Research on aerodynamic influence of contra-rotating propfan slipstream on propfan engine nacelle inlet

In order to investigate the aerodynamic influence of contra-rotating propfan (CRP) slipstream on propfan engine nacelle, numerical simulation research on the CRP/Nacelle integrated configuration was carried out by using sliding mesh unsteady method. The aerodynamic performance of integrated configuration under different working conditions was studied, and compared with nacelle without CRP. Results show that the total pressure recovery coefficient of inlet port increases due to the work done by CRP. However, as the inhale of the wake vortex, the flow structure inside the inlet becomes more complex because of the mixing of wake vortex and wall separation vortex, which leads to a significant increase of distortion index and loss coefficient. Compared with isolated nacelle, the slipstream causes the total pressure recovery coefficient of inlet port increased by 10.2% and 13.4% under cruise condition and takeoff condition, which is accompanied by the intensification of flow loss and non-uniformity inside the inlet, resulting in a significant increase of the total pressure loss coefficient and total pressure distortion index.


Introduction
The propfan engine is considered as one of the alternatives of the next generation of commercial aviation propulsion because of its huge potentiality on reducing fuel consumption [1].As the main source of engine thrust, contra-rotating propfan (CPR) is the most important aerodynamic component of propfan engine.The nacelle of tractor-propfan is installed in slipstream region, which means the aerodynamic influence of CRP slipstream on nacelle cannot be ignored in real process.Airflow processes tangential rotation speed and spiral streamline after passing through CRP, which will influent the aerodynamic characteristics of nacelle cowl.More importantly, the inlet port is completely submerged in the downstream flow field of CRP, and slipstream can make the flow field inside the inlet port very complex, which may cause bad vortex distortion and affect the normal operation of the engine [3].Therefore, it's of great significance to carry out research on aerodynamic influence of CRP slipstream on propfan engine nacelle.
Since the 21st century, scholars from various countries have conducted extensive research on the flow field characteristics of CRP slipstream.NASA has carried out extensive numerical simulation and experimental research on the F31/A31 propfan.Claus [4] and Stephens[5] compared the numerical simulation and experimental results of the F31/A31 contra-rotating propfan under takeoff and cruise conditions, verifying the credibility of the numerical simulation method.Housman [6] used the highorder precision overlapping grid Discretization method to simulate the CFD of F31/A31 propfan.
Figure 1 shows the contour of the vorticity of the simulation results.Van Zante [7] used PIV (Particle Image Velocimetry) technology to study the characteristics of blade tip vortices and wake of F31/A31 propfan.Brehm [8] used an immersive boundary method to calculate the flow field of CRP.The French Aerospace Lab Delatre [10] conducted numerical simulations of HTC5 propfan to investigate the effects of different torque ratios on noise and flow characteristics.DLR (Deutsches Zentrum für Luftund Raumfahrt) Sturmer [12] used PIV technology to study the interaction between the wake and tip vortices of the front rotor blades and the rear rotor, and compared the PIV results with numerical simulation results.Although research institutions around the world have carried out extensive research on slipstream characteristics of CRP, there is still limited research on the downstream aerodynamic components affected by CRP slipstream.Therefore, some research on the coupling aerodynamic effects of traditional propellers and aircraft components was collected as a reference for this study.Ruiz Calavera [15] studied the aerodynamic performance of the propeller and inlet port based on the military transport aircraft A-400M.The simulation results obtained by momentum source method and sliding mesh unsteady method were compared with the experimental results, and the results showed that the simulation results obtained by the sliding grid unsteady method were more similar to the experimental results.Zhou [17] used MRF(Multiple Frame of Reference) method and sliding mesh unsteady method to simulate the internal and external flow of the propeller/inlet configuration, and preliminarily studied the propeller slipstream and the aerodynamic interference mechanism between the inlets.Wen [18] studied influence of slipstream of propeller on inlet port based on the sliding mesh unsteady method.The results show that the acceleration effect of propeller slipstream increases the total pressure in the inlet, while the torsion effect and blade wake decrease flow quality of the inlet.
Related studies have shown that sliding mesh unsteady method can effectively simulate the slipstream of propellers and obtain the temporal variation of transient aerodynamic parameters in the flow field [19].In this paper, a propfan engine nacelle is taken as the research object, and the aerodynamic influence of CRP slipstream on nacelle is carried out based on sliding mesh unsteady method under takeoff and cruise conditions.

