Transverse Flow Field Research Articles

Active and passive microrheology with large tracers in hard colloids.

The dynamics of a tracer particle in a bath of quasi-hard colloidal spheres is studied by Langevin dynamics simulations and mode coupling theory (MCT); the tracer radius is varied from equal to up to seven times larger than the bath particles radius. In the simulations, two cases are considered: freely diffusing tracer (passive microrheology) and tracer pulled with a constant force (active microrheology). Both cases are connected by linear response theory for all tracer sizes. It links both the stationary and transient regimes of the pulled tracer (for low forces) with the equilibrium correlation functions; the velocity of the pulled tracer and its displacement are obtained from the velocity auto-correlation function and the mean squared displacement, respectively. The MCT calculations give insight into the physical mechanisms: At short times, the tracer rattles in its cage of neighbours, with the frequency increasing linearly with the tracer radius asymptotically. The long-time tracer diffusion coefficient from passive microrheology, which agrees with the inverse friction coefficient from the active case, arises from the transport of transverse momentum around the tracer. It can be described with the Brinkman equation for the transverse flow field obtained in extension of MCT, but cannot be recovered from the MCT kernel coupling to densities only. The dynamics of the bath particles is also studied; for the unforced tracer the dynamics is unaffected. When the tracer is pulled, the velocity field in the bath follows the prediction of the Brinkman model, but different from the case of a Newtonian fluid.

Bivariant species mixing and pressure drop within a hybrid periodic modulated microslit

In this article, a mathematical study is presented on electroosmotic flow (EOF) of power law fluids driven by an external electric field, where the heterogeneity of a microslit is created by multiple wavy triangular modulated polarized wall hurdles. The primary aim of this paper is to demonstrate and depict the mixing performance that generating more retention time and enhancing the interface area, which is evaluated both analytically and computationally. EOF in microchannels is restricted to low Reynolds numbers with a relatively high viscosity effect, which could predict advection domination in flow mixing due to heterogeneities that can supersede the need of flow turbulence. The numerical experiment is performed for the flow phenomena of this novel alternating microgrooves' patterning to generate an intensively transverse flow field, which represents strong reverse flow due to a higher pressure drop. The geometry modification and potential heterogeneity are the key factors to disturb the flow stream by fluid folding and stretching, leading to significant improvement in mixing efficiency. The numerical computations are performed for the nonlinear coupled Nernst–Planck–Navier Stokes equations using a control volume approach over a staggered grid algorithm to elaborate the performance of the electric potential distribution, the external electric field, the flow field, and the species concentration, which are the major contributors of the mixing efficiency. The evaluated results confirm that surface modulation substantially reduces the mass flow rate, effectively resulting in an increase in the retention time of the flow diffusion, which is justified through analytical testing. The nonlinear coupling effects are found to be more pronounced for shear thickening fluids rather than shear thinning and Newtonian solutions, resulting in a low torque corresponding to equilibrium conditions. To achieve a targeted mixing performance, it is observed that flow behavior indices should be optimized in terms of aversion of flow behavior index, viscous dissipation, and yield stress effect.

Numerical simulation of boiling two-phase flow in the subchannel under static state and rolling motion

The design and operation of the offshore floating reactor cannot follow those of the land-based reactor affected by the complex marine environment. This study investigated subcooled flow boiling in the subchannel under static state and rolling motion numerically. Clear interfaces between the vapor and liquid phases are obtained.The numerical result of void fraction deviates from the experimental value by 11.9% at a given elevation. The transverse flow fields and phase-distributions are centrosymmetric under static state. The vapor only exists near the rod surfaces under static state, which is in the shape of “H”. Under rolling motion, flow fields and phase distributions vary periodically with time and the positions of the subchannel, which are significantly different from static state. At the same elevation, the time-averaged void fraction under roll motion is slightly higher than that under static state.

Poynting singularities in the transverse flow-field of random vector waves.

