Fluid Buoyancy Research Articles

Acoustic-gravity waves in ocean and atmosphere generated by an underwater source

Air-water interface becomes anomalously transparent, and the power flux in the wave transmitted into the air increases dramatically, when a compact sound source in water approaches the interface within a fraction of wavelength [O.A. Godin, Phys. Rev. Lett. 97, 164301 (2006)]. The anomalous transparency of the ocean-atmosphere interface has important implications for detection of underwater explosions and monitoring of compliance with the Comprehensive Nuclear Test Ban Treaty. At wave frequencies below 0.1 Hz, it becomes necessary to take gravity into account. Then fluid buoyancy and compressibility simultaneously serve as restoring forces, and mechanical perturbations in the water and in the air propagate as acoustic-gravity waves (AGW). It was previously shown [I. Fuks and O.A. Godin, Proc. OCEANS'11, MTS/IEEE, Kona, HI, Sept. 2011] that, in the case of a shallow source in an ocean of an infinite depth, a sharp peak in the power flux into air appears at frequencies close to a cutoff frequency of about 4mHz of a surface acoustic-gravity wave. In this paper, we extend these results to the ocean of a finite depth where the AGWs interact with an elastic bottom.

Characterization of Iron–Phosphate–Silicate Chemical Garden Structures

Chemical gardens form when ferrous chloride hydrate seed crystals are added or concentrated solutions are injected into solutions of sodium silicate and potassium phosphate. Various precipitation morphologies are observed depending on silicate and phosphate concentrations, including hollow plumes, bulbs, and tubes. The growth of precipitates is controlled by the internal osmotic pressure, fluid buoyancy, and membrane strength. Additionally, rapid bubble-led growth is observed when silicate concentrations are high. ESEM/EDX analysis confirms compositional gradients within the membranes, and voltage measurements across the membranes during growth show a final potential of around 150-200 mV, indicating that electrochemical gradients are maintained across the membranes as growth proceeds. The characterization of chemical gardens formed with iron, silicate, and phosphate, three important components of an early earth prebiotic hydrothermal system, can help us understand the properties of analogous structures that likely formed at submarine alkaline hydrothermal vents in the Hadean-structures offering themselves as the hatchery of life.

Modeling of Artificial Aurelia aurita Bell Deformation

AbstractRecently, there has been significant interest in developing underwater vehicles inspired by jellyfish. One of these notable efforts includes the artificial Aurelia aurita (Robojelly). The artificial A. aurita is able to swim with similar proficiency to the A. aurita species of jellyfish even though its deformation profile does not completely match the natural animal. In order to overcome this problem, we provide a systematic finite element model (FEM) to simulate the transient behavior of the artificial A. aurita vehicle utilizing bio-inspired shape memory alloy composite (BISMAC) actuators. The finite element simulation model accurately captures the hyperelastic behavior of EcoFlex (Shore hardness-0010) room temperature vulcanizing silicone by invoking a three-parameter Mooney-Rivlin model. Furthermore, the FEM incorporates experimental temperature transformation curves of shape memory alloy wires by introducing negative thermal coefficient of expansion and considers the effect of gravity and fluid buoyancy forces to accurately predict the transient deformation of the vehicle. The actual power cycle used to drive artificial A. aurita vehicle was used in the model. The overall profile error between FEM and the vehicle profile is mainly due to the difference in initial relaxed profiles.

Design and Implementation of a New Six-DOF Maglev Positioner With a Fluid Bearing

This paper proposes a new six degrees-of-freedom (6-DOF) electromagnetic precision positioner, made of a hybrid mechanism utilizing both a magnetic driving force and the uplifting force of the fluid, for which a new structure, the electromagnetic actuator, and an effective controller have been developed. The concept of the mechanism design involves not only the magnetic driving mechanism, but also the fluid buoyancy and damping properties, which help to counterbalance the weight of the platen so as to achieve a very low steady-state power consumption. The four goals of the new system design include the following: 1) to have a large range of motion (at the mm level); 2) to achieve precision positioning; 3) to design a compact but low-cost mechanism; and 4) to achieve low power consumption. In this system, there are a total of eight permanent magnets (PMs) attached to the movable carrier, and eight electromagnetic coils appropriately mounted on a fixed base. After exploring the characteristics of the magnetic forces between PMs and electromagnetic coils, the general 6-DOF dynamic model of this system is derived and analyzed. Then, because of the naturally unstable behavior and uncertainties of the underlying system, a robust adaptive sliding-mode controller is proposed to guarantee system stability for both regulation and tracking tasks. Finally, extensive experiments have been conducted to demonstrate the performance of the proposed system. The experimental results show that the range of motion is 3 mm × 3 mm × 4 mm, and the tracking error in each axis is kept to within 10 μm, which reaches the limit of our optical sensors. These experimental results demonstrate satisfactory performance of the positioner in terms of theoretical analysis and experimental results.

