Air-induced inverse Chladni patterns

When very light particles are sprinkled on a resonating horizontal plate, inverse Chladni patterns are formed. Instead of going to the nodal lines of the plate, where they would form a standard Chladni pattern, the particles are dragged to the antinodes by the air currents induced by the vibration of the plate. Here we present a detailed picture of the mechanism using numerical simulations involving both the particles and the air. Surprisingly, the time-averaged Eulerian velocity, commonly used in these type of problems, does not explain the motion of the particles: it even has the opposite direction, towards the nodal lines. The key to the inverse Chladni patterning is found in the averaged velocity of a tracer particle moving along with the air: this Lagrangian velocity, averaged over a vibration cycle, is directed toward the antinodes. The Chladni plate thus provides a unique example of a system in which the Eulerian and Lagrangian velocities point in opposite direction

Effects of Cooling During Exercise on Thermoregulatory Responses of Men With Paraplegia.

BACKGROUND: People with spinal cord injury (SCI) have an altered afferent input to the thermoregulatory center, resulting in a reduced efferent response (vasomotor control and sweating capacity) below the level of the lesion. Consequently, core body temperature rises more rapidly during exercise in individuals with SCI compared with people who are able-bodied. Cooling strategies may reduce the thermophysiological strain in SCI. OBJECTIVE: The aim of this study was to examine the effects of a cooling vest on the core body temperature response of people with a thoracic SCI during submaximal exercise. METHODS: Ten men (mean age=44 years, SD=11) with a thoracic lesion (T4-T5 or below) participated in this randomized crossover study. Participants performed two 45-minute exercise bouts at 50% maximal workload (ambient temperature 25°C), with participants randomized to a group wearing a cooling vest or a group wearing no vest (separate days). Core body temperature and skin temperature were continuously measured, and thermal sensation was assessed every 3 minutes. RESULTS: Exercise resulted in an increased core body temperature, skin temperature, and thermal sensation, whereas cooling did not affect core body temperature. The cooling vest effectively decreased skin temperature, increased the core-to-trunk skin temperature gradient, and tended to lower thermal sensation compared with the control condition. LIMITATIONS: The lack of differences in core body temperature among conditions may be a result of the relative moderate ambient temperature in which the exercise was performed. CONCLUSIONS: Despite effectively lowering skin temperature and increasing the core-to-trunk skin temperature gradient, there was no impact of the cooling vest on the exercise-induced increase in core body temperature in men with low thoracic SCI

Modelling and experimental investigation of a thermally driven self-oscillating pump

This paper explores the pumping characteristics and behaviour of a thermally driven self-oscillating pump. The pump consists of a single wickless capillary tube with a circular cross-section. The tube is closed at one end and has a T-section with two check valves at the other end to provide for a one directional flow. An experimental setup was built to investigate the output mass flow and pressure head of the pump. During the experiments, the performance of the check valves had a negative influence on the output mass flow. To determine this influence, a video analysis of the fluid oscillation without the check valves was conducted and compared to results with check valves. The average output mass flow with valves was approximately 0.0010. kg/s with a maximum measured pump flow of 0.0013. kg/s. The maximum pressure head delivered was 0.25. bar. A numerical model of the vapour bubble oscillation was developed to get a better understanding of the pump and its working principles. The model is based on the conservation of mass, momentum and energy, and resulted in a non-linear system of coupled differential equations. Overall, the experiments conducted with the thermally driven self-oscillating pump have shown that the pump has good potential to be used in aerospace applications

Development of a novel hybrid discrete particle-immersed boundary model for fluidized bed membrane reactors

In recent years many different types of fluidized bed membrane reactors have been proposed, for example for the production of ultra-pure hydrogen. For the development and optimization of these novel fluidized bed membrane reactors, fundamental knowledge on the effect of the presence of – and permeation of gas through – the immersed membranes on the hydrodynamics and heat and mass transfer characteristics of the fluidized bed is essential, yet largely lacking in the literature. In this work a hybrid 3D soft-sphere Discrete Particle - Immersed Boundary Model has been developed to investigate the hydrodynamics in great detail, specifically focusing on bubble formation/annihilation close to the submerged membranes, bubble size distribution and particle mixing as a function of the permeation ratio. In this paper, the details of the numerical implementation of the immersed boundary method are described and validated, and first results on the effect of gas extraction and gas addition through the membranes on the fluidized bed hydrodynamics are presented

Inversion of Chladni patterns by tuning the vibrational acceleration

Inverse Chladni patterns, i.e., grains collecting at the antinodes of a resonating plate, are traditionally believed to occur only when the particles are small enough to be carried along by the ambient air. We now show—theoretically and numerically—that air currents are not the only mechanism leading to inverse patterns: When the acceleration of the resonating plate does not exceed g, particles will always roll to the antinodes, irrespective of their size, even in the absence of air. We also explain why this effect has hitherto escaped detection in standard Chladni experiments