drag under wet conditions

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pipoloko
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Joined: 24 Dec 2012, 20:15

drag under wet conditions

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Hey guys anybody knows reports/articles on the influence of moisture air on CD/CL?
Any particular simulator /excel to analyse a coast down test data?
thanks

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Vyssion
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Joined: 10 Jun 2012, 14:40

Re: drag under wet conditions

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pipoloko wrote:
21 May 2017, 19:35
Hey guys anybody knows reports/articles on the influence of moisture air on CD/CL?
Any particular simulator /excel to analyse a coast down test data?
thanks
Technically, CL and CD are fixed coefficients which are independent of outside influences, so they won't change.

I don't know of any studies on it in particular, however, if you would like to know how "humidity" alone influences it decreases air density when compared to the same volume of dry air at the same temperature.

Water vapour is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. So when water vapour increases, the amount of Oxygen and Nitrogen decrease per unit volume due to the water vapour displacing it from the control volume; i.e. density decreases because the mass of the control volume decreases. Oxygen has an 15.999 atomic unit mass while Nitrogen has a 14.007 atomic units mass. Since both these elements are diatomic (O2 and N2), the atomic mass of diatomic Oxygen is ~32 and the diatomic mass of Nitrogen is ~28. Water vapour (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen weighs 1.0079 atomic units while Oxygen is 15.999 atomic units. Water vapour atoms have an atomic mass of 1.0079 + 1.0079 + 15.999 = ~18.0 atomic units. At 18.0 atomic units, water vapour is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). And so at a constant temperature, the more water vapour that displaces the other gases, the less dense that air will become.

This means that from a purely density basis, if all other factors are kept the same, the lift force and drag force exerted on the vehicle will decrease linearly with density.

In terms of actual water droplets in the air and how they hit surfaces as the cars move through the spray etc, there is a small amount of momentum which each droplet carries that is converted into a force upon contact in the opposite direction of travel (broadly speaking). So it isn't really a case that the lift and drag forces will change based on water spray, but rather that the water will become an additional external force acting negatively upon the vehicle.

I have no idea how much this affects performance, but there is at least "some" effect present, despite the impact on tyre grip being substantially more dominating in terms of slower speeds and thus inversely squared reduction in lift and drag forces.
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