[MetroMPG]

For further reading, an excerpt of a discussion found here:

Drag CRISIS! The Coefficient of Drag is NOT a Constant! « Realworldnumbers

(Phil - are you the "Phil" in the discussion on that page too?)

[Re:] the variability of Cd – we all know that it does: it just doesn’t generally vary that much in the range of vehicle speeds that we are interested in. I’d like to add in a few comments:

1. [...] When we test cars (I work with Saloons and SUVs mostly) we see very little variation between 50 km/h and 100 km/h (our usual test speed. At this speed Re will be in the range 106 to 107). Variations are of the order of 0.005, or less, in Cd.

2. At practical road speeds we have very little (perhaps even no) laminar boundary layer -so boundary layer transition is of little relevance. Many (depending on the vehicle) of our separation lines are geometrically fixed; also gaps, seals, finishers etc act to ‘trip’ the boundary layer. Aside from maybe a small region around the bonnet (hood) leading edge radius we don’t really see laminar oundary layers (you do see some re-laminarisation on door mirror casings sometimes). The upshot of this is that we see relatively little change in flow topology in the speed range of interest. Hence Cd doesn’t vary much, as I’ve said.


3. Cd should fall slightly with speed as, for cars, the total drag force that is being non-dimensionalised by area and dynamic head comprises pressure drag, skin friction drag and a small amount of (vortex) induced drag (perhaps). Whilst the pressure drag force scales with the square of velocity the skin friction drag does not: it increases with velocity raised to a power of less than unity. Thus it is being ‘over corrected’, for want of a better term, by the velocity squared term in the denominator of the drag equation. Hence, although the force increases with velocity, its contribution to the total coefficient diminishes. However this variation in Cd will be small as skin friction drag is worth something around 5% of the total for contemporary cars; so a variation in 5% of the total will be small!


4. On the road the car will be subject to relatively large levels of atmospheric turbulence, plus turbulence generated by vehicle wakes and road-side objects. This will also ensure that the flow is post-critical and Cd will not tend to vary much during typical high-speed or constant-speed cruising.


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