62 replies to this topic

[Ben]

Just wondering if people here can clear up some confusion I'm having regarding the density of air vs. the humidity level.

The following is from Dallara's GP2 Aero Manual:

Typically,
Higher Air Temperature by 10° F ( 5.5 °C) reduces downforce and drag by 3.0 %
Higher Air Pressure by 1” Hg increases downforce and drag by 3.0%
Higher Air Relative Humidity by 50% increases downforce and drag by 0.5%

So basically it's saying that humid air is more dense, hence downforce goes up.

However this site:

http://wahiduddin.ne...ty_altitude.htm

Seems to be suggesting that humid air is less dense than dry air.

Who's right?

Ben

[JtP1]

Well, clouds float. So theoretically humid air is less dense.

[McGuire]

The Dryer the air, the Denser the air.

[Canuck]

One need only measure the humidty around PII and correlate that with the well established density in his vicinity.

[Greg Locock]

Air is mostly N2, molecular mass 28. Water is H2O, molecular mass 18. Density of a gas is proportional to its average molecular mass. Humid air is a mixture of dry air and water vapour, a gas.

Dallara is wrong with chocolate sprinkles.

[McGuire]

You can see that relative humidity is not the most straightforward way to measure water vapor content in air, or water vapor's effect on ambient air pressure -- as compared to say, dew point.

For an engine, wet air does at least three things: it lowers air density and therefore total oxygen content; it provides evaporative cooling to the intake charge; reduces peak combustion temperatures and pressures. In many cases within a normal range, the net effect can be an incremental increase in power -- which is not currently reflected in most output correction factors.

Nitro drag racing engines process huge quantities of air, which makes them very sensitive to air quality in general. Tuners there track absolute humidity as measured in grains of water per pound, a mode of measurement once found mainly in the HVAC business.

[Ben]

Originally posted by Greg Locock
Air is mostly N2, molecular mass 28. Water is H2O, molecular mass 18. Density of a gas is proportional to its average molecular mass. Humid air is a mixture of dry air and water vapour, a gas.

Dallara is wrong with chocolate sprinkles.

Good - so is the A1 manual and all Dallara's other manuals. When it comes to properties of air I trust the pilots every time :-) Thanks for the reality check guys.

NB The same error (i.e. wet air is denser) is repeated in Jorge Segers' book. Mainly because he ignores the vapour pressure for the water.

Ben

[phantom II]

Originally posted by Ben


Good -I trust the pilots every time :-) Thanks for the reality check guys.

Ben

[Fat Boy]

I'm honored to have a statement of mine as a sig line. The one that Ben quoted is a pretty good sentiment, but damn, I wish I would have written it so it wasn't so clumsy to read.

[Greg Locock]

Try Mar's law

http://spacecraft.ss...akins_laws.html number 6

[cheapracer]

Originally posted by Fat Boy
I'm honored to have a statement of mine as a sig line. The one that Ben quoted is a pretty good sentiment, but damn, I wish I would have written it so it wasn't so clumsy to read.

It's quite clear and clever actually, kudo's.

[murpia]

Interesting to hear Dallara are wrong, although in my experience many racecar manuals are guilty of over-simplification and over-linearisation...

If I measure dynamic air pressure using a pitot-static tube, will that account for all relative humidity effects, as well as ambient pressure and temperature effects?

Or, if I need to correlate data from a 'dry' to a 'damp' day should I have measured the relative humidity also and applied some other correction factor?

(This is for aerodynamics, not engine power...)

Regards, Ian

[Ben]

Originally posted by phantom II

Not you...

Ben

[Ben]

Originally posted by cheapracer


It's quite clear and clever actually, kudo's.

That's why I liked it - so relevant to the job :-)

Ian - I guess if you measure the pressure differential with a pitot then you'd be measuring something that was proportional to rho*V^2 hence you'd need to calculate the density vs. humidity, pressure and temp. But that's me guessing off the top of my head.

