Quote:
Originally Posted by Phase
define non ventilated cavities. what are your thoughts on the rear of the new prius?
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Any boat-tail, or box-cavity must not allow any porosity/ aspiration / contamination to the very-vulnerable aft-body flow.
The surface must be air-tight, whether the skin of an actual tail, or each cell of a box-cavity.
If not, the existence of the voids induces an adverse pressure spike or gradient, triggering flow separation right then and there.
There's very little 'reason' for flow to remain attached to an aft-body.
Only the gentle reduction in cross-section of a 'streamlined' contour can allow the feeble flow to receive enough momentum interchange from the nearest streamline to prevent the air from just 'climbing' up the rear of the car, on it's way to the windshield header, where the pressure minimum occurs.
There exists, what Tamai refers to as a 'shape factor 'H'', which determines whether separation will occur. It requires CFD to work it.
Tamai comments, on page-62, ' The actual mechanism of turbulent separation and its reattachment is beyond the scope of this book.'
In the absence of a wind tunnel, or CFD, we're admonished to stick with rules-of-thumb regarding aft-body surface patch tangent angles. That's exactly what the ASTs are predicated upon.
Hucho had an associate write an entire chapter on boundary-layer theory in his 2nd-Edition, which does explain the mechanism for TBL separation.
Hucho, himself, studied under Professor Hermann Schlichting, who wrote the book, 'BOUNDARY-LAYER THEORY', which would be a reference requirement for any aerodynamicist ( I traveled to the University of Texas at Arlington, Texas, and photo-copied the entire book in their library, along with Fachsenfeld's.