Body effect on inflow distortion in civil engine intake under crosswind condition

The effect of body on the inlet aerodynamic performance of distortion under crosswind condition for a high pass ratio turbofan engine was investigated. The work is carried out by using computational fluid dynamics on civil nacelle and body. The investigation suggests that: due to the shelter of the body, the inlet distortion on the windward side and leeward side is very different under crosswind conditions. The nacelle on the windward side mainly takes in air from the side while the engine on the leeward side takes in air from above and below. The existence of body makes it easier to produce fuselage vortex and tail vortex near the nacelle. The body has an adverse impact on the intake of the engine on the leeward side and improves the requirement of the crosswind resistance of the upper section of the nacelle.


Introduction
The primary role of the air intake is to provide the fan with adequate flow uniformity over a wide range of operating conditions.Modern civil turbofan engines operate at high bypass ratios and have larger diameters, thus requiring shorter lengths to compensate for increased intake drag.Shorter air intakes reduce the ability to distribute evenly, which results in easier separation of airflow over the cabin compared to conventional air intakes.In order to meet the needs of the integrated design and analysis of the airframe and the engine, CFD and wind tunnel tests were carried out in the process of civil aircraft.Recently, CFD tools have also been applied to simulate inlet vortices with relative success.Trapp et al. [1] performed RANS simulations to evaluate vortex behavior around the nacelle under crosswind conditions, and validated the numerical results with measured data and analytical solutions.Cao et al. [2] conducted a numerical study on the interaction between airflow separation on the air intake and the fan at high incidence.Zantopp [3] conducted an experimental study to study the formation of ground vortex from air inlets under crosswind conditions.Kuen [4] studied the effect of inlet disturbances caused by crosswinds on the operation of fan blades.Verm [5] studied the effect of total pressure distortion on single-stage compressors and low bypass ratio fans.Mauro [6] investigated the conditions under which distortion occurs in the intake air of a civil aircraft engine, investigating two sources of distortion: ground vortex intake and high angle of incidence flight conditions.Menezes [7] analyzed the root cause of the EMB-145 engine core speed being higher in the right engine than the left engine through CFD analysis and aircraft testing, considering the crosswind effect.The effect of bodywork on intake tract distortion under crosswind conditions has been rarely studied before.This paper reports the results of a numerical study of fuselage and nacelle configurations with radial total pressure deformation.This paper is organized as follows.The numerical model and benchmark test cases are first introduced.Afterwards, the numerical method used here is validated by comparison with known test data.Finally, the effect of the fuselage on the intake distortion of the engine nacelle under crosswind is investigated.

Physical model
A typical civil aircraft engine power nacelle with a large bypass ratio is selected as the research object to study the influence of the airframe on the crosswind intake distortion of the civil aircraft intake.The power nacelle model is shown in Figure 1.The nacelle includes an air inlet, an air intake cone, a fan cover, an external outlet and an inner outlet.Considering the effect of ground effect, the ratio of intake port height H to nacelle diameter D is H/D=0.93.

