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What are shockwaves?
Shock waves are strong compression waves caused by a sudden release of energy (such as an explosion) or by objects traveling at supersonic speed. Shock waves are features exclusive to the supersonic regime, and they always travel at a speed greater than Mach 1. They cause an instantaneous increase in properties such as pressure, temperature and density of the medium through which it traverses. Despite the destructive applications, shock waves can also be used constructively in a wide variety of fields. Shock tubes are devices that help in the generation and study of shock waves in a controlled manner.
Brief Description of Applications of Shock Waves
Aerodynamics
a) Basic studies on shock waves – Shock waves as an aerodynamic feature is still an area of research focus. The mechanics of flow within the very small thickness of the wave is not fully understood. At higher Mach numbers, it induces non-equilibrium in the flow with respect to the temperature. Such studies are generally addressed using direct spectroscopic measurements of the medium immediately downstream of the shock wave. Coherent Anti-Stokes Raman spectroscopy (CARS) is a commonly used method for this. Moreover, shock movement through media other than gases is a fairly new field of study. The physics of shock propagation through liquids and solids are still relatively unknown.
b) High speed flows – Shock tunnels are devices capable of generating highly energetic flows of very high Mach number. At present, the fastest sustained flight aircraft that is a reality travels with only Mach 3.1. Using shock tunnels, immense amount of research is conducted to push this envelope to above Mach 5. The measurements used in such tunnels include pressure, heat flux, pitching moments, lift and drag etc. At these high velocities, the aerodynamic heating is so high that no known substance can sustain it without change in shape or evaporation of material. Thus, aerodynamic shape adjustments have to be made to account for this bottleneck.
 
Chemical Kinetics
a) Pyrolysis – Decomposition of hydrocarbons and other compounds at high temperatures is an important field of study. This can be achieved by passing shock waves through these chemicals, thereby inciting instant rise of temperature, leading to reaction. By analyzing the final products, chemists are able to predict the reaction path and their rates.
b) Fuel ignition delay – In high efficiency fuels, such as rocket fuels, the delay between introduction of spark in it and the actual ignition is a very important parameter. This delay time can be accurately measured by passing a shock wave in a shock tube through the fuel. After the sudden rise in temperature, there will be a small time after which the fuels combust. This delay time can be measured from the pressure signal recorded at the inner walls of the shock tube.
 
Medical Applications
a) Lithotripsy – Kidney stones are generally formed in large enough masses to get stuck within the organ without being able to move further. Focussed shock waves of the right strength can be used to rupture these stones into tiny pieces without affecting the human tissue around. These small pieces can then be passed along the urinary tract without pain.
b) Traumatic brain injury – Injuries due to sudden impact on the head can be replicated using shock waves. These experiments are generally carried out on laboratory rats, which are later dissected and checked for growth of the trauma.
c) Needle-less drug delivery – Painless drug delivery can be achieved by delivering the medicine through the surface of the skin with the assistance of a weak shock wave. The medicine then reaches a special group of cells known as ‘Langerhans’s cells’, which will then be spread throughout all the organs of the body, just as an injection into the blood stream would do. This method is cheap, painless and safe.
d) Artificial Insemination of Cows – Shock assisted insemination of cows were seen to be far more efficient than conventional methods without damage to the sperm tissue or discomfort to the cow.
 
Material Studies
a) Shock impact on materials – Impact of strong shocks on hard materials such as ceramics causes physical and chemical changes on its surface, even change in surface morphology. Studies into this field would be critical in designing heat shields for re-entry satellite vehicles etc.
b) Projectile launcher – Shock waves can be used to propel small projectiles with high velocities in excess of Mach 1. The effect of impact of such a projectile of certain materials leads to an interesting field of research. Such an impact has the ability to change the properties of the material. Studies in this regard will help in creating workarounds for unwanted impact on fast moving aircrafts.
c) Blast mitigation studies – Many defence departments around the world are conducting research into designing the best possible material to regulate the strength of a blast wave so as to protect personnel from bomb blasts. Vests for soldiers and buildings in dangerous zones may be fabricated from these materials so that the transmitted impact of the blast wave is a minimum, thereby saving lives. Shock tubes can be used to simulate these conditions and then figure out the most efficient material for this purpose.
 
Instrumentation
a) Dynamic calibration of sensors – Shock waves can be used to conduct dynamic calibration of piezoelectric or piezo resistive pressure sensors and accelerometers so as to figure out the maximum frequency response of the device. This becomes an important parameter when the measurement involves highly fluctuating input, such as pressure measurements inside a combustion chamber.
b) Dust removal from micro channels in silicon chips – At small scales, the strength of adhesion becomes so high that removing dust particles from micro and Nano channels become very difficult. Targeted shock waves can be used to clear up these channels.
 
