SESSION VIII

SHOCK WAVES

SESSION CHAIRMAN" FRANKLIN K. MOORE Cornell University, Ithaca, New York

[facing p. 512

J. Quant. Spectrosc. Radiat. Transfer. Vol. 8, p. 513. Pergamon Press, 1968. Printed in Great Britain

SHOCK WAVES

INTRODUCTORY REMARKS FRANKLIN K. MOORE

Cornell University, Ithaca, New York

THIS afternoon's session concerns shock waves with radiative transfer. The problem is to examine the coupling which may exist between energy transfer by radiation and mechanical energy transfer by the shock-wave mechanism. Workers in this field are now busily engaged with matters such as precursor ionization, non-L.T.E, effects and so forth. However, it is probably fair to say that only quite recently, since 1963 or so, has a complete picture emerged for even the simplest case of shock wave structure modification by radiative transfer. I am referring especially to the work of Zeldovich, Heaslet and Baldwin, Raizer, and Mitchner and Vinokur. Perhaps, therefore, I should take a moment to sketch the result for strong plane shocks in a gray gas, with L.T.E. and other simplifying approximations.

If we first imagine a shock wave to produce a certain temperature jump in the absence of radiation, then if we "turn on" the radiation, the gas approaching the shock wave must increase in temperature, undergo its jump at the shock, and then (if the shock is strong enough) decrease in temperature because the gas behind is losing heat by radiation. Thus, a temperature "overshoot" is implied for a purely gasdynamic reason. If the shock is extremely strong, the radiation is very intense, and a parameter defined as the ratio of black body radiative flux behind the shock wave to the mechanical energy flux of the shock wave could be large. In such a case, even though the initial and final states must satisfy the Rankine-Hugoniot relations, the shock wave structure would begin with a very extensive thermal precursor region, followed by a jump and then a very abrupt fall of temperature to the final value. The precursor is optically thick and brings the temperature to a level, just before the jump, equal to the final Rankine-Hugoniot temperature. Thus, the jump together with the subsequent s'udden fall has the character of an "isothermal shock". The jump-fall part of the structure is optically thin.

Now, the foregoing is for a strictly one-dimensional shock wave which is perhaps a thing not to be found in real life. It is difficult to imagine that a very great investment of thermal energy in an optically thick precursor could actually take place in a real situation. Rather, the energy radiated forward will tend to be diverted to absorbent surfaces or to free space. So, an extrem'ely strong adiabatic Rankine-Hugoniot shock such as fluid mech- anics people are used to is not to be expected, if its structure is substantially affected by radiative transfer.

I should like now to introduce the session. There are five papers, and they emphasize non-L.T.E, considerations, non-grey gases and non-adiabatic processes. These compli- cations add higher orders of physical complexity beyond the purely gasdynamic and geometrical matters I have just mentioned.

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