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Old 01-09-2010, 02:34 PM   #6 (permalink)
aerohead
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Continued from previous day

Quote:
Originally Posted by aerohead View Post
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 -------------------------------
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 -----------------------------
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