Rarefied Gas Dynamics

for exa mple, high­ altitude hype rsoni c flow fields, the reflectiv e and reactive characteristi cs of gases interacting with soli d and li qui d surfac es, energy- transfer phenomena in molecula r collisions, aerosol dynamics, cluste r formation and topology, flows induced by evapor ation and conde nsation, upper-atmosp heric dy­ namics, and the attai nment of milli -Kelvi n temperatures by flow cooli ng te chniques. Other sub jects in the field incl ude vacuum- pump perfor­ mance, spa cecraft con tamination, a var iety of interactions due to thru st er plumes, spa cecraft charging, and gas and isotope sepa rat ions. Underlying all of these sub jects is the cen tral them e of the field of rarefie d gas dynamics: the study of gas flow phenomena in which the discrete mole cular nature of the gas cannot be sa fel y ignored. The field has a rich heritage of analysi s appl ied to the st udy of flows where conce pt s and techniques related to none qui li brium st atistical mechanics are important. An equally respe cted tradition is the deve lopment and application of instrume ntation techniques that can be use d to study the details of mole cula r motion in gas flows, as well as to study flow-ge nerated popul ations of internal energy st ates and the charac teristics of gase s after surfac e encounters. Rarefie d gas dynami cs is founded on the pion eering work in the ki netic theory of gases begun in the la tter half of the nin eteenth century, which has cont inue d to the present. What today we call rarefied gas dynami cs was discusse d as early as 1934 by Zahm (1934), but research began in earnest shortly after World War II (Tsie n 19 46). Because of growing interest in space and flight at extre me altit ude, attention was drawn to a flow regim e that had not been se rio usl y consi dered as a conce rn of either aer ody nami cs or gas dynamics. In this flow re gime, the mole cula r mea n free path in the gas, .A., become s si gni ficant compa red with either a charac