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[en] The investigation of flows of superfluid helium (He II) constitutes an active and challenging research field. Flow visualization techniques, which allow following the motion of relatively small particles suspended in the fluid, have been specifically giving in recent years significant contributions to our understanding of the underlying physics. It has been shown, for example, that the derived flow-induced properties, such as the velocity statistical distribution, depend on the length scale probed by the particles, for both thermally and mechanically driven flows of He II. Quantum features appear indeed at small enough length scales, smaller than the quantum length scale of the flow, the average distance between quantized vortices, while, at larger scales, a classical (viscous-like) picture emerges, especially in the case of mechanically driven flows. The visualization results obtained to date firmly support the view that the investigation of particle dynamics in quantum flows is not only interesting in its own right but it may also lead to the deeper understanding of fluid turbulence in general.
[en] We have built a simple mechanical system to emulate the fast-start performance of fish. The system consists of a thin metal beam covered by a urethane rubber, the fish body and an appropriately shaped tail. The body form of the mechanical fish was modeled after a pike species and selected because it is a widely-studied fast-start specialist. The mechanical fish was held in curvature and hung in water by two restraining lines, which were simultaneously released by a pneumatic cutting mechanism. The potential energy in the beam was transferred into the fluid, thereby accelerating the fish. We measured the resulting acceleration, and calculated the efficiency of propulsion for the mechanical fish model, defined as the ratio of the final kinetic energy of the fish and the initially stored potential energy in the body beam. We also ran a series of flow visualization tests to observe the resulting flow patterns. The maximum start-up acceleration was measured to be around 40 m s-2, with the maximum final velocity around 1.2 m s-1. The form of the measured acceleration signal as function of time is quite similar to that of type I fast-start motions studied by Harper and Blake (1991 J. Exp. Biol. 155 175-92). The hydrodynamic efficiency of the fish was found to be around 10%. Flow visualization of the mechanical fast-start wake was also analyzed, showing that the acceleration peaks are associated with the shedding of two vortex rings in near-lateral directions.
[en] The novel technique of particle image displacement velocimetry allows visualization of two-dimensional flows as well as the quantification of the instantaneous velocity and vorticity fields. The method's principles of operation are then presented; the method is shown to be suited to the gathering of both temporal and spatial data, as is required in the case of complex flow fields. 6 references
[en] Flow visualization tests were performed in a one-fifth scale adiabatic hydraulic model of a MAPLE reactor to investigate the effectiveness of bypass (or countermomentum) and suction flows in suppressing the core jet in an open-chimney-in-pool arrangement. The bypass-flow requirements for suppression of core flow from the open chimney for various core flows, chimney heights, suction locations and suction angles are presented. The experimental data trends are in qualitative agreement with the results of numerical simulations. Overall, it is concluded that, with the proper chimney design, the core jet can be successfully confined in a MAPLE-type open-chimney-in-pool arrangement
[en] Highlights: • Flow characteristics of the MPHP with reentrant cavities are investigated. • Reentrant cavities in the MPHPs are shown to promote nucleation and early startup. • The thermal resistance of the MPHP with reentrant cavities is decreased by up to 57%. • A single MPHP incorporating various sizes of reentrant cavities is proposed. - Abstract: This study is performed to investigate the effect of the size of reentrant cavities on the thermal performance of a micro pulsating heat pipe (MPHP). The flow and thermal characteristics of the MPHPs with each MPHP having a different size of reentrant cavities, along with a MPHP without reentrant cavities are experimentally obtained and compared. Silicon-based MPHPs with and without reentrant-type artificial cavities inside the channels are fabricated using MEMS techniques. The MPHPs have rectangular channels which are engraved on a silicon wafer with a hydraulic diameter of 667 . Ethanol is used as the working fluid. To allow for flow visualization, the etched micro-channels are covered with a transparent glass. Each MPHP with reentrant cavities has reentrant cavities of one size, which are either 10, 20, 30, or 40 , respectively. Reentrant cavities in the MPHPs are shown to promote nucleation and early startup. Furthermore, the thermal resistance of the MPHP with reentrant cavities is decreased by up to 57%. As the size of the reentrant cavities increases, lower input power is required for startup, and the MPHP with the largest cavities (40 ) shows the earliest startup. On the contrary, as the size of the reentrant cavities decreases, a reduction of the thermal resistance of the MPHPs is maintained to higher input power, and the MPHP with the smallest cavities (10 ) shows the lowest thermal resistance at high input power (>12 W). Finally, a single MPHP incorporating various sizes of reentrant cavities (10, 20, 30, and 40 ) is shown to exhibit extended operating range and enhanced thermal performance simultaneously.
