Jet Regime Research Articles

Model development in OpenFOAM to predict spillway jet regimes

Hydropower is the most important renewable energy source. Though hydropower provides abundant benefits, dams have also altered natural flow conditions affecting fish habitat. Elevated total dissolved gas (TDG) can result in gas bubble disease in affected fish. TDG production depends on the gas volume fraction and bubble depth in the tailrace, which are a function of spillway jet regimes. This paper presents a model developed in OpenFOAM to predict spillway jet regimes. The model utilizes the volume of fluid method to capture the dynamic free surface. A Large Eddy Simulation model, together with Detached Eddy Simulation, was used for turbulence closure. The model adequately reproduced jet regimes observed in a reduced-scale laboratory model. Differences in jet regimes predicted at reduced and prototype scales were observed. Results suggest that turbulence, not scaled in the laboratory model, plays an important role in the flow characteristics downstream of spillways.

Regimes during liquid drop impact on a liquid pool

Water drops falling on a deep pool can either coalesce to form a vortex ring or splash, depending on the impact conditions. The transition between coalescence and splashing proceeds via a number of intermediate steps, such as thick and thin jet formation and gas-bubble entrapment. We perform simulations to determine the conditions under which bubble entrapment and jet formation occur. A regime map is established for Weber numbers ranging from 50 to 300 and Froude numbers from 25 to 600. Vortex ring formation is seen for all of the regimes; it is greater for the coalescence regime and less in the case of the thin jet regime.

Large-scaled simulation on the coherent vortex evolution of a jet in a cross-flow based on lattice Boltzmann method

Large eddy simulation (LES) is performed on a jet issued normally into a cross-flow using lattice Boltzmann method (LBM) and multiple graphic processing units (multi-GPUs) to study the flow characteristics of jets in cross-flow (JICF). The simulation with 8 1.50?10 grids is fulfilled with 6 K20M GPUs. With large-scaled simulation, the secondary and tertiary vortices are captured. The features of the secondary vortices and the tertiary vortices reveal that they have a great impact on the mixing between jet flow and cross-flow. The qualitative and quantitative results also indicate that the evolution mechanism of vortices is not constant, but varies with different situations. The hairpin vortex under attached jet regime originates from the boundary layer vortex of cross-flow. While, the origin of hairpin vortex in detached jet is the jet shear-layer vortex. The mean velocities imply the good ability of LBM to simulate JICF and the large loss of jet momentum in detached jet caused by the strong penetration. Besides, in our computation, a high computational performance of 1083.5 MLUPS is achieved.

Flows produced by the combined oscillatory rotation and translation of a circular cylinder in a quiescent fluid

AbstractFlows produced by a circular cylinder undergoing oscillatory rotation and translation in a quiescent fluid have been studied via direct numerical simulations. The incompressible Navier–Stokes equations were solved for large dimensionless time windows using an immersed boundary method with adaptive Cartesian grid refinement. Parametric studies were conducted in two dimensions on the Reynolds number, Keulegan–Carpenter number and phase shift. In addition to the previously reported net thrust case (Blackburn et al., Phys. Fluids, vol. 11, 1999, pp. 4–6), the study catalogued the appearance of several streaming jet regimes with varying deflection angles, deflected and horizontal vortex shedding regimes, and a double mirrored jet regime with varying inter-jet angles, as well as several chaotic cases. Visualizations are presented to clarify each observed flow regime and to illustrate the parameter space. Connections are drawn between these canonical bluff-body deflected wakes and a similar phenomenon observed in aerofoils oscillating at high reduced frequencies in a cross-flow. Also, the discovery of the streaming jet regimes with varying deflection angles opens the door for using these flows as a low-Reynolds-number propulsive mechanism requiring only a two-degree-of-freedom actuator. Simulation results suggest that the flow phenomena observed in two dimensions persist in three dimensions, despite spanwise fluctuations.