EDIT: Yes it looks like you need to calculate density from pressure/temp/humidity to use the pitot pressures to solve for velocity: http://www.grc.nasa....lane/pitot.html

The fact that the Dallara manual and the A1 manual (John Tavis? ex-Lola and Penske) plus Segers' book suggests it's a common mistake. Makes you wonder if the engine performance increase is canceled out by the drag increase or that the whole effect's so damn small that most people write about it and buy weather stations to look clever and never actually use the info to alter a fat lot (cynical, moi...)

Ben

[murpia]

Originally posted by Ben
Ian - I guess if you measure the pressure differential with a pitot then you'd be measuring something that was proportional to rho*V^2 hence you'd need to calculate the density vs. humidity, pressure and temp. But that's me guessing off the top of my head.

EDIT: Yes it looks like you need to calculate density from pressure/temp/humidity to use the pitot pressures to solve for velocity: http://www.grc.nasa....lane/pitot.html

No, I'm not planning to calculate V from dynamic pressure. In fact, I sometimes calculate rho from V^2 if I have GPS velocity (using a linear regression, it's really helpful to build confidence in your pitot calibration). But, in the main I just use dynamic pressure to correlate downforce run to run without splitting it up into it's components.

What I'm wondering is if CzA (downforce coefficient) is still considered constant if the rho term in my dynamic pressure measurement has a large relative humidity component. We'll assume for now it's not so damp that the water vapour condenses in the low pressure regions around the car. I think I might spot that...

The fact that the Dallara manual and the A1 manual (John Tavis? ex-Lola and Penske) plus Segers' book suggests it's a common mistake. Makes you wonder if the engine performance increase is canceled out by the drag increase or that the whole effect's so damn small that most people write about it and buy weather stations to look clever and never actually use the info to alter a fat lot (cynical, moi...)

Ben

You have the new A1GP manual already?...

Regards, Ian

[Greg Locock]

The way I'd look at it is the wing works in a Bernouilli like way as does the pitot, so the pitot measures the right property.

[phantom II]

You betcha life on it.

Originally posted by Greg Locock
The way I'd look at it is the wing works in a Bernouilli like way as does the pitot, so the pitot measures the right property.

[McGuire]

Originally posted by Ben


Makes you wonder if the engine performance increase is canceled out by the drag increase or that the whole effect's so damn small

Well, the thing about relative humidity is that it can only vary one hundred percent. If you have one standard atmosphere (59 F, sea level, 29.92 in hg baro, zero percent relative humidity) and the relative humidity increases to 100 percent, the relative air density is decreased around .6 percent (equal to around 200 feet in altitude). Watkins Glen has around 80 feet in elevation change. I think Sears Point is around 160 feet.

[RDV]

ooops, late into this thread as too busy and sidetracked into the aptly named idiots thread...
the whole discussion on humidity is something I have never resolved, either by theory or practice..

In theory humid air should entail less drag and downforce, but the effect on car itself is very very difficult to measure on track, other perturbing factors being much greater and we're not even talking of adding the engine performance changes...and separating all the variables is bad enough as it is....in wind tunnel we have controlled(we hope) standard humidity and temp...(sort off I've worked in some real doozies(TM-FBoy))

So generally accept theory is right, and a s racing cars are not mission-critical like planes the difference in humidity not really a practical problem....in the case of dry to rain cases when it can change enormously all the other factors are even bigger(i.e. wet tires, raising the car so you're not surf-planing, etc.)

[murpia]

Originally posted by Greg Locock
The way I'd look at it is the wing works in a Bernouilli like way as does the pitot, so the pitot measures the right property.

Originally posted by RDV
In theory humid air should entail less drag and downforce, but the effect on car itself is very very difficult to measure on track, other perturbing factors being much greater and we're not even talking of adding the engine performance changes...and separating all the variables is bad enough as it is....in wind tunnel we have controlled(we hope) standard humidity and temp...(sort off I've worked in some real doozies(TM-FBoy))

So generally accept theory is right, and as racing cars are not mission-critical like planes the difference in humidity not really a practical problem....

