Intake Characteristics and Inhomogeneity of Supersonic Passage

The type, quantity, location, geometry and size of the intake passage will directly affect the quality of the engine, and then affect the whole propulsion system and even the performance of the aircraft. In this paper, the intake characteristics and inhomogeneity of three different passage structures are studied under supersonic conditions. The numerical simulation models of the intake passages of F-16, F-18 and J-10 were established, and the steady-state calculations were carried out under the condition of Mach number 1.2. In the three intake passages, the total pressure decreases gradually along the flow direction, and the pressure loss near the wall is large, while the total pressure value far from the wall is large, and the pressure field is relatively uniform. The F-18 intake passage has relatively small inlet pressure and total inlet and outlet pressure loss. In terms of inlet uniformity, the intake passage of F-16 has the highest value, F-18 has lower value, and J-10 has the lowest level of inhomogeneity.


Introduction
Intake passage is an important component of aero-engine, and its performance is closely related to the working characteristics of the engine.The basic requirements for gas turbine engine intake passage are as follows: small internal and external losses in the processes of air compression and air supply to engine; the air flow is stable under all flight conditions and engine operating conditions; outlet flow inhomogeneity and unsteadiness are within allowable levels of the engine.
The researches of aero-engine intake passage are carried out simultaneously with the research of scramjet.In the history of intake passage research, there have been many design schemes.Holland conducted experimental study and numerical simulation on the three-dimensional side-pressure intake passage under Mach number 10 and Mach number 6. Bourdeau et al. [1] studied the optimization design of the three-dimensional supersonic intake passage.Zha et al. [2] numerically simulated the influence of inflow angle of attack on the intake passage non-start in the high-speed sky/earth roundtrip system.Michael et al. [3] studied the performance of an intake passage with square to circular structure and fixed geometric dimensions.The three-dimensional side pressure intake passage is a new design concept, as an important aerodynamic component of scramjet, it has not been formally tested in flight tests.
Since the 1970s, NASP Langley Research Center has designed various intake passage models and carried out a series of experimental studies, obtaining a large amount of data.At the end of 1960s, Gregory and Johnston successively proposed the concept of intake passage integrated with the body, which greatly influenced the later intake passage design and experiments.Since the concept of integration of body and propulsion system came into being, it has been widely recognized and become a criterion for intake passage design.Kumar and Trexler [4] analyzed the performance of the scramjet intake passage by using the three-dimensional N-S equation solver, and they also analyzed the influence of front and back sweeping on the side plates on the performance of intake passage.Fang [5] used the software platform WIND to complete the numerical simulations on side pressure intake passages with different configurations of side plate and lip plate.Garrison [6] presented the numerical simulation results of shock/turbulent boundary layers in the intake passage.Professor Li Hua from National University of Defense Technology carried out the first numerical simulations of hypersonic intake passage isolation sections in China, and studied shock waves, boundary layer interference and vortex structures in the rectangular isolation sections under the condition of boundary layer incoming flow from precursor.Professor Liang Jian han et al. completed the free jet experiments of threedimensional side pressure intake passage under multiple Mach numbers of incoming flow.Dr. Fan Xiao qiang et al. [7] studied the influence of transition device for axisymmetrical intake passage precursor and the influence of wind tunnel pulse starting process on the entire starting process of experimental intake passage model.Bei hang University and NPU have developed hypersonic numerical simulation software, and numerically explored the design and optimization methods of hypersonic intake passage.At present, the research and analysis of flow field in aero-engine intake passage are mainly performed by numerical simulation.
In this paper, the working performances of the intake passages of three types of fighter jets under different Mach number conditions are studied.Through the numerical calculations on the intake passage model with different boundary parameters, the details of flow field, the data of total pressure loss across inlet and outlet and the velocity field inhomogeneity at the outlet section are obtained.Through the calculation and analysis of these data, the intake characteristics and inhomogeneity of the supersonic passage are obtained.

Geometric Models
The fluid model size data of the intake passages of American F-16, F-18 and J-10 fighter jets investigated in this paper are all from the solid models of the practical intake passages, with a scale of 1:48.The models have been shown in figure 1.