Features of two-phase flow in a rectangular microchannel with the height of 300 μm

The two-phase flow in a narrow short horizontal channel of rectangular cross section with the width of 10 mm and height of 300 μm was studied experimentally. The features of the jet regime were studied in detail. It is shown that an increase in superficial velocity of jet pulsation frequency increases, and the ratio of unwetted area of the upper channel wall to the wetted area of the upper channel wall decreases.

A Case Study of Mid-Atlantic Nocturnal Boundary Layer Events during WAVES 2006

AbstractThe Water Vapor Variability-Satellite/Sondes (WAVES) 2006 field campaign provided a contiguous 5-day period of concentrated high-resolution measurements to examine finescale boundary layer phenomena under the influence of a summertime subtropical high over the mid-Atlantic region that is characterized by complex geography. A holistic analytical approach to low-level wind observations was adopted to identify the low-level flow structures and patterns of evolution on the basis of airmass properties and origination. An analysis of the measurements and the other available observations is consistent with the classic depiction of the daytime boundary layer development but revealed a pronounced diurnal cycle that was categorized into three stages: (i) daytime growth of the convective boundary layer, (ii) flow intensification into a low-level jet regime after dusk, and (iii) interruption by a downslope wind regime after midnight. The use of the field campaign data allows for the differentiation of the latter two flow regimes by their directions with respect to the orientation of the Appalachian Mountains and their airmass origins. Previous studies that have investigated mountain flows and low-level jet circulations have focused on regions with overt geographic prominence, stark gradients, or frequent reoccurrences, whereby such meteorological phenomena exhibit a clear signature and can be easily isolated and diagnosed. The results of this study provide evidence that similar circulation patterns operate in nonclassic locations with milder topography and atmospheric gradients, such as the mid-Atlantic region. The new results have important implications for the understanding of the mountain-forced flows and some air quality problems during the nocturnal period.

Diffusion and Filamentary Regime of an Atmospheric Pressure DC Plasma Jet in Nitrogen and Air

An atmospheric pressure direct current plasma jet is produced in nitrogen and air. The active region and afterglow region of the plasma jet at various discharge currents and gas flow rates are investigated. Depending on the gas flow and the current, the discharge can be sustained at diffusion or filamentary modes.

How does a shear boundary layer affect the stability of a capillary jet?

The stability of a capillary jet with a shear boundary layer growing over its free surface is studied theoretically. The effect of an infinitely thin boundary layer is first examined from the corresponding singularly perturbed eigenvalue problem. Then, a linear stability analysis of the Navier-Stokes equations for a finite boundary layer thickness verifies the consistency of the previous asymptotic study. We show that, for sufficiently long distances from the boundary layer origin, Rayleigh's dispersion relation is recovered, which means that the layer does not affect the jet's stability in that case. However, for distances on the order of the jet's radius, the perturbation growth factor, the most unstable wave number, and the range of unstable wave numbers increase, while the convective-to-absolute instability transition delays drastically. These results establish a general framework to explain a variety of capillary jet phenomena, from the extended jetting regime of capillary jets in gas co-flows to fundamental features of the dripping faucet.

The Transition to a Subtropical Jet Regime and Its Maintenance

Abstract An abrupt transition from a merged jet regime to a subtropical jet regime is analyzed using a two-layer modified quasigeostrophic (QG) spherical model. Unlike the common version of QG models, this model includes advection of the zonal mean momentum by the ageostrophic mean meridional circulation, allowing for a relatively realistic momentum balance in the tropics and subtropics. The merged jet is a single jet inside the Ferrel cell created by a merging of the subtropical and eddy-driven jets, and the subtropical jet is a mainly thermally driven jet at the Hadley cell edge. The maintenance of each type of jet depends on the dominant baroclinic modes. In the merged jet regime, the spectrum is dominated by intermediate-scale (wavenumbers 4–6) fast waves at the midlatitudes that grow close to the jet maximum. In the subtropical jet regime, the spectrum is dominated by long (wavenumbers 1–3) slow westward-propagating waves at high latitudes and somewhat weaker intermediate-scale slow waves at the midlatitudes. In the subtropical jet regime, waves equilibrate at weaker amplitudes than in the merged jet regime. A mechanism is found that explains why baroclinic instability is weaker in the subtropical jet regime, although the vertical shear of the mean flow is stronger, which has to do with the lower-level potential vorticity (PV) structure. The relevance of these results to the real atmosphere seams to hold in local zonal sections but not for the zonal mean.

