flow separation:Tiger Woods meets Cinderella,or 5-seconds to a perfect roofline
 

There has been a lot of discussion surrounding both boundary layer separation and also Kammbacks,rooflines,etc.,in the search for the perfect form.There has also been challenges with respect to wording,phraseology,linguistics,terminology,etc.,w hen making descriptions of forms (i.e.:curved,parabolic,cambered,curvilinear,ellipt ical,etc.).

I've been racking my brain trying to come up with a "mind's picture" which might achieve universal recognition and get everybody on the same page with respect to "imagery".And while Henry Landa warns of the use of "rules-of-thumb",maybe we members need at least some basic reference to build off of.

So here's my attempt.

(1) Tiger Woods : Hopefully,everyone is acquainted with the game of golf.And if so,everyone is aware that at this time,Tiger Woods is a major player in the sport.Also,members here at ecomodder have already been acquainted with the role dimples play with respect to a golfball's turbulent boundary layer and its impact on drag and range.

Perhaps members have also had a peek at least an introductory text on fluid dynamics and the iconic images from the U.S.Naval Research facility at Pasadena,California,depicting the bowling-ball in the water tunnel.In the side-by-side photos,one can see the difference in boundary layer separation for both laminar,and turbulent flow.When artificial roughness is introduced to the bowling-ball (they glue sand to the leading face of the ball),a turbulent boundary layer forms early and separation is moved from 80-degrees behind the stagnation point,to 115-degrees behind the stagnation point.This is exactly what the dimples on the golf ball do.

(2) Cinderella: In the fairy tale of Cinderella,she must be home from the Prince's ball at the castle by Midnight, or else her carriage will turn into a pumpkin.Thesis: If one invisions Cinderella's clock face nemesis as a golf ball,and the ball is travelling in the direction of 3-O'clock,then 3-O'clock is also the stagnation point,12-O'clock is the point of the ball's frontal area,and at 5-seconds after "midnight",the boundary layer has separated,and begins the turbulent wake (profile drag).

If you take the image of the clock face,and imagine the line which bisects 12 and 6 O'clock depicts any vehicle body at the point where it's roof "peaks".Then if you draw a horizontal line from the rim of the clock face where it intersects the 5-second mark,left to the vertical 12/6 line,this defines the "ground plane",and the curved line from 12-O'clock,rotating clockwise,around to the five-seconds after point defines the" ideal roofline" of any vehicle.

If a scale form of any vehicle is placed such that the bottom of it's wheels are on the "ground plane",and the "peak" of it's roof is at 12-O'clock position,then the curved line described by the rim of the clock face describes it's "ideal" roofline. The bedcover on the T-100 is basically defined by this" form".You can call the the line anything you want.Perhaps "curvilinear" is the most correct,however I'm no Noam Chomsky.

Perhaps we can refer to it as the clock- face -shape.Interestingly enough,if you "mirror" this form,and add a prolate-ellipsoid nose to it,you basically arrive at the "ideal" low-drag ,free-stream,aerodynamic form,of minimum profile drag and minimum skin friction.

I hope this excercise helps clear up some confusion.I know its "geeky",just that maybe everyone can connect with the "imagery" and make the connection.Happy modding,Phil.

Thanks Phil!

I'll use this Cinderella version as a bedtime story for my son :p

Phil, thanks for that analogy. But I'm a bit confused. If the ball is moving in the direction of 3 o'clock, wouldn't the separation point be 5 seconds BEFORE midnight? Doesn't the air cling to the ball for a short distance past maximum circumference?

Also, since water is 800 times as dense as air, wouldn't the Reynolds numbers for the bowling ball and golf ball be very different? Or was this compensated for in the bowling ball analysis?

Thanks.

Hi Patrick! From the images,with the laminar boundary layer,flow does separate before midnight(90-degrees).If 3-O'clock denotes 0-degrees(the stagnation-point),flow separates at 80-degrees from there (clockwise).With the sand glued to the front of the bowling ball,or dimples with the golfball,a turbulent boundary layer is established immediately,which forestalls flow separation until the bell has sounded for Cinderella at 5-seconds after 12:00.When the flow breaks off,the wake is smaller and has a higher base pressure and consequently lower profile drag.For the golfball,drag is cut in half,and its range (for a club head velocity of 110-mph) is doubled,from 150-yards,to 300-yards.Drag reduction is so remarkable with the turbulent boundary layer,artificial roughening of the golfball is critical to it's performance even at the low Reynold's Number it experiences for it's size and velocity.I should have posted the velocity of the submerged ball.And you're correct that under water,the bowling ball it would see much higher Reynold's Number.

