Jet Velocity Profile Research Articles

Application of particle image velocimetry for measuring He II thermal counterflow jets

The particle image velocimetry (PIV) technique was successfully applied for measuring the velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen–deuterium solid particles were used as tracer particles for PIV measurement. In the application, the normal component velocity was measured. The jet velocity profile and spatial decay of the jet velocity were compared with those of turbulent round jets of ordinary viscous fluids. The velocity measured near the jet nozzle exit was compared with the theoretical prediction for the normal component flow velocity.

Theory and kinematic measurements of the mechanics of stable electrospun polymer jets

We present a simplified approach to understanding the mechanics of stable electrospinning jets based on electrohydrodynamic theory that explicitly incorporates the extensional rheology of polymeric fluids. Flow regimes of electrospun jets are identified by analogy to uniaxial extension of a fluid jet. These flow regimes predict the limiting kinematics of electrospinning jets and identify dimensionless parameters important to the control and operation of electrospinning processes. In situ kinematic measurements validate model assumptions and scaling predictions, and allow the reduction of entire jet radius and velocity profiles to several key parameters. The model predictions are shown to hold both above and below the entanglement concentration, as well as for solutions with added electrolyte and increased conductivity. The analysis also enables direct measurement of the apparent extensional viscosity of solutions at the high extension rates experienced during electrospinning. Finally, dimensional analysis of the model yields a correlation for electrospun fiber diameter in terms of measurable fluid properties, controlled process parameters, and measured jet variables, demonstrating the influence of mechanics in the straight portion of the jet on ultimate fiber morphology.

Transient Ceiling Jet Temperature and Velocity Profiles in the Presence of an Upper Layer: Comparison with Predictions by LAVENT and JET Computer Fire Models

Results of a scaled experimental study are compared with the predictions produced by the LAVENT and JET computer fire models. As in most large-scale fire testing, characterization of heat release rate (HRR) and accuracy of its measurement creates a lot of uncertainty for validation of these codes. Sensitivity of LAVENT results to the assumed radiative loss of 35% is examined by varying the total HRR which corresponds to varying the convective HRR. Comparisons show that LAVENT uses the wall-jet theory at its core for prediction of the ceiling jet flow and thus does not capture the ceiling jet profile well and overpredicts the upper layer temperature during the development of the layer. The velocity is generally overpredicted since the retardation of the jet momentum by the upper layer does not seem to be modeled accurately. JET has a better user interface and slightly different parameters, but does not provide ceiling jet profiles. The major difference between JET and LAVENT include the ceiling jet temperature and velocity algorithms, the fusible link algorithms, and the use of a variable radiative fraction as a function of fire size and type.

Evaluation of high-velocity plunging jet-issuing characteristics as a basis for plunge pool analysis

This paper presents experimental work on the characteristics of jets issuing from water-releasing structures. Systematic experimental tests have been performed at near-prototype velocities of up to 30 m/s using a cylindrical jet nozzle. Dynamic pressures were measured along the diameter at the nozzle outlet with an acquisition rate of up to 2 kHz, for different upstream supply conditions. These measurements allowed assessing the mean and the fluctuating characteristics of the jet. The influence of supply circuit secondary currents, aeration and geometrical contraction on jet turbulence intensity and velocity profiles is discussed. The initial turbulence intensity varies from 2 to 8%, whereas the kinetic energy correction factor ranges from 1.0 to 1.1. Statistical analysis of pressure measurements shows that jet core pressures follow a Gaussian distribution fairly well for non-exceedence probabilities between 0.1 and 0.999. The experimental results and an extensive literature survey are used to define issuance parameters relevant for engineering practice.

可視化法を用いた極低温熱流動現象の研究 Ｈｅ ＩＩ熱カウンター流ジェット計測へのＰＩＶの応用

The particle image velocimetry (PIV) technique was successfully applied for measuring the velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen-deuterium solid particles were used as tracer particles for PIV measurement. In the application, the normal component velocity was measured. The jet velocity profile and spatial decay of the jet velocity were compared with those of round jets of ordinary viscous fluids. The velocity that was measured near the jet nozzle was compared with the theoretical prediction.

The effect of jet velocity profile on the characteristics of thickness and velocity of the liquid sheet formed by two impinging jets

In this study, the effect of jet velocity profile on the thickness and velocity of the liquid sheet formed by two low-speed impinging jets was investigated. In addition to the constant jet velocity and the Poiseuille parabolic profile, the jet velocity profile measured experimentally was considered to account for the real nonuniform jet velocity profile. For three jet velocity profiles, the distributions of the thickness and velocity of the liquid sheet were analytically predicted by solving conservation equations for mass, momentum, and energy. The predicted results were compared with the previous experimental results. The jet velocity profile affected the resulting thickness and velocity characteristics of the liquid sheet. The distributions of the thickness and velocity of the liquid sheet predicted by using the measured jet velocity profile produced more acceptable results, which agreed better with the experimental observations, than those obtained by using the constant jet velocity, which has been commonly used in previous theoretical works.

