Water Velocity Vector Research Articles

Heat transfer, dynamic characteristics and disturbance to water of bubbles generated on vertical surfaces featuring extreme wettability patterns

Surfaces with mixed wettability patterns are one of the potential ways to improve energy conversion efficiency of industrial equipment. It can significantly change the boiling heat transfer proformance of the surface by affecting the bubble dynamics, as well as the flow field properties. Accordingly, this work simultaneously investigates the thermodynamic, bubble dynamics, and water velocity vector field of the bubbles generated on the designed surfaces, through a combination of thermal parameter measurements, high-speed photography, and particle image velocimetry (PIV). The surfaces are designed in six types, and coated by alternating superhydrophilic materials (contact angle of 9.8°) and superhydrophobic materials (contact angle of 154.8°) at intervals with widths of 5 mm, 10 mm, and 15 mm, respectively. The results showed that the different wettability of the pattern at the bubble generation leads to different bubble formation modes, which in turn affects the heat transfer and the initial dynamics of the bubble. The different width of the pattern affects the bubble floatation and water motion. After comparison, it is found that the bubbles have the best heat transfer proformance when they are generated on a superhydrophilic pattern with a width of 15 mm. However, regardless of pattern wettability and width, the bubble frequency, size, and velocity, as well as the water vorticity and velocity, increase with decreasing supercooling, and first increase and then decrease with increasing heat flux. Based on the experimental results, the correlations of bubble velocity and water velocity are modified by introducing the influence factor of wettability patterns. It is tested that it is not only adaptable to the data of this experiment, but can also be generalized to the data of other works.

Analysis of Sub-Atmospheric Pressures during Emptying of an Irregular Pipeline without an Air Valve Using a 2D CFD Model

Studying sub-atmospheric pressure patterns in emptying pipeline systems is crucial because these processes could cause collapses depending on the installation conditions (the underground pipe covering height, type, fill, and pipeline stiffness class). Pipeline studies have focused more on filling than on emptying processes. This study presents an analysis of the following variables: air pocket pressure, water velocity, and water column length during the emptying of an irregular pipeline without an air valve by two-dimensional computational fluid dynamics (2D CFD) model simulation using the software OpenFOAM. The mathematical model predicts the experimental values of the study variables. Water velocity vectors are also analysed within the experimental facility, assessing the sensitivity of the drain valve to different openings and changes in water column length during the hydraulic phenomenon.

Side wall water outlet design for silicon wafer wet cleaning bath

For the wet cleaning of 300-mm-diameter semiconductor silicon wafers, an additional water outlet in the wet cleaning bath was designed, particularly focusing on the pinhole arrangements on the side walls by the concept of the overlapping water removal regions. The water velocity vectors and the virtual particle paths based on numerical calculations showed that the water injected from two bottom inlets went up through the narrow space between the wafers, then horizontally flowed along the water surface and exited the bath. The calculations additionally indicated that the water recirculation in the upper half region of the bath could be reduced depending on the side wall pinhole arrangement. Simultaneously, the percent of removed particles was increased. Based on the experiments, the water in the bath remained colorless after the continuous supply of a blue-colored ink tracer for 90 s. For smoothly transporting away the contaminants, the side wall outlet design contributed to the simple water motions with less recirculation.

