Pursuit watts/cda?

[acoggan]

Originally Posted by AndyFroncioni

Use iAero/iBike.
iAero does tend to "clean up" some Aerolab files, but not ever as much as I'd like. If your out-and-back is straight and almost perfectly aligned with the wind direction, you might get ok results. But how common is that?

Well that depends on where you test, now doesn't it? (I picked my "natural wind tunnel" because it is nicely aligned with the prevailing wind in that immediate vicinity...indeed, it has to be, because it is just that, i.e., a tunnel through the trees.)

Originally Posted by AndyFroncioni

3) Use a numerical "zippering" approach for out-and-backs.
Pretend that at every point in an out-and-back, the wind term is (CdA*0.5*RHO*(v-w_i)^2) going out and (CdA*0.5*RHO*(v+w_i)^2) coming back. w_i is the wind at point i.

This way, you could theoretically start from the turn-around point and "zipper" the "out" ve curve with the "back" ve curve, one point at a time. I've attached the out and back file.

Unfortunately, I've not had a lot of luck with this yet.

FWIW, I've been doing essentially as you describe with data from regression tests almost from day 1, and have found that it works quite well as long as the wind is aligned with the test segment as described above.

That said, it is a bit of a moot point, because pure head/tailwinds have little effect on the estimated CdA value, at least if you do runs in each direction.

[AndyFroncioni]

Originally Posted by acoggan

Well that depends on where you test, now doesn't it? (I picked my "natural wind tunnel" because it is nicely aligned with the prevailing wind in that immediate vicinity...indeed, it has to be, because it is just that, i.e., a tunnel through the trees.)

Ah, clever. Crap -- I wish I'd thought of that! :-)

Yes, I guess there are a lot of cases where it might be possible to use the natural terrain to "shape" the wind. I find any kind of wind difficult to manage, but my test venues are quite open to wild yaw variations.

Headwinds, btw, tend to give you a virtual elevation that's too high by a factor of:

h = 0.5 * CdA * rho * v * ( v+w)^2 * T / ( m * g ) ,

where
v = ground speed
h = the virtual additional height
w = the headwind
T = time spent riding at the given headwind

Example: CdA = 0.300, rho = 1.2, v = 10 m/s, w = 5 m/s, m = 80kg, T= 10 s

gives us a virtual elevation height of:

h = 1.72m

In other words, 10s into a 5m/s headwind gives you a false virtual elevation of 1.72m.

I've used this with a bad file, some historical wind date, and Google Earth to estimate the CdA for Tony O'Keeffe ( TONY O'KEEFFE - TRIATHLETE - HOME ). I don't have an error estimate for this approach, though. I'm just saying that sometimes, you have to get creative.

Cheers,

Andy

[AndyFroncioni]

Originally Posted by acoggan

Well that depends on where you test, now doesn't it? (I picked my "natural wind tunnel" because it is nicely aligned with the prevailing wind in that immediate vicinity...indeed, it has to be, because it is just that, i.e., a tunnel through the trees.)

Ah, clever. Crap -- I wish I'd thought of that! :-)

Yes, I guess there are a lot of cases where it might be possible to use the natural terrain to "shape" the wind. I find any kind of wind difficult to manage, but my test venues are quite open to wild yaw variations.

Headwinds, btw, tend to give you a virtual elevation that's too high by a factor of:

h = 0.5 * CdA * rho * v * ( v+w)^2 * T / ( m * g ) ,

where
v = ground speed
h = the virtual additional height
w = the headwind
T = time spent riding at the given headwind

Example: CdA = 0.300, rho = 1.2, v = 10 m/s, w = 5 m/s, m = 80kg, T= 10 s

gives us a virtual elevation height of:

h = 1.72m

In other words, 10s into a 5m/s headwind gives you a false virtual elevation of 1.72m.

I've used this with a bad file, some historical wind data, and Google Earth to estimate the CdA for Tony O'Keeffe ( TONY O'KEEFFE - TRIATHLETE - HOME ). I don't have an error estimate for this approach, though. I'm just saying that sometimes, you have to get creative.

Cheers,

Andy
andyfroncioni.com