Mean Flow in a Submerged Hydraulic Jump with Baffle Blocks

AbstractThe flow in a submerged hydraulic jump with three-dimensional (3D) baffle blocks was experimentally studied. An incoming jet with varying supercritical Froude numbers was deflected with one or two rows of 3D baffle blocks for different downstream water depths. Depending on the submergence factor, two flow regimes were observed for which the time-averaged 3D velocity field was measured with an acoustic Doppler velocimeter (ADV). Measurements were conducted at different stations located both upstream and downstream of the blocks and above the crest of the blocks. Velocity measurements were made in a plane located at the center of the middle block (i.e., the centerplane of the flume) and, also, in a plane located at the midpoint between the blocks (off-centerplane). The mean flow pattern was found to be significantly different for the two flow regimes. In the deflected surface jet (DSJ) regime, which occurred at low submergence factors, the major flow features were a small surface roller upstream of ...

Numerical Modelling of the Hydrodynamics and Total Dissolved Gas Downstream of Hells Canyon Dam

A computational fluid dynamics model was developed to predict the flow pattern and total dissolved gas (TDG) distribution downstream of Hells Canyon Dam on the border of Idaho and Oregon, USA. Free surface simulations in the tailrace were carried out to predict spillway jet regimes with and without deflectors. Model results agreed with deflector performance curves obtained in a 1:48 laboratory scale model. The distribution of TDG was predicted with a mixture model that accounts for the two-phase flow in the tailrace and the mass transfer between bubbles and water. Attenuation of the turbulence at the free-surface and the effect of the bubbles on the turbulence was included to capture water attraction towards spillway surface jets, which affects the amount of water exposed to bubbles and downstream TDG dilution. The model takes into account changes in bubble size with dissolution and pressure. Good agreement was obtained between predicted TDG and available measurements in the field.

Numerical study of the reentrant jet and twin vortex flow regimes in the ventilated cavitation

Ventilated cavitation is generated artificially by injection of air through very precise openings in the profile. The main parameters of a ventilated cavitation are the gas supply injected rate and the gas leakage rate at rear of the cavity. The unsteady ventilated cavitation is characterized by gas regimes, of which there is three types [3]: portion gas-leakage, twin vortex gas-leakage, gas-leakage from pulsating surpercavities. The experimental observations show, in the twin vortex gas-leakage case, for low ventilation air flow coefficient it is the reentrant jet regime which appears. The present numerical study concerns the twin vortex gas leakage regime around the conical head bodies. The study is based on the 2D numerical code IZ models unsteady cavitating flows. In the present work, dissolved non-condensable gas is taken into account in the transport equation-based model in this numerical code. The numerical results are compared and validated with experimental measures [1] and an analysis of the transition between the reentrant jet and twin vortex is presented.

Laboratory and modeling studies on the effects of water and soot emissions and ambient conditions on the properties of contrail ice particles in the jet regime