Geometric model
The geometry in this article adopts self-designed propfan engine nacelle and CRP, and nacelle inlet is spoon shaped with a boundary layer splitter.The design point of the CRP is consistent with that of the nacelle.The nacelle model is shown in Figure 2 (a) and 2 (b), while the model coupled with CRP is shown in Figure 2 (c).The specific parameters are shown in Table 1. the model coupled with CRP is shown in Figure 2 (c).The front and rear rotor both have 8 blades, and the corresponding blade radius at the blade tip is 2 meters.The installation angle at 75% of the blade height is 47.15drgee under cruise condition, and under takeoff condition is 29.75degree.Looking from front to back, the front rotor rotates clockwise and the rear rotor rotates counterclockwise.
p is the total pressure at inlet entrance.

Numerical method
The numerical calculation adopts the sliding grid unsteady method to solve the Reynolds averaged N-S equation with the SST k  − turbulence model.Figure 3 shows a schematic diagram of the calculation domain and boundary condition.The entire calculation domain is cylindrical, with a length of 10 times the length of the nacelle before and after, and a maximum radius of 10 times the length of the nacelle on the left and right.The far field inlet A and far field C are both far field boundary condition, and the inlet outlet B and far field outlet D are outlet boundary condition.The grid of the coupling configuration between CRP and nacelle is generated into a rotating domain and a stationary domain, as shown in the Figure 4, both using structured grids.The rotating domain is a cylindrical area that wraps around CRP.The mesh near the wall is encrypted.The height of the first layer of mesh is 0.01mm, ensuring that Y + meets the requirements of the turbulence model.
The sliding mesh model is an unsteady computational model, with the basic idea that after each time step, the grid in the rotating region rotates relative to the stationary region once (the rotation angle is determined by the rotational speed and physical time step), and the rotation of the rotating component is simulated through the relative slip of the grid.In the sliding mesh model, the flow fields in the stationary and rotating regions are calculated separately.Due to the relative slip of the grids in the two regions, the distribution of nodes at the interface changes continuously with time steps.At next time step, the flow parameters in the two regions are interpolated and transferred at the new interface, coupling the flow fields in the two regions at each time step.When sliding mesh technology is applied to two or more computational regions, each computational region has at least one boundary interface, which is adjacent to the other unit region.These two computational regions move relative to each other along the interface.In this article, the unsteady calculation timestep is set to 0.016667s, which means that CRP rotates 0.9 degree per timestep and iterates 20 steps internally.  .Numerical calculation grid.The feasibility of the CRP numerical method in this paper is verified by using the F7/A7 experimental model and experimental data from reference [22].The F7/A7 was a 8X8 configuration CRP, designed at Mach 0.72 with rotation speed 1275 rpm. Figure 5(a) shows the comparison between the experimental and numerical simulation results of the corrected power coefficient PQA under different advance ratios.It can be seen that the numerical simulation of PQA under different pitch ratios is in good agreement with the experimental results.At the design point, the relative error of PQA is only 1.2%.It can be seen that the numerical simulation method used in this article can simulate the flow characteristics of CRP.
The validation of the nacelle numerical calculation method is carried out using experimental data from reference [23], and the results are shown in Figure 5(b).It can be seen that the static pressure distribution on the cowl obtained from the numerical calculation is basically consistent with the experimental data, indicating that the numerical calculation method is reasonable.

