Originally Posted by aerohead
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Walter Lay of Michigan University published his research on this topic in 1933.
Lay's research was verified by Carr, at MIRA in 1968-9.
Buchheim et al. expanded on this body of knowledge in the early 1980s at Volkswagen AG with the Audi 100-II/ 100-III R&D.
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Not knowing what your 'scientific studies' are, you're advised to re-read them, looking for references to 'windshield inclination angle'-related limitations due to:
* light diffusion/refraction
* solar heating
* attached flow 'saturation.'
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While a 'steep' windshield inclination angle IS good for flow attachment over the windshield-to-roof transition, anything beyond the radius which first produces attached flow is 'superfluous', adding zero additional drag improvement, either 'locally', or 'downstream'.
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And this 'degree' of flow attachment doesn't grant the designer any special license to violate the fundamental of fluid mechanics in relation to the roofline contour.
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1) Walter Lay constructed a Cd 0.12, quasi- half-body car model #22 long-tail, of 'optimum' 'aerofoil' shape.
2) With model#10 ,by mutilating the forebody to include a 'JEEP' windshield, the drag increased to Cd 0.24, a 'doubling.'
3) By adding additional inclination to this windshield with model# 14, drag fell to Cd 0.17.
4) By further relaxing the angle to 45-degrees, with model# 18, the car returned to Cd 0.12, 'WITHOUT' the aerofoil-shaped windshield.
5) With the model# 18 windshield angle, flow attachment had become 'saturated.'
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There's a probability that there's no difference in 'aerodynamics' between the 2022 Prius, and the 2023 Prius, on account of any windshield difference.
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Without a 'blueprint' I can't tell you anything about the new roofline capabilities. It would be pure speculation.
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* Before discussing a tail 'elongation,' you'd want to fully belly-pan the Prius, including a 'working' diffuser.
* Reduce the 'ventilation' of the wheels.
* And full rear skirts would be highly recommended.
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Then, if your elongation is 'streamlined', with fully-attached flow along a pressure-recovering contour, at that point, the Cd will be a function of the new wake area, compared to the OEM wake area.
Top, side, and diffuser geometry of the tail extension will have to be scientifically 'precise.'
And it needs to be completely 'airtight.'
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