Characteristics Features of Hypersonic Flow

There are certain physical phenomena that essentially differentiate the hypersonic flows as compared to the supersonic flows. Even though, the flow is treated as supersonic, there are certain special features that appear when the speed of the flow is more than the speed of sound typically beyond the Mach number of 5. Some of these characteristics features are listed here;

Thin shock layer : It is known from oblique shock relation that the shock wave angle decreases with increase in the Mach number for weak shock solution. With progressive increase in the Mach number, the shock wave angle reaches closer to the flow deflection angle . Again, due to increase in temperature rise across the shock wave, if chemical reaction effects are included, the shock wave angle will still be smaller. Since, the distance between the body and the shock wave is small, the increase in the density across the shock wave results in very high mass fluxes squeezing through small areas. The flow region between the shock wave and the body is known as thin shock layer as shown in Fig. 4.6.1(a). It is the basic characteristics of hypersonic flows that shock waves lie closer to the body and shock layer is thin. Further, the shock wave merges with the thick viscous boundary layer growing from the body surface. The complexity of flow field increases due to thin shock layer where the boundary layer thickness and shock layer thickness become comparable.

Fig. 4.6.1: Few important phenomena in a hypersonic flow: (a) Thin shock layer; (b) Entropy layer; (c) Temperature profile in a boundary layer; (d) High temperature shock layer; (e) Low density effects.

Entropy layer : The aerodynamic body configuration used in hypersonic flow environment is typically blunt to avoid thin shock layers to be closer to the body. So, there will be a detached bow shock standing at certain distance from the nose of the body and this shock wave is highly curved (Fig. 4.6.1-b). Since, the flow process across the shock is a non-isentropic phenomena, an entropy gradient is developed that varies along the distance of the body. At the nose portion of the blunt body, the bow shock resembles normal to the streamline and the centerline of the flow will experience a larger entropy gradient while all other neighboring streamlines undergo the entropy changes in the weaker portion of the shock. It results in an entropy layer that persists all along the body. Using the classical Crocco's theorem , the entropy layer may be related to vorticity. Hence, the entropy layer in high Mach number flows, exhibits strong gradient of entropy which leads to higher vorticity at higher magnitudes. Due to the presence of entropy layer, it becomes difficult to predict the boundary layer properties. This phenomenon in the hypersonic flow is called as vortcity generation. In addition to thin shock layer, the entropy layer also interacts with viscous boundary layer that leads to very complicated and unknown flow features.

Viscous-Inviscid interaction : When a high velocity, hypersonic flow is slowed down in the vicinity of the aerodynamic body due to viscous effects within the boundary layer, the major portion of the kinetic energy is transformed into the internal energy of the gas known as viscous dissipation leading to increase in temperature. For a cold wall, the typical temperature profile in a boundary layer is shown in Fig. 4.6.1(c). Since, the pressure is constant in the normal direction through the boundary layer, the increase in temperature results decrease in density. In order to pass through a given mass flux at reduced density, the thickness of the boundary layer must be larger. Thus, the displacement thickness increases, causing the body shape to appear much thicker and displacing outer inviscid flow. Hence, the free stream flow encounters an inflated object which changes the shock shape and in turn boundary layer parameters such as surface pressure, wall heat flux, skin friction etc. Again, when the boundary layer becomes thick, it essentially merges with the thin shock layer. Thus, there are major interactions of viscous boundary layer, thin shock layer and outer inviscid flows. This phenomenon is known as viscous-inviscid interaction and has important effect on the surface pressures and the stability of hypersonic vehicles.