Inverse modeling of velocities and inferred cause of overwash that emplaced inland fields of boulders at Anegada, British Virgin Islands

A combination of numeric hydrodynamic models, a large-clast inverse sediment-transport model, and extensive field measurements were used to discriminate between a tsunami and a storm striking Anegada, BVI a few centuries ago. In total, 161 cobbles and boulders were measured ranging from 1.5 to 830 kg at distances of up to 1 km from the shoreline and 2 km from the crest of a fringing coral reef. Transported clasts are composed of low porosity limestone and were derived from outcrops in the low lying interior of Anegada. Estimates of the near-bed flow velocities required to transport the observed boulders were calculated using a simple sediment-transport model, which accounts for fluid drag, inertia, buoyancy, and lift forces on boulders and includes both sliding and overturning transport mechanisms. Estimated near-bed flow velocities are converted to depth-averaged velocities using a linear eddy viscosity model and compared with water level and depth-averaged velocity time series from high-resolution coastal inundation models. Coastal inundation models simulate overwash by the storm surge and waves of a category 5 hurricane and tsunamis from a Lisbon earthquake of M 9.0 and two hypothetical earthquakes along the North America Caribbean Plate boundary. A modeled category 5 hurricane and three simulated tsunamis were all capable of inundating the boulder fields and transporting a portion of the observed clasts, but only an earthquake of M 8.0 on a normal fault of the outer rise along the Puerto Rico Trench was found to be capable of transporting the largest clasts at their current locations. Model results show that while both storm waves and tsunamis are capable of generating velocities and temporal acceleration necessary to transport large boulders near the reef crest, attenuation of wave energy due to wave breaking and bottom friction limits the capacity of storm waves to transport large clast at great inland distances. Through sensitivity analysis, we show that even when using coefficients in the sediment-transport model which yield the lowest estimated minimum velocities for boulder transport, storm waves from a category 5 hurricane are not capable of transporting the largest boulders in the interior of Anegada. Because of the uncertainties in the modeling approach, extensive sensitivity analyses are included and limitations are discussed.

LOCOMOTION OF ARTICULATED BODIES IN AN IDEAL FLUID: 2D MODEL WITH BUOYANCY, CIRCULATION AND COLLISIONS

Articulated solid bodies (ASB) is a basic model for the study of shape-changing underwater vehicles made of rigid parts linked together by pivoting joints. In this paper we study the locomotion of such swimming mechanisms in an ideal fluid. Our study ranges over a wide class of problems: several ASBs can be involved (without being hydrodynamically decoupled), the fluid-bodies system can be partially or totally confined and fluid circulation, buoyancy force and possible collisions between bodies are taken into account. We derive the Euler–Lagrange equation governing the dynamics of the system, study its well-posedness and describe a numerical scheme implemented in a Matlab toolbox (Biohydrodynamics Toolbox).

Transmission of acoustic-gravity waves through air-water interface.

It was demonstrated recently that air-water interface, which is usually an almost perfect reflector of acoustic waves, becomes anomalously transparent and the power flux in the wave transmitted into air increases dramatically, when a compact sound source in water approaches the interface within a fraction of wavelength [O. A. Godin, Phys. Rev. Lett. 97, 164301 (2006)]. Powerful underwater explosions and certain natural sources, such as underwater landslides, generate very low-frequency waves in water and air, for which both fluid buoyancy and compressibility simultaneously serve as restoring forces. In this paper, analysis of sound transmission through air-water interface is extended to acoustic-gravity waves (AGWs). It is found that, as for sound, the interface becomes anomalously transparent for sufficiently shallow compact sources of AGWs. Depending on the source type, the increase in wave power flux into air due to diffraction effects can reach several orders of magnitude. Physical mechanisms responsible for the anomalous transparency are discussed. Excitation of an interface wave by an underwater source is shown to be an important channel of AGW transmission into atmosphere, which has no counterpart in the case of sound.

