Full Velocity Field Research Articles

Turbulent transport in an inclined jet in crossflow

The present study experimentally investigates a turbulent jet in crossflow relevant to film cooling applications. The jet is inclined at 30°, and its mean velocity is the same as the crossflow. Magnetic resonance imaging is used to obtain the full three-dimensional velocity and concentration fields, whereas Reynolds stresses are obtained along selected planes by Particle Image Velocimetry. The critical role of the counter-rotating vortex pair in the mixing process is apparent from both velocity and concentration fields. The jet entrainment is not significantly higher than in an axisymmetric jet without crossflow, because the proximity of the wall inhibits the turbulent transport. Reynolds shear stresses correlate with velocity and concentration gradients, consistent with the fundamental assumptions of simple turbulence models. However the eddy viscosity is strongly anisotropic and non-homogeneous, being especially low along the leeward side of the jet close to injection. Turbulent diffusion acts to decouple mean velocity and concentration fields, as demonstrated by the drop in concentration flux within the streamtube issued from the hole. Volume-averaged turbulent diffusivity is calculated using a mass–flux balance across the streamtube emanating from the jet hole, and it is found to vary slowly in the streamwise direction. The data are compared with Reynolds-Averaged Navier–Stokes simulations with standard k−ε closure and an optimal turbulent Schmidt number. The computations underestimate the strength of the counter-rotating vortex pair, due to an overestimated eddy viscosity. On the other hand the entrainment is increasingly underpredicted downstream of injection. To capture the correct macroscopic trends, eddy viscosity and eddy diffusivity should vary spatially in different ways. Therefore a constant turbulent Schmidt number formulation is inadequate for this flow.

Leaving flatland: Diagnostics for Lagrangian coherent structures in three-dimensional flows

Finite-time Lyapunov exponents (FTLE) are often used to identify Lagrangian Coherent Structures (LCS). Most applications are confined to flows on two-dimensional (2D) surfaces where the LCS are characterized as curves. The extension to three-dimensional (3D) flows, whose LCS are 2D structures embedded in a 3D volume, is theoretically straightforward. However, in geophysical flows at regional scales, full prognostic computation of the evolving 3D velocity field is not computationally feasible. The vertical or diabatic velocity, then, is either ignored or estimated as a diagnostic quantity with questionable accuracy. Even in cases with reliable 3D velocities, it may prove advantageous to minimize the computational burden by calculating trajectories from velocities on carefully chosen surfaces only. When reliable 3D velocity information is unavailable or one velocity component is explicitly ignored, a reduced FTLE form to approximate 2D LCS surfaces in a 3D volume is necessary. The accuracy of two reduced FTLE formulations is assessed here using the ABC flow and a 3D quadrupole flow as test models. One is the standard approach of knitting together FTLE patterns obtained on adjacent surfaces. The other is a new approximation accounting for the dispersion due to vertical (u,v) shear. The results are compared with those obtained from the full 3D velocity field. We introduce two diagnostic quantities to identify situations when a fully 3D computation is required for an accurate determination of the 2D LCS. For the ABC flow, we found the full 3D calculation to be necessary unless the vertical (u,v) shear is sufficiently small. However, both methods compare favorably with the 3D calculation for the quadrupole model scaled to typical open ocean conditions.

The effect of velocity shear on dynamo action due to rotating convection

AbstractRecent numerical simulations of dynamo action resulting from rotating convection have revealed some serious problems in applying the standard picture of mean field electrodynamics at high values of the magnetic Reynolds number, and have thereby underlined the difficulties in large-scale magnetic field generation in this regime. Here we consider kinematic dynamo processes in a rotating convective layer of Boussinesq fluid with the additional influence of a large-scale horizontal velocity shear. Incorporating the shear flow enhances the dynamo growth rate and also leads to the generation of significant magnetic fields on large scales. By the technique of spectral filtering, we analyse the modes in the velocity that are principally responsible for dynamo action, and show that the magnetic field resulting from the full flow relies crucially on a range of scales in the velocity field. Filtering the flow to provide a true separation of scales between the shear and the convective flow also leads to dynamo action; however, the magnetic field in this case has a very different structure from that generated by the full velocity field. We also show that the nature of the dynamo action is broadly similar irrespective of whether the flow in the absence of shear can support dynamo action.

