Reflection of a shock wave from the shock tube end wall:

A shock wave which is reflected from the end wall of the shock tube then travels towards the diaphragm location. The region behind this reflected shock in termed as the region five (5). The gas in this region is at rest, thus. The reflected shock wave travels into the oncoming flow of velocity . Thus the shock jump relations can be applied to the relations across the reflected shock wave as,

	(3.1.13)
	(3.1.14)

 

36. 2. Variants of Shock tube.

Strength of the primary shock is defined by the higher pressure ratio between region 2 and 1 (Eq. 35.2) or in other words the higher pressure rise due to motion of the primary shock. Stronger primary shock is quicker and faster. Enhanced shock strength is many a times requirements of experimentalists to satisfy the necessary requirement. As we observed that the strength of the primary shock can be increased if the ratio of acoustic speeds of driver gas to driven gas is high. Hence for a same driven gas we need to have higher acoustic speed of the driver gas. There for following strategies become useful to increase the speed of the primary shock wave.

1. Increse in pressure of the dirver for the same driven gas pressure. This strategy leads to increase in the driving pressure ratio for increasing the strength of primary shock.

2. Use of lighter driver gas like hydrogen and helium. However hydrogen might lead to explosion for air as driven gas conditions so helium is invariently used to satisfy this strategy. Helium as driver gas is helpful for increasing the acoustic speed by the virtue of higher specific heat ratio and higher gas constant.

3. Heating of the driver gas increases its temperature and hence the acustic speed which in turn increases the strength of the primary shock wave.

The strategy of driver gas heating can be implimented with helium driver for integration of two strategies to get very high shock speed. There are many ways to increase the driver gas temperature. Simplest way is to incorporate the driver gas heating through electric heater placed in the driver section or attached to the walls of the driver section. Another way of driver gas heating is the use of double diaphragm shock tube which has an extra section behind the driver section and they are separated by one more diaphragm. Bursting of the extra diaphragm leads to a shock wave propagating in the original driver section which in turn increases the pressure and temperature of this driver gas. Reflection of this shock wave from diaphragm section between real driver and driven sections rises the pressure to the extend to bust this diaphragm. However this diaphragm rupture has been achieved by high pressure and temperature driver which is responsible for high speed primary shock wave in the driven section. Use of combustion reaction in the driven section can also be helpful to raise the pressure and temperature of the driver gas at the time of diaphragm rupture. Adiabatic compression of the driver gas can also be achieved by compressing the driver gas using a piston placed at end of the driver section and moving the same using an external reservoir.

36. 3. Applications of shock tube:

Shock tube is very useful equipment and can be implemented for various applications in its various variants. Shock tube in its simplest format is used for the external flow aerodynamics studies in the higher subsonic, transonic and lower supersonic Mach numbers. Higher diameter tubes are advisible in these cases to avoid the domination of viscous effects. For the experimentation in supersonic or hypersonic flow regimes, driven tube end is connected to a convergent divergent nozzle along with the test section and dump tank to comprise a shock tunnel. This formation helps to expand the stanganant gas behind the reflected shock through the nozzle to achieve high speed flow in the test section. Shock tube can also be used for chemical kinetic studies for measurement of reaction rates. Use of shock tube for contact less drug delivery has also been thought for its application in medical sciences.