My concern is correlating downforce measurements from different days with different relative humidities, so it is a practical problem for me. This is something that I did a month or so ago: Day 1 was dry and bright and Day 2 was cloudy and the track surface was still damp in the shade. So no doubt Day 2 had more moisture in the air. If I can do a better job with another correction factor in the analysis, I want to hear about it...

The test procedure we follow means engine performance changes are irrelevent, but we rely absolutely on our pitot pressure measurement to correlate run-to-run and day-to-day. It's not always been possible to run the same reference configuration on different days (although we did on the two days mentioned above).

There are some assumptions built into 'standard' Bernoulli - incompressibility and non-viscous flow are the main ones as I understand it. I suspect any effect that relative humidity has on these is rather complex and not practical to condense to a single correction (fudge) factor. Viscous effects primarily affect boundary layers I believe - possibly this would have a bigger effect on a high-drag device like a racecar than a low-drag device like an aircraft? Also I suspect there are lower Cp regions on a racecar than an aircraft, again could condensation maybe have a local effect?

Opinions?

Regards, Ian

[RDV]

murpia-Day 1 was dry and bright and Day 2 was cloudy and the track surface was still damp in the shade. So no doubt Day 2 had more moisture in the air. If I can do a better job with another correction factor in the analysis, I want to hear about it...

....what I said, have had this often, and never managed to be accurate enough to see the humidity in down-force or drag, the macro effects of air density and temp swamping out any measurable humidity effects to any degree of confidence, same for the engine output....what annoys me more is all my engine bods waffle like mad on this....

[JtP1]

I had a discussion with someone who had a Dallara, but he couldn't shed any light on the subject.

Now I don't believe the Dallara manual is wrong and I don't believe the figures given are from purely theoretical calculations or pure track testing. So the figures probably come from a wind tunnel.

One of the problems of a wind tunnel is unless using a full size model, the scale effect of the air enters the equation. There are a set of equations for adjusting the figures to suit the scale effect, the first time I found them was in a simplistic technical manual from 1932. I was looking up Clark Y sections when I found it.

So finding the right person, who knew the right person. I asked "how do F1 teams allow for this effect in the wind tunnel?". The answer is "they thin the air" I suspect by heating, thus Ron's pond at the Paragon centre. Another method would be to raised the relative humidity

To get back to the original point. What I think is really happening, is the car is operating in a ground effect enviroment close to the ground and this throws up the supposed annomaly.

[Charles E Taylor]

Air Density vs. Humidity


All you will ever need to know about the effects of the atmosphere on vehicle performance may be found here.

http://www.spaceagec...ol.com/Litroom2


You may find some of the links extremely interesting, if a little dated





Enjoy yourselves.






Charlie

[SpaMaster]

Originally posted by Ben
Just wondering if people here can clear up some confusion I'm having regarding the density of air vs. the humidity level.

The following is from Dallara's GP2 Aero Manual:

Typically,
Higher Air Temperature by 10° F ( 5.5 °C) reduces downforce and drag by 3.0 %
Higher Air Pressure by 1” Hg increases downforce and drag by 3.0%
Higher Air Relative Humidity by 50% increases downforce and drag by 0.5%

So basically it's saying that humid air is more dense, hence downforce goes up.

However this site:

http://wahiduddin.ne...ty_altitude.htm

Seems to be suggesting that humid air is less dense than dry air.

Who's right?

Ben

I think we are talking about two different issues here. The former one is about humid air (air with increased humidity) and the latter one is moist air. The latter one considers constant pressure thereby replacing air (N2/O2) molecules with H2O molecules which has less molecular weight. So the density would become less.

The former one talks about increasing humidity. It basically mean introducing more H2O molecules to a given environment. Maximum amount of watervapour that can be present in air for a given temperature before they condense (with increased pressure) is related to 100% (relative) humidity. So, if we go from 40% to 60% meaning 50% increase in humidity, it basically means introduction of more H2O, meaning more mass. You should actually see it as increase in pressure. So it would the air more dense.

