Under the microgravity environment, liquid can be treated without container due to weightless. Therefore physical properties of a material are expected to be measured with a high accuracy. In surface tension measurement of levitated droplet, it is calculated by using the formula derived under the assumption of linearity of the resonant frequency. However, it is indicated that the resonant frequency of a droplet changes with oscillation and rotation. So the purpose of this study is to examine effect of oscillation and rotation on the resonant frequency experimentally. A droplet is levitated by using electrostatic levitator, and induced to oscillate and rotate. Resonant frequency, aspect ratio, amplitude and rotational velocity are measured from droplet motion. It is confirmed oscillation and rotation effect on aspect ratio and resonant frequency. And the frequency shift becomes zero under the appropriate combination of amplitude and rotational velocity. It is experimentally clarified that the resonant frequency is strongly related with time average of aspect ratio.
We conducted laboratory experiments on the flow around a circular cylinder moving at a constant velocity U0 close to an air-water interface in order to understand interference between vortex-shedding frequency from the cylinder and the free surface wave. The vortex street and the free surface profile were visualized for Reynolds number (Re = U0d/ν) between 200 and 15000 and for Froude number (Frh = U0/(hg)1/2) between 0.01 and 3.2 (d and h are the cylinder diameter and the distance between the top of the cylinder and the undisturbed water surface, respectively). At large Reynolds numbers (> 104), the surface deformation becomes substantial in the downstream of the cylinder and intermittent bubble entrainment by a jet-like flow is observed. The wave oscillation and the vortex shedding frequency are synchronized when the bubble entrainment is observed. From the Strouhal-Reynolds number relationship with gap ratio (a = h/d) between 0.25 and 2.0, the Strouhal number of the wave oscillation and that of vortex shedding increase with decreasing the gap ratio a.
Although several models such as the Rosin-Rammler equation and the Nukiyama-Tanazawa equation have been proposed to represent size distributions of particles or droplets, there are few size distribution models for micro-bubbles. In this study, we therefore investigate the applicability of the available size distribution models to micro-bubbles generated by a pressurized dissolution method. Experiments are conducted for several liquid volumetric fluxes in the decompression nozzle. Measurements of diameters of micro-bubbles are carried out in a downstream region of the decompression nozzle and in the upper tank by using phase Doppler anemometry (PDA). The experimental results indicate that (1) the Nukiyama-Tanasawa equation well represents the size distribution of micro-bubbles generated by the pressurized dissolution method, whereas the Rosin-Rammler equation fails in the representation, (2) the size distribution of micro-bubbles can be evaluated by using the Nukiyama-Tanasawa equation without measuring individual bubble diameters, provided that the mean bubble diameter and the skewness of the bubble distribution are given, and (3) an evaluation method of visibility based on the bubble size distribution and the bubble number density is proposed, and the evaluated visibility agrees well with the visibility measured in the upper tank.
Microbubbles oscillate nonlinearly in the ultrasound field. By irradiating ultrasound, microbubbles reflect superharmonic or subharmonic signals. In particular, microbubbles coated by a protein, lipid, or polymer shell show a mechanical property different from that of micobubbles without shell. In this research, nonlinear oscillations of shell-coated microbubbls were compared with those of gas microubbles. We use the modified Rayleigh-Plesset equation introduced by P. Marmottant, et al. which describes the dynamics of microbubbless coated with lipid shell that has viscosity and elasticity. The dynamical behavior of insonified shell-coated microbubbles was investigated by numerical simulation. Our numerical results suggest the possibility that the oscillation center shifts to the side of bubble compression, and that the response of a shell-coated microbubble has both hardening and softening effects, because the surface tension changes with the bubble radius if the bubble shell exists. Moreover, these results are in qualitative agreement with experimental ones.
