Incident shock zone

One effect that has been studied extensively is the build-up of a boundary layer on the shock tube walls. The source of the problem is the fact that the walls are at room temperature throughout the experiment. Hence, there is a severe temperature gradient between the wall and the shocked gas. A layer begins to form on the walls behind the shock wave and grows in thickness as the distance between the shock front and contact surface increases. The thickness of the boundary layer affects the calculation of the observation time in the incident shock zone along with the temperature and density of the gas. In addition to these corrections, allowance has to be made for the temperature decrease which accompanies the endothermic process of dissociation. A review [8] of the experimental and theoretical work accomplished in this area, along with practical formulae that enable shock tube workers to estimate the magnitude of the deviations from ideal behaviour, has been published. 

The shock front continues down the tube until it collides with the end wall. It is reflected back up the tube, passing once again through the incident zone, raising it to a higher temperature and pressure, and creating the reflected shock zone. Measurements may be made on the reflected zone gas as a function of time. Since the observation time is short with respect to human response, the data are taken from photographs of oscilloscope traces. 

The amplified signals are recorded on a raster pattern upon which timing marks have been superimposed. Although an ideal shock wave would display a constant velocity, a real shock wave is usually attenuated with respect to its velocity. This non-ideal behaviour must be recorded in order to make corrections to the calculation of the density, pressure, and temperature of either the incident or reflected shock zones. 

In Fig. 2.41, O represents the zone of not being bothered I refers the zone in with incident shock waves passing through and reflected shock waves not reaching ... 

In case of colder and wet vapor, the 2-dimensional wedge can produce the compressed and supersonic condensing flow at the inclined surface as shown in Fig. 6. We can see immediately after the attack of incident shock wave a very rapid condensation zone that propagates from the apex of wedge at series No.3,4,5,6 and the left hand side of the wedge becomes strongly... 

The thickness of the compression zone is about 10" cm at atmospheric pressure thus the time for compression is about lO" sec. Since the wave length of visible light is of this order of magnitude, the optical reflectivity method was developed. Experiments were carried out in which the angle of incidence on the shock front was kept constant, and the angle of observation varied. In the case of simple air shock... 

Nevertheless several investigations of H2-O2—diluent mixtures reacting in incident or reflected shock waves have employed ultraviolet emission to observe the length of the induction period,the time constant of the exponential growth of the emission intensity with fixed quenching and detection conditions,and the postignition zone under very low density conditions. Many of the results are in accord with proportionality of the OH chemiluminescence intensity to the product [0][H]. 

---