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"The effect of background turbulence on the scalar field of an axisymmetric turbulent jet is investigated experimentally. The present investigation builds on the work of Gaskin et al. (2004), who studied the concentration and velocity fields of a plane jet in a shallow coflow with different turbulence levels and Khorsandi et al. (2013), who studied the velocity field of an axisymmetric turbulent jet emitted into a turbulent background. Different driving algorithms for a large RJA were tested and the statistics of the turbulence generated downstream of the RJA were compared to characterize the algorithms' performance. Variations in the spatial configuration of jets operating at any given instant, as well as in the statistics of their on/off times were studied. The algorithm identified as RANDOM generated the closest approximation of zero-mean-flow homogeneous isotropic turbulence. The flow generated by the RANDOM algorithm had a relatively high turbulent Reynolds number (ReT = uTl/[nu] = 2360, where uT is a characteristic RMS velocity, l is the integral length scale of the flow, [nu] is the kinematic viscosity of the water) and the integral length scale (l = 11.6 cm) is the largest reported to date. Thus, RANDOM algorithm was used to generate the background turbulence for the investigation of scalar mixing within a turbulent jet.The effect of background turbulence on the mixing of a passive scalar within a turbulent jet at different Reynolds numbers was investigated. To this end, planar laser-induced fluorescence was employed to obtain concentration measurements of dye (disodium fluorescein, Schmidt number = 2000) within the jet. Two jet Reynolds numbers (Re=UjD/[nu], where Uj is the jet exit velocity, D is the nozzle diameter and [nu] is the kinematic viscosity of the jet fluid, water) were studied: 10600 and 5800. The resulting statistics of the scalar fields showed that the mean concentrations of jets emitted into turbulent backgrounds were lower than those of jets emitted into a quiescent background near the centerline. However, near the edges of the jet (r/x>0.15), the concentrations were higher for the jets issued into turbulent surroundings. The RMS concentrations of the jet emitted into a turbulent background significantly increased. Examination of the probability density functions of concentration revealed a higher degree of intermittency of the scalar field. The probability of low concentrations increased in the presence of background turbulence although the maximum concentrations were comparable to those of the jet emitted into a quiescent background. Flow visualizations revealed meandering of the jet issued into background turbulence, which is associated with the increased probability of lower concentrations and higher intermittency. Additionally, the widths of the jets emitted into a turbulent background were increased. For the lower jet Reynolds number, the described effects were more evident and the jet structure was destroyed by the background turbulence within the measurement region, resulting in flat radial profiles of both the mean and RMS concentrations. Comparison of the results of the scalar field with those of the hydrodynamic jet of Khorsandi et al. (2013) revealed a similar behavior of the two fields. However, the most significant difference was the larger radial extent of the profiles of mean and RMS concentrations, which resulted from the meandering of the jet and increased transport of scalar by turbulent diffusion. The flow visualizations suggest that the entrainment and mixing in the jet in a turbulent background changes with the destruction of jet structure, from jet driven entrainment to become potentially dominated by i) increased lateral advection of the jet by large scales of the background turbulence during the meandering of the jet, which is subsequently mixed by its smaller scales, and ii) turbulent diffusion that is significantly enhanced by the turbulent background." --
In many cases, turbulence is superimposed on an unsteady organized motion of the mean flow. In the past, these turbulent flows have been studied by time or ensemble averaging methods and some decomposition techniques such as Proper Orthogonal Decomposition (POD).In this study a new decomposition technique called the Turbulence Filter will be used to decompose the forced turbulent jet flows. The technique decomposes the velocity field into two parts, one is fluctuating (turbulent) part and the other is more organized (forced) part. Within this context several experiments of organized turbulent jet have been carried out. In these experiments, variable frequency and amplitude oscillationare imposed to the jet. The jet velocity field is measured in 1D and 2D.Hot-wire anemometer technique was used in one-dimensional measurements and Particle Image Velocimetry (PIV) technique was used for the 2D velocity fields. The Turbulence Filter technique was very successful in the decomposition of these 1D and 2D turbulent flows for even at very big noise ratios. In 1D measurements, two different forcing methods were applied to the jet flow in order to obtain organized jetflow. A holed plate was used for triangular forcing and an elliptic plate was used for sinusoidal forcing. The axial distance was varied by using a traverse mechanism. In the experiments, Re number and the forcing frequency of the signal were varied. The multiple hot-wires (six probes) were used to investigate the evolution of the signal along the radial distance.The POD and Turbulence Filter decomposition methods and wavelet transform are applied to coaxial jet flows for various downstream positions. The data was obtained from cross-wire measurements, so axial and radial velocity components were acquired.As a first application for 2D velocity field, the Turbulence Filter is applied to impinging jet flow. After decomposition of the velocity field, large-scale and small- scale structures are obtained and their velocity fields are investigated. In the second application for the 2D flow, an analysis of turbulence on premixed jet flames havebeen carried out. PIV images of velocity field in premixed flame of 4% propane in air are decomposed by Turbulence Filter and thus small and large-scale structures are obtained. The effects of large-scale velocity fields and strain rates on the flame structure and burning rate are evaluated.
