Kwing-So Choi

The return to isotropy of homogeneous turbulence

Three types of homogeneous anisotropic turbulence were produced by the plane distortion, axisymmetric expansion and axisymmetric contraction of grid-generated turbulence, and their behaviour in returning to isotropy was experimentally studied using hot-wire anemometry. It was found that the turbulence trajectory after the plane distortion was highly nonlinear, and did not follow Rottas linear model in returning to isotropy. The turbulence wanted to become axisymmetric even more than it wanted to return to isotropy. In order to show the rate of return to isotropy of homogeneous turbulence, a map of the ratio of the characteristic time scale for the decay of turbulent kinetic energy to that of the return to isotropy was constructed. This demonstrated that the rate of return to isotropy was much lower for turbulence with a greater third invariant of the anisotropy tensor. The invariant technique was then applied to the experimental results to develop a new turbulence model for the return-to-isotropy term in the Reynolds stress equation which satisfied the realizability conditions. The effect of the Reynolds number on the rate of return to isotropy was also investigated and the results incorporated in the proposed model.

Near-wall structure of a turbulent boundary layer with riblets

A detailed wind tunnel study has been carried out on the near-wall turbulence structure over smooth and riblet wall surfaces under zero pressure gradient. Time-average quantities as ‘well as conditionally sampled profiles were obtained using hotwire/film anemometry, along with a simultaneous flow visualization using the smoke-wire technique and a sheet of laser light. The experimental results indicated a significant change of the structure in the turbulent boundary layer near the riblet surface. The change was confined within a small volume of the flow close to the wall surface. A conceptual model for the sequence of the bursts was then proposed based on an extensive study of the flow visualization, and was supported by the results of conditionally sampled velocity fields. A possible mechanism of turbulent drag reduction by riblets is discussed.

TURBULENT BOUNDARY-LAYER CONTROL BY MEANS OF SPANWISE-WALL OSCILLATION

An experimental investigation into the changes in turbulence structure of the boundary layer over a wall oscillating in spanwise direction was carried out in a wind tunnel using hot-wire anemometry and flow visualisation. The main purpose of this investigation is to confirm recent numerical results which seem to indicate that the turbulent skinfriction drag can be reduced by up to 40 percent over the oscillating wall. The results from the present investigation clearly indicate that the logarithmic velocity profiles are shifted upwards and turbulence intensities reduced by the spanwise-wall oscillation, confirming the basic conclusions of recent direct numerical simulation. Also, the skinfriction reductions as much as 45% are observed in the present experiment at an optimum speed of wall oscillation. The flow-visualisation study indicates that the longitudinal vortices in the near-wall region of the boundary layer are twisted towards the direction of spanwise-wall motion with oscillation. As a result, the vortices are realigned into the cross-flow direction, resulting in a reduction of turbulent wall-skin friction by spanwise-wall oscillation. A conceptual model for a turbulent boundary layer over an oscillating wall is proposed to examine the mechanism of turbulent drag reduction by spanwise-wall oscillation.

Turbulent drag reduction using compliant surfaces

Over the past forty years intensive investigations into the use of compliant surfaces have been undertaken, both theoretically and experimentally, in order to obtain turbulent drag reduction in boundary–layer flows. Although positive results were found in some of the studies, none of these had been successfully validated by independent researchers. In this paper the results are reported of a recent investigation carried out by the authors to verify the experimental results of Semenov in 1991 and Kulik and co–workers in 1991, who successfully demonstrated the ability of compliant surfaces to reduce the skin–friction drag and surface–flow noise in a turbulent boundary layer. A strain–gauge force balance was used in the present study to directly measure the turbulent skin–friction drag of a slender body of revolution in a water tunnel. Changes in the structure of turbulent boundary layer over a compliant surface in comparison with that over a rigid surface were also examined. The results clearly demonstrate that the turbulent skin friction is reduced for one of the two compliant coatings tested, indicating a drag reduction of up to 7 per cent within the entire speed range of the tests. The intensities of skin–friction and wall–pressure fluctuations measured immediately downstream from the compliant coating show reductions in the intensities of up to 7 and 19 per cent, respectively. The results also indicate reductions in turbulence intensity by up to 5 per cent across almost the entire boundary layer. Furthermore, an upwards shift of the logarithmic velocity profile is also evident indicating that the thickness of the viscous sublayer is increased as a result of turbulent drag reduction due to the compliant coating. It is considered that the results of the present experimental investigation convincingly demonstrate for the first time since the earlier work in Russia (by Semenov and Kulik) that a compliant surface can indeed produce turbulent drag reduction in boundary–layer flows.

