Bulk Fluid Velocity Research Articles

Understanding the flow behavior around marine biofilms

In vitro platforms capable of mimicking the hydrodynamic conditions prevailing in natural aquatic environments have been previously validated and used to predict the fouling behavior on different surfaces. Computational Fluid Dynamics (CFD) has been used to predict the shear forces occurring in these platforms. In general, these predictions are made for the initial stages of biofilm formation, where the amount of biofilm does not affect the flow behavior, enabling the estimation of the shear forces that initial adhering organisms have to withstand. In this work, we go a step further in understanding the flow behavior when a mature biofilm is present in such platforms to better understand the shear rate distribution affecting marine biofilms. Using 3D images obtained by Optical Coherence Tomography, a mesh was produced and used in CFD simulations. Biofilms of two different marine cyanobacteria were developed in agitated microtiter plates incubated at two different shaking frequencies for 7 weeks. The biofilm-flow interactions were characterized in terms of the velocity field and shear rate distribution. Results show that global hydrodynamics imposed by the different shaking frequencies affect biofilm architecture and also that this architecture affects local hydrodynamics, causing a large heterogeneity in the shear rate field. Biofilm cells located in the streamers of the biofilm are subjected to much higher shear values than those located on the bottom of the streamers and this dispersion in shear rate values increases at lower bulk fluid velocities. This heterogeneity in the shear force field may be a contributing factor for the heterogeneous behavior in metabolic activity, growth status, gene expression pattern, and antibiotic resistance often associated with nutrient availability within the biofilm.

Influence of thermal conductivity on transient mixed convection in a vented cavity with a hollow cylinder and filled with CNT-water nanofluid

This study is aimed to perform a numerical time-dependent investigation thermal conductivity effect of the annular cylinder within a vented cavity using CNT based-water nanofluid. For demonstrating the effect of thermal conductivity, four distinct hollow cylinder materials such as Ks = 0.5(Plastic tiles), Ks = 0.84(Clay tiles), Ks = 1.1(Concrete tiles), and Ks = 2(Slate tiles) are introduced together with a suitable variation of dimensionless time (0 ≤ τ ≤ 1). The governing equations of the model with associated boundary conditions is solved using finite element based Galerkin's weighted residual method. Different contour plots for thermal and flow field transformation and mean Nusselt number, mean fluid temperature, bulk convective field temperature, temperature gradient, pressure gradient, vortices, and fluid velocity magnitude are presented for qualitative and quantitative thermal performance analysis. With the decrease of solid thermal conductivity, 27.3% thermal transport enhancement is noted from the heated surface of the cylinder. However, a 16.3% increase in the bulk fluid temperature has been recorded with the increase in cylinder conductivity. The numerical outcomes from this investigation propose a better thermo-fluid efficiency compared to the existing methodology which can be suggestive to engineers and researchers for designing heat exchangers, heat pipes, and other thermal systems.

The influences of boundary layer thickness on the characteristics of saltation sand flow – A large eddy simulation study

In this paper, Large Eddy Simulation (LES) of turbulence and Lagrangian model of sand particle motion are adopted to study the characteristics of wind-blown sand flow for different boundary layer thickness. The simulations are conducted within computational domain height (boundary layer thickness) of δ = 0.5 m, 1 m, 5 m and 12 m, respectively. It is found by comparing the computational results that the mass flux and sand transport rate increase with the increase of boundary layer thickness for the same frictional wind velocity, and the fluctuation of particle velocity and sand transport rate increase significantly too. The spatial scales of particle structure, defined by the correlation of sand particle concentration, significantly increase with δ, so does the time scale of statistical stability of sand transport rate. For two computational domains of δ = 1 m and 12 m, the statistical relative error of sand transport rate reduce to less than 5 % only when the average time goes higher than 45 δ/Ub, where Ub is the bulk fluid velocity. In the context of turbulence, it might take about 10 mins to obtain reliable sand statistics in the neutral atmospheric boundary layer whose boundary layer thickness is about 100–200 m.

