Local Measurements in Cavitating Flow by Ultra-Fast X-Ray Imaging

The present paper presents an experimental method to measure velocity fields in a cavitating flow. Dynamics of the liquid phase and of the bubbles are both investigated. The measurements are based on ultra fast X-ray imaging performed at the APS (Advanced Photon Source) of the Argonne National Laboratory. This is collaboration between research teams devoted to fluid mechanics (LML laboratory, Laboratory for water and turbine machines) and experts in X-ray imaging (French atomic commission, Argonne National Laboratory). The experimental device consists of a millimetric Venturi test section associated with a transportable hydraulic loop. Various configurations of velocity, pressure, and temperature have been investigated. This first paper focuses on the experimental equipment and process, and also the description of the image processing which is performed to analyze the results and obtain the velocity fields of both phases within the cavitating areas. Promising preliminary results are also presented.

A New Cavitation Model Based on Evaporation and Condensation Theory

Cavitation is a phenomenon which occurs where the local pressure falls off under the vapor pressure. Over the past few years, numerical simulation models for cavitation have been developed significantly in order to investigate the mechanism of cavitation. In the paper, A local homogeneous cavitation model based on the theory of evaporation and condensation has been deduced, which is used to describe the phase change between water and vapor. The RNG k–ε turbulence model is used to simulate the turbulent flow and the finite volume method is employed to discrete the governing equations. The effects of surface tension of water, pressure fluctuations and non-condensable gases are included in the mass transfer cavitation model. Also in order to neglect the effects of the quantities such as the bubble number and bubble diameter, which is difficult to measure, the relations between the aerodynamic drag and surface tension forces is used to describe the bubble diameter. In order to evaluate the new cavitation model, the two phase cavitation flows around a NACA0015 hydrofoil at different attack angle and different cavitation number are simulated by the new cavitation model, and compared with references, which showed good agreement with the experiments.

Dielectrophoretic Mixing With Novel Electrode Geometry

Electrokinetic mixing of analytes at micro-scale is important in several biochemical applications like cell activation, DNA hybridization, protein folding, immunoassays and enzyme reactions. This paper deals with the modeling and numerical simulation of micromixing of two different types of colloidal suspensions based on principle of dielectrophoresis (DEP). A mathematical model is developed based on Laplace, Navier-Stokes, and convection-diffusion-migration equations to calculate electric field, velocity, and concentration distributions, respectively. Mixing of two colloidal suspensions is simulated in a three-dimensional computational domain using finite element analysis considering dielectrophoretic, gravitational and convective (advective)–diffusive forces. Phase shifted AC signal is applied to the alternating electrodes for achieving the mixing of two different colloidal suspensions. The results indicate that the electric field and DEP forces are maximum at the edges of the electrodes and become minimum elsewhere. As compared to curved edges, straight edges of electrodes have lower electric field and DEP forces. The results also indicate that DEP force decays exponentially along the height of the channel. The effect of DEP forces on the concentration profile is studied. It is observed that, the concentration of colloidal particles at the electrodes edges is very less compared to elsewhere. Mixing of two colloidal suspensions due to diffusion is observed at the interface of the two suspensions. The improvement in mixing after applying the repulsive DEP forces on the colloidal suspension is observed. Most of the mixing takes place across the slant edges of the triangular electrodes. The effect of electrode pairs and the mixing length on degree of mixing efficiency are also observed.