Characteristics of tsunami motion and energy budget during runup and rundown processes over a plane beach

Motions of tsunami waves during runup and rundown processes on the uniform sloping beach are studied numerically by the fully nonlinear and highly dispersive Boussinesq equations. We first study the leading-depression N-wave defined by solitary theory. The shoreline movement and its moving speed are analyzed during the wave approaching the coastal area. Details on the flow field and the energy transformation are obtained in terms of the reconstruction of the full velocity field by Boussinesq equations. In addition, solitary wave is studied as a comparison. The different energy budget explains the phenomenon that the N-wave leads much larger runup than the solitary wave in some specific situations. The investigation is then extended to the patterns of the energy transformation of N-shape waves, hump-like waves, and sinusoidal long waves. These waves are of the same order in scale with the recent giant tsunamis. The results show that the potential energy of the N-shape tsunami waves nearly reaches the maximum while that of the hump-like tsunami waves does not reach the maximum at the maximum runup, meanwhile the kinetic energy of both waves does not go to zero. For the sinusoidal wave train, however, its potential energy reaches the maximum and its kinetic energy goes to zero exactly at the maximum runup. To understand the responses caused by the variation of the waveforms, effects of the nonlinearity and the dispersion on the energy budget are studied in the geophysical tsunamis order. Moreover, the regularities of the energy budget of some general cases including the leading-depression N-waves (LDNs), the leading-elevation N-wave, and generalized LDNs are investigated extensively. The mechanism of the energy budget of these waves is quite different as just considering the potential energy at the maximum runup.

THREE-DIMENSIONAL VELOCITY AND DENSITY RECONSTRUCTIONS OF THE LOCAL UNIVERSE WITH COSMICFLOWS-1

This paper presents an analysis of the local peculiar velocity field based on the Wiener Filter (WF) reconstruction method. We used our currently available catalog of distance measurements containing 1797 galaxies within 3000 km s−1: Cosmicflows-1. The WF method is used to recover the full three-dimensional peculiar velocity field from the observed map of radial velocities and to recover the underlying linear density field. The velocity field within a data zone of 3000 km s−1 is decomposed into a local component that is generated within the data zone and a tidal one that is generated by the mass distribution outside that zone. The tidal component is characterized by a coherent flow toward the Norma–Hydra–Centaurus (Great Attractor) region, while the local component is dominated by a flow toward the Virgo Cluster and away from the Local Void. A detailed analysis shows that the local flow is predominantly governed by the Local Void and the Virgo Cluster plays a lesser role. The analysis procedure was tested against a mock catalog. It is demonstrated that the WF accurately recovers the input velocity field of the mock catalog on the scale of the extraction of distances and reasonably recovers the velocity field on significantly larger scales. The Bayesian WF reconstruction is carried out within the ΛCDM WMAP5 framework. The WF reconstruction draws particular attention to the importance of voids in proximity to our neighborhood. The prominent structure of the Local Supercluster is wrapped in a horseshoe collar of under density with the Local Void as a major component.

Experimental and numerical investigation of the internal kinematics of a surf-zone plunging breaker

Over the last couple of decades both the qualitative and quantitative understanding of breaking waves in the surf zone have greatly increased. This is due to the advances in experimental and numerical techniques. However, few comparisons between these two different investigative techniques for surf-zone breaking waves have been reported.In this study, a comparison is made between the experimental and numerical investigation of the internal kinematics of a surf-zone plunging breaker. The full-field velocity measuring technique known as Particle Image Velocimetry (PIV) is used in the experiments. In the hybrid numerical scheme, the main model solves the Navier–Stokes equations using a Finite Volume method and the free-surface is simulated using a Volume of Fluid (VOF) method. An important feature of this work is that, unlike in most other comparisons between numerical and experimental results, the exact geometry of the physical wave flume and the exact motion of the physical wavemaker are duplicated in the numerical wave tank. To achieve this, an additional numerical model using a Boundary-Integral Method (BIM) is employed to generate the input conditions for the Navier–Stokes solver.Very good agreement was found for all comparisons: free-surface elevations, velocity vector maps, velocity profiles and velocity-magnitude contours. However, some small discrepancies were observed. In the free-surface elevation comparisons, a slight time lag was observed in the numerical results and it is suggested that this was due to the small amount of smoothing applied in the BIM to enable it to continue to supply input data to the Navier–Stokes solver well beyond the breaking of the wave. In addition, some small differences were also found between the numerically predicted velocity distributions and those measured in the experiments. These disagreements occurred mostly in the aerated region and it is proposed that they could be caused by errors in the PIV velocity data due to air bubble effects. However, they could also be attributed to the fact that no turbulence model is used in the numerical scheme and it is these aerated areas where the turbulence levels are the highest.