Drag reduction and the Vogel exponent of a flexible beam in transient shear flows

Interactions between a flexible beam and a fluid in a channel are of great relevance to biological hairy surfaces, aquatic vegetation, marine life (e.g., fish gills), and many industrial systems alike. While steady state response of a beam to such flows is fairly well-explored, their behavior in the transient regime is not fully understood. A series of numerical simulations are performed to study the laminar Couette flow of an incompressible viscous fluid past an elastic beam in a two-dimensional channel. The flexible beam is perpendicular to the direction of flow, and its base is fixed to the stationary bottom of the channel. We measure the evolution of the Vogel exponent, drag reduction, and reconfiguration number during the transient and steady-state response of the fluid–structure system for different geometrical and physical properties. Our benchmark shows a good agreement between numerical and experimental observations. Our results show that the system's steady-state response at different bulk-fluid velocities can be reproduced by investigating the shear flow response during the transient regime. We define a new variable that characterizes the evolution of the local velocity profile in the proximity of the free end of the beam and use that to characterize the transient-regime response. The analysis yields insight into the competing effects of elasticity of the beam and non-linear flow response.

Comparison of Prospective and Retrospective Gated 4D Flow Cardiac MR Image Acquisitions in the Carotid Bifurcation.

To evaluate the agreement of 4D flow cMRI-derived bulk flow features and fluid (blood) velocities in the carotid bifurcation using prospective and retrospective gating techniques. Prospective and retrospective ECG-gated three-dimensional (3D) cine phase-contrast cardiac MRI with three-direction velocity encoding (i.e., 4D flow cMRI) data were acquired in ten carotid bifurcations from men (n = 3) and women (n = 2) that were cardiovascular disease-free. MRI sequence parameters were held constant across all scans except temporal resolution values differed. Velocity data were extracted from the fluid domain and evaluated across the entire volume or at defined anatomic planes (common, internal, external carotid arteries). Qualitative agreement between gating techniques was performed by visualizing flow streamlines and topographical images, and statistical comparisons between gating techniques were performed across the fluid volume and defined anatomic regions. Agreement in the kinematic data (e.g., bulk flow features and velocity data) were observed in the prospectively and retrospectively gated acquisitions. Voxel differences in time-averaged, peak systolic, and diastolic-averaged velocity magnitudes between gating techniques across all volunteers were 2.7%, 1.2%, and 6.4%, respectively. No significant differences in velocity magnitudes or components ([Formula: see text], [Formula: see text], [Formula: see text]) were observed. Importantly, retrospective acquisitions captured increased retrograde flow in the internal carotid artery (i.e., carotid sinus) compared to prospective acquisitions (10.4 ± 6.3% vs. 4.6 ± 5.3%; [Formula: see text] < 0.05). Prospective and retrospective ECG-gated 4D flow cMRI acquisitions provide comparable evaluations of fluid velocities, including velocity vector components, in the carotid bifurcation. However, the increased temporal coverage of retrospective acquisitions depicts increased retrograde flow patterns (i.e., disturbed flow) not captured by the prospective gating technique.

Evolution of magnetic deformation in neutron star crust

ABSTRACT We examine the magnetic field evolution occurring in a neutron star crust. Beyond the elastic limit, the lattice ions are assumed to act as a plastic flow. The Ohmic dissipation, Hall drift, and bulk fluid velocity driven by the Lorentz force are considered in our numerical simulation. A magnetically induced quadrupole deformation is observed in the crust during the evolution. Generally, the ellipticity decreases as the magnetic energy decreases. In a toroidal-field-dominated model, the sign of the ellipticity changes. Namely, the initial prolate shape tends to become oblate. This occurs because the toroidal component decays rapidly on a smaller time-scale than the poloidal dipole component. We find that the magnetic dipole component does not change significantly on the Hall time-scale of ∼1 Myr for the considered simple initial models. Thus, a more complex initial model is required to study the fast decay of surface dipoles on the above-mentioned time-scale.

