Tomographic Piv Measurements of Turbulent Fountains With Refraction Index Matching

Earl, Thomas; Paetzold, J.; Cochard, Steve

doi:10.1615/jflowvisimageproc.2014011727

Cited by 3 publications

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“…This approach allows for the measurement error to be assessed a posteriori and aggregates the systematic and random errors associated with the experimental apparatus and