Adaptive Sliding-Mode Control for a Precision Positioner with Hybrid Mechanism

This paper proposes a novel six-degree-of-freedom (DOF) electromagnetic precision positioner made of a hybrid mechanism utilizing both magnetic driving force and fluid upper lifting power, in which the new structure, the electromagnetic actuator, and the effective controller are developed. The concept of the mechanism design not only involves the magnetic driving mechanism but also the fluid buoyancy and damping properties, of which the latter help counter-balance weight of the platen so as to achieve very low steady-state power consumption. The four goals of novel system design include: (1) to have large moving range (in mm level), (2) to achieve precision positioning, (3) to design compact but low-cost mechanism, and (4) to achieve low power consumption. The experimental results show that traveling range is 3mm×3mm×4mm, and the tracking error in each axis is kept within 10µm, which is up to the limitation of our optical sensors.

シミュレーション解析による水中ドルフィンキック時の足関節底屈角度の増加がパフォーマンスに与える影響

The purpose of this study was to investigate the relationship between plantar flexion angle and underwater dolphin kick performance using a simulation model, SWUM (SWimming hUman Model), in which the entire body was represented as a series of elliptic cylinders. Three unsteady fluid forces (added mass force, normal and tangential resistive force), buoyancy and gravity acting on the elliptic cylinders were computed from the shape and density of the cylinders, and from the joint motion for one cycle. Eight elite competitive swimmers participated in this study. Their body characteristics were measured and input as the simulation data (the shape and density of the elliptic cylinders). The joint motion data for one cycle were obtained by the 2-D DLT method during the underwater dolphin kick. To recreate the dynamics of the underwater dolphin kick in the simulation, three fluid coefficients, used to calculate the three unsteady fluid forces, were identified to minimize the difference between measured and simulated swimming velocity by the least-squares method. This identification was conducted for each of the input data. Simulations of the increased plantar flexion angle in the knee extension phase were executed with the optimal fluid coefficients for each swimmer in order to analyze the change in swimming velocity and thrust generated by the feet. The conclusions can be summarized as follows : (1) Swimming velocity during the underwater dolphin kick increased due to the increment of the thrust generated by the feet after an increase in the plantar flexion angle. (2) It was suggested that flexibility of plantar flexion is important, although it is not a determinant of underwater dolphin kick performance. (3) Swimmers with low flexibility of plantar flexion tended to achieve a faster swimming velocity after increasing the plantar flexion angle.

CFD Modelling of Liquid Metal Flow and Heat Transfer in Blast Furnace Hearth

An in-depth understanding of the liquid metal flow and heat transfer is essential in order to identify the key mechanisms for the hearth erosion of a blast furnace. In this study, a comprehensive computational fluid dynamics model is described which predicts the flow and temperature distributions of liquid iron in blast furnace hearth, and the temperature distribution in the refractories. The new model addresses conjugate heat transfer, natural convection and turbulent flow through porous media, with its main features including improved transport equations (a modified k–ε turbulence model and thermal dispersion term) and a three-dimensional, high-resolution grid. The new turbulence model and terms take account of the effect of microscopic flows around coke particles and allow unified treatment of coke bed and coke-free layer. The predicted results show a well-organized flow pattern: two large-scale recirculation zones are separated vertically at the taphole level. This flow pattern controls the temperature distribution in the liquid phase, so that the temperature remains nearly uniform in the upper zone, but changes mainly across the lower zone. The effects of several factors were examined, such as cases comparing fluid buoyancy with constant fluid density as well as the shape and position of the coke free zone (i.e. based on reported dissection studies). Natural convection is found to be most important for the liquid metal flow patterns observed. Comparison with the plant data shows that the refractory pad temperature is under-predicted when assuming intact hearth lining. The pad temperature is very sensitive to the erosion of protection layer in the hearth lining.