Calculation method and calculation conditions
The ANSYS ICEM software is used to generate a structured computational grid for the full 3D flow field of the fuselage and nacelle.Fig. 2 shows the schematic diagram of the computational grid for the fuselage and nacelle surfaces.In order to ensure the accuracy of the calculation results and capture the flow process of jet and mixing well, the distance from the far-field boundary of the computational domain to the nacelle is set to 250 times the equivalent diameter of the nacelle.In order to accurately simulate the complex flow in the boundary layer, the grid near the wall of the nacelle is refined.The grid height of the first layer is about 0.01 mm, and the near wall is about y+ ≈ 1.In addition, the meshes of the inlet, lip, inner and outer culverts, and the areas near the ground where the flow field parameters change violently are properly refined.The total number of three-dimensional computing grids in the entire flow field is about 38 million.The software used for numerical simulation is ANSYS CFX, and CFX uses lattice-based finite volume method to discretize the Reynolds-averaged Navier-Stokes equations.The computational domain to be solved is first divided into different types of mesh elements, and then control volumes are built around each mesh node.The flux of the control volume is calculated by integrating points on the face between the control volumes.The time discretization adopts the fully implicit time advancing format, and the spatial discretization adopts the high-order upwind format.The Reynolds stress in the momentum equation is handled by the two-equation turbulence model SST combined with the automatic wall function.The model uses the k-ω model in the near-wall region and the k-ε model in the far-wall region.In the near-wall region, it not only has the characteristics of high calculation accuracy of the kω model, but also overcomes the kω model in the far-wall region.The model is sensitive to incoming flow ω.In the flow field calculation, the residuals of the whole flow field and the inlet and outlet flow are monitored, and the convergence is based on the fact that each residual index decreases by 4 orders of magnitude or the residuals do not decrease after continuing iterative iterations and the inlet and outlet flow remains stable.
The boundary conditions are set as follows: the far-field inlet boundary condition adopts the velocity inlet condition; the far-field outlet boundary condition adopts the pressure boundary condition; the calculation parameters are given the parameters such as free flow static temperature, static pressure, and flow velocity.Crosswind conditions are imposed in the calculation, and the direction of the crosswind is from right to left (driver's perspective, the right side is the windward side).The inlet boundary conditions of the intake duct adopt the pressure boundary conditions, which are continuously adjusted with the iterative process to match the flow rate of the fan surface; the boundary conditions of the outer duct inlet adopts the total temperature and total pressure boundary conditions; the boundary conditions of the inner duct inlet adopt the total temperature and total pressure boundary conditions; the nacelle object surface boundary condition applies the solid-wall no-slip adiabatic condition.The ground boundary condition is set to the no-slip wall boundary condition.

Calculation results and discussion
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Verification
In order to verify the ability of CFD software to solve the flow problems related to the power nacelle, the experimental data of the axisymmetric turbofan power simulator (TPS, turbine powered simulator) wind tunnel of Japan Institute of Aeronautics and Astronautics (JAXA) was selected to verify the numerical example. .The calculation adopts the working condition of calculation number 13 (corresponding to the experimental number 516) in the literature [8], the calculation Mach number is 0.801, the Reynolds number is 2 million, the temperature is 315.74K, the attack angle is 0 °, the sideslip angle is 0°, and the mass flow coefficient is 0.31678.The ratio of the total pressure of the external nozzle to the static pressure of the incoming flow is 0.92978, the ratio of the total temperature to the static temperature of the incoming flow is 1.01198, the ratio of the total pressure of the internal nozzle to the static pressure of the incoming flow is 0.68668, and the ratio of the total temperature to the static temperature of the incoming flow is 0.99310 .Fig. 4 shows the distribution curves of the calculated and experimental results of the pressure coefficients on the nacelle fan cover and the inner fairing.It can be seen from the figure that the calculated pressure coefficients in the inner and outer flow areas of the nacelle are basically consistent with the experimental values.It can be considered that the numerical method, grid model and the treatment measures of the inlet and exhaust boundary conditions used in this paper can be more accurate.Simulate the flow field characteristics of the nacelle intake, shroud and exhaust system.

