  Shock tubes have emerged as an effective tool for applications in various
fields of research and technology. The conventional mode of shock tube
operation employs a frangible diaphragm to generate shockwaves. The last
half-century has witnessed significant efforts to replace this
diaphragm-bursting method with fast-acting valves. These diaphragmless methods
have good repeatability, quick turnaround time between experiments, and produce
a clean flow, free of diaphragm fragments in contrast to the conventional
diaphragm-type operation. The constantly evolving valve designs are targeting
shorter opening times for improved performance and efficiency. The present
review is a compilation of the different diaphragmless shock tubes that have
been conceptualized, developed, and implemented for various research endeavors.
The discussions focus on essential factors, including the type of actuation
mechanism, driver-driven configurations, valve opening time, shock formation
distance, and operating pressure range, that ultimately influence the shockwave
parameters obtained in the shock tube. A generalized mathematical model to
study the behavior of these valves is developed. The advantages, limitations,
and challenges in improving the performance of the valves are described.
Finally, the present-day applications of diaphragmless shock tubes have been
discussed, and their potential scope in expanding the frontiers of shockwave
research and technology are presented.