CRP slipstream analysis
Figure 6 shows the distribution of the vorticity absolute value isosurface of CRP /nacelle coupled configuration under different flight conditions.Figure 6(a) shows the vorticity isosurface of cruise condition (Ma=0.785,Altitude=10668m), while Figure 6(b) shows the vorticity isosurface of the takeoff condition (Ma=0.2,Altitude=0m).At the tip and root of the CRP, blade tip vortices and hub vortices are generated respectively.And a trailing edge shedding vortex is also generated in the middle of the blade.Under cruise condition, the tip vortices generated by the rear blades and front blades are mixed with each other in the slipstream region, forming network structure of vortices.But the situation under takeoff condition is completely different.Due to low incoming flow velocity, the pitch of the tip vortex is smaller than that under cruise conditions.And tip vortices in the slipstream region do not form obvious network structure because airflow flows through the front rotor and the flow pipe contracts which leads the front and rear blade tip vortices are not at the same height.Meanwhile, the blade efficiency is much lower under takeoff condition, which leads to more trailing edge shedding vortices, that means vortex structure in slipstream region will be extremely complex.
Figure 7 shows vorticity distribution at the meridional plane of coupled configuration of CRP /nacelle under different flight conditions.The vortices generated by the front and rear rotors have opposite directions.It can be seen that the slower the incoming flow speed, the denser the vortex distribution in the slipstream region.Due to the obstruction of the nacelle, the tip vortex is symmetrically distributed, while the hub vortex is asymmetrically distributed on the surface of the nacelle.The tip vortex of the front rotor has a significant contraction effect under takeoff condition, while the tip vortices of the front and rear rotors are at the same height under cruise condition.The blade tip vortex is located far from the nacelle, so the hub vortex and blade trailing edge shedding vortex are the main factors affecting the aerodynamic performance of the nacelle.Figure 10 shows the streamline distribution of the inlet entrance of isolated nacelle model and CRP /nacelle coupled configuration under cruise condition.It can be seen that the inlet entrance only has a very small circumferential velocity, and the streamline distribution is also symmetrical without CRP.However, in the model with CRP, the torsion effect of CRP gives the airflow partial velocity to the right and downward directions.At the same time, it can be found that there is a large circumferential velocity gradient region at the left of inlet entrance, which indicates that there is wake shedding vortex generated by the blades., where L represents the length of the inlet duct.In Figure 11(a) and 12(a), it can be seen that without CRP, the vortex structure in the inlet mainly comes from the separated vortices on two sides of the wall.According to the numerical calculation settings mentioned earlier, the blade rotates through one channel every 50 timesteps.Therefore, the unsteady flow of wake vortices in the inlet is analyzed at intervals of 10 timesteps.

Flow field characteristics
. Vorticity distribution at different cross sections under cruise condition.Figure 13 and Figure14 show the distribution of total pressure recovery coefficient and Mach Number at inlet exit of isolated nacelle inlet and CRP /nacelle coupled configuration inlet under takeoff and cruise conditions.In Figure 13(a) and Figure14(a), there is low energy flow accumulation region above the inlet exit with a pair of vortices on both sides rotating in opposite directions, which consistent with Figures 11(a) and 12(a).From Figures 13(b) and 14(b), it can be seen that due to the influence of CRP slipstream, the low-energy flow in the boundary layer at the right wall of the inlet is mixed with the core stream, which leads that the separation phenomenon is weakened and total pressure recovery coefficient increases compared to the configuration without CRP, as previously mentioned.However, due to being on the leeward side, the low-energy flow in the boundary layer cannot obtain energy, while also accelerating separation due to the induction of small vortices.The total pressure recovery coefficient at inlet exit with CRP is low in the left and high in the right.Besides, it can be found that the total pressure at inlet exit with CRP is greater than that without CRP because of the work done by CRP.

Inlet performance analysis
Figure 15 shows the line chart diagram of the total pressure recovery coefficient at different cross section of isolated nacelle inlet and CRP /nacelle coupled configuration inlet under takeoff and cruise conditions.It can be seen that the total pressure recovery coefficient of coupled configuration is significantly higher.At the same time, the total pressure recovery coefficient of the inlet under takeoff condition is higher than that under cruise condition because CRP does more work under takeoff condition.Table 2 presents the performance parameters of both configurations at inlet exit.Although the nacelle inlet with CRP configuration has a high total pressure recovery coefficient, the suction of wake vortex makes the flow field inside the inlet very complex, and the internal flow loss increases sharply.The total pressure loss coefficient in the inlet under takeoff conditions increases by 191.7%.In the same time, the intensification of the non-uniformity of the airflow at inlet exit is reflected in the significant increase of total pressure distortion index.According to the numerical calculation results, the total pressure distortion index under takeoff condition can increase by 119.6% and the cruise condition can increase by 52.8%.16(a) is the line chart diagram showing the change of inlet exit aerodynamic performance with different sideslip angle under cruise condition.It can be seen that whether the flow sideslip angle is positive or negative, the total pressure recovery coefficient at the inlet exit always decreases.This is because the incoming sideslip angle will disturb the speed triangle of the CRP, making the CRP deviate from the normal operating point, reducing the capacity of blades to perform work, and at the same time, the number of breakaway vortices at the trailing edge of the blade increases, leading to an increase of the total pressure distortion index.

