Characteristics of a Jet in the Vicinity of a Free Surface

2012 ◽  
Vol 134 (3) ◽  
Author(s):  
Jiahao Tian ◽  
Vesselina Roussinova ◽  
Ram Balachandar
Keyword(s):  
Shear Stress ◽  
Free Surface ◽  
Normal Component ◽  
Central Plane ◽  
Free Jets ◽  
Piv Measurements ◽  
Free Jet ◽  
Double Peaks ◽  
Surface Jet ◽  
Surface Jets

In this study, the characteristics of a round turbulent jet in the vicinity of a free surface are investigated. The jet issued from a nozzle located at a depth five times the nozzle diameter (d = 10 mm) below and parallel to the free surface. The jet exit velocity was 2.8 m/s and the resulting Reynolds number was 28,000. Instantaneous two-dimensional PIV measurements were obtained in the vertical central plane and in several horizontal planes at various distances (y/d = 0,±1,±2,±3± 4) from the axis of the nozzle. All fields-of-view were positioned at streamwise locations in the range of 28 < x/d < 62, where the jet interacts significantly with the free surface. The results reveal that the behavior of the surface jet is very similar to that of the free jet before it interacts with the free surface which occurs at about x/d = 30. Beyond this, the velocity normal to the free surface is diminished and those parallel to the free surface are enhanced in the region near the free surface. In the horizontal plane near the free surface (y/d = +4), the spreading of the surface jet is significantly greater than that of the free jet. The mean lateral flow in this region tends to be outward everywhere for the surface jet, while the opposite trend occurs in the free jet. Turbulence intensities in all three directions are reduced by the effect of the free surface confinement. Near the free surface, at y/d = +4, unlike the single peak streamwise turbulence intensity profile noticed in the case of the free jet, the off-axis double peaks reappear in the case of the surface jet. The magnitude of shear stress in the vertical central plane of the surface jet is smaller than that noticed in the free jet near the free surface. In identical horizontal planes, the shear stress (-uw¯) profiles are similar in both free jets and surface jets in regions where the interaction with the free surface is not significant (x/d ≈ 30). As the downstream distance increases near the free surface, the magnitudes of the shear stress profiles are larger compared to that of the free jet. An increase in the normal component of vorticity is observed in the horizontal planes near the free surface.

Toward Modular Prediction of Free-Surface Jet Array Cooling: The Hydraulic Jump Location and Non-Monotonous Heat Transfer

2016 ◽  
Author(s):  
Herman D. Haustein
Keyword(s):  
Heat Transfer ◽  
Free Surface ◽  
Hydraulic Jump ◽  
Liquid Extraction ◽  
Jet Interaction ◽  
Free Jet ◽  
Key Issues ◽  
Surface Jet ◽  
Prediction Approach ◽  
Jet Array

The present study develops the ground work for modular prediction of free-surface jet arrays. Jet arrays generate one of the highest single-phase heat transfer rates, while covering reasonably large areas with good thermal uniformity, relevant to electronics cooling. However, due to liquid evacuation problems, free-jet arrays suffer from flooding, cross-flow and jet interaction, together with the large amount of influencing geometrical parameters, this makes them very difficult to predict. For the modular prediction approach to be applied, key issues are here addressed: experiments were conducted employing de-ionized water in both single and basic multiple-jet array (2×2, with local liquid extraction in the jet interaction zones) configurations. Modular conditions, wherein all jets are similar to each other, were created experimentally in a consistent fashion, by use of liquid extraction in the jet-interaction zones. Based on present and previous experimental data the influencing parameters on the pre-jump depth were identified. This description was then used to predict the location of the hydraulic jump (as dependant on the measured post-jump depth). The model combines elements of two previous approaches the shallow-water vs. jump conservation model, and obtains good agreement with available data. In addition conditions were shown for maximizing the distance at which the hydraulic jump occurs — to the point that the supercritical flows of adjacent jets touch (standing fountain type jump). This not only permits prediction of the supercritical flow heat transfer distribution over almost the entire array area, but also reduces the low heat transfer post-jump regions to a minimum. Finally, a more universal single-jet heat transfer model was developed incorporating inherent self-similarities recently identified by the authors and considering all relevant parameters: jet velocity profiles, nozzle-plate spacing, and inclination relative to gravity, to predict stagnation heat transfer as well as its radial decay. It is further identified that the influence of inclination is also of vital importance to free-surface jets (breakage of symmetry) and must be examined in future studies. By addressing these three key issues the foundation for a modular prediction of heat transfer under a free jet array is laid.

