[Vman455]

Originally Posted by aerohead

1) Most of the 'flow' on an automobile is 'laminar.'

None of the textbooks on vehicle aerodynamics I have claims this. Quite the opposite, in fact; most of the flow around a car is turbulent (aside from a thin laminar layer next to the body surface), and transitions to a turbulent layer based on Reynolds number, which is proportional to the distance from the leading edge of the body. But this is a good thing, since a turbulent boundary layer will follow curves that a laminar one won't!

The laminar state of the boundary layer flow is stable against disturbances for certain conditions only. At a distance x = xtr from the leading edge of the plate a transition to the so-called turbulent state of the boundary layer takes place. The transition between the two states of the boundary layer flow is largely governed by the value of the Reynolds number.... In general, for medium Reynolds number transition from laminar to turbulent occurs in the region of minimum pressure, and with increasing Reynolds number the transition moves upstream.

...

Turbulent boundary layers can withstand much steeper adverse pressure gradients without separation than laminar boundary layers.

(Hummel, Dietrich, "Some Fundamentals of Fluid Mechanics," in Aerodynamics of Road Vehicles, 4th ed., ed. Hucho [Warrendale: SAE International, 1998], 66-68).

Near the front edge, the air flows smoothly with no turbulent perturbations, and appears to behave rather like a stack of flat sheets or laminae sliding over each other with friction, the outer ones moving faster than the inner ones. This type of flow is therefore called laminar flow. Further along, as indicated in Fig. 1.7, there is a sudden change or transition to a turbulent type in which random motion is superimposed on the average flow.

(Barnard, R.H., Road Vehicle Aerodynamic Design, 3rd ed., [St. Albans: MechAero, 2009], 9-10).

Further, researchers recognized as far back as the 1970s that there is an additional problem of cars operating in a turbulent atmosphere, i.e. the free stream flow, rather than being laminar as in a wind tunnel, is in fact turbulent. P.W. Bearman outlined the problem and attempted to begin to address it in his paper "Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies" at the 1976 GM conference:

Road vehicles are exposed to turbulence. generated both by the natural wind and by other vehicles, and under some conditions this may affect their mean-drag characteristics. It is shown that the free stream turbulence can change separation and reattachment positions on bluff bodies by modifying the boundary layer, free shear layer and wake development.

(Bearman, P.W. "Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies," in Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, ed. Sovran et al, [New York: Plenum Press, 1978], 112).

But it is a problem that still vexes engineers. How do we model dynamic atmospheric turbulence--which by its nature is random--in a systematic manner? Various turbulence models are used in CFD analysis, but these are approximations. I don't know of any wind tunnels that have some device for creating free stream turbulence, but perhaps there are some.

Maintaining laminar flow as far back from the front of the car as possible by smoothly rounding the front end is one strategy to achieve lower drag, where laminar flow can be encouraged (given acceptable environmental conditions) before the inevitable transition to turbulence:

Using the techniques of computational fluid dynamics described in the final chapter, it is possible to design a front-end profile that provides a steadily decreasing pressure almost up to the front screen. This not only inhibits separation, but produces a low rate of boundary layer growth, and provides the possibility of producing a significant area of low-drag laminar boundary layer.

(Barnard, R.H., Road Vehicle Aerodynamic Design, 3rd ed., [St. Albans: MechAero, 2009], 80).

Lastly, it is quite easy to observe on the road whether the flow at any point on a car is laminar or turbulent: simply tape wool tufts onto the body surface and then observe their behavior. If a tuft points perfectly in one direction with no movement, the flow is laminar; in laminar flow, pathline = streamline = streakline, and there is no mixing. If it moves, vibrates or fluctuates at all, the flow is turbulent. In fact, now that I think about it, it may be possible to observe the point of transition at the front of the car on a calm day--but you would need a road completely free of other traffic and a day with absolutely no wind.