Full-field measurements of self-excited oscillations in momentum-dominated helium jets

Flow structure of momentum-dominated helium jets discharged vertically into ambient air was investigated using a high-speed rainbow schlieren deflectometry (RSD) apparatus operated at up to 2,000 Hz. The operating parameters, i.e., Reynolds number and Richardson number were varied independently to examine the self-excited, flow oscillatory behavior over a range of experimental conditions. Measurements revealed highly periodic oscillations in the laminar region at a unique frequency as well as high regularity in the flow transition and initial turbulent regions. The buoyancy was shown to affect the oscillation frequency and the distance from the jet exit to the flow transition plane. Instantaneous helium concentration contours across the field of view revealed changes in the jet flow structure and the evolution of the vortical structures during an oscillation cycle. A cross-correlation technique was applied to track the vortices and to find their convection velocity. Time traces of helium concentration at different axial locations provided detailed information about the oscillating flow.     

Direct Numerical Simulation of Transitional Noncircular Buoyant Reactive Jets

The near field dynamics of transitional buoyant reactive jets established on noncircular geometries, including a rectangular nozzle with an aspect ratio of 2:1 and a square nozzle with the same cross-sectional area, are investigated by three-dimensional spatial direct numerical simulations. Without applying external perturbations at the inflow boundary, large vortical structures develop naturally in the flow field due to buoyancy effects. Simulation results and analysis describe the details and clarify mechanisms of vortex dynamics of the noncircular buoyant reactive jets. The interaction between density gradients and gravity initiates the flow vorticity. Among the major vorticity transport terms, the gravitational term mainly promotes flow vorticity in the cross-streamwise direction. For the baroclinic torque, it can either create or destroy flow vorticity depending on the local flow structure. The vortex stretching term has different effects on the streamwise and cross-streamwise vorticity. Streamwise vorticity is mainly created by vortex stretching, while this term can either create or destroy cross-streamwise vorticity. Under the coupling effects of buoyancy and noncircular nozzle geometry, three-dimensional vortex interactions lead to the transitional behavior of the reactive jets. Simulations also show that the rectangular jet is more vortical than the square jet. The rectangular jet has a stronger tendency of transition to turbulence at the downstream due to the aspect ratio effect. Mean flow property calculations show that the rectangular buoyant reactive jet has a higher entrainment rate than its square counterpart. Received 13 December 2000 and accepted 24 July 2001     

Turbulence measurements from a plane air jet with buoyancy induced curvature

An experimental investigation of the turbulence structure of a heated plane air jet discharged at various angles into quiescent surroundings is described. Hot-wire anemometry was used to obtain the profiles of mean and turbulent velocities and temperature normal and tangential to the curved path of the flow. Measurements in the buoyancy induced curved region of the jet show the relative influence of the stability induced by both buoyancy and jet curvature on the turbulence structure.     

Computational study of buoyancy effects in a laminar starting jet

Vortical structures formed in evolving jets are important in applications such as fuel injection in diesel engines and fuel leaks. When the jet fluid is different from the ambient fluid, the buoyancy can play an important role in determining the jet flow structure, and hence, the entrainment and fluid mixing processes. In the present study, a jet of helium injected in air is investigated, with emphasis placed on delineating the buoyancy effects on vector–scalar fields during the starting phase. We utilize a computational model, previously validated to predict the flow field of low-density gas jets. The model incorporates finite volume approach to solve the transport equation of helium mass fraction coupled with conservation equations of mixture mass and momentum. Computations were performed for a laminar jet to characterize the advancing jet front, and to capture the formation and propagation of vortex rings and the related pinch-off process. Results show significant effects of buoyancy on jet advancement, as well as on vorticity and helium concentration in the core of the vortex rings.     

