An experimental study was carried out on a twin-disc machine to study the influence of various parameters on the resistance to micropitting of steel discs. The slide-to-roll ratio (SRR = sliding speed/mean rolling speed) was shown to have a great influence on surface-initiated pitting, which occurs even for low sliding speeds. Metallographic studies revealed the disadvantages of atmospheric pressure heat treatments compared to low-pressure ones: discontinuities such as oxides under the surface favor crack propagation. The load applied on the discs-and thus the contact pressure-did not affect the micropitting of the surfaces in the range studied. The effects of surface machining and surface treatments were finally considered: ground surfaces showed better resistance to micropitting than surfaces obtained by turning, and the benefits of shot peening were exposed. The influence of surface roughness and material properties on the resistance to pitting is discussed in the light of these results.
Aerodynamic interference can occur between high-speed bodies when in close proximity. A complex flowfield develops where shock and expansion waves from a generator body impinge upon the adjacent receiver body and modify its aerodynamic characteristics. The aims of this paper are to validate a computational prediction method, to use the predicted solutions to interpret the measured results and to provide a deeper understanding of the associated flow physics.
Aerodynamic interference can occur between high-speed slender bodies when in close proximity. A complex flowfield develops where shock and expansion waves from a generator body impinge upon the adjacent receiver body and modify its aerodynamic characteristics in comparison to the isolated case. The aim of this research is to quantify and understand the multi-body interference effects which arise between a finned slender body and a second disturbance generator body. A parametric wind tunnel study was performed where the effects of receiver incidence and axial stagger were considered. Computational Fluid Dynamic simulations showed good agreement with the measurements and these were used in the interpretation of the experimental results. The overall interference loads for a given multi-body configuration are found to be a complex function of the pressure footprints from the compression and expansion waves emanating from the generator body as well as the flow pitch induced by the generator shockwave. These induced interference loads change sign as the shock impingement location moves aft over the receiver and in some cases cause the receiver body to become statically unstable. Overall, the observed interference effects can modify the subsequent body trajectories and may increase the likelihood of a collision. Nomenclature a = sonic velocity, ms -1
No abstract
The supersonic flow around a rocket piloted by thrusters has been investigated. Steady Reynolds-averaged Navier-Stokes computations and pressure measurements in a wind tunnel, using pressure taps and pressuresensitive paint, have been performed. For experimental investigations, real hot-gas thrusters have been replaced by cold-gas jets using a lower total pressure and a different gas. The uniqueness of this work involved calibrating those cold-gas thrusters in order to reproduce an aerodynamic interference very similar to the hot-gas jets case. This has been achieved by modifying the section ratio of the nozzles in order to control the wave celerity ratio between freestream and jet flows. A good agreement has been obtained between numerical simulations of hot-gas and cold-gas jets flows, as well as between computations and measurements of the pressure coefficient around the body in the wind tunnel. Thus, this method has provided a good approximation for the interaction between hot-gas jets and crossflow by using lower-pressure and lower-temperature jets.the body, m J = momentum flux similarity number JR = momentum flux ratio M = mass flux similarity number M th = thruster ejection Mach number M 1 = freestream Mach number NCR = nozzle celerity ratio NJR = nozzle momentum flux ratio NMR = nozzle mass flux ratio NPR = nozzle pressure ratio NrTR = nozzle temperature ratio P = pressure similarity number PR = total pressure ratio P j = nozzle exit static pressure, Pa P o , P 1 = freestream static pressure, Pa P t = total pressure, Pa P ti = thruster stagnation pressure, Pa Re D = diameter-based Reynolds number r th = specific thruster gas constant, J kg 1 K 1 r 1 = specific freestream gas constant, J kg 1 K 1 T = temperature similarity number T j = nozzle exit static temperature, K T t = total temperature, K T ti = thruster stagnation temperature, K T 1 = freestream static temperature, K U = velocity component along the x axis, m s 1 U o , U 1 = freestream velocity component along the x axis, m s 1 u f = friction velocity component along the x axis, m s 1 W = velocity component along the mean ejection direction, m s 1 W j = nozzle exit velocity component along the mean ejection direction, m s 1 x = horizontal coordinate along the body axis, m y = horizontal coordinate perpendicular to the body axis, m y = dimensionless wall distance z = vertical coordinate, m th = thruster gas isentropic coefficient 1= freestream gas isentropic coefficient = kinematic viscosity, m 2 s 1 = density, kg m 3 j = nozzle exit density, kg m 3 1 = freestream density, kg m 3 w = wall shear, Pa
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