Streamlines
In this paper, the typical ground take-off state is used to analyze the air intake distortion characteristics of the nacelle.The calculation parameters are: height 0 m, static pressure 101325 Pa, and static temperature 288.15K.The reduced flow rate of engine power is about 533 kg/s, the angle of attack is 0°, and the side slip angle is 90°.The flow diagram of the ground crosswind condition when the crosswind speed is 30 knots is shown in Figure 4.The part of the side flow near the engine lip first enters the windward side engine, and the part farther from the engine lip bypasses the fuselage , into the leeward side engine.Due to the shielding of the fuselage, the air intake of the nacelles on the windward side and the leeward side under crosswind conditions is very different.A nacelle on the leeward side has a very different intake in crosswind conditions than a nacelle alone.Figure 5 shows the intake range of the nacelles on the windward side and the leeward side.The leeward side nacelle absorbs the airflow from the front and farther, and the intake range is wider and is more affected by the uneven airflow around it.And its suction is affected by both the ground and the fuselage.It is easier to generate fuselage vortices.Generally, in order to reduce drag, the top section of the nacelle is thinner, and the crosswind section is the main windward side of the crosswind, and the design is thicker, but for the leeward side engine, the top section is the main windward side.The thin profile of the top is not good for the air intake distortion under crosswind conditions, and the experiment and CFD simulation of the single nacelle cannot reflect this effect.The influence of the fuselage on the intake of the leeward side engine cannot be ignored.

Separation
According to the overall design and safe operation requirements, the air intake should meet the engine air intake quality requirements at least at 20 knots of crosswind under ground conditions.In this paper, the influence of crosswind speed on intake distortion is firstly studied under the certified engine operating conditions, and the range of the analyzed crosswind speed is 10-30 knots.For the convenience of explanation, the following describes the size of the crosswind speed in conjunction with the imperial unit (knot) commonly used in the evaluation of the crosswind speed of civil aircraft.Figure 6 shows the distribution of total pressure recovery at the outlet of the left and right air ducts, that is, the fan section when the crosswind speeds are 15 knots, 20 knots, 25 knots and 30 knots, respectively.The red area in the figure is a high value and the blue The color area is the low value, the left side is the leeward side, and the right side is the windward side.It can be seen from Figure 6 that the left and right engine nacelles have low total pressure areas near the wall, which is due to the decrease in total pressure caused by the loss of viscosity due to the airflow.The distribution of total pressure at the outlet of the right engine (windward side engine) is basically similar, and the low total pressure area causing the increase of the distortion index is mainly distributed near the wall and the ground vortex area.The size and intensity of the ground vortex area in the low pressure area near the wall increase with the increase of the crosswind speed.The left engine is used as the wind-driven side engine.When the crosswind speed is less than 25 knots, the ground vortex phenomenon is not as obvious as that of the right engine.At the crosswind speed of 25 knots, an obvious low total pressure area appears on the top of the nacelle.At the crosswind speed of 30 knots, the The low total pressure area becomes larger.The distortion index will increase significantly at this time.This is due to the thin profile at the top of the nacelle, where the airflow separates as it accelerates.

Vortex
Figure 7 shows the recovery distribution of the total pressure at the intake port and the vorticity isosurface when the crosswind speed is 30 knots, and the isosurface shows the area where the force has vortices.It can be seen from the figure that the ground vortex suction phenomenon is obvious, resulting in an increase in the stagnant pressure loss on the fan surface.The fuselage vortex shedding after flowing through the nose may be sucked into the left engine (by the wind side engine), causing separation and aggravating total pressure distortion.The existence of the fuselage makes the flow field environment where the engine on the wind-driven side is located more complicated, which adversely affects the air intake quality of the wind-driven side engine.

Conclusion
The influence of the fuselage on the air intake is reflected in the change of the main air intake channel (air intake direction) of the engine on the wind side under the crosswind condition, the expansion of the incoming air flow range, the generation of fuselage vortices, and the increase in the incoming flow.
The non-uniformity of the air intake is ultimately reflected in the increase of the intake distortion index, which has an adverse effect on the intake air quality.Therefore, the influence of the fuselage must be considered when designing the nacelle air intake.

Figure 2 .
Figure 2. Schematic diagram of surface meshes for fuselage and nacelle calculation.

Figure 3 .
Figure 3.Comparison of nacelle pressure coefficient and test value.

4 .
(a) Nacelle on the windward side (b) Nacelle on the leeward side Figure Flow diagram of ground crosswind condition.

Figure 6 .
Figure 6.Distribution of total pressure recovery on exit of inlet.