Landracing Forum

Tech Information => Aerodynamics => Topic started by: jdolecki on December 10, 2018, 09:21:40 AM

Title: Air tempeture effect on air flow
Post by: jdolecki on December 10, 2018, 09:21:40 AM
Does the temperature of the air flowing across a body have different effects on the aerodynamics?

Basically are they better on a cooler day verses a hot day?

What about engine exhaust, exiting a car mid body and flowing over the body verses coming straight out the back?

I know is kind of a general question but i don't know the answer.

thanks, john




Title: Re: Air tempeture effect on air flow
Post by: TheBaron on December 10, 2018, 10:13:10 AM
Air drag, lift, and downforce all go up and down with the air temperature.....

The "rule of thumb" rate is 1% per 11 degrees F of change...

Exhaust thrust is very real with larger supercharged power plants....

Exhaust should be directed as aft as possible and into a "low pressure"
region if possible (such as the wake of the vehicle)....

The WWII P-51's forward thrust (at extreme altitudes where the propeller became more and more inefficient) it was estimated that 20% was contributed by the exhaust and the radiator ducting.....

Robert "Smitty" Smith
Red Baron Race Team
Title: Re: Air tempeture effect on air flow
Post by: mtiberio on December 10, 2018, 10:13:18 AM
since hot air is less dense than cool air, I'd say yes.
Title: Re: Air tempeture effect on air flow
Post by: Bratfink on March 08, 2019, 04:05:33 PM
Humidity, temperature and ambient air pressure all play a part.

Drag Force = 1/2 * Density * Velocity^2 * Cd * Frontal Area.

Density = pressure(absolute) / Specific Gas Constant * Temp(absolute).

Temp goes up, density goes down and so does drag.

However if pressure goes up it increases density and drag.

Title: Re: Air tempeture effect on air flow
Post by: Stan Back on March 08, 2019, 05:13:57 PM
. . . and then there's Bolivia – which, I'm told, fooled everyone.
Title: Re: Air tempeture effect on air flow
Post by: SPARKY on March 09, 2019, 11:49:46 AM
SB did the HP go up as well as the drag ?  :-o
Title: Re: Air tempeture effect on air flow
Post by: Stan Back on March 09, 2019, 04:29:44 PM
I seem to remember accounts of unblown bikes going faster there – apparently the drag was reduced by such an extent that the horsepower loss was less a consequence than anticipated.
Title: Re: Air tempeture effect on air flow
Post by: SPARKY on March 09, 2019, 08:34:31 PM
isn't that the same as B'ville cleaner cars go faster in the AM  dirtier  cars in the afternoon?
Title: Re: Air tempeture effect on air flow
Post by: Stan Back on March 09, 2019, 08:53:45 PM
Huh?
Title: Re: Air tempeture effect on air flow
Post by: DallasV on March 09, 2019, 11:14:48 PM
Having a naturally aspirated roadster (brick) I will gladly take more drag from dense air to better feed the noisemaker. Those slick little skinny rolling pencils, especially with forced air, probably are on the other side of the fence.