Samik Bhattacharya

Effect of Three-Dimensional Plasma Actuation on the Wake of a Circular Cylinder

Dielectric-barrier discharge plasma actuators were mounted on a circular cylinder in a square wave pattern for active forcing of the cylinder wake. The intermittent spacing of the buried electrodes created a spanwise-modulated blowing profile with a spanwise wavelength of four cylinder diameters, such that three-dimensional instabilities in the wake were targeted for control. Considerable spanwise variation in the wake was achieved with low-power forcing when the actuators were mounted at the location of maximum shear layer receptivity. This spanwise variation was a direct consequence of the difference in the vorticity levels of the shed vortices from the cylinder. High power forcing nearly eliminated vortex shedding, leading to a considerable amount of drag reduction as measured in the intermediate wake.

Investigation of the cylinder wake under spanwise periodic forcing with a segmented plasma actuator

The wake response to three-dimensional forcing of flow over a circular cylinder was studied. Spanwise-segmented dielectric-barrier discharge plasma actuators were mounted on the cylinder in a square wave pattern for active forcing of the cylinder wake. The buried electrodes were placed periodically to create a spanwise-modulated blowing profile, with the aim of targeting three-dimensional instabilities in the wake. Considerable spanwise variation in the wake was achieved, which was a direct consequence of the difference in the location of shed spanwise vortices from the cylinder, along with the generation of streamwise vorticity. Two distinct power levels were used for forcing the flow, with different flow response observed between the two conditions. With low power, the segmented forcing caused the large-scale spanwise structures in the forcing region to lead those in the no-forcing region, with an accompanying shift away from the centerline and generation of streamwise vorticity. While vortex shedding was not substantially attenuated with low-power forcing, the shedding in the near wake was significantly attenuated with high-power forcing. This attenuation in the shedding strength was accompanied by a decrease in the peak shedding frequency, indicating an increase in the formation length. High-power forcing caused elongation of the Karman vortices due to the induced strain field and strong differential development of the wake shedding frequency. In both forcing regimes, the wake three-dimensionality increased as shown by the increased width of the spectral peaks.

Control of Cylinder Wake Using Three Dimensional Disturbances

Wake of a circular cylinder subjected to acoustically driven disturbances introduced from sinusoidal slits was investigated at Reynolds number of 24,000 and 40,000. Results showed that the amplitude of the primary shedding frequency was eliminated when the forcing frequency was twice the fundamental frequency. This is attributed to the mechanism of redistribution of energy from the large coherent structures to the smaller ones. Furthermore, the introduction of three dimensional disturbances accelerated the separating shear layers which narrowed the wake and made it uniform in terms of mean velocity and reduced drag.

The Optimum Wavelength of Spanwise Segmented Plasma Actuator Forcing of a Circular Cylinder Wake

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, 2d, 4d 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 4d actuator was operated at a high power setting. The strength of vortex shedding was considerably attenuated in the near wake during forcing with the 4d 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 4d actuator created distinct patterns of streamwise vorticity in the wake with a spanwise wavelength of 4d which resembled mode A structures. It is argued that the effectiveness of the 4d 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.

Study of the Wake of a Circular Cylinder under Spatially and Temporally Modulated Plasma Actuation

The wake of a circular cylinder was forced with a temporally-modulated, spatiallysegmented dielectric barrier discharge (DBD) plasma actuator. The buried electrode was located at only certain spanwise locations along the cylinder such that spatially-modulated plasma was created. Along with this, the supply signal was amplitude-modulated with a square wave of twice the vortex shedding frequency. It was found that significant changes in wake properties occurred based on the modulation frequency and the power of actuation. When high-power, modulated forcing was employed with the segmented actuator, vortex shedding was significantly attenuated. A clear difference in the most prominent frequency peak was observed in the spectra between locations on the cylinder where plasma formed and where it did not. In the near-wake of the cylinder, the sub-harmonic mode was found to be stronger behind the non-plasma-forming region compared to the plasma-forming region. The high-power, modulated forcing also resulted in less drag compared to the unforced case as calculated by numerically integrating the velocity profile at a streamwise distance of x/d=40, where d is the cylinder diameter. In the case of low-power, modulated forcing, the unsteadiness in the wake increased. The unsteadiness was caused by sudden attenuation of the shedding peak caused by the emergence of a secondary peak in the spectra. The lowpower, modulated forcing also increased the velocity defect and consequently resulted in higher drag. The performance of the segmented actuator was compared with a straight actuator where the buried electrode covered the whole span of the cylinder. It was found that for the same induced velocity, a ‘W’-shaped velocity profile was obtained in the cylinder wake with a straight actuator and modulated forcing at both low and high power. Modulated forcing with the straight actuator increased the drag compared to the unforced case.