Impact and Outcomes of a Flow Visualization Course

For the past six years, a course on flow visualization has been offered to mixed teams of graduate and undergraduate engineering and fine arts photography students at the University of Colorado. The course has significant technical content on flow visualization and photographic techniques, and includes some emphasis on documentation and the interpretation of results, particularly with respect to atmospheric dynamics as revealed by clouds. What makes this course unusual is the emphasis on the production of images for aesthetic purposes: for art. While a number of art/science collaborations are growing worldwide, both in professional and academic communities, typically scientists are expected to contribute technical support while artists produce art. A particularly unusual aspect of this course is that all students are expected to demonstrate both aesthetic sensibility and scientific discipline. Another is that students are not constrained to study specific phenomena or use specific techniques; instead, creativity is required. A major outcome from this course is a series of stunning images. In addition, anecdotal evidence suggests that this course has a lasting impact on students’ perception of fluid physics, which can be contrasted to the effect of traditional introductory fluids courses. This raises the question of whether this impact is significant with respect to students’ understanding and appreciation of fluid mechanics, and if so, what aspect of the flow visualization course is most important? A survey instrument is being designed to quantify whether students’ awareness of fluid mechanics in the world around them changes when they take these courses and if students’ attitudes towards fluids is changed when they take these courses.

Ultrasonically Driven Cavitating Atomizer: Prototype Fabrication and Characterization

A new device, ultrasonic cavitating atomizer (UCA), has been developed that uses ultrasonically driven cavitation to produce fine droplets. In the UCA the role of cavitation is explicitly configured to enhance the breakup of the liquid jet exiting the nozzle into fine droplets; the pressure modulation also assists the breakup process. The experimental study involves the fabrication of a prototype and the building of an experimental rig to test the prototype using water as the working fluid. The parameters tested include liquid injection pressure, horn tip frequency and liquid flow rate. The result shows improvement in the atomization of water with the application of ultrasonic cavitation.

Modeling and Analysis of a Flow Divider Valve

Flow divider valves are often used in hydraulic systems to synchronize actuators. The basic structure of the flow divider valve is by incorporating a compensating spool to maintain equal pressure drops across metering orifices. Ideally, flow divider valve splits a single source flow into two parts under a specified ratio regardless of load conditions. In practical applications, any change in load pressure will cause force imbalance on the compensating spool, which will alter the flow rates through the metering orifices and affect the control accuracy consequently. In this study, the steady and dynamic performances of a flow divider valve are simulated numerically by solving the characteristic equations. The parameters studied in this research are centering spring constant, compensating spool mass, and metering orifice area. The simulation results show that flow force is the key factor to affect the flow division accuracy. Flow division error increases with increasing the load pressure differential, centering spring constant, and metering orifice area. Even though decreasing the spring force or the metering orifice area can reduce division error, the spring force still needs to be large enough to overcome the spool static friction and the orifice area cannot be too small to lose energy efficiency. Dynamic division error increases with increasing load pressure differential and metering orifice area but with decreasing spool mass. Increasing load pressure differential, spool mass, and metering orifice area will enhance the oscillatory tendency and increase the valve settling time. The centering spring constant has no obvious effect on the valve dynamic response.

Design of a Propeller Fan Using 3-D Inverse Design Method and CFD for High Efficiency and Low Aerodynamic Noise

Three-Dimensional Inverse Design Method, where the 3-D blade profile is designed for a specified blade loading distribution, has been applied for designing a propeller fan rotor with high efficiency and low noise. A variety of the blade loading distributions (pressure jump across the blade), vortex pattern (forced vortex, free vortex, and compound vortex) and the stacking conditions (sweep angles) were specified and the corresponding 3-D blade configurations were obtained. Among the 22 different designs, 14 propeller fan rotors including the reproduced baseline fan were manufactured by a rapid prototyping based on a selective laser sintering system (SLS) and tested. It was confirmed experimentally that the best design achieved about 5.7 points improvement in the peak total-to-static efficiency and the 2.6dB(A) reduction in aerodynamic noise. The flow mechanisms leading to the higher efficiency and lower aerodynamic noise were discussed based on experiments and the RANS steady flow simulations. Based on these investigations, design guidelines for the inverse design of propeller fan rotors with higher efficiency and lower aerodynamic noise were proposed.

