Drag Reduction and Thermal Protection of the Combination of Aero Disk, Lateral Jet, and Rear Jet for Hypersonic Vehicle

This study proposes a combined scheme based on a spike-aero-disk, a lateral jet, and a rear jet to enhance hypersonic vehicles’ drag reduction and thermal protection performance. Numerical simulations were conducted using CFD methods to validate the scheme’s capabilities. The results demonstrate a significant improvement in drag reduction and thermal protection compared to the basic scheme with only a spike-aero disk. Furthermore, under the same mass flow rate conditions, the combined scheme with an extra rear jet is compared to a scheme with a spike-aero-disk-lateral jet, revealing a reduction of approximately 23.4% in the peak Stanton number, indicating a remarkable enhancement in drag reduction and thermal protection performance. The simulation results show that the use of lateral jet and rear jet improves the overall thermal protection ability and drag reduction ability of the vehicle.


Introduction
In recent years, hypersonic vehicles have gained significant strategic importance in aerospace.Due to their superior performance and wide range of potential applications, hypersonic vehicles are those capable of flying at Mach 5 and above speeds in vast airspace.Compared to conventional vehicles, hypersonic vehicles had much faster flight speeds and higher maneuverability, attracting widespread attention in domestic and international research.
In hypersonic flight, vehicles often face severe shockwave drag and heating, presenting significant challenges to the overall performance of hypersonic vehicles.The high-temperature and high-pressure environment can damage the vehicle's scheme and safe flight.Intense shockwaves also increase flight drag, severely impacting the vehicle's speed and payload capacity.Therefore, methods for vehicle protection are of great importance.Various protection schemes, such as leading-edge cavities, opposing jets, and spikes, have been extensively studied.
Spike is an effective means for reducing drag in current vehicle designs.They improve the vehicle's overall performance by altering surround flow field characteristics.However, spikes do not provide significant thermal protection.Various methods, such as opposing jets, lateral jets, and transpiration cooling, can complement the spike's thermal protection capability and enhance the vehicle's overall drag reduction and thermal protection.
In this regard, some researchers have proposed combined schemes of spikes and jets.Ma proposed a spike-aero-disk and oblique jet combination [1] .This scheme induces significant changes in the flow field and achieves sound vehicle protection.Meng analyzed the role of lateral jets and spikes in vehicle protection [2] .Xu and Wang et al. considered spikes combined with opposing jets to improve the aircraft's performance [3,4] .Esmail Zadeh used four aero disks to reduce blunt surface pressure and temperature [5] .Zhang used a combination of multiple jets to improve the vehicle's flight attitude [6] .The study conducted by Guo et al. proposed a novel concept called spike-aero-disk-channel [7] .Their research focused on investigating the effects of channel convergence half-angle and lateral jet position on various aspects, including the flow field, drag reduction, and thermal protection.
Compared with the traditional method of surface coating anti-ablation material to protect the surface of the vehicle, the method of adding a jet can protect the surface result of the vehicle for a longer time.The cost of using anti-ablation materials is larger than jet flow, requiring a high level of technology, and the way of adding a jet cleverly avoids these problems, and only needs to replace the jet working fluid regularly.Coupled with the jet flow, it also significantly reduces the motion pressure of hypersonic vehicles, which means that when the vehicle is flying, the propulsion required by the aircraft is reduced, and less fuel can be carried to ensure the normal flight of the vehicle.
These results suggest that the spike-aero-disk-channel concept proposed by Guo et al. exhibits improved aerodynamic performance and thermal reduction characteristics compared to the conventional spike-aero-disk design.
Zhu et al. proposed a combined scheme that uses a spike with an aero disk and rear jets to improve the vehicle's drag reduction and thermal protection performance, achieving the expected results [8] .The rear jets enhanced the thermal protection performance of the spike-aero-disk and the vehicle.However, the rear jets increased the drag on the vehicle surface.This paper proposes a combination scheme based on a lateral jet plus an extra rear jet to reduce the drag.The combination involves introducing rear jets through slots behind the aero disc and lateral jets on the side surface of the spike.The aim is to utilize lateral jets to radially push the shockwave away, achieving good thermal protection on the spike-aerodisk and blunt-body surface while reducing the drag.In this paper, Computational Fluid Dynamics (CFD) numerical simulation methods are applied to explore.The overall drag reduction and thermal protection performance of the combined scheme.

Simulation Model
The combined scheme used in this study is shown in Figure 1

Numerical Method
The Computational Fluid Dynamics (CFD) method was employed in this study.A two-dimensional axisymmetric model was used, with the SST k-ω turbulence and Reynolds-averaged Navier-Stokes model.The solver employed was based on double precision, density-based, and implicit schemes.The AUSM flux scheme was utilized, along with second-order upwind spatial discretization.The air was assumed to follow the ideal gas model, and the air viscosity was determined using the Sutherland viscosity law.Additionally, the highest temperature did not exceed 2500 K, so chemical reactions were not considered.Only the variation in specific heat ratio due to high-temperature real gas effects was taken into account, and it was approximated using a segmented seventh-degree polynomial fit.