Evaluation of Turbulent Kinetic Energy Dissipation Rate in a Free Jet Flow from PIV Measurements

2012 ◽  
Vol 7 (1) ◽  
pp. 53-69
Author(s):  
Vladimir Dulin ◽  
Yuriy Kozorezov ◽  
Dmitriy Markovich
Keyword(s):  
Energy Dissipation ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Dissipation Rate ◽  
Energy Dissipation Rate ◽  
Turbulent Velocity ◽  
Small Scale ◽  
Velocity Fluctuations ◽  
Piv Measurements ◽  
Free Jet

The present paper reports PIV (Particle Image Velocimetry) measurements of turbulent velocity fluctuations statistics in development region of an axisymmetric free jet (Re = 28 000). To minimize measurement uncertainty, adaptive calibration, image processing and data post-processing algorithms were utilized. On the basis of theoretical analysis and direct measurements, the paper discusses effect of PIV spatial resolution on measured statistical characteristics of turbulent fluctuations. Underestimation of the second-order moments of velocity derivatives and of the turbulent kinetic energy dissipation rate due to a finite size of PIV interrogation area and finite thickness of laser sheet was analyzed from model spectra of turbulent velocity fluctuations. The results are in a good agreement with the measured experimental data. The paper also describes performance of possible ways to account for unresolved small-scale velocity fluctuations in PIV measurements of the dissipation rate. In particular, a turbulent viscosity model can be efficiently used to account for the unresolved pulsations in a free turbulent flow

Bedforms in a turbulent stream: formation of ripples by primary linear instability and of dunes by nonlinear pattern coarsening

2010 ◽  
Vol 649 ◽  
pp. 287-328 ◽  
Author(s):  
ANTOINE FOURRIÈRE ◽  
PHILIPPE CLAUDIN ◽  
BRUNO ANDREOTTI
Keyword(s):  
Grain Size ◽  
Shear Stress ◽  
Free Surface ◽  
Stability Analysis ◽  
Sediment Transport ◽  
Phase Shift ◽  
Linear Instability ◽  
Basal Shear ◽  
Flat Sand ◽  
Nonlinear Pattern

It is widely accepted that both ripples and dunes form in rivers by primary linear instability; the wavelength of the former scaling on the grain size and that of the latter being controlled by the water depth. We revisit here this problem in a theoretical framework that allows to give a clear picture of the instability in terms of dynamical mechanisms. A multi-scale description of the problem is proposed, in which the details of the different mechanisms controlling sediment transport are encoded into three quantities: the saturated flux, the saturation length and the threshold shear stress. Hydrodynamics is linearized with respect to the bedform aspect ratio. We show that the phase shift of the basal shear stress with respect to the topography, responsible for the formation of bedforms, appears in an inner boundary layer where shear stress and pressure gradients balance. This phase shift is sensitive to the presence of the free surface, and the related effects can be interpreted in terms of standing gravity waves excited by topography. The basal shear stress is dominated by this finite depth effect in two ranges of wavelength: when the wavelength is large compared to the flow depth, so that the inner layer extends throughout the flow, and in the resonant conditions, when the downstream material velocity balances the upstream wave propagation. Performing the linear stability analysis of a flat sand bed, the relation between the wavelength at which ripples form and the flux saturation length is quantitatively derived. It explains the discrepancy between measured initial wavelengths and predictions that do not take this lag between flow velocity and sediment transport into account. Experimental data are used to determine the saturation length as a function of grain size and shear velocity. Taking the free surface into account, we show that the excitation of standing waves has a stabilizing effect, independent of the details of the flow and sediment transport models. Consequently, the shape of the dispersion relation obtained from the linear stability analysis of a flat sand bed is such that dunes cannot result from a primary linear instability. We present the results of field experiments performed in the natural sandy Leyre river, which show the formation of ripples by a linear instability and the formation of dunes by a nonlinear pattern coarsening limited by the free surface. Finally, we show that mega-dunes form when the sand bed presents heterogeneities such as a wide distribution of grain sizes.

