Large-eddy Breakup Devices Research Articles

The possibility of drag reduction by outer layer manipulators in turbulent boundary layers

Experiments were carried out with the aim of investigating the possibility of obtaining a net drag reduction on a finite body by manipulating the outer layer structure of the turbulent boundary layer. The experiments were carried out in a 260 m long towing tank, where large eddy breakup devices (LEBU’s) were used in single and tandem configurations on a large flat plate. The total drag was measured directly by a force gauge, and the geometrical configuration parameters, as well as the chord Reynolds number, were varied over wide ranges. The highest chord Reynolds number tested was 260 000, which is within the range of interest for practical applications. Also, the device drag was measured directly in flight, which enabled evaluation of the performance of the manipulators, as well as determination of the total skin friction reduction. Despite a substantial skin friction reduction for certain configurations (7% averaged over the part downstream from the manipulators) as well as a low device drag, no drag reduction was found. The present results, and a critical evaluation of previously reported data, make any substantial net drag reduction by use of LEBU’s at high Reynolds numbers seem implausible.

Application of unsteady aerodynamics to large-eddy breakup devices in a turbulent flow

The modifications to the vertical velocity of a turbulent flow by the large-eddy breakup devices (LEBU) were analyzed using linear three-dimensional unsteady aerodynamics. A Fourier component of the input turbulence convected over these devices was considered and the modified vertical velocity in the far downstream wake was solved. It is shown that the amplitude of the vertical velocity behind the device is reduced, the effect increasing with increasing stream and spanwise wavenumber. Two plates placed far apart in series produce a reduction that is the square of that for a single plate. The presence of the ground plane decreases the effectiveness of the LEBU in reducing the amplitude of the vertical velocity in the far wake.

Turbulent Reynolds analogy factors of stacked large-eddy breakup devices

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Turbulent boundary-layer wall pressure fluctuations downstream of a tandem LEBU

An initial measurement of turbulent wall pressure fluctuations downstream of tandem large eddy breakup devices (LEBUs) indicates a significant reduction by comparison with the reference case of a flat plate; the average magnitude of the reduction is 12.5 percent. Peak reduction is at 7-8 kHz, and is of the order of the C(f) reduction due to the tandem LEBUs. These data indicate a secondary benefit derivable from LEBUs, in addition to their skin friction reduction effect.

Direct drag measurements for a flat plate with passive boundary layer manipulators

Drag measurements for a flat plate equipped with passive boundary layer turbulence manipulators, the so-called Large Eddy Break Up (LEBU) devices, have been carried out in a towing tank at plate length Reynolds numbers of 5–20 million. The towing tank facility allowed direct drag measurements as opposed to indirect methods based on, e.g., measurement of momentum loss thickness of the boundary layer. The LEBU devices consisted of two hydrofoils arranged in tandem, with a spacing of five to eight boundary layer thicknesses at the manipulator position (δm) and a height in the boundary layer of 0.5δm–0.9δm. Four different manipulator configurations were tested, with and without a trip wire on the test plate. The LEBU foil section was 10% thick and symmetric. The ratio between the LEBU chord and δm ranged from 0.8 to 1.1, and the chord Reynolds number ranged from 25 000 to 100 000. Only for the lowest Reynolds number was a 2% drag reduction obtained. However, the main body of the experiments showed no drag reduction in contrast to previously reported experiments.

The effect of large-eddy breakup devices on oncoming vorticity

Experiments have shown that a large-eddy breakup device consisting of a short splitter plate placed in the turbulent boundary layer over a plane wall can lead to a reduction in drag. We investigate the idealized problem of an incident line vortex convected past such a device. The vorticity shed from the trailing edge of the plate is found to cancel the effect of the incident vortex and to reduce velocity fluctuations significantly in the vicinity of the wall.