Computational Domains
The mesh configurations and boundary conditions of the investigated intake passages are shown in figure 2. The inlet boundary condition is pressure inlet and the outlet boundary condition is pressure outlet.Multiple interior faces are created with equal distances to calculate the local and averaged velocity inhomogeneity of each section.

Working Conditions and Parameter Definitions
In this paper, the altitude of 6000 meters is selected as the working height of the aircraft intake passages, and three Mach numbers, 0.8, 1.0 and 1.2, are selected as the working conditions of the numerical simulations.The total pressure loss between the inlet and outlet and the inhomogeneity of outlet section of the three intake passages as well as their variations with the working conditions are obtained.The atmospheric parameters at 6000 meters are as follows: where, Ts is the static temperature, Ps is the static pressure, a is the local sound speed and ρ is the air density.
Physical parameters used in the numerical simulations in this paper are calculated by the following formulas: 0.16(Re ) where, T0 is the total temperature, P0 is the total pressure, Ma is the Mach number, and Vm is the averaged flow speed.
Before analyzing the results of numerical calculations, the performance indicators should be defined to facilitate the analysis and comparison of results and get more convincing conclusions.The performance indicators studied in this paper are the averaged velocity inhomogeneity of the compressor inlet section and the local inhomogeneity of 5% compressor inlet section.The average inhomogeneity is defined as follows: where, ̅  is the area-weighted averaged velocity on the interested area,  ̅ max is the area-weighted averaged velocity on the area where   >  ̅  ,  ̅ min is the area-weighted averaged velocity on the area where   <  ̅  .The formula of the area-weighted averaged velocity is as follows: The definition of the local inhomogeneity of the velocity field on the 5% cross section is basically the same as that of the averaged velocity inhomogeneity on the whole surface, except that only a small circular surface equivalent to 5% of the entire cross section area is taken to calculate the local velocity inhomogeneity.

Flow Field Analysis of the Intake Passages
In this section, the distributions of total pressure, static pressure and velocity vectors on the selected section are analyzed.The pressure contour of each section reflects the overall pressure distribution in the flow process, and also reflects the non-uniformity of the flow field.The velocity vector distribution reflects the general velocity distribution in the flow process, and its inhomogeneity is a judging criterion on the rationality of the structural design of the intake system.As shown in figure 3(a), the cross section of F-16 intake passage is gradually expanding.For supersonic flow field, the fluid velocity increases gradually from the inlet to the outlet, but due to the high velocity and the presence of viscosity and boundary layer, the fluid velocity near the wall is relatively low.It can be seen that the velocities on the exit are in a relatively small range, indicating that the exit velocity field is relatively uniform.Figure 3(b) is the total pressure distribution in the supersonic flow field of the intake passage.It can be seen that the total pressure decreases gradually along the flow direction, and the total pressure loss is larger and the total pressure gradient is larger near the wall.By contrast, the total pressure value of the flow field far from the wall is larger but the pressure field is more uniform.
As shown in figure 3(c), the F-18 intake passage has a gradually expanding cross-section.It can be seen that the velocity of the flow field of F-18 intake passage increases greatly when the cross section suddenly shrinks in the middle, and then the variation of pipe diameter is small and the velocity becomes stable.The velocity distributions of the outlet section are similar at different positions, indicating that the outlet velocity field is relatively uniform.Figure 3(d) is the schematic diagram of the total pressure distribution of the supersonic flow field in the F-18 intake passage.It can be seen that the total pressure decreases gradually along the flow direction, and the total pressure loss near the wall is larger and the total pressure gradient is also larger.By contrast, the total pressure value of the flow field far from the wall is larger while the pressure field is more uniform.
As shown in figure 3(e), J-10 intake passage has a gradually expanding cross-section.For supersonic flow field, the fluid velocity gradually increases with the flow development, but the fluid velocity close to the wall surface is low due to the presence of viscosity and boundary layer.It can be seen that the outlet velocity has a relatively small range, indicating that the exit velocity field is relatively uniform.Figure 3(f) is a schematic diagram of the total pressure distribution in the supersonic flow field of J-10 intake passage.It can be seen that the total pressure decreases gradually along the flow direction and the pressure loss near the wall is large.The axial size of J-10 intake passage is large, hence the flow field can be fully developed, but it also increases the pressure loss.In contrast, the total pressure far from the wall surface is larger and the pressure field is more uniform.1 shows the total pressure values on the inlet and outlet sections and the total pressure losses of different intake passages.It can be seen that the F-18 intake passage has relatively small inlet pressure and total pressure loss between the inlet and outlet.The F-16 intake passage has the same inlet pressure as the J-10, but the F-16 intake passage has the greatest pressure loss.It can be seen from figure 4 above that the variation trend of the cross-stream inhomogeneity along the F-16 intake passage is roughly as follows: the averaged inhomogeneity on the cross-stream sections along the streamwise direction basically maintains a constant level when Z<120mm, and then slowly declines.The maximum value of the 5% local inhomogeneity ranges from 8.59% to 11.53%, and the average inhomogeneity ranges from 8.39% to 9.90%.As can be seen from figure 5, the variation trend of the inhomogeneity on different cross-stream sections of F-18 intake passage is roughly as follows: the inhomogeneity rises slowly within a distance of about 350mm from the inlet section, reaches the peak value, and then begins to decline.The maximum values of the 5% local inhomogeneity were within 5.76%-8.91%,and the average inhomogeneities were within 5.26%-9.13%.It can be seen from figure 6 that the variation trend of the inhomogeneity different cross-stream sections of J-10 intake passage is roughly as follows: the inhomogeneity shows a downward trend from the inlet surface, and tends to flatten when it drops to a certain value and remains stable.The maximum values of the 5% local inhomogeneity were in the range of 7.52%-8.51%,and the average inhomogeneities were in the range of 7.27%-7.92%.