Grid Independence Validation
A block-schemed grid was utilized, and it conforms to the requirements for employing the SST k-ω model.The first layer of grids normal to the wall was maintained with a y+ value near 1 to ensure accuracy.Three sets of grids were prepared: coarse, medium, and fine, as shown in  3 and Figure 4 depict the distribution of the wall pressure coefficient (Cp) and Stanton number along the surface of the blunt body, respectively.It can be observed that the pressure coefficient and Stanton number distributions show similar trends among the three different grid sizes.The pressure coefficient values are similar to each other.However, there is a significant difference in the peak Stanton number between the coarse grid and fine grid, and medium grid.On the other hand, the results obtained from the fine grid and medium grid are very similar.Considering the economic problem, the use of a fine grid will increase the calculation amount of the simulation.Therefore, this study selects the medium grid size to ensure computational accuracy while saving computational resources.

Numerical Method Validation
To validate the accuracy of the numerical method used in this study, a comparison is made with experimental results obtained from the study conducted by Jiang [9] , which investigated the flow around a drag reduction strut as a spike without and with a jet.The experimental setup involved a drag reduction strut with a length of L = 80 mm, a diameter of d = 12 mm, and a blunt body with a diameter of D = 80 mm.The experiments were conducted at flight Mach number 6, a flight altitude of 30 km, in a wind tunnel with a total temperature of 465 K.The static pressure in the flow field was 1357 Pa.
Figure 5 compares the density contours obtained from the numerical method and the experimental Schlieren image.The numerical method captures the shock wave formed at the head of the spike, the separation shock wave generated by flow separation at the tip of the strut, and the reattachment wave near the shoulder of the scheme.The shape and location of the density contour are in good agreement with the experimental Schlieren results.As shown in Figure 6, the selected characteristic point's numerical values are consistent with the experimental results obtained from the case without jet flow using a single drag reduction strut.The xaxis represents the ratio of the arc length measured from the geometric stagnation point along the surface of the blunt body to the diameter of the blunt, while the y-axis represents the pressure distribution along the windward surface.The numerical simulation results exhibit the same trend as the experimental results without jet flow, showing an initial increase followed by a decrease, with a peak at the shoulder region of the blunt body.The specific numerical values also exhibit slight differences, indicating good agreement between the data from the numerical simulation and the experimental case without a jet.
Considering that this study involves the interaction between the jet flow and the free flow for thermal protection, it is necessary to validate the accuracy of the numerical calculation method used in simulating the results of this interaction.Hayashi's hypersonic jet flow experiment is selected for verification [10] .In this experiment, the opposing jet flow consists of nitrogen gas with a Mach number of 1, a total temperature is 300 K, and a pressure ratio of 0.6.The free flow's Mach number is 3.98, the total pressure is 1.37 MPa, and the total temperature is 397 K.As shown in Figure 7 and Figure 8, the simulation results exhibit a consistent overall trend with the experimental results.The Stanton number values between computational and experiment agree with minor differences.

Flow field Characteristics
The study investigates the flow characteristics, drag reduction, and thermal protection of a combination scheme consisting of a spike-aero-disk, rear jet, and lateral jet, compared to the basic model with only a spike-aero-disk.The drag reduction and heat protection are also compared with the configuration that includes a spike-aero-disk and lateral jet with the same mass flow rate of jets.The mass flow rates for each configuration are shown in Table 3.

