Effects of Dissolved Gas on Unsteady Cavitating Flow Around a Clark Y-11.7% Hydrofoil

In the present study, the effects of dissolved gas content on the unsteady cavitating flow around a Clark Y-11.7% hydrofoil are investigated in a cavitation tunnel. Lift and drag forces in various cavitating conditions are directly measured by strain gauges attached on the cantilever supporting the hydrofoil. In addition, the cavitating flow is filmed from the top and the side simultaneously using two high speed video cameras. The high (roughly 6–8ppm) and low (1–2ppm) DO conditions are examined to obtain the qualitative tendencies of the effects of dissolved gas on unsteady cavitation behavior and lift/drag characteristics. It is found that that the relationship between the cavitation behavior and the lift/drag fluctuations does not qualitatively differ in the two different DO conditions, while the amplitude is slightly larger in the low DO condition. At transitional cavity oscillation, in the both DO conditions, the lift/drag coefficients increase during the growth stage of sheet/bubble cavities on the hydrofoil and they decrease when the developed super-cavity disappears. Moreover, it seems that the amplitude of the lift/drag forces in the low DO condition is larger than in the high DO condition but the frequency of lift force fluctuation is not very different.

Pressure and Shock Waves in Bubbly Liquids

It is well known that sound propagation in liquid media is strongly affected by the presence of gas bubbles that interact with sound and in turn affect the medium. An explicit form of a wave equation in a bubbly liquid medium was obtained in this study. Using the linearized wave equation and the Keller-Miksis equation for bubble wall motion, a dispersion relation for the linear pressure wave propagation in bubbly liquids was obtained. It was found that attenuation of the waves in bubbly liquid occurs due to the viscosity and the heat transfer from/to the bubble. In particular, at the lower frequency region, the thermal diffusion has a considerable affect on the frequency-dependent attenuation coefficients. The phase velocity and the attenuation coefficient obtained from the dispersion relation are in good agreement with the observed values in all sound frequency ranges from kHz to MHz. Shock wave propagation in bubbly mixtures was also considered with the solution of the wave equation, whose particular solution represents the interaction between bubbles. The calculated pressure profiles are in close agreement with those obtained in shock tube experiments for a uniform bubbly flow. Heat exchange between the gas bubbles and the liquid and the interaction between bubbles were found to be very important factor to affect the relaxation oscillation behind the the shock front.

Flow Visualization of Pipe Flows Obtained by MR Imaginary

In this study, magnetic resonance imaging (MRI), a noninvasive medical diagnostic imaging technique, was evaluated as a noncontact measurement method for fluid machinery. In this report, various simple flow fields are investigated, and a labeled water mass is tracked and visualized in two-dimensional images by the time–spatial labeling inversion pulse (time-SLIP) method. In this article, steady and pulsating pipe flows in a straight tube and in abruptly contracting and expanding channels were tested and compared with particle image velocimetry measurements or numerical simulations to evaluate their validity. In addition, as feasibility test, a rotating water turbine and a fluidic diode with a strong swirling flow were tested to estimate this method’s applicability to fluid machines. The results indicate that the time-SLIP method of tracking labeled water mass is sufficiently accurate for use in simple fluid machinery with slow flows.

Experimental Study of Turbulent Swirling Flow in a Vertical Tube for Heat Transfer Enhancement Using Ultrasonic Velocity Profiler (UVP)

Two-phase swirling flow through a pipe is a complex turbulent flow and its prediction is still challenging. The present paper describes the experimental investigation of the air-water two phase swirling flow in vertical co-current flow. Swirling flow is induced by a twisted tape in a 20 mm inner diameter pipe. The flow is investigated using Ultrasonic Velocity Profiler (UVP), which allows the measurement of liquid and gas velocities simultaneously. Furthermore, simultaneous measurement of void fraction is performed using Wire Mesh Sensor (WMS). The experimental results reveal that swirling flow has significant impact on bubbles’ distribution. In low liquid flow rate, the average bubble velocity is fairly uniform along the radial position and void fraction increases in the near wall region. However, increasing liquid flow rate at constant gas flow rate leads to increase in void fraction in the core region, this is mainly due to drift velocity which is affected by centrifugal force. Experimental findings and parametric trends based on the effects of swirling flow are summarized and discussed.