[Ben]

Originally posted by SpaMaster

I think we are talking about two different issues here. The former one is about humid air (air with increased humidity) and the latter one is moist air. The latter one considers constant pressure thereby replacing air (N2/O2) molecules with H2O molecules which has less molecular weight. So the density would become less.

The former one talks about increasing humidity. It basically mean introducing more H2O molecules to a given environment. Maximum amount of watervapour that can be present in air for a given temperature before they condense (with increased pressure) is related to 100% (relative) humidity. So, if we go from 40% to 60% meaning 50% increase in humidity, it basically means introduction of more H2O, meaning more mass. You should actually see it as increase in pressure. So it would the air more dense.

Any references for that?

Ben

[SpaMaster]

Originally posted by Ben


Any references for that?

Ben

For the latter moist air part, the page you provided itself would suggest what I mentioned. I don't have a reference for the second part, really. It's based on my understanding of increase in humidity. I would try to get some reference for that. But you can see in the link for moist air part, how the writer chooses to use moist air instead of humid air.

[Ben]

Originally posted by SpaMaster

For the latter moist air part, the page you provided itself would suggest what I mentioned. I don't have a reference for the second part, really. It's based on my understanding of increase in humidity. I would try to get some reference for that. But you can see in the link for moist air part, how the writer chooses to use moist air instead of humid air.

I don't really follow. The page I linked to just mentions that density goes down with increased humidity.

Ben

[SpaMaster]

Originally posted by Ben


I don't really follow. The page I linked to just mentions that density goes down with increased humidity.

Ben

Okay, the page has a section "Moist Air is Less Dense...", right? It starts as "As you may have noticed, moist air is less dense than dry air...."

There you will find the following:

According to the ideal gas law, a cubic meter of air around you, wherever you are right now, has a certain number of molecules in it, and each of those molecules has a certain weight.

Most of the air is made up of nitrogen molecules N2 with a somewhat lesser amount of oxygen O2 molecules, and then other molecules such as water vapor.

Since density is weight divided by volume, we need to consider the weight of each of the molecules in the air. Nitrogen has an atomic weight of 14, so an N2 molecule has a weight of 28. For oxygen, the atomic weight is 16, so an O2 molecule has a weight of 32.

Now along comes a water molecule, H2O. Hydrogen has an atomic weight of 1. So the molecule H20 has a weight of 18. Notice that a water molecule is lighter weight than either a nitrogen molecule or an oxygen molecule.

Therefore, when a given volume of air, which contains only a certain number of molecules, has some water molecules in it (which are very light weight), it will weight less than the same volume of air without any water molecules.

So this moist air is strictly replacing nitrogen with watervapour.

Now increase in humidity is different. Let's consider a nice place well in the middle of a plain at a temperature of 30 deg C. Let's say it's relative humidity is 30%. Now, let's say, by some magic, I am able to shift this place to an equatorial coastal place like Singapore at same temperature (30 deg C). Now the relative humidity can be as high as 95%. What does this mean? Water from sea has evaporated into the local atmosphere causing increased humidity. Increased humidity basically means new water molecules have come into the local atmosphere. This is basically increased mass, so increased density. Here, watervapour is not replacing nitrogen, but it is just added to nitrogen.

I think you may be confusing by what the quoted para says by 'fixed number of molecules'. At any given T and P, a given volume can have only a fixed number of molecules, irrespective of the type of gas. This is the moist air case. At any given T, you can keep on introducing watervapour molecules until they would reach saturation and condense. This is the humid air case. We can explain the humid air case differently. For any given amount of watervapour present in the local atmosphere, I can keep on decreasing T until when it starts to condense - which is dew point.

So the humid air case is an open system with inter-connected weather patterns. Moist air case is a closed system proving a unit volume at a given T&P would have only fixed number of molecules irrespective of their molecular weight. Moist air case says : Give me propylene gas or watervapour, either way you can have only the same number of molecules for a given T&P. So the context of both cases are different.

[McGuire]

Originally posted by SpaMaster

I think we are talking about two different issues here. The former one is about humid air (air with increased humidity) and the latter one is moist air. The latter one considers constant pressure thereby replacing air (N2/O2) molecules with H2O molecules which has less molecular weight. So the density would become less.

