Mach Waves

Consider an aerodynamic body moving with certain velocity (V) in a still air. When the pressure at the surface of the body is greater than that of the surrounding air, it results an infinitesimal compression wave that moves at speed of sound (a). These disturbances in the medium spread out from the body and become progressively weaker away from the body. If the air has to pass smoothly over the surface of the body, the disturbances must ‘warn' the still air, about the approach of the body. Now, let us analyze two situations: (a) the body is moving at subsonic speed ; (b) the body is moving at supersonic speed .

Case I : During the motion of the body, the sound waves are generated at different time intervals as shown in Fig. 4.4.1. The distance covered by the sound waves can be represented by the circle of radius . During same time intervals , the body will cover distances represented by, . At subsonic speeds , the body will always remains inside the family of circular sound waves. In other words, the information is propagated through the sound wave in all directions. Thus, the surrounding still air becomes aware of the presence of the body due to the disturbances induced in the medium. Hence, the flow adjusts itself very much before it approaches the body.

Case II : Consider the case, when the body is moving at supersonic speed . With a similar manner, the sound waves are represented by circle of radius after different time intervals. By this time, the body would have moved to a different location much faster from its initial position. At any point of time, the location of the body is always outside the family of circles of sound waves. The pressure disturbances created by the body always lags behind the body that created the disturbances. In other words, the information reaches the surrounding air much later because the disturbances cannot overtake the body. Hence, the flow cannot adjust itself when it approaches the body. The nature induces a wave across which the flow properties have to change and this line of disturbance is known as “Mach wave”. These mach waves are initiated when the speed of the body approaches the speed of sound . They become progressively stronger with increase in the Mach number.

Fig. 4.4.1: Spread of disturbances at subsonic and supersonic speeds.