The equivalence of the Lagrangian-averaged Navier-Stokes-α model and the rational large eddy simulation model in two dimensions

In the large eddy simulation (LES) framework for modeling a turbulent flow, when the large scale velocity field is defined by low-pass filtering the full velocity field, a Taylor series expansion of the full velocity field in terms of the large scale velocity field leads (at the leading order) to the nonlinear gradient model for the subfilter stresses. Motivated by the fact that while the nonlinear gradient model shows excellent a priori agreement in resolved simulations, the use of this model by itself is problematic, we consider two models that are related, but better behaved. The rational LES model that uses a sub-diagonal Pade approximation instead of a Taylor series expansion, and the Lagrangian averaged Navier-Stokes-α model that uses a regularization approach to modeling turbulence. In this article, we show that these two latter models are identical in two dimensions.

Spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 from InSAR and GPS observations

In this study, the spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 is monitored using interferometric synthetic aperture radar (InSAR) data acquired by the European Remote-Sensing Satellites (ERS-1 and ERS-2) and the Environmental Satellite (ENVISAT). These data are combined with continuous global positioning system (GPS) measurements to identify four discrete episodes of caldera subsidence and uplift, these episodes are: 1992–1995 (subsidence of 2.7 cm/year), 1996–2000 (subsidence of 0.5 cm/year, with local uplift of 1.7 cm/year at Norris), 2000–2004 (subsidence of 0.7 cm/year, with local uplift of 0.6 cm/year at Norris), and 2004–2009 (uplift of 3–8 cm/year, with local subsidence of 1–4 cm/year at Norris). We construct the full three-dimensional velocity field of Yellowstone deformation for 2005–2006 from ascending and descending ENVISAT orbits. The InSAR three-dimensional velocity field and three-component GPS measurements indicate that the majority of the observed deformation (3–8 cm/year) across the Yellowstone caldera and near Norris Geyser Basin (NGB) occurred in the vertical direction between the summers of 2005 and 2006. During this time, significant lateral displacements of 3–7 cm/year also occurred in the east–west direction at the southeastern and western rims of the Yellowstone caldera and in the area between Hebgen Lake and NGB. Minor north–south displacements of about 0.2 cm/year also occurred, however, in the southwestern section of the caldera and near Yellowstone Lake during the same period. The calculated three-dimensional velocity field for 2005–2006 implies the existence of two pressure-point sources, beneath the two structural resurgent domes in the Yellowstone caldera, connected by a planar conduit, rather than a single, large sill as proposed in previous studies. Furthermore, no measurable displacements occurred along any fault zone across the caldera during the entire period of observation (1992–2009). Therefore, we infer that magmatic and hydrothermal processes beneath the Yellowstone caldera and NGB were the main sources of deformation.