In a present study, a micro particle concentration in the microchannel with 12 embedded sensor electrodes is measured based on the temporally transitional cross-sectional capacitance measurement. The accurate capacitance measurement system is designed based on the stray immune electrical circuit. The temporal transition of the cross-sectional capacitance of deionized water and polystyrene particle at the adjacent electrode pair is measured at the cross-section near the microchannel outlets. The experiment result shows the cross-sectional capacitance is decreased gradually when the presence of lower dielectric material in the microchannel flow. The particles concentration is calculated in the terms of particle volume fraction based on the cross-sectional capacitance measurement. The particle volume fraction is slightly proportional with the time measurement because the particles slowly migrated and transported in the microchannel flow. The particle volume fraction at near the microchannel wall is 13.3% increased after the temporal transition at 8,000 ms and fully to concentrate at 13,000 ms at the cross-section V; near the outlets.
A planar measurement system for sediment fluxes is developed to quantify the 2D sediment transport in laboratory flumes. The system consists of image-based techniques respectively for concentration and velocity measurements. Negative images of sediment particles in motion are taken by a high speed camera using an electro-luminescence sheet as a backlight source. Sediment concentration is estimated from its pre-calibrated relationship with light attenuation through water volume containing sediment particles. Sediment transport velocities are simultaneously measured by PIV applied to the same images. Spatial distribution of sediment fluxes can be obtained by simply multiplying the sediment concentration and velocity at respective locations. Simple tests are carried out to verify the performance of the developed system. The results suggest that the system is capable of quantifying sediment fluxes with up to 15 % erorr in the range of less than 2.5 in light attenuation factor A.
A wire-mesh sensor (WMS) can acquire a void fraction distribution at a high temporal and spatial resolution and also estimate the velocity of a vertical rising flow by investigating the signal time-delay of the two WMSs. The authors propose to extend this time series analysis to estimate the multi-dimensional velocity profile via cross-correlation analysis between a point of upstream WMS and multiple points downstream. Moreover, bubbles behave in various ways according to size, which is used to classify them into certain groups.
In this study, SPH (Smoothed Particle Hydrodynamics) simulations of binary droplets collision in free space were carried out including effects of surface tension formulated based on Cahn-Hilliard free energy theory, and the effects of the differences in density and surface tension of fluids on the collision behavior were investigated. The spatial gradient appropriate for dealing with particles with different densities was derived by using modified discrete equations. The simulation result of head-on collision between identical micrometer-scale droplets corresponded well to the experimental result. Upon collision of droplets with different surface tension coefficients, the liquid-liquid interface line at the free surface moved toward the liquid with higher surface tension coefficient immediately after the contact due to the Marangoni effect, and then the liquid with lower surface tension coefficient was covered with that with higher one.
Direct numerical simulations are conducted for the propagation of pressure waves in a bubbly liquid by taking the compressibility of both gas and liquid phases into account. In the simulation, the interaction of multiple in-line bubbles with an incident shock wave in a channel is computed with the improved ghost fluid method. When the collapse time of each bubble (Tb) is nearly equal to the propagation time of the incident shock wave that passes through each bubble (Ts) (i.e., when the pressure amplitude of the incident shock wave is sufficiently high), the shock waves generated from the upstream collapsing bubbles affect strongly the motion of downstream bubbles. On the other hand, if Ts/Tb <<1, the pressure wave associated with the bubble oscillations propagates downstream after the incident shock wave passes through the bubbles. The propagation speeds for various void fractions obtained from the direct simulation are compared with those defined for isothermal homogeneous mixture. The results show that both agree with each other with less than 10 %.
In order to realize CO2 ocean sequestration, it is important to reveal the knowledge of CO2 hydrate film thickness which forms on the interface between CO2 and water. However, knowledge of CO2 hydrate film thickness under various parameter conditions such as temperature, surrounding fluid velocity on the film, and elapsed time from hydrate formation is little. The objective of the present study is to investigate the influence of elapsed time from CO2 hydrate formation. CO2 hydrate film thickness is measured under differential fluid velocity conditions by laser interference method in time series. And CO2 hydrate film thickness is estimated based on hydrate film formation and decomposition model. As the result, the measured CO2 hydrate film thickness increase under no fluid velocity condition in time series and CO2 hydrate film thickness is maintained constant under fluid velocity condition in time series. The estimated CO2 hydrate film thickness show the same tendency of measured CO2 hydrate film thickness.
