Experimental Investigation of Boundary Layer Instabilities on the Free Surface of Non-Turbulent Jet

Shear instabilities induced by the relaxation of laminar boundary layer at the free surface of a high speed liquid jet are investigated experimentally. Physical insights into these instabilities and the resulting capillary wave growth are gained by performing non-intrusive measurements of flow structure in the direct vicinity of the surface. The experimental results are a combination of surface visualization, planar laser induced fluorescence (PLIF), particle image velocimetry (PIV), and particle tracking velocimetry (PTV). They suggest that 2D spanwise vortices in the shear layer play a major role in these instabilities by triggering 2D waves on the free surface as predicted by linear stability analysis. These vortices, however, are found to travel at a different speed than the capillary waves they initially created resulting in interference with the waves and wave growth. A new experimental facility was built; it consists of a 20.3 × 146.mm rectangular water wall jet with Reynolds number based on channel depth between 3.13 × 104 to 1.65 × 105 and 115. to 264. based on boundary layer momentum thickness.

A Study of Cavitation Flow in a Centrifugal Pump at Part Load Conditions Based on Numerical Analysis

In order to predict cavitation performance of the centrifugal pump, including cavitating structures and vapour volume at the blade suction side, as well as its relationship with the backflow in the impeller eye, a 3D numerical simulation of detailed steady and unsteady cavitating flow was applied to reproduce its inner flow fields at part load conditions (0.5Qd and 0.4Qd). The comparisons of cavitation characteristics of the current centrifugal pump at an on-design point (1.0Qd) and a high flow rate (1.2Qd) were achieved as well. In addition, Frequency analysis of pressure fluctuations at the blade passages and the inlet pipe were also obtained during cavitation for a flow coefficient of 50%. The results further show that successive blade cavitation patterns and the creeping cavitation number dropping appear for a wide range of flow rates when the inlet total pressure decreases from cavitation inception to the breakdown of the centrifugal pump, as is quite different from that when cavitation occurs at 1.0Qd or 1.2Qd. Unbalanced attached cavities on the blade suction side were also observed at 0.5Qd. Meanwhile, the unsteady behaviour of cavities attached to the blade suction side and cavitation number dropping depend on the flow rate and cavitation number. Another significant characteristic of the phenomenon is that all the domain frequencies in blade passages and inlet pipe at part load conditions are 0.048Hz∼48.285Hz, which is typically lower than the shaft rotational frequency of the model centrifugal pump.

Predicting Bubble Migration due to Bjerknes Force in a Complex 3D Geometry: Numerical and Experimental Results

While the Bjerknes force is not the only force experienced by a bubble subjected to an acoustic field; studies of bubble translation in non-flowing fluid have identified Bjerknes force as being the most influential. Therefore, Bjerknes force can be used to trap bubbles in predefined locations of maximum and minimum absolute pressure. Specifically challenging is to determine these locations in complex geometries because direct measurement of the acoustic pressure for the whole system is generally not possible. The objective of this research is to numerically predict Bjerknes force effect on bubble migration and accumulation in a complex 3D geometry that includes piezoelectric materials, elastic materials and a fluid media. A numerical solution of the acoustic pressure field was obtained for this geometry, valid in the range of small pressure oscillations. Additionally, using the linearized Rayleigh-Plesset equation, which gives the volumetric oscillations of a bubble subjected to an acoustic field, the Bjerknes force was numerically computed. By knowing the Bjerknes force, a bubble migration pattern upon entering the system was predicted. A CMOS high speed camera was used to experimentally monitor bubble multimode excitation and bubble response to a stationary pressure field validating our numerical results. Results are presented for experiments conducted for a 1mm bubble diameter with acoustic fields ranging from 7 to 10 kHz which correspond to values of the structure and/or the bubble’s resonant frequency.

Measurements of Gas Bubble Size Distributions in Flowing Liquid Mercury

Pressure waves created in liquid mercury pulsed spallation targets have been shown to induce cavitation damage on the target container. One way to mitigate such damage would be to absorb the pressure pulse energy into a dispersed population of small bubbles, however, measuring such a population in mercury is difficult since it is opaque and the mercury is involved in a turbulent flow. Ultrasonic measurements have been attempted on these types of flows, but the flow noise can interfere with the measurement, and the results are unverifiable and often unrealistic. Recently, a flow loop was built and operated at Oak Ridge National Labarotory to assess the capability of various bubbler designs to deliver an adequate population of bubbles to mitigate cavitation damage. The invented diagnostic technique involves flowing the mercury with entrained gas bubbles in a steady state through a horizontal piping section with a glass-window observation port located on the top. The mercury flow is then suddenly stopped and the bubbles are allowed to settle on the glass due to buoyancy. Using a bright-field illumination and a high-speed camera, the arriving bubbles are detected and counted, and then the images can be processed to determine the bubble populations. After using this technique to collect data on each bubbler, bubble size distributions were built for the purpose of quantifying bubbler performance, allowing the selection of the best bubbler options. This paper presents the novel procedure, photographic technique, sample visual results and some example bubble size distributions. The best bubbler options were subsequently used in proton beam irradiation tests performed at the Los Alamos National Laboratory. The cavitation damage results from the irradiated test plates in contact with the mercury are available for correlation with the bubble populations. The most effective mitigating population can now be designed into prototypical geometries for implementation into an actual SNS target.

