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The Secret of Shock Waves: Why Does High Mach Number Flight Change the Properties of Fluids?
With the continuous advancement of science and technology, aerospace engineers have gradually deepened their research on high Mach number flight. When a vehicle exceeds the speed of sound, and especially when it reaches the famous "high Mach numbers", the properties of the fluid undergo changes that cannot be underestimated. The effects of high Mach numbers, defined as speeds exceeding five times the speed of sound, on aircraft performance and the flow field around it are dramatic. These changes involve the core principles of fluid mechanics, which not only challenge traditional scientific cognition but also pave the way for the development of future aviation technology.
Characteristics of flow field
While there is debate over the exact definition of high-Mach flow, some physical phenomena can generally be identified that are difficult to ignore analytically in supersonic flows.
The particularity of high Mach flow is reflected in the shock layer, aerodynamic heating, entropy layer, real gas effect and low density effect.
Small shock wave spacing
As the Mach number of an object increases, the density behind the shock wave in front of the object also increases, which is based on the principle of conservation of mass, accompanied by a decrease in volume behind the shock wave. At higher Mach numbers, the distance between the shock wave and the object will decrease, a phenomenon that is particularly evident in high Mach number flights.
Formation of the Entropy Layer
As the Mach number increases, the entropy change of the shock wave also increases, which leads to the formation of strong entropy gradients and strong turbulence, and closer mixing with the boundary layer. This condition directly affects the aerodynamic characteristics of the object and puts higher demands on the design of the aircraft.
Sticky Interaction
In high Mach number flows, part of the large kinetic energy is converted into the internal energy of the fluid due to the viscosity effect. This change is manifested in the increase of the fluid temperature. As the temperature increases, the density of the boundary layer decreases, which causes the boundary layer to become thicker and often merge with the shock wave near the leading edge of the fuselage.
High Temperature Flow
Due to the effect of viscous dissipation, high Mach number flow will cause the fluid temperature to rise suddenly, which will lead to non-equilibrium chemical flow properties, including molecular vibration excitation, dissociation and ionization, which will cause convection and radiation. The influence of heat flow.Classification of Mach number categories
When discussing these high-speed flows, scientists usually classify them by their Mach number. After entering the transonic region, the fluid begins to exhibit complex behavior, which makes traditional calculation methods gradually ineffective at this stage.
NASA defines "high" as the Mach number range of Mach 10 to 25, and re-entry speeds as those exceeding Mach 25.
Similarity Parameters
High Mach number flows require higher dimensional similarity parameters for classification, which can help simplify the countless test cases to obtain comparable flow behavior. Scientists need to consider the real gas effects of the fluid, and under non-equilibrium flow conditions, the number of variables required to describe the gas state can be as high as ten to hundreds.
Regions of high Mach number flow
High Mach number flows can be roughly divided into several regions, each exhibiting different physical properties and flow behavior:
Perfect gas: The flow range considered as an ideal gas.Bi-temperature ideal gas: The temperatures of rotation and vibration need to be considered separately.Dissociated gas: The molecules of the gas begin to dissociate, affecting surface heating.Ionized Gas: Separate modeling of electron and gas components.Radiation Dominated Region: Heat conduction transforms into radiation dominated transfer.
With more in-depth research on high Mach number phenomena, future aerospace technology may be able to break through current boundaries and explore the possibility of flight at higher speeds. However, the physical principles behind these changes are still worth further thinking and understanding. How will future aerospace technology be affected by these flow characteristics?
