Aerodynamic Pressure Distribution
 

This is something we haven't got into yet,but I figured we'd get there sometime,so here's a bit of info.
*As your vehicle attacks the ambient air mass as you drive,the body of your vehicle will displace the bundle of imaginary streamline filaments which comprise the flow field.
*As the streamlines are displaced,they must accelerate from rest,to some quantity of velocity,respective to the degree of displacement.
*Static pressure is converted to velocity pressure,or dynamic pressure,as seen within the drag force equation.
*The highest static pressure (zero velocity) occurs at the forward stagnation point.
*The lowest static pressure ( highest velocity) typically occurs just before the roof peak/windshield header.
This is one of the most important things to know as an aero-modder.
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All the air around your vehicle wants to flow to this area of lowest pressure!
Let me repeat that.
All the air around your vehicle wants to flow to the area around your windshield header!
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*The reason the air doesn't all flow there is due to momentum-induced kinetic energy being delivered to the turbulent boundary layer from beyond it,in the laminar inviscid flow.
*You might imagine the flow nearest the boundary layer as billions of miniature attack aircraft strafing the boundary layer with a continuous spray of kinetic energy bullets.
*If the attack angle of the bullets is not too steep,the bullets will penetrate into the boundary layer.
*If the contour of the aft-body in particular is too steep,the bullets will ricochet off the boundary layer losing their ability to force the boundary layer down and rearward.
*Without the continuous influx of bullets,the boundary layer will by necessity,move in the direction of lowest pressure attempting to reach equilibrium.
*If so,then if the flow is downstream of this windshield area when the contour is too steep,the air will begin to flow backwards towards the windshield,triggering separation.
*This counter-flow of the separation spawns the eddies which bloom into full-blown turbulence.
*Unless the contour relaxes to a point where the bullets can again impart energy to the boundary area,the flow is lost forever,creating the turbulent wake which causes the pressure drag that streamlining attempts to reduce or eliminate.
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*In fluid mechanics terms,the air against the body boundary is always at rest due to viscous affects at the boundary 'wall
*Raising pressure requires the flow to decelerate.
*But the flow is already at zero velocity.It can't slow any when it's already at rest.
*Without the injection of kinetic energy from the outer flow,separation is triggered.
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Following are some diagrams which I've altered and colorized.The blue areas are positive static pressures acting against the body surface,the yellow areas are negative pressures pulling at the body surface.
*You'll note the low pressure spike near all the windshield headers:
*This first image is from the Ford Motor Company's Jay C. Kessler and Stanley B. Wallis,in SAE Paper # 660464,"Aerodynamic Test Techniques",1966,Figure 10.
http://i1271.photobucket.com/albums/...ad2/12-197.jpg
This second image is an adaptation from Hucho,from a 1977 research paper,SAE # 770389,by F.K.Schenkel,"The Origins of Drag and Lift Reductions on Automobiles with Front and Rear Spoilers". You can see from the blue at the tail that the spoiler is providing some positive pressure at the rear.
http://i1271.photobucket.com/albums/...ad2/12-195.jpg
This third image is from the same paper,and they've added a taller spoiler which while providing more down-force,is adding to the overall drag.
http://i1271.photobucket.com/albums/...ad2/12-194.jpg
This fourth image is from GM's "Aerodynamics for Body Engineers," by Kent B. Kelly and Harry J. Holcombe, SAE Paper #649A,1963,illustrating the pressure distribution for the 'new' Corvette Stingray.
http://i1271.photobucket.com/albums/...ad2/12-193.jpg
This 5th image is for a Jaray Combination Form car of the 1930s,from Sighard Hoerner's book,"Aerodynamic Drag,Fig.9.4,1951.
http://i1271.photobucket.com/albums/...ad2/12-192.jpg
This 6th image is of an 1961 Ferrari 250 GT,from Professor Alberto Morelli and Pininfarina,included in "The Time Tunnel-An Historical Survey of Automotive Aerodynamics",by Karl E. Ludvigsen,Mobility Systems Co.,SAE Paper # 700035,1970.
http://i1271.photobucket.com/albums/...ad2/12-191.jpg
This 7th image is the 'alpha' car from F.K.Schenkel's research,without any front or rear spoiler.
http://i1271.photobucket.com/albums/...ead2/12-19.jpg
This 8th image is adapted from Hucho's 2nd Edition book,Fig.6.8.I've un-nested the 1965 Volkswagen 1600 Sedan from the 3-vehicle graph to isolate it as an individual table,adding an oversize image of my own of the car.
http://i1271.photobucket.com/albums/...d2/12-1910.jpg
This 9th image is also an adaptation from Fig.6.8,un-nesting the 1968-74 Volkswagen Transporter from the 3-vehicle table,and adding my own oversize vehicle image.
http://i1271.photobucket.com/albums/...ad2/12-199.jpg
This 10th image is again,also from the Fig.6.8,isolated,with my larger Porsche 914 Coupe image.

