube.com/watch?v=-SbnPu89 ... re=related[/youtube]

An Australian Team has already attempted to make a nozzle for their car, however I think that it actually made their car slower. Hopefully with your help we can design one that works well.



I Have found some figures that may help about the properties of the C02 cartridge:

a)Pressure upstream of the nozzle throat is 8 MPa.
b) Free stream pressure is assumed to be 101 kPa.
c) Mass flow rate is assumed constant at 0.08 kg/s
d) Temperature upstream of the nozzle is assumed 300 K
e) Fluid is carbon dioxide: specific heat ratio is 1.289 and the gas constant is 188.9 J/(kg*K)
f) Exit velocity 560m/s, exit mach number of 3.4

Cartridge Dimentions:

Length: 64mm
Width: 20mm
Mass: 0.025kg
Capacity:0.008kg (8g of CO2 emmitted over race)
Starter: Striker

I would be really gratefull if anyone knew any equations related to this area of aerodynamics.

Thank you very much for your help.

[tok-tokkie]

The challenge is to make the car accelerate better than any of the others. The challenge is not how much air you can blow backwards. The venturi is brilliant at increasing the air volume. What you want is to increase the thrust which is not the same.

In fact your car is just a rocket sledge & rockets provide thrust through the conservation of momentum. Wiki: http://en.wikipedia.org/wiki/Conservati ... r_momentum

[bettonracing]

The larger the velocity differential between the CO2 'exhaust' & the air immediately surrounding it, the more thrust You'll have (generally speaking) by taking advantage or air's viscosity & Newton's third law. Adding the 'aspirator' nozzle shown in the video will speed up the air immediately surrounding the CO2 exhaust, likely reducing thrust.

Regards,

H. Kurt Betton

P.S. Be cognisant of the spirit of the competition. There's a fine line between asking experts for help, & asking experts for the answer. I don't think You've done anything wrong yet, but Your open-ended question is borderline and someone could respond with 'the answer'.

[Rugby GP]

Thanks for your replies.

This why I think that it might work:

In the video, the nozzle increases the volume and mass of gas that enters the bag.

Therefore a larger mass of gas is moved backwards (into the bag)

Therefore a larger thrust is produced (Newton's 3rd Law)

Can anyone see where I'm going wrong here.

Tok-Tokie: Surely if the mass of air going backwards is increased, then the thrust will increase

Betton racing: surely if there is a large volume/second, medium speed flow at the end of the exhaust, it will produce more thrust than a tiny volume/second, high velocity?
I see what you are saying about asking for help/answer, and have changed my original post accordingly. Your comment will become useful when I am asking companies to sponsor my team.

[flynfrog]

you are trading mass for velocity. Also keep in mind that your rate of gas leaving your car is constantly dropping so it would be difficult to tune even if there was an optimal point

[Izzy410]

Not sure this help but,

remember like flynn said the gas that exits degrade in time and the only way to compensate is to use an adjustable nozzle but that's impossible by rules.
what you can try is to find an optimum opening size around the nozzle. This could be done by windtunnel testing. but how to test it is beyond me.

[bettonracing]

Air propulsion (in open air) works because of the speed differential between the exhaust and the air immediately surrounding it. The exhaust can only 'push' because it has something to push against (i.e. shearing the ambient fluid). For discussion sake let's define the "shearing interface" as a cylinder that is the diameter of exhaust tip. This shearing interface is where the pushing happens (to satisfy Newton's 3rd law).

The propulsion force is a function of the shearing force & the surface area of the shearing interface. Shearing force is a function of fluid viscosity & speed of the exhaust.

Hint: You want to optimise shearing force with shearing interface surface area. The aspirator increases surface area (larger volume/ mass) but significantly reduces the shearing force (by reducing velocity).

^ The above is a hurried response. If something isn't clear let me know. Read up on air inducement in the mean time. The Dyson bladeless fan uses the same principle and may help give some leads to technical info/data.

Regards,

H. Kurt Betton

[Caito]

Nice explanation betton.

Do you have any related 'math'. Just to make ballpark calculations to watch the tradeoff between surface area(volume) and shear force(speed)


thank you!

[ashf1mclaren]

hi all, i would have to say that the funnel nozzle he later uses draws in air the accelerates it through the gradual closing, the Australian teams design does not feature very well a nossle which would draw in air flow, it would seem to be focused on the co2 air flow also it only features on part of the design shown in the clip, there is know booster shaped nozzle exit on the opposite side to the gradual closing, i think the with this sudden expansion of space would increase air flow speed. but as stated as the co2 is not a continuous flow it may lead to some advantage at the start and a drag disadvantage at the end canceling any gain out

[Rugby GP]

Here's my new website: We managed to get the fastest car in the UK! but we didn't use the nozzle

http://www.wix.com/thoraj/rugbygp