Shock waves and its effect on supersonic flight
Definition of shock wave – shock waves are defined as a compressional large amplitude wave evolved by abrupt change of pressure and density in a narrow region travelling through a medium, especially in air, caused by earthquake/explosion or by an object moving faster than the speed of sound.
The thickness of the shocks is comparable to the mean free path of the gas molecules in the flow field.
Formation – the flow consists of a large number of fluid molecules in unit volume and the transport of mass, momentum and energy takes place through the motion of these molecules. Also, the molecules carry the signals about the presence of the cylinder around the flow field at a speed equal to speed of sound. As shown in fig the
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The lines which can be drawn at any flow field with the inclination of μ are said Mach lines of Mach waves. In the nonuniform flow field, μ varies with M and the Mach lines are curved. In the field of fluid flow at any point P, there is always two lines which intersect the streamline at the angle μ. Considering in the three- dimensional flow, the flow of supersonic stream is always associated with two families of mach lines represented in plus and minus signs. The mach lines with + sign run to the right of the streamline when viewed through the flow direxn and the lines with “–“ sign run to the left. These mach lines are also called characteristics.
These charecteristics play an important role in the compression and expansion process in the sense that it is only through these lines that it is possible to retard or accelerate a supersonic flow isentropically.
Supersonic Compression – in supersonic flow compressions are not generally isentropic. Usually they take place through shock waves. But in some cases the compression is isentropic where flow turning is achieved through large number of weak oblique shock as shown in