Effects of jet velocity profiles on a round jet in cross-flow

This paper reports the results of an experimental investigation on the effects of jet velocity profiles on the flow field of a round jet in cross-flow (JICF) using laser-induced fluorescence and digital particle-image velocimetry techniques (DPIV). Tophat and parabolic jets were considered, with the momentum ratios (MRs) ranging from 2.3 to 5.8. Results show that the thicker shear layer associated with a parabolic JICF is able to delay the formation of leading-edge and lee-side vortices when compared to the tophat JICF at the corresponding MR. As a result, there is an increase in jet penetration and a reduction in the near-field entrainment of cross-flow fluid by a parabolic JICF. Also, the less coherent nature of the leading-edge and lee-side vortices in a parabolic JICF is more likely to break up sporadically into smaller-scaled vortices. In addition, DPIV results show that a parabolic JICF exhibits not only a faster velocity recovery of cross-flow fluid at the jet lee-side than the corresponding tophat JICF, it also consistently registers a higher magnitude of the peak average vorticity than the tophat JICF for all MR considered. Despite these differences, the time-averaged flow topology for both cases share many salient features.

Kelvin-Helmholtz disruptions in extended magnetized jet flows

We numerically investigate the long-term temporal evolution of magnetized jets where the computational domain covers multiple wavelengths (up to 10) of the fastest growing Kelvin-Helmholtz unstable mode. The dynamical importance of the magnetic field, which is initially uniform and flow-aligned, varies over a significant range: the plasma β in the jets ranges from O(1000) (essentially hydrodynamical) down to O(1) (equipartition jets). Our calculations of two-dimensional, longitudinally periodic, extended slab configurations identify an inverse cascade process in the overall disruption to a broadened and heated jet flow. This process occurs for transonic and supersonic flows as well, with rapid shock-dominated transients appearing in supersonic cases, and with characteristic differences depending on the initial jet width. For configurations with a jet velocity profile having a radius that is much larger than the vorticity thickness of the flow, the cascade proceeds early through pairing/merging of individual mode structures on both jet boundaries. Jets with radii of the order of the vorticity thickness are strongly unstable to sinuous deformations with boundary layer-layer interactions between vortex (transonic, weak magnetic field) and shock (supersonic, strong field) structures in a few sound crossing times. We back up these findings for planar jets with selected three-dimensional simulations of extended cylindrical jet configurations. These tend to have more small-scale fluctuations in their relaxed endstates. The timescales and overall scenario for the helical disruptions agree well with the 2D studies. This allows us to discuss the possible implications of our results in the context of magnetohydrodynamic stability of astrophysical jets.

On a model equation that reflects some of the shear flow hydrodynamic stability properties

A model equation is proposed in the paper that mimics some of the shear flow hydrodynamic stability properties. It contains the basic velocity profile which can be arbitrarily chosen, and a nonlinear term, whose form can be appropriately adjusted to any particular problem. Full linear and weakly nonlinear theories for the Bickley jet velocity profile are elaborated. The solution of the linear problem is obtained in terms of associated Legendre functions. Within the weakly nonlinear theory a Landau equation is derived that describes the evolution of the perturbations near the critical wave number. The conditions for supercritical stability and subcritical instability are revealed. AMS Mathematics Subject Classification (2000) 76E05, 76E30.

A Specially Designed Air-Assisted Sprayer to Improve Spray Penetration and Air Jet Velocity Distribution Inside Dense Nursery Crops

New sprayers are needed to deliver droplets uniformly within dense nursery crops to obtain healthy and marketable plants in the nursery industry. An air-assisted sprayer with five-port nozzles was developed and investigated to improve spray penetration into dense nursery canopies. Spray deposits at the top, middle, and bottom of canopies were characterized using fluorescence detection, and were compared at three nozzle heights in a commercial nursery field. Dynamic air velocities corresponding to the deposit sampling locations inside and outside the canopy were measured at the time when the sprayer passed over the canopies. Air jet velocity profiles from nozzle outlets to 0.79 m below the nozzles were determined experimentally and mathematically. The measured air jet velocity decreased from 40.1 to 19.4 m/s when the distance from the orifice increased from 0.33 to 0.79 m. The peak velocities at the top and middle elevations for both inside and outside the canopy decreased as the nozzle height above the ground increased, but the peak velocities at the bottom elevation for both situations were not significantly decreased as the nozzle height increased. The average period of time for dynamic air velocities higher than 1 m/s inside the canopy was 1.9 s at the bottom, 3.8 s at the middle, and 1.1 s at the top of canopies. The mean spray deposit inside taxus canopies with leaf area index of 5.96 increased in an exponential function as the peak air velocity increased. The spray penetration capability and spray deposition uniformity inside taxus canopies were greatly improved with the five-port air-assisted sprayer.

