Similarity laws have opened the door for fluid dynamic experiments using subscale models. These laws enable the study of dynamically similar flows with geometrically similar models that are a fraction of the full-scale size with a concomitant reduction in required power. In some instances these similarity laws require experimental facilities that operate with fluids under very high temperatures and pressures in order to push the similarity variables to full-scale values or at least to useful values.
High-enthalpy flow conditions encountered in hypersonic flight require testing facilities that produce chemically reacting flows and plasmas. Ignition processes and chemical kinetics generally require high temperature and pressure facilities. Shock tubes provide a reliable and repeatable means to produce conditions for high enthalpy flows and chemical kinetics. The short duration of any flow state in a shock tube, usually of the order of milliseconds, presents measurement challenges for fluid properties and reaction rates. Similarly, extremely fast response times are required to measure transient flow quantities in turbomachinery. The exploitation of material properties to create useful test conditions often introduces 'side effects' that must be accounted for in models and measurements. Paradoxically, the most difficult part of some experiments can be the understanding and management of these effects. For example, the thermodynamic state of a working fluid might be far from that encountered by a full-scale system, or the mechanical properties and dynamical behaviour of structural materials might determine the feasibility of meaningful measurements. For modelling and risk management, the behaviour of candidate materials and working fluids must be thoroughly understood.
In this special feature, we survey some of the facilities and methods that have been developed for the measurement of fluid properties and processes under a wide range of conditions. Our focus is on facilities developed for aero/fluid dynamic systems that range from high-Reynolds number facilities that employ high-pressure water and dense gases, to shock tubes for chemical kinetics and high-enthalpy flows, to a low-pressure shock tube for instrument calibration. The authors in this issue were asked to present new results that highlight some of the unique features of their test facilities that both explore and exploit the physical and thermodynamic properties of fluids.
I thank the authors for their response to this call, and on their behalf I offer my sincere thanks to the editorial staff of Measurement Science and Technology for their extremely efficient and professional support in assembling this special feature. I also extend a personal thanks to Professor John Foss for first suggesting this special feature and for his support while it was assembled.