Exhaust Flow and Aero

[superford317]

 Now this is where all of the bad things start to happen, the free stream air flow treats the boundary layer as if it were the actual vehicle surface, so when the boundary layer grows in size as it becomes turbulent and wants to totally separate from the vehicle surface and become even larger, drag increases dramatically and at the same time pressure drag increases behind the vehicle.
Even though turbulent boundary layers cause more drag than laminar boundary layers because of higher skin friction, it moves the point at which the boundary layer would otherwise separate from the vehicle causing lower drag pressure, it is still much better than the boundary layer becoming totally separated from the vehicle surface. The boundary layer if controlled can cause a reduction in drag. The longer the boundary layer can be forced to stick to the vehicle surface and conform to the surface shape before separating, the drag will be lower, even if it is only one inch on each side and one inch at the top you will still have a lower drag than if it were allowed to totally separate.
By re energizing the slowing boundary layer and forcing it to be turbulent for a longer period it can delay the flow separation and even finesse it to become reattached to the vehicle surface, thereby lowering the vehicle drag.
To get the best drag reductions, the size of the riblets will have to be tailored to the boundary layer thickness and the speed of the vehicle and the pattern of the riblets on the vehicle surface will have to be a random pattern , if the riblets are in uniform rows the drag will actually increase dramatically.
What the riblets do are, turbulence is increased on the peak of the rib but laminar flow is maintained in the groove and even though the surface area is increased the shear stress is lowered.
The riblets will lower the thickness of the boundary layer and reduce the surface turbulence. The effectiveness of the riblets to reduce drag can be on the order of 5% to 14% depending on many variables from air density, vehicle speed, shape and to how clean the riblets are, to work properly the grooves need to be very clean. In a turbulent boundary layer riblets can reduce friction drag to a level below what a flat plate is.
In the early 1980’s the initial R&D for the riblets was done at NASA langley research center. When the 3M Company read the research papers they started development of a riblet adhesive film that could be applied over surfaces. The adhesive film was named “Scotchcal MarineDrag Reduction Tape”  the first known use of the 3M drag reduction tape was in 1987 in the Americas Cup on the ship piloted by Dennis Connor on the ship “The Stars and Stripes” after winning 4 straight races the rules were changed and the riblet tape was prohibited.
3M does not sell the tape or market it, the price of manufacture was said to be to cost prohibitive so production ceased.
I have used vortex generators, turning vanes, boundary layer suction, surface riblets and .040 air jets blowing at 50psi, all to keep the boundary layer energized and attached to the vehicle surface to reduce drag.

[joea]

"""I have used vortex generators, turning vanes, boundary layer suction, surface riblets and .040 air jets blowing at 50psi, all to keep the boundary layer energized and attached to the vehicle surface to reduce drag"""

how did the vehicle (s) perform before compared to before after..ie what kind of results..?..

curious..

[SPARKY]

lol    is that #2 or #3 joe

[joea]

...i told my wife.."ill love you no matter how big your butt might get"...

doesnt mean it worked real well.....

i guess i like to know abit about how the f' something turned out....

[superford317]

Streamliners and golf balls are very different. Pressure drag behind the ball causes more drag than friction drag. By upsetting the boundary layer, a turbulent flow is set up, that actually weakens the effect of flow separation. The wake shrinks, thus drag drops.
Streamliners have an efficient shape and getting laminar flow with minimum separation over the body is the goal. Golf balls are spheres and have an inefficient shape. Dimples can make a golf ball travel up to twice the distance.
It all depends on whether moving the point of flow separation will give a drag reduction, and whether making the boundary layer turbulent will move the location of the separation.  A car with a sharp corner somewhere near the back will cause the flow to separate there, whether it is laminar or turbulent, so forcing it to be turbulent cannot reduce drag. The shape of a streamliner is much different. There could well be regions on the streamliner where the separation point could be moved further back through the use of dimples or some other kind of device, but the drag reduction would certainly not be as dramatic as it is for a sphere. Shear stress is higher in turbulent flow than in laminar flow. Unless you can benefit by moving the separation point back, the dimples cause a drag increase, rather than a decrease. Golf balls and streamliners operate at Reynolds’s numbers that are much different.
The dimples on a golf ball create turbulence. Without them the air would lose energy and actually separate from the golf ball and increase drag dramatically. The dimples Add energy to the air and it stays attached to the golf ball all the way around. This actually reduces the drag.
The dimples disturb the boundary layer so that airflow separation happens further down, the turbulent air has more energy and the wake is smaller.
Drag on a sphere is primarily created by pressure drag on its back half that is created because the boundary layer separating and forming a wake and creating a low pressure area behind the sphere. If we could somehow minimize that separation, the drag would be significantly reduced. The boundary layer is a thin layer of air that lies very close to the surface of a body in motion. It is within this layer that the pressure gradient develops that causes the airflow to separate from the surface.
The reason we do not see dimples on streamliners is that these only work well on non streamlined bodies. The primary form of drag on these kinds of shapes is caused by pressure drag. Streamliners are dominated by friction drag. These streamlined bodies, have a shape that creates a much more gradual pressure gradient. This promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.

