Boundary Layer Flow Separation Control by Synthetic Jet Actuators

In this research the effectiveness of synthetic jet actuators (SJAs) for the boundary layer flow separation control over a circular hump model at chord Reynolds number in range of 105 with speed in range of 10 m/s in turbulent regime is studied both experimentally and numerically. A low speed closed circuit wind tunnel with velocity range of 0-25 m/s, as well as a circular hump model have been designed and manufactured to enable the research. Improvement of the performance of synthetic jet actuators is achieved by geometrical optimization of SJAs through a series of Hot-Wire Anemometry (HWA) experiments in quiescent conditions (no cross flow). The influence of different geometrical and operational parameters including actuator position, the ratio of the peak exit jet velocity of actuators to the free stream velocity of cross flow (VR), actuation frequency and waveform on the flow separation control are investigated by both Hot-Wire Anemometry (HWA) and Particle Image Velocimetry (PIV) techniques. The results revealed that the best location of SJAs to attain the best performance is somewhere upstream and close to the separation point. Improvement of aerodynamic performance of active flow control has been achieved by optimization of both geometrical and operational parameters of active flow control.

The results show that the best performance of synthetic jet actuators in control of flow separation over the hump model occurs at optimum value of velocity ratio of 1.85 with 42.6 and 44.2% reduction of the length of recirculation region by using sine and square wave excitation of SJAs, respectively. This achievement is a unique improvement in control of flow separation. The Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are utilized to numerically investigate boundary layer flow separation over a hump model and its active control. The OpenFoam software is used to numerically simulate and analyze the flow by K-Omega SST model. The SJAs have been fully simulated in this study by considering both cavity and membrane oscillations. Also, Merging and Stitching techniques have been utilized to generate the computational grid for actuated case which were very useful to greatly reduce the computational costs. The three-dimensional Computational Fluid Dynamics (CFD) simulations of unactuated case, as well as the simulation of interaction of vortical structures generated by synthetic jet actuators with cross flow revealed more information about the flow physics of separation phenomenon and its control and suggest some ideas (e.g. geometrical optimization of SJAs) to improve the aerodynamic performance of active flow control. The full simulation of synthetic jet actuators and their interaction with cross flow, alongside utilization of Merging and Stitching techniques for mesh generation is arguably the first attempt of this kind. For completeness, the accuracy of URANS technique as well as the effectiveness of synthetic jet actuators have been evaluated by comparison of numerical predictions with experimental data. The instantaneous and time-averaged results are in reasonable agreement with experimental results and depict the successful performance of SJAs to delay flow separation by interaction of vortical structures with separated shear flow. Although, the predictions of separation flows in fluid dynamics field always is challenging, but the comparison of numerical results with experimental data shows that the K-Omega SST model can be used as a good strategy for prediction of flow separation and its control.