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Old 05-01-2008, 05:57 PM   #1 (permalink)
aerohead
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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.

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