Observation of a bistable flow in a hemispherical cavity.

Snedeker, Richard S.; Donaldson, Coleman duP.

doi:10.2514/3.3528

Cited by 23 publications

(8 citation statements)

References 2 publications

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“…The visualizations and velocity measurements from Gromov et al [29] confirmed and extended the results of Snedeker et al [86].…”

Section: Heat Transfer Enhancement Using Dimplessupporting

confidence: 78%

“…Nevertheless, main attention of papers published in the literature has been paid to the heat and mass transfer effects averaged in time whereas the important unsteady processes including the vortex formations and their role in the heat transfer enhancement have not been thoroughly investigated. Snedeker et al [86] documented an unusual asymmetric distribution of the flow in a three-dimensional surface depression. Two stable vortex positions for special dimple configurations have been estimated whereas the axis of the vortex has a certain angle towards the main flow.…”

Section: Heat Transfer Enhancement Using Dimplesmentioning

confidence: 99%

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Flow Structures and Heat Transfer on Dimpled Surfaces

Turnow¹,

Zhdanov²,

Kornev³

et al. 2011

Proceeding of Seventh International Symposium on Turbulence and Shear Flow Phenomena

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Dimples are known to increase heat transfer with a low induced hydraulic loss in narrow channels. Main attention of papers published in the literature has been paid to the heat and mass transfer effects averaged in time whereas the unsteady processes and their role in the heat transfer enhancement have not been thoroughly investigated. Following the investigations of unsteady effects and its direct impact on heat transfer enhancement is the main subject of this thesis. Vortex structures and heat transfer mechanism in a turbulent flow over a single dimple and a staggered dimple array in a narrow channel have been investigated using Large Eddy Simulation (LES), Reynolds-averaged Navier-Stokes (RANS) simulation, Laser Doppler Velocimetry (LDV), time resolved pressure measurements and dye visualizations. Considered are dimples with a depth to diameter ratio h/d = 0.26 for Reynolds numbers in the range of Re d = 10000 to Re d = 40000. The numerical methods have been validated with experiments, DNS data and empiric correlations published in literature. The flow and temperature fields are captured by LES using dynamic mixed model applied both for the velocity and temperature. A test bench has been installed at the University of Rostock to validate the numerical methods and to investigate flow structures over dimpled surfaces using modern measurement techniques. The conducted time resolved pressure measurements and analysis of the time signals recorded from LDA measurements give a deep insight into flow physics.In addition to the direct analysis of the flow field, the three-dimensional Proper Orthogonal Decomposition (POD) method has been carried out on LES pressure and velocity fields to identify spatio-temporal structures hidden in the random fluctuations.A multi-objective design optimization based on mesh motion algorithm has been performed within the framework OpenFOAM for further improvement of the heat exchanger performance. Based on mesh motion technique and selected parameters for optimization, the surface can adopt an arbitrary shape and is not limited by any kind of interpolation. The aim of the procedure is to find the geometry most favorable to maximize heat exchange while obtaining a minimum hydraulic loss.

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“…The visualizations and velocity measurements from Gromov et al [29] confirmed and extended the results of Snedeker et al [86].…”

Section: Heat Transfer Enhancement Using Dimplessupporting

confidence: 78%

Section: Heat Transfer Enhancement Using Dimplesmentioning

confidence: 99%

Flow Structures and Heat Transfer on Dimpled Surfaces

Turnow¹,

Zhdanov²,

Kornev³

et al. 2011

Proceeding of Seventh International Symposium on Turbulence and Shear Flow Phenomena

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show abstract

“…The first pervasive research related to dimple concavities was performed by Snedeker and Donaldson [32] investigating the flow structures on a dimpled surface. Terekhov et al [33,34] experimentally examined the flow structures in a single dimpled surface by tracking hydrogen bubbles, using laser Doppler anemometry method and soot-oil visualization technique.…”

Section: -K)mentioning

confidence: 99%

Thermo-Hydraulic Performance Enhancement of Finned Elliptical Tube Heat Exchangers by Utilizing Innovative Dimple Turbulators

Lotfi

Sundén

2019

Heat Transfer Engineering

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As a new type of fin structure in finned tube heat exchangers, dimple turbulators exhibit excellent potential for thermo-hydraulic performance enhancement. A three-dimensional numerical simulation study was conducted to investigate the influences of five kinds of innovative concave dimple turbulators (CDTs), namelyelliptical dimple, conical frustum dimple, trapezoidal prism dimple, leeward triangular dimple and upward triangular dimple (UwTD) on the thermohydraulic performance enhancement in a plate fin-and-elliptical tube (PFET) heat exchanger, where CDTs are textured on the fin surface transversely between the elliptical tubes. The computational results are analyzed by considering the performance evaluation criterion for the PFET heat exchangers with different types of CDT shapes. The present investigation demonstrates that the heat transfer enhancement is intimately pertained to ejection with longitudinal counter-rotating flow, strengthened secondary flow and vortex structures at the downstream rim of CDT. A parametric study on the CDTs indicated that the UwTD vortex turbulators give better thermo-hydraulic performance under the present conditions. The numerical simulation results illustrated different secondary flow structures and heat transfer characteristics of the CDTs with various shapes, which disclosed the influential mechanisms of differently shaped dimple turbulators on the heat transfer augmentation in PFET heat exchangers.

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“…According to the most common paradigm for such systems, a unique stationary turbulent state can be identified given a certain stirring mechanism and a set of equilibrium plasma parameters. Multistable solutions -for which identical parameters admit distinct turbulent states -are known to occur in neutral fluids [1][2][3], where they are associated with bifurcations and hysteretic behaviour [4,5]. Multistability has also been reported in weakly magnetised systems of charged fluids [6,7].…”

Section: Introductionmentioning

confidence: 99%

Bistable turbulence in strongly magnetised plasmas with a sheared mean flow

Christen,

Barnes,

Hardman

et al. 2021

Preprint

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The prevailing paradigm for plasma turbulence associates a unique stationary state to given equilibrium parameters. We report the discovery of bistable turbulence in a strongly magnetised plasma. Two distinct states, obtained with identical equilibrium parameters in first-principle gyrokinetic simulations, have turbulent fluxes of particles, momentum and energy that differ by an order of magnitude -with the low-transport state agreeing with experimental observations. Occurrences of the two states are regulated by the competition between an externally imposed mean flow shear and "zonal" flows generated by the plasma. With small turbulent amplitudes, zonal flows have little impact, and the mean shear causes turbulence to saturate in a low-transport state. With larger amplitudes, the zonal shear can (partially) oppose the effect of the mean shear, allowing the system to sustain a hightransport state. This poses a new challenge for research that has so far assumed a uniquely defined turbulent state.

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Observation of a bistable flow in a hemispherical cavity.

Cited by 23 publications

References 2 publications

Flow Structures and Heat Transfer on Dimpled Surfaces

Flow Structures and Heat Transfer on Dimpled Surfaces

Thermo-Hydraulic Performance Enhancement of Finned Elliptical Tube Heat Exchangers by Utilizing Innovative Dimple Turbulators

Bistable turbulence in strongly magnetised plasmas with a sheared mean flow

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