**1. Introduction**

The inlet distortion is a common phenomenon in takeoff and flight. Some specific conditions, such as takeoff, aircraft inhaling missile exhausts, and aircraft use of the S-duct inlet, may lead to combined distortions at the aero-engine inlet, including the total pressure distortion, total temperature distortion, and swirl distortion. The inlet distortion negatively affects aero-engine performance, which directly threatens aero-engine stability and safety. Therefore, it is necessary to study the influences of combined distortions on aero-engine stability.

Researchers have studied the influences of combined distortions on the stability of the engine compression system for many years. They focus on the total pressure and total temperature distortion instead of other combined distortions. Braithwaite and Braithwaite *et al.* obtained the operating parameters of the engine under the 180° total pressure and total temperature distortion in different phases by testing engines TF30- P-3 and YTF34. The pressure distortion required to stall the compressor increases as the temperature distortion decreases for aligned high temperatures and low pressures. The required pressure distortion increases with the increased temperature distortion for the opposed high-temperature low-pressure case. The most persistent distortion occurs when the high temperature and low pressure are in the same 180° sector; the distortion attenuates the most when the low-pressure zone is opposite to the high-temperature zone [1, 2].

Mehalic studied the temperature and pressure distortion of the engine by testing engine PW1128. When the low-pressure zone coincides with the high-temperature

zone, it will cause the high-pressure compressor to stall. When the low-pressure zone is opposite to the high-temperature zone, the influence on the stability of the highpressure compressor will be weakened [3]. Davis used a stage-by-stage parallel compression system simulation and found the largest loss in stall-margin results under the steady-state pressure and temperature distortion when the high-temperature and lowpressure regions coincide. The stability limit has been lowered for the steady-state pressure distortion and temperature ramps. Stability is related to the position of the time for the dynamic combined distortion of the pressure oscillation and temperature gradient (the linear increase of temperature over time) when the temperature gradient is applied in the pressure oscillation cycle [4]. Modified parallel compressor theory can simulate more complex distortion patterns, including temperatures, pressures, and swirls. However, radial distortion effects cannot be included easily due to the changes in radial work distributions [5].

Frohnapfel *et al.* conducted experimental investigations and found that it is impossible to completely decouple total pressure and swirl distortions. A total pressure distortion typically creates a secondary flow field as air mixes from high to lowpressure regions within the distortion. Similarly, flow losses of axial flow at the inlet associated with turning under the swirl distortion for swirl distortion testing disturb the flow pressure field. The radial and tangential flow angles at the fan outlet are significantly influenced by the inlet-swirl distortion; whereas, the total and static pressure ratios of the fan are significantly influenced by the total pressure distortion at the inlet [6].

Huang found that the effect of combined distortions on engine stability is a complex nonlinear relationship rather than a simple addition of pressure and temperature distortion effects [7]. Ye *et al.* found that a compressor system creates maximum stability margin degradation when pressure distortion and temperature distortion phases coincide based on the numerical simulations of the 2D Euler equation, and the stability margin can be improved in some phases if a combined-distortion phase is changed suitably [8]. Xie *et al.* used the parallel compressor method and found that the influence of combined distortions on the fan and compressor pressure ratio is the same as that of the single-pressure distortion [9].

The parallel compressor model is often used in studying combined distortion. Pearson and McKenzie first proposed the parallel compressor theory in 1959 [10]. Reid [11], Mazzawy [12], and other researchers supplemented the model [13]. The theory shows that a compressor under the distortion can be treated as two or more sub compressors operating in parallel, and different levels of pressure- or temperaturedistortions may be imposed upon their inlets, e.g., some take in distorted/undistorted air. Each sub compressor is assumed to operate independently of the other sub compressor except for the exit boundary where either a uniform static pressure or a uniform Mach number is imposed. As a result, each segment of the total system simulation will operate as a separate compression system. The entire compression system is considered to be unstable when one sub-compressor reaches the stability limit in this classical form. The mean operating point at instability is a weighted average of the low flow sector operating at the uniform flow stability boundary and the high flow sector operating at some other points far from the stability limit using the approach (see **Figure 1**) [14].

Research on the influences of combined distortions on the stability of the aeroengine compression system mainly adopts the parallel compressor method and the penetrating force method. However, the 3D computational fluid dynamics (CFD) numerical simulation is rarely used. Besides, research on combined distortions focuses

**Figure 1.** *Parallel compressor theory [14].*

on the total pressure and total temperature distortion instead of combined distortions including the swirl distortion. The 3D CFD method was utilized to simulate the whole circumference of the rotor and study the influences of combined distortions on fan performance in the work. Valuable results were obtained.