Impinging laminar jets at moderate Reynolds numbers and separation distances

An experimental and numerical study of impinging, incompressible, axisymmetric, laminar jets is described, where the jet axis of symmetry is aligned normal to the wall. Particle streak velocimetry (PSV) is used to measure axial velocities along the centerline of the flow field. The jet-nozzle pressure drop is measured simultaneously and determines the Bernoulli velocity. The flow field is simulated numerically by an axisymmetric Navier-Stokes spectral-element code, an axisymmetric potential-flow model, and an axisymmetric one-dimensional stream-function approximation. The axisymmetric viscous and potential-flow simulations include the nozzle in the solution domain, allowing nozzle-wall proximity effects to be investigated. Scaling the centerline axial velocity by the Bernoulli velocity collapses the experimental velocity profiles onto a single curve that is independent of the nozzle-to-plate separation distance. Axisymmetric direct numerical simulations yield good agreement with experiment and confirm the velocity profile scaling. Potential-flow simulations reproduce the collapse of the data; however, viscous effects result in disagreement with experiment. Axisymmetric one-dimensional stream-function simulations can predict the flow in the stagnation region if the boundary conditions are correctly specified. The scaled axial velocity profiles are well characterized by an error function with one Reynolds-number-dependent parameter. Rescaling the wall-normal distance by the boundary-layer displacement-thickness-corrected diameter yields a collapse of the data onto a single curve that is independent of the Reynolds number. These scalings allow the specification of an analytical expression for the velocity profile of an impinging laminar jet over the Reynolds number range investigated of .

The preheated Airy wall jet

The Airy jet is a wall-bounded flow belonging to the similarity class of the well known free jet but, in contrast to the latter, its far field behavior is an algebraically decaying rotational flow. The velocity and temperature distributions of a preheated Airy jet flowing over an insulated wall are investigated using both analytical and numerical methods, and are compared with those of the classical (preheated) exponentially decaying wall jet. For the same value of the dimensionless skin friction parameter, the maximum of the similar velocity profile of the Airy jet exceeds that of the classical wall jet by approximately 20%. The dimensionless temperature along the insulated wall scales for large values of the Prandtl number with Pr2/3 for both jets, while for small values of the Prandtl number the temperature scales with Pr1/3 for the Airy jet and goes to 1 for the classical wall jet.

Study of trajectories of jets in crossflow using direct numerical simulations

This paper studies the trajectories and near field of round jets in crossflow. Incompressible direct numerical simulations are performed at velocity ratios of 1.5 and 5.7 and the effects of jet velocity profile and boundary layer thickness on the jet trajectory are examined. The ‘rd’ scaling used at present (Margason 1993) does not contain any information on these parameters, and trajectories scaled by rd do not collapse. The trajectory is strongly influenced by the near field which depends on both the jet velocity profile and the crossflow boundary layer. A length scale is proposed to describe the near field of the jet. An analytical expression is proposed for this length scale which is a measure of the relative inertia of the jet and the crossflow. Incorporating this length scale significantly improves the scaling of the trajectories.

CFD MODELLING TO TRACK THE PATH OF PESTICIDE DROPLETS FROM AN AIR-ASSISTED ORCHARD SPRAYER

To predict the fate of pesticide droplets generated by an air-assisted orchard sprayer, a CFD model was developed. The modelling was based on a Lagrangian multiphase flow model. The effects of fan rotational and sprayer ground speeds on the droplet distribution were studied. The simulation was done for fan speeds of 1360 and 1836 rev/min, and tractor speeds of 1.67 and 2.58 m/s. To consider the speed of the tractor a moving coordinate system was implemented. At the exit of the fan, the vertical velocity profile of the air jet was measured and used as an inlet boundary condition for the model. The droplets were injected from eight 80° hollow cone nozzles with a 5 bar pressure. The distribution of the sprayed droplets was highly influenced by fan rotational and sprayer ground speeds. With an increase in sprayer ground speed, there was an increase in the deflection of droplets opposite to the driving direction. The distance the droplets travelled to the direction of the jet was increasing with an increase in fan speed, but it was decreased with an increase in tractor ground speed. There was a good agreement between the predicted and measured vertical droplet distributions. Canopy interaction was not considered at this stage.

