aerodynamics question

[qwerty]

Hi all, im doing a physics experiment on soda bomb cars.. for those who dont know its a soda bulb placed in the back of a wooden small car,.. it travels around 20 metres in 1-2 seconds..

 

For our physics experiment, we have CHOSEN to do this and we have to like.. talk about different effects on the car, one is how weight affects it.. ie, adding different weights on to the car and checking how this affects the time, deceleration, initial vel and final vel, etc.

 

Now, we need at least 3 things to talk about, another could be different surfaces, or different inclines... but the one i need help with is aerodynamics. We could attach differnt shaped things onto the car to make it less aerodynamic? but how would we be able to measure the 'aerodynamicness' of it? would there be any formulas to use? maybe we could set it up on a flat surface and use a hairdrier to blow air at it, and see which attachment we have used on the car would make the car move the furtherest by the hairdrier?

 

any ideas? thanks in advance, John.

[mezarashi]

From what I know, fluid dynamics is a relatively complex subject. Although not the most difficult subject the background knowledge required to do any of its analysis is vast. Normally as cars are tested in wind tunnels, you will observe how much drag or force the wind is putting on the car in test. If a car is very aerodynamic, it should experience less drag at highspeeds. The problem in your case is that the car performs at relatively highspeeds, 20 meters in a couple of seconds? I'm not sure if that qualifies for the air turning turbulent instead of laminar. Both require different equations of analysis (one bases itself on fluid viscosity-laminar while the other on particle inertia-turbulent).

 

It is somewhat possible to model the effects using a smaller model in what mechanical engineers call Dimensional Analysis and Similitude. Kind of like how engineers predict how a river or dam will flow using a smaller scale model with different fluids. That topic is however, can be half a semesters worth of college material.

 

I would suggest sticking to studies related to frictional forces, or do a qualitative aerodynamic study

[J.C.MacSwell]

Hi all' date=' im doing a physics experiment on soda bomb cars.. for those who dont know its a soda bulb placed in the back of a wooden small car,.. it travels around 20 metres in 1-2 seconds..

 

For our physics experiment, we have CHOSEN to do this and we have to like.. talk about different effects on the car, one is how weight affects it.. ie, adding different weights on to the car and checking how this affects the time, deceleration, initial vel and final vel, etc.

 

Now, we need at least 3 things to talk about, another could be different surfaces, or different inclines... but the one i need help with is aerodynamics. We could attach differnt shaped things onto the car to make it less aerodynamic? but how would we be able to measure the 'aerodynamicness' of it? would there be any formulas to use? maybe we could set it up on a flat surface and use a hairdrier to blow air at it, and see which attachment we have used on the car would make the car move the furtherest by the hairdrier?

 

any ideas? thanks in advance, John.[/quote']

 

Your basic drag equation is the drag coefficient X velocity squared X frontal area X density of air divided by 2. A lot of factors go into the coefficient such as Mezarashi pointed out. It changes (often abruptly as the flow may change abruptly) with shape, size and velocity even though some of these are already somewhat in the formula.

[fuel pod]

I would say that the above two replies would do..pretty well.....simply the drag equation can be used..

u should get hold of drag coefficents values for simple shapes like cone ,hemisphere...

its better to get drag co-efficent vs Reynold no. graphs....(these are attained by the use of the analysis mentioned above keeping in account a lot of factors)

use the shape in front of the body...neglect the rest of body profile and friction..

get the value for reynold no. from the graph and simply put in the drag equation..to ge the drag force on the body..

 

by the way reynold`s no=

velocity of (object/fluid) X Density of the fluid X dia / kinematic viscosity