A conservative phase-space moving-grid strategy for a 1D-2V Vlasov–Fokker–Planck Solver

We develop a conservative configuration- and velocity-space (i.e., phase-space) moving-grid strategy for the Vlasov–Fokker–Planck (VFP) equation in a planar geometry. The velocity-space grid is normalized and shifted in terms of the thermal speed and the bulk-fluid velocity, respectively. The configuration-space grid is moved according to a mesh-motion-partial-differential equation (MMPDE), which equidistributes a monitor function that is inversely proportional to the gradient-length scales of the macroscopic plasma quantities. The resulting inertial terms in the transformed VFP equations are discretized to ensure the discrete conservation of mass, momentum, and energy. To satisfy the discrete conservation theorems in the presence of phase-space mesh motion, we employ the method of discrete nonlinear constraints – explored in previous studies – but the underlying symmetries are determined in a much more efficient manner than before. The conservative grid-adaptivity strategy provides an efficient scheme that resolves important physical structures in the phase-space while controlling the computational complexity at all times. We demonstrate the favorable features of the proposed algorithm through a set of test cases of increasing complexity. The problems test independent components of the algorithms, as well as the integrated capability on settings relevant to inertial confinement fusion.

On-chip Mixing, Pumping and Concentrating Effects by Using AC Electrothermal Flow

Background:Microfluidic manipulation (including: pumping, mixing and concentrating effects) is highly challengeable for bioengineering and on-chip analysis applications such as point-of-care immune-detection systems. In this research we propose a configurable electrode structure to form various manipulation effects including pumping, mixing and concentrating processes by applying an Alternate Current (AC) electrokinetically-driven flow.Methods:By applying an inhomogeneous electric field causes temperature rise accompanied by temperature gradients generation inside the microchannel. As a result, an AC electrothermal flow generates inside the channel, which is efficient to generate mixing, pumping and concentrating effects.Results:The proposed system is studied numerically by Finite-Element-Method, Based on the results, a) bulk fluid velocity of 100 µm/s is achieved by exciting the electrodes in pumping mode, b) complete mixing efficiency is observed in mixing mode, c) for antibody-antigen binding process (concentrating mode), the surface reaction increases by the factor of 9 after 5 seconds of sample loading. Results reveal that the system is highly efficient for bio-fluid mediums.Conclusion:AC electrothermal fluid manipulation process was investigated numerically inside a microchannel for biological buffers. Back and forth fluid motions, clockwise/counter-clockwise rotational vortexes and also antibody-antigen linking enhancement were achieved by engineering the specific electrode patterns. The manipulation efficiency improves by increasing both the amplitude of electric potential and the ionic strength of biofluid. As a result, our proposed configurable device is of interest for onchip immunoassays and point-of-care devices.

Phase lock-on in periodically excited Bingham flow past a confined cylinder

The focus of the work is the “phase lock-on”, a well-known phenomenon observed in oscillating dynamical systems where the forcing and output frequencies are synchronized. This phenomenon appears to be universal with the frequencies related by a rational ratio. We considered the case of an incompressible flow past a confined cylinder where the incident bulk flow is subjected to periodic velocity perturbations. While we considered both Newtonian and non-Newtonian (Bingham) fluids, the main interest was on the Bingham case. Numerical simulations were conducted at a fixed Reynolds number of Re = 100 based on the bulk fluid velocity and cylinder diameter. The relative amplitude of the periodic velocity perturbation was considered in the range 0.10–0.30 and the forcing frequency relative to the natural unforced shedding frequency was in the range 1.4–2.3. These choices yielded fixed lock-on periodic states that correspond to the number 2 on the horizontal axis of an Arnold tongue graph. Yield stress effects are shown for Bingham numbers in the range 0–0.5.

Hydraulic resistance of in-tube cooling supercritical water accompanying out-tube pool boiling

The experiment was conducted by immersing a smooth horizontal tube in a pool tank to simulate the flow condition of Passive Residual Heat Removal System (PRHRS) in a SuperCritical Water-cooled Reactor (SCWR). Hydraulic resistance and friction factor of in-tube cooling supercritical water accompanying out-tube pool boiling were investigated in this study with test pressure ranging from 23 to 28 MPa and mass flux ranging from 600 to 1000 kg·m−2·s−1. The influence of pressure and mass flux on pressure drop in adiabatic and cooling flows was analyzed. This paper also discussed the effect of deceleration in the cooling flow and assessed various friction-factor correlations by employing the experimental data. Results showed that the friction factor of the adiabatic flow exists a steep “pit” approaching to pseudocritical region. A noticeable “Λ-shaped” profile was observed in the vicinity of the pseudocritical temperature, due to deceleration-effect of frictional pressure drop in the cooling flow. The deceleration factor of supercritical cooling flow led to the axial fluid element shrinkage and radial bulk fluid velocity parabolic distribution. Taking into account the effect of deceleration-factor, a modified correlation was proposed for in-tube cooling supercritical water accompanying out-tube pool boiling, of which the average error and root mean square error are −2.51% and 15.28% respectively.

