[Dgate]

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

 

[Insightful Trekker]

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 :?:

HTH!

 

[Dougie]

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.

 

[kapps]

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.

 

[ElectricTroy]

Does the Insight even have a "turbelent wake"? I thought the shape of the car formed a very clean "tail" behind the car... hence no need for dimples.

 

[Dgate]

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...

This is exactly contrary to the research that is being carried out at this link
http://www.physorg.com/news11095.html

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 !

DGate

 

[clett]

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.

 

[Insightful Trekker]

And then there are the real world conditions of cross winds etc. Further reducing any advanced aero benefits.

 

[Dgate]

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

 

[kapps]

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.

 

[Insightful Trekker]

Well Dgate,

I hate <sincerely> to keep popping your dimples (back out ), but your own examples should be"troubling" you. Yup :!: Dimples on a golf ball when _driving_ (the long high speed shot) make a difference, chipping or putting - nada. 300 MPH automobiles :?: Yup :!: Greater attention to aero and maybe even dimples would be an improvement. But IMO at 55-75 MPH what your proposing won't be measurable _except_ maybe under controlled conditions.

Its an interesting concept. And with some work may have some specialized aviation applications (certian places on an aircraft). But just how the dimples are applied and the possible consequences of metal fatigue may be the insurmountable problems there.

Good Luck :!:

 

[Dgate]

John the article stated "between" 55mph and 300mph and this is on a golf ball size object not car size which would be different.Less or more :?:

"Any" advantage in the normal speed window would be desirable,after all the ethos of the Insight design was to chip away at everything affecting efficiency.
Each item,LRR tires,negative front end tracking etc etc taken on their own must have seemed trifling but altogether make for impressive results.
If each of these items could be honed to a finer degree(thats why everyone runs higher pressure in their tires :idea: ) its a plus,a bit like an athelete striving for that final fittness plateau.

DGate

 

[clett]

Similar to the dimple proposal is the idea of "riblet" sheets to reduce drag. This concept was discovered when it was noticed that sharks uses these to reduce drag by altering the boundary layer.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=33951

"Sharks pay a large metabolic cost to overcome the drag on their body surface. The skin scales of some sharks possess tiny ridges that run parallel to the longitudinal body axis. The grooved body surface reduces drag through its influence on the boundary layer (10). Riblet sheets, modeled on shark skin, reduced the fuel consumption of an Airbus 320 when placed over the wings and fuselage."

 

[Dgate]

Fascinating article clett which shows we should be looking at nature more for answers on improving our lives and solving some of our current problems.

DGate

 

[nemystic]

Wow,
This is another interesting thread! It reinforces my thinking that Insight owners are the true champions of diminishing returns.
I did a quick search, but unfortunately couldn't find "Invisible Gold's" "wind tunnel" photo where the wind driven snow in a back-yard venturi very graphically illustrated how the Insight has nearly optimal aerodynamics.
The "square back" of most vehicles is inherently inefficient. Simply stated, there's a big low pressure area behind it. The pressure differential effectively increases the drag coefficient.
On recumbent bicycles, a tail sock (sometimes referred to as a rear faring) enables a very significant reduction in overall drag, apparently more so than a front faring.
Some of the most elegant and efficient designs are based on the examples of nature. My Hobie Mirage kayak, with the hydro-sail pedal drive, based on penguin flippers is a fine example.

My Insight has some ugly dents on the left side. These are a result of an unfortunate freak accident inside the garage a few years ago. I haven't repaired them, mostly because it would be quite an ordeal with the aluminum body and the major painting effort that would be required. I've also rationalized it with the "dolphin / golf ball" dimpled surface effect that reduces drag.
Have the dents improved fuel economy? Who knows?
Perhaps anyone who gets 16mpg in a pick-up truck will believe anything that someone who gets 90mpg on the summer commute will tell them.

What's the conclusion from this thread?
'Perhaps one more way to improve MPG: leave your Insight outdoors during a hailstorm. :roll:

 

[Insightful Trekker]

'Perhaps one more way to improve MPG: leave your Insight outdoors during a hailstorm. :roll:

Funny you should mention it.

Kathleen, membername "Sputnik" did just such. IIRC no MPG improvement resulted :!:

 

[Dougie]

The "conclusion" from this thread is that Honda spent a bunch of money to hire aerodynamic engineers who thought about all these questions (and a bunch more that haven't been mentioned yet, such as how to avoid airflow stalls in the radiator opening), did their math homework, and designed a car that has cost to optimal aerodynamic performance.

The biggest and easiest remaining improvements come with impractical costs: Covering the front wheels, closing the underbody wheel openings, reducing the capacity of the cooling system...

More dents aren't going to help.

 

[Guillermo]

I agree with Dougie,
The areas were the most aerodynamic improvements can be made to the Insight is by smoothing out the air flow under the car, redirecting engine compartment air to exit out of strategically placed vent holes in the hood (instead of under the car) then by removing the side mirrors.

 

[sputnik]

'Perhaps one more way to improve MPG: leave your Insight outdoors during a hailstorm. :roll:

Funny you should mention it.

Kathleen, membername "Sputnik" did just such. IIRC no MPG improvement resulted :!:

Yup, about a year ago both Papaya and Silver were caught in two severe hail storms. Papaya had the body damage repaired (paintless dent removal) but little Silver still resembles an odd metallic golf ball.

We did not noticed any increase in MPG in the months following the dimple treatment. Just recently David's MPG has skyrocketed, but that seems to correspond to a change in route to work...he has the road nearly to himself for close to 10 miles, no stop lights and can lean burn most of the way home!

 

[Dgate]

Aaaaah but are the dimples of uniform placement and 30 mm across :?:
plus the glass area has to be dimpled as well

DGate