Conclusion
This article uses the sliding mesh unsteady method to study the impact of the slipstream of CRP on the flow field structure and aerodynamic performance of the nacelle inlet by comparing the numerical simulation results of isolated nacelle and nacelle/CRP coupling configuration under different flight conditions.The main conclusions are as follows: The main factors affecting the flow field structure and aerodynamic performance of the nacelle inlet are the hub vortices and blade trailing edge shedding vortices in the slipstream region of the CRP and the slipstream effect is not consistent under different operating conditions.Under cruise condition, the blade tip vortices of the front and rear rotors are at the same height, so that the blade tip vortices in the slipstream region are mixed with each other, forming a network structure and transmitting downstream.While under takeoff condition, the slipstream tube has significant contraction and acceleration effects.The blade tip vortices and trailing edge vortices of the front rotor contract after detachment, leading to more trailing edge vortices and hub vortices being sucked in by the inlet.The downstream blade tip vortex structure is no longer a network, but rather a pitch like structure dominated by the blade tip vortices of the rear rotor.
The slipstream effect of CRP causes the airflow at the inlet entrance to have a significant tangential velocity which is influenced by the rotation direction of the rear rotor.The wake vortex sucked in by the inlet move from left to right.It is worth noting that the inlet always sucks in a pair of opposite vortices at the same time, which is caused by the mixing of the front and rear wake.The wake vortex accelerates the mixing of low-energy flow in the boundary layer, wall separation vortex and main flow, and improves the accumulation problem of low-energy flow on the windward side.However, the small vortex on the leeward side does not have a large amount of energy, which cannot improve the lowenergy fluid cluster, and may even induce the early generation of wall separation vortices.The suction of the slipstream wake vortex leads to a very complex flow field structure in the inlet, exacerbating the non-uniformity of airflow.The vorticity density during takeoff is greater than that during cruise, so the impact of the takeoff state slipstream on the nacelle inlet is greater.
Compared with the isolated nacelle configuration, under cruise condition, the CRP slipstream increases the total pressure recovery coefficient at the inlet exit by 10.2%, but the distortion index increases by 52.8%, and the total pressure loss coefficient in the inlet increases by 175%.Meanwhile, under takeoff condition, the counter rotating propeller fan slipstream increases the total pressure Coefficient of restitution at the inlet outlet by 13.4%, but the distortion index increases by 119.6%, and the total pressure loss coefficient in the inlet increases by 197.7%.In addition, changing the angle of attack and sideslip angle can lead to a decrease in the aerodynamic performance of the nacelle inlet under cruise condition, and the sideslip angle has a greater impact on the inlet performance.In general, even though the work done by the CRP will bring benefits to the total pressure recovery coefficient, the slipstream will aggravate the airflow loss and non-uniformity.In the actual work process of the propfan nacelle, it is necessary to consider flow control technology to suppress the adverse effects of slipstream.

Figure 1 .
Figure 1.CRP vortex structure isosurface.Although research institutions around the world have carried out extensive research on slipstream characteristics of CRP, there is still limited research on the downstream aerodynamic components affected by CRP slipstream.Therefore, some research on the coupling aerodynamic effects of traditional propellers and aircraft components was collected as a reference for this study.Ruiz Calavera[15] studied the aerodynamic performance of the propeller and inlet port based on the military transport aircraft A-400M.The simulation results obtained by momentum source method and sliding mesh unsteady method were compared with the experimental results, and the results showed that the simulation results obtained by the sliding grid unsteady method were more similar to the experimental results.Zhou[17] used MRF(Multiple Frame of Reference) method and sliding mesh unsteady method to simulate the internal and external flow of the propeller/inlet configuration, and preliminarily studied the propeller slipstream and the aerodynamic interference mechanism between the inlets.Wen[18] studied influence of slipstream of propeller on inlet port based on the sliding mesh unsteady method.The results show that the acceleration effect of propeller slipstream increases the total pressure in the inlet, while the torsion effect and blade wake decrease flow quality of the inlet.Related studies have shown that sliding mesh unsteady method can effectively simulate the slipstream of propellers and obtain the temporal variation of transient aerodynamic parameters in the flow field[19].In this paper, a propfan engine nacelle is taken as the research object, and the aerodynamic influence of CRP slipstream on nacelle is carried out based on sliding mesh unsteady method under takeoff and cruise conditions.

