# Acoustic Improvement of Stator-Rotor Interaction with Nonuniform Trailing Edge Blowing
## 1 Introduction  
The continued growth of the aviation industry resulted in aviation noise being recognized as a harmful factor in everyday life. The International Civil Aviation Organization (ICAO) and airworthiness agencies of individual countries have issued a number of aircraft airworthiness standards in order to control civil aircraft noise. However, noise is not only an issue in the context of civil aviation. Even for military aircraft, noise reduction is desirable to delay the fatigue of materials and facilitate acoustic stealth. The fan and the compressor are the main sources of noise on an aeroplane. Nevertheless, the noise reduction of compressors is difficult, and has attracted substantial attention due to the complex geometry of the compressors and their multistage structure. The trailing edge of rotating or stationary blades generates a wake that affects the aerodynamic and acoustic performances of any downstream blades. When a spinning rotor blade sweeps through the viscous vortex wake shed from an upstream stator, it becomes subject to unsteady loads, as illustrated in Figure 1. In an axial engine, auxiliary airflow is commonly used to control flow. Suction and pulsed blowing flow shows a robust ability to delay separation [1]. Tip blowing can control the tip leakage in a high-pressure turbine cascade [2]. However, the blowing flow technology has been adopted mostly for turbine blade cooling [3]. The compressor interstage region or stator/rotor interaction area is the core noise source in a compressor. Nacelles with liners are currently the most widely used noise reduction measures on an aeroplane. However, lined nacelles only reduce noise after it has been generated. Obviously, there exists the alternative solution of lowering compressor noise by suppressing it at the location of the source in the stator-rotor interaction area. One technique to facilitate such noise reduction is trailing edge blowing (TEB), due to its potential to compensate the momentum loss in the wake region. The novel technology of \"trailing edge blowing\" for the purpose of improving the flow field emerged at the end of the last century. It was discovered when it was first applied to turbine machinery at the Massachusetts Institute of Technology that it results in greater thrust for engines. [PERSON] [4] and [PERSON] [5] used TEB to reduce the rotor-stator interaction noise of a turbo fan engine, and they concluded that TEB is the preferable methodology for flow control when compared with boundary-layer suction. [PERSON] [6] from Virginia Polytechnic and State University continued this work to reduce downstream rotor noise with uniform TEB from the wake of an upstream stator. This was followed up by [PERSON] [7,8], who investigated the effects of TEB on turbo fan engine noise, and concluded that it could be effective to uniform rotor wakes. Meanwhile, [PERSON] [9] used less than 1% mass flow to reduce forward radiated noise by 8-9 dB. [PERSON] [10] used TEB for a delta wing to reduce unsteady flow. In 2009, [PERSON] [11] illustrated the performance of TEB when applied in different positions on a fan through calculations and experiments. In 2012, [PERSON] [12] researched Mach number effects on noise. The above references reveal that TEB was mostly used on turbines with the purpose of making the flow field more uniform, but with rare concurrent acoustic research on stator-rotor interaction. [PERSON] [13] conducted a series of experiments on a supersonic compressor with 6 dB of noise reduction. [PERSON] [14] continued [PERSON]'s research in an anechoic chamber of Virginia Tech, and found that the strongest noise reduction of the first four order blade-passing frequency (BPF) was 3-5 dB. [PERSON] [15] used non-inserting microphones for [PERSON]'s experiments. Thereafter, the research team at Virginia Tech [16, 17, 18, 19] studied flow control methods, airfoil design, and acoustic measurement methods, and concluded that TEB has an obvious influence on acoustics and aerodynamics.