Page 9 - Combined_73_OCR

S = Car Reference Area (21.8 ft2 full scale)

= Vehicle Rake Angl C D

CORRECTIONS AND CORRELATION - When testing scale models mounted over

fixed ground plane in closed jet wind tunnels, numerous corrections

must be considered in order to relate the tests to the actual vehicle.

Reynolds Number corrections and boundary layer simulation at the ground

plane were already discussed in the sections on model description and

installation. In addition, corrections for bouyancy and blockage are

also normal required.

Bouyancy effects are caused by the longitudinal static pressure

gradient in the test section. This gradient is caused by boundary laye

growth along the tunnel walls and the loss in total pressure due to

friction. Since the tunnel static pressure becomes more negative along

the length of the model, the model drag is increased. By measuring the

static pressure gradient in the test section and applying the methods of

correction outlined in Pope (3), bouyancy can be easily evaluated.

When the model is placed in the closed test section, the n o s-

sectional area of the test section is effectively reduced and the local

velocities at the model increased. This 0 ) ffect is referred to as

’’solid blocking and results in a velocity at the model that is highe

than the indicated velocity measured upstream of the model. In addition

to ’’solid blocking”, the fact that the air is slowed down in the wake of

the model and, consequently, speeded up outside the wake, results in an­

other correction called ’’wake blocking”. If the model is quite large in

cross-sectional area (greater than 1 or 2%) relative to test section area,

blockage effects must be accounted for.

-8-