Heat Transfer From Novel Target Surface Structures to a 3×3 Array of Normally Impinging Water Jets

Impinging jet arrays provide a means to achieve high heat transfer coefficients and are used in a wide variety of engineering applications such as electronics cooling. The objective of this paper is to characterize the heat transfer from an array of 3×3 submerged and confined impinging water jets to a range of target surface structures. The target surfaces consisted of a flat surface, nine 90 deg swirl generators, a 6×6 pin fin array, and nine pedestals with turn-down dishes that turned the flow to create an additional annular impingement. In order to make comparisons with a previous single jet study by the authors, each impinging jet within the array was geometrically constrained to a round, 8.5 mm diameter, square-edged nozzle at a jet exit-to-target surface spacing, of H/D=0.5. A custom measurement facility was designed and commissioned in order to measure the heat transfer coefficient and the pressure loss coefficient of each of the target surface augmentations. The heat transfer results are presented in terms of Nu/Pr0.4, and the pressure results are presented in terms of pressure loss coefficient. Comparing the array of jets to a single jet showed a decrease in heat transfer. Full field velocity magnitude images showed that this decrease in heat transfer was caused by neighboring jet interference cross-flow coupled with a greater back pressure effect. The analysis of the different target surface augmentations showed that the performance of the pedestal with the turn-down dish was the least compromised by the addition of the surrounding jets. It showed both the highest fin efficiency of 95.1% and fin effectiveness of 2.27. However, it showed the highest overall pressure loss coefficient compared with the other target surfaces, and therefore the nine 90 deg swirl generators performed the best in terms of both pressure loss coefficient and thermal performance. The findings of this paper are of practical relevance to the design of primary heat exchangers for high-flux thermal management applications, where the boundaries of cooling requirements continue to be tested.

Mixing of dense binary suspensions: Multi‐component hydrodynamics and spatial phase distribution by PEPT

AbstractThe three‐component flow field and spatial phase distribution of binary mixtures of glass particles suspended in water in a stirred vessel have been resolved using positron emission particle tracking (PEPT). The Lagrangian flow data provided by the technique have been converted to give a detailed Eulerian description of each of the three phase components of the flow generated by a pitched blade turbine. For the first time, it has been possible to determine the full 3D velocity and concentration fields of the liquid phase and both the solid components within opaque dense slurries of this type containing up to 40 wt % solids. Spatial distributions of local time‐averaged slip velocity have also been obtained for each solid component showing wide variations. The detailed PEPT measurements have enabled the solids mass balance and the mass continuity of the three phase components to be accurately verified throughout the vessel. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Three-dimensional flow in electromagnetically driven shallow two-layer fluids

Recent experiments on a freely evolving dipolar vortex in a homogeneous shallow fluid layer have clearly shown the existence and evolution of complex three-dimensional (3D) flow structures. The present contribution focuses on the 3D structures of a dipolar vortex evolving in a stable shallow two-layer fluid. Experimentally, Stereoscopic Particle Image Velocimetry is used to measure instantaneously all three components of the velocity field in a horizontal plane and 3D numerical simulations provide the full 3D velocity and vorticity fields over the entire flow domain. Remarkably, the experimental results, supported by the numerical simulations, show to a large extent the same 3D structures and evolution as in the single-layer case. The numerical simulations indicate that the so-called frontal circulation in the two-layer fluid is due to deformations of the internal interface. The 3D flow structures will also affect the distribution of massless passive particles released at the free surface. With numerical studies it is shown that these passive particles tend to accumulate or deplete locally where the horizontal velocity field is not divergence-free. This is in contrast with pure two-dimensional incompressible flows where the divergence of the velocity field is zero by definition.

PTV experiments of subcooled boiling flow through a vertical rectangular channel

Time resolved Particle Tracking Velocimetry (PTV) experiments were carried out to investigate turbulent, subcooled boiling flow of refrigerant HFE-301 through a vertical rectangular channel with one heated wall. Measurements were performed with liquid Reynolds numbers (based on the hydraulic diameter) of Re = 3309, 9929 and 16,549 over a wall heat flux range of 0.0–64.0 kW/m 2. Turbulence statistics are inferred from PTV full-field velocity measurements. Quantities such as: instantaneous 2D velocity fields, time-averaged axial and normal velocities, axial and normal turbulence intensities, and Reynolds stresses are obtained. The present results agree well with previous studies and provides new information due to the full-field nature of the technique. This work is an attempt to provide turbulent subcooled boiling flow data for validation and improvement of two-phase flow computational models.

