Performance comparison of Stellite 6® deposited on steel using supersonic laser deposition and laser cladding

Stellite 6® powders were deposited on carbon steel using Supersonic Laser Deposition. The microstructure and performance of the coatings were examined using SEM, optical microscopy, EDS, XRD, microhardness testing and pin-on-disc wear testing. The results showed that the microstructure and wear behaviour of the most successful SLD deposition conditions with N2 at a pressure of 30bar, a temperature of 450°C and a deposition power of 1.5kW were compared with that of optimised laser cladding. © 2012 Elsevier B.V.

Comparison of bleed and micro-vortex generator effects on supersonic boundary-layers

An experimental study of bleed and vortex generators in supersonic ow has been conducted. Methods were developed to analyze and directly compare the two systems' effects on turbulent boundary layers to better understand their potential to mitigate ow separation. LDA was used to measure two components of velocity in the boundary-layer for three cases|baseline, with bleed, or with a VG|at Mach numbers of 1.3, 1.5 and 1.8. The bleed system was comprised of a series of 2mm diameter normal holes operated at different suction rates, removing up to 10% of the incoming boundary layer. Three VG shapes were tested only at Mach 1.5 and 1.8. Measurements of the evolution of Hi and Cf downstream of each device indicate that Hi is not an appropriate parameter to gauge the effectiveness of vortex generators due to boundary layer wake distortion. The skin friction coeficient Cf may be a more appropriate measure. Similar increases in Cf were generated by VGs and bleed. The recovery to baseline conditions downstream of bleed was sensitive to Mach number, and more investigation of that effect will be required. Copyright © 2012 by University of Cambridge.

Comparison of Micro-Vortex generators in supersonic flows

An experimental comparison of several vortex generator geometries was conducted at Mach 1.5, 1.8, and 2.5 to better understand downstream vortex development as a function of device shape and Mach number. The devices had heights less than that of the boundary-layer ("micro"-vortex generators) and were either vane-shaped or of the alternative microramp geometry. LDV was used to measure two components of velocity at several stations downstream of the devices. The velocity data were then fitted to a vortex model so that vortex parameters such as circulation, core radius, and trajectory were estimated. Mach number dependence was seen for all parameters. Vortex height and core radius both tended to decrease slightly with increasing Mach number. A critical vane angle for maximum circulation was observed and also decreased with increasing Mach number. Circulation was seen to scale with wall-friction velocity for Mach 1.5 and 1.8 but not 2.5. © 2012 by W.R. Nolan and H. Babinsky.

Flow physics of a normal-hole bled supersonic turbulent boundary layer

An experimental study on normal hole bleed in a supersonic turbulent boundary layer has been conducted. A combination of LDV, Schlieren imagery and oil flow visualization were used to provide a better understanding of the three-dimensional flow field surrounding a supersonic bleed array. Experiments were performed at Mach numbers of 1.8 and 2.5, while previously published results at Mach numbers of 1.3 and 1.5 were also incorporated. The bleed system was capable of removing up to approximately 10% of the incoming boundary layer through a tunnel-spanning array of discrete holes with diameters the same order of magnitude of boundary layer displacement thickness. Inspection of boundary layer profiles downstream of the bleed region indicates that vorticity generated by the discrete holes can have a substantial influence on changes to the boundary layer shape factor and skin friction coefficient, through modification of the lower 20% of the boundary layer. This vorticity was visualized through oil-flow visualization, and LDV measurements, showing the development of two vortices off each bleed hole, and corresponding upwash and downwash regions with far-reaching three dimensional effects. © 2013 by J. M. Oorebeek and H. Babinsky.

A single-Stream jet noise prediction model for a family of chevron nozzles

This study develops a single-stream jet noise prediction model for a family of chevron nozzles. An original equation is proposed for the fourth-order space-time cross-correlations. They are expressed in flow parameters such as streamwise circulation and turbulent kinetic energy. The cross-correlations based on a Reynolds Averaged Navier-Stokes (RANS) flowfield showed a good agreement with those based on a Large Eddy Simulation (LES) flowfield. This proves that the proposed equation describes the cross-correlations accurately. With this novel source description, there is an excellent agreement between our model's far-field noise predictions and measurements1 for a wide range of frequencies and radiation angles. Our model captures the spectral shape, amplitude and peak frequency very well. This establishes that our model holds good for a family of chevron nozzles. As our model provides quick and accurate predictions, a parametric study was performed to understand the effects of a chevron nozzle geometry on jet noise and thrust loss. Chevron penetration is the underpinning factor for jet noise reduction. The reduction of jet noise per unit thrust loss decreases linearly with chevron penetration. The number of chevrons also has a considerable effect on jet noise.

