A few threads ago I made a comment about recent research into aerodynamics on dimpled surfaces producing less drag than smooth surfaces.
Although this research was not directed at golf ball design it prompted several remarks comparing the two.
The below is from a different source relating to golf ball dimples which should substantiate the original statement.
[Wind tunnel experiments show that the dimples on a nonspinning golf ball actually decrease the drag force, allowing the ball to slip through the air with less resistance than a smooth ball, but only at speeds between about 55 mph and 300 mph. Below 55 mph, both the smooth and dimpled balls slip through the air with about the same resistance (drag force).
It is at the critical speed of about 55 mph that the nonspinning, dimpled golf ball passes the critical "Reynolds number"(aerodynamic jargon for "force barrier"), reducing the drag force significantly. The smooth ball goes through a similar force barrier, but at a critical speed of about 300 mph, and at this speed has less drag than the dimpled ball. It is at these critical speeds that the drag tail (turbulence behind the ball) suddenly decreases in size. The separation point of air causing the turbulent tail or wake rapidly moves from a point about 80 degrees from the air flow direction to about 110 degrees around the back of the ball. This is sometimes referred to as "delayed separation" (see illustration on page 6). When this happens, the drag force decreases by almost 40% of that just prior to this critical speed.
The drag force on the golf ball will then slowly increase as the speed increases. A golf ball well-struck off a driver is launched at about 160 mph and lands at about 70 mph. Thus, the ball would be in this decreased drag zone for most of its flight, taking advantage of the phenomenon.]
If we knew the speed (Reynolds Number)at which least resistance occured on the insight it would provide another tool for obtaining max fuel efficiency.
Or could the Insight surface (skin drag) be improved with a dimpled surface ?
DGate
2000 Insight
Mini City El..Electric
87 Cit 2CV
Daihatsu K Class
If we knew the speed (Reynolds Number)at which least resistance occured on the insight it would provide another tool for obtaining max fuel efficiency.
Its genernally agreed that aero is not a significant MPG factor below 40 MPH. Yes a lower CD at higher speeds will reduce the MPG losses due to aero drag. The faster you travel the greater the aero factors significance.
With a uniformly shaped object in flight several other more complex aero factors can be eliminated from consideration. The Insights shape is much more complex aerodynamically.
IMO at its theoretical _best_ dimpling would only slow the MPG loss at higher speeds. But optimal MPG in cars as we now know them will still only be achievable when aero drag is not a significant factor (low speed). Its _always_ a "negative" (MPG consuming).
But its easy to test for yourself Anybody got a ball peen hammer
The Reynolds number for a car, taking into account its entire length, is in the neighborhood of a few million. However, aerodynamics is extremely complicated, and just knowing the Reynolds number for the entire car (or for a local area) is not going to give you useful information.
I'm convinced that the Honda engineers had access to a considerable amount of information that you're just starting to poke at, and that if there was utility in putting dimples on the car, they would have done it.
For example, the Honda Dream solar cars in the 1990s were extremely advanced, and obviously came out before the Insight was designed. They were designed to have absolutely the best possible aerodynamic performance. An excellent reference is "The Leading Edge" by Goro Tamai.
Besides, you don't have to do this experiment, all you have to do is look under your front bumper, where there are little bumps that do the same thing as the dents in golf balls.
Dimpling will work only on certain areas. Sphere's are the objects most affected by it. I see you understand the critical Reynolds number where the laminar flow in the boundary layer begins to break down and become turbulent. This reduces the "form drag" of the sphere. Form drag is what's caused by the turbulent wake. The other type of drag is "skin friction." This is the actual friction between the fluid and surface and is usually less than form drag. While dimpling reduces form drag, it increases skin friction drag. On an object that has been designed to be very aerodynamic, adding dimples may not do much to the turbulent wake but it will still increase the skin friction drag, thus increasing total drag. It's all very complex and needs a lot of windtunnel testing to work out.
__________________
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All cars have a turbulent wake even if they are the perfect teardrop shape as in over long salt flat record breakers because they sit on wheels, this alone upsets the air flow.
kapps writes:
While dimpling reduces form drag, it increases skin friction drag...
Which is why I originally posted this questioning our normal perception of aerodynamics.
Maybe the golf ball comparison is not legitimate and has gotten everyone off track.
If this research idea proves correct it would completely change our idea of designing for less drag.Just because it has been done a certain way in the past does not mean there is no room for breakthroughs.If we accept the past as written in stone we stand still !
One way of reducing the form drag, to mimic the events that occur above the Reynolds threshold, is to force carefully directed jets of compressed air from nozzles at the rear of the vehicle. It was shown to effectively make the vehicle much "longer" (aerodynamically speaking, which improves Cd), there was less turbluence in the wake and drag from low pressure at the rear was reduced a lot. This tech was developed for trucks but (I think) the IP was bought out by F1 teams.
John this is true but also applies to the nut behind the wheel,hills,weather etc, we must however strive to make the car as aerodynamic efficient as poss.
Any improvement is a positive even after deducting the negatives of the real world environment.
Clett writes [One way of reducing the form drag,is to force carefully directed jets of compressed air from nozzles at the rear of the vehicle]
In fact the tri engined Dassult Falcon jet was designed with this in mind so the center jet exhaust thrust fills this wake area thus eliminating it and reducing drag.
Dgate, your right about the differences between spheres and other surfaces. On sphere's, you want to break down the boundary layer at as low a speed as possible. That's what reduces drag. The article you posed says that they used roughness to increase the speed at which the boundary layer breaks down. The aerodynamics off an airfoil mean you want to keep the laminar flow as fast as possible while on sphere's, you want to get rid of it.
__________________
-2000 NF Red
-64.6 LMPG
-JVC El Kameleon HU, JVC 12 disc changer, Infinity Kappa 3-ways up front and Alpine 2-ways in the rear
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What would be the Insights "Reynolds Number"?
Anybody got a ball peen hammer 