[en] The structure of Benard--Marangoni convection cells can be controlled by periodic topographic patterns on the heated surface that generates the convection. When the periodicity of the topographic pattern matches the intrinsic periodicity of the convection cells, a convective pattern is formed that is 1:1 commensurate with the topographic pattern. Arrays of hexagonal, square, and triangular convection cells were generated over the appropriately designed topographic patterns, and visualized by infrared imaging. For imposed patterns with periodicity in two dimensions, as the ratio of the intrinsic and perturbing length scales changes, the pattern of the convection cells shows sharp transitions between different patterns commensurate with the imposed pattern. For imposed patterns with periodicity in one dimension (i.e., lines) the convection cells use the unconstrained dimension to adapt continuously to the external perturbation. Topographically controlled convection cells can be used to assemble microscopic particles into externally switchable regular lattices
[en] Highlights: • PIV technique was used to visualize the vortex generations and interaction of a solitary wave and a submerged thin plate under different angles. • Increasing the plate angle in both directions will lead to decrease the clockwise vortex strength that is formed at the rear of the obstacle. • Changing the thin plate angle in the direction of wave propagation made a new counter clockwise vortex at the bottom of the channel. • Changing the plate angle towards the negative direction reduced more wave height and wave speed. • The maximum wave deceleration occurred at Δ?? = −30°, and the maximum wave acceleration occurred on Δ?? = 60°. - Abstract: This research presents an experimental investigation into the interaction of a solitary wave and a submerged thin plate under different angles. Experiments are conducted to measure both velocity and vorticity by using the Particle Image Velocimetry (PIV) technique. Effects of changing the thin plate angle on the wave height and wave speed are analyzed through the use of wave height gages. Vortices are generated when the solitary wave is transmitted over the obstacle. Understanding the formation and location of the vortices will help analyze the obstacle effect on the flow. In the initial stage, a thin plate is located vertically, and flow structures are visualized. The plate is then deviated from the vertical direction towards positive angles (the direction of wave propagation) and towards negative angles in the opposite direction. In order to study the effects of the plate angle on the flow structures, experiments are carried out for three positive and three negative angles. Comparison of the formed vortices at different angles shows the formation of an additional vortex near the bottom of the channel for positive angles, as opposed to negative angles. The larger the angle is, the less the formation time of the vortex at the bottom of the channel will be. The study of the clockwise vortices formed behind the obstacle shows that increasing the plate angle in both directions decreases their strength. The clockwise vortices of negative angles are stronger than those of positive angles. In addition, changing the plate angle to the negative direction causes more wave height and wave speed reduction than changing it to the positive direction.
[en] Coherent structures in the near-wall region of turbulent channel flow are investigated using the technique of simultaneous visual and LDV measurements. The visualizations of the coherent structure were done in the streamwise view using a fluorescent dye illuminated by a sheet of laser or halogen light. Some new aspects of the correlation between characteristic velocity signals of u, v and the structures of streamwise vortices are inferred from the present experiment. The results indicate that the velocity signals associated with a bursting event are formed when the pairs of counterrotating streamwise vortices pass through, which generate near the viscous sublayer and develop away from the wall in the near-wall region of turbulent channel flow
[en] Vortex rings were formed with a piston-cylinder mechanism in the presence of uniform background co-flow supplied through a shroud surrounding the cylinder. The jet and co-flow were started simultaneously. Ratios of the co-flow to jet velocity (Rv) in the range 0-1 were considered. The formation number (F) as a function of Rv was determined using the procedure of Gharib et al. [J. Fluid Mech. 360, 121 (1998)] and a generalized definition of formation time. The results show a sharp decrease in F as Rv increases from 0.5-0.75, suggesting possible performance limitations for pulsed-jet propulsion