Numerical investigations of electrothermally actuated moving contact line dynamics: Effect of property contrasts

This article reports interfacial electro-thermo-chemical-hydrodynamics of binary fluids with contrasting viscosity, thermal conductivity, and electrical conductivity of fluids under AC electrokinetics, called alternating current electrothermal (ACET) mechanism, over wetted substrates. The interfacial kinetics of the two phases are modulated by the coupled influence of electrothermal, viscous, and capillary forces. Numerical investigations of contact line dynamics reveal that at low viscosity of displaced fluid, viscous drag force significantly reduces leading to faster progression of the contact line. Larger viscous drag force at higher viscosity of the displaced fluid resists the interface motion to travel along the capillary. ACET forces are the direct consequences of the thermal and electric fields. For low thermal conductivity of the displaced fluid, the temperature gradient becomes much stronger leading to higher ACET forces and contact line velocity. Below a threshold limit of thermal conductivity, stronger electrothermal forces cause misbalance between contact line velocity and bulk fluid velocity, which, in turn, trigger an interesting phenomena of interface breaking. Mismatch in electrical conductivity generates electrical stresses across the interface that deforms the interface profile and causes boosting impact across the interface leading to an increase in contact line velocity. The net force across the interface changes the direction depending on the deviation of electrical conductivity ratio from unity. Finally, we observe that larger channel height and wider electrode spacing decrease the net force on the bulk fluid and contact line velocity.

Direct numerical simulation of particle-laden turbulent channel flows with two- and four-way coupling effects: budgets of Reynolds stress and streamwise enstrophy

The budgets of the Reynolds stress and streamwise enstrophy are evaluated through direct numerical simulations for the turbulent particle-laden flow in a vertical channel with momentum exchange between the two phases. The influence of the dispersed particles on the budgets is examined through a comparison of the particle-free and the particle-laden cases at the same Reynolds number of Reb = 5600 based on the bulk fluid velocity and the distance between the channel walls. Results are obtained for particle ensembles with four response times in simulations with and without streamwise gravity and inter-particle collisions at average mass (volume) fractions of 0.2 (2.7 × 10−5) and 0.5 (6.8 × 10−5). The particle feedback force on the flow of the carrier phase is modeled by a point-force approximation (PSIC-method). It is shown that all the terms in the budgets of the Reynolds stress components are decreased in the presence of particles. The level of reduction depends on the particle response time and it is higher under the effects of gravity and inter-particle collisions. A considerable reduction in all the terms of the streamwise enstrophy budget is also observed. In particular, all production mechanisms, and mainly vortex stretching, are inhibited in the particulate flows and thus the production of streamwise vorticity is significantly damped. A further insight into the direct particle effects on the fluid turbulence is provided by analyzing in detail the fluid–fluid, fluid–particle and particle–particle correlations, and the spectra of the fluid–particle energy exchange rate. The present results indicate that the turbulence production, dissipation and pressure–strain term are generally large quantities, but their summation is relatively small and comparable to the fluid–particle direct energy exchange rate. Consequently, the particle contribution can potentially increase or decrease the fluctuating fluid velocities and eventually control the direction of fluid turbulence modification.

