Laser-induced uorescence (LIF) and laser Doppler velocimetry (LDV) are used to explore the structure of a turbulent boundary layer over a wall made up of two-dimensional square cavities placed transversely to the flow direction. There is strong evidence of occurrence of outflows of fluid from the cavities as well as inflows into the cavities. These events occur in a pseudo-random manner and are closely associated with the passage of near-wall quasi-streamwise vortices. These vortices and the associated low-speed streaks are similar to those found in a turbulent boundary layer over a smooth wall. It is conjectured that outflows play an important role in maintaining the level of turbulent energy in the layer and enhancing the approach towards self-preservation. Relative to a smooth wall layer, there is a discernible increase in the magnitudes of all the Reynolds stresses and a smaller streamwise variation of the local skin friction coefficient. A local maximum in the Reynolds shear stress is observed in the shear layers over the cavities.

An experimental investigation has been carried out on a supersonic jet of air issuing from an M = 1.44 converging-diverging rectangular nozzle of aspect ratio 4. Particle image velocimetry measurements of the flow field along with near-field acoustic measurements were made. The effect of injection of a small amount of water (∼5% of the mass flow rate of the jet) into the shear layer of the jet, on the unsteady flow structure and sound generation were examined. The presence of water droplets in the jet modified the turbulence structure significantly, resulting in axial and normal r.m.s. velocity reductions of about 10% and 30%, respectively, as compared to that of a normal jet. An even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The near-field noise levels (OASPL) were found to reduce by about 2-6 dB depending on the location of the injection and the water mass flow rate. Far-field acoustic measurements carried out on a heated M = 0.9 (jet exit velocity = 525 m s −1) jet show significant (6 dB) reductions in the OASPL with moderate amounts of water injection (17% of the mass flow rate of the jet) suggesting that the technique is viable at realistic engine operating conditions. 132 A. Krothapalli and others confined to a definite wedge sector. They emanate from the region within the first few diameters downstream of the nozzle exit. These are generated by small-scale disturbances (or eddies) that are being convected at supersonic speeds so that they emit Mach waves in the direction defined by a disturbance convection velocity and the atmospheric speed of sound (Phillips 1960; Ffowcs Williams 1965; Ffowcs Williams & Maidanik 1965). Although these waves lie in an important range of the spectrum (1 ∼ 4 kHz in the case of full scale engines), they may not have enough intensity at far distances to contribute significantly to the far-field noise. These waves are eliminated by surrounding the jet with a gas stream that has a higher speed of sound, thus resulting in subsonic convection velocities of the small disturbances, as demonstrated by Oertel & Patz (1981) and more recently by Papamoschou (1997). The second field is highly directional, peaking at smaller angles relative to the jet axis (or larger angles to the inlet axis). This noise field is generated from large-scale instabilities reaching peak amplitude in the region somewhat upstream of the end of the potential core. These sources are associated with the unsteady flow on a scale that is comparable with the local shear layer width (Bishop, Ffowcs Williams & Smith 1971). The spectral intensity of this sound field generally contains two distinct peaks (Laufer, Schlinker & Kaplan 1976). One is associated with highly directional Mach waves characterized by high positive pressure peaks in the far-field microphone signal (Ffowcs Williams, Simson & Virchis 1975; Laufer et al. 1976). These Mach waves are of significant strength as compared to those seen very close to the jet exit as discussed above. It is found th...

Supersonic impinging jets, such as those occurring in the next generation of short takeoff and vertical landing aircraft, generate a highly oscillatory ow with very high unsteady loads on the nearby aircraft structures and the landing surfaces. These high-pressure and acoustic loads are also accompanied by a dramatic loss in lift during hover. Previous studies of supersonic impinging jets suggest that the highly unsteady behavior of the impinging jets is due to a feedback loop between the uid and acoustic elds, which leads to these adverse effects. A unique active control technique was attempted with the aim of disrupting the feedback loop, diminishing the ow unsteadiness, and ultimately reducing the adverse effects of this ow. Flow control was implemented by placing a circular array of 400-¹m-diam supersonic microjets around the periphery of the main jet. This control approach was very successful in disrupting the feedback loop in that the activation of the microjets led to dramatic reductions in the lift loss (40%), unsteady pressure loads (11 dB), and near-eld noise (8 dB). This relatively simple and highly effective control technique makes it a suitable candidate for implementation in practical aircraft systems.

A simple surface-mounted tapered tab has recently attracted uids research both for its ability to enhance mixing and heat transfer (for which it is known as high-e ciency vortab mixer) and for its generation of coherent structures that are topologically similar to those found in natural turbulent boundary layers. Two types of structures, namely pressure-driven counter-rotating vortex pair (CVP) and hairpin vortices were previously identiÿed in the tab wake, but the contribution of individual structures to the mixing enhancement process and how they interact are not known. In the present study, ow visualization using a planar laser-induced uorescence (PLIF) technique is carried out to probe into the ow dynamics in the wake of the mixing tab. By injecting dye at an appropriate location and illuminating the ow in various planes, the structures are visualized clearly. The results show, in contrary to earlier observations, that the two types of structures dominate di erent regions. At the Reynolds number of 700 based on tab height (h), the CVP has more in uence in the region 0 ¡ x=h ¡ 1:5. The counter-rotating action of the vortex pair induces a pumping action along the symmetry by which the low-speed uid from the boundary layer is transported to the high-speed outer shear layer. The displaced uid is entrained by the recirculating counter-rotating vortices and is mixed well while convecting downstream. Beyond this region, fully developed hairpin structures contribute more to mixing in a similar way as in a turbulent boundary layer. It is observed that the shedding frequency of hairpin vortices is slightly higher than the pumping frequency of the counter-rotating vortex pair. It is also observed that the hairpin structures loses their identity beyond x=h ¿ 15, and there is no large-scale cross-stream mixing visible in this region.

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We report here an experimental study of the behaviour of a fully developed axisymmetric turbulent jet whose buoyancy is enhanced by volumetric heating over the region between two streamwise stations. The buoyancy enhancement is achieved by ohmic heating of an electrically conducting liquid jet, and the measurements are made using a laser Doppler velocimeter. It is found that, with heating, the axial component of mean velocity can increase appreciably relative to the unheated jet; however the turbulent intensity (normalized by the jet centreline velocity) decreases. The shape of the normalized mean velocity distribution across the jet is not significantly affected by the heating, but that of the fluctuating velocity is. The decay of the centreline velocity is considerably slowed down, or even reversed, due to the heating; similarly the spread rate is arrested at larger values of the Richardson number. As a result of the enhanced buoyancy the mass flux in the jet at first increases more rapidly than in the unheated jet but further downstream remains nearly constant over a distance of the order of the length of the heat injection region.

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The characteristics of supersonic impinging jets are investigated using Particle Image Velocimetry (PIV). The purpose of the experiments is to understand the jet induced forces on STOVL aircraft while hovering close to the ground. For this purpose, a large diameter circular plate was attached at the nozzle exit. The oscillations of the impinging jet generated due to a feedback loop are captured in the PIV images. The instantaneous velocity field measurements are used to describe flow characteristics of the impinging jet. The important flow features such as oscillating shock waves, slipstream shear layers and large scale structures are captured clearly by the PIV. The presence of large scale structures in the impinging jet induced high entrainment velocity in the near hydrodynamic field, which resulted in lift plate suction pressures. A passive control device is used to interfere with the acoustic waves travelling in the ambient medium to suppress the feedback loop. As a consequence, the large scale vortical structures disappeared completely leading to a corresponding reduction in the entrainment.