Abstract. Contrails and contrail-induced cirrus clouds are identified as the most uncertain components in determining aviation impacts on global climate change. Parameters affecting contrail ice particle formation immediately after the engine exit plane (< 5 s in plume age) may be critical to ice particle properties used in large-scale models predicting contrail radiative forcing. Despite this, detailed understanding of these parametric effects is still limited. In this paper, we present results from recent laboratory and modeling studies conducted to investigate the effects of water and soot emissions and ambient conditions on near-field formation of contrail ice particles and ice particle properties. The Particle Aerosol Laboratory (PAL) at the NASA Glenn Research Center and the Aerodyne microphysical parcel model for contrail ice particle formation were employed. Our studies show that exhaust water concentration has a significant impact on contrail ice particle formation and properties. When soot particles were introduced, ice particle formation was observed only when exhaust water concentration was above a critical level. When no soot or sulfuric acid was introduced, no ice particle formation was observed, suggesting that ice particle formation from homogeneous nucleation followed by homogeneous freezing of liquid water was unfavorable. Soot particles were found to compete for water vapor condensation, and higher soot concentrations emitted into the chamber resulted in smaller ice particles being formed. Chamber conditions corresponding to higher cruising altitudes were found to favor ice particle formation. The microphysical model captures trends of particle extinction measurements well, but discrepancies between the model and the optical particle counter measurements exist as the model predicts narrower ice particle size distributions and ice particle sizes nearly a factor of two larger than measured. These discrepancies are likely due to particle loss and scatter during the experimental sampling process and the lack of treatment of turbulent mixing in the model. Our combined experimental and modeling work demonstrates that formation of contrail ice particles can be reproduced in the NASA PAL facility, and the parametric understanding of the ice particle properties from the model and experiments can potentially be used in large-scale models to provide better estimates of the impact of aviation contrails on climate change.

Are the Winters 2010 and 2012 Archetypes Exhibiting Extreme Opposite Behavior of the North Atlantic Jet Stream?*

AbstractThe atmospheric circulation over the North Atlantic–European sector experienced exceptional but highly contrasting conditions in the recent 2010 and 2012 winters (November–March, with the year dated by the relevant January). Evidence is given for the remarkably different locations of the eddy-driven westerly jet over the North Atlantic. In the 2010 winter the maximum of the jet stream was systematically between 30° and 40°N (south jet regime), whereas in the 2012 winter it was predominantly located around 55°N (north jet regime). These jet features underline the occurrence of either weak flow (2010) or strong and persistent ridges throughout the troposphere (2012). This is confirmed by the very different occurrence of blocking systems over the North Atlantic, associated with episodes of strong cyclonic (anticyclonic) Rossby wave breaking in 2010 (2012) winter. These dynamical features underlie strong precipitation and temperature anomalies over parts of Europe, with detrimental impacts on many socioeconomic sectors. Despite the highly contrasting atmospheric states, mid- and high-latitude boundary conditions do not reveal strong differences in these two winters. The two winters were associated with opposite ENSO phases, but there is no causal evidence of a remote forcing from the Pacific sea surface temperatures. Finally, the exceptionality of the two winters is demonstrated in relation to the last 140 years. It is suggested that these winters may be seen as archetypes of North Atlantic jet variability under current climate conditions.

Effects of jet/vortex interaction on contrail formation in supersaturated conditions

The effects of the interaction between a model of aircraft trailing vortex and engine jets on the formation of a contrail in supersaturated ambient air are studied using Eulerian-Lagrangian two-phase flows large-eddy simulations. The three-dimensional structure of the contrail, the mean flow properties, and the statistical correlations between ice and water vapor are analyzed for different jet and vortex parameters and different soot particle numbers. The interaction is characterized by the entrainment of the jets by the trailing vortex. In the four-engines case, particles exhausting the outboard jets are exposed to local higher supersaturation due to the temperature drop in the low-pressure vortex core region. The soot particle number affects both the structure and the global characteristics of the contrail. The increase in soot loading results in stronger vapor depletion – which leads to larger ice production – and to smaller ice particles for a given amount of vapor exhaust. The fraction of activated particles decreases with soot loading because of the increased competition for the vapor available for condensation. Of particular interest is the interaction between the jet/vortex turbulence and ice microphysics (activation, condensation, particle size distribution, and optical depth). It is found that the characteristic timescales of mixing and condensation can be of the same order in the jet regime. For the present high ambient supersaturation conditions, the competition between the entrainment of humid ambient air and vapor depletion plays an important role in determining the initial growth of the contrail and the spatial and size distributions of ice particles.