In order to utilize the full potential of tailored flows of electromagnetic energy at the nanoscale, we need to understand its general behavior given by its generic representation of interfering random waves. For applications in on-chip photonics as well as particle trapping, it is important to discern between the topological features in the flow-field of the commonly investigated cases of fully vectorial light fields and their 2D equivalents. We demonstrate the distinct difference between these cases in both the allowed topology of the flow-field and the spatial distribution of its singularities, given by their pair correlation function g(r). Specifically, we show that a random field confined to a 2D plane has a divergence-free flow-field and exhibits a liquid-like correlation, whereas its freely propagating counterpart has no clear correlation and features a transverse flow-field with the full range of possible 2D topologies around its singularities.

Numerical investigation of supersonic transverse jet interaction on CPU/GPU system

The main purpose of this paper is to develop a double-precision parallel algorithm implemented on graphics processing units (GPUs) for quick and accurate numerical simulations of large-scale supersonic transverse jet interaction problems. The finite volume method based on structured grid is considered; the AUSM + UP upwind scheme and the explicit multistage Runge–Kutta method are used for spatial discretization and time discretization, respectively. The turbulent solution is solved by K–ω SST two-equation model. Numerical investigation is performed for a supersonic missile body. Numerical results show that performing calculations on GPU can accurately capture the complex wave structures and vortex structures in the supersonic transverse jet flowfield. For single-GPU implementation, parallel computing can achieve an acceleration ratio of 90 times or more, and four GPU parallel computing can achieve an acceleration ratio of 106–245 times. Thus, GPU parallel computing can achieve a large-scale and efficient solution to supersonic transverse jet interaction problems.

Impingement of propeller jet on a vertical quay wall

This study experimentally investigates the mean and turbulent flow fields of a propeller jet impinging on a vertical quay wall. Four impingement distances, namely, wall clearances (= longitudinal distance between the propeller and vertical wall), were used to examine their influence on the flow behaviors. In order to investigate the characteristics of the three-dimensional impinging jet, both the streamwise (jet central plane) and transverse (impingement plane) flow fields were measured for each impingement distance using the particle image velocimetry (PIV) technique. Based on the wall clearance, three regions are identified, namely free, impingement and wall jet regions. Additionally, the results show that the evolution of the impinging jet is governed by two mechanisms: jet diffusion and wall obstruction. Their relative importance under varying wall clearances was interpreted in terms of the mean flow patterns and energy dissipation. Moreover, the proper orthogonal decomposition (POD) method is employed to identify the dominant flow structures associated with these two mechanisms in terms of the POD modes and their corresponding dominant frequency, which in turn provides a quantitative means to single out the individual flow mechanism.

Continuous Focusing of Microparticles in Horizontally Actuated Rectangular Channels

We present a device to continuously focus microparticles in a liquid-filled open channel subjected to lateral vibration at frequencies of the order of 10 Hz. The vibration generates a capillary wave at the liquid-air interface. This capillary wave leads to a net motion of microparticles over multiple cycles, causing collection under the displacement nodes of the capillary wave. These accumulated particles are observed as a concentrated stream in the presence of a continuous flow along the open channel, which means that the channel can be designed such that the focused particle stream exits through one outlet, while the filtrate is removed via interspaced outlets on each side of the particle stream. A numerical model is proposed, which superimposes the periodic flow field due to the capillary wave and the inlet-induced transverse flow field between the inlet and the outlet. The model is utilized to predict the smallest distance from the inlet at which the focused stream of particles is obtained, termed here the collection length. In addition, experiments are performed for different channel and inlet sizes, vibration actuation amplitudes, flow rates, and particle sizes. By considering the design factors extracted from the modeling data, we demonstrate that the resulting device is capable of continuous particle collection down to 1 μm diameter, at flow rates of up to 1.2 ml/min.