Figure 3 .
Figure 3. Domain of the numerical simulation.
Figure 4. Numerical calculation grid.The feasibility of the CRP numerical method in this paper is verified by using the F7/A7 experimental model and experimental data from reference[22].The F7/A7 was a 8X8 configuration CRP, designed at Mach 0.72 with rotation speed 1275 rpm.Figure5(a)shows the comparison between the experimental and numerical simulation results of the corrected power coefficient PQA under different advance ratios.It can be seen that the numerical simulation of PQA under different pitch ratios is in good agreement with the experimental results.At the design point, the relative error of PQA is only 1.2%.It can be seen that the numerical simulation method used in this article can simulate the flow characteristics of CRP.The validation of the nacelle numerical calculation method is carried out using experimental data from reference[23], and the results are shown in Figure5(b).It can be seen that the static pressure distribution on the cowl obtained from the numerical calculation is basically consistent with the experimental data, indicating that the numerical calculation method is reasonable.
(a) Comparison and verification result of corrected power coefficient.(b) Comparison and verification result of static pressure distribution on the cowl.

Figure 5 .
Comparison of experiment and CFD results.

Figure 8 andFigure 8 .Figure 9 .
Figure 9  show the distribution of Mach number and pressure at the meridional plane of CRP /nacelle coupled configuration and isolated nacelle model under takeoff and cruise conditions.In Figure8(a) and 8(b), the Mach number of slipstream region is significantly greater than that of incoming flow, and the slipstream tube contracts, because the CRP works on the airflow, which leads the airflow velocity increases and the flow area decreases according to the continuity equation.Under cruise condition shown in Figure9(a) and 9(b), airflow has higher velocity and CPR does less work, thus the contraction effect of the slipstream tube is not significant.But the acceleration effect of the slipstream increases the airflow velocity near the wall of the nacelle cowl, leading to a more significant local high-speed and low-pressure region at the outer lip of inlet and lower wall contraction section of cowl.Besides, slipstream has an improvement effect on the problem of low-energy flow accumulation in boundary layer splitter.Furthermore, for the internal flow of the inlet, the slipstream of CRP increases the airflow velocity and uneven distribution of pressure as shown in Figure8(d) and Figure 9(d).(a)Mach Number(Isolated Nacelle) (b) Mach Number(CRP/Nacelle) (c) Pressure(Isolated Nacelle) (d) Pressure(CRP/Nacelle) Mach number and pressure distribution at the meridional plane under takeoff condition.(a) Mach Number(Isolated Nacelle) (b) Mach Number(CRP/Nacelle) (c) Pressure(Isolated Nacelle) (d) Pressure(CRP/Nacelle) Mach number and pressure distribution at the meridional plane under cruise condition.

Figure 10 .
Figure 10.Streamline distribution of the inlet entrance under cruise condition.Figure 11 and Figure12 show the distribution of vorticity at different cross section of isolated nacelle inlet and CRP /nacelle coupled configuration inlet under takeoff and cruise conditions.Each cross section is located at / 0, 0.25, 0.5, 0.75,1.0xL =

Figure 11 .
Figure 11.Vorticity distribution at different cross sections under takeoff condition.

Figure 15 .
Figure 15.Total pressure recovery coefficient at different cross section.
Figure 16(b) shows the Line chart diagram of inlet exit aerodynamic performance varying with angle of attack under cruise condition.It can be found that the aerodynamic performance of the inlet varies relatively little with angle of attack of the incoming flow.As the angle of attack increases, the total pressure recovery coefficient decreases slightly and the distortion index increases.(a) Aerodynamic performance of inlet exit at different sideslip angle.(b) Aerodynamic performance of inlet exit at different angle of attack.

Figure 16 .
Figure 16.Aerodynamic performance of inlet exit at different flow conditions.

Table 1 .
Design Parameters of Nacelle.