J. H. Chou

On low-frequency modulations and three-dimensionality in vortex shedding behind a normal plate

(Received 10 December 2003 and in revised form 1 October 2004) 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.

CONTROL OF SEPARATED FLOW BY A TWO-DIMENSIONAL OSCILLATING FENCE

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 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.

Nonuniform motion of leading-edge vortex breakdown on ramp pitching delta wings

Vortex flow characteristics on ramp pitching delta wings of sweep angles from 59 to 70 deg were studied by flow visualization and LDA measurement. Experimental observations indicate that the movement of the breakdown point is strongly dependent on the sweep angles and the pitching rates. The moving behavior of the breakdown point can be characterized by the occurrence of two delays. The first delay is due to the fact that the flow initially takes a length of time comparable to C/U» to respond to the pitching motion. The second delay exists only for specific sweep angles and occurs in the course of pitching-up motion. It is characterized by the phenomenon that the breakdown point is moving slowly or even standing still above the wing surface for a certain length of time. The chordwise location corresponding to this occurrence shifts upstream as the pitching rate increases. This delay diminishes as the sweep angle increases. The LDA data obtained further reveal that both delays are associated with the underdevelopment of the primary vortex.

Flow distortion in a circular-to-rectangular transition duct

Experiments were made for three circular-to-rectangular transition ducts with different transition lengths at Reynolds numbers which ranged from 4 × 10 3 to 2 × 10 4 . The Reynolds number is based on the inlet boundary-layer thickness and a reference freestream velocity measured upstream of the transition duct. The secondary flow pattern developed at the exit cross-sectional plane was mapped out in detail by a three-dimensional velocity measurement technique

A T-shaped vortex shedder for a vortex flow-meter

Abstract A configuration of a T-shaped vortex shedder is developed with the goal of improving the quality of the vortex shedding signal measured. This T-shaped vortex shedder comprises a trapezoidal cylinder which is fixed in shape and an extended plate attached behind whose length is variable. The vortex shedding frequency is deduced from the differential pressure signal measured, which corresponds to the pressure difference resulting from the two sides of the trapezoidal cylinder. By varying the length of the extended plate, the optimal situation is found to be when the length of the extended plate falls in the range of 1.56 to 2.0 times the width of the vortex shedder, when the low-frequency variations embedded in the pressure signal are significantly suppressed.

Instantaneous vortex-shedding behaviour in periodically varying flow

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 × 103 and 2.46 × 104, whereas the frequency ratio, Fs/Fo, ranged from 0.29 to 14.64. Fs denotes the natural vortex-shedding frequency referred to the mean flow, and Fo 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 vortex-shedding phenomenon observed into three regimes, namely, quasi-steady vortex shedding for Fs/Fo > 4.37, hysteresis vortex shedding for Fs/Fo= 1.56-4.37 and non-interactive vortex shedding for 0.29 < Fs/Fo < 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 Fs/Fo. Further, the variations of non-dimensionalized instantaneous vortex-shedding frequencies obtained in the accelerating and decelerating portions of the periodically varying flow are found to depend on Fs/Fo and ΔU0/Ū0. In the regime of non-interactive vortex shedding, the vortex-shedding frequency tends to vary with the mean velocity of periodically varying flow, excluding the occurrences of primary and secondary lock-on at Fs/Fo= 0.97-1.03 and 0.495-0.514, respectively.

Intermittent switching of gap flow downstream of two flat plates arranged side by side

Abstract Intermittent, side-to-side switching of the gap flow downstream of two side-by-side flat plates was studied for gap ratios between 1·50 and 1·85. On the side of the gap to which the flow is deflected, called the narrow-wake side, periodic vortex shedding is detected at the outer edges of the wake shear layer and the gap flow. On the other side, called the wide-wake side, no periodic vortex shedding is measured; instead flow developing in the near wake region is characterized by turbulent fluctuations. A specially designed data reduction scheme was developed in this work to detect the instant of gap flow switching. It is found that the number of events of gap flow switching increases with the gap ratio, and the statistics of time intervals between neighboring switching events appears to be a Poisson distribution.

A study on signal quality of a vortex flowmeter downstream of two elbows out-of-plane

Abstract Experiments were made with a vortex meter situated downstream of two elbows out-of-plane, at Reynolds numbers between 7.8×10 4 and 2.1×10 5 . Analysis on the signals obtained from a piezo-electric sensor embedded in the vortex shedder shows that the signal quality, concerned with the vortex shedding frequency component, deduced in the vicinity of 15 pipe diameters downstream is superior to that at other locations, and is remarkably better than that found in the region of fully-developed turbulent pipe flow. The vortex shedding frequencies measured in this region, in terms of the Strouhal value, fall within 0.129±0.001, that 0.129 is the reference value reduced in the fully-developed turbulent pipe flow region. Also noted, the uncertainty interval associated with the vortex shedding frequency measured in this region is smaller than that reduced in the region of fully-developed turbulent pipe flow.

Low-frequency fluctuations in the near-wake region of a trapezoidal cylinder with low aspect ratio

Abstract Base pressure signals obtained on a trapezoidal cylinder with an aspect ratio 4.7, at Reynolds numbers of the order of 104, were examined after being low-pass filtered at different cut-off frequencies. For the range of the Reynolds numbers studied, the integral time scales of the low-passed fluctuations versus the cut-off frequencies chosen was found to fall on a curve. Cross-correlation between the pressure fluctuations measured at the cylinder base and the velocity fluctuations detected in the near-wake region were examined with the signal traces low-passed at different cut-off frequencies. Accordingly, an optimal cut-off frequency was defined as the one corresponding to the highest correlation obtained. The integral time scale of the low-passed fluctuations in reference to the optimal cut-off frequency was found to be about two times the vortex-shedding period. The low-frequency variations measured appeared to be insensitive to the artificial tripping to the sidewall boundary layer, and exhibited a global behavior in a near-wake region.

Flopping phenomenon of flow behind two plates placed side-by-side normal to the flow direction

Experiments were made for the flow over two side-by-side normal plates for which the gap ratios are in the range 1.4–2.1 and the Teynolds numbers are at 6.6 × 103 and 1.8 × 104. At low gap ratios, i.e., 1.4–1.6. the gap flow appears always to be biased and flip-flops to the preferred side non-periodically with respect to time. As the gap ratio becomes larger, the percentage of time occupied by the gap flow in the biased state decreases and the non-biased state of the gap flow becomes prevalent. A comparison of the experimental results obtained under five free stream turbulence conditions further shows that the addition of artificial disturbance into the free stream promotes gap flow flopping at low gap ratios.