PIV study of velocity distribution and turbulence statistics in a rod bundle

In reactor fuel assembly in PWR, spacer grids with mixing vanes are used in rod bundles to enhance flow mixing and heat transfer. Thus, the spacer grids have significant influence on the economy and safety of reactors. This paper focuses on visualization research on the single phase flow in rod bundle with spacer grids by Particle Image Velocimetry (PIV), and a measurement of multi-subchannels in the both transverse and longitudinal directions was conducted at the Reynolds number of 10,400. Matching Index of Refraction (MIR) technique was implemented to accurately measure the internal flow field in the rod bundle. The flow statistics of rod bundle were obtained, such as mean velocity, turbulence intensity and Reynolds stress. The velocity distribution and turbulence statistics in the longitudinal flow field are presented downstream the spacer grid. The streamwise development of symmetrical vortex array in the transverse flow field is also evaluated. A comparison of Reynolds stresses in the longitudinal and the transverse flow field is performed to reveal the momentum transfer process in three-dimensional flow field. Experimental results benefit the development and evaluation of spacer grid, and also provide the whole-field validation for numerical simulations.

Plane boundary effects on characteristics of propeller jets

The flow properties of a propeller jet in the presence of a plane bed boundary were investigated using the particle image velocimetry technique. Three clearance heights, Z b = 2D p, D p, and 0.5D p, where D p = propeller diameter, were used to examine boundary effects on the development of the jet. In each case, the mean flow properties and turbulence characteristics were measured in a larger field of view than those used in past studies. Both the streamwise and transverse flow fields were measured to obtain the three-dimensional characteristics of the propeller jet. Similar to a confined offset jet, the propeller jet also exhibits a wall attachment behavior when it is placed near a plane boundary. As a result, in contrast to its unconfined counterpart, the confined propeller jet features three regions, namely the free jet, impingement and wall jet regions. The study shows that the extent of each region varies under different clearance heights. The development of the mean flow and turbulence characteristics associated with varying clearance heights are compared to illustrate boundary effects in these regions. In the impingement region, the measured transverse flow fields provide new insights on the lateral motions induced by the impingement of the swirling jet. In the wall jet region, observations reveal that the jet behaves like a typical three-dimensional wall jet and its axial velocity profiles show good agreement with the classical wall jet similarity function.

Numerical simulation of transverse jet flow field under supersonic inflow

Transverse jet flow field under supersonic inflow is simulated numerically for studying the characteristic of fuel transverse jet and fuel mixing in scramjet combustion chamber. Comparison is performed between simulated results and the results of references and experiments. Results indicate that the CFD code in this paper is applicable for simulation of transverse jut flow field under supersonic inflow, but in order to providing more effective numerical predictive method, CFD code should be modified through increasing mesh density and adding LES module.

Optical Display of Transverse Jet Flow Field Under Supersonic Inflow

Optical display and measurement of transverse jet flow field under supersonic inflow is performed with continuous xenon lamp color schlieren, strobo light color schlieren and strobo light monochrome schlieren. Characteristic of fuel transverse jet in scramjet combustion chamber is studied. Results indicate that wave structure can be displayed through images obtained from long time exposure of schlieren, exposure time in dozens of nanoseconds is suitable for the display of turbulent structure, and comparing with color schlieren, amounts of information obtained from monochrome schlieren are few, whose sensitivity is much lower, either.

Constraints on the Jet-Medium Coupling from Measurements at RHIC and LHC

The measured data on the nuclear modification factor for pions and reconstructed jets as well as on the high-pT elliptic flow at RHIC and LHC energies are compared to results from a linear pQCD and a highly non-linear hybrid AdS holographic model of jet-energy loss. We find that the high-pT ellitic flow requires to include realistic medium transverse flow fields and a jet-medium coupling including the effects of the energy of the jet, the temperature of the bulk medium, and non-equilibrium effects close to the phase transition. We extend our jet-energy loss model that is coupled to state-of-the-art hydrodynamic prescriptions to backgrounds generated by the parton cascade BAMPS. We demonstrate that the results for the hydrodynamic and the parton-cascade backgrounds show a remarkable similarity. Unfortunately, the results for both the pion and a parton-jet nuclear modification factor are insensitive to the jet-path dependence of the models considered.

