Turbulent Round Jet Research Articles

Influence of ambient conditions on evaporation and transport of respiratory droplets in indoor environment

Respiratory droplets are playing a significant role in the transmission of any flu type disease as well as SARS-Cov-2 virus. The presence of pathogens affects the evaporation of the liquid droplets along with ambient temperature and relative humidity (rh). Complete evaporation of droplets leads to the formation of aerosol or droplet nuclei which remain suspended in the air for a longer period of time and get spread over larger distances increasing the risk of disease transmission. In present work, a droplet evaporation model has been formulated considering the droplet as a salt solution and the formation of crystals has been taken into account which will be analogous to the aerosol formation. After the establishment of the evaporation model, the trajectories of the droplets are investigated considering a turbulent round jet model during exhalation. Aerosols are found to be spreading over distances of 8 to 9 m which is quite alarming. Large droplets get converted to smaller ones but the viral loading of the large droplets is much higher than the smaller as viral loading is proportional to initial size. This is highlighted by the viral load contour and the mean diameter line contour for a half-height window. Different weather conditions are investigated to observe the evaporation of droplets and the formation of aerosols in order to qualitatively analyse the risks associated with each city in specific weather conditions. Hot and dry conditions are most favourable to aerosol formation.

Calibration and data reduction for X-hotwires using cross validation

A strategy for calibration of X-wire probes and data inversion is described in this article. The approach used has elements of full velocity vs yaw-angle calibration with robust curve fitting. The responses of an X-wire probe placed in a calibration jet are recorded for a set of velocity and yaw inputs followed by fitting cross-validated splines. These spline functions trained from calibration data are evaluated for the probe responses during measurement. X-wire probes are calibrated for low to moderate velocities (0.65 m/s to 32 m/s) and yaw angles in the range −40° to 40° and comparisons with conventional interpolation schemes are made. The proposed algorithm can be extended to calibration of other multiple wire probes and for higher velocities. Some measurements in a single round turbulent jet flow at high Reynolds number using the proposed inversion algorithm are also presented. The present scheme is found to perform better particularly at low flow magnitudes and/or extreme flow angles than the schemes used previously.

A Study of RANS Turbulence Models in Fully Turbulent Jets: A Perspective for CFD-DEM Simulations

This paper presents an analysis of linear viscous stress Favre averaged turbulence models for computational fluid dynamics (CFD) of fully turbulent round jets with a long straight tube geometry in the near field. Although similar work has been performed in the past with very relevant solutions, considerations were not given for the issues and limitations involved with coupling between an Eulerian and Lagrangian phase, such as in fully two-way coupled CFD-DEM. These issues include limitations on solution domain, mesh cell size, wall modelling, and momentum coupling between the two phases in relation to the particles size. Therefore, within these considerations, solutions are provided to the Navier–Stokes equations with various turbulence models using a three-dimensional wedge long straight tube geometry for fully developed turbulence flow. Simulations are performed with a Reynolds number of 13,000 and 51,000 using two different tube diameters. It is found that a modified k-ε turbulence model achieved the most agreeable results for both the velocity and turbulent flow fields between these two flow regimes, while a modified k-ω SST/BSL also provided suitable results.

Direct numerical simulation of forced turbulent round jet: Effect of flow confinement and varicose excitation

Direct numerical simulation of a turbulent round jet subjected to varicose excitation has been carried out. The effect of domain size and waveform used for providing varicose excitation have been studied with the help of time-averaged mean, fluctuating quantities, and instantaneous isosurfaces of the Q-criterion. Initial evolution of the jet suggests that the secondary instability is delayed in time with an increase in the domain size irrespective of the waveform. It has also been observed that the secondary instability manifests stronger for the square wave based excitation as compared to sinusoidal excitation for the smaller and medium domains. In addition, simulations demonstrate that the aforementioned secondary instability is sustained in the long term for small and medium domains. In the case of a confined domain, simulations indicate that square wave based excitation leads to greater enhancement in mixing and entrainment characteristics of the jet when compared to sinusoidal excitation. We demonstrate that sine pulsing at the inlet excites energy up to the second harmonic of the preferred mode while square pulsing excites energy (at least) up to the fifth harmonic which results in more energetic small-scales structures in the far field which in turn augment the mixing characteristics of jet. Qualitative assessment of vortical structures indicates that differently excited jets gradually become similar in the far field of large domains owing to the availability of sufficient amounts of fluid for entrainment. This behavior has also been quantitatively established by means of axial and lateral profiles of both time-averaged as well as fluctuating quantities characterizing the pulsed jet.

