Secondary wake instabilities of a blunt trailing edge profiled body as a basis for flow control

The secondary instability in the wake of a two-dimensional blunt body with a chord to thickness ratio of 46.5 was experimentally investigated for Reynolds numbers of 3500, 5200 and 7000 based on the blunt trailing edge height $h$. Planar, stereoscopic and high-speed particle image velocimetry (PIV) measurements were performed to characterise the wake and upstream boundary layer. The same mode B secondary instability that is found in the cylinder wake was found to be present in the wake of the elongated body studied here. The most probable wavelength of the secondary instability, defined as the spanwise distance between adjacent streamwise vortex pairs in the wake, was found to range from $0.7h$ to $0.8h$ by applying a spatial autocorrelation to the spanwise–wall-normal instantaneous fields of the $Q$-criterion. The temporal evolution of the secondary wake vortices was investigated using time-resolved stereoscopic PIV measurements and it was shown that the vortices maintain both their directions of rotation and spanwise positions during the primary vortex shedding cycles. In agreement with previous literature, the secondary instability did not greatly change as the upstream boundary layer transitioned from laminar to turbulent. Moreover, any upstream boundary layer structures were found to rapidly evolve into wake structures just past the blunt trailing edge. The wavelength of the secondary instability was shown to match the spanwise distance between adjacent low-speed zones of streamwise velocity in the wake. These undulating velocity patterns proved to be a viable method for determining the secondary instability wavelength; however, this type of analysis is highly sensitive to the energy content used for data reconstruction when proper orthogonal decomposition is applied beforehand.

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“…Re(h) 5 × 10 4 by Naghib-Lahouti et al (2012, 2014. Both investigations found secondary instability wavelengths varying from 2.0 to 2.5h.…”

Section: Introductionmentioning

confidence: 89%

Secondary instabilities in the wake of an elongated two-dimensional body with a blunt trailing edge

2018

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“…3 While mode B emerges at a relatively low Re, the signature of mode B structures persists at Re on the order of 1000 and even 10 000. 5,6 …”

Section: A Three-dimensionality Of a Cylinder Wakementioning

confidence: 99%

“…5,15,16 Considerable insight into the effect of spanwise-segmented forcing of the cylinder wake can be obtained from the work of Kim and Choi, 16 who carried out a numerical investigation on three-dimensional forcing of a flow over a circular cylinder. Sinusoidal blowing and suction through a straight slot on the surface of the cylinder was implemented as a boundary condition in the simulation.…”

Section: B Spanwise Forcingmentioning

confidence: 99%

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

Bhattacharya

Gregory

2015

Physics of Fluids

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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 Kármán 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.

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“…Compared to studies with passive forcing, there is a dearth of active forcing experiments in the literature with varying spanwise forcing wavelength. One such experiment with active forcing has been reported by Naghib-Lahouti et al, 11 where they explored the possibility of controlling the wake of a blunt trailing edge body by exciting a secondary instability in the flow through a series of injection slots. The wavelength of the perturbation was kept same as the mode B instability wavelength, and they obtained 19% reduction in drag at a Re of 815.…”

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confidence: 97%

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

Bhattacharya

Gregory

2013

51st AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

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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. NomenclatureA cylinder = frontal area of cylinder A plasma = area of plasma region b = width of a plasma region C d = section drag coefficient C μ = blowing coefficient d = cylinder diameter f = frequency f st = vortex shedding frequency h = circumferential extent of a plasma region l = spanwise length of the cylinder n = number of plasma regions on one side of the cylinder Re = Reynolds number based on cylinder diameter St = Strouhal number t = time u = fluctuating component of U U = streamwise velocity component U = mean of transverse velocity component U  = freestream velocity U j = plasma actuator jet velocity v = fluctuating component of V V = transverse velocity component 2 V = mean of transverse velocity component x = streamwise direction y = transverse direction z = spanwise direction  = spanwise wavelength of actuator  j = density of induced jet   = freestream density < x > = mean streamwise vorticity

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Secondary wake instabilities of a blunt trailing edge profiled body as a basis for flow control

Cited by 27 publications

References 40 publications

Secondary instabilities in the wake of an elongated two-dimensional body with a blunt trailing edge

Secondary instabilities in the wake of an elongated two-dimensional body with a blunt trailing edge

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

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

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