Role of Buoyancy on Instabilities and Structure of Transitional Gas Jet Diffusion Flames

Transitional jet diffusion flames provide the link between dynamics of laminar and turbulent flames. In this study, instabilities and their interaction with the flow structure are explored in a transitional jet diffusion flame, with focus on isolating buoyancy effects. Experiments are conducted in hydrogen flames with fuel jet Reynolds number of up to 2,200 and average jet velocity of up to 54 m/s. Since the fuel jet is laminar at the injector exit, the transition from laminar to turbulent flame occurs by the hydrodynamic instabilities in the shear layer of fuel jet. The instabilities and the flow structures are visualized and quantified by the rainbow schlieren deflectometry technique coupled with a high-speed imaging system. The schlieren images acquired at 2,000 frames per second allowed exposure time of 23 μs with spatial resolution of 0.4 mm. Results identify a hitherto unknown secondary instability in the flame surface, provide explanation for the observed intermittency in the breakpoint length, show coherent vortical structures downstream of the flame breakpoint, and illustrate gradual breakdown of coherent structures into small-scale random structures in the far field turbulent region.     

Behavior of near-field dilution of thermal buoyant jet discharged horizontally in compound open-channel

The RNG κ-ε model considering the buoyancy effect, which is solved by the hybrid finite analytic method, is used to simulate the mixture of the horizontal round thermal buoyant jet in compound open channel flow. The mixing features near the spout and flowing characteristic of the secondary currents are studied by numerical simulation. Meanwhile, （1） the distribution of the measured isovels for stream-wise velocity, （2） secondary currents, （3） the distribution of the measured isovels for temperature of typical cross-section near the spout, were obtained by the three-dimensional Micro ADV and the temperature measuring device. Compared with experimental data, the RNG κ-ε model based on buoyancy effect can preferably simulate the jet which performs the bifurcation phenomenon, jet reattachment （Conada effect） and beach secondary currents phenomenon with the effect of ambient flow, buoyancy, and secondary currents of compound section and so on.     

Direct numerical simulation of the near field dynamics of a rectangular reactive plume

Spatial direct numerical simulation (DNS) is used to study the near field dynamics of a buoyant diffusion flame established on a rectangular nozzle with an aspect ratio of 2:1. Combustion is represented by a one-step finite-rate Arrhenius chemistry. Without applying external perturbations at the inflow boundary, large vortical structures develop naturally in the flow field, which interact with the flame and temporally create localized holes within the reaction zone in which no chemical reactions take place. The interaction between density gradients and gravity plays a major role in the vorticity generation of the buoyant plume. At the downstream of the reactive plume, a more disorganized flow regime characterized by small scales has been observed, following the breakdown of the large vortical structures due to three-dimensional (3D) vortex interactions. Analysis of energy spectra shows that the spatially developing reactive plume has a tendency of transition to turbulence under the effects of combustion-induced buoyancy. The buoyancy effects are found to be very important to the formation, development, interaction, and breakdown of vortices in reactive plumes. In contrast with the relaminarization effects of chemical exothermicity via viscous damping and volumetric expansion on non-buoyant jet diffusion flames, the tendency towards transition to turbulence in reactive plumes is greatly enhanced by the buoyancy effects.     

低速轴对称层流射流流动形态和失稳机制的实验研究

本文应用染色液和悬浮粒子显示方法,进一步实验研究轴对称层流射流的流动形态及其失稳机制。首次成功地在从一定口径的喷嘴流出的低速轴对称层流射流中观察到环形回流流动。给出了射流随速度演化及实验容器边界对其流动形态影响的显示照片,发现实际射流的轴对称波动及失稳过程正好对应射流在容器底部产生的环形旋涡的生长和破碎过程。本文认为由于实验空间有限尺度对流动的限制改变了原来射流的流动形态和流场空间的拓扑性质,射流与实际边界的相互作用对实际射流的失稳和转捩有重要的影响。     

