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John Moore - One of the best experts on this subject based on the ideXlab platform.
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Education Committee Best Paper of 1995 Award: Methods of Classical Mechanics Applied to Turbulence Stresses in a Tip Leakage Vortex
Journal of Turbomachinery, 1996Co-Authors: Joan G. Moore, S. A. Schorn, John MooreAbstract:Moore et al. measured the six Reynolds Stresses in a tip leakage vortex in a linear turbine cascade. Stress tensor analysis, as used in classical mechanics, has been applied to the measured Turbulence Stress tensors. Principal directions and principal normal Stresses are found. A solid surface model, or three-dimensional glyph, for the Reynolds Stress tensor is proposed and used to view the Stresses throughout the tip leakage vortex. Modeled Reynolds Stresses using the Boussinesq approximation are obtained from the measured mean velocity strain rate tensor. The comparison of the principal directions and the three-dimensional graphic representations of the strain and Reynolds Stress tensors aids in the understanding of the Turbulence and what is required to model it.
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Methods of Classical Mechanics Applied to Turbulence Stresses in a Tip Leakage Vortex
Volume 5: Manufacturing Materials and Metallurgy; Ceramics; Structures and Dynamics; Controls Diagnostics and Instrumentation; Education; IGTI Scholar, 1995Co-Authors: Joan G. Moore, S. A. Schorn, John MooreAbstract:Moore et al. measured the six Reynolds Stresses in a tip leakage vortex in a linear turbine cascade. Stress tensor analysis, as used in classical mechanics, has been applied to the measured Turbulence Stress tensors. Principal directions and principal normal Stresses are found. A solid surface model, or 3-d glyph, for the Reynolds Stress tensor is proposed and used to view the Stresses throughout the tip leakage vortex. Modelled Reynolds Stresses using the Boussinesq approximation are obtained from the measured mean velocity strain rate tensor. The comparison of the principal directions and the 3-d graphical representations of the strain and Reynolds Stress tensors aids in the understanding of the Turbulence and what is required to model it.Copyright © 1995 by ASME
Jonathan D. Nash - One of the best experts on this subject based on the ideXlab platform.
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The role of Turbulence and internal waves in the structure and evolution of a near-field river plume
Ocean Science, 2020Co-Authors: Rebecca Adam Mcpherson, Craig Stevens, Joanne O’callaghan, Andrew J. Lucas, Jonathan D. NashAbstract:Abstract. An along-channel momentum budget is quantified in the near-field plume region of a controlled river flow entering Doubtful Sound, New Zealand. Observations include highly resolved density, velocity and Turbulence, enabling a momentum budget to be constructed over a control volume. Estimates of internal Stress ( τ ) were made from direct measurements of Turbulence dissipation rates ( ϵ ) using vertical microstructure profiles. High flow speeds of the surface plume over 2 m s −1 and strong stratification ( N 2 ∼ 10 - 1 s −2 ) resulted in enhanced Turbulence dissipation rates ( ϵ > 10 - 3 W kg −1 ) and internal Stress ( τ > 10 - 2 m 2 s −2 ) at the base of the surface layer. Internal waves were observed propagating along the base of the plume, potentially released subsequent to a hydraulic jump in the initial 1 km downstream of the plume discharge point. The momentum flux divergence of these internal waves suggests that almost 15 % of the total plume momentum can be transported out of the system by wave radiation, therefore playing a crucial role in the redistribution of momentum within the near-field plume. Observations illustrate that the evolution of the momentum budget components vary between the distinct surface plume layer and the turbulent, shear-stratified interfacial layer. Within the surface plume, a momentum balance was achieved. The dynamical balance demonstrates that the deceleration of the plume, driven by along-channel advection, is controlled by Turbulence Stress from the plume discharge point to as far as 3 km downstream. In the interfacial layer, however, the momentum equation was dominated by the Turbulence Stress term and the balance was not closed. The redistribution of momentum within the shear-stratified layer by internal wave radiation and other hydraulic features could account for the discrepancy in the budget.