Significant of Isothermal Flow Studies for High Swirling Flow in Unconfined Burner <sup></sup>

This paper is presents numerical simulation of isothermal swirling turbulent flows in a combustion chamber of an unconfined burner. Isothermal flows of with three different swirl numbers, SN of axial swirler are considered to demonstrate the effect of flow axial velocity and tangential velocity to define the center recirculation zone. The swirler is used in the burner that significantly influences the flow pattern inside the combustion chamber. The inlet velocity, U0 is 30 m/s entering into the burner through the axial swirler that represents a high Reynolds number, Re to evaluate the differences of SN. The significance of center recirculation zone investigation affected by differences Re also has been carried out in order to define a good mixing of air and fuel. A numerical study of non-reacting flow into the burner region is performed using ANSYS Fluent. The Reynolds–Averaged Navier–Stokes (RANS) realizable k-ε turbulence approach method was applied with the eddy dissipation model. An attention is focused in the flow field behind the axial swirler downstream that determined by transverse flow field at different radial distance. The results of axial and tangential velocity were normalized with the U0. The velocity profiles’ behaviour are obviously changes after existing the swirler up to x/D = 0.3 plane. However, their flow patterns are similar for all SN after x/D = 0.3 plane towards the outlet of a burner.

The Initial Flow of Classical Gluon Fields in Heavy Ion Collisions

Using analytic solutions of the Yang-Mills equations we calculate the initial flow of energy of the classical gluon field created in collisions of large nuclei at high energies. We find radial and elliptic flow which follows gradients in the initial energy density, similar to a simple hydrodynamic behavior. In addition we find a rapidity-odd transverse flow field which implies the presence of angular momentum and should lead to directed flow in final particle spectra. We trace those energy flow terms to transverse fields from the non-abelian generalization of Gauss' Law and Ampere's and Faraday's Laws.

The Effect of Velocity in High Swirling Flow in Unconfined Burner

This paper presents a numerical simulation of swirling turbulent flows in combustion chamber of unconfined burner. Isothermal flows with three different swirl numbers using axial swirler are used to demonstrate the effect of flow in axial velocity and tangential velocity on the center recirculation zone. The significance of center recirculation zone is to ensure a good mixing of air and fuel in order to get a better combustion. The inlet velocity, U0 is 30 m/s entering into the burner through the axial swirler that is represents a high Reynolds number. A numerical study of non-reacting flow in the burner region is performed using ANSYS Fluent. The Reynolds–Averaged Navier–Stokes (RANS) standard k-ε turbulence approach method was applied with the eddy dissipation model. The paper focuses the flow field behind the axial swirler downstream that determined by transverse flow field at different on radial distances. The results of axial and tangential velocity were normalized with the inlet velocity. The velocity profiles are different after undergoing the different swirler up to the burner exit. However, the results of velocity profile showed that the high SN gives a better swirling flow patterns.

Longitudinal Dispersion in River Flows Characterized by Random Large-Scale Bed Irregularities: First-Order Analytical Solution

AbstractA stochastic Lagrangian approach is proposed for the analytical derivation of a longitudinal dispersion coefficient that accounts for both transverse and longitudinal flow field variability in straight-axis open channels characterized by longitudinal large-scale bed heterogeneity, that is, by longitudinal bed irregularities over a wide range of representative lengths, from the simple grain roughness to large undulations (up to order of kilometers), possibly related to topographical discontinuities or to extended and inhomogeneous depositional processes carried out by natural or anthropogenic agents. The resulting dimensionless expression, involving the average Chezy coefficient and the ratios of river width and longitudinal heterogeneity correlation length to the average flow depth, is obtained at the first order in the depth fluctuations as a time-dependent function given by the sum of three distinct components. The first main component is related to the transverse velocity distribution and would...

