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

Abstract

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, such that three-dimensional instabilities in the wake were targeted for control. Two distinct power levels of 5.6 and 13.6 watts were used for forcing the flow. Considerable spanwise variation in the wake was achieved with the lower 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 considerable amount of drag reduction as measured in the far wake. Nomenclaturewire voltage 2 f = frequency f st = vortex shedding frequency L = length along the actuator (spanwise direction) Re = Reynolds number t = time T s = period of vortex shedding u = fluctuating component of U ũ = periodic component of u u = random component of u U = streamwise velocity component U  = freestream velocity U j = plasma actuator jet velocity U = streamwise velocity component U = mean of streamwise velocity component v = fluctuating component of V V = transverse velocity component V = mean of transverse velocity component x = streamwise direction y = transverse direction z = spanwise direction  = spanwise wavelength of actuator ω z = spanwise vorticity

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

“…This particular finding has not been observed for other cases of experiments, namely low-and high-power continuous forcing and high-power, modulated forcing. It should be mentioned here that in earlier experiments with low-power continuous forcing 15 , the random attenuation of shedding peaks was not observed as was found in Figure 24.…”

“…A particular characteristic of the flow in the low-power modulated forcing case will now be discussed. For all other test conditions (also for earlier continuous forcing experiments 15 ) it has been observed that close to the cylinder (less than x/d of 10) the time-averaged velocity profiles obtained by x-wire traverse were very regular in the sense all the data points could be nicely fitted by a single curve (let it be called a common fitting curve). This was because of the ordered nature of periodic shedding close to the cylinder.…”

“…Similar results were obtained in earlier work with continuous high power forcing, where significant attenuation was found. 15 However, there is a distinct difference in the behavior of the shedding frequency when the continuous and modulated forcing cases are compared. Figure 9 and Figure 10 indicate a difference in shedding frequency between the two transverse locations (probe 1 and probe 2) for modulated forcing, but the spectra for continuous forcing 15 show no difference in the resulting shedding frequency for the two regions.…”

“…15 However, there is a distinct difference in the behavior of the shedding frequency when the continuous and modulated forcing cases are compared. Figure 9 and Figure 10 indicate a difference in shedding frequency between the two transverse locations (probe 1 and probe 2) for modulated forcing, but the spectra for continuous forcing 15 show no difference in the resulting shedding frequency for the two regions. Figure 11 and Figure 12 show this difference more clearly: the peak frequency in the modulated, no-plasma region is 17 Hz, but probe 2 shows a peak at 23 Hz ( Figure 11), but the continuous forcing results in shedding peaks at 17 Hz for both regions ( Figure 12).…”

“…During previous work with continuous forcing with segmented actuators, 15 it was observed that the power level of the plasma actuation was an important parameter for the forcing experiments. At power levels lower than 7 or 8 watts, the vortex shedding was not attenuated or cancelled, although there was considerable influence in the near wake.…”