Axisymmetric jet manipulation using multiple unsteady minijets

A turbulent round jet with a Reynolds number of 8000 based on the minijet exit diameter is experimentally manipulated using 1–6 unsteady radial minijets. Five control parameters are investigated, including the minijet number or configuration (N), the duty cycle (α) of minijet injection, and the mass flow rate, excitation frequency, and diameter ratio (Cm,N, fe/f0, and d/D) of an individual minijet to a main jet. The decay rate (Ke) of the jet-centerline mean velocity under manipulation strongly depends on these five parameters. The typical flow structures or conceptual flow models are proposed for each minijet configuration based on extensive measurements. Three distinct mechanisms behind the highly effective manipulation are identified and discussed from the flow structures. It has been found that the optimal fe/f0 and α, i.e., (fe/f0)opt and αopt at which the maximum Ke or Ke,max occurs, do not vary with N, though optimal Cm,N or (Cm,N)opt does. Empirical scaling analysis performed under (fe/f0)opt reveals that the relationship Ke = g1 (N, Cm,N, α, d/D) may be reduced to Ke/Ke,max = g2 ζ, where g1 and g2 are different functions, and the scaling factor ζ = Cm,N/(Cm,N)opt, representing physically the minijet penetration depth. Both Ke,max and (Cm,N)opt are found to depend on N, α, and d/D. Discussion conducted based on Ke/Ke,max = g2 ζ provides important insight into the jet control physics. It is also discussed how fe/f0 and α are connected to the vortex interaction and, hence, Ke.

Mapping of Energy Cascade in the Developing Region of a Turbulent Round Jet

Mapping of Energy Cascade in the Developing Region of a Turbulent Round Jet

The effects of rough surfaces on heat transfer and flow structures for turbulent round jet impingement

Heat transfer and flow structures for round jet impingement over rough surfaces have been investigated using the shear stress transport model (SST) with transition model. This model is first evaluated against the experimental data and other numerical results for the smooth surface with Reynolds number Re = 23,000 for the nozzle-plate spacing of 2. Based on the above evaluation, the effects of rough surfaces on the mean velocity, turbulence field, skin friction and heat transfer are investigated with equivalent sand grain roughness heights ks of 50, 100, 250 and 500 μm. Two trends of the effects of roughness on the velocity field are found. First, the mean velocity over rough surfaces becomes steeper than that over the smooth surface in very near the wall. However, for ks ≥ 250 μm, the velocity gradient in the axial direction does not increase with an increase of ks. Second, the decay of the mean velocity is found in the deceleration region, but minimal in the acceleration zone. In addition, it is observed that the increase of roughness height leads to the reduction of the wall shear stress in the stagnation region. On the contrary, downstream, large ks usually contributes to an augmentation of wall shear stress. For heat transfer, the enhancement of heat transfer due to roughness varies from 2.53% to 6.08% compared with the case of smooth surface. However, this enhancement is non-monotonic due to the change of the second peak of heat transfer which is believed to be the key factor.

Quantifying eddy structures and very-large-scale motions in turbulent round jets

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Turbulence properties in jets with fractal grid turbulence

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Numerical simulation and self-similarity of the mean mass transfer in turbulent round jets

The paper investigates the mean mass transfer/passive scalar spreading in turbulent submerged round jets. Two regions of flow are present in the jet evolution: the Near-Field Region (NFR) and the Fully Developed Region (FDR). This group of research investigates from some years the mean evolution of turbulent rectangular jets with the new physical finding that two sub-regions (not a single one) are present in the NFR. The first region of the two is the newly discovered Undisturbed Region of Flow (URF), while the second one is the known Potential Core Region (PCR). In a recent paper we showed that the flow evolution of turbulent round jets, as far as momentum spreading is concerned, is self-similar also in the NFR. Literature shows that mass transfer spreading is self-similar only in FDR. The present paper presents new mean mass transfer results of the numerical Large Eddy Simulation (LES) in turbulent round jets. Four Reynolds numbers, from 2492 to 19,988, and two laminar Schmidt numbers, 1 and 10, are investigated. The first novel result of this paper is that mass transfer is self-similar in the NFR. The second result is that two new analytical models describe the passive scalar spreading in the URF and PCR. The third result is that two new self-similar laws describe the passive scalar spreading in the FDR. The fourth result states that the well-known power-law relationship, between passive scalar and axial momentum in the FDR, holds regardless of the modeling of turbulent viscosity and turbulent Schmidt number.