Experimental investigation of lubrication and cooling effects of high-speed rotating machines

This study discusses the lubrication and cooling effects of high-speed rotating machines. The optimal channel design of the rotating test section with ribs will enhance cooling purposes. Therefore, the negative effects of thermal expansion and distortion of rotating elements are inhibited, thus decreasing the damage percentage of rotating machining tools, and improving processing precision and efficiency. To analyze the cooling behavior of the working fluid, the experimental conditions are required to fit the practical operating parameters of the high-speed rotating shaft. The experimental test analyzes axial and radial local Nusselt number distribution of the test section of the high-speed rotating cylinder. The experimental results show that Rib 4 case provides the best performance on the cooling area, showing a 20–40% removal of heat compared to that of the smooth wall. Given various fluid velocities (Reynolds number, Re) and buoyancy parameters (Grashof number, Gr), Rib 4 also achieves the highest cooling rate. Furthermore, we found that the larger the Re and Gr are, the larger cooling efficiency is. In conclusion, the experimental results provide some useful reference data for similar high-speed rotating lubrication and cooling problems.

A pendulum experiment on added mass and the principle of equivalence

The concept of added mass in fluid mechanics has been known for many years. A familiar example is the accelerated motion of a sphere through an ideal (inviscid and irrotational) fluid, which has an added mass equal to one-half the mass of the fluid displaced. The period of oscillation of a simple pendulum in a vacuum is independent of its mass because of the equivalence of gravitational and inertial masses. In contrast, in a fluid both buoyancy and added mass affect the period. We present experimental results on simple pendula of different materials oscillating in various fluids. The results agree fairly well with the results obtained for the added mass in an ideal fluid.

Investigation of compliancy effects on the inviscid instability in fluid flow over a flat plate with heat transfer

PurposeA detailed investigation of the effects of surface flexibility on the inviscid instability of boundary‐layer flow over a horizontal flat plate with heat transfer when the fluid buoyancy is large is undertaken. The aim of this study is to determine whether the inviscid disturbances that arise at this limit are stabilized or destabilized by surface compliancy.Design/methodology/approachFor large positive buoyancy numbers, the motion of the disturbances is governed by the Taylor‐Goldstein (TG) equation. Using the Chebyshev collocation spectral method, the eigen‐solutions of the TG equation are obtained and compared with known results from boundary‐layer flow over rigid flat plates.FindingsThe numerical results show that the effects of surface compliancy are important for small wave numbers and that for the inviscid modes, increasing surface parameters has the effect of destabilizing the flow. For large compliancy parameters the flow structure is indistinguishable from that which obtains in boundary layer flow over rigid surfaces.Research limitations/implicationsThe multiplicity of the compliant wall parameters makes a full parametric study that should show the effect of varying the compliance of the boundary surface on the stability of the flow mathematically intractable. In this study only a brief parametric study, for selected parameters is presented.Originality/valueIt is believed that this paper would be of immense value to applied mathematicians and engineers working in the areas of boundary layer stability and natural convection flows. In the last four decades there has been overwhelming interest shown by researchers in convective boundary‐layer flow. This has largely stemmed from the numerous applications of such flows in geophysics and engineering problems. Many of these applications, for example, polymer and food processing involve the flow of a fluid over a flexible surface. This research aims to bring the dynamics of the surface as a possible mechanism to stabilize convective boundary layer flows and prevent separation.

Performance of a variety of low Reynolds number turbulence models applied to mixed convection heat transfer to air flowing upwards in a vertical tube

The study reported here is concerned with mixed convection heat transfer to air flowing upwards in a vertical tube. Computational simulations of experiments from a recent investigation have been performed using an ‘in-house’ code which was written specifically for variable-property, developing, buoyancy-influenced flow and heat transfer in a vertical passage. The code incorporates a selection of two-equation, low Reynolds number turbulence models. The objective of the study was to evaluate the models in terms of their capability of reproducing the effects on turbulent heat transfer of non-uniformity of fluid properties and buoyancy. Direct comparisons have been made between results from the experimental investigation and those obtained by computational modelling for a range of conditions. The trends of impairment and enhancement of heat transfer owing to the influence of buoyancy found in the experiments were captured to some extent in the simulations using each of the models. However, none reproduced observed behaviour correctly over the entire range of buoyancy influence.

