Flow Characteristic Study on Wake Flow of High-speed vehicles

Based on the general CFD software NNW-FlowStar of national numerical wind tunnel engineering, this paper adopts anisotropic unstructured hybrid mesh to simulate the wake field characteristics of aircraft in high-speed flight. NNW-FlowStar software in high resolution numerical format, gaussian type node gradient calculation method, massively parallel and a series of numerical algorithm in terms of development and improvement to ensure the fine simulation of the hypersonic flow field. The numerical method can clearly aircraft wake flow field, the main shock, the corner expansion area, shear layer and the shock wave, clearly shows the wake flow field structure. With the increase of Mach number, the wake flow is getting more and more strong, the backwater area range is more and more small, after the stagnation point, the shock wave is also much greater compressibility.


Introduction
When the high-speed vehicle flies in a dense atmosphere, there are wakes composed of hightemperature gas molecules and ions behind the vehicle.Studying the structure and characteristics of the wake flow field is not only necessary to explain various phenomena in the wake, but also of great significance to solve the problems of aerodynamics, heat and aircraft recognition at the bottom of the aircraft.
In terms of wake flow field, the downstream flow field with supersonic velocity and small flow parameter changes is generally referred to as the far tail, while the area connecting the bottom of an object and the far tail is referred to as the near tail near-tail flow with complex structure, including shock wave, shear layer and vortex interference, which is a difficulty in wake flow research.At present, many works are concentrated in this field.
There are already many people researched the wake flow, Deiwert and Thomas studied the bottom flow of the jet in the transonic zone [1,2]; Lombard researched the cone wake with high Mach number and analyzed the influence of Mach number, Reynolds number and bluntness ratio [3]; Tassa studied the influence of real gas on the wake [4].
In this paper, the typical wake flow characteristics of high-speed aircraft are studied and the flow field structure is analyzed in detail.The structure of the article is as follows: Section 2 is an introduction to numerical methods, Section 3 is the results and discussion, and finally section 4 is a brief conclusion.
( ) Where, , and g v denotes the wall velocity and HQ () denotes the inviscid flux vector, v HQ () denotes the viscous flux vector.Here,  , ( ,, u v w ) and E denote the density, velocity of three directions, and specific total energy of the fluid.n denotes the outward pointing normal unit vector of the boundary.The fluxes can be described as :

Geometry and Computational meshes
The vehicle of THAAD (Terminal High Altitude Area Defense) system is a representative high-speed vehicle.A simplified model of THAAD vehicle is used in this paper to research the wake flow characteristics at high speed.We focus on the flow characteristics of wake flow, and in order to study the flow characteristics detailly, the model is further simplified to a 2-D model.Fig. 2~3 show the grid distribution of the vehicle, the computational mesh is unstructured hybrid mesh, including triangular prism, tetrahedral, and pyramid grid.It can be seen that the grids near the head of aircraft are specially refined to simulate the flow characteristics.In order to get the detailed wake characteristics of high-speed aircraft, the local grids near the wakes are specially refined.

Validation of numerical method
FlowStar software belongs to scientific computing software, the ultimate goal of the software is to provide high reliability data for the process to use.In the development process of FlowStar software, a large number of standard models and engineering actual shape assessment and calibration have been carried out.Figure 4 gives out FlowStar soft thing to AIAA (Aircraft Industries Association of America) the fifth calculation flow of strength resistance prediction work conference [10] and AIAA second high The official statistical results of the lift prediction working conference [11] showed that the FlowStar software performed very well, and the results were near the statistical average of all the results of the conference, which was comparable to the well-known international CFD software in the industry [12].(1) Strong shear layer.It can be seen that the supersonic wake is very long, the velocity of the air flow in the middle of the wake is significantly lower than that outside the wake, and the Mach number in the middle of the wake is only half of that of the incoming flow.The strong shear of the wake may interfere with the following vehicle or with the separation of supersonic stages.
(2) Strong vortexes.There is a dead water zone at the bottom of the wake, the airflow velocity in the dead water zone is extremely low, there is strong shear flow inside and outside the dead water zone, and the axisymmetric vortex in the dead water zone.Fig. 6 shows a typical flow field structure.As soon as the high-speed air flows around the corner, expansion waves form at the bottom of the aircraft, while the flow vortex region is rapidly generated at the bottom.The vortex center is a stable spiral point, namely, the streamline points to the vortex center.The boundary of the vortex is the shear layer.After passing through the dead water zone, the highspeed air flows squeeze each other and form a shock wave after the stagnation point.The velocity of the air flow inside the shock wave is relatively low.The flow velocity in the main wake zone is about half of that of the incoming flow, and there is a strong shear boundary between the main wake zone and the outside.

The effect of Mach number
Figure 7~9 show the wake field characteristics under different Mach numbers, which are 3.0, 5.0 and 7.0 respectively.It can be seen that, from the perspective of wake shear, with the increase of Mach number, wake becomes stronger and stronger, and the influence area of wake becomes larger and larger.It seems that wake becomes more and more "robust".From the perspective of the range of dead water zone, with the increase of Mach number, the compression of shear layer is more obvious, the range of dead water zone is smaller and smaller, and the compressibility of shock wave after the stagnation point is stronger.

Conclusion
Based on the general CFD software NNW-FlowStar of national numerical wind tunnel engineering, this paper adopts anisotropic unstructured hybrid mesh to simulate the wake field characteristics of aircraft in high-speed flight.Through the numerical simulation, the following conclusions can be drawn: (1) NNW-FlowStar software in high resolution numerical format, gaussian type node gradient calculation method, massively parallel and a series of numerical algorithm in terms of development and improvement to ensure the fine simulation of the hypersonic flow field.
(2) The numerical method can clearly aircraft wake flow field, the main shock, the corner expansion area, shear layer and the shock wave, clearly shows the wake flow field structure.
(3) With the increase of Mach number, the wake flow is getting more and more strong, the backwater area range is more and more small, after the stagnation point, the shock wave is also much greater compressibility.

Figure 1 .
Figure 1.The geometry of vehicle.

Fig. 5
Fig.5shows typical wake characteristics under Mach 5.0 inflow condition.As you can see, the wake flow has two obvious characteristics:(1) Strong shear layer.It can be seen that the supersonic wake is very long, the velocity of the air flow in the middle of the wake is significantly lower than that outside the wake, and the Mach number in the middle of the wake is only half of that of the incoming flow.The strong shear of the wake may interfere with the following vehicle or with the separation of supersonic stages.(2)Strong vortexes.There is a dead water zone at the bottom of the wake, the airflow velocity in the dead water zone is extremely low, there is strong shear flow inside and outside the dead water zone, and the axisymmetric vortex in the dead water zone.

Figure 6 .
Figure 6.flow structure of wake flow.

Figure 9 .
Figure 9. wake flow characteristics at different Mach number 7.0.