Measurements in High Void-Fraction Bubbly Wakes Created by Ventilated Supercavitation

A study of ventilated supercavitation in the reentrant jet regime has been carried out in the high-speed water tunnel at St. Anthony Falls Laboratory as the hydrodynamics part of an interdisciplinary study on stability and control of high-speed cavity-running bodies. The work is aimed at understanding the interaction between a ventilated supercavity and its turbulent bubbly wake, with the goal to provide the information needed for the development of control algorithms. Particle image velocimetry (PIV) measurements in high-void fraction bubbly wakes created by the collapse of ventilated supercavities are reported. Bubble velocity fields are obtained and are shown to submit to the same high Reynolds number similarity scaling as the single-phase turbulent axisymmetric wake. A grayscale technique to measure local average void fraction is outlined. Results of a time-resolved PIV experiment at 2000 Hz, using an adaptive masking scheme based on a sliding intensity threshold filter, are also presented.

Erratum to “Integral solutions for some turbulent quantities of small-scale hydrogen leakage: Non-buoyant jet or momentum-dominated regime of buoyant jet” [Int J Hydrogen Energy 34 (2009) 1607–1612], by M.F. El-Amin and H. Kanayama

Erratum to “Integral solutions for some turbulent quantities of small-scale hydrogen leakage: Non-buoyant jet or momentum-dominated regime of buoyant jet” [Int J Hydrogen Energy 34 (2009) 1607–1612], by M.F. El-Amin and H. Kanayama

Study of Unforced and Modulated Film-Cooling Jets Using Proper Orthogonal Decomposition—Part I: Unforced Jets

The effects of jet flow-rate modulation were investigated in the case of a 35 deg inclined jet in cross-flow over a flat plate using Mie scattering visualizations, time-resolved flow rate records, and large eddy simulations (LES). An unforced jet study was conducted over a wide range of blowing ratios to provide a baseline for comparison to the pulsed results. The two distinct and well known steady jet regimes (attached jet with high film cooling performance for BR < 0.4 and detached jet with poor film cooling performance for BR > 1.0) were related to the dynamics of characteristic vortical structures, significant in the transition from one regime to the other. Similarity of the inclined jet results with a past vertical jet study are also put in perspective when comparing wall adiabatic effectiveness results. 3D proper orthogonal decomposition (3D-POD) was performed on LES results of an unforced case at BR = 0.15 to provide an analysis of dominant modes in the velocity and temperature fields. Error calculations on the reconstructed fields provided an estimation of the number of modes necessary to obtain satisfactory reconstruction while revealing some of the shortcomings associated with POD.

Investigating the particle dispersion in a spout-fluid bed using particle trajectory

Abstract Particle dispersion in a spout-fluid bed was investigated using time series of particle trajectory by evaluation of dispersion coefficient based on Einstein’s expression. A series of experiments were performed in a lab-scale spout-fluid bed using microwave heating and infrared thermal imaging (MH-ITI) technique for recording the position of a tracer. The influence of gas velocity on the particle dispersion in different flow regimes has been taken into account. The results show that the particle dispersion behavior depends on the flow regime. In the flow regime of internal jet, the increase of gas velocity improves the axial dispersion. In the case of jet in fluidized bed with bubbling, increasing fluidizing gas velocity promotes the radial dispersion at the lower part of bed and the axial dispersion at the upper part of bed. In the flow regime of jet in fluidized bed with slugging, the effect of fluidizing gas is related to the slug pattern. The result also indicates that the transition of flow regime can be evaluated from the particle trajectory.

New Approximations and Calibration Methods to Provide Routine Real-Time Polarimetry on JET

The increasing importance of providing reliable polarimetric measurements in real time, for both machine protection and plasma control, has motivated the development of a quick version of the calibration algorithms for JET polarimeter. This new code, which interprets the calibration procedure performed before each shot, is based on a physical equivalent model of the diagnostic optical path and is valid for any operational regime of JET. It provides results before the plasma breakdown, and, with its estimates of the optical paths parameters, the polarimetric measurements have an accuracy more than sufficient for real-time purposes. New approximate equations have been validated in order to obtain the line integrated density from the newly calibrated horizontal chords, so that also these polarimetric measurements can also be used for density feedback and machine protection. The availability of reliable polarimetric measurements in real time opens new perspectives also to the determination of the plasma boundary, the magnetic equilibrium, and their use in advanced feedback control schemes.