An experimental study of Rayleigh-B

Due to the very fine space resolution and high noise-reduction capability of the improved laboratory-made laser Doppler anemometer, the temporal mean velocity and velocity fluctuation in uniform, rough bed open channel turbulent flows were measured in full detail and a large amount of information about the wall region was obtained. The influence of boundary roughness on flow characteristics was examined by changing the relative smoothness scale, H/ks, of the flow. It is proposed that there are three sorts of bed roughness, namely, the large scale roughness, the mediuin one and the small one. The small scale roughness is of more universal significance to engineering practice, and much attention is devoted to the characteristics of the flow over boundaries of such a sort of roughness.

In this paper, the systematic longitudinal turbulent intensity measurements of flow in a gravel open-channel have been conducted with a highly accurate one-dimensional LDA-system. The channel bed was roughened with gravels of uniform and non-uniform diameters respectively, and a wide range of H/Ks were considered. In comparison with the smooth bed results, it was found that the bed roughness makes the flow turbulence become well-distributed in the region near the bed and in the case of small H/Ks values. With regard to the flow over a non-uniform gravel bed, only the flow region below the maximum gravels is affected by the rough bed, and outside this region, the turbulent intensity distributions could collapse on to a single curve.

Experimental investigations were conducted on a NACA0012 airfoil to determine the influences of mounting angles and mounting locations on the lift-enhancing effects of Gurney flaps at a Reynolds number of 2.1

The laser Doppler velocimeter (LDV) and hydrogen bubble ow visualization techniques have been used to measure the characteristics of turbulent boundary layer ow over riblets surface and to observe its coherent structure in the near wall region. The experimental results indicate that, in comparison with the turbulent boundary layer ow over a smooth surface, greater values are obtained for the thickness of viscous sublayer, the thickness of bu er region and the integral constant C in the log-law. Moreover, the maximum value of turbulent intensity u0=U is increased by 6.7%, but its position is decreased by 6.4%, the skewness factor and the atness factor are hardly a ected by the riblets surface. The non-dimensional value of low-speed streak spacing is reduced by 20%, this is also an indication of turbulent drag reduction.

Flow visualization was used to study the effects of a vectored trailing edge jet on the leading edge vortex breakdown of a 65 delta wing. The experimental results indicated that there is little effect of the jet on the leading edge vortex breakdown when the angle of the vectored jet is less than 10. With the increase of the vectored angle B, the effect of the jet on the flow becomes stronger, i.e., the jet delays the leading edge vortex breakdown in the direction of the vectored jet, and accelerates breakdown of the other leading edge vortex. Moreover, the effect of the jet control tends to be weaker with the angle of attack.

An experimental wind-tunnel investigation was undertaken to determine the effects of Gurney flaps on a 40-deg cropped nonslender delta wing at a chord Reynolds number of 250,000. In the experiment, the height of the Gurney flaps was varied from 0.01C to 0.05C, and the sideslip angle of the model was selected as 0, 5, 10 and 20 deg. In addition, the 0.05C Gurney flap was serrated with different heights of 0.01C to 0.05C separately. In comparison with the baseline clean configuration results, it was found that the model with plate Gurney flaps can indeed increase the lift-to-drag ratio at moderate-to-high lift coefficients for the wing, and the greatest increment was obtained for the 0.01C Gurney flap. The effect of Gurney flap on the increment of lift-to-drag ratio tends to be not significant with the increase of sideslip angle. Moreover, the 0.05C serrated Gurney flap provides the best performance among the serrated Gurney flaps.

The effects on the wall pressure of the plate caused by the introduction of a gap near the ground and the flaps were investigated experi mentally. The mean pressure on the plate surface was found to decrease with the size changing of the gap near the ground when the gap size (g) is larger than the flap height (h), but the one in the recir culation region increase with the size of the gap when the gap size is about 0<g<2h. Three kinds of now strncture appear with increasing the gap sizes, and the transit point is at g/h-0.2, 0.8 respectively. For the zigzag tlap (d/h=0.19, d is the height of the zigzag), the vmation of wall pressure with g/h is similar to the flat flap in general. The zigzag affects the mean wall pressure in the region of 0<x/h<20 (the i'ecirculation zone) and can decrease the fluctu atthg pressure in the region ofx/h > 20 downstream.

The dye-injection flow visualisation technique was used to investigate the effect of the apex flap on the leading-edge vortex breakdown over a cropped 76/40 double delta wing. The angle of. attack of the experimental model varied from 20 to 40o, and tile length of the apex flap was 25%c and 50%c rcspcctivcly. By changing the angle of the apex flap, we found that the apex flap is an efficient method to control the leading edge vortex breakdown, and rbat there exists an angle of the apex flap at which the value of the leading edge vortex breakdown delay reaches maximum. Moreover, it is found that, for a< 28, the small apex flap is a better choice for delaying the vortex breakdown; for a> 28, the large apex flap is superior to the small one.