http://i1271.photobucket.com/albums/...ad2/12-198.jpg
Here,with the Z-car,you can see the streamline contraction near the roof peak which would indicate the high velocity/low pressure.
http://i1271.photobucket.com/albums/...ntitled314.jpg
Here you can see the separation line on the Volkshart Saggitta where the contour is too steep
http://i1271.photobucket.com/albums/...ntitled313.jpg
The VW XL1 has a relaxed contour which allows the continuous kinetic transfer into the boundary layer.
http://i1271.photobucket.com/albums/...ohead2/209.jpg
VW's 1981 long-tail Flow body walks a fine line and comes in as a simple body with Cd 0.14
http://i1271.photobucket.com/albums/.../Untitled5.jpg
Paul Jaray's 'pumpkin seed' of 1922 set the stage with Cd 0.13 (Inspiration for the 'Template').
http://i1271.photobucket.com/albums/...ntitled5-1.jpg

can you dumb this down more for me, or would that pretty much be a book?

feelin kinda like homer....
Dr. Julius Hibbert: Homer, I'm afraid you'll have to undergo a coronary bypass operation.
Homer Simpson: Say it in English, Doc!
Dr. Julius Hibbert: You're going to need open-heart surgery.
Homer Simpson: Spare me your medical mumbo jumbo!
Dr. Julius Hibbert: We're going to cut you open and tinker with your ticker.
Homer Simpson: Could you dumb it down a shade?

Is it fair to say that molecules, in addition to moving normal to the direction of travel, also get sucked forward and back in the direction of travel?

Renormalizing all that in my head is tough. Could you edit the post and number the illustrations for reference purposes? In fact here's a test:

1. http://i1271.photobucket.com/albums/...d2/12-1910.jpg
2. http://i1271.photobucket.com/albums/...ad2/12-199.jpg
3. http://i1271.photobucket.com/albums/...ad2/12-198.jpg

Yes you can. Just place each IMG separated by a Return, Go Advanced and wrap the whole block with Ordered List. Voila!

Thanks for including the Notchback. :thumbup: Here's mine:

http://ecomodder.com/forum/member-fr...-outa-town.jpg

You can almost make out the Reggae/Deadhead sticker on the hood.

dumb down
 

*It's not a book,but at least a chapter
*The streamline filaments we visualize with smoke all have the same amount of energy,either as potential energy or kinetic energy,or some combination of both.They are isoenergetic.
*Danial Bernoulli demonstrated that when we move air,it loses some of it's static pressure and gains velocity pressure.
*At the forward stagnation point on your car,the pressure acting on this point is the ambient barometric pressure of the atmosphere plus the dynamic pressure.The 'Total' pressure.
*The Dynamic Pressure is in the Drag Power formula,and is ( 1/2 rho V-squared ).
*Near the windshield area,the air is going as fast as it's going to go and has virtually zero static pressure,and all velocity pressure (which an airspeed indicator,manometer,or MAGNEHELIC would operate on).
*With the tables shown,the aerodynamicists have drilled holes down the centerline of a full-scale automobile mockup,installed specially shaped static pressure taps under each hole,which lead off through plastic tubing to a bank of manometers.
*At a given air velocity,the bank of manometers are photographed simultaneously,and the readings captured.
*The length of the distance away from ,or underneath the surface of the car indicates the amplitude of the pressure,whether positive or negative,based upon the Dynamic Pressure.
*Each reading indicates the pressure coefficient at that location.
http://i1271.photobucket.com/albums/...ead2/12-19.jpg
*The blue spike leading out from the front grille indicates the pressure which rams air through the cooling system,AC condenser,intercooler,and fluid coolers.
*The yellow areas indicate negative pressures measured at the body surface at the sampling locations.
*On this particular car,you notice that beyond the hood/cowl/windscreen bottom,there's no positive pressure acting on the body.
*This car would have 'lift' issues,front and rear,easily remedied with an airdam and small rear spoiler.
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*For streamlining,you want gentle teardrop taper which while providing a small pressure rise,does not exceed what the turbulent boundary layer can handle.
*The air directly against the skin of your car is already at a standstill,do to the viscous affects of the air.
*If the body cross-section changes too rapidly,Bernoulli's Theorem mandates that the air decelerate.
*This is impossible,since the air is already at rest.
*And since the angle is too steep,the free-stream outside the boundary layer cannot successfully transfer momentum energy (bullets) to pin the air against the body,the boundary layer just lifts away,and heads for the windshield area,while being smacked by the oncoming flow,causing it to roll up into eddies,then turbulence.
*Here is the 1963 Corvette Stingray.With a 'Template' comparison we can anticipate rear lift issues,and with the pressure profile you can see that there exists no positive pressure over the back of the car.
http://i1271.photobucket.com/albums/...ad2/12-193.jpg
If you fast-forward to 2014,you can see how GM has lofted the aft-body of the Corvette Stingray,adding high-speed stability in the bargain.
http://i1271.photobucket.com/albums/...Untitled15.jpg
*They get some mpg potential as well.

