Module 4 : COMPRESSIBLE FLOW

Lecture 6 : Hypersonic Flow: Part - I

 

Introduction to Hypersonic Flow

The hypersonic flows are different from the conventional regimes of supersonic flows. As a rule of thumb, when the Mach number is greater than 5, the flow is classified as hypersonic. However, the flow does not change its feature all of a sudden during this transition process. So, the more appropriate definition of hypersonic flow would be regime of the flow where certain physical flow phenomena become more important with increase in the Mach number. One of the physical meanings may be given to the Mach number as the measure of the ordered motion of the gas to the random thermal motion of the molecules. In other words, it is the ratio of ordered energy to the random energy as given in Eq. (4.6.1).

(4.6.1)

In the case of hypersonic flows, it is the directed/ordered kinetic energy that dominates over the energy associated with random motion of the molecules. Now, recall the energy equation expressed in the form of flow velocity (V), speed of sound and stagnation speed of sound .

(4.6.2)

Eq. (4.6.2) forms an adiabatic ellipse which is obtained for steady flow energy equation. When the flow approaches the hypersonic limit, the ratio becomes . Then, Eq. (4.6.2) simplifies to the following expression.

(4.6.3)

In other words, the entire kinetic energy of the flow gets converted to internal energy of the flow which is a function total temperature of the flow.

The study/research on hypersonic flows revels many exciting and unknown flow features of aerospace vehicles in the twenty-first century. The presence of special features in a hypersonic flow is highly dependent on type of trajectory, configuration of the vehicle design, mission requirement that are decided by the nature of hypersonic atmosphere encountered by the flight vehicle. Therefore, the hypersonic flight vehicles are classified in four different types, based on the design constraints imposed from mission specifications.