Modeling Free Jet Trajectory at an Overfall and Resulting Shear Stress Distribution in the Plunge Pool

1993 ◽  
Vol 36 (5) ◽  
pp. 1309-1318 ◽  
Author(s):  
A. W. Fogle ◽  
J. C. McBurnie ◽  
B. J. Barfield ◽  
K. M. Robinson
Keyword(s):  
Shear Stress ◽  
Stress Distribution ◽  
Shear Stress Distribution ◽  
Free Jet ◽  
Plunge Pool ◽  
Jet Trajectory

scholarly journals Assessment of CFD Solvers and Turbulent Models for Water Free Jets in Spillways

Fluids ◽  
2020 ◽  
Vol 5 (3) ◽  
pp. 104
Author(s):  
António Muralha ◽  
José F. Melo ◽  
Helena M. Ramos
Keyword(s):  
Experimental Data ◽  
Sensitivity Analysis ◽  
Velocity Field ◽  
Numerical Simulations ◽  
Free Water ◽  
Concentration Profiles ◽  
Air Concentration ◽  
Free Jets ◽  
Free Jet ◽  
Turbulent Models

The capability of two different OpenFOAM® solvers, namely interFoam and twoPhaseEulerFoam, in reproducing the behavior of a free water jet was investigated. Numerical simulations were performed in order to obtain the velocity and air concentration profiles along the jet. The turbulence intensity was also analyzed. The obtained results were compared with published experimental data and, in general, similar velocity and air concentration profiles were found. InterFoam solver is able to reproduce the velocity field of the free jet but has limitations in the simulation of the air concentration. TwoPhaseEulerFoam performs better in reproducing the air concentration along the jet, the results being in agreement with the experimental data, although the computational runs are less stable and more time consuming. The sensitivity analysis of the inlet turbulent intensity showed that it has no influence in the characteristics of the jet core. With this research it is possible to conclude that: interFoam with k-Epsilon (k-ε) turbulence model is the best choice if the goal of the numerical simulations is the simulation of the velocity field of the jet. Meanwhile, twoPhaseEulerFoam with mixturek-Epsilon (mk-ε) shall be considered if the objective is the simulation of the velocity field and the air concentration.

The Aerodynamic Approach to Furnace Design

1959 ◽  
Vol 81 (4) ◽  
pp. 361-369 ◽  
Author(s):  
J. H. Chesters
Keyword(s):  
Open Hearth ◽  
Open Hearth Furnace ◽  
Hearth Furnace ◽  
Low Velocity ◽  
Droplet Dynamics ◽  
Free Jets ◽  
Free Jet ◽  
Side Walls ◽  
Air Streams ◽  
Fuel Stream

Flow patterns and mixing in actual furnaces can be best appreciated by starting with free jets and proceeding via jets in simple envelopes to jets (cold or alight) fed with surrounding air streams and impacting on surfaces. The fuel stream in an open-hearth furnace behaves initially as a free jet, entraining the relatively low velocity air around it, but on hitting the bath it splashes and runs forward and up the side walls. The gases reaching the roof eject flux droplets and then divide, part recirculating to meet the oncoming air and part joining the main flow to the exit. Future progress requires more knowledge of droplet dynamics, and demands more symmetrical flow, control of recirculation, or radical changes.

Hydrodynamics of quenching with impinging free-surface jet

2012 ◽  
Vol 55 (13-14) ◽  
pp. 3677-3685 ◽  
Author(s):  
Nitin Karwa ◽  
Lukas Schmidt ◽  
Peter Stephan
Keyword(s):  
Free Surface ◽  
Surface Jet
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