Transient positively and negatively buoyant turbulent round jets

 The transient character of the jet issuing from an upward nozzle centered at the bottom of a vertical cylindrical tank into bulk liquid of a different density was measured using flow visualization and PIV for varying densimetric Froude numbers by varying the jet Reynolds numbers and the ratios of fluid densities. Positively buoyant jets penetrate to the free surface, driven by both momentum and buoyancy in the upward direction. The lighter jet fluid stratifies in a layer above the bulk liquid. Upon starting, a negatively buoyant jet has three stages. First the jet penetrates to its maximum height in the tank. Then the jet penetration decreases due to the downward backflow of heavier fluid surrounding the jet, which reduces the jet’s upward momentum. Finally the jet penetration height fluctuates around a mean value about 70% the maximum height of penetration. For small negative Froude numbers, the flow is fountain-like. The downward flow turns radially outward as it reaches the bottom of the tank and eventually an annular recirculation zone forms at the bottom of the tank with vortical motion opposite the vorticity of the jet. For large negative Froude numbers, the spreading of the jet extends far enough so the annular downward flow is along the walls of the tank resulting in a large annular recirculation zone. The penetration depth, h, and time, t, scale with buoyancy flux, F, and the jet momentum flux, M, as hM ^-3/4∣F∣^1/2 and t∣F∣/M to collapse the transient jet penetration height data onto a single curve over a wide range of Froude numbers for either positively or negatively buoyant jets. Received: 8 June 1998/Accepted: 3 February 1999     

Experiments with large diameter gravity driven impacting liquid jets

 The phenomenon of a liquid jet released under gravity and falling through or impacting onto another liquid before colliding with an obstructing solid surface has been studied experimentally under isothermal conditions. Usually the jet diameter was sufficiently large to ensure jet coherency until collision. Direct flow visualization was used to study jets released into water pools with no air head space and jets impacting onto water pools after falling through an air head space. It is shown that distances predicting the onset of buoyancy and the entrainment of air using derivations from continuous plunging jets, are not applicable for impacting jets. The morphology of jet debris after collision with the solid surfaces correlates with the wetting properties of the jet liquid on the surface. Received: 28 November 1997 / Accepted: 21 May 1998     

Near-field development of gas-phase horizontal laminar jets with positive and negative buoyancy measured with filtered Rayleigh scattering

Near-field mixing characteristics of horizontally issuing jets, alternatively positively and negatively buoyant, are explored. The cross-sectional mass fraction of a buoyant horizontal jet consisting of helium flowing into ambient air is measured using a non-intrusive technique, filtered Rayleigh scattering, for Reynolds numbers ranging from 50 to 1,200, Froude numbers ranging as low as 0.71, and Schmidt numbers on the order of unity for all tests. Several corresponding experiments were carried out using carbon dioxide in place of helium in order to determine whether the direction of the buoyancy changes the characteristic shape of the jet cross-section. Consistent with the literature, mixing rates were consistently higher on the side of the jet where instability, due to density stratification, was present. At jet Froude numbers ranging between 1.5 and approximately 3, the jet cross-section takes a shape consistent with a single plume of fluid being ejected from the core in a vertical direction—upward for a jet with positive buoyancy and downward for a jet with negative buoyancy. Remarkably, for Froude numbers less than unity, the distortion of the jet is quite different in that two separate plumes emanate from each side of the jet while ejection from the center is suppressed. Both the positively and negatively buoyant jet cross-sections exhibited this trait, suggesting that the mechanism that determines the cross-sectional shape of the jet core is only mildly influenced by centripetal effects brought about by streamline curvature. The location of the jet centroid at varied streamwise locations was computed from the mass fraction data, yielding jet trajectory.     

