A study on three dimensional forcing of a cylinder wake was conducted in order to determine the optimum spanwise forcing wavelength. The forcing was implemented by mounting dielectric barrier discharge plasma actuators on the cylinder in a square wave pattern. Actuators of wavelength 1d, 74, Ad and 6d (d = cylinder diameter) were used to study the influence of forcing on the wake at a Reynolds number of 4700. The actuators were operated at two distinct power levels. Drag measured at an intermediate wake distance showed that maximum drag reduction was obtained when the Ad actuator was operated at a high power setting. The strength of vortex shedding was considerably attenuated in the near wake during forcing with the Ad actuator in the high power case. Forcing with 6d actuators caused significant variation in the spanwise properties of the wake which was evident even at a streamwise distance of 40 cylinder diameters. Particle image velocimetry was used to obtain cross flow images in the near wake. It was found that forcing with the Ad actuator created distinct patterns of streamwise vorticity in the wake with a spanwise wavelength of Ad which resembled mode A structures. It is argued that the effectiveness of the Ad actuator compared to other wavelengths was due to the formation of streamwise mode A structures in the near wake which hindered the development of spanwise Karman vortices.
展开▼