Recent increase in the size of space platforms requires the management of larger amount of waste heat under high heat flux conditions and the transportation of it along a long distance to the radiator. Flow boiling applied to the thermal management system in space attracts much attention as promising means to realize high-performance heat transfer and transport. However, gravity effects on the two-phase flow phenomena and corresponding heat transfer characteristics have not yet been clarified in detail. In the present paper, flow boiling heat transfer characteristics are investigated in the microgravity environment by the parabolic flight experiments.
Numerical simulation tool based on commercialized code STAR-CD was developed to analyze thermo-fluid behavior in liquid storage tank for liquid rocket propulsion system. In this tool, liquid and gas phase were separately calculated since two phase flow calculation usually requires enormous CPU power to obtain solution for reasonable time. This paper describes thermo-fluid behavior in gas phase. Experiment by using subscale tank and LN2 as a simulated liquid was conducted for the purpose of evaluating results by numerical calculations. It was shown that history of temperature distributions in gas phase and solid walls agreed well with experimental ones. In addition of this, accumulated consumed mass of gas helium could be estimated through this simulation tool within about 2% difference from measured value.
The innovative technology is proposed for micro particle mixing and transportation in a plasma tube using electrostatic force coupled with plasma actuation on the inner surface of a tube. This Plasma tube (Dielectric Barrier Discharge tube) is composed of a pair of spiral electrodes both inside and outside of the tube for the generation of DBD. The fundamental characteristics of a plasma tube such as discharge characteristics and induced flow velocity were clarified experimentally. Furthermore, three dimensional electrical potential distribution as well as particle trajectory inside the plasma tube was shown by computational simulation.
In this study, the laser tomography method was applied to measure the void fraction distribution of the water jet in radial direction and axial direction. The continuous laser beam emitted by Nd:YAG green laser (wavelength=532nm, beam diameter=0.25mm) is introduced into the water jet in normal to the flow direction. The laser beam is emitted toward the photo detector, which is located at the opposite side. The photo detector can detect the transmissivity of the laser beam passing across the cross section of the jet. By detecting the laser at the photo detector, we can get the transmissivity distribution of the jet in radial direction. By using this measured transmissivity distribution data, we can reconstruct the void fraction distribution inside of the jet by using the tomography method. For the image reconstruction, we applied ML-EM method (Maximum Likelihood — Expectation Maximization) and assumed that the water jet droplet follows some specific distribution. By using these techniques, the void fraction distribution of the extreme high speed water jet can be successfully measured.
Growth characteristics of instability waves of a planer liquid sheet formed with surfactant solution have been clarified. It has been widely reported that a liquid sheet jet oscillates in sinuous shape because of Kelvin-Helmholtz instability and the growth rate of the instability wave depends on surface tension. Therefore, if the liquid is surfactant solution, the growth characteristics can depend on time-dependent surface tension referred to as dynamic surface tension, which originates from time delay required for adsorption of surfactant molecules to newly formed surface. In this study, the wave growth affected by the dynamic surface tension has been investigated as follows. At first, the relationship between the growth rate of the wave and time-invariant surface tension was determined using ethanol aqueous solutions with different concentrations. Secondly, the dynamic surface tension of the surfactant solution was measured, and surface tension varying with the distance from the nozzle was estimated. Thirdly, spatially varying amplitude of the wave was calculated from the results of above two steps. Finally, the calculated wave amplitude was compared with measured amplitude of the surfactant solution. The good agreement between calculated amplitude and experimental one proved that growth characteristic for surfactant solution depends on dynamic surface tension.