The mechanism of turbulent drag reduction with wall oscillation

An extensive study of the near-wall structure of turbulent boundary layer over a spanwise oscillating wall was conducted using hot-wire measurements in a wind tunnel in order to better understand the mechanisms involved in turbulent drag reduction under these conditions. The results showed that the logarithmic velocity profiles were shifted upwards and the turbulence intensities reduced, suggesting that the viscous sublayer was thickened as a result of drag reduction with wall oscillation. The probability density functions of velocity fluctuations exhibited long tails of positive probability, reflecting increases in the skewness and kurtosis within the viscous sublayer. The measured thickness of the Stokes layer was comparable to that of the viscous sublayer of the boundary layer when the turbulent drag reduction was observed with the wall oscillation. At the same time, the Reynolds number of the Stokes layer was found well below the critical value, so that the Stokes layer was expected to remain laminar if there were no boundary layers over the oscillating surface. These are considered to be important conditions in obtaining turbulent drag reduction with spanwise-wall oscillation. The present study also showed that a net spanwise vorticity was generated by the periodic Stokes layer over the oscillating wall just outside the viscous sublayer, which reduces the mean velocity gradient of the boundary layer near the wall. At the same time, the spanwise vorticity hampers the longitudinal vortices in the viscous sublayer to stretch in the streamwise direction, reducing the streamwise vorticity associated with these vortices. The near-wall burst activity was weakened as a result of this, resulting in the reduction of turbulent skin-friction drag. A remarkable change in the burst signature in the near-wall region of the boundary layer was observed.

Near-wall structure of turbulent boundary layer with spanwise-wall oscillation

An investigation was carried out with an aim to better understand the mechanism of turbulent drag reduction with spanwise-wall oscillation, by carefully analyzing the experimental data of the near-wall structure of the boundary layer modified by the wall motion. It was found that the mean velocity gradient of the turbulent boundary layer was reduced close to the wall, and the logarithmic velocity profile shifted upwards by the wall oscillation. We argue that these changes in the mean velocity profile are mainly due to a negative spanwise vorticity created in the near-wall region of the boundary layer over the oscillating wall. The resultant near-wall velocity reduction seems to have weakened the near-wall turbulence activity by hampering the stretching of the quasistreamwise longitudinal vortices, leading to a reduction in skin-friction drag. Indeed, the signatures of the sweep events over the oscillating wall indicated that the duration and the strength were reduced by 78% and 64%, respectively. The redu...

Drag reduction of turbulent pipe flows by circular-wall oscillation

An experimental study on turbulent pipe flows was conducted with a view to reduce their friction drag by oscillating a section of the pipe in a circumferential direction. The results indicated that the friction factor of the pipe is reduced by as much as 25% as a result of active manipulation of near-wall turbulence structure by circular-wall oscillation. An increase in the bulk velocity was clearly shown when the pipe was oscillated at a constant head, supporting the measured drag reduction in the present experiment. The percentage reduction in pipe friction was found to be better scaled with the nondimensional velocity of the oscillating wall than with its nondimensional period, confirming a suggestion that the drag reduction seem to be resulted from the realignment of longitudinal vortices into a circumferential direction by the wall oscillation.

Turbulent drag reduction by Lorentz force oscillation

An experimental investigation into an electromagnetic technique to reduce skin-friction drag of turbulent boundary layers was conducted with an electroconductive solution in water, with potential applications to ships and underwater vehicles. We used an array of actuators made of permanent magnets interleaved with copper electrodes, which are set flush with the wall surface across the flow. This setup created the Lorentz force in the cross-flow direction within a thin region near the wall. More than 40% of turbulent skin-friction reductions were observed when the electromagnetic force is oscillated across the flow.

Turbulent boundary-layer control with plasma actuators.

This paper reviews turbulent boundary-layer control strategies for skin-friction reduction of aerodynamic bodies. The focus is placed on the drag-reduction mechanisms by two flow control techniques—spanwise oscillation and spanwise travelling wave, which were demonstrated to give up to 45 per cent skin-friction reductions. We show that these techniques can be implemented by dielectric-barrier discharge plasma actuators, which are electric devices that do not require any moving parts or complicated ducting. The experimental results show different modifications to the near-wall structures depending on the control technique.

Flow control around a circular cylinder using pulsed dielectric barrier discharge surface plasma

Dielectric barrier discharge (DBD) plasma actuators have been used to control the flow around a circular cylinder at Re=15 000, where the near-wake structure was studied using time-resolved particle image velocimetry with simultaneous measurements of the dynamic lift and drag forces. It was shown that the vortex shedding was suppressed when the surface plasma placed near the natural separation point was activated in a pulsed mode at nondimensional frequency, fp+, above 0.6 with a force coefficient, Cp, greater than 0.05%. Plasma actuator performance on flow control was summarized by mapping the changes in drag and lift fluctuations as a function of the forcing frequency and the force coefficient. They showed that more than 70% reduction in lift fluctuations was obtained with up to 32% drag reduction at fp+=2.0 and Cp=0.32%. Here, narrowing of the wake was observed as the plasma promoted shear-layer roll-ups at the forcing frequency. This, however, did not affect the shear layer on the opposite side of the...