Re: difference in the strength of cold/dry vs warm/humid win
Posted: Thu Jan 17, 2013 10:02 pm
here's some calcs i did a while back on another forum.
let us use a couple of formula's do calculate that actual dynamic pressure the wind can applied to the kite.
Dynamic pressure is a defined property of a moving flow of gas or liquid and can be expressed as
pd = 1/2 ñ v2 (1)
pd = dynamic pressure (Pa)
ñ = density of fluid (kg/m3)
v = velocity (m/s)
Denisity can be calculated with the idea gas law.
p=p/(RT)
p=Pressure
r=individual gas constantj/kgdegk)
T=Absolute temp K
So. . . . .
16 deg density = 1.22kg/m2
32 deg desity = 1.15kg/m2
lets use 15m/s (30 knots) as a constant is our dynamic pressure calc.
dynamic pressure 16 deg = 137
dynamic pressure 32 deg = 130
Thats 5% difference.
So what else effects the density of the air?
The pressure and the humidity. Pressure is easy.An increase in pressure increases the density.
Humidity is somewhat more confusing. Water vapor is actually lighter than air. So the more humid the air the less dense the air is. .... that i can't be bothered calculating.
The end result is this. Because the pressure increase is 1/2 mass x velocity squared the impact of small changes in temp/pressure/humidity is small. The greatest impact is a change in velocity.
ok i cheated for the humidty calc and found this calculator on the interweb
http://wahiduddin.net/calc/calc_da_rh.htm
The figure i'm interested in is the the density kg/m2.
the change in denisty from 0% dry air to 100% humidity changes dependant on pressure and temp.
But the highest change i could see was a 2% change in density. . ...
Really it means it has very little impact on the final figures.
If we stacked all the variables up in a worst case scenario. so 20Deg temp swing. high pressure dry air at 10 deg c and low pressure moist air at 30deg c then you stack up to a 12% reduction in force at the high temp.
PS Peter,,,, i'm not an engineer so feel free to tear my calcs or logic to peices if its wrong.
let us use a couple of formula's do calculate that actual dynamic pressure the wind can applied to the kite.
Dynamic pressure is a defined property of a moving flow of gas or liquid and can be expressed as
pd = 1/2 ñ v2 (1)
pd = dynamic pressure (Pa)
ñ = density of fluid (kg/m3)
v = velocity (m/s)
Denisity can be calculated with the idea gas law.
p=p/(RT)
p=Pressure
r=individual gas constantj/kgdegk)
T=Absolute temp K
So. . . . .
16 deg density = 1.22kg/m2
32 deg desity = 1.15kg/m2
lets use 15m/s (30 knots) as a constant is our dynamic pressure calc.
dynamic pressure 16 deg = 137
dynamic pressure 32 deg = 130
Thats 5% difference.
So what else effects the density of the air?
The pressure and the humidity. Pressure is easy.An increase in pressure increases the density.
Humidity is somewhat more confusing. Water vapor is actually lighter than air. So the more humid the air the less dense the air is. .... that i can't be bothered calculating.
The end result is this. Because the pressure increase is 1/2 mass x velocity squared the impact of small changes in temp/pressure/humidity is small. The greatest impact is a change in velocity.
ok i cheated for the humidty calc and found this calculator on the interweb
http://wahiduddin.net/calc/calc_da_rh.htm
The figure i'm interested in is the the density kg/m2.
the change in denisty from 0% dry air to 100% humidity changes dependant on pressure and temp.
But the highest change i could see was a 2% change in density. . ...
Really it means it has very little impact on the final figures.
If we stacked all the variables up in a worst case scenario. so 20Deg temp swing. high pressure dry air at 10 deg c and low pressure moist air at 30deg c then you stack up to a 12% reduction in force at the high temp.
PS Peter,,,, i'm not an engineer so feel free to tear my calcs or logic to peices if its wrong.