EXPERIMENTAL AND MODELING OF FLOW OVER LABYRINTH AND PLAIN STEPPED FALLS

The characteristics of flow over plain and labyrinth stepped falls is investigated experimentally and simulated numerically using commercial package ANSYS CFX. The aim is to verify the numerical model by assessing its accuracy and dependence in modeling the flow over this kind of hydraulic structures. Eight physical models of rectangular labyrinth stepped fall cycles with general slope (1V:1H and 1V:2H), different step numbers (5 and 10) and two different cycle widths (0.06 and 0.1 m) were tested and simulated in laboratory flume. For the purpose of comparison four models of plain stepped falls are constructed with the same number of steps and slopes as the labyrinth ones, they were tested and simulated. The simulation is based on the RNG k-e turbulence model, three dimensional volume of fluid method (VOF) and incompressible flows. To verify the numerical models, all experimental data including water surface profile, hydraulic head and total calculated energy dissipation were compared with the corresponding results predicted by the numerical model. The comparison showed good agreement between experimental and numerical results via applying statistical tests. The new labyrinth stepped fall model was more effective for dissipating the relative energy as compared with plain stepped falls. The results showed that with decreasing number of steps, downstream slope and increasing the length magnification ratio, the relative energy dissipation is increased. The numerical results illustrated that the local direction of flow depends on the depth of water on the steps and the width of the labyrinth cycle. Also it was observed that the local flow directions, which are occurred on streamlines and contours, are dependent on impingement angle of water velocity vectors which varies between 74 and 62.5 degree.

CFD simulation of multiphase (liquid–solid–gas) flow in an airlift column photobioreactor

A 2D computational fluid dynamics simulation was carried out using a multiphase flow model with an Eulerian–Eulerian approach for a microalgae culture in an airlift column photobioreactor. Simulation was performed for a $$0.0625\,l/l_{\mathrm{culture}}\cdot \hbox {min}$$ inlet airflow. Air, water and microalgae velocity contours showed less gas phase present in the downcomer than in the riser, suggesting the necessity of vigorous mixing in the ascendant portion if homogeneous water and solid flow is to be achieved. Air velocity is smaller in the downcomer (shorter velocity vectors) than in the riser. Water velocity vectors point always in the expected direction, down in the downcomer and up in the riser. Microalgae paths, perhaps due to the small size of the microorganisms, follow the water velocity vectors. As there are fewer hydraulic restrictions to the liquid phase in the riser, a large amount of energy is dissipated by gas–liquid interactions. In the downcomer region, the gas phase is almost nonexistent, and bubble collisions are almost nonexistent as well. A quasi-stagnation zone was found at the lower section of the downcomer, showing that the design requires improvement. Finally, the turbulent kinetic energy is larger at the top and middle region of the riser; meanwhile, it is lower at the downcomer. Similar results were observed for the energy dissipation rate.

CFD simulation of multiphase flow in an airlift column photobioreactor

<p>A Computational Fluid Dynamics (CFD) simulation using <em>CFX, ANSYS 11.0</em>, has been carried out using a multiphase flow model with an Eulerian-Eulerian approach for an airlift column photobioreactors (PBR). Transient simulations were performed for three inlet air flow, 2, 3 and 5 l/min. The contours for gas holdup, air and water velocity showed that the presence of gas phase (air bubbles) is lower in the downcomer but larger in the riser, which leads to require a vigorous mixing in the riser that will be sufficient for a continuous flow. For air, velocity vectors show that they are smaller in the downcomer than in the riser. Nevertheless, water velocity vectors are organized, pointing down in the downcomer and up in the riser. Water shear stress rate contours analysis showed that, shear stress rate regions are considerably larger in the riser, but lower in the downcomer. Due to fewer restrictions to the liquid phase in the riser, a large amount of energy is dissipated by gas liquid interactions. In the downcomer region, gas phase is almost inexistent, and so are the bubble collisions. Finally, the kinetic energy is larger at the top region of the riser, meanwhile is lower at the downcomer. Similar results are observed for energy dissipation rate.</p> <p> </p>

Implicit Bidiagonal Numerical Scheme for Simulation of 2D Flood Waves

This paper is concerned with a mathematical model for numerical simulation of 2D flood waves due to partial dam-break. The governing water equations are solved by an implicit bidiagonal numerical scheme, based on the MacCormack’s predictor-corrector technique. The mathematical model is used to compute 2D flood waves due to partial instantaneous symmetrical dam-break in a rectangular open channel with a rectangular cylinder barrier downstream. Results, in terms of water velocity vectors and contours of water depth, water surface, following dam-break phenomena, are investigated in the two-dimensional problems.

