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Hypersonic, Experimental and Computational Capability, Improvement and Validation

For the purpose of discussion, aerothermodynamic
phenomena are classified into seven types: aerodynamic
parameters, viscous/shock interaction, boundary-layer
transition, forebody-heating/heat-transfer, radiation and
ablation, lee and base-region flow, and low-density flow.
These seven types are listed in the first column of Table l, are
illustrated schematically in Figures 1 — 7, and are discussed in
the following. This discussion defines the basis for the first
phase of the Working Group 18 activities.

Aerodynamic parameters including lift, drag, and moment
coefficients, in the real flight of blunt vehicles such as Apollo
and slender vehicles such as the Shuttle Orbiter, are different
from those deduced from calculations or experimental studies
made in the perfect-gas facilities (Hillje and Savage 1968;
Maus et al. 1984; Park 1990). The difference is particularly
prominent in the trim angles of attack of these vehicles at
hypersonic Mach numbers: typically the trim angle is larger
than predicted by 2° to 4°. The real-gas phenomena are
believed to influence the pressure distribution via the changes
in the effective specific heat ratio, 3/, which occur mostly
under equilibrium or near-equilibrium conditions for a blunt
body, and via the geometrical imbalance in pressures due to
the nonequilibrium phenomena which occur mostly for a body
with sharp leading edge.

The effective specific-heat ratio, 7, is well defined for
equilibrium or near-equilibrium real-gas flow as well as for
perfect-gas flow. When real-gas processes absorb energy,
both the specific heat at constant pressure and that at constant
volume increase, and yin a real-gas is smaller than in a
perfect gas. The pressure distribution in such a real gas could
be calculated approximately by assuming that the gas is
perfect but that yvaries appropriately from point to point,
always with a value less than that in a perfect gas, if such a 7
distribution can be determined a priori. For a perfect gas of
constant 7/, the pressure at the stagnation point or on a flat
plate at an inclined angle with an attached oblique shock can
be expressed as a function of the assumed 7.