Aerodynamic Streamlining Template Part-B

[aerohead]

Background: For a "streamlined car",we can turn to the man who first used the term in 1922,Paul Jaray.Jaray was an aeronautical engineer with the Zeppelin Werke,involved in airship design which includes "Los Angeles" built for the U.S.under the Versailles Treaty.--------------------------------------------- ( from Hucho ) It was Jaray who recognized that the flow around a body of revolution ( spheres,cones,ellipsoids,cylinders,etc.) of very low drag in free air,is not axially symmetrical close to the ground.As a result,the drag increases.--------------- Where ground clearance approaches zero,the optimum shape for a low drag is a half-body,which forms a complete body of revolution together with it's mirror-image produced through reflection from the roadway.THIS IS EXTREMELY IMPORTANT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!! Refer to Figure 4.118------------------------------------------------------------------------ Also from Hucho,is a table which shows one of the lowest Cd belonging to a "streamline body" of Length/Diameter ratio of 2.5:1,@ Cd0.04.This image is from Hoerner's book of 1965,which is also in his 1951 book,and comes from : Goethert,Drag Measurements on Streamline Bodies in the High-Speed Tunnel of the Deutsche Versuchsanstalt fur Luftfahrt,Berlin,Zentrale fur Technisch-Wissenschaftliches Berichts 1944,p.377.------------------------------- I've taken this profile as the "minimum" for use in the template as it is drag-free(profile) and has the minimum wetted area to also minimize friction drag.-------------------------- The linear progression combines mirror-image of the 2.5:1 streamline body to create the "pumpkin-seed" Jarayesque form above the ground,with curves just "slow" enough to guarantee arrached flow.------------------------------------ You will see Fig.4.119 from Hucho which illustrates a 1966 VW Beetle in "mirror" and will notice that from the graph,at super-critical flow,the ellipsoid of lowest drag also occurs at a fineness-ratio of 2.5:1.------------------------- From Marchaj' book I've included a table of "sections" which includes a drag table showing form-drag and friction drag combining for "Total drag". You will see that a drag minimum occurs for wing-loke structures at a fineness ration(aspect-ratio/thickness-ratio) of 3.92:1.----------------------------- I've included some tables from Fluids texts which shows some forms and their respective drag coefficients in both 2-and 3-dimensional flow,at varying Reynolds Number.---------------------------------------------- I placed Sunraycer under the template,it was a perfect match.Bill Watson's airship "White-Dwarf" of 1984 is about 2.57:1 ratio.Watson worked with Burt Hibbs,aerodynamacist for Sunraycer.Both of AeroVironment.-------------------------------------------------- This shape seems to end up on some of the most efficient vehicles known in the world,I believe it to be a shoe -in for aero-modding.-------------------------------------- Those with more advanced aerodynamic toolbags will will no doubt venture out into some of the more exotic shapes,but for amateurs,I believe this form can serve us well.------------------------------------------ Good design and engineering calls for the bare minimum to get the job done.Using the minimum will guarantee lowest weight,most efficient use of material and energy and lowest economic and environmental impact.

[jcfdillon]

As a kid I was fascinated by paper airplanes for a while, and I noticed then that the tube shaped paper airplanes were the ones that traveled the furthest, had the most stable flight, and followed the straightest path, out of all of the paper airplanes I tried. So, my question is whether anyone has used large straight air tubes or air channels for cars, aligned precisely in the direction of travel? I am not sure if a round tube or square tube would be best, but inside needs to be straight and smooth. Thank you.

[aerohead]

Originally Posted by jcfdillon

As a kid I was fascinated by paper airplanes for a while, and I noticed then that the tube shaped paper airplanes were the ones that traveled the furthest, had the most stable flight, and followed the straightest path, out of all of the paper airplanes I tried. So, my question is whether anyone has used large straight air tubes or air channels for cars, aligned precisely in the direction of travel? I am not sure if a round tube or square tube would be best, but inside needs to be straight and smooth. Thank you.

For drag reduction,the point of separation must be moved rearward along a path which maximizes boundary layer attachment while reducing the body cross-section to convert kinetic energy into static pressure behind the body.
The 'Template' does this very thing.
If you rob air from the outside of the body and allow it to flow through,you kill off the very kinetic energy you'd be harvesting to protect the boundary layer.
with a straight tube you'd be doing nothing to reduce the body cross-section,and internal skin-friction along the inner wall of the tube would be killing off kinetic energy,converting it to heat,which is necessary to dilute the low base pressure behind the tube.
Then there's the issue of interior space within the vehicle (occupants,luggage,car seats,groceries,etc.).

[freebeard]

I noticed then that the tube shaped paper airplanes were the ones that traveled the furthest, had the most stable flight, and followed the straightest path, out of all of the paper airplanes I tried.

It sounds like the question assumes the benefit would be stability. The paper airplane is in free air. A vehicle is moving through turbulent air next to the ground surface. That changes things.


Some benefit may come from a short duct (less skin friction) to move air from a high pressure to low pressure area. There are cross-sections between round and square.

[aerohead]

Originally Posted by freebeard

It sounds like the question assumes the benefit would be stability. The paper airplane is in free air. A vehicle is moving through turbulent air next to the ground surface. That changes things.


Some benefit may come from a short duct (less skin friction) to move air from a high pressure to low pressure area. There are cross-sections between round and square.

Do we know if the lower image is an actual photograph,or an air-brushed rendering depicting an anticipated flow?
I'm struggling with suspension of disbelief with respect to the flow.All of it.

[freebeard]

Give in to your disbelief. Here's a shot looking down the throat of the duct. [for the record, Ferrari F12 Berlinetta]

The air isn't kicking up above the wheel center-line and diving toward the duct; it's moving sideways and down. If it really works.

What did you think of the Wind-Electric hybrid? The cover date is unreadable, but there's an article about CB radio. I'm guessing mid-80s. That high body with wheel fairing became standard practice on solar racers much later.

[aerohead]

Originally Posted by freebeard

Give in to your disbelief. Here's a shot looking down the throat of the duct. [for the record, Ferrari F12 Berlinetta]

The air isn't kicking up above the wheel center-line and diving toward the duct; it's moving sideways and down. If it really works.

What did you think of the Wind-Electric hybrid? The cover date is unreadable, but there's an article about CB radio. I'm guessing mid-80s. That high body with wheel fairing became standard practice on solar racers much later.

Thanks for the Ferrari image.The jury's (in my head ) still out on that one.
I have an article on the loop-wing wind car somewhere in my rat's nest.
There is a rumor that the guy came through Lubbock,Texas in the late 1970s and was pulled over by a State Trooper for speeding.
When asked if he knew what the speed limit was,he said 82 mph,The officer,chuckling, corrected him,stating that the highway number was 82,but that the speed limit was 55 MPH.
No citation was issued.
'gotta love rumors!

[SvdM]

Looks like they're 'stealing' higher pressure (higher viscosity?) air away from the side-mirror?

[freebeard]

Stealing or bleeding would be a good description, but the whole A-pillar area is a problem, not just the mirror.

I don't think viscosity changes much, it's pressure vs velocity.