Control Mechanism Of Thermal Actuator In High-speed Planar Shear Flow And Round Jet

Active flow control technology has become one of the primary methods to improve the aircraft performance and has been widely applied to mixing enhancement and noise reduction. In this paper, large eddy simulation(LES) is used to explore the effect of thermal actuator on high speed and high Reynolds number planar shear flow and free round jet flow. This study is aimed at finding the mechanism of mixing enhancement and noise reduction in the shear layer due to the action of the thermal actuator. The effect of the actuation intensity, actuation mode and actuators configuration are analyzed. The details are shown as follows.(1) A Mach 1.3 planar free shear layer is simulated with LES. The time averaged flow field and instantaneous flow field are analyzed to explore the flow characteristics. The frequency spectrum is calculated. In the two dimensional simulation, it is shown that as the vortices move downstream, the pressure potential energy of vortices is converted into kinetic energy, and the fluctuation frequency decreases while the magnitude increases. The vorticity and turbulent kinetic energy distribution is analyzed in the three dimensional simulation. It is found that the strip-like spanwise vortical structures dominate in the head region of the shear layer, and the development of the streamwise vorticity causes the instability of these structures, leading to the formation of hairpin vortices.(2) Large eddy simulation(LES) is used to explore the effect of the thermal actuator on a Mach 1.3 planar free shear flow at different actuation modes. The steady actuation and unsteady actuation with the sine function are used in the two dimensional simulation. It is found that for the two dimensional shear flow, the higher steady actuation magnitude leads to the higher dominant frequency, but the fluctuation amplitude at this frequency decreases; while the unsteady actuation can also suppress the high peak of the original dominant fluctuation and lead to higher dominant frequency, especially the downstream. The case with the lower actuation frequency causes more peaks with the lower magnitude. In the three dimensional simulation, the pulse actuation mode is used and the effects of the pulse amplitude, pulse frequency and duty cycle on the development of the vortices are explored. Results show that the growths of the shear layer thickness, turbulent kinetic energy and vorticity are enhanced by the unsteady actuation. The higher pulse magnitude causes the higher velocity and pressure fluctuation, and the actuation causes a qualitative change when the pulse magnitude and duty cycle are large enough. The frequency spectrum analysis indicates that although the actuation frequency is much lower than the frequency of the shear layer, the actuator can suppress the high frequency fluctuation effectively.(3) Large eddy simulation(LES) is used to simulate an axisymmetric jet and three dimensional supersonic round jet. In the axisymmetric jet, the fluctuation property of different regions of the jet is analyzed. Results show that the pressure fluctuation in the vicinity of the axis is caused by the flow entrainment, and the fluctuation frequency is low. However the pressure fluctuation within the shear layer is effected by the vortical motion and shear stress, and possesses a wide range of frequencies. The analysis of the three dimensional flow field and fluctuation property indicates that the streamwise vorticity promotes the spatial transport of momentum, resulting in the growth of shear layer and pressure fluctuation.(4) Large eddy simulation(LES) is used to study the effect of different actuation modes on a Mach 1.3 free round jet, the effect of the pulse magnitude and pulse frequency are investigated in both axisymmetric and three-dimensional simulations and the control mechanism of the spatial actuation modes on the vortical structure is explored in the three-dimensional analysis. Results show that as the pulse frequency increases, vortices shed at upper location with higher shedding frequency in the asymmetric simulation. In the three dimensional simulation, the near-field and far-field flow properties are analyzed for the actuation modes of m=+-1 and m=+-4. It is found that the thermal actuators produce high temperature region which acts like a physical bump leading to the formation of the high vorticity region. This helps to promote the growth of the shear layer and mixing enhancement. The Q criterion is used to identify the vortical structure. It is shown that the actuation causes stronger fluctuation of the radial and circumferential velocity which helps to form the hairpin vortices. Different vortices distribution and aerodynamic noise frequency spectrum are found for the actuation modes of m=+-1 and m=+-4. It is shown that the aerodynamic noise frequency spectrum is quite wide, and the actuation can affect the noise characteristics and suppress the high frequency component effectively.