# I How much wind force does a moving vehicle generate?

1. Aug 15, 2016

### Q-man

Does anyone know how to calculate how much force (or wind) a moving vehicle generate?
I am trying to figure out how much wind does a moving car (or any vehicle) generate when traveling at certain speed.

2. Aug 16, 2016

### benny_91

Are you talking about how much wind resistance a car has to overcome?? In that case there are two types of drag forces: Pressure drag and friction drag. The formulae for these are available in every fluid mechanics text.

3. Aug 17, 2016

### Q-man

Hi Benny_91,
Thank you for responding to my posting. I want to measure how much energy is generated when a car is travels at at a certain speed (e.g. sixty miles per hour).
Imagine when you're standing on the side of the road, when a car travels at sixty miles per hour passing by and you feel the energy (wind) from the car passing by. I want to measure or calculate that energy or how much wind when a car is traveling at a certain speed.

4. Aug 17, 2016

### benny_91

The force required by the car to travel at a constant velocity minus the force required to overcome rolling friction at the wheels will give the resistance force offered by the wind. This force multiplied by the constant velocity of the car will give the energy generated or energy transferred to the air per second. This energy mostly appears as heat or kinetic energy.

5. Aug 17, 2016

### RonL

I think what you are looking for will be hard to define, how will you determine the area to measure ? but using the same math as when calculating windmill energy is what I think you will need.

6. Aug 17, 2016

### Staff: Mentor

Why do you want to calculate this? It is not energy that is easily harvested.

7. Aug 17, 2016

### Khashishi

8. Aug 18, 2016

### lychette

perhaps he/she is just interested in the calculation?

9. Aug 18, 2016

### Staff: Mentor

That certainly could be, but we sometimes get folks here wanting to figure out how much energy they can recover by putting a fan/windmill on a car...

10. Aug 18, 2016

### lychette

OK....I wonder what they wanted

11. Aug 18, 2016

### CWatters

This link has a breakdown showing where the energy goes...

http://www.consumerenergycenter.org/transportation/consumer_tips/vehicle_energy_losses.html [Broken]

Last edited by a moderator: May 8, 2017
12. Aug 18, 2016

### RonL

Is there not a difference, as to drag from air staying in contact with the skin surface of a moving object ? and the buffeting of the large air mass that has been pushed aside by said object ?

13. Aug 18, 2016

### CWatters

What do you mean by "difference"?

Any energy imparted to the air by the motion of the car must ultimately come from the fuel used to propel the car. Where else could it come from?

14. Aug 18, 2016

### CWatters

PS The air "pushed aside" by the car doesn't stay pushed aside. There is no vacuum behind the car.

If you tried to harness the energy in the air pushed aside you might be able to capture more than the 2.5% I referred to BUT this would come at the expense of increasing the drag and energy wasted by the car. There is never any free lunch.

15. Aug 18, 2016

### RonL

I can see where most might assume a harvest from the car, I took it to be a harvest on solid ground as the car moved past.

16. Aug 19, 2016

### CWatters

Either way the energy still comes from the car.

17. Aug 19, 2016

### Staff: Mentor

Or on the side of the road.

18. Aug 19, 2016

### sophiecentaur

It's the same with slipstreaming behind another vehicle. The 'pull' you get comes from the muscles of the poor guy, cycling in front. Skeins of geese have the strongest flyers near the front and they are also donating energy to those behind. The leader could always be going faster and cheaper without hangers on.
I must say, the concept of 'harvesting' energy is continually being brought up by people as if it's the answer to all our energy needs. If there is wasted energy in any process, the best solution by far is usually just to make that process more efficient. The only example that I can think of where this doesn't apply is in Combined Heat and Power systems, in which the Efficiency of the generation system is limited by thermodynamic constraints. In that case, the basic Efficiency is truly disastrous and getting a bit of home heating out of it just ameliorates the situation a bit.

19. Aug 19, 2016