Variation of velocity profile of jet and its effect on interfacial stability

Linear stability theory is used to study a double fluid model for a liquid jet surrounded by a coaxial gas steam. Under the different pressure gradients for liquid and gas flow, the variation of the velocity profile in the model and the thickness of the shear layer were investigated. The effects of such variation on the interfacial stability were discussed with the application of Chebyshev spectral collocation method.

Phenomenological model for determining velocity field of LPG jet in combustion chamber of direct injection S.I. engine

A phenomenological model has been established to predict the velocity distribution of LPG (Liquefied Petroleum Gas) jet in combustion chamber of spark ignition (SI) engine. A shaped coefficient \(\beta\) governing the similarity of velocity profiles of LPG jets has been defined based on the theoretical and experimental analyses of turbulent diffusion jets. The results show that \(\beta\) is constant for steady jet but it is not the case for unsteady one. The model will enable us to calculate the velocity profiles of LPG jet after ending injection. This is necessary for research of stratified combustion in direct injection LPG SI engines.

WALL JET DEVELOPMENT IN A TURBULENT RECIRCULATING CAVITY FLOW

A turbulent wall jet within a confining recirculating cavity was examined using flow visualization, particle image velocimetry (PIV), and laser Doppler velocimetry (LDV). The PIV technique was used macroscopically for flow visualization as well as at a much finer resolution to determine detailed velocities within the developing wall jet. Comparison data collected using laser Doppler velocimetry (LDV) was used for validation purposes. The wall jet was well characterized using conventional unbounded wall jet parameters. Wall jet velocity profiles collapsed onto a single profile with the application of the appropriate parameters, indicating some degree of similarity after the initial jet region and up to the mid wall distance. Distances past this region were characterized by a rapid decay in the jet caused by the adverse pressure gradient created by the bounding wall. It was found that the wall jet growth rate within the recirculating cavity was greater than that of an unbounded wall jet in a stagnant fluid. This phenomenon is attributed to the recirculating flow, confining boundaries, and the initial jet exit velocity conditions.

Forcing a planar jet flow using MEMS

We present the results of an experimental study in which a planar laminar jet of air was forced by an array of micro-electromechanical systems (MEMS) micro-actuators. In the absence of forcing, the velocity profile of the experimental jet matched the classic analytic solution. Driving actuators on either side of the jet in-phase or anti-phase, respectively, excited the symmetric or anti-symmetric mode of instability of the jet. Asymmetric forcing, using MEMS actuators on only one side of the jet, was also investigated.

Variability of Turbulence and Its Outer Scales in a Model Tropopause Jet

Abstract Variability of flow regimes and turbulence scalings in a model of an inhomogeneously stratified, tropopause jet is investigated through high-resolution, forced, three-dimensional numerical simulations (with 512 or 1024 vertical levels). Multivalued scaling branches, with respect to the local gradient Richardson number (Rig), are shown to occur in several turbulent quantities (such as variances, fluxes, mixing efficiency, outer scales, and their ratios, etc.). Two distinct scaling curves are found for the upper and lower flanks of the jet, each showing a clear branch switching at a critical height. Distinct scaling curves in the upper and lower flanks of the jet can be related to the doubling of buoyancy frequency (N) in the background profile (across the model tropopause). The vertical levels corresponding to the inflection points (maximal shearing) in the quasi-equilibrium turbulent mean jet velocity profile is identified as the best criterion for branch switching along each of the scaling curve...

Effect of Particle Properties on Falling Velocity of Powder Jet

We measured the velocity and number density of powder jet falling from a hopper by using a phase Doppler anemometer and obtained the following results. The center line velocity of powder jet near the hopper exit is close to the free falling velocity of a single particle in disregared of the fluid drag, which is independent of the diameter, density and mass flow rate of particle, and far from the exit tends to approach the terminal settling velocity of a single particle. When the particle diameter is small and the mass flow rate is increased, the ratio of the center line velocity to the terminal velocity increases. The axial fluctuation velocity profile of powder jet takes a minimum at the jet axis and after taking a maximum decrases toward the outer edge. The profile becomes flat when the mass flow rate is small and the falling distance is increased. The number density profile takes a maximum near the jet axis and rapidly decreases toward the outer edge. The center line number density of the small particle is larger than that of the large particle. Itis inferred that the dimensionless mass flow rate of the entrained air is increased with increasing the falling distance, being independent of particle size and density.