Color schlieren deflectometry study of jet mixing: effect of buoyancy and perforation

Mixing of jets is crucial for optimal performance of many industrial applications and there is a need to optimize both nozzle geometry and flow conditions. The present study reports the influence of buoyancy and perforation on mixing between a jet and its environment. Optical techniques are ideal for the study of jet mixing due to their non-intrusive and inertia free properties. The present study gives an account of mixing between helium jet and the ambient fluid using a combination of color schlieren deflectometry and radial tomographic mathematics. Four different perforation sizes have been used and the experiments are performed for Reynolds numbers 21–676 and Richardson numbers 3.27–0.0015. Color schlieren images show distinct influence of perforation and flow conditions (Richardson number). Oxygen concentration and jet width quantify effectiveness of jet mixing. Buoyancy plays an important role in mixing at high Richardson number. Perforation improves jet mixing i.e. there is about 120% increase in jet width and the size of perforation plays an important role.     

Influence of the wall boundary layer thickness on a gas jet injected into a liquid crossflow

The effects of the wall boundary layer thickness on the development of an axisymmetric gas jet injected into a confined vertical water flow were investigated. The variations in the wall boundary layer were made by using suction at the wall through rectangular profiled slots. The water velocity around the two-phase jet was studied for several boundary layer thickness values by laser Doppler velocimetry. The gas jet outline was extracted by image processing applied on visualisations for a wide variety of water, gas and suction conditions. These comparisons showed that the boundary layer has no influence on jet development. The data showed that the interactions between flows near the injection do not develop downstream, which accounts for the absence of the two classical contrarotating vortices in the medium field of the jet. The influence of the gas pocket, and then of buoyancy, is predominant over other phenomena. Received: 15 December 1999 / Accepted: 29 August 2000     

Effect of the coflow stream on a plane wall jet

We propose in this work to study an isothermal and a non-isothermal laminar plane wall jet emerging in a coflow steam. The numerical solution of the governing equations was performed by a finite difference method. In this work, we are interested in the study of the influence of Grashof numbers on the wall jet emerging in a medium at rest. Further, we will examine the effect of the coflow stream on the behavior of the dynamic and thermal properties of the wall jet subjected to a constant temperature. A comparison with a simple wall jet is carried out. The results show that for a buoyant wall jet, two parameters can influence the flow: the inertial and buoyancy forces. The velocity effect indicates that the potential core length increases with the velocity ratio. We are also showed that when using a momentum length scale, the normalized longitudinal maximum velocity can reach an asymptotic curve at different velocity ratios.     

A novel formula to describe the velocity profile of free jet flow

Having a potential core, the velocity profile in initial zone of incompressible submerged jet flow is different from that in fully developed region. In the former researches, the two regions were studied separately, even a short part between the two regions being considered as a transition region. The velocity profile in fully developed region looked as a Gaussian distribution, which is valid when jet initial region is comparatively short. But when the size of initial zone is long enough not to be able to be neglected, especially for large-size exit, this kind of assumption is not acceptable. Based on the analysis of flow structure of jet flow, a new velocity profile formula of submerged jet flow was proposed, which unites the initial, transition, fully developed regions of jet flow via modifying Gaussian distribution with a radial adjusting coefficient. For the round jet with the medium or high range of Reynolds number, the radial adjusting coefficient is a power function of reciprocal of jet distance. And then some literature experimental data were applied in verification, and the new formula exhibited a good calculation result. This work opened that the jet flow velocity profile at any site along the flow distance can be described via a same formula.     

Formation of Coanda jet from sharp-edged swirl nozzle with base plate

Experiments were conducted using tufts and PIV to determine the conditions for which a swirled gas jet issuing from a sharp-edge nozzle, in flush with a base plate, would form a Coanda jet. The flow field was also simulated. The inception of the Coanda jet was observed to be associated with the formation of a recirculation bubble at the nozzle exit. A threshold value of swirl number, which increased monotonically with Reynolds number, was required for the formation of the Coanda jet. The Coanda jet was associated with hysteresis. The flow features and transition from a diverging jet to a Coanda jet are discussed.     