Numerical simulation of vapor bubble condensation in turbulent subcooled flow boiling

Subcooled flow boiling is a significant heat transfer regime in pressurized water reactors (PWRs) and also occurs in boiling water reactors (BWRs) prior to the onset of saturated nucleate boiling. In subcooled boiling, individual bubbles experience a life-cycle of nucleation from the wall, (possible) departure from the wall, and transport along the channel during condensation/collapse. The present paper focuses on the last stage of this life cycle. CFD analysis of a single bubble behavior in upward subcooled flow boiling was performed using volume-of-fluid (VOF) interface tracking method and large eddy simulation (LES) turbulence model. High pressure steam-water conditions at high flow velocity were simulated due to their relevance to the nuclear application. The numerical solutions were compared with terminal velocity correlations. Bubble behavior was investigated at different system pressure, subcooling temperature, bubble diameter, pipe diameter, and bulk fluid velocity, corresponding to bulk Reynolds number range of 0–410,000. The effects of these parametric variations on bubble rise velocity, deformation rate, trajectory, and distortion are presented. Results show that bubble rise velocity increases with bulk velocity, and bubble size, but decreases with increasing pipe diameter. A reverse bubble velocity trend with varying system pressure is observed with and without bulk velocity. Bubble condensation rate decreased with increasing system pressure, bubble size and pipe diameter, but increased with increasing subcooling temperature and bulk fluid velocity.

Measurement of soot concentration and bulk fluid temperature and velocity using modulated laser-induced incandescence

The use of a modulated LII two-colour technique to measure soot temperature in a laminar diffusion flame is described, and the results compared to CARS experiments. A sinusoidal modulated diode laser is used to excite the soot, and both the modulated LII intensity and its relative phase to the excitation source are measured with a lock-in amplifier and recorded. Modulation frequencies from 25 to 200,000 Hz were employed. The temperature is derived from the ratio of the modulated LII radiation signal at 445 and 750 nm, and the results compared to values obtained by CARS spectroscopy. The modulated LII temperatures were largely independent of modulation frequency and agreed well with the CARS temperatures. A theory is developed to explain the dependence of the phase delay of the modulated LII signal (with reference to that of the laser excitation source) on gas replacement time in the sample volume, soot cooling rate and soot volume fraction. The theory is shown to give a reasonable fit to the experimental results at all frequencies. At lower frequencies, the phase delay is dominated by the gas replacement time in the sample volume and at higher frequencies by the cooling rate of the heated soot. Time constants for both processes and the soot volume fraction are derived from the data and shown to be largely in agreement with the expected values. Using modulated LII-determined soot volume fraction and inverted and scatter corrected line-of-sight attenuation-determined absorption coefficients, the soot refractive index absorption function E(m) was measured to be between 0.45 and 0.42 over the wavelength range of 436–825 nm.

Investigation of bubble behaviours in wet central heating systems

A series of experimental measurements has been conducted in order to investigate the bubble behaviours through the horizontal pipe line of the domestic wet central heating systems. Obtained results exposed the effect of 90 degree bend, buoyancy forces on bubbly two phase flow patterns and effect of velocity on void fractions and bubble diameters. Distance chosen for the first sight glass (HSG0) was sufficient enough to note the effect of 90 degree bend on void fraction patterns. Due to the effect of 90 degree bend, position of the peak void fractions across the pipe section lowers, with an increase in bulk fluid velocity. Bubbles tend to flow for longer distance at the bottom of the pipe section. Buoyancy force effect is demonstrated with figures for highest bulk fluid velocity at three different positions. Analysis of four different flow rates at two different saturation ratios show reduction for average bubble diameters and void fractions when bulk fluid velocity increases. An attempt to predict bubble dissolution rates across the horizontal pipeline of the system is made, however results show some uncertainties.

Two-phase distribution in the vertical flow line of a domestic wet central heating system

The theoretical and experimental aspects of bubble distribution in bubbly two-phase flow are reviewed in the context of the micro bubbles present in a domestic gas fired wet central heating system. The latter systems are mostly operated through the circulation of heated standard tap water through a closed loop circuit which often results in water supersaturated with dissolved air. This leads to micro bubble nucleation at the primary heat exchanger wall, followed by detachment along the flow. Consequently, a bubbly two-phase flow characterises the flow line of such systems. The two-phase distribution across the vertical and horizontal pipes was measured through a consideration of the volumetric void fraction, quantified through photographic techniques. The bubble distribution in the vertical pipe in down flow conditions was measured to be quasi homogenous across the pipe section with a negligible reduction in the void fraction at close proximity to the pipe wall. Such a reduction was more evident at lower bulk fluid velocities.

