Most practical flows in engineering applications are turbulent, and exhibit separation which is generally undesirable because of its adverse effects on performance and efficiency. Therefore, control of turbulent separated flows has been a topic of significant interest as it can reduce separation losses. Often, flow control work employs passive techniques to manipulate the flow. These approaches do not require any additional energy source to achieve the control, but are accompanied by additional viscous losses. However, it is more desirable to employ active techniques as these can be turned on and off, depending on the flow control requirement. Use of synthetic jets has gained popularity in recent times for active flow control because of their ability to transfer linear momentum to the flow system without net-mass injection across the boundary in the vicinity of separation. The present work is Case 3 of the 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop, http://cfdval2004.larc.nasa.gov/case3.html, conducted by NASA for the flow over a wall-mounted hump. This flow is characterized by a simple geometry, but, nevertheless, is rich in many complex flow phenomena such as shear layer instability, separation, reattachment, and vortex interactions. The baseline case and control case with steady suction has been successfully simulated by Gan et al., (2007 and 2008). The present work is focused on implementing a synthetic jet to achieve flow control. The jet was simulated by implementing an analytical sinusoidal velocity boundary condition at the surface of the jet exit. The jet-exit velocity has a parabolic profile across the control slot, and a sinusoidal temporal variation. The flow is simulated at a Reynolds number of 371,600, based on the hump chord length, C, and a Mach number of 0.04. The synthetic control jet exits through a slot located at approximately 0.65 C. Solutions are obtained using the three-dimensional RANS SST turbulence model, and the DES and LES turbulence modeling approaches. Multiple turbulence modeling approaches help to ascertain what techniques are most appropriate for capturing the physics of this complex separated flow. The location of the reattachment behind the hump is compared with experimental results. The successful control of this turbulent separated flow leads to a reduction in the reattachment length, compared with the baseline case. Velocity contours at several streamwise locations are presented and compared to experimental results. Mean flow parameters such as pressure coefficients and skin-friction coefficient are presented. The paper includes detailed comparisons of turbulent parameters such as the Turbulent Kinetic Energy (TKE) and Reynolds stress profiles, with experimental results. Instantaneous vorticity contours are presented from the simulations. Discussion are presented of the effects of synthetic jet control on flow separation and reattachment and the resulting enhancement of performance and efficiency.
Numerical study of active flow control using synthetic jets