Civil Engineering
a) De-clogging of blocked bores – Over times; the water channels that feed bore wells get clogged up due to particular blockage. Shock waves sent down these bores can open up these channels and restore water supply in the wells.
 
Industrial Applications
a) Tea industry – Shock application on tea leaves before the withering process reduces the drying time considerably, increasing the time efficiency of a tea factory by as much as 300%. A chemical study conducted on the shock-impacted tealeaves has suggested that the anti-cancerous property of the tea has been accentuated.
b) Wood industry – Wood used for making pencils and furniture has to be impregnated with chemicals that make it stronger and more resistant to termite attacks, moisture damage etc. These chemicals can be efficiently and quickly induced into the wood fibres with the application of shock waves. Sandalwood gives more yield from shock treated wood in comparison to the conventional method.
c) Juice extraction – Shock waves have been used to extract pure juice from apple, bitter gourd etc. Direct impact of shock waves of the right strength causes the cells within the fruit or vegetable to rupture, thereby releasing the juice while the outer shell remains unaffected. After the process, by simply broaching the skin of the fruit, the juice can be poured out.
Footnote: The list of applications enumerated here is not exhaustive. There is a multitude of other applications that are currently known and more yet to be discovered. For further explanation of the applications listed here, please refer “Shock Waves Made Simple” by C.S. Kumar, K. Takayama and K.P.J. Reddy from Wiley Publishers.
PUBLICATIONS:
In a very short time since the invention many research papers have been published on Reddy tube.
Partial list of these publications are given below:
  1. Reddy K.P.J. and Sharath N, “Manually operated piston-driven shock tube”, Current Science, 104 (2), pp 172-176, 2013.
  2. Reddy K.P.J. and Babu R. "Reddy tube driven table top hypersonic shock tunnel", Proceedings of the International Symposium on Shock Waves 29, Wiscosin, USA, 2013.
  3. Kumar, C. S, Takayama, K, Reddy, K. P. J, “Shock Waves Made Simple,” Wiley Estern, New Delhi (2013).
  4. Surana K.S., Reddy K.P.J., Joy A.D. and Reddy J.N., "Riemann shock tube: 1D normal shocks in air, simulations and experiments", International Journal of Computational Fluid Dynamics, DOI:10.1080/10618562.2014.927056, 2014.
  5. Dhananjaya I. Bhat, Dhaval Shukla, Anita Mahadevan, N. Sharath, K.P.J. Reddy, “Validation of a blast induced neurotrauma model using modified Reddy tube in rats: A pilot study”, The Indian Journal of Neurotrauma, 11, 91-96, 2014.
  6. C S Kumar and Reddy K.P.J. “Experiments in hand operated, hypersonic shock tunnel facility”, Shock Waves,
    DOI: 10.1007/s00193-015-0608-x, 2015.
  7. Reddy K. P. J, Sharath N, Babu R and C S Kumar, “Experiments using Reddy Tube Driven Table Top Hypersonic Shock Tunnel”, Proceedings of the International Symposium on Shock Waves 30, Tel Aviv, Israel, 2015.
  8. Reddy KPJ, Sharath N, Babu R and C S Kumar, “Table top shock tube and hypersonic shock tunnel facilities”, Proceedings of the 11th International Workshop on Shock Tube Technology, Gottingen, Germany, 2016.
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Video Links Useful Links:
   
– Huge explosion and shock waves
https://www.youtube.com/watch?v=KC7zdZ00eZ4
– Research facility for shock waves in LHSR, IISc
http://www.aero.iisc.ernet.in/~lhsr/web/index.htm
– Film on shock waves by vigyan prasar, shows India’s work
on shock wave research
https://www.youtube.com/watch?v=K0PHI3FlJbg
 
– Nevada chemical plant explosion showing destructive force
of the shock wave
https://www.youtube.com/watch?v=_KuGizBjDXo
 
– Shock wave produced by atomic test, code named Shock Ravel
https://www.youtube.com/watch?v=UTdy1Yp1h5A
 
– Simulation of shock waves by analyzing flow through a nozzle
https://www.youtube.com/watch?v=cK7g4xio0WU
 
– Video showing shock wave formation in transonic fight
https://www.youtube.com/watch?v=K08Gc0tKWoA
 
– Technical talk on Shock Waves at Bangalore Science Forum 2013
https://www.youtube.com/watch?v=MXzKOWVEnHE