Bluff-Body Wake Control
If the cross section of a body perpendicular to the flow is significant, it is called a bluff body. Wind moving past skyscrapers and flow over a moving truck or car are some examples of bluff-body flow. Because of the shape of these bodies, there exists a region of considerable size behind them where the flow is slowed down. This region is termed the wake. Due to the existence of a large wake, bluff bodies experience pressure drag. Sometime, large vortices are shed periodically behind the body. This phenomenon is called vortex shedding. It creates an unsteady, periodic force which makes the bluff body vibrate and may lead to structural failure. Two kinds of control can be implemented to control bluff-body flows. One is called passive control where the geometry of surface is modified, for example by putting tabs, streaks or dimples on the surface. In the other method, external energy is input to some type of electronic actuator which operates at a specific condition. This is called active control. Some examples are blowing and suction, synthetic jets, and plasma actuators.
In our group, research is focused on active control of bluff-body flow using dielectric barrier discharge (DBD) plasma actuators mounted on a circular cylinder. DBD actuators are constructed by separating two electrodes (called the exposed and buried electrodes) with a dielectric material. Figure 2 shows a schematic of a DBD actuator. When high voltage is applied between them, plasma is created in front of the exposed electrode. The ionized particles collide with the neutral air molecules and create a body force. This whole action gets revealed in the form of an air jet. This jet can be used for flow control purpose.
Segmented Plasma Actuator
The current research employs the concept of segmented actuation (Gregory et al., 2008). To achieve this, buried electrodes have been mounted on the cylinder only at specific spanwise locations. The main idea behind this kind of actuator design is to promote three-dimensionality in the wake. Greater three-dimensionality encourages greater momentum transport in the wake and consequently less pressure drag.
Initial results from this research indicated considerable drag reduction achieved with segmented actuation compared to conventional two-dimensional forcing (with a continuous buried electrode rather than a segmented one). Vortex shedding was greatly attenuated at a power level of 13 watts. At a lower power level of 5 watts, the wake responded differentially to the forcing based on the spanwise location across the cylinder. In the region behind cylinder locations without plasma formation, the vortices migrated closer to the wake centerline whereas behind regions of plasma formation on the cylinder, they were displaced away from the centerline.
Attention is being focused on understanding the relation between actuator performance and the wavelength of actuation, as well as the effect on wake modes. The preliminary mechanism for two-dimensional forcing has been assumed by some researchers as a phase-mismatch in the Karman vortex street. However, it is yet to be confirmed whether the same mechanism is responsible in the present case of three-dimensional forcing or if it is related to some other cause such as modification of the separation point. Hot-wire anemometry and particle image velocimetry are being deployed to understand the complicated flow behavior in the wake of the circular cylinder.
- Gregory, J.W., Porter, C.O., Sherman, D.M., and McLaughlin, T.E., 2008, “Circular Cylinder Wake Control using Spatially Distributed Plasma Forcing,” AIAA 2008-4198, 4th AIAA Flow Control Conference, Seattle, WA.
- "Effect of Three-Dimensional Plasma Actuation on the Wake of a Circular Cylinder," AIAA 2012-0907, 50th AIAA Aerospace Sciences Meeting, Nashville, TN. doi: 10.2514/6.2012-907
- "Study of the Wake of a Circular Cylinder under Spatially and Temporally Modulated Plasma Actuation," AIAA 2012-2957, 6th AIAA Flow Control Conference, New Orleans, LA. doi: 10.2514/6.2012-2957