Analysis on the Outflow Inhomogeneity of Intake Passages
The F-16 intake passage has the highest velocity inhomogeneity, F-18 has the second highest velocity inhomogeneity, and J-10 has the lowest level.The F-16 intake passage has a high level of inhomogeneity in the front segment, and the velocity inhomogeneity begins to decline in the back segment, while the F-18 intake passage has a generally upward trend of inhomogeneity along the streamwise direction.The intake velocity inhomogeneity of J-10 shows a continuous descending trend, so the intake passage of J-10 has the best inlet uniformity.

Conclusions
In this paper, the performances of three types of fighter jets' intake passages under the supersonic condition (Ma=1.2) are studied.The intake passages are modeled, the boundary parameters are calculated and the numerical simulations of flow field are conducted to obtain the data of intake passages' pressure and velocity distributions.Through the calculation and analysis of these data, the intake characteristics and inhomogeneity are obtained.In the three intake passage structures, the total pressure decreases gradually along the flow direction, and the total pressure loss near the wall is larger and the total pressure gradient is also larger.In contrast, the total pressure value of the flow field far from the wall is larger and the pressure field is more uniform.In terms of total pressure loss, the F-18 intake passage has the smallest inlet pressure and total pressure loss between the inlet and outlet, J-10 has a larger intake pressure loss, and F-16 has the largest pressure loss.In terms of inlet uniformity, F-16 has the highest inlet uniformity, F-18 has a poorer inlet uniformity, and J-10 has the lowest level.

Figure 1 .
Figure 1.Three dimensional models of the intake passages.

Figure 2 .
Figure 2. Mesh configurations and boundary conditions of the intake passages.

Figure 3 .
Figure 3.The distributions of velocity vectors and total pressure in the intake passages.

Figure 4 -
6 show the variation of velocity field inhomogeneity of the cross sections along the intake passages of F-16, F-18 and J-10 fighter jets with the coordinate value of the section with the Mach number fixed at 1.2.

Figure 4 .
Figure 4.The variation of the velocity inhomogeneity with coordinates along the flow direction in the F-16 intake passage with M=1.2.

Figure 5 .
Figure 5.The variation of the velocity inhomogeneity with coordinates along the flow direction in the F-18 intake passage with M=1.2.

Figure 6 .
Figure 6.The variation of the velocity inhomogeneity with coordinates along the flow direction in the J-10 intake passage with M=1.2.

Table 1 .
The results of inlet and outlet pressures and total pressure losses.