Extending the Frequency Limits of "Postage-Stamp PIV" to MHz Rates

The effective frequency limits of postage-stamp PIV, in which a pulse-burst laser and very small fields of view combine to achieve high repetition rates, have been extended by increasing the PIV acquisition rate to very nearly MHz rates (990 kHz) by using a faster camera. Charge leaked through the camera shift register at these framing rates but this was shown not to bias the measurements. The increased framing rate provided oversampled data and enabled use of multi-frame correlation algorithms for a lower noise floor, increasing the effective frequency response to 240 kHz where the interrogation window size begins to spatially filter the data. The velocity spectra suggest turbulence power-law scaling in the inertial subrange steeper than the theoretical -5/3 scaling, attributed to an absence of isotropy.

Symmetrical Taylor impact studies of copper

An integrated investigation of rod-on-rod (symmetric Taylor) impact of annealed copper was conducted using the single-stage gas-gun facility at the Cavendish Laboratory as a validation study of the Armstrong–Zerilli constitutive model, as modified by Goldthorpe. Two main techniques were used for obtaining data from the experiments: high-speed photography (up to 20 million frames s −1 framing rate) and a velocity interferometer system for any reflector (VISAR). The symmetric configuration was used to minimize friction effects and eliminate target indentation seen in classic Taylor tests, where a rod is fired against a massive target block. However, the need for coaxial alignment of the two rods made the experiments considerably more challenging to perform than the classic case. The propagation of plasticity along the rods was monitored using high-speed photography and VISAR. It was found to propagate with a logarithmically decelerating velocity. The rod profiles and VISAR traces can be understood in terms of material properties such as strain hardening. No asymmetry between the responses of the two rods involved (moving and stationary) was observed within the resolution of the techniques employed. A modified Armstrong–Zerilli material model for copper predicted intermediate profiles well, but slightly overestimated the material strength.

Time-Resolved PIV Measurements Within a Triple Impeller Stirred-Tank

Time-resolved velocity measurements using particle image velocimetry (PIV) were performed in a triple impeller stirred-tank to evaluate flow structure and flow dynamics. Using a high framing rate digital camera and a high pulse rate laser, the flow field at a planar location along the diameter of the tank was over-sampled to capture the time evolution of vortical structures in the flow. Several tests were performed for rotational speeds ranging from 175 RPM to 1575 RPM. The working fluid was glycerin at ambient conditions. Time averaged results over several blade passage cycles reveal that the vortical toroidal structure and frequency changes significantly over the RPM range investigated. A point of transition exists past which symmetry in the tank is lost, as a result of jet-jet interaction between adjacent Rushton turbines. While the flow in the region near and in between the turbines is unsteady and periodic due to blade passage, further away in the core of the mixing vessel the flow reaches a state of uniformity, as revealed from the reconstructed volumetric velocity field determined from the time-resolved PIV measurements.

Anode Boundary Layer Effects in Plasma Spray Torches

The control over coating quality in plasma spraying is partly dependent on the arc and jet instabilities of the plasma torch. Different forms of instabilities have been observed with different effects on the coating quality. We report on an investigation of these instabilities based on high-speed end-on observation of the arc. The framing rate of 40,500 frames per second has allowed the visualization of the anode attachment movement and the determination of the thickness of the cold gas boundary layer surrounding the arc. The images have been synchronized with voltage traces. Data have been obtained for a range of arc currents, mass flow rates, for different gas injectors and for anodes displaying different amounts of wear. The analysis of the data has led to quantitative correlations between the cold gas boundary layer thickness and the instability mode for the range of operating parameters. The arc instabilities can be seen to enhance the plasma jet instabilities and the cold gas entrainment. These results are particular useful for guiding plasma torch design and operation to minimize the influence of plasma jet instabilities on coating properties.