Statistics of surface gravity wave turbulence in the space and time domains

We present experimental results on simultaneous space–time measurements for the gravity wave turbulence in a large laboratory flume. We compare these results with predictions of the weak turbulence theory (WTT) based on random waves, as well as with predictions based on the coherent singular wave crests. We see that the both wavenumber and frequency spectra are not universal and dependent on the wave strength, with some evidence in favour of the WTT at larger wave intensities when the finite-flume effects are minimal. We present further theoretical analysis of the role of the random and coherent waves in the wave probability density function (p.d.f.) and the structure functions (SFs). Analysing our experimental data we found that the random waves and the coherent structures/breaks coexist: the former show themselves in a quasi-Gaussian p.d.f. core and the low-order SFs and the latter in the p.d.f. tails and the high-order SFs. It appears that the x-space signal is more intermittent than the t-space signal, and the x-space SFs capture more singular coherent structures than the t-space SFs do. We outline an approach treating the interactions of these random and coherent components as a turbulence cycle characterized by the turbulence fluxes in both the wavenumber and the amplitude spaces

Effect of wall surface wettability on collective behavior of hydrogen microbubbles rising along a wall

This paper presents an experimental study of the influence of wall surface wettability on the behavior of hydrogen microbubbles rising along a nearly vertical wall. Multiple optical diagnostics, including particle tracking velocimetry, have been employed for the study. The microbubble behavior observed along three different kinds of wall surfaces (hydrophobic, hydrophilic, and super-hydrophilic) was characterized by the microbubble-wall attachment, bubble size distribution, bubble coalescence, and microbubble layer formation. Microbubbles rising along the wall with poor wettability soon attach to the wall and grow to millimeters in size as a result of bubble coalescence. Such millimeter-sized bubbles detach from the wall because of their increased buoyancy, and eventually enhance transverse diffusion of microbubbles, which is known as the sweep-out effect. In contrast, in the case of very good wettability, almost no microbubbles attach to the wall and smoothly form a thin microbubble layer in the wall proximity. The observed phenomena contradict our intuitive expectation of the effect of surface wettability on gas bubbles, and hence may be regarded as a feature of microbubbles that distinguishes them from large bubbles

Effect of heated wall inclination on natural convection heat transfer in water with near-wall injection of millimeter-sized bubbles

Natural convection heat transfer from a heated wall in water with near-wall injection of millimeter-sized bubbles is studied experimentally. Velocity and temperature measurements are conducted in the nearwall region. In the range of the heated wall angles from 0 to 40 degrees from the vertical, the heat transfer coefficient increases by up to an order of magnitude with bubble injection. The ratio of the heat transfer coefficient with bubble injection to that without injection increases with the wall inclination angle. Based upon measured liquid temperature distributions and liquid flow velocity profiles, enhancement of heat transfer by bubble injection is explained by two mechanisms. First, wall-parallel transport of cold liquid into the thermal boundary layer is enhanced by the bubble-driven flow. Second, wall-normal mixing of warm liquid and cold liquid occurs, as a result of wall-normal velocity fluctuations of the liquid phase activated by a combination of bubble rising motion, vortex shedding from the bubbles, and unsteady vortices formed within the boundary layer. The unsteady vortices travel along the wall together with the bubbles, primarily contributing to the enhancement of heat transfer at higher wall inclination angles

A galloping energy harvester with flow attachment

Aeroelastic energy harvesters are a promising technology for powering wireless sensors and microelectromechanical systems. In this letter, we present a harvester inspired by the trembling of aspen leaves in barely noticeable winds. The galloping energy harvester, a curved blade oriented perpendicular to the flow, is capable of producing self-sustained oscillations at uncharacteristically low wind speeds. The dynamics of the harvesting system are studied experimentally and compared to a lumped parameter model. Numerical simulations quantitatively describe the experimentally observed dynamic behaviour. Flow visualisation is performed to investigate the patterns generated by the device. Dissimilar to many other galloping harvester designs, the flow is found to be attached at the rear surface of the blade when the blade is close to its zero displacement position, hence acting more closely to aerofoils rather than to conventionally used bluff bodies. Simulations of the device combined with a piezoelectric harvesting mechanism predict higher power output than that of a device with the square prism

