IIT Jodhpur
Seminar on "Transient Flow Characteristics in High-Speed Confined Jets" - 27 September 2018

Date and Venue Information:
27 September 2018, Seminar Hall, 4:30 PM, Department of Mechanical Engineering

Title of Seminar:
"Transient Flow Characteristics in High-Speed Confined Jets" by Arun Kumar

The present study investigates three fundamental aspects of high-speed flows in confined ducts which find applications in high altitude testing facilities, vacuum pumps, fluidic actuators and straight ejector diffuser. These are, 1) secondary flow evolution, 2) primary jet expansion characteristics and 3) the transformations of shock structures with the increasing jet total pressure. It is seen that the secondary flow exhibits a large re-circulation bubble (primary re-circulation bubble) during the initial start-up. With increase in primary jet expansion, this bubble splits into two. It is also seen that along with the primary re-circulation bubble there exists another re-circulation bubble in the outer duct upstream (secondary re-circulation bubble). With further increase in pressure, the main jet expands to the outer duct. It should be noted that the know-how of dynamics of these re-circulation bubbles in the secondary flow evolution is very useful in understanding the starting transients involved in zero secondary flow or vacuum ejectors. Pressure measurement results show that the vacuum generation in zero secondary flow ejectors is non-monotonic with increasing jet total pressure, with three distinct stages of evacuation. These three different stages are found to be caused by the existence of the primary and secondary re-circulation bubbles, the splitting of primary re-circulation bubble into two and the attainment of frozen state at the jet impingement point with the outer duct, respectively. The primary jet expands asymmetrically and is found to get amplified due to shock-shear layer interaction. However, no asymmetric expansion has been found in 3-D sudden expansion jets. The present study also demonstrated a novel control strategy to make symmetric expansion in plane sudden expansion geometries using insert plates at duct exit.
The shock characteristics in underexpanded confined jet show a transformation from Regular reflection (RR) to Mach reflection (MR) and vice versa, with increase in jet total pressure. This is analogous to the shock transformations in open jets but in a reverse order. However, the cause for the MR-RR transformation in the present case seems to be different from the corresponding open jet scenario. It is found that during this transformation, neither the upstream Mach number nor the incident shock angle is varying, which is in contradiction to classical shock transformations observed till date. Thus the present transformation is found to be a total pressure variation induced one, which is found to be a new kind of shock transformation.
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Brief Profile:
Dr. Arun Kumar completed his Ph.D. from the Department of Aerospace Engineering, IIT Madras in February 2018. Currently, he is working as a Post-Doctoral Fellow at the Department of Aerospace Engineering, IIT Madras. Dr. Arun has been actively involved in wide spectrum of research activities on high speed flows using experimental and computational techniques, particularly in supersonic confined jets and shock wave reflections.  His research  activities on confined jet provides an in-depth understanding on the recirculation bubble dynamics, the secondary fluid entrainment characteristics and the base pressure evolution in backward step kind sudden expansion geometries and has been selected by Physics of Fluids Journal as ‘Editors Pick’ in June 2018 Edition. These studies are of great importance in designing and developing various high-speed engineering applications like High Altitude Testing (HAT) facilities, hypersonic wind tunnels and vacuum pumps. He is also interested in developing and optimizing various high-speed engineering devices like vacuum ejectors and fluidic actuators.  Dr. Arun is also having a strong research background in shock wave dynamics using complex experimental techniques and in developing experimental facilities for generating shock/blast waves. His recent research works involves unsteady shock reflections and refraction over various interfaces, shock wave diffraction over under water bodies, shock bubble interactions and asymmetric expansion in high-speed confined jets. He has been recently awarded with INSPIRE FACULTY Fellowship funded by Department of Science and Technology.