Does downforce increase when it rains?

[bhall II]

All things equal, yes, downforce (and drag) increase in the rain. This is because it raises the fluid (air) density
represented by 'p' in the Lift coefficient and Drag Equations.

Avogadro's law: equal volumes of gases at equal temperature and pressure will have an equal number of molecules.

That means the water molecules associated with humidity must displace other molecules in the air. This reduces air density, because the molecular mass of water (~18) is lower than that of the molecules displaced, principally dinotrogen (~28) and dioxygen (~32).

[ChrisDanger]

All things equal, yes, downforce (and drag) increase in the rain. This is because it raises the fluid (air) density
represented by 'p' in the Lift coefficient and Drag Equations.

Avogadro's law: equal volumes of gases at equal temperature and pressure will have an equal number of molecules.

That means the water molecules associated with humidity must displace other molecules in the air. This reduces air density, because the molecular mass of water (~18) is lower than that of the molecules displaced, principally dinotrogen (~28) and dioxygen (~32).

When we calculate drag and use , is this assuming zero humidity? The above (correctly) suggests that air density decreases with humidity, but the vehicle is still travelling through a fluid/air mixture, not just the remaining air. Common sense says that the more water there is in the air, the more dense the mixture will be. Surely? At the one extreme imagine driving through pure water. Your downforce (and drag!) will be huge. Does this not suggest that increasing humidity will result in greater downforce (all else being equal, which they obviously wont be)?

[hollus]

Every molecule of water as gas displaces a molecule of oxygen or nitrogen, and both were heavier than the molecule of water, hence the density goes down. Up to more than one percent, actually.
If the water is part of the gas (moisture), it reduces its density, if the water is liquid, then it is not part of the gas and it might as well be solid. Would you say that downforce increases driving through a sandstorm? Curiosly, it probably does.

[roon]

At the one extreme imagine driving through pure water. Your downforce (and drag!) will be huge.

At which point I will pose the question: Does driving through bubbly water reduce downforce?

Hollus, I like your analogy of driving through a sandstorm. A different way to visualize the question at hand.

[Paul]

If we compare humid air with rain, I'd say downforce should increase as heavy (compared to air) water droplets are deflected upwards and thus must create downward force acting on the vehicle. How those droplets affect aerodynamic elements, however, seems more difficult to comprehend. Would an airfoil soaked in water change its characteristics, would an important vortex generator work as intended- that I'd like to know.

[roon]

Paul, I mostly agree with you. We might also consider that some of the raindrops will not even hit the car, but be carried with the airstream. The full force of a falling drop might never meet the car, or at least will be influenced by its local airstream before impact.

Regarding vortex formation, raindrops might have a negative influence, if we consider that they could have a dampening effect (no pun intended). Each raindrop imposing drag, a braking force, taking away energy from the air.

[Gatecrasher]

Every molecule of water as gas displaces a molecule of oxygen or nitrogen, and both were heavier than the molecule of water, hence the density goes down. Up to more than one percent, actually.
If the water is part of the gas (moisture), it reduces its density, if the water is liquid, then it is not part of the gas and it might as well be solid. Would you say that downforce increases driving through a sandstorm? Curiosly, it probably does.

A mole of H2O (18mL) and one of N2 (22.4L at STP) do not have the same volume, so you can't use this to assume density.

[hollus]

To summarize all answers: The rain as liquid will not produce downforce the way a gas does. It will by hitting the car while moving downwards and being projected upwards by impact, that adds some grams of downforce, less than 0.1% of what the car normally produces.
To make this, you pay many costs: grip, mechanical, shape changes, possible disruptions to airflow and, in most cases, the price of higher humidity and lower barometric pressure, both of which reduce density and thus gas flow related downforce. The cost of each of these is likely to be more than 0.1% of the downforce.
At least that is how I see it.
Note: humidity = water in the gas phase.

[Andres125sx]

To summarize all answers: The rain as liquid will not produce downforce the way a gas does. It will by hitting the car

Far from an aerodinamicist here, so take this with a grain of salt, but I was thinking about this as well.

Rain drops are not a gas, so they´re not flowing around the wings, flaps, etc. but hitting them and as Hollus pointed being proyected, so then only the gas continue flowing around the wing, flaps, etc. So the car is actually acting like a filter, taking the water out, thus reducing air density.

And then we should think that proyected water will be affecting the airflow and reducing aero efficiency. Add to that the water wich stick on the car is also reducing wings, flaps. etc skin smoothness and increasing... (sorry don´t know the english term, boundary layer drag maybe?), and reducing aero efficiency further

[zac510]

This is going to be one of those "will the airplane take off" kind of threads isn't it?

[Andres125sx]

This is going to be one of those "will the airplane take off" kind of threads isn't it?

No, it´s not going to be, it is

[Nickel]

All of those stating an increase in air density: pardon?

Rain is associated with low pressure systems. It's like being at a slightly higher altitude. Many of you have it backwards.

[Shooty81]

Let's assume the droplets follow the air around them ideally, then we can do some math:

3.6 mm rain per hour or 1gram per second and square meter

Falling at 6m/s

Means 0.167 g per cubic meter of falling rain in the air or 0.000167 kg/cubic meter.

Compare that to air density of 1.2 kg/cubic meter, and you see the effect is not very big (0.01%).

Probably the effect of less ideal surfaces when they are wet will be bigger.

So I think downforce will be a bit less in the rain.

[J.A.W.]

To summarize all answers: The rain as liquid will not produce downforce the way a gas does. It will by hitting the car

Far from an aerodinamicist here, so take this with a grain of salt, but I was thinking about this as well.

Rain drops are not a gas, so they´re not flowing around the wings, flaps, etc. but hitting them and as Hollus pointed being proyected, so then only the gas continue flowing around the wing, flaps, etc. So the car is actually acting like a filter, taking the water out, thus reducing air density.

And then we should think that proyected water will be affecting the airflow and reducing aero efficiency. Add to that the water wich stick on the car is also reducing wings, flaps. etc skin smoothness and increasing... (sorry don´t know the english term, boundary layer drag maybe?), and reducing aero efficiency further

Yet some of those 'shimmering' paint finishes that 'POP!' to the eye, also appear to shed water very effectively.. so not looking so 'dirty' - post wet race..

Fluid dynamics is a complex issue,& very small droplets ( as spray) ( & in ambient racing temps)..
do likely/mostly conform too, as they are compelled to follow the vanes/strakes/airfoils etc,
along with the aero-flow - as it pressure/velocity streams, due to these devices - accordingly.

[PlatinumZealot]

Rain drop are a contaminant that disrupt the design flow structures. There is absolutely no way it increases down-force.