Superimposition ( Rolex meets Big-Ben )
 

Just greasing the wheels for tomorrow.
I finally put some graphics together for this thread.The "word-pictures" seem to hit dead-ends,so between wearing out Al's copy machines,a pair of scissors,White-out,and Scotch Tape,I've finally got something to sink our eyes into.
There's been enough talk of golf-ball dimples,turbulent boundary layers and critical Reynolds Numbers that maybe we can make a connection to all this now.
THEORY Re-hash:
* All spheres behave identically above critical Reynolds Number,regardless of size.
* Spheres are so well understood that they may be used as calibration models for any wind tunnel.
*The 'fixed' separation point of a sphere offers a naturally defined turbulence-free architecture by employing a single superimposition,as Salvador Dali might do with one of his canvases.
*Familiar forms we can use to visualize the the superimposition are the dial face of both the analog wristwatch,and wall clock.

CONDITIONS:
The "flow" conditions for our example can be interpreted either as a dial face moving horizontally to the left,with the forward stagnation point located at 9:00 O'clock,or the dial face stationary,as in a wind tunnel,with the air blowing horizontally from the left.In both instances,air velocities are such to produce Reynolds Number above critical.
RESULTS:
* In all situations,attached flow exists from the forward stagnation point,up and over,to a point at 115-degrees of rotation,occurring both at 4-seconds after 12:00 and 6:00( down and under ).
* Photographs of this flow can be seen in the FLOW-IMAGES thread galleries for both air and water.Golf-ball images can be GOOGLED.
SUPERIMPOSITION
* The area of the sphere's aft-body,between 12/6 and 4-seconds after is what is of interest to aero-modders
* Enlargement and enhancement of this section can provide the architecture for a separation -free automotive roof and aftbody structure.
* Suitable enlargement and superimposition of a properly-scaled smaller watch dial "leading-edge" creates a proto-teardrop of which the aft-body may serve as a design template.
OUTCOME
* The teardrop created possesses the hemispherical nose reported in fluid texts to provide adequate and minimum low drag,separation-free penetration for up to transonic velocity.
* The teardrop's aft-body is also separation-free and may be used with high-confidence as a template for modeling automotive forms.
-------------------- WILL CONTINUE -------------------------------

Continued from previous day
 

SPECIFICATIONS
* The 'teardrop' represents a separation-free body of revolution of 36% thickness.
* Length/ Diameter = 2.78:1
* Frontal Area occurs @ 18% body length behind nose.
* Aft-body boat tail constitutes 82% of body length.
* At "automotive" scale,critical Reynolds Number is achieved at 20-mph.
* At 20-mph,frontal drag coefficient is constant and estimated at 0.04 in free-air.
* Frontal drag coefficient in ground-effect is estimated @ 0.08.
* Boat tail terminal angle is 22-degrees.
ANCILLARY CONSIDERATIONS
* When viewed in 2-D the superimposition mimics Abbott and Von Doenhoff's,"Transformation of a Circle into a Wing Section."
* By default,the form also satisfies the Kutta-Joukowsky condition for ensuring smooth flow at the trailing edge.
* The form approximates the NACA 0021 Section.
* A coincidence of the Rolex/Big-Ben form,is that it embodies W.A.Mair's critical 22-degree boat tail angle.
* The form also embodies the L/D drag minimum,as reported by Hoerner/Hucho ( see Fig.4.119,p.200,Aerodynamics of Road Vehicles,Hucho,1987.
CONCLUSIONS
* In the absence of more sophisticated streamlining tools,the dial-face may serve the would-be aero-modder with a design template that may be used with a high degree of confidence,much the way Volkswagen AG's handbook:"No Guesswork" aids their factory-trained technicians when entering into an overhaul.
* The "GO/NO-GO" simplicity of the clock-face-derived template makes for expeditious fabrication of pre-test prototypes,providing a historically-proven technological basis for project development.

-------- IMAGES WILL BE ADDED ASAP -----------------------------

IMAGES: Template Development
 

Here's the linear progression of the development of the clock-face template.
As you see,there's not much to it.That's the whole point.Perhaps you'll see a similarity to other low drag forms.

SEX for ALL (made you look!) check/respond ????????
 

Please take a look,see if you could work with it.It took me 35-years to come up with it.

OK here's a response, with graphics.

I guess it's more of a question,

You state somewhere in here that the wheels of the vehicle should rest on the ground plane.
Yet you say that flow separation occurs at 4 seconds after midnight.

It seems to me that these 2 statements contradict in the sense that air will either remain attached to a given shape until it reaches 4 seconds past midnight (115° beyond stagnation) on a curve (Red), or, you need to overlay a huge wing section onto a given vehicle (Green), and if you fall below this line at any point, you have flow separation, even if it occurs at some point less than 115° (which in this case is on the ground 20 feet behind the truck).

Which one is it? Red or Green? Or am I missing something? (Again, as usual)


http://ecomodder.com/forum/attachmen...1&d=1264316383

I'll take the dark red:

http://ecomodder.com/forum/attachmen...1&d=1264426206

I'm not sure if Aerohead is back yet so give him a bit of time to respond.