Vortex shedding and its control | Prof. S. D. Sharma | 2017

Periodic vortex shedding, also known as Karman vortex shedding, is a common phenomenon that occurs in the wake of a nominally two dimensional bluff body due to interaction between two separated shears layers from opposite sides. We have employed numerous methods, both active and passive, to suppress this alternate vortex shedding in order to alleviate its detrimental effects such as high form drag, flow induced vibration and noise. In active control method, only one of the shear layers at the trailing edge of a blunt based aerofoil was either sucked in or tangentially blown just before the separation. With increasing strength of the suction and blowing, a complete suppression of the vortex shedding could be achieved. The efficacy of the steady blowing technique was further significantly improved by making it pulsating. The pulsating blowing was applied in open-loop mode with two control parameters – blowing strength and pulsating frequency. For the frequency of blowing being twice the unforced vortex shedding frequency and the active phase of the cycle being just 20%, not only a complete suppression of the vortex shedding was achieved but also the wake was found to gain momentum. In passive control method, the spanwise vortex dislocation was introduced by certain geometric cutouts provided at the model trailing edge. The serration shapes were found to introduce strong streamwise vortices in pairs of either co-rotating or contra-rotating by choice. The regular vortex shedding was effectively suppressed resulting in about 34% reduction in the form drag. Moreover, the pairs of contra-rotating vortices were found to generate lift even at zero angle of incidence due to wake vectoring (downwash) effect. Further, following the technique of active control, only one of the separated shear layer was cancelled by placing a short plate at an angle in its path which resulted in a complete suppression of the vortex shedding. Periodic vortex shedding, also known as Karman vortex shedding, is a common phenomenon that occurs in the wake of a nominally two dimensional bluff body due to interaction between two separated shears layers from opposite sides. We have employed numerous methods, both active and passive, to suppress this alternate vortex shedding in order to alleviate its detrimental effects such as high form drag, flow induced vibration and noise. In active control method, only one of the shear layers at the trailing edge of a blunt based aerofoil was either sucked in or tangentially blown just before the separation. With increasing strength of the suction and blowing, a complete suppression of the vortex shedding could be achieved. The efficacy of the steady blowing technique was further significantly improved by making it pulsating. The pulsating blowing was applied in open-loop mode with two control parameters – blowing strength and pulsating frequency. For the frequency of blowing being twice the unforced vortex shedding frequency and the active phase of the cycle being just 20%, not only a complete suppression of the vortex shedding was achieved but also the wake was found to gain momentum. In passive control method, the spanwise vortex dislocation was introduced by certain geometric cutouts provided at the model trailing edge. The serration shapes were found to introduce strong streamwise vortices in pairs of either co-rotating or contra-rotating by choice. The regular vortex shedding was effectively suppressed resulting in about 34% reduction in the form drag. Moreover, the pairs of contra-rotating vortices were found to generate lift even at zero angle of incidence due to wake vectoring (downwash) effect. Further, following the technique of active control, only one of the separated shear layer was cancelled by placing a short plate at an angle in its path which resulted in a complete suppression of the vortex shedding.