[SPARKY]

YEP reincarnation is a fact!

[dw230]

I'm confused(daily state), the OP claims not to know about our classifications yet, the signature claims a 1000 HP lakester build in progress.

DW

[maguromic]

Dan (Grasshopper), Your confusion will go away when you walk on the salt without leaving a foot print.  Tony

[Stan Back]

Soggy is good?

[superford317]

As i stated in some of my preavious posts, i have never been to a LSR race, never seen the cover of a rule book. All i know is what i see from this forum and the pictures.
With many years spent at the track, more time spent in a class room than i prefer, tunnel testing and CFD. one can make very well educated assumptions and observations.
As someone in an earlier post hinted at, you don’t have to read any of this or you can assume it is all a lie, do your own research and gather your own opinions.
As evidence, 4600 hits in the past month must mean someone is interested and think it is worth reading.
Aerodynamics is aerodynamics.
i am building a bellytank for enjoyment and the thrill of the speed, i dont know about the classifications.
later i may try to run in a specific class for a set speed goal.
Some day i would like to travel to the great white dyno but will probably never race outside the ECTA.
Since i began my work for the US government in iraq i have been to the USA for 18 days since 2006, i havnt turned a wrench on my tank since then.

[superford317]

The riblets should not be placed on the entire vehicle surface, that will only worsen the aero problems. Some thought and research will have to be done to determine where they will need to be placed.
The riblets are only miniature vortex generators, depending on the application they are not much more than scratches on the vehicle surface.
Some sort of testing will have to be done to determine the optimal size of the riblets, wind tunnel testing, CFD or tuft testing and using trial and error. There are many variables that will determine the size of the finished riblets, the final deciding factor is the height of the boundary layer.
The riblets should be between 75% and 85% of the height of the localized boundary layer thickness they are placed in. On a vehicle with numerous boundary layer separation problems and locations, there will be many different sizes of riblets.
Vehicle speed, shape, air density, surface texture and surface angle of attack, along with a list of other minor details, will determine the height of the boundary layer and the riblet will be designed to fit the particular boundary layer it will be placed in.
Removable body panels can be constructed with different riblet designs that are optimized for a specific track and speed range, with a specific air density range in mind. 
The optimal distance the riblets layout should be placed from the transition point where the boundary layer begins to separate from the vehicle surface, should be between 4 and 6 times the height of the boundary layer.
A universal size riblet can be used but the efficiency will be greatly reduced, a generic riblet may have the effectiveness of 10% to 30% of a designer riblet. Depending on how lucky you get.
As long as the riblets are placed in a very random pattern and not in organized rows and placed reasonably close to the transition area some gain will be noticed.
On UAV’s that had equipment attached to them later that had not been in the original design from the factory, designer riblets made a 8% reduction in fuel consumption or a gain in top speed of 3% to 5% and thereby increased there loiter time.
On track performance gains from the use of riblets will vary with the down force package used and the particular track with top speeds attained, the number of turns and the length of the straights, as well as all of the other variables mentioned above. In a particular race where the aero package was pushed beyond what it was designed for, the use of pre planned body panels with designer riblets brought the fuel consumption and speeds back into previous parameters. 
The more research and development a particular vehicle has done to its aero package the less need there will be for the riblets as the body shape and design have already been optimized.
When things are attached to a vehicle surface that were not in the original design criteria and start to affect the aerodynamics and upset the air flow, that is when localized use of the riblets can be used to help bring the airflow back into order.

[SPARKY]

Brings to mind somthing that PP told me one time---you can have the millions you spent trying to achieve perfection in the wind tunnels, nullified by the sun hitting one side of your car and the panel distorting 

[jl222]

 Building a bellytank without a rule book but does research on aero

  Name some cars or your BS.


             JL222

[Blue]

 Building a bellytank without a rule book but does research on aero

  Name some cars or your BS.


             JL222

It's pretty clear that "superford317" is an external payload integrator on UAV's.  He deals in a lot of separation effects.  I would welcome him coming to LSR and improving the knowledge base of aerodynamics.  However, like John, I think he needs a little more time applying these ideas to ground vehicles before making blanket statements.  LSR and UAV's move in the same air, yet suffer many different effects stemming from their different mission requirements.   

As far as suction or blowing, this is called "active circulation control" in the industry.  We've been working on it for 70 years, and the effects and maintenance issues are well quantified.  A short summary of the current state of the art:

1. Suction is not worth the maintenance/reliability of the slots and/or holes.
2. Blowing is capable of massive increases in lift in takeoff/landing configurations and is not worth the power in cruise conditions.
3. Achieving efficient blowing effect (C_u) with low temperatures and power levels is the current frontier.  Current studies are focused on pulsed jets to improve the circulation control effects vs. power required.

It is possible that LSR vehicles could use active circulation control to affect separation and make significant reductions in drag.  It would require a lot of full scale testing with tufts and an understanding of jet effects.

[superford317]

"BLUE" you are very close in your statement
Before my present line of work, i spent considerable time in 3 different racing leagues before i was approached for my present work.
in your post you are hitting on my present work that i am prepapring for my next line of posts for land racing.
i have read a lot of your posts BLUE and you seem to be very knowledgable also.