EFFECTS OF THERMAL DIFFUSION AND SUBSTRATE TEMPERATURE ON SILICON DEPOSITION IN AN IMPINGING-JET CVD REACTOR

A numerical/computational model and the associated FORTRAN computer code have been developed to simulate the chemical vapor deposition processes. The model includes variable fluid properties and buoyancy forces in the hydrodynamic model and does not use Bousinesq approximation. The model is applied to predict the silicon deposition rate from gaseous silane supplied with a hydrogen carrier gas in an atmospheric-pressure circular impinging-jet reactor. A wide range of wafer temperature (933-1733 K) with three different silane inlet mass fractions is investigated. The significance of the thermal diffusion or Soret effect on the silicon deposition rate is also conducted. Grid-independent solutions are obtained. It is found that while the deposition rate increases greatly with the wafer temperature, the silane inlet mass fraction does not have a significant effect. Small changes in the deposition rates, along with very small thermal diffusion ratios, are observed if the Soret effect is included in the model. Thus, the Soret effect is found to be negligible for this reactor configuration over the range of wafer temperatures and silane inlet concentrations investigated.

IMPORTANCE OF GRID REFINEMENT IN NUMERICAL MODELING OF CHEMICAL VAPOR DEPOSITION PROCESSES

The importance of grid resolution near the substrate surface for accurate prediction of the deposition rates in chemical vapor deposition modeling has been demonstrated. The exercise is conducted through numerical modeling of the chemical vapor deposition of silicon in an atmospheric-pressure, circular, impinging-jet reactor. Silicon is deposited from gaseous silane (SiH 4 ) supplied in a dilute condition premixed in a hydrogen carrier gas. The substrate temperature is kept fixed at 1,333 K. The model includes variable fluid properties and buoyancy forces in the hydrodynamic model. The Bousinesq approximation is not used because the temperature gradient is large. In addition to the hydrodynamic and thermal solution, both gas-phase reactions in the bulk gas and surface reactions on the susceptor are included in the model. The mesh-independent solution and the deposition rate of silicon on the wafer surface are presented. It is observed that a very fine mesh near the substrate surface, within the concentration boundary layer for the intermediate species such as silylene (SiH 2 ), is required to establish grid independency and accurate prediction of the deposition rate. For the specific deposition process modeled in this study, about 7 control volumes had to be placed within the SiH 2 concentration boundary layer at the substrate surface.

Complex behavior of magma-hydrothermal processes: role of supercritical fluid

Magmas emplaced into the upper portions of the earth’s crust are accompanied by extensive hydrothermal activity. Hydrothermal activity is represented as a system of coupled processes that dissipate thermal, mechanical, and chemical energy into the magma’s lithocap, primarily by convection of H2O-rich fluids. To investigate dynamical behavior of the system, a serial experiment was undertaken in which T(t) and P(t) values are computed for a pluton location during the time the region was subjected to near-critical hydrothermal convective flow. The consequent evolution of fluid buoyancy, ∇xρf, ion stability, ΔḠ°, and fracture extension, δL/L0 during this time indicates that variations in density gradients increase smoothly until 70,000 yr then burst into frequent, ≈100-yr oscillations. Oscillations first increase in magnitude then decrease. Oscillatory behavior of state conditions derived from numerical experiments illustrate resonant effects in chemical equilibrium and fracture extension processes and show the sensitivity of the stable mineral assemblage to either of the competing chemical and mechanical transport processes. An oscillatory zoned tourmaline that formed at near-critical conditions of H2O from the Geysers Geothermal deposit appears to provide evidence of nonlinear systematics in hydrothermal activity.Mathematical analogs to this system demonstrate that processes in this system record their dynamical behavior in the supercritical region and suggest that alteration events are generated by the complex, “chaotic” behavior of these processes. This type of behavior appears to be further augmented by strong divergence of H2O-fluid properties toward ± infinity at commonly encountered state conditions in the shallow reaches of magma-hydrothermal activity. System behavior elucidated here arises from affording for connectivity of processes by numerical experiments of hydrothermal activity for a region near the contact of a magma and its lithocap. The cumulative data from numerical experiments, equation-of-state (EOS) relationships, geologic and geochemical observations support the proposition that magma- hydrothermal processes should be thought of as complex dynamical systems whose behavior at state conditions near the supercritical region of the fluid is likely chaotic.