Influence of cluster-assembly parameters on the field emission properties of nanostructured carbon films

Supersonic cluster beam deposition has been used to produce films with different nanostructures by controlling the deposition parameters such as the film thickness, substrate temperature and cluster mass distribution. The field emission properties of cluster-assembled carbon films have been characterized and correlated to the evolution of the film nanostructure. Threshold fields ranging between 4 and 10 V/mum and saturation current densities as high as 0.7 mA have been measured for samples heated during deposition. A series of voltage ramps, i.e., a conditioning process, was found to initiate more stable and reproducible emission. It was found that the presence of graphitic particles (onions, nanotube embryos) in the films substantially enhances the field emission performance. Films patterned on a micrometer scale have been conditioned spot by spot by a ball-tip anode, showing that a relatively high emission site density can be achieved from the cluster-assembled material. (C) 2002 American Institute of Physics.

Normal shock boundary layer control with various vortex generator geometries

Various vortex generators which include ramp, split-ramp and a new hybrid concept "ramped-vane" are investigated under normal shock conditions with a diffuser at Mach number of 1.3. The dimensions of the computational domain were designed using Reynolds Average Navier-Stokes studies to be representative of the flow in an external-compression supersonic inlet. Using this flow geometry, various vortex generator concepts were studied with Implicit Large Eddy Simulation. In general, the ramped-vane provided increased vorticity compared to the other devices and reduced the separation length downstream of the device centerline. In addition, the size, edge gap and streamwise position respect to the shock were studied for the ramped-vane and it was found that a height of about half the boundary thickness and a large trailing edge gap yielded a fully attached flow downstream of the device. This ramped-vane also provided the largest reduction in the turbulent kinetic energy and pressure fluctuations. Additional benefits include negligible drag while the reductions in boundary layer displacement thickness and shape factor were seen compared to other devices. © 2010 by Sang Lee.

Jet diameters and velocity profiles in continuous ink-jets

Theoretical predictions of the diameters of continuous ink-jets downstream of long nozzles are generalized to include the important cases of ink-jet fluids and shorter nozzles where the velocity profile at the nozzle exit is undeveloped (non-parabolic). Comparisons of the new predictions with experiments and simulations are made for fairly long nozzles with tapered profiles and short nozzles with conical profiles; experimental and simulated profiles are also compared downstream of the nozzle exit for both industrial and large scale ink-jet print heads. Precise measurements of the un-modulated jet diameters downstream of the nozzle exit can set really useful limits to the possible shapes of the flow profile right at the nozzle exit, and in particular allow some assessment of the axial velocity gradients and fluid shear rates at the nozzle exit where direct speed measurement is usually impractical. Simulations allow further study of the relaxation of the velocity profile downstream of the nozzle exit, and are reported for both un-modulated and modulated CIJ jetting. Implications of this work include speeding up CIJ simulations, absolute calibration of the applied CIJ system modulation, and the likely magnitude of dynamic surface tension effects on observed CIJ satellite speeds.

Normal shock boundary layer control with various vortex generator geometries

Various vortex generators which include ramp, split-ramp and a new hybrid concept "ramped-vane" are investigated under normal shock conditions with a diffuser at Mach number of 1.3. The dimensions of the computational domain were designed using Reynolds Average Navier-Stokes studies to be representative of the flow in an external-compression supersonic inlet. Using this flow geometry, various vortex generator concepts were studied with Implicit Large Eddy Simulation. In general, the ramped-vane provided increased vorticity compared to the other devices and reduced the separation length downstream of the device centerline. In addition, the size, edge gap and streamwise position respect to the shock were studied for the ramped-vane and it was found that a height of about half the boundary thickness and a large trailing edge gap yielded a fully attached flow downstream of the device. This ramped-vane also provided the largest reduction in the turbulent kinetic energy and pressure fluctuations. Additional benefits include negligible drag while the reductions in boundary layer displacement thickness and shape factor were seen compared to other devices. © 2011 Elsevier Ltd.