Fingering of a cavitation bubble in a thin gap : ejection of the reversed boundary layer into the bulk flow

The flow around a bubble, precipitously expanding in a thin gap between flat walls, was found to have a peculiar feature: distinct fingering occurs at the bubble wall, which was observed through the ultra-high speed optical visualization. The effect is attributed to the reversal of the flow within boundary layers, which provokes the growth of instabilities at the inflection point and, when the surface tension is low enough (the local Weber number is high enough), leads to the fingering. In this paper, we show the high speed recordings of the fingering and model the evolution of the radial velocity to quantitatively confirm feasibility of the proposed instability mechanism

Gas-liquid hydrogenation in continuous flow – The effect of mass transfer and residence time in powder packed-bed and catalyst-coated reactors

Catalyst-coated tube reactors have been compared with the reactors packed with catalyst powder in alkyne semi-hydrogenation over a 5 wt% Pd/ZnO catalyst and cinnamic ester full hydrogenation over a 2.4 wt% Pd/C catalyst. The “powder packed-bed” reactors (packing with catalyst powder below 30 μm) showed irreproducible performance in time due to mobility of the catalyst layer in the bed which altered the fluidic path and therefore affected the mean liquid residence time and the dispersion. The catalyst-coated tube reactors demonstrated an ideal plug-flow behaviour (Péclet number > 120), while the powder packed-bed showed a considerable back-mixing (Péclet ~ 25). Under all conditions studied, the reaction rate in the powder packed-bed was limited by external mass transfer, while in the coated tube – by the intrinsic kinetics. The coated tubes demonstrated a much lower pressure drop, an improved alkene selectivity, and a 5 times higher throughput compared to the powder packed-bed. The dilution of the catalyst bed with glass beads improved the throughput 4-fold at the expense of 4-fold increase in the pressure drop. In full hydrogenation reaction, the catalyst-coated tube showed a 14 times higher throughput than in the powder packed-bed at the full alkyne conversion. A reactor model for the catalyst-coated tube has been proposed that takes into account the change in the fluid velocity during the reaction. The model described the reaction kinetics demonstrating that the catalyst-coated tubes can be used as a tool to obtain kinetic data in gas-liquid reactions in flow

Gravity wave turbulence in a laboratory flume

We present an experimental study of the statistics of surface gravity wave turbulence in a flume of a horizontal size 12×6  m. For a wide range of amplitudes the wave energy spectrum was found to scale as Eω∼ω-ν in a frequency range of up to one decade. However, ν appears to be nonuniversal: it depends on the wave intensity and ranges from about 6 to 4. We discuss our results in the context of existing theories and argue that at low wave amplitudes the wave statistics is affected by the flume finite size, and at high amplitudes the wave breaking effect dominates

Formation-breakdown cycle of turbulent jets in a rotating fluid

Results of comprehensive particle image velocimetry measurements investigating the dynamics of turbulent jets in a rotating fluid are presented. It is observed that background system rotation induces a time-periodic formation–breakdown cycle of the jets. The flow dynamics associated with this process is studied in detail. It is found that the frequency of the cycle increases linearly with the background rotation rate. The data show that the onset of the breakdown phase and of the reformation phase of the cycle can be characterized in terms of a local Rossby number employing an internal velocity and a length scale of the jet. The critical values for this local Rossby number, for onset of breakdown and reformation, scale linearly with a global Rossby number based on the flow conditions at the source. The analysis of the experimental data suggests centrifugal instability as the potential origin of the formation–breakdown cycle

Enhanced droplet size control in liquid-liquid emulsions obtained in a wire-guided X-mixer

The droplet size in a liquid‐liquid emulsion can be controlled by placing a metal wire along the centerline of an X‐mixer. Droplets gradually form when flowing along the wire, with droplet separation occurring at the tip of the wire rather than at the channel intersection in the X‐mixer. The droplet size is now defined by the Plateau‐Rayleigh instability developing in the axisymmetric annular flow region rather than by a sophisticated and hardly predictable three‐dimensional flow at the channel intersection. The wire‐guided droplet formation allows for fine control of the droplet size by changing the wire diameter, the position of the wire tip, and the flow rates. Further control of the droplet size can be achieved by adjusting the surface tension by adding a surfactant