bradl016 wrote:
So, with the extensive physics that my botany undergrad taught me -
Aren't there two types of air flow? Laminar (some drag) and turbulent (heavy drag)? Turbulent occurs more frequently at high speeds, right?
So, do these assumptions of the time saved at 30 kph adjust for the smaller turbulent drag at lower speed? Is that represented by the exponential curve for wind resistance? Does aero equipment work to control laminar air flow from becoming turbulent?
Hmm. Now that I'm thinking of it, maybe that's why one poster said that you save more time 'to a point'. After that point, the air flow is so smooth that TT equipment doesn't do anything.
Sorry to spout of a series of inane questions. Your question got the old noodles thinking.
You're sort of on the right track.
As cyclists we travel through largely laminar air flow. But our bodies, our bikes, parts of our bikes like the leading edges of wheels, handlebar and tubes and even things like exposed brake housings and cables disturb that laminar flow and create turbulence on their trailing edges. It's the pressure differential between that laminar flow striking the leading edges and the lower pressure turbulent wake behind the trailing edges along with some viscous skin drag of the air flowing around each object that creates air resistance.
Depending on the dimensions and shape of each of those items the turbulent region stretches out like an eddy or wake behind the trailing edge and eventually the flow 'reconnects' and becomes largely laminar again. The cross sectional area or actually three dimensional volume of that turbulent region dictates how low that trailing edge pressure gets and to a large extent how large the overall pressure differential or IOW the large part of the aero drag. Airfoils and other oblong high aspect ratio shapes help minimize the trailing edge turbulence which minimizes the pressure differentials and of course the aero drag.
So yeah, laminar vs turbulent flow is a big part of the aero puzzle. But at the speeds we travel the surrounding air is largely in a laminar flow region for all of us, fast or slow and all of us generate trailing edge turbulence with our bodies, bikes and individual parts of the bikes. That doesn't really change for fast vs. slow riders but in certain situations, particularly crosswind situations there are differences related to the effective angle that the apparent wind (vector sum of our velocity relative to the ground and ambient wind velocity) strikes us or the yaw angle. For the same partial or full crosswind faster riders will experience lower yaw angles or more direct effective headwinds and slower riders will experience greater yaw angles (more of a crosswind). That impacts aerodynamics as components like deep section wheels can be designed to perform better in pure headwinds or better in high yaw angle situations.
So it's not like fast riders get less relative advantage from an aero position, they get plenty of advantage relative to their speed. It's just that slower riders get more of an absolute time savings because they're out there for so much longer. In the example given above both riders saved roughly 5.5% from their slower time with the decreased CdA, it's just that 5.5% of the larger number (the slower rider's time at higher CdA) is more total seconds or more absolute time.
-Dave
[edit] typo...
is proportional to 
instead of 
"