The former one talks about increasing humidity. It basically mean introducing more H2O molecules to a given environment. Maximum amount of watervapour that can be present in air for a given temperature before they condense (with increased pressure) is related to 100% (relative) humidity. So, if we go from 40% to 60% meaning 50% increase in humidity, it basically means introduction of more H2O, meaning more mass. You should actually see it as increase in pressure. So it would the air more dense.

That does not logically follow. Whether they call it "moist air" or "humid air," both sources clearly cite the same standard to measure it: relative humidity. So there is no difference between "humid air" and "moist air" here --and if there were a difference, it will not be quantified by the standard of relative humidity.

I think we can clear this up by reviewing what is relative humidity.

[SpaMaster]

Originally posted by McGuire



That does not logically follow. Whether they call it "moist air" or "humid air," both sources clearly cite the same standard to measure it: relative humidity. So there is no difference between "humid air" and "moist air" here --and if there were a difference, it will not be quantified by the standard of relative humidity.

I think we can clear this up by reviewing what is relative humidity.

Relative humidity is the percentage of amount of water vapour present to the maximum amount of watervapor that can be present at a given temperature. This maximum amount of water that can be present is given by the saturation vapor pressure at given T.

The moist air section of the web-page does not say anything about humidity, as I saw.

May be, the following section from wikipedia, would help better understanding.

Only the physical properties of water are considered when determining the relative humidity of an air water mixture. Air simply acts as a transporter of water vapour not a holder of it. In fact, water vapor can be present in an airless volume and therefore the relative humidity of this volume can be readily calculated.

[Greg Locock]

My thermodynamics book does not differentiate between moist air, and humidity.

[McGuire]

Originally posted by SpaMaster

The moist air section of the web-page does not say anything about humidity, as I saw.

It is specifically covered in the web page, quoting:

Actual Vapor Pressure from Relative Humidity:

Relative humidity is defined as the ratio (expressed as a percentage) of the actual vapor pressure to the saturation vapor pressure at a given temperature.

To find the actual vapor pressure, simply multiply the saturation vapor pressure by the percentage and the result is the actual vapor pressure. For example, if the relative humidity is 40% and the temperature is 30 deg C, then the saturation vapor pressure is 42.43 mb and the actual vapor pressure is 40% of 42.43 mb, which is 16.97 mb.


Density Calculations:

Now that the actual vapor pressure is known, we can calculate the density of the combination of dry air and water vapor as described in equation 4.

The total measured atmospheric pressure is the sum of the pressure of the dry air and the vapor pressure:

(8) P = Pd + Pv

where: P = total pressure
Pd = pressure due to dry air
Pv = pressure due to water vapor

So, rearranging that equation, we see that Pd = P-Pv. Now we have all of the information that is required to calculate the air density.

As relative humidity increases, air density decreases. That is the long and the short of it. I can almost follow the pretzel logic that allows you to conclude the opposite, but I can't go there with you. We would have to burn all these damn science books.

[McGuire]

Originally posted by Greg Locock
My thermodynamics book does not differentiate between moist air, and humidity.

I think in part our friend is confusing H2O and water vapor. However, often it rains with the relative humidity less than 100 percent. Water droplets that condensed in a region of 100 percent relative humidity in a higher altitude fall straight through relatively dry air at a lower altitude.

[SpaMaster]

McGuire, the passage you posted would calculate the dry air density. Density of the air without watervapour. But the combined density would go up. The total pressure would go up. That's what Dallara manual would have implied by the set-up data, I guess. More pressure meaning more downforce.

Moist air section of the web page calculated density considering both air molecules and watervapour molecules.

[SpaMaster]

Originally posted by Greg Locock
My thermodynamics book does not differentiate between moist air, and humidity.

Personally I don't see any difference either. But I mentioned it to denote the two different sections Ben posted and the different contexts in which they were explained.

