Supersonic velocity means that the speed exceeds the propagation speed of sound in the air, which may occurshock waveA shock wave is a wave that propagates in a fluid at a speed higher than the speed of sound and compresses the fluid.The shock wave in the gas is the most obvious. There is a very thinWavefrontseparate.The pressure in front of and behind this wave front is different, with sudden changes.The greater the pressure ratio,shock waveThe stronger.
shock wave
Gases, liquids and solidsmediumMedium pressure (or stress), density and temperature jump on the wave frontCompressional wave。also calledshock wave。During explosion, impactSupersonic flowThe shock wave will appear during the process.When a series of compression waves propagate in a compressible medium, the higher the degree of compression of the medium, the faster the compression wave will propagate. Later compression waves propagate in the previously compressed medium, and later waves will catch up with the previous waves, so the compression wave system will overlap at a certain position to form a strongJerkCompressedshock wave。Sudden changes occur in a very thin layer, calledShock layerAnd its thickness is moleculeMean free pathTherefore, the shock wave can be approximated as a strong discontinuity without thickness.If the shock waveWavefrontWavelikeDirection of movementVertical, forNormal shock wave；Otherwise, it is oblique shock wave.
correlation theory
According to the conservation relation of mass, momentum and energy that must be obeyed by the medium movement, we can get a group of contact shock waves before and aftermediumThe relationship between the speed u, pressure p, density ρ, specific enthalpy (enthalpy per unit mass) h and other parameters is the Rankine Hugoniot relationship.This set of equations is applicable to gas, liquid and solid, and their specific expressions are different due to different equations of state for different media.The tangential velocity has no change, and the jump of normal velocity has substantial significance.Normal shock waveThe flow ahead is supersonicshock waveThen it becomes subsonic, and its pressure, density and temperature increase,Total pressureThe total temperature remains unchanged when the temperature drops.This feature is consistent withThe second law of thermodynamics, i.ePrinciple of entropy increase，Supersonic flowAfter the shock wave, part of the mechanical energy is irreversibly converted into heat energy, indicating that the total pressure drops;But it also conforms toconservation of energy, the total energy remains unchanged, that is, the total temperature remains unchanged.The stronger the shock wave,Entropy increaseThe larger.
Wave resistance
The gas shock wave will cause great resistance on the supersonic aircraft, which is calledWave resistance。Piping (e.gSupersonic wind tunnel、Jet engineThe shock wave in) will reduce the equipment efficiency.Generally, measures shall be taken to eliminate shock wave or reduce its strength.
Generate clouds
At the same time, the supersonic may also produce a cloud on the supersonic object, but the specific reason has not been identified by scientists, which needs new research in the future.[1]
Applied to aircraft
In Aerodynamic Design of Aircraft, Dietrich Kch ǔ emann, an outstanding aerodynamic scientist, divided the aircraft into four systems: classical system, swept back system, slender system and wave riding system.This classification covers the scope of aircraft development in the next 100 years from an unprecedented macro perspective.In particular, the latter three systems describe supersonic aircraft.[2]
Relational calculation
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In the coordinate system moving with the shock wave, the shock wave is fixed.At the point P on the shock wave in Figure 1, connect the medium velocity v, pressure p, density ρ and ratio before and after the shock waveenthalpyH (enthalpy per unit mass of substance)mass conservation, but conservation andConservation of energyThe equations are:
Shock wave relation
Subscripts 1 and 2 represent the parameters before and after the shock wave respectively, and n and t represent the components in the normal direction n and tangent direction t of the shock wave at point P.These basic relations are applicable to any medium, including gas, liquid and solid, but can have different expressions depending on the medium (see shock wave in solid).These relations are often called Rankin Hugonew relations.In order to make the above equations closed, the equation of state of the medium should also be supplemented.The equation of state of gas is well studied, and the equation of state of solid and liquid under high temperature and pressure needs further study.
Figure 1
For the complete gas with constant specific heat, using the corresponding equation of state, we can directly solve the relationship between the gas flow parameters behind the oblique shock wave:
Relational expression
type
Is the critical sound speed (corresponding to the sound speed when Ma=1);Ma1 is the wave front airflowmach number；β is shock wave relativeYu BoForward airflowInclination angle(Figure 1);T. S and p0 are respectivelyThermodynamic temperature, specific entropy (entropy of matter per unit mass) and total pressure;γ is the specific heat ratio.When β equals 90 °, these relations become normal shock relations.
In the normal shock wave, there is a relationship v1v2=c * 2 or λ 1 λ 2=1, which is called the velocity coefficient in the formula λ=v/c *. When the velocity is equal to the sound velocity, λ=1. This relationship shows that the supersonic flow (λ>1) changes into subsonic flow (λ<1) through the normal shock wave, and the opposite change is impossible.From the relationship between entropy increase (△ S=S2-S1) and Mach number in front of the positive shock wave (Figure 2), if the wave front is subsonic flow (Ma1 ＜ 1), then △ S ＜ 0, which violatesThe second law of thermodynamics, so it is impossible.
Figure 2
From the mass conservation equation, the relationship between the angle of deflection δ of the gas flowing through the shock wave and the tilt angle β of the shock wave can be obtained directly:
The relationship between the angle δ of flow after passing through the shock wave and the tilt angle β of the shock wave
For a complete gas with constant specific heat, this relationship is reduced to:
The relationship between the angle δ of flow after passing through the shock wave and the tilt angle β of the shock wave
Corresponding to a certain Ma1, there is a maximum angle δ max.When the air flow with Mach number of Ma1 encounters a sharp wedge with half the top angle of α (Fig. 3), if α<δ max, a oblique shock wave attached to the top of the wedge will be formed;If α ＞δ max, an isolated bow shock wave will be generated in front of the wedge.
Figure 3
Supersonic velocity measurement
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For a long time, the supersonic flow field has been "invisible and untouchable". Its airflow speed can reach thousands of kilometers per second, the temperature range can span thousands of degrees Celsius, and the turbulence fluctuation frequency can reach 1MHz. To accurately obtain the distribution of the velocity field, density field, turbulence fluctuation, aero optical wavefront and other parameters of the flow under such extreme mechanical conditions, traditional test and measurement technologies are powerless,It has always been regarded as a bottleneck problem puzzling the development of relevant aerospace weapons, aerospace and missile equipment.
Aiming at the broad prospect of the application of supersonic flow field measurement technology, this project, under the leadership of professor Yi Shihe, an aerodynamics expert, has proposed new principles and methods of planar laser scattering technology of nano tracing, invented NPLS test system integration and operation technology, supersonic flow field nano tracing technologyNanotracer supersonic flow field tomography technology has developed the "supersonic flow field NPLS test system", which has realized the visualization of aircraft flow field and the high-resolution test measurement of flow field velocity field, density field, turbulence fluctuation, aero optical wavefront and other parameters, and achieved the research goal of "clear and visible, fine measurement" of supersonic flow field.The key technical problems related to aerodynamic design and precision guidance, which have long puzzled the development of major domestic models, have been solved at one fell swoop, and the independent innovation and R&D capability of China's aerospace vehicle development has been significantly improved.[3]