Self-similarity for statistical properties in low-order representations of a large-scale turbulent round jet based on the proper orthogonal decomposition

This study is an experimental investigation into the self-similarity behavior of first and second order statistical quantities derived from a jet flow based on a) the original data and b) its low-order representations derived from the Proper Orthogonal Decomposition (POD) and c) a comparison of both. The flow under investigation is an air-helium turbulent round jet with Re≈15400 emerging from a tube into an ambient containing identical gas mass fraction and temperature as the jet at a constant pressure. Instantaneous two-dimensional velocity field measurements were obtained for downstream distances of 5.5d to 17.4d in the plane of the axis of the jet, via Particle Image Velocimetry. The snapshot POD algorithm was then applied to this data set to generate low-order representations with rank approximations 1, 5, 10 and 50. These then serve as the basis to derive the respective (rank truncated) statistical properties. All properties are non-dimensionalized with a self-similar framework as obtained from the original jet data. It is found that the statistical properties obtained from the low-order representations a) resemble in shape the asymptotic outline of the original jet and b) that the maximum values (for a given low-order representation) exhibit asymptotic states with increasing downstream distances. This is a strong indication that i) self-similar behavior is equally found in the low-order representations and that ii) this finding is mainly controlled by the large-scale vortices. The sole exception is the axial velocity root-mean-square values, where a distinct dip in the center line of the flow is found. This dip is successively filled up by smaller-scale turbulence for higher order truncations. Additionally, a new criterion – based on the maximum cross-correlation obtained through successive time traces of the temporal POD modes – is suggested to distinguish physically relevant modes from the POD basis in a more quantitative and explicit manner compared to traditional energy-based criteria.

Proper orthogonal decomposition analysis of the large-scale dynamics of a round turbulent jet in counterflow

The understanding of the large-scale dynamics of the turbulent jet in counterflow remains limited. By employing proper orthogonal decomposition and spectral proper orthogonal decomposition on large eddy simulation data, new insights are presented. The fundamental mode dynamics are described as varying penetration, precession, and an alternating stretching-contracting motion. Furthermore, intermittency in the temporal evolution of these modes is identified.

Cold flow characteristics of a novel high-hydrogen Micromix model burner based on multiple confluent turbulent round jets

As a promising commercial hydrogen-rich gas turbine combustion technology, micro-mixing combustion has been characterized for its excellent performance with low NOx emissions. New flame stabilization mechanism of micro-mixing flames may produce new design criteria. In order to explore that, cold flow characteristics of a novel Micromix model burner based on multiple confluent round jets has been studied experimentally and numerically, which is considered to be the basis for the exploration. A three-dimensional laser Doppler velocimetry system (3D-LDV) was used to measure the flow field of the model burner. It was found that the cold flow characteristics of the burner were different from the twin plane jets, the twin round jets, and the low Reynolds number confluent round jets. Compared to which, the interior of the micro-mixing nozzle is at a very high turbulence intensity level, and the jets merging point of the burner moved upstream; however, the position of the combined point of the burner was close to the confluent round jets. There is no recirculation region between jets near the burner outlet when the nozzle spacing was equal to 3 times the nozzle diameter and the Reynolds number was less than 16,702. The steady computational Reynolds averaged equations (RANS) model results were used to compare with the experimental results. It was found that the RANS results can match the experimental results well, and the three RANS models predict the spatial mixing deficiency less than 1% at the outlet, indicating that the fuel and air were almost completely premixed uniformly.

Quantification of turbulent structures in and around the boundary region of a turbulent round jet released into counter-flow

This experimental study reports the turbulent flow structure of the interaction of turbulent round jet issued into a counter turbulent flow for improved understanding of the turbulent mixing characteristics including pollutant dispersal at the estuarine region for tidal river flows. A high-frequency data acquisition system (Acoustic Doppler velocimeter) was employed to quantify the turbulent flow structures within the mixing region of the turbulent jet. It was found that the cross-sectional mean axial jet velocity decayed faster in counter turbulence compared to a quiescent background. A reduced occurrence of engulfment and nibbling processes were evident which signified that the entrainment rate of the moving fluid reduced owing to the jet released into the counter flow. Further, the intermittency and diffusion depth of eddies decreased for jets issued against the current. The kinetic energy budget revealed that the pressure diffusion term of the turbulent kinetic energy budget equation supplied energy to the jet from the mean background flow at the self-similar region of the jet.

Coupled population balance and large eddy simulation model for polydisperse droplet evolution in a turbulent round jet

A hybrid approach is developed to simulate the transport and breakup of droplets in a turbulent round jet. The inflow size distribution is obtained from a one-dimensional parcel model that is coupled to a coarse large-eddy simulation (LES) of a turbulent jet. The LES results are compared to experimental data and show good agreement. Additionally, LES allows us to quantify the distributions of the mean and variability of key quantities of the polydisperse distribution.

Effect of Coflow Turbulence on the Dynamics and Mixing of a Nonbuoyant Turbulent Jet

AbstractThe effect of a turbulent coflow on a turbulent round jet is investigated experimentally. The primary objective of this work is to study the evolution of the turbulent jet as the level of t...

