Page 11 - Combined_73_OCR

Vj4 = Model volume (13.78 ft^)

n ’ o = Model uncorrected drag coefficient
(.35 assumed)

The term of the equation containing the shape factors is the solid block­

age portion, while the term containing the uncorrected drag is the wake

blockage portion. Herriot’s method is based on the use of a source-

ink distribution to represent the body and an infinite distribution

of image to simulate tunnel walls. Because the body of revolution
shapes considered by Herriot represent relatively clean aerodynamic

c n hapes, question is raised as to the accuracy of this approach as

c d ppiied to automobile wind tunnel work.

A further extension of Thom's work is found in the velocity ratio

method proposed by Hensel (6). In Hensel’s report, calculated ratios

velocity changes at the model to those at the tunnel walls are pre-
O
ented. By measuring the velocity increases at the tunnel walls, the

velocity increase at the model can be determined using this approach.

Major short-comings of this approach are the difficulty in separating

wake blockage effects from the wall velocities measured and the fact

that bodies of revolution are the aerodynamic shape considered. If

it is assumed that the wall velocities reflect total blockage effect

and that velocity ratios for bodies of revolution are reasonably re­

presentative of automobile velocity ratios, Hensel’s method can be

applied quite easily as shown below:

Kt = 1 + 2 (4)

where ua = Velocity increase at model
due to model blockage

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