[McGuire]

Originally posted by SpaMaster
McGuire, the passage you posted would calculate the dry air density. Density of the air without watervapour. But the combined density would go up. The total pressure would go up. That's what Dallara manual would have implied by the set-up data, I guess. More pressure meaning more downforce.

Moist air section of the web page calculated density considering both air molecules and watervapour molecules.

From the web page:

Moist Air is Less Dense...

As you may have noticed, moist air is less dense than dry air. It may seem reasonable to try to argue against that simple fact based on the observation that water is denser than dry air... which is certainly true, but irrelevant.

Solids, liquids and gasses each have their own unique laws, so it is not possible to equate the behavior of liquid water with the behavior of water vapor.

The ideal gas law says that a certain volume of air at a certain pressure has a certain number of molecules. That's just the way this world works, and that simple fact is expressed as the ideal gas law, which was shown above in equation 1.

Note that this is the gas law... not a liquid law, nor a solid law, but a gas law. Hence comparisons to a liquid are of little help in understanding what is going on in the air, and may simply result in more confusion.

According to the ideal gas law, a cubic meter of air around you, wherever you are right now, has a certain number of molecules in it, and each of those molecules has a certain weight.

Most of the air is made up of nitrogen molecules N2 with a somewhat lesser amount of oxygen O2 molecules, and then other molecules such as water vapor.

Since density is weight divided by volume, we need to consider the weight of each of the molecules in the air. Nitrogen has an atomic weight of 14, so an N2 molecule has a weight of 28. For oxygen, the atomic weight is 16, so an O2 molecule has a weight of 32.

Now along comes a water molecule, H2O. Hydrogen has an atomic weight of 1. So the molecule H20 has a weight of 18. Notice that a water molecule is lighter weight than either a nitrogen molecule or an oxygen molecule.

Therefore, when a given volume of air, which contains only a certain number of molecules, has some water molecules in it (which are very light weight), it will weight less than the same volume of air without any water molecules.

[Ben]

Originally posted by SpaMaster

Okay, the page has a section "Moist Air is Less Dense...", right? It starts as "As you may have noticed, moist air is less dense than dry air...."

There you will find the following:
So this moist air is strictly replacing nitrogen with watervapour.

Now increase in humidity is different. Let's consider a nice place well in the middle of a plain at a temperature of 30 deg C. Let's say it's relative humidity is 30%. Now, let's say, by some magic, I am able to shift this place to an equatorial coastal place like Singapore at same temperature (30 deg C). Now the relative humidity can be as high as 95%. What does this mean? Water from sea has evaporated into the local atmosphere causing increased humidity. Increased humidity basically means new water molecules have come into the local atmosphere. This is basically increased mass, so increased density. Here, watervapour is not replacing nitrogen, but it is just added to nitrogen.

I think you may be confusing by what the quoted para says by 'fixed number of molecules'. At any given T and P, a given volume can have only a fixed number of molecules, irrespective of the type of gas. This is the moist air case. At any given T, you can keep on introducing watervapour molecules until they would reach saturation and condense. This is the humid air case. We can explain the humid air case differently. For any given amount of watervapour present in the local atmosphere, I can keep on decreasing T until when it starts to condense - which is dew point.

So the humid air case is an open system with inter-connected weather patterns. Moist air case is a closed system proving a unit volume at a given T&P would have only fixed number of molecules irrespective of their molecular weight. Moist air case says : Give me propylene gas or watervapour, either way you can have only the same number of molecules for a given T&P. So the context of both cases are different.

I'm not buying it. I'm with Greg no thermodynamic book differentiates between "moist" and "humid" and without a reference I'm just going to conclude that you're wrong - sorry.

Ben

[McGuire]

Originally posted by Ben


I'm not buying it. I'm with Greg no thermodynamic book differentiates between "moist" and "humid" and without a reference I'm just going to conclude that you're wrong - sorry.

Ben

I have to admit I can't follow the poster's path of reasoning.