Table 3 Mass Flow Rate of different jet flow Jet
Mass Flow Rate (kg/s) The lateral jet of the spike-spike-aero-disk-lateral-rear jet 0.01195 The rear jet of the spike-spike-aero-disk-lateral-rear jet 0.01195 The lateral jet of the spike-aero-disk-lateral jet 0.02390 As shown in Figure 9, the basic model with the spike-aero-disk compresses the free flow at the head of the spike-aero-disk, resulting in detached bow shocks.The flow decelerates after passing through the shocks while remaining supersonic.
In the combined model of spike-aero-disk, rear jet, and lateral jet, the fluid field undergoes significant changes compared to the basic model.The free flow is compressed into detached bow shocks at the head of the spike-aero-disk and experiences oblique shocks after being blocked by the boundary of the rear jet flow.The rear jet flow rapidly expands, interacts with the spike-aero-disk surface, and generates oblique shocks.The jet flow attaches to the rod surface until it reaches the lateral jet flow.The boundary layer separates and propagates under the influence of the rear jet flow, then is further pushed away radially by the lateral jet flow, resulting in the free flow moving further away from the spike surface and increased angle of the leading-edge shock wave.
As depicted in Figure 10, both combined models with jet, exhibit typical under-expanded jet schemes at the outlet, forming Mach discs.A typical under-expansion jet structure is formed at the outlet of the rear jet and the lateral jet.Two Mach disks can be captured.After ejecting from the nozzle, the reverse jet is accelerated to supersonic speed by expansion.Then decelerated to subsonic speed under the action of high strength Mach disk, and continues to expand and accelerate upstream.The supersonic free flow encounters the rear jet after being strongly compressed by the bow shock wave and forms a contact surface.Both models effectively redirect the free flow from the spike surface through the lateral jet flow.
However, introducing a rear jet flow in the combined model leads to more changes in the flow scheme.After being affected by the rear jet flow, the free flow undergoes flow separation and interacts with the lateral jet flow, forming an upward-tilted shear layer and a recirculation region between the lateral flow and the aero disc.Furthermore, under the combined influence of the free flow and the rear jet flow, the lateral jet flow changes direction and tilts downstream towards the spike surface, resulting in a reattachment shock wave.The lateral jet flow pushes the shock wave away from the scheme surface, further increasing the angle of the leading-edge shock wave and weakening the intensity of the reattachment wave.
Additionally, the lateral jet flow effectively protects the surface of the spike-aero-disk and the blunt body from aero heating.In summary, the introduction of multiple jets has a significant improvement on the blunt surface of the vehicle, and the characteristics of the blunt external flow field change.The free flow is separated under the action of the drag-reducing rod, and under the joint action of the rear jet and the lateral jet, the shock wave is further separated from the vehicle structure, which greatly reduces the pressure of the interface between the vehicle and the free flow, and the aerodynamic heating problem caused by hypersonic flow is also alleviated.As depicted in Figure 1, adding a rear jet flow reduces the Stanton number on the scheme surface, but it increases the pressure coefficient on the vehicle surface.The spike-aero-disk-lateral -rear jets model exhibits a lower pressure coefficient on the vehicle surface compared to the basic scheme, with its peak value reduced by approximately 38.9%.However, compared to the spike-lateral jet scheme, the spike-lateral jet-rear jet scheme has a bigger pressure coefficient on the scheme surface, with its peak value increased by approximately 11%.
Figure 12 Pressure coefficient distribution for different models

Conclusion
This study investigated the performance of a scheme for a vehicle's drag reduction and thermal protection, based on the combination of rear jet, lateral jet, and spike-aero-disk.CFD was used to explore the flow characteristics and compare their performance among the spike-aero-disk-lateral -rear jets, spike-aero-disk-lateral jets, and the basic scheme.When the mass flow is the same.The following conclusions were drawn: 1) Compared to the basic scheme, the spike-aerodisk-lateral-rear jets and the spike-aero-disk-lateral jet achieved significant drag reduction and thermal protection effects.The pressure coefficient and Stanton number were significantly reduced, with the Stanton number peak decreased by approximately 10 times.Compared to the basic scheme, the spike-aerodisk-lateral-rear jets scheme exhibited a reduction of about 38.9% in the pressure coefficient on the scheme surface.
2) Introducing a rear jet significantly improved the thermal protection capability of the vehicle.The spike-aerodisk-lateral-rear jets exhibited a reduction of approximately 23.4% in the Stanton number peak compared to the spike-aero-disk-lateral jet model.
3) Compared with the spike-aero-disk-lateral jet scheme, the introduction of the rear jet not only enhanced the thermal protection capability but also increased the pressure coefficient on the blunt body.It indicates a decrease in the drag reduction capability of the vehicle.The spike-aerodisk-lateral-rear jets had a bigger pressure coefficient on the blunt-body and an increase of about 11% in the peak value.
. The following dimensions are defined: blunt-body diameter D = 50 mm, spike length L = D = 50 mm, lateral jet hole center distance from the blunt-body wall Li = 0.5D = 25 mm, aero-disk diameter d = 0.08D, and distance between the center of the rear jet and the spike Ln = 0.0044D = 0.22 mm.

Figure 1
Figure 1 Physical model and boundary conditions.The free flow conditions were selected with a flight altitude of 25 km, a flight Mach number of 5, an incoming pressure of 2550 Pa, and an incoming temperature of 221.5 K.The parameters of the flows are listed in Table1.The grid and local zoom-in are shown in Figure2.The wall condition was set as a no-slip isothermal wall with a temperature of 300 K.

Figure 3
Figure 3 Distribution of pressure coefficients on the blunt-body for different grids.

Figure 4
Figure 4 Distribution of Stanton number on the blunt-body for different grids.

Figure 5
Figure 5 Density contour and experimental schlieren of the spike.

Figure 6 .
Figure 6.The computational and experimental Pressure along the scheme.

Figure 7
Figure 7 Comparison of Density contour between computational and experimental results.

Figure 8
Figure 8 Computational and experimental Stanton number along the scheme.

Figure 9
Figure 9 Characteristics of the flow field around the basic model and spike-aero-disk-rear-lateral jets model