Estimation of Uncertainty Bounds From Cross Correlation Peak Ratio for Individual PIV Measurements

Numerous studies have established firmly that particle image velocimetry (PIV) is a robust method for non-invasive, quantitative measurements of fluid velocity, and that when carefully conducted, typical measurements can accurately detect displacements in digital images with a resolution well below a single pixel (in some cases well below a hundredth of a pixel). However, previously these estimates have only been able to provide guidance on the expected error for an average measurement under specific image quality and flow conditions. This paper demonstrates a new method for estimating the uncertainty bounds to within a given confidence interval for a specific, individual measurement. Here, the ratio of primary to secondary peak heights in a phase-only generalized cross-correlation is shown to correlate strongly with the range of observed error values for a given measurement, regardless of flow condition or image quality. Using an analytical model of the relationship derived from synthetic data sets, the uncertainty bounds at a 95% confidence interval are then computed for several artificial and experimental flow fields, and the resulting errors are shown to match closely to the predicted uncertainties. While this method stops short of being able to predict the true error for a given measurement, knowledge of the uncertainty level for a PIV experiment should provide great benefits when applying the results of PIV analysis to engineering design studies and CFD (computational fluid dynamics) validation efforts.

Experimental Investigation of Falling Liquid Film in Vertical Downward Two-Phase Pipe Flow

In this research the hydrodynamics of falling liquid film in a vertical downward two-phase flow (liquid-gas) is experimentally studied. The 4 inch clear PVC test section is 6.1 meters long, with a length to diameter ratio (L/D) of 64. The fluids utilized are compressed air, water, Conosol mineral oil (light oil) and Drake mineral oil (heavy oil). The superficial liquid velocities tested range from 12 to 72 cm/s while the superficial gas velocities range from 0.2 to 29 cm/s. The vertical facility is equipped with the state-of-the-art instrumentation for two-phase flow measurements, the capacitance Wire-Mesh Sensor (WMS), allowing two-phase flow measurements with conducting and non conducting fluids. Experimental results show that the liquid film thickness has a quasi-linear relationship with the superficial liquid velocity for the air-water case. For the air-oil cases, at superficial liquid velocities higher than 50 cm/s, the liquid film thickness trend is affected by the liquid droplet entrainment. Furthermore, it was found that the liquid droplet entrainment increases as the superficial liquid velocity increases or the surface tension decreases. Details of the liquid droplets traveling in the gas core, wave formation, wave breakup and film thickness evolution are observed in the WMS phase reconstruction.

Investigation of Relative Importance of Some Error Sources in Particle Image Velocimetry

Several error sources are analyzed for 2-component PIV, including: calibration, magnification variation, perspective, resolution, and the correlation algorithm noise floor. Several of these error sources are compared with previously published estimates. New experimental data and methods for measuring the contribution of each source to velocity uncertainty are presented. The calibration uncertainty on the velocity measurement was found to be small (so long as reasonable care is taken in the calibration) and independent of the calibration target for a 2-component PIV setup. The perspective error and magnification variation were both calculated and experimentally found to be small. The light sheet thickness only has an effect when the thickness is greater than 1% of the distance from the light sheet to the camera lens plane. The spatial resolution may be so coarse as to not capture the smaller eddies in the flow, thus attenuating the measured fluctuations. The noise floor was found to contribute significantly to the uncertainty in the velocity measurements in sub-pixel displacement.

Stability of an Evaporating Liquid Film Under Nonequilibrium Conditions With Variable Gravity

An evolution equation describing the dynamics of an evaporating liquid film has previously been developed from the governing equations of fluid dynamics after the application of the lubrication approximation and the choice of a viscous time scale. The authors have solved the evaporating liquid film evolution equation with a validated numeric program. Different mechanical boundary conditions were successfully applied and their effect on the film dynamics was examined. The evolution equation has also been modified to include buoyancy driven instabilities. This paper outlines a linear stability analysis that was performed on the time dependent, evaporating liquid film evolution equation. The effect of the evaporation rate, departure from equilibrium at the interface and variable gravity is examined by solving the equation as an initial value problem.

Experimental Analysis of a Bathtub Vortex in a Simplified Cowl Box of Automotive Vehicles

On automotive vehicles, the cowl box is a volume located at the bottom of the windshield, under the cowl top grille. It provides external fresh air to the HVAC (Heating, Ventilating and Air Conditioning) unit and it is used to collect water coming from the windshield under rain conditions. This box is designed as a tranquillisation chamber to segregate water from air and avoid the ingress of rainwater into the HVAC unit. However, as the area is awkward to access with measuring devices, our knowledge about the physics of flow in the cowl box is limited. The present work aims to advance our knowledge through experimental work on the air/water flow in a simplified cowl box in order to optimize the box size and improve numerical models. This paper will focus on the analysis of the bathtub vortex, which is potentially responsible for insufficient draining of the water collected in the cowl box. The experimental set-up consists of a Plexiglas parallelepiped representing a simplified cowl box with top cowl grille, HVAC inlet and drain. A blower generates airflow through the HVAC inlet. A water sheet, with controlled flow rate, is created on an inclined plane representing the windshield. Velocity measurements of all components are obtained by PIV (Particle Image Velocimetry) in the liquid phase and the surface level is recorded by a capacitance probe near the drain. Moreover, contour detection of the vortex core is achieved using a high-speed camera. Results show a relationship between the pressure loss generated by the airflow in the cowl box, the water level and the vortex structure. The modification of the vortex structure as well as the modification of velocity components near the air core are visible only in transient stages. These experimental results give us today some insight to understand the physical phenomena occurring in the cowl box.