Probe for Measuring Groundwater Velocity at the Centimeter Scale

A novel method of measuring small-scale groundwater velocities in unconsolidated noncohesive media uses the travel time of a tracer pulse between an injection port and two detectors located on the surface of a cylindrical probe, called a point-velocity probe (PVP), as the basis for velocity estimation. The direction and magnitude of the water velocity vector were determined to within +/- 9% of magnitude and +/- 8 in direction, on average, in ten laboratory tank tests conducted with the PVP, when the velocities were between 5 and 98 cm/ day. Numerical simulations supported the accuracy of the underlying theory for interpretation of the PVP data and indicated that the technology is capable of measuring velocity at a very fine scale (0.5 cm around the circumference). The benchtop and modeling investigations indicated that the probe is moderately sensitive to the condition of the porous medium immediately next to the cylinder surface, suggesting that challenges exist for the deployment of the instrument in the field.

Water Transport in an Unsaturated Medium to Roots with Differing Local Geometries

AbstractPearl millet [Pennisetum glaucum (L.) R. Br.: syn. P. americanum (L.) Leeke] develops substantially lower predawn leaf water potentials than cowpea [Vigna unguiculata (L.) Walp.] under conditions of soil dehydration, but overall root length densities are greater in millet than in cowpea. The hypothesis was tested that, due to differences in root geometry and root length density, root systems of millet become less efficient in water uptake than those of cowpea as soil dries. Plants were grown in an artificial medium in pots or 1.15‐m tubes in a greenhouse and subjected to soil drying. Root geometries, root length densities, and water potentials of cowpea and millet were measured as the rooting medium dried. Distribution of roots was quantified by calculating a clumping ratio (total root length density divided by the root length density in the region surrounding individual root segment axes) at several depths within the profile. Overall root length densities were higher in millet than in cowpea, and root distributions differed. Millet roots were clumped locally along the root axis and globally within the profile, with the highest root length densities at the surface. Cowpea root lengths were one‐half as dense as millet along the root axis and distributed fairly uniformly throughout the profile. These data were used in two‐dimensional modeling of horizontal water transport from soil to branched segments of root axes of cowpea and millet to test the hypothesis. Simulations in which millet water potential became substantially lower than cowpea, and millet root length was twice as great as cowpea, predicted little difference in cumulative water flow to the roots of millet or cowpea during soil dehydration. This prediction is consistent with observations. Simulated water velocity vector fields indicated that millet's dense and clumped local root geometry became less efficient in water uptake under soil water deficits than cowpea's less dense and more evenly distributed root geometry because water uptake by millet was mainly at the root tips; in contrast, most of the root length of cowpea contributed to water uptake. Simulations conducted with millet root axes having different root length densities indicated that the low efficiency of millet in water uptake was due to the combined effects of local clumping and high root length density. When input parameters for roots and soil were varied, conclusions from the simulations remained the same.

Pure drift current and mass transport in coexistent waves

The analysis of a pure drift current is presented, considering energy relationships, in addition to momentum relationships, originally treated by V. W. Ekman [1905]. The turbulent viscosity, varying with depth, is related to the kinetic energy dissipation, and is found to vary exponentially with the angle of rotation of the water velocity vector. The analysis is limited to fully-developed drift current, with fully developed surface wave system. Only a small part of the observable current velocity is found to be caused by the drift current, and most of it is due to the mass transport in waves. Almost the entire energy, transferred from the wind to water, is absorbed by waves, and very small part is absorbed by the drift current. In fully developed waves, about 2/3 of the energy is transmitted by normal air pressures on the wave surface, and about 1/3 is transmitted by tangential drag and the mass transport velocity. Present analysis yields the current velocity at the surface, and the form of the energy dissipation with depth, in reasonable agreement with available observational data.