Hi Phil,

I read through this particular post several times, and have to mention something.

When you said, "All the air around your vehicle wants to flow to the area around your windshield header!", I have a simple comment.

I work in a Flow Lab, and have worked with sonic nozzles and subsonic orifices for many years.

Theory:

Any time one wants to get air molecules to move, a differential pressure (dP) is required.

In this graphic, the molecular flow is from left to right, and
- P1 is inlet pressure (usually gage + baro)
- P2 is throat pressure (or lack of, and can be vacuum)
-- an extremely high vacuum for sonic nozzles
-- can be close to 28 inHg vacuum or higher
- P3 is exhaust pressure (typically at baro or ambient)

http://ecomodder.com/forum/attachmen...1&d=1419462642

Sonic nozzles are typically used for controlling bulk or mass flow, depending on the application. We use them to calibrate mass flow meters in the Lab.

The only reason that flow transfers from P1 past P2, to P3 is dP or delta pressure. The flow at the throat (P2) can be almost 100% kinetic energy, with no pressure component, and P1 and P3 are mostly potential energy, with no kinetic.

When it is stated that the air molecules "want" to move from P1 to P2 due to the low pressure there, then why would the flow at P2 want to progress towards P3? Location P3 is at higher pressure, and based on your observation, P3 flow would want to reverse and flow towards P2.

This does not happen is life so another mechanism causes the general flow from P1 past P2 to P3. It is dP or delta pressure that causes generalized flow across any flow device, be it a flow orifice, nozzle, or any other flow restrictor.

Summing up, I would say that any dP around our vehicle moving down the road causes a generalize flow around the vehicle. The pressure at the front of the vehicle has to be higher at the front, than at the rear for the dP to be enough to cause flow around the object.

Phil, thanks for all the nice graphics in your post.

Jim.

Well, I'd number the pictures for you if I could. :( But the one I wanted to point out is the one you used as an example. :)

By way of your explanation—the discontinuities around the bumpers would be due to lack of sensor data.

Do you think the abstraction where they have the pressure radiating normal to the surface, or the one with a single straight line is less misleading?

molecules/numbering
 

I've gone back in an numbered the early images,with source credits.Hope that helps.
And yes,the air can move in all directions,including sideways (transverse contamination),depending on pressures and their location on the body.
Post-1985 cars,with body camber,typically have the boundary layer held against the body by a favorable pressure gradient created by the attacking 'source' flow,right up to the position of maximum cross-section.
It's in the aft-body where we are cautioned,as here,we are in an unfavorable pressure gradient,and any 'fast' contour, would create too fast of a pressure rise, overwhelming the turbulent boundary layers ability to receive momentum energy from 'above.'