Discrete particle model for sheet flow sediment transport in the nearshore

Fully three‐dimensional discrete particle computer simulations of high‐concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments. A simple impulse‐momentum approach explains simulation results and forms the basis for adding an acceleration‐related term to widely used energetics sediment transport formulae. Transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores. Simulations integrate F = ma and a corresponding set of equations for the torques for each sphere. Normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point, respectively; particle interactions are both inelastic and frictional. Pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag, added mass, and buoyancy forces on particles. Transport properties of the simulated granular‐fluid assemblage are robust to large variations in material properties of the particles. Simulated transport rates agree with available experimental data for unsteady transport of coarse sands; the mode of bed load motion, dispersion of bed load particles, and particle segregation by size and density are qualitatively consistent with available particle‐scale observations of bed load transport of natural particles.

Mechanisms for generating overpressure in sedimentary basins: A reevaluation: Reply

Overpressure can be produced by the following processes: (1) increase of compressive stress, (2) changes in the volume of the pore fluid or rock matrix, and (3) fluid movement or buoyancy. Loading during burial can generate considerable overpressure due to disequilibrium compaction, particularly during the rapid subsidence of low- permeability sediments. Horizontal stress changes can rapidly generate and dissipate large amounts of overpressure in tectonically active areas. Overpressure mechanisms involving change in volume must be well sealed to be effective. Fluid volume increases associated with aquathermal expansion and clay dehydration are too small to generate significant overpressure unless perfect sealing occurs. Hydrocarbon generation and cracking to gas could possibly produce overpressure, depending upon the kerogen type, abundance of organic matter, temperature history, and rock permeability; however, these processes may be self-limiting in a sealed system because buildup of pressure could inhibit further organic metamorphism. The potential for generating overpressure by hydrocarbon generation and cracking must be regarded as unproven at present. Fluid movement due to a hydraulic head can generate significant overpressure in shallowly buried, well-plumbed basins. Calculations indicate that hydrocarbon buoyancy and osmosis can generate only small amounts of localized overpressure. The upward movement of gas in an incompressible fluid also could generate ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received October 17, 1995; revised manuscript received September 4, 1996; final acceptance January 20, 1997. 2Department of Geological Sciences, Durham University, South Road, Durham DH1 3LE, United Kingdom. Osborne e-mail: M.J.Osborne@ durham.ac.uk; GeoPOP web site http://www.dur.ac.uk/~dgl0zz7/ We wish to thank the companies that support the Geosciences Project on Overpressure (GeoPOP) at the universities of Durham, Newcastle, and Heriot-Watt: Agip, Amerada Hess, Amoco, ARCO, Chevron, Conoco, Elf Exploration, Mobil, Norsk Hydro, Phillips Petroleum UK Company Limited, Statoil, and Total. We also thank Neil Goulty (Durham) for commenting on an earlier draft of this paper. Osborne thanks Gordon Macleod (Newcastle) for help with geochemical modeling.

The Stokes problem for a dusty fluid in the presence of magnetic field, heat generation and wall suction effects

This work focuses on the laminar flow of a two‐phase particulate suspension induced by a suddenly accelerated infinite vertical permeable surface in the presence of fluid buoyancy, magnetic field, heat generation or absorption, and surface suction or blowing effects. The governing equations for this modified Stokes problem are developed based on the continuum representation of both the fluid and the particle cloud. Appropriate dimensionless variables are introduced. The resulting dimensionless equations are solved numerically by an accurate implicit finite‐difference method for two situations. The first case corresponds to an impulsive start of the surface from rest while the second case corresponds to a uniformly accelerated surface. The numerical results for these cases are illustrated graphically. Comparisons with previously published work are performed and the results are found to be in good agreement. Typical fluid‐ and particle‐phase velocity and temperature distributions as well as wall shear stress and heat transfer results are reported for various values of the particle loading, Hartmann number, wall mass transfer coefficient and the heat generation or absorption coefficient.