Measurement of Three-Dimensional Particle Distribution by Tomographic Digital Holography

This paper presents a measurement of three-dimensional particle distribution by tomographic digital holography. Holographic patterns of particles (particle Reynolds number Rep=0.79, average diameter dp=73.5 μm) settling in a static flow were recorded by two high speed cameras orthogonal to each other. Cameras were calibrated by a proposed method using a calibration device and a pattern matching. The calibration accuracy of the method is within 0.34 times of particle diameter.

Ultra-High Frequency Sound Waves for Microparticle Separation

In this study, we have demonstrated a particle separation device taking advantage of the ultra-high frequency sound waves. The sound waves, in the form of surface acoustic waves, are produced by an acoustofluidic platform build on top of a piezoelectric substrate bonded to a microfluidic channel. The particles’ mixture, pumped through the microchannel, is focused using a sheath fluid. A travelling surface acoustic wave (TSAW), propagating normal to the flow, interacts with the particles and deflect them from their original path to induce size-based separation in a continuous flow. We initially started the experiment with 40 MHz TSAWs for deflecting 10 μm diameter polystyrene particles but failed. However, larger diameter particles (∼ 30 μm) were successfully deflected from their streamlines and separated from the smaller particles (∼ 10 μm) using TSAWs with 40 MHz frequency. The separation of smaller diameter particles (3, 5 and 7 μm) was also achieved using an order of magnitude higher-frequency (∼ 133 MHz) TSAWs.

Effects of Diffuser Length on Cloud Cavitation in an Axisymmetrical Convergent-Divergent Nozzle

Unsteady behavior of periodic cloud cavitation is typically observed in the field of fluid machinery under a high speed liquid flow such as a cavitating hydrofoil as well as cavitating water jet. The instability of cloud cavitation remains to be completely solved though it has been confirmed that there are two instabilities which is an intrinsic instability of cavitation and a system instability. Sato, et al. have found through previous investigations that the pressure wave at the collapse of shedding clouds can make a trigger to cause a reentrant motion. In the present study, the authors focus on a cavitating water jet to investigate the cavitation aspects in an axisymmetrical convergent-divergent nozzle and examine an unsteady behavior of cloud cavitation through high speed video observation and image analysis based on the frame difference method. Especially, the authors study the effect of nozzle divergent part (diffuser) as well as the upstream pressure effect on cloud cavitation in the nozzle. As a result the authors have found that there are two kinds in the shedding pattern and the reentrant motion pattern for cloud cavitation depending on the nozzle diffuser length.

Study of Partial Cavitation on a Plane-Convex Hydrofoil With Mesh Development by Using GMSH Free Software

Commercial programs are widely used to do unstructured and structured meshes for CFD simulations. However, grids and meshes based on free-open source software (FOSS) give to researchers and engineers the possibility to adapt and improve the meshing process for special study cases with a high Reynolds numbers, such as unsteady partial cavitating flows. In order to improve the grid qualities, the FOSS GMSH has been used to do three types of grid, unstructured hexahedral mesh, hybrid mesh and structured hexahedral mesh for the simulation of partial cavitation around a plane-convex hydrofoil. Numerical simulations have been carried out by using the FOSS OpenFOAM based on the Zwart cavitation model and the implicit large eddy simulation (ILES). The results show that the structured mesh provides the best simulating to experimental data. On the other hand, the hybrid mesh induces unreliable results at leading edge without shedding.

Effects of Contraction Ratio on Elastic Instability of Hyaluronate Solution in a Micro Channel

Flow behaviors of sodium hyaluronate (HA-Na) in water solution and in phosphate buffered saline (PBS) solution as a viscoelastic fluids in planer abrupt contraction-expansion channels has been observed in this study. Especially, the effect of the geometry of the flow path on the flow behavior was focused on. The corner vortices in the corner of the upper region in the abrupt contraction-expansion channels were also analyzed to quantify the flow characteristics. The elasticity numbers of the solution, which is affected by rheological properties of the solution and the channel geometry had a big influence on the fluidity, that is, stable or unstable, when the concentration of the solution in lower. It was concluded that such stable and unstable flows are categorized on Wissenberg-Reynolds number space.