P2
 

Thanks Jim.
With respect to these automotive pressure histories,we're encouraged to focus on boundary layer phenomena.
http://i1271.photobucket.com/albums/...ntitled9-2.jpg
*In this 1st wind tunnel photo of spheres,we see the non-dimpled flow separation before the max camber position on a smooth,laminar boundary layer golf ball.The air from the back of the ball is traveling against the flow towards the high-velocity/low-pressure 'windshield area' of the golf ball,triggering separation and high drag.
*In the second image,we've dimpled the ball (driven our car beyond 20-mph) and the inviscid laminar outer flow field is transferring momentum energy into the Reynolds number-induced turbulent boundary layer,allowing flow attachment back to 4-seconds after 12:00 on 'top',and 4-seconds before 6:00 on 'bottom.'
*If you measure the local tangent angle at the separation point you get 22-degrees.
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*All the air around the sphere still wants to flow to the original laminar boundary layer separation line,but it is prevented by the momentum interchange into the turbulent boundary layer AS LONG AS the body doesn't reach a 'fast' contour (pseudo-fastback).
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*The yellow spike at the windshield top in all the pressure tables is a reminder of where this 'low' exists on the car,and if we compromise the shape of the car,this is where the air in the turbulent boundary layer is going to attempt to get to.
*If we keep increasing the radius of the aft-body contour,we eventually reach a crude streamline body of revolution,and the wake is eliminated.
*This is what Hucho's books are all about,and underlies all automotive streamlining.
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*In your wind tunnel,the air mover,or vacuum vessel which drives the supersonic flow,is powering the delta-P which enables the flow.Work is added to the system just as the brake horsepower of the cars engine.
*The wind tunnel is also a closed system and you have conservation of air mass volume.
*Whatever passes P1 must equal P2 and P3,by definition.
*The total energy of any given imaginary streamline filament will be equal to any other streamline element under steady-state conditions.
*This is the isoenergetic concept established by Bernoulli.
*P1 is accelerated from a low velocity/high-pressure,to mostly velocity at P2,then decelerated to a lower velocity/higher pressure again at P3.
*This is exactly what Daniel Bernoulli demonstrated.
*You have lost some energy to surface friction and viscous friction.This energy is lost forever (2nd-law of Thermo).
*In your transonic and supersonic work you've got to deal with shockwave losses as well.
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*Our engine or motor power induces motion.
*Motion displaces the air mass.
*Displacement creates acceleration
*Acceleration creates velocity
*Velocity affects pressure
*The difference between the forward stagnation point and base pressure of the wake is the pressure drag which is the major portion of the total drag,of which streamlining (by definition) attempts to reduce or eliminate.
*If the aft-body is correct,there'll be zero separation,almost total pressure recovery,and the base pressure will be close to that of the forward stagnation point,killing pressure drag.

bumpers/abstraction
 

Schenkel didn't seem to get into the bumpers per se
http://i1271.photobucket.com/albums/...ead2/12-19.jpg
Morelli took some measurements although they're handled individually.
http://i1271.photobucket.com/albums/...ad2/12-191.jpg
Here's a 1959 Chevy Bel Air,and it looks like GM investigated the bumper regions
http://i1271.photobucket.com/albums/...ntitled1-8.jpg
Here's their 1960 Corvair Coupe
http://i1271.photobucket.com/albums/...ntitled2-8.jpg
*Since the bumpers of the day were not integrated into the body,they'd be essentially 2-dimensional themselves and the investigators might have just blown them off.
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As to the pressure representations,the length of the elements depict the amplitude of the pressure as a function of the dynamic pressure.
*At 60-mph,the forward stagnation point would essentially be the dynamic pressure.
*The direction of each element would reflect the resultant force direction of the pressure acting on the surface boundary of the body.
*The 'LONG' yellow spike at the windshield indicates that the velocity of the highly accelerated local flow, exceeds the 'road speed' of the air.Flow over a roof might be 1-1/2 X the velocity of the uninterrupted flow.Same for the A-pillars.

Nice thread, sort of wish there were extreme examples such as the Aero-Template car and a Pick-Up Truck for comparison.

Is it accurate or ignorant to say/think that avoiding large negative pressures will in turn avoid producing large positive pressures?

That is to say, if you never accelerate the air to begin with, it will not have to slow down later to equalize the mass air flow (correct term?).

EDIT:
The so-called perfect shape would have quite a bit of lift, perhaps we should turn this thing upside down?

https://surjeetyadav.wordpress.com/author/surjeetyadav/
https://surjeetyadav.files.wordpress...14/02/ge-7.png
https://surjeetyadav.files.wordpress...14/02/ge-6.png

Nice little paper on down-force without wings:
https://lucky13racing.wordpress.com/

avoiding large negative pressures
 

*avoiding large negative pressures wouldn't be the issue.We'd have to have zero shape (a flat car).
*3-D streamline shapes do not produce lift as in a 2-D airfoil.The nose is held down by the favorable pressure gradient in the forebody.The tail is held down by the near-fully recovered static pressure of the separation-free aft-body.
(the T-100 showed essentially zero lift at 135-mph according to DARKO).The T-100 is also almost 50-50 weight distribution on the axles.
Here you can see the negative-lift positive pressure (blue) cancelling out the positive-lift negative pressure (yellow) acting on the 3-D body
http://i1271.photobucket.com/albums/...ad2/12-192.jpg
*If we invert the streamline body,we lose attachment over the top (lift) and we force the total pressure under the vehicle (more lift).
*The streamline half-body remains very low drag.If you have dynamic stability issues,it's due to a conflict between center of pressure and center of gravity.

