[aerohead]

1) In order for the 'scale' flow to represent 'real flow' ( verisimilitude ) we need a minimum Reynolds number ( Rn ) of around 1,000,000, which is the 'supercritical Rn' for a fully developed turbulent boundary layer to exist.
2) This is essential if 'real flow' effects are desired in a scale model.
3) You'll need the formula for Rn.
4) You'll need the kinematic viscosity of the 'fluid' your choosing for the water table.
5) Knowing the kinematic viscosity, and supercritical Rn requirement, you reverse-engineer the water table flow velocity requirement necessary to balance the equation.
6) If you fail to satisfy all conditions of verisimilitude, what you observe will be 'bogus.' And there's no way you could trust scaling up your results in order to create a 1:1-scale 'body'.
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7) If testing for a 'teardrop' trailer body, 2-D flow will not reveal the extremely high vortex-drag which exists in 3-D flow. This was tested to high precision by Fachsenfeld, who presents the wind tunnel photographs, plus associated drag tables, in his self-published ,1951, Aerodynamiks Des Kraftfahrzeugs.