Transient Flow Patterns of Multiple Plasma Synthetic Jets Under Different Ambient Pressures

The plasma synthetic jet is a new active flow control technique, which has great potentials for supersonic flow control A plasma synthetic jet actuator (PSJA) for supersonic flow control which operates under low ambient pressure is designed In order to explore the transient jet flowfield, high-speed Schlieren and electronic measurement systems are utilized to test the single-shot operating characteristics of the actuator under two ambient pressure conditions. The evolution of the jet boundary, which depicts the transient jet flowfield and can be used to estimate the flow control capability, is captured. It is found that the ambient pressure is the primary reason to affect the arc energy deposition, which directly determines the velocity of blast wave and jet front. In addition, the flow patterns of PSJA under various ambient pressures show some similarities. The jet evolution can be divided into three stages, i.e. pressure dominant stage, the inertia dominant stage and vortex ring dominant stage. During the pressure dominant stage, the PSJA could be treated as jet. For the inertia dominant stage, the Froude number decreases from 2263.82 to 21.73, indicating the inertia effect gets weakened with an intensified buoyancy effect, but the inertia effect still holds the dominant position, and this stage may be considered as the end mark of the injection process. As for the vortex ring dominant stage, large scale vortex ring becomes significant and the jet front velocity is very low but with apparent fluctuation.     

The effect of buoyancy on flow fluctuations for horizontal plane jets in low speed applications

Low speed jets have important applications in chemical process, power and aerospace industries. Velocity fluctuations in low speed laminar jets have been investigated experimentally and theoretically, in the present work. The effects of buoyancy on the mean and fluctuating components of velocity have been highlighted. It is observed that even for forced convection dominated flow, convective instabilities and the resulting local velocity fluctuations are significantly influenced by buoyancy. Both the dominant frequency and the amplitude of velocity fluctuations depend on the jet exit temperature and spatial location within the jet. For isothermal jets, the dominant frequency of oscillation increases almost linearly with Reynolds number, while for buoyant jets nonlinearity exists at lower Reynolds numbers. Numerical simulations of the present study are found to be reasonably successful in predicting the oscillatory behavior of both isothermal and non-isothermal laminar free jets accurately.     

Buoyancy effects on turbulent swirling flows

A three-parameter model of turbulence applicable to free boundary layers has been developed and applied for the prediction of axisymmetric turbulent swirling flows in uniform and stagnant surroundings under the action of buoyancy forces. The turbulent momentum and heat fluxes appearing in the time-averaged equations for the mean motion have been determined from algebraic expressions, derived by neglecting the convection and diffusion terms in the differential transport equations for these quantities, which relate the turbulent fluxes to the kinetic energy of turbulence, k, the dissipation length scale of turbulence, L, and the temperature covariance, T ′². Differential transport equations have been used to determine these latter quantities. The governing equations have been solved using fully implicit finite difference schemes. The turbulence model is capable of reproducing the gross features of pure jet flows, buoyant flows and swirling flows for weak and moderate swirl. The behaviour of a turbulent buoyant swirling jet has been found to depend solely on exit swirl and Froude numbers. The predicted results indicate that the incorporation of buoyancy can cause significant changes in the behaviour of a swirling jet, particularly when the buoyancy strength is high. The jet exhibits similarity behaviour in the initial region for weak swirl and weak buoyancy strengths only, and the asymptotic case of a swirling jet under the action of buoyancy forces is a pure plume in the far field. The predicted results have been found to be in satisfactory agreement with the available experimental data and in good qualitative agreement with other predicted results.     

The role of inlet location on the dynamics of thin two-dimensional buoyant surface jets

The transition between the motion of a subsurface buoyant jet and a supersurface buoyant jet has been studied. Measurements indicate that the transition region is sharp and characterized by an intermediate regime where both a wall-hugging flow and the more standard outlet flow are present. The point of transition was found to exhibit significant hysteresis depending on whether the receiving pool height was increasing or decreasing with time. Measurements of the temperature field downstream show that the effects of the different regimes on the density stratification can persist for long distances.