PEPT measurements of solid–liquid flow field and spatial phase distribution in concentrated monodisperse stirred suspensions

The technique of positron emission particle tracking (PEPT) has been used to determine the two-phase flow field and spatial phase distribution in a stirred vessel containing coarse glass particles suspended in water. The Lagrangian flow data provided by PEPT have been converted to give a detailed Eulerian description of the two-phase flow generated by a pitched blade turbine (PBT) operating in up-pumping or down-pumping mode. For the first time, it has been possible to determine the full 3D velocity and concentration fields of both the liquid and the solid phase within an opaque dense slurry of this type containing up to 40 wt% solids. The detailed PEPT measurements have also enabled the solids mass balance and the mass continuity of the two phases to be accurately verified throughout the vessel. The data show that the pumping effectiveness of the PBT is unaffected at moderate solid fractions, but is considerably reduced at high fractions and even more so in the up-pumping mode. A uniformity index based on the variance of the local solids concentration in the vessel shows that, overall, a down-pumping PBT achieves a significantly better homogeneity than an up-pumping PBT.

Experimental observations of flow modifications in nanofluid boiling utilizing particle image velocimetry

In order to understand the effects of nanofluids upon boiling, pool boiling experiments involving both pure water and water based nanofluids were investigated. Point temperature and full velocity field measurements were obtained during pool boiling conditions using state-of-the-art dynamic particle image velocimetry (DPIV). Specifically, results of nanofluid's effect upon nucleate boiling heat transfer are investigated, in an effort to find engineering solutions in critical applications such as nuclear energy. A change in the hydrodynamic behavior of bubbles was observed with the introduction of nanoparticles. The measured velocity fields exhibited different fluid behavior for the various cases investigated which in turn highlights the complexity of the fluid flow in the pool boiling experiment. The results show a relation between wall temperature and nanofluid concentration.

Measurements of jet flows impinging into a channel containing a rod bundle using dynamic PIV

In this investigation, full-field velocity profiles of jet flows impinging into a rod bundle were measured using Particle Image Velocimetry (PIV) technique in a facility with Matched-Index of Refraction. The test section consisted of a 1 m long rectangular channel with a cross-sectional area of 76.2 × 76.2 mm 2. Water was pumped through either single or double jets located on the top wall of the channel. The Reynolds numbers of the inlet jets were varied from 4470 to 13,400. Inside the channel adjacent to the inlet jets, a rod bundle was configured with 29 staggered tubes of 10.67 mm outside diameter (O.D.). The time-resolved velocity fields were obtained for regions within the rod bundle from which the vorticity fields and the turbulent characteristics of those regions were calculated. The results are useful for developing new models to help predict the flow patterns in the lower plenum of Gas Cooled Reactors.

Non-intrusive experimental investigation of flow behavior inside a 5 × 5 rod bundle with spacer grids using PIV and MIR

The validity of the simulation results from computational fluid dynamics (CFD) is still under scrutiny. Some existing CFD closure models for complex flow produce results that are generally recognized as being inaccurate. Development of improved models for complex flow simulation requires an improved understanding of the detailed flow structure evolution with dynamic interaction of the flow multi-scales. Thus, the goal of this work is to contribute to a better understanding of presupposed and existent events that could affect the safety of nuclear power plants. The fundamental phenomena of fluid flow in rod bundles with spacer grids can be elucidated by using state-of-the-art measurement techniques. This study aims to develop an experimental data base with high spatial and temporal resolution of fluid flow velocity inside a 5 × 5 rod bundles with spacer grids. The full-field detailed data base is intended to validate CFD codes at various temporal-spatial scales. Measurements are carried out using dynamic particle image velocimetry (DPIV) technique inside an optically transparent rod bundle utilizing the matching index of refraction (MIR) approach. This work presents full field velocity vectors and turbulence statistics for a rod bundle under single phase flow conditions.