This book contains contributions by former students, colleagues and friends of Professor John L. Lumley, on the occasion of his 60th birthday, in recognition of his enormous impact on the advancement of turbulence research. A variety of experimental, computational and theoretical topics, including turbulence modeling, direct numerical simulations, compressible turbulence, turbulent shear flows, coherent structures and the Proper Orthogonal Decomposition are contained herein. The diversity and scope of these contributions are further acknowledgment of John Lumley's wide ranging influence in the field of turbulence. The large number of contributions by the authors, many of whom were participants in The Lumley Symposium: Recent Developments in Turbulence (held at ICASE, NASA Langley Research Center on November 12 & 13, 1990), has presented us with the unique opportu nity to select a few numerical and theoretical papers for inclusion in the journal Theoretical and Computational Fluid Dynamics for which Professor Lumley serves as Editor. Extended Abstracts of these pa pers are included in this volume and are appropriately marked. The special issue of TCFD will appear this year and will serve as an additional tribute to John Lumley. As is usually the case, the efforts of others have significantly eased our tasks. We would like to express our deep appreciation to Drs. R.
"The passive scalar field of an axisymmetric turbulent jet and an isokinetic jet in an approximately homogeneous isotropic turbulence (HIT) with negligible mean flow is studied experimentally. The present research builds on that of Khorsandi et al. 2013 and Perez-Alvarado 2016, who studied the velocity field and the passive scalar field of an axisymmetric turbulent jet in a turbulent ambient, respectively. The primary objective is to deduce the jet structure, and to study the jet mixing in the HIT ambient by following the meandering path of the jet, i.e. conditional on the jet centroid. The secondary objective, complementing the first, is to study the diffusion of a momentumless patch of a passive scalar in the HIT ambient.The effect of a turbulent ambient on the dynamics and mixing of the passive scalar field of an axisymmetric turbulent jet is investigated. The experiments were conducted either in a quiescent or a turbulent ambient. The turbulent ambient was generated by a random jet array to achieve an approximately zero-mean-flow HIT ambient in the measurement plane. Two jet Reynolds numbers of Re = 5800 and 10600 were studied. Planar laser-induced fluorescence was used to measure the concentrations of the passive scalar dye (Sc = 2000) at orthogonal cross-sections of the jet at axial distances of x/d = 20, 30, 40, 50, 60. The statistics of the passive scalar field were conditioned on the jet centroid and were compared to the Eulerian statistics and to those of the jet in a quiescent ambient. The use of the centroidal analysis allowed the structure of the jet in the HIT ambient to be deduced, for which a two-region model was proposed. In the first region, following the developing region of the jet, the ambient turbulence progressively disrupts the jet structure and results in a faster concentration decay compared to the quiescent ambient. At a critical downstream distance, where the relative turbulence intensity between the ambient and the jet (ξ = urms,HIT/ urms,jet ) exceeds 0.5, the HIT ambient has destroyed the jet structure and the second region starts. In the second region, the turbulent diffusion is the only mechanism to transport the passive scalar field. The first-order centroidal statistics of the scalar field show self-similarity and self-preservation before jet break-up. The width of the jet is larger in the HIT ambient compared to that in a quiescent ambient and grows with axial distance but remains unchanged beyond jet break-up. Using the present passive scalar data and the velocity data from Khorsandi et al. 2013, it is argued that the momentum-driven entrainment of the jet in the HIT ambient is reduced compared to that in a quiescent ambient, and that the entrainment ceases beyond the jet break-up. The entrainment of the smaller scales of the ambient turbulence leads to a wider range of centerline concentrations and rms concentrations within the jet, and they are hypothesized to increase local concentration gradients and reduce the jet mixing.Diffusion of a patch of a passive scalar in the HIT ambient is studied. A high-Sc number passive scalar dye (Sc = 2500) is released isokinetically from a large diameter jet (d = 29.97 mm), and an orthogonal view of the passive scalar field is obtained using planar laser-induced fluorescence. The temporal evolution of the scalar patch is due to molecular diffusion and to turbulent diffusion in a quiescent ambient and in the HIT ambient, respectively. Time-averaged statistics of the passive scalar field are assessed at t = 0.2, 1, 1.8, 2.6, 3.4 s using a centroidal analysis. The mean concentration decays quickly and the rms concentration increases within the scalar patch. Compared to the quiescent ambient case, a wider range of the concentrations is present at the centroid of the scalar field. The size of the scalar patch increases with time, which is attributed to an increasing turbulent diffusivity for times shorter than the integral time scale of the turbulence"--
This book presents methodologies for analysing large data sets produced by the direct numerical simulation (DNS) of turbulence and combustion. It describes the development of models that can be used to analyse large eddy simulations, and highlights both the most common techniques and newly emerging ones. The chapters, written by internationally respected experts, invite readers to consider DNS of turbulence and combustion from a formal, data-driven standpoint, rather than one led by experience and intuition. This perspective allows readers to recognise the shortcomings of existing models, with the ultimate goal of quantifying and reducing model-based uncertainty. In addition, recent advances in machine learning and statistical inferences offer new insights on the interpretation of DNS data. The book will especially benefit graduate-level students and researchers in mechanical and aerospace engineering, e.g. those with an interest in general fluid mechanics, applied mathematics, and the environmental and atmospheric sciences.
The first part aims at providing the physical and theoretical framework of the analysis of density variations in fully turbulent flows. Its scope is deliberately educational. In the second part, basic data on dynamical and scalar properties of variable density turbulent flows are presented and discussed, based on experimental data and/or results from direct numerical simulations. This part is rather concerned with a research audience. The last part is more directly devoted to an engineering audience and deals with prediction methods for turbulent flows of variable density fluid. Both first and second order, single point modeling are discussed, with special emphasis on the capability to include specific variable density / compressibility effects.