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Research Papers: Flows in Complex Systems

# Active Control of a Backward Facing Step Flow With Plasma Actuators

[+] Author and Article Information

CONICET
Buenos Aires, Argentina

Roberto Sosa, Guillermo Artana

CONICET
Buenos Aires, Argentina

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received August 23, 2013; final manuscript received April 30, 2014; published online September 10, 2014. Assoc. Editor: Mark R. Duignan.

J. Fluids Eng 136(12), 121105 (Sep 10, 2014) (9 pages) Paper No: FE-13-1519; doi: 10.1115/1.4027598 History: Received August 23, 2013; Revised April 30, 2014

## Abstract

Active control over a backward facing step flow is studied experimentally by means of plasma based devices. The Reynolds number based on the step height $h$ is 1520. An electrohydrodynamic actuator (EHD), dielectric barrier discharge (DBD) type, is flush mounted to the step wall. The DBD configuration adds momentum locally, normal to the separated shear layer, thus producing strong modifications downstream. The actuation is periodic and its frequency and amplitude are scrutinized to characterize the flow behavior under forcing. Measures of velocity fields for these flows are obtained from particle image velocimetry (PIV). As reported by previous works, the reattachment length shows an important reduction for an optimum forcing frequency. This value closely matches the shear layer flow natural frequency. On the other hand, the flow is less sensitive to the forcing amplitude though the analysis allows us to optimize the actuation in order to save power consumption.

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## Figures

Fig. 1

(a) Outline of the test section geometry and position of the DBD actuator. The imaging region is represented schematically by the laser sheet. (b) Detail of the DBD plasma actuator. (c) Scheme of the burst modulation signal mode.

Fig. 2

Time averaged velocity modulus contours for the forcing jet produced by the DBD actuator (ff = 10Hz, DC = 50%). Induced flow rate G has been calculated through the plane marked with the dashed line.

Fig. 3

Contour of the velocity modulus U/U∞ and streamlines for the nonforced mean flow for Re = 1520. The recirculation length xr≃5.70h.

Fig. 4

Symbols • represent the recirculation region length for different Re numbers for nonforced flow. Symbol ★ stands for the selected case to be controlled.

Fig. 5

Dashed line: Power spectrum signal for ux estimated from a Pitot probe located at x/h = 4. Solid line: The same data smoothed.

Fig. 6

Symbols: PIV mean velocity profiles u¯x(y) for different positions x/h. Solid lines: Data is fit by f(y) = c1+c2*tanh(c3*y+c4).

Fig. 7

Linear stability analysis on experimental mean flow for the nonforced case, at Re = 1520. The selected frequency in (d) fn = 9.8 Hz matches the Pitot measures.

Fig. 8

Recirculation bubble length xr modifications under actuation for three forcing amplitudes and a range of frequencies 0.1<f+<3.4

Fig. 9

Evolution of the mean flow, represented by contours of the nondimensional velocity modulus U/U∞ and streamlines. Different forcing frequencies are considered.

Fig. 10

Momentum thickness evolution downstream. Nonforced case and three forcing frequencies for a fixed forcing amplitude (%DC = 25). Dashed lines show the corresponding recirculation length for each case.

Fig. 11

Momentum thickness evolution downstream. Nonforced case and three forcing amplitudes for a fixed forcing frequency f+ = 1.

Fig. 12

Instantaneous vorticity contours for the natural flow and three forcing frequencies for DC = 50%. Some Q contours (solid lines) identify the main vortex of the flow. Formation length is appreciable as well as an approximate measure for the wavelength λ.

Fig. 13

Instantaneous vorticity contours for the more unstable mode of the linear stability problem for the nonforced case. Experimental scales δω and Um are used on Eq. (7).

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