| Flow separation is widely observed in the field of aviation, aerospace, offshore engineering and civil engineering. In this paper, experimental observations and numerical simulations are applied to cases with flow separation. Further, active flow control techniques are applied to controlling separated flows of backward-facing step, and backward-facing step flow control using synthetic jet is preliminarily studied. Major works and conclusions are listed in the following passage:(1) The rationality of thin layer approximation, along with its physical meaning, is carefully analyzed. Several typical cases of attached flows are studies using both thin layer approximation and full Navier-stokes methods. Numerical simulation using RANS method could obtain accurate lift/drag, pressure and friction characteristics. For separated zones of the flow field, the accuracy of numerical simulation decline, numerical methods which can capture vortical characteristics of these zones should be introduced. For a certain turbulence model, the lift/drag coefficients derived from both thin layer approximation and full N-S are similar. Full N-S discretization of viscous term corresponds to closer simulation results to experimental data. For attached flows, the differences of simulation results of lift/drag coefficients are primarily caused by turbulence models. SST model derives relatively larger drag coefficient than SA turbulence model. Mesh size has great impact on simulation results, and clear shock structures are observed in denser grid system. Therefore, the influence of grid size on flow field structures should be studied, and the convergence of grid size should be ensured in simulation works.(2) NACA0021 with massive separation is simulated with URANS and DES turbulence models. Simulation results using URANS and DES models are compared and the influence of span width is studied. For massively separated flows, the precision of lift/drag characteristics, surface pressure distribution and power spectral characteristics derived by URANS method cannot meet engineering requirements. Simulations using DES series turbulence models are more accurate, with significantly reduced error, and engineering requirements could be fully satisfied. Show liter increase impact resistance coefficient and pressure distribution is not obvious to the length, but the calculation accuracy of the power spectral density can be significantly improved. Therefore, simulation models with appropriated span width should be used to obtain accurate results.(3) SAS turbulent model is used to simulate NACA0021 with massive separation, the impact of grid density and span width on results is studied. SAS has advantages capturing unsteady aerodynamic characteristics of massively separated flows and is grid adaptable. The lift/drag characteristics, pressure distribution and power spectral density of lift are in good agreement with experimental data. Vortex structures derived by ?2 criterion are smoother than by Q criterion, and no threshold should be pre-assigned, thus ?2 criterion is more suitable in identifying vortex structures. Second order extrapolation should be used to ensure accuracy of identified vortex structures at interfaces of two blocks. Refined vortex structures are resolved by finer grid, but accuracy of lift/drag coefficients and pressure distribution worse slightly compared with standard grid size. The main and secondary peaks of power spectral density of lift coefficient are in good agreement with the experimental data. With increased span width, the precision of pressure distribution, lift/drag coefficients and power spectral of lift could be improved. The simulation result with span width L = 4c is the best among all cases.(4) URANS and IDDES are applied to simulate aerodynamic configuration of semi YF-22 at low Mach number and high angles of attack with massive separation. While RANS model can capture lift and drag characteristics accurately, the moment at high angles are dramatically higher than experimental data. IDDES model ensures the accuracy of both lift/drag and moment coefficients at high angle of attack. By analyzing the characteristics of spatial vortices and surface pressure, the improvement of accuracy of moment coefficient at high angles is explained. IDDES model obtains weaker strake vortices and higher pressure on the upper surface of inlet than URANS, leading to smaller moment coefficient, which is closer to experimental data.(5) Baseline flow field of backward-facing step is simulated using RANS method. Two methods are applied to simulate synthetic jet flow- boundary with sinusoidal varying velocity and deforming grid. For the baseline flow, wall shear stress and velocity profiles at different stations can be accurately simulated, and the results obtained by SA and SST turbulence model are similar. When velocity boundary condition is used, periodic vortex formation and shedding phenomenon are observed. Flow control effect is remarkable with a very strong negative pressure area in the vicinity of the downstream region. When deformable mesh is used to simulate synthetic jet, flow patterns in the cavity are complex, but pressure is essentially the same. Accurate modelling of reciprocator is needed to simulate acoustic streaming effects of synthetic jet with piston moving up and down.(6) Active flow control, which has great application prospects in aerodynamic design, can restrain flow separation and reduce drag. A newly developed synthetic jet device with non-linear oscillation of the reciprocating piston actuator into the pipe is introduced and applied to control flow field of backward-facing step. An in-looped design optimization system based on experimental data adopting hybrid searching algorithm is constructed and applied to optimize parameters of this synthetic jet device. The optimum state based on experiment restrains separation dramatically, which validates the efficiency of the design optimization strategy and the excellent performance of synthetic jet device. The optimum jet slot angle is 127.5° and the optimum frequency is 35 Hz. Then, power consumed in driving reciprocator is considered to derive a multi-objective optimum scheme. With theoretical analysis and experimental data of velocity profiles and Reynolds stress distributions, flow control mechanism of synthetic jet device is preliminarily revealed. The optimization process and the analysis of optimum state provide guidance to the design of active flow control devices. |
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