[SpaMaster]

OK, guys, here we are . It's not easy to get the ideas across on something like this. The context of both cases are different, that's what I meant. That's why Dallara and A1 manual and other boooks may suggest otherwise. While calculating downforce for race car, the guys would worry more about overall pressure/density than dry air density, right? A thermodynamic book would be clear on interpreting the dry air density separately. I believe both are right.

[phantom II]

I have been trying to find an article by John Roncz who has done extensive research on this matter. He has designed over 50 aircraft.
I think moist air is an indication of saturation which is accompanied by a temperature and pressure drop. High humidity can include high pressure such as today in S Florida. 89’f due point 79 and 29.95“hg. In less than a week, all data will be the same except for dew point. In aircraft terms, I will have a longer take off roll today and a reduction in climb rate in measurable terms than next week. Even though my indicated speed is the same, my true airspeed will be higher and a higher power setting will be required because of increased drag. The actual stall speed will be higher even though the indicated airspeed says the same.
Some airfoils will require different trim settings in high humidity and especially moist air and/or in transitions thru critical airspeeds. In some light twins, engine out at VMC can be critical in high humidity conditions. Engine power lower and stall speed higher. Recipe for disaster even if you fly by the numbers.
The wing tip acts as a condensation nucleus and creates saturation which shortens or destroys the boundary layer. The short spans and long chords of race car wings reduces this effect and in most cases we are dealing with unmeasurable negligibilities especially with the span variations in chord and incidence.
The higher the aspect ratio, the more the airfoil is effected by not only humidity, but moist air and rain. In other words, ideal atmospheric conditions are required to perform some aerobatic maneuvers be they low pressure or high, high humidity up to and including saturation, depends on the airfoil and the desired maneuvre.
I think much research is been done in this regard with race cars. All you need is 100th of a second, right?
About doubts that some have that a F1 car can’t drive on a ceiling, my late and dear friend was an aero dynamist at NASA. He mentioned the density change that takes place during airflow which gravity acts upon. Can’t remember the details. Moist air will make it even less possible. If you want a real treat in aerodynamics, seek out lectures by John Ronz and Burt Rutan. Maybe you can write him and ask him his opinions about race cars. He may begin a new career.

Originally posted by SpaMaster
OK, guys, here we are . It's not easy to get the ideas across on something like this. The context of both cases are different, that's what I meant. That's why Dallara and A1 manual and other boooks may suggest otherwise. While calculating downforce for race car, the guys would worry more about overall pressure/density than dry air density, right? A thermodynamic book would be clear on interpreting the dry air density separately. I believe both are right.

[McGuire]

Originally posted by SpaMaster
OK, guys, here we are . It's not easy to get the ideas across on something like this. The context of both cases are different, that's what I meant. That's why Dallara and A1 manual and other boooks may suggest otherwise. While calculating downforce for race car, the guys would worry more about overall pressure/density than dry air density, right? A thermodynamic book would be clear on interpreting the dry air density separately. I believe both are right.

The Dallara material refers specifically to relative humidity.

[Ben]

Originally posted by McGuire


The Dallara material refers specifically to relative humidity.

Quite...

Ben

[SpaMaster]

Originally posted by McGuire


The Dallara material refers specifically to relative humidity.

I know that..

Increase in relative humidity does not mean the pressure is constant. Relative humidity is given for a particular temperature.

But, if we want to keep the pressure constant for a given temperature by replacing air molecules with watervapour molecules, the overall density of the system would decrease.

[johnny yuma]

water vapour DISPLACES molecules of nitrogen,oxygen and trace gases from any given VOLUME of air when humidity is higher.The water vapour is not "added" to the piece of air we inhabit,it must displace heavier gases to occupy its space.

However,if condensation is present such as clouds,mist,fog whatever this is not actually a gas so will be obeying different laws. If this is what Dallara are talking about perhaps they are approaching rightness from a wrong direction.Surely driving at speed through water droplets would increase drag ?

[Ben]

Originally posted by johnny yuma
water vapour DISPLACES molecules of nitrogen,oxygen and trace gases from any given VOLUME of air when humidity is higher.The water vapour is not "added" to the piece of air we inhabit,it must displace heavier gases to occupy its space.