Solutions for the diurnally forced advection‐diffusion equation to estimate bulk fluid velocity and diffusivity in streambeds from temperature time series

Key Points A new equation is presented to analyze temperature time series from streambeds Explicit solutions calculate both water flux and thermal diffusivity directly Additional applications, error estimation, and validation are provided

A liquid cooling solution for temperature redistribution in 3D IC architectures

The advancement of contemporary three-dimensional integrated circuit (3D IC) technologies offers a promising solution for the insatiable demand of the consumer electronics market. The increased complexity of 3D IC design permits the execution of multiple applications at greater speeds whilst remaining within the design constraints of energy consumption, yield and time-to-market. However, the increased computing performance and compact size may introduce a thermal barrier inhibiting performance, particularly in the case where multiple logic die are stacked and co-aligned hotspots are induced. To mitigate this thermal barrier a novel integrated active thermal solution is investigated in this paper whose purpose is to alleviate hotspots in a contemporary two-die 3D IC architecture. The solution employs a series of integrated microchannels, which permits the transfer of heat, via a coolant, from lower to upper strata. This microfluidic system is driven by a series of integrated AC electrokinetic pumps embedded in the channel walls. Recent advancements in electrokinetic micropump technology have allowed greater increases in fluid velocity – to an order of mm/s – while operating within the voltage constraints of a 3D IC. Numerically qualitative and quantitative temperature distributions are presented for a 3D IC chip design both with and without microchannels for a constant heat flux on the active layer of each silicon chip. The implementation of a microchannel network is shown to alter the thermal distribution map within a 3D IC package creating hot and cold zones with variations on temperature of −14.6°C≤ΔT≤9.8°C with a ΔTmax of −6.5°C in the silicon die stack (equivalent to a total maximum heat flux, qmax″, of approximately 112.5W/cm2). Increasing bulk fluid velocity, within the range 1.3mm/s≤uavg≤13mm/s, can vary the area of the cold zone enhancing heat transfer and reducing the temperature of the die stack without an overall temperature change in the package.

The production spectrum in fusion plasmas

Significant broadening of the energy spectrum of the products of nuclear reactions occurs in fusion plasmas. We provide a method for calculating the shape of this production spectrum for arbitrary plasma distribution functions. The method is exact and can be used for both isotropic and anisotropic distributions. We derive expressions for Maxwellian (both stationary and moving with a bulk fluid velocity), bi-Maxwellian and beam–target plasmas. The neutron spectrum produced by the D + D → He3 + n reaction is studied as an example. It is shown that the neutron spectrum produced from a Maxwellian plasma becomes asymmetric at high plasma temperatures with a long high-energy tail. The effect of bulk fluid velocity on the neutron spectrum is shown to be significant in some cases. In particular, the spectrum produced by an imploding shell has a much greater FWHM than the spectrum obtained from a stationary plasma. The spectrum produced by a beam–target interaction shows significant anisotropy in the high-energy tail as the viewing angle varies from perpendicular to parallel to the beam direction.

A Null-Buoyancy Thermal Flow Meter With Potential Application to the Measurement of the Hydraulic Conductivity of Soils

A null-buoyancy thermal flow sensor is described; it is designed specifically for the measurement of very slow downward fluid flows in a vertical pipe in which a glass-rod thermistor is concentrically located. Sensor power dissipation in this thermistor is adjusted so that the upward thrust of the buoyant thermal plume from the warm thermistor sensor exactly counterbalances the downward bulk fluid velocity. This results in flow stagnation at the sensor tip characterized by a local peak in the sensor's temperature. Experimental results agreed with CFD and engineering models to suggest that the required power level needed to counterbalance the downward velocity at this null-buoyancy point depends upon the square of the velocity over the narrow velocity range between 0.25 mm/s and 2.5 mm/s. The flow sensor has been designed to have the potential to measure the infiltration rate of water into different soil types by applying it to a simpler form of the common disc permeameter (tension-infiltrometer) used to determine the hydraulic conductivity of soils.