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The role of Turbulence and internal waves in the structure and evolution of a near-field river plume
2019Co-Authors: Rebecca Adam Mcpherson, Andrew J. Lucas, Craig L. Stevens, Joanne M. O'callaghan, Jonathan D. NashAbstract:Abstract. An along-channel momentum budget is quantified in the near-field plume region of a controlled river flow entering Doubtful Sound, New Zealand. Observations include highly resolved density, velocity and Turbulence, enabling a momentum budget to be constructed over a control volume. Estimates of internal Stress (τ) were made from direct measurements of Turbulence dissipation rates (ε) using vertical microstructure profiles. High flow speeds of the surface plume over 2 m s−1 and strong stratification (N2 ~ 10−1 s−2) resulted in enhanced Turbulence dissipation rates (ε > 10−3 W kg−1) and internal Stress (τ > 10−2 m2 s−2) at the base of surface layer. An observed transition from a supercritical to sub-critical flow regime in the initial 1 km indicates the presence of an internal hydraulic jump and the subsequent release of internal gravity waves. The momentum flux divergence of these internal waves suggests that almost 15 % of the total plume momentum can be transported out of the system by wave radiation, therefore playing a crucial role in the redistribution of momentum within the near-field plume. Observations illustrate that the evolution of the momentum budget components vary between the distinct surface plume layer and the turbulent, shear-stratified interfacial layer. Within the surface plume, a momentum balance was achieved. The dynamical balance demonstrates that the deceleration of the plume, driven by along-channel advection, is controlled by Turbulence Stress from the plume discharge point to as far as 3 km downstream. In the interfacial layer however, the momentum equation was dominated by the Turbulence Stress term and the balance was not closed. The redistribution of momentum within the shear-stratified layer by the observed hydraulic jump and internal wave radiation could account for the discrepancy in the budget.
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The role of Turbulence Stress divergence in decelerating a river plume
Journal of Geophysical Research, 2012Co-Authors: Levi Kilcher, Jonathan D. Nash, James N. MoumAbstract:[1] Turbulence controls the composition of river plumes through mixing and alters the plume's trajectory by diffusing its momentum. While believed to play a crucial role in decelerating river-source waters, the Turbulence Stress in a near-field river plume has not previously been observationally quantified. In this study, finely resolved density, velocity, and Turbulence observations are combined with a control-volume technique to describe the momentum balance in the Columbia River's near-field plume during 10 tidal cycles that encompass both large and small river flow. Turbulence Stress varies by 2–3 orders of magnitude, both within a given ebb and between ebbs with different tidal or river forcing; its magnitude scales with the strength of the instantaneous ebb outflow, i.e., high Stresses occur during peak flow of strong ebbs. During these periods, the momentum equation is represented by a balance between Stress divergence and plume deceleration. As the flow relaxes, the Stress divergence weakens and other terms (pressure gradient and Coriolis) may become appreciable and influence plume deceleration. While the momentum balance could not be closed during these weaker flow periods, during strong tidal pulses the time scale for decay based on observed Stress is significantly less than a tidal half-period, indicating that Stress divergence plays a fundamental role in the initial deceleration of the plume.
Radhakrishna Sureshkumar - One of the best experts on this subject based on the ideXlab platform.
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Dynamics of hairpin vortices and polymer-induced turbulent drag reduction.
Physical review letters, 2008Co-Authors: Kyoungyoun Kim, Ronald J. Adrian, S. Balachandar, Radhakrishna SureshkumarAbstract:It has been known for over six decades that the dissolution of minute amounts of high molecular weight polymers in wall-bounded turbulent flows results in a dramatic reduction in turbulent skin friction by up to 70%. First principles simulations of turbulent flow of model polymer solutions can predict the drag reduction (DR) phenomenon. However, the essential dynamical interactions between the coherent structures present in turbulent flows and polymer conformation field that lead to DR are poorly understood. We examine this connection via dynamical simulations that track the evolution of hairpin vortices, i.e., counter-rotating pairs of quasistreamwise vortices whose nonlinear autogeneration and growth, decay and breakup are centrally important to Turbulence Stress production. The results show that the autogeneration of new vortices is suppressed by the polymer Stresses, thereby decreasing the turbulent drag.