This falsifies that Colani Bonneville car, or is it an edge case?

http://ecomodder.com/forum/member-fr...wu5wo1-500.jpg

colani
 

Colani's body is more sophisticated than a simple inverted airfoil.His inclination would keep the top in a favorable pressure gradient,basically forbidding separation,and the underside 'fast' diffuser would help compensate for shoving all the air under there.I suspect that it would be pretty stable.

I am reading about the laminar and turbulent boundary layers and pressure again in other sources, and I came back to EM to see what we have discussed in the past. This short thread had a lot of great images and effective dialogue between different ideas. (Also, it has a reference to The Simpsons... so, you are welcome!)

And Notchbacks!

So, two serious questions. At the surface of the car body, the flow stream of the boundary layer stalls to zero because of vicosity and friction. The shear effects of that stalled air slows the stream above it, and to a lesser degree the stream above that. The velocity of the stream keeps turbulence down, but nonetheless turbulance develops in the boundary layer as the flow moves downstream. The boundary layer expands because it is becoming more turbulent, and the larger the area of the body, especially length, the more unstable it will become until it actually fully separates due to the turbulance. I think that is mostly correct, and if there are errors I don't know how to judge where I have made them. Maybe it is not turbulance that causes expansion, but something to do with heat and friction that causes both turbulance and expansion.

First question (vocabulary): that stalled flow at the wall is sometimes described as having "separated." I have also seen it claimed that it is capable of localized reversed flow. However, that useage of term "separation" can be confusing because it may not be refering to the same effects as when we talk about "separation" of the entire boundary layer that happens when/if the flow encounters a major change in body contour and pressure, right?

Second question: obviously there is friction drag produced by the stalled flow at the body surface and its interation with the rest of the boundary layer. That is not insignificant drag, either, apparently. What, however, is the relationship between the growing turbulance in the expanding boundary layer and actual separation drag at the rear window, c-pillars, and/or tail of the road vehicle? Is there one?

I hope this makes sense. No time to edit. Daughter and I gotta work on her car.

Thanks for replies.

Until someone who knows what they're talking about comes along...

It's spelled turbulence.
The way I account for things in my own thinking is to distinguish laminar, attached turbulent and detached turbulent flow.

edit:
I could take this to a print shop and have it printed on contact paper.
https://cdn.shopify.com/s/files/1/24...g?v=1561090696
https://cdn.shopify.com/s/files/1/24...g?v=1561090696

Haha. Well, I said I didn't get to edit, so...

And the session working with my daughter on her car didn't get so far either. Funny how her promise to work on it with me waned after it was paid for.... hmmmm....

Yeah. I didn't realize you were in the Homer category with me on this.

Anyway, thanks for the Feynman cite: "turbulence as the most important unsolved problem in classical physics." That's as interesting and more topical than Homer Simpson. :)

Oh Lord, it's hard to be humble... :)

:)
If humility were easy for us, perhaps we'd be fish.

I hadn't seen this thread before, and I note it started 6 years ago. I am not going to enter into a debate about theory, but simply say two things.

1. When you can measure the surface pressures on your own car, at any location, at any speed, whenever you feel like it, a lot of this discussion starts to look very academic. You don't need to theorise or guess - just measure what is actually happening. It is only surface body pressures, and the direction in which the resulting forces act, that determine thrust/drag, and lift/downforce. Nothing else.

2. Pressure distribution on cars depends on shape. Yes, you can generalise (ie use rules of thumb) or look at published pressure distributions for other cars, but literally every car is different. The pressure distribution will depend - among other factors - on whether there is attached or separated flow, the thickness of the boundary layer, and the positive/negative camber of the body's individual parts. And all that applies equally underneath the car. Because of the large panel areas involved, even tiny changes in pressures can have a big impact on the resulting aerodynamic forces. That makes generalisations often quite deceptive.

TL;DR: just measure them.

I hear you, Julian. I like testing. Gonna do some this summer, if I get to do what I plan, it'll be ABA bidirectional testing of my decklid spoiler design using both throttle stop and coast down methods. Maybe just throttle stop, depending on conditions and time.