Pressure Gage Measurements in a Dry to Wet Environment

Pressure gages are used in many fluid measurements applications. One such application is the measurement of wave impact pressures on structures. This application poses a unique problem of measuring pressures in a “wet to dry” environment. Often there is a thermal drift component in the pressure readings that makes it difficult to extract the actual pressure rise due to wave loading. These types of measurements also require high response rates to measure the detail of the short duration impacts, usually on the order of one to twenty kilohertz. Several bench tests were carried out in at the Naval Surface Warfare Center, Carderock Division, in 2008 to investigate various types of gages to find a robust gage that could withstand this type of application. Three different gages were used in this investigation. The first sensor (gage 1) is a dynamics general purpose ICP (integrated circuit piezoelectric) pressure sensor, capable of making very high frequency dynamic pressure measurements, rated to 200 psi. The second sensor (gage 2) is a voltage compensated, media isolated piezoresistive sensor, rated to 15 psi. This gage had a pressure port which was filled with water during testing to eliminate air compression effects. The third sensor (gage 3) is a semiconductor pressure gage rated to 25 psi (172.4 kPa). A water “drop” test setup was constructed of 2 inch (5.1 cm) PVC pipe. The pressure gages were mounted to the bottom of the setup facing up, and the pipe was filled from the top, with a quick acting gate valve located 2 feet (0.61 m) from the pressure sensors. Once the pipe was filled with the desired amount of water, the gate valve was opened as quickly as possible, and the impact force was measured. Vent pipes were mounted to a “cross” fitting in the vertical pipe which allowed for the air to escape. Several water “drop” tests were performed with this setup. From these tests, the thermal drift of gage 1 is evident. Gage 3 exhibits similar behavior. Gage 2 captures the water pressure impact, and then returns to a small positive static pressure as a result of the water that is sitting above it. Of the three sensors, gage 2 appears to be the most temperature stable.

Investigation of the Fluid Flow in a Rotor-Stator Cavity With Inward Through-Flow

The present paper covers fluid flow in rotor-stator cavities with inward through-flow. First, a general introduction into the physics of the cavity boundary layer flow is given. The structure of the flow is very complex and depends on different dimensionless parameters. For practical applications, simple and robust calculation procedures are crucial for design purposes. Two basic modelling approaches are compared (3 layer model of Kurokawa [14] and “one layer” approach of Mo¨hring [17]) with experimental data from the literature. The flow models are classified in context of the simplified equations of motion by emphasizing the main assumptions and simplifications in their derivation. Further on, for the one layer model, the use of the logarithmic law for the velocity distribution close to the wall is proposed instead of the classic 1/7 power law. The modified flow model is validated against experimental data for different parameter combinations, yielding better agreement for moderate inlet rotation. Finally numerical simulations have been performed in order to investigate the discrepancies between measured and calculated core rotation distributions for strong inlet swirl. It is supposed that the assumption of radial equilibrium in the core region is not necessarily appropriate for evaluation of the core rotation. Further on, it is clarified in which situations the tangential velocity component of the absolute velocity at the impeller outlet can be used as a boundary condition for the flow model.

Free-Surface Measurements in a Tow Tank Using LiDAR

Light Detection and Ranging, or LiDAR, is a remote sensing technique that can be utilized to collect topographic data. These systems have been used extensively to measure open ocean and ship generated waves. Recently LiDAR systems have been used to measure the transom wave of the R/V Athena I and ambient ocean waves. This work has primarily focused on providing the time averaged, and spectral content of the wave field, by scanning the laser to measure wave profiles evolving in time. This paper describes recent efforts to utilize LIDAR systems to measure free-surface elevations in laboratory tow tanks. LiDAR measurements are limited to the white-water breaking regions of the flow, due to the limited strength of the signal return from non-breaking regions. In extending LiDAR measurements to a laboratory tow tank environment the lack of surface roughness and hence the lack of surface light scatterers needed to be addressed. A number of laboratory measurement applications will be described including a tow tank measurement similar to the R/V Athena I effort, and also measurement of regular and irregular breaking waves.