Experimental study and large eddy simulation for the turbulent flow around four cylinders in an in-line square configuration

The turbulent flows around four cylinders in an in-line square configuration with different spacing ratios of 1.5, 2.5, 3.5 and 5.0 have been investigated experimentally at subcritical Reynolds numbers from 11,000 to 20,000. The mean and fluctuating velocity distributions were obtained using the laser Doppler anemometry (LDA) measurement. The digital particle image velocimetry (DPIV) was employed to characterize the full field vorticity and velocity distributions as well as other turbulent quantities. The experimental study indicated that several distinct flow patterns exist depending on the spacing ratio and subcritical Reynolds number for turbulent flow. The three-dimensional numerical simulations were also carried out using the large eddy simulation (LES) at Reynolds number of 15,000 with the spacing ratio of 1.5 and 3.5. The results show that the LES numerical predictions are in good agreement with the experimental measurements. Therefore, the three-dimensional vortex structures and the full field instantaneous and mean quantities of the flow field such as velocity field, vorticity field, etc., which are very difficult to obtain experimentally, can be extracted from the simulation results for the deepening of our understanding on the complex flow phenomena around four cylinders in in-line configuration.

State and parameter estimation of spatiotemporally chaotic systems illustrated by an application to Rayleigh–Bénard convection

Data assimilation refers to the process of estimating a system's state from a time series of measurements (which may be noisy or incomplete) in conjunction with a model for the system's time evolution. Here we demonstrate the applicability of a recently developed data assimilation method, the local ensemble transform Kalman filter, to nonlinear, high-dimensional, spatiotemporally chaotic flows in Rayleigh-Bénard convection experiments. Using this technique we are able to extract the full temperature and velocity fields from a time series of shadowgraph measurements. In addition, we describe extensions of the algorithm for estimating model parameters. Our results suggest the potential usefulness of our data assimilation technique to a broad class of experimental situations exhibiting spatiotemporal chaos.

Positron emission particle tracking in a mechanically agitated solid–liquid suspension of coarse particles

The technique of positron emission particle tracking (PEPT) is a unique non-intrusive Lagrangian flow visualisation technique which allows probing of opaque fluids and within opaque apparatus. It uses a single positron-emitting particle as flow tracer which is tracked in 3D space and time to reveal its full Lagrangian trajectory. PEPT was used to study the mixing of a concentrated suspension of coarse glass particles, containing up to 10.6 wt% solids, in a vessel agitated by a pitched blade turbine operating in both up-pumping and down-pumping modes. The Lagrangian information obtained was used to obtain a detailed Eulerian description of the two-phase flow inside the vessel. For the first time, it has been possible to determine the full 3D velocity and concentration fields of both the liquid and the solid phase within an opaque flow of this type.

The three-dimensional structure of an electromagnetically generated dipolar vortex in a shallow fluid layer

Many experiments have been performed in electromagnetically driven shallow fluid layers to study quasi-two-dimensional (Q2D) turbulence, the shallowness of the layer commonly is assumed to ensure Q2D dynamics. In this paper, however, we demonstrate that shallow fluid flows exhibit complex three-dimensional (3D) structures. For this purpose we study one of the elementary vortex structures in Q2D turbulence, the dipolar vortex, in a shallow fluid layer. The flow evolution is studied both experimentally and by numerical simulations. Experimentally, stereoscopic particle image velocimetry is used to measure instantaneously all three components of the velocity field in a horizontal plane, and 3D numerical simulations provide the full 3D velocity and vorticity fields over the entire flow domain. It is found that significant and complex 3D structures and vertical motions occur throughout the flow evolution, i.e., during and after the forcing phase. We conclude that the bottom friction is not the main mechanism leading to three-dimensionality of the flow but rather the impermeability of the boundaries. It is further shown that free-surface deformations, i.e., gravity waves, are of minor importance too as a mechanism to generate 3D motion. Furthermore, it is demonstrated that the observations are not due to three-dimensionality introduced by the forcing mechanism but intrinsically due to the flow dynamics itself. The flow evolution is analyzed with respect to its quasi-two-dimensionality by adopting the ratio of “horizontal” to “vertical” kinetic energies, the normalized horizontal divergence, and a measure of the relaxation to a Poiseuille-like profile. An important observation is that, although the relative magnitude of the vertical velocity as compared to the horizontal flow components decreases for decreasing fluid depth, the vertical profile of the horizontal flow relaxes rather slowly to a Poiseuille-like profile, i.e., not faster than the bottom friction time scale.