The effect of the geometric features of the turbulent/non-turbulent interface on the entrainment of a passive scalar into a jet

We consider the scalar concentration field in the proximity of the turbulent/non-turbulent interface (TNTI) of a round momentum-driven turbulent jet at Re = 10 600. Orthogonal cross sections of the jet are taken at 50 nozzle diameters from the nozzle exit using planar laser-induced fluorescence. The conditional scalar concentration is evaluated along the interface-normal direction, identifying the thickness of the TNTI region as 0.64λ (where λ is the Taylor microscale). Conditioning the scalar concentration within the TNTI revealed higher values of the passive scalar in the vicinity of the boundary elements shaped by large vorticity structures, i.e., isosurface points with low curvature (flat regions), small interface angle, and large radial distance from the jet centerline. In contrast, small vorticity structures near the boundary manifesting with high interface curvature, high interface angle, and small radial distance are associated with lower concentration values. Using the current experimental resolution, we find that high concentrations near the far boundary points persist up to a distance of 0.40λ–0.48λ into the TNTI region, after which boundary points closer to the jet centerline exhibit larger concentration values along the interface-normal direction, similar to the fully turbulent region. The cross correlation analysis showed that in regions characterized by low streamwise momentum, there are positive, albeit small, scalar correlations between the non-turbulent and the TNTI regions. The latter may imply local detrainment of the fluid particles containing the scalar at far radial positions.

Numerical predictions of near field behavior of variable density non-reacting turbulent round jets

Non-reacting turbulent round jets with density variations are computationally investigated in the near field region. Helium, Air and Carbone dioxide are the considered gases emitted from the nozzle exit and discharged in a weakly co-flow air stream, leading to three different density ratios; 0.14 [Helium/Air], 1.0 [Air/Air] and 1.52 [CO2/Air]. Two closure turbulence models are tested; the first order standard k-ε model and the second order Reynolds Stress Model (RSM) and confronted to experimental data. Predicted results elaborated by the RSM model are found to be in good agreement with experiments. Mean and turbulent flow properties, obtained for the velocity and scalar concentration field, are presented for the three simulated jets. It is found that the lighter gas (Helium) tends to mix more rapidly with the co-flow ambient air than heavier gases (Air and CO2). The entropy generation growth is shown to be greater for the Helium jet.

An analytical description of the energy balance in turbulent, round, free jets

This brief contribution provides a quantification of the terms of the turbulent kinetic energy transport equation for a round steady turbulent free jet. The analysis is based on the assumption of flow self-similarity, and it is performed by means of a simple analytical asymptotic analysis. The results are in good agreement with the experimental findings of Panchapakesan and Lumley [J. Fluid Mech. 246, 197–223 (1993)] and with the large eddy simulations of Bogey and Bailly [J. Fluid Mech. 627, 129–160 (2009)], hence providing a theoretical interpretation of such findings.

Velocity Field and Scalar Field Measurements of Turbulent Buoyant Round Jets in a Two-Layer Stratified Environment

Results reported in the literature have shown that the turbulence models currently implemented in computational fluid dynamics (CFD) commercial codes (e.g., ANSYS-CFX, STAR-CCM+, and FLUENT) have a tendency to overestimate thermal stratification and underestimate turbulent mixing when buoyancy effects become dominant with respect to momentum effects. Also, standard large eddy simulation models cannot fully capture the behavior of jets interacting with stratified environments because the assumption of turbulence isotropy of the smaller scales breaks down. Because of light diffraction and image distortion, it is challenging to apply nonintrusive optical flow measurements, like particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), to get experimental data for CFD validations when there are density variances involved in the flow. However, a refractive index matching (RIM) technique that has been recently developed in our Experimental and Computational Multiphase Flow Laboratory allows us to perform high-resolution measurements of velocity fields and scalar fields for turbulent buoyant jet flow in the presence of density differences as high as 8.6%. To form a fully turbulent round free jet, an experimental facility was designed with a jet nozzle diameter of 2 mm, located at the bottom of a cubic tank with 30-cm side length. The jet flow is established by a servo-engine-driven piston to eliminate possible fluctuations introduced by the motor. A high-fidelity synchronized PIV/PLIF system was utilized in conjunction with RIM to measure the velocity and concentration fields in the self-similar regions of a jet flow with a density difference of 3.16% for aqueous solutions. With Reynolds numbers of 4000 and 10 000, the jet impinging with a two-layer stably stratified environment is compared to the positively buoyant jet with lighter fluid injected into denser surroundings. Detailed quantifications of the measurement uncertainties are also carried out. The experimental results are presented in terms of turbulent statistics and the analysis of jet penetration depths.