The shock wave may be described as a compression front in a supersonic flow field, and the flow process across the front results in an abrupt change in fluid properties. The thickness of the shock wave is comparable to the mean free path of the gas molecules in the flow field. In other words, shock is a thin region where large gradients in temperature, pressure and velocity occur, and where the transport phenomena of momentum and energy are important. The normal shock wave is a compression front normal to the flow direction. However, in a wide variety of physical situations, a compression wave inclined at an angle to the flow occurs. Such a wave is called an oblique shock. Indeed, all naturally occurring shocks in external flows are oblique.