Effect of surface temperature on attached flow
 

I have been wondering the last few days and so far I have been unsuccessful in finding any real information on whether or not an increase in surface temperature has a postive impact on attached flow.

For example, could it matter what color your car is? If your car is black and you live in a sunny climate, will the increased surface temperature of the car help reduce viscosity of the airflow over the car thus reducing drag? I realize at highway speed the cooling effect of the air over the surface will neutralize any solar heating, however in city driving could this make a difference?

I haven't seen any "real" info on the topic, but I doubt there's any significant effect of the temp of the vehicle's surface on airflow simply because the passing air isn't in the vicinity long enough to change temperature.

I don't think the car's surface would affect it, but the local air temp would slightly vary the density of air which makes it harder/easier for the car to push through the air.

effect
 

I don't know of any investigation ever performed.I don't think it's an oversight.In the city,where it might have an effect,the speeds would be so slow as to lose any benefit.----------------- And as already mentioned,at higher speeds,the residence time of the air over the car is so brief,as to render the effect null.---------------------- My intuition tells me that this is probably a dead end,mpg wise.

If you have access to a introductory Heat Transfer textbook, you should be able to find out more information about this topic. Most hx books contain at least baisc info about it and may direct you to further reading. Your local library may not have them, but should be able to order them on inter-library loan. Heated surfaces would have some positive effect but on ordinary vehicles the difference wouldn't be enough to notice because the points of separation are very distinctly due to geometry. ie behind the rear-view windows and cabin.

I'm sure if results were to be taken in the lab, positive results could be recorded for shapes that are more aerodynamic which do not regularly experience flow separation but my guess is that positive results would be obtained only at higher temps than would occur from solar heating.

if the surface temperature is increase, we heat the surface and insulate from inside, so that the passenger won't feel hot?

does this decrease the drag?

Sure! Snow and ice melt off black vehicles sooner and more completely, so that makes 'em sleeker.

I have not been here long, but I do get a giggle when Frank gives one of his 'short and sweet' answers.

NACA did some tests and the results in drag were not all too great.

EDIT: I meant to state that they had little effect

http://ntrs.nasa.gov/archive/nasa/ca...1979070772.pdf

Colder air is denser and has less viscosity, therefore would remain attached longer in adverse pressure gradients and have a thinner boundary layer. I think that a cold surface would have less drag for body dominated by surface drag and not much effect on a blunt body like a car.