An observationally validated theory of viscous flow dynamics at the ice-shelf calving front

AbstractAn analytical theory is developed for ice flow velocity in a boundary layer couplet at the calving front. The theory has simple quantitative characteristics that relate ice front velocity to thickness, strain rate and shelf width, matching one set of empirically derived relationships (Alley and others, 2008) and implying that these relationships predict ice velocity rather than calving rate. The two boundary layers are where longitudinal and transverse flow fields change from the interior flow to patterns consistent with the calving-front stress condition. Numerical simulations confirm the analytical theory. The quantitative predictions of the theory have low sensitivity to unmeasured parameters and to shelf plan aspect ratio, while its robustness arises from its dependence on the scale invariance of the governing equations. The theory provides insights into calving, the stability of ice-shelf calving fronts, the stability of the grounding line of laterally resisted ice streams, and also suggests that the calving front is an instructive dynamical analogue to the grounding line.

‘Snap-shot’ velocity vector mapping using echo-planar imaging

A ‘snap-shot’ ultra-fast MRI velocimetry technique based upon the echo-planar imaging (EPI) pulse sequence is presented. The new technique is an extension of the GERVAIS pulse sequence previously developed by Sederman et al. (2004) and is capable of acquiring both reference and velocity encoded phase maps following a single excitation for generation of three-component velocity vectors in under 125 ms. This approach allows velocity images of systems with a dynamic, non-periodic geometry to be obtained by MRI. The technique proved to be accurate within 5% error by comparison with Poiseuille flow in a pipe and for the transverse plane flow field in a Couette cell. It was further applied to obtain the velocity field around an impeller in a stirred vessel; an unsteady yet periodic system which otherwise could only be studied by triggered acquisitions. Good agreement was evident between the present technique and triggered conventional velocity encoded pulse sequences. Finally, new experimental data attainable only by the new sequence is demonstrated as the flow field within a mobile droplet of oil is captured as it rises through a column of water. The technique promises to be highly useful in velocimetric measurements of dynamic, non-periodic systems, and in particular for the characterisation of multiphase flow systems.

A Test of Three Optical Flow Techniques—LCT, DAVE, and NAVE

A time sequence of high-quality images currently produced by high-resolution observations either from the ground or in space may be utilized to determine the transverse flow field on the plane of the sky with the help of optical flow techniques. We have examined the performance of three different methods—a well-known technique called local correlation tracking (LCT), a recently developed technique called the differential affine velocity estimator (DAVE), and a new technique called the nonlinear affine velocity estimator (NAVE)—using three kinds of image data: mapping-based synthetic images, a set of MHD simulation data, and real images (magnetograms) taken by the Solar Optical Telescope on board Hinode. We have generalized the model equation of image evolution by adding to the continuity equation a source term that is proportional to the image value. Synthetic images were constructed based on the analytical solution of this equation with different velocity profiles: uniform, affine, or nonaffine. The tests with the synthetic data indicated that NAVE is very good at detecting subpixel motions, superpixel motions, and nonuniform motions, while LCT is not good at detecting nonuniform motions, especially around critical points, and the performance of DAVE is degraded in the presence of superpixel motions. In all the methods, the performance became worse as the velocity field deviated more from an affine one. We also found that the MHD simulation data we used are not quite suited for discriminating between the three methods, maybe because the data do not contain enough structural information to be used for tracing. In contrast, the determination of velocity fields from the real image data was somewhat sensitive to the technique adopted. The technique of NAVE with the source term produced velocity fields that are the most consistent with the data.

Couette flow of a Bingham plastic in a channel with equally porous parallel walls

In the present paper the flow of a Bingham fluid between two parallel porous walls is studied. One of the walls moves with constant velocity parallel to the other, which is fixed, while a longitudinal pressure gradient exists, as well as a transverse flow field due the porosity of the walls. An exact analytical solution is given for the u-velocity field, which has four different forms depending on the values of the three dimensionless parameters, which are the Bingham, Couette and Reynolds numbers.