Reynolds number calculation

[aerohead]

Originally Posted by alambe

While we're doing the math, what is the effect of temperature on skin friction and pressure drag. I'm thinking warm day vs cool day vs cold snap like we recently had.

(I should do it myself, but it's been so long I don't even know where to look for the numbers)

In 35-years I've seen but a single table for air density as a function of local temp and baro. pressure,which is what you need to know.
If you have a local dynamometer shop you might pester them in to showing you how to do the conversion.
Once you know your density (rho) you can plug that into your formulas.

[RobertSmalls]

Density Altitude Calculator - English/Metric, Relative Humidity

PV=nRT tells you that for an ideal gas (which air is, to a good approximation), density is linear with barometric pressure.

[jime57]

Originally Posted by some_other_dave

One of the fun parts with that equation is that exhaust gas is HOT. And it cools relatively rapidly as it moves through the exhaust system. So its density changes by a decent amount...

-soD

Which leads to a considerable amount of head scratching by designers of high performance exhaust manifolds(headers). It gets so complex that most designers just give up on calculations and develop the diameters and lengths experimentally on an engine dynamometer. Throw in stepped diameters and the problem becomes totally intractable.

[gone-ot]

...and, because of the exhaust large temperature change, most people simply use an approximated "average" value.

[aerohead]

Originally Posted by RobertSmalls

Density Altitude Calculator - English/Metric, Relative Humidity

PV=nRT tells you that for an ideal gas (which air is, to a good approximation), density is linear with barometric pressure.

Isn't the linear relationship valid if only barometric pressure is the single variable?

[RobertSmalls]

Originally Posted by aerohead

Isn't the linear relationship valid if only barometric pressure is the single variable?

Yes. Here's the relationship with two variables:

PV=nRT
nR/V=P/T

where nR/V can be equated with density, and T is absolute temperature, in Rankine or Kelvins.

If you want to add a third variable like humidity, or if the ideal gas approximation isn't good enough, it's time for a lookup table. I don't know what method the Javascript calculator I linked to uses.