However,if condensation is present such as clouds,mist,fog whatever this is not actually a gas so will be obeying different laws. If this is what Dallara are talking about perhaps they are approaching rightness from a wrong direction.Surely driving at speed through water droplets would increase drag ?

Good answer - The manual talks about relative humidity though so they clearly don't mean condensed water droplets in the air - I still think they just forgot the vapoir pressure of the water or something.

Just rechecked Jorge Segers' book and he's actually got the equation right, but there are just some typos (i.e. numbers) that make the section confusing.

Ben

[gordmac]

Where do the displaced molecules go?
If you boiled water in a sealed room would the density go up?
Essentially are you talking about a constant pressure process where it would seem density goes down or a constant volume process?

[Greg Locock]

That's right, near the ground atmospheric pressure is a constant pressure process (the pressure is identically equal to the weight of air in the column above, per unit area). That's slightly tautological, as it uses an odd definition of weight that includes centripetal effects and the variation of gravity with altitude, but at least it gets the idea across.

[johnny yuma]

Originally posted by gordmac
Where do the displaced molecules go?
If you boiled water in a sealed room would the density go up?
Essentially are you talking about a constant pressure process where it would seem density goes down or a constant volume process?

I think Robert Stephenson understood pretty well if you boiled water in a sealed room (boiler) the pressure would go up way above Atmospheric and push a train along very nicely-but this is because of the change of state from a liquid to a gas in a sealed area,ideally with no air there in the first place.However if you boiled water in a sealed room full of air I would expect density and pressure in that room to rise together.When you opened the door the "new" water vapour molecules would gradually migrate out displacing nitrogen etc thereby making the local air a bit less dense until the vapor condensed again and the nitrogen etc returned proportionally.
The situation in the room is not analagous with vapourisation of water in the atmosphere because only gravity keeps the atmosphere together,and it can expand and contract readily as conditions change.

[McGuire]

Originally posted by johnny yuma
water vapour DISPLACES molecules of nitrogen,oxygen and trace gases from any given VOLUME of air when humidity is higher.The water vapour is not "added" to the piece of air we inhabit,it must displace heavier gases to occupy its space.

However,if condensation is present such as clouds,mist,fog whatever this is not actually a gas so will be obeying different laws. If this is what Dallara are talking about perhaps they are approaching rightness from a wrong direction.Surely driving at speed through water droplets would increase drag ?

Exactly right. And on Pole Day at Indy a few of the sharpest guys are tracking the dew point and watching the cars for vapor trails, looking to trim out the last 1/4 turn of wing. But that is very fine whittling indeed... and it has nothing to do with relative humidity, just as you say, nor does it bear any relevance to road race setup.

Again, relative humidity can vary 100 percent at most, which may equate to a few hundred feet of altitude. The effect on drag/downforce is hardly measurable in a road course setup, as RDV noted. But water is not relative humidity. Relative humidity is evaporated water in the air. Water droplets are condensed H2O in the air... and on the ground. So when we are dealing with a significant amount of ambient moisture, like in a wet setup, first we are not talking about relative humidity. It can be raining with the relative humidity less than 100 percent. And if there is any moisture on the track, the huge decrease in surface coefficient dwarfs any consideration of condensate drag and now the last thing we care about is a quarter turn of wing. Now we are installing treaded tires, unhooking the antiroll bars, and piling on all the downforce we have.

[Monstrobolaxa]

The drag and downforce (lift) formulas are simplified equations...

One of the things that you notice in very humid places like Sepang (90%) and warm temps you have difficulty breathing....more then in a less humid place at the same temperture.

Another thing is that it's harder to breath when it's hotter...then when it's colder! That explains why people with breathing and heart problems have more problems during a hot day then a cold day...

If you look at the viscocity of air when it's hot the viscocity is much higher then when air is cold.

I suspect that the question is not down do air density, but down to viscocity. Which in the simplified equation is down to a change of Cd...

If you run in a more viscous fluid (air/water vapour mixture) you'll have more drag and downforce

Correct me if I'm wrong...