In this study spanwise correlation measurements and smoke flow visualization were performed on vortex shedding behind a normal plate. For Reynolds numbers in a range between 1800 and 27 000, the hot-wire signals measured were analysed by a wavelet transformation, from which the instantaneous properties of vortex shedding were obtained and examined. Results show that the phase difference of vortex shedding detected at two spanwise locations, separated by twice the characteristic length, can be as high as 35 •. A correlation analysis further shows that large spanwise phase differences occur when small fluctuating amplitudes in the vortex shedding signals are measured. Smoke-wire visualization performed at Reynolds number 1800 indicates that the formation of shedding vortex can be divided into two distinct situations, namely, one featuring a long formation region, called Mode L; and the other featuring a short formation region, called Mode S. In Mode S, the three-dimensionality of vortex formation appears to be very pronounced, and the secondary vortices are clearly present in the separated shear layer. The events of Mode S occupy less than 5% of the total time measured, and are called the burst events in this study.

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Control of separated flow behind a backward-facing step using a two-dimensional oscillating fence installed upstream has been investigated in this work. Parameters of the flow considered included the reduced frequency of the oscillating fence, the distance from the oscillating fence to the backward-facing step, the ratio of the maximum height of the oscillating fence to the step height, and the Reynolds number. It was found that with the experimental parameters properly selected the time-mean reattachment length of the separation region could be reduced over 40%, compared to the case without the presence of an oscillating fence. The evolution of unsteady flow behind a backward-facing step was further studied in detail by a phase-averaging measurement technique. The results obtained indicate that suppression of the separated flow behind the step is mainly due to the downwash motion induced by the vortical structure released upstream from the oscillating fence, when it convects over the step. Nomenclature / = frequency of the oscillating fence h = instantaneous height of the fence h f = maximum height of the fence h s = height of backward-facing step, =1.5 cm I r = inter mitt ency function defined in Eq.(2) K = reduced frequency, =fhf/U 0 K cr = critical reduced frequency, above which organized vortical structure develops behind the oscillating fence as it extends into the flow L s = distance from the oscillating fence to the step R eQ = Reynolds number, = U 0 /0v t = time T = time period of the oscillating motion of the fence U, u = time-mean stream wise velocity and streamwise velocity fluctuation U 0 = freestream velocity measured at the inlet of the test section U a = streamwise growth rate of the vortical flow structure behind the oscillating fence X = streamwise coordinate X r = time-mean reattachment length of the separated flowbehind the backward-facing step, without the presence of the oscillating fence, but with a squarewave trip upstream X' r = time-mean reattachment length of the separated flow behind the backward-facing step measured Y = vertical coordinate Z = spanwise coordinate 6 = boundary-layer momentum thickness measured at the step, without the presence of the oscillating fence v = kinematic viscosity of air (/> = phase angle of the oscillating motion of the fence Q z -time-mean vorticity in the Z direction -= time-mean quantity < > = ensemble-averaged quantity AX r = the reduction of the time-mean reattachment length measured, -X' r -X r

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Vortex shedding behind a stationary T-shaped cylinder in a circular pipe subjected to periodically varying flow was studied at Reynolds numbers between 6.17 × 10 3 and 2.46 × 10 4 , whereas the frequency ratio, F s /F o , ranged from 0.29 to 14.64. F s denotes the natural vortex-shedding frequency referred to the mean flow, and F o denotes the frequency of periodically varying flow. By adopting the Hilbert transform to analyse the velocity signals measured, the instantaneous vortex-shedding frequency was obtained. Based on this quantity, one could categorize the vortexshedding phenomenon observed into three regimes, namely, quasi-steady vortex shedding for F s /F o > 4.37, hysteresis vortex shedding for F s /F o = 1.56-4.37 and noninteractive vortex shedding for 0.29 < F s /F o < 1.56. In the regime of quasi-steady vortex shedding, the instantaneous vortex-shedding frequency follows the periodically varying flow without phase lag. In the regime of hysteresis vortex shedding, the instantaneous vortex-shedding frequency lags behind the periodically varying flow. Phase lag roughly varies linearly with F s /F o . Further, the variations of nondimensionalized instantaneous vortex-shedding frequencies obtained in the accelerating and decelerating portions of the periodically varying flow are found to depend on F s /F o and ∆U 0 /Ū 0 . In the regime of non-interactive vortex shedding, the vortexshedding frequency tends to vary with the mean velocity of periodically varying flow, excluding the occurrences of primary and secondary lock-on at F s /F o = 0.97-1.03 and 0.495-0.514, respectively.

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