Quote:
Originally Posted by miket
What is the optimum finess ratio for a given internal volume. I know the optimum fineness ratio for a given height what what about for a given volume? would a shorter and longer shape be better than a taller and less long shape?
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For volume I believe the sphere represents the structure of minimum surface area to contain a given volume.
If I'm reading into your question,which fineness ratio for a given volume produces the least drag?,then we need to qualify the question.
For automobiles,the ultimate fineness ratio is one which produces an 'effective' 'free-air' fineness ratio of around 2.27:1.
A body length/height=5 is a relatively good metric to shoot for.
If you go below,you invite separation and high pressure drag.
If you go over,you invite additional skin friction with attendant drag increase.
If any part of the aft-body exceeds a tangent angle of 22-degrees you can expect separation.
The aft-body of the Aerodynamic Streamlining Template' respects all the criteria required for minimum drag for a road vehicle.
Dr. Hucho says the body ahead of the point of maximum cross-section "has only minor influence on the total drag.The main contributions to the drag force originate from the rear part of the body......it is very important to design a rear body surface which brings the divided streamlines smoothly together.Optimum shapes are 'streamlined' bodies having a very slender rear part.......... Lower drag can only be achieved by extending the length of the vehicle's body...........The drag coefficient for ...cars may be plotted against vehicle length.....(For) correlation...between greater body lengths and lower aerodynamic drag.....if the evaluation is limited to vehicles developed for the lowest possible drag coefficient,this expected trend is in fact confirmed."
The 2.27:1 streamlined body has the same Cd 0.04 as the 2.5:1 streamlined body used for the 'Template'.
The aft-body of the 2.27:1 streamlined body violates Mair's 22-degree angle at useful lengths of the 'Template'.
The 2.5:1 body chosen for the 'Template' is the shortest body,which can produce the lowest drag coefficient in ground-effect ( Cd0.08 ),along with respecting tangent angle limitations as spelled out by Dr.Mair for the elimination of flow separation.
NOTE: There are structures which have been shown to demonstrate lower Cd's,but they are purely 'laminar' structures,and once accelerated to critical Reynolds number,demonstrate no advantage over the 2.5:1 streamline body form.