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dynamics of hairpin vortices and polymer induced turbulent drag reduction
Physical Review Letters, 2008Co-Authors: Ronald J. Adrian, S. Balachandar, Radhakrishna SureshkumarAbstract:It has been known for over six decades that the dissolution of minute amounts of high molecular weight polymers in wall-bounded turbulent flows results in a dramatic reduction in turbulent skin friction by up to 70%. First principles simulations of turbulent flow of model polymer solutions can predict the drag reduction (DR) phenomenon. However, the essential dynamical interactions between the coherent structures present in turbulent flows and polymer conformation field that lead to DR are poorly understood. We examine this connection via dynamical simulations that track the evolution of hairpin vortices, i.e., counter-rotating pairs of quasistreamwise vortices whose nonlinear autogeneration and growth, decay and breakup are centrally important to Turbulence Stress production. The results show that the autogeneration of new vortices is suppressed by the polymer Stresses, thereby decreasing the turbulent drag. It is known that turbulent friction drag can be reduced by up to 70% in aqueous and organic liquids by the dissolution of minute amounts (10 –50 ppm) of high molecular weight polymers to wall-bounded flows [1]. While the technological relevance of polymer drag reduction (DR) cannot be over-emphasized (e.g., it is used to save billions of dollars in energy costs associated with intercontinental crude oil transportation), the complex physics of how macroscopic flow and nanoscale polymers interact to reduce turbulent shear (Reynolds) Stress has motivated over 60 years of fundamental research. Although the mechanisms of flowpolymer Stress coupling have been deciphered experimentally in inertialess nonlinear flows [2], they remained largely unknown in turbulent flows until faithful direct numerical simulations (DNS) were performed in the midnineties for polymer DR in channel flows. DNS studies showed, consistently with experimental observations, that the decrease of the turbulent Reynolds shear Stress is accompanied by the weakening of near-wall vortices. Further, they underscored a direct correlation between DR and the enhancement of the extensional viscosity, a measure of the fluid’s ability to resist elongational deformation [3,4]. More recent DNS studies [5,6] have revealed that the body forces due to polymeric Stresses oppose the vortical motions of quasistreamwise vortices (QSV) in the buffer layer, i.e., the intermediate layer that mediates momentum exchange between the near wall and core fluid in channel or pipe flow. These findings from DNS are corroborated by the analysis of the effect of elasticity on Newtonian coherent structures. For instance, Roy et al. [7] suggests that the self-sustaining process of wall Turbulence [8] becomes weaker due to the polymer forces opposing both biaxial and uniaxial extensional flow regions around QSV. Similarly, exact coherent states (ECS), which are closely related to the buffer layer Turbulence [9], can be suppressed entirely by polymer forces if elastic effects are sufficiently large [10]. Above the buffer layer, hairpin vortices are more prevalent than QSV, and they are responsible for the production of the Reynolds shear Stress in the log layer where the mean velocity has a logarithmic distribution. In the polymer drag-reduced flows, the hairpins in the log layer are also weakened by polymer counter torques [11], as is the case for QSV in the buffer layer. In addition, some portion of the total DR is attributed to the reduction in the number of energetic vortices with increasing elasticity, as deciphered by extensive flow visualizations. In the Newtonian flow, hairpins can autogenerate to form a ‘‘hairpin packet’’ which is an important feature of wall-bounded Turbulence that has been shown to explain many experimental measurements such as inordinately large amount of streamwise kinetic energy in very long streamwise wave length, the occurrence of multiple ejection events in turbulent bursts, formation of new QSV, and characteristic angles of inclined hairpin vortices [12]. Further, the hairpin vortex packet makes a significant contribution to the mean Reynolds shear Stress [13], which has two parts: the coherent Reynolds Stress caused by nonlinear interactions among individual vortices within the packet and incoherent Stress originated from velocity fluctuations induced by each individual vortex [12]. Despite the insights gained from the above studies, the generation and evolution of the turbulent vortices in the presence of a simultaneously evolving polymer conformation or Stress field and how they contribute to DR is poorly understood. In this Letter, we portray, for the first time, the nonlinear autogeneration of new vortices and formation of hairpin packets in the presence of polymer Stress by performing a series of computationally demanding dynamical simulations and explain the effect of such dy
Joan G. Moore - One of the best experts on this subject based on the ideXlab platform.
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Education Committee Best Paper of 1995 Award: Methods of Classical Mechanics Applied to Turbulence Stresses in a Tip Leakage Vortex
Journal of Turbomachinery, 1996Co-Authors: Joan G. Moore, S. A. Schorn, John MooreAbstract:Moore et al. measured the six Reynolds Stresses in a tip leakage vortex in a linear turbine cascade. Stress tensor analysis, as used in classical mechanics, has been applied to the measured Turbulence Stress tensors. Principal directions and principal normal Stresses are found. A solid surface model, or three-dimensional glyph, for the Reynolds Stress tensor is proposed and used to view the Stresses throughout the tip leakage vortex. Modeled Reynolds Stresses using the Boussinesq approximation are obtained from the measured mean velocity strain rate tensor. The comparison of the principal directions and the three-dimensional graphic representations of the strain and Reynolds Stress tensors aids in the understanding of the Turbulence and what is required to model it.
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Methods of Classical Mechanics Applied to Turbulence Stresses in a Tip Leakage Vortex
Volume 5: Manufacturing Materials and Metallurgy; Ceramics; Structures and Dynamics; Controls Diagnostics and Instrumentation; Education; IGTI Scholar, 1995Co-Authors: Joan G. Moore, S. A. Schorn, John MooreAbstract:Moore et al. measured the six Reynolds Stresses in a tip leakage vortex in a linear turbine cascade. Stress tensor analysis, as used in classical mechanics, has been applied to the measured Turbulence Stress tensors. Principal directions and principal normal Stresses are found. A solid surface model, or 3-d glyph, for the Reynolds Stress tensor is proposed and used to view the Stresses throughout the tip leakage vortex. Modelled Reynolds Stresses using the Boussinesq approximation are obtained from the measured mean velocity strain rate tensor. The comparison of the principal directions and the 3-d graphical representations of the strain and Reynolds Stress tensors aids in the understanding of the Turbulence and what is required to model it.Copyright © 1995 by ASME
James N. Moum - One of the best experts on this subject based on the ideXlab platform.
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The role of Turbulence Stress divergence in decelerating a river plume
Journal of Geophysical Research, 2012Co-Authors: Levi Kilcher, Jonathan D. Nash, James N. MoumAbstract:[1] Turbulence controls the composition of river plumes through mixing and alters the plume's trajectory by diffusing its momentum. While believed to play a crucial role in decelerating river-source waters, the Turbulence Stress in a near-field river plume has not previously been observationally quantified. In this study, finely resolved density, velocity, and Turbulence observations are combined with a control-volume technique to describe the momentum balance in the Columbia River's near-field plume during 10 tidal cycles that encompass both large and small river flow. Turbulence Stress varies by 2–3 orders of magnitude, both within a given ebb and between ebbs with different tidal or river forcing; its magnitude scales with the strength of the instantaneous ebb outflow, i.e., high Stresses occur during peak flow of strong ebbs. During these periods, the momentum equation is represented by a balance between Stress divergence and plume deceleration. As the flow relaxes, the Stress divergence weakens and other terms (pressure gradient and Coriolis) may become appreciable and influence plume deceleration. While the momentum balance could not be closed during these weaker flow periods, during strong tidal pulses the time scale for decay based on observed Stress is significantly less than a tidal half-period, indicating that Stress divergence plays a fundamental role in the initial deceleration of the plume.
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Enhanced Turbulence due to the superposition of internal gravity waves and a coastal upwelling jet
Journal of Geophysical Research, 2007Co-Authors: G. S. Avicola, James N. Moum, A. Perlin, Murray D. LevineAbstract:to vertical aspect ratios of 10 2 to 10 3 (median value � 300). These patches are clearly defined by regions of low Richardson number and occur where and when the linear superposition of the three dominant shear constituents (near-inertial, M2, and thermal wind) interferes constructively. This is most pronounced at the base of the coastal jet, where the thermal wind shear is largest. While the effect of the Turbulence Stress divergence on the jet is small compared to geostrophy (� 1%), it is significant in the second-order force balance governing secondary circulation. The timescale associated with the decay of the thermal wind shear via Turbulence Stress is O(10) days. We confirm that the vertical salt flux due to mixing is comparable to the net Ekman transport of salt onto the shelf within the bottom boundary layer. Because numerical models of coastal circulation lack Turbulence in midwater column, any vertical transport of scalars, including salt and heat, must be achieved inshore of the 40-m isobath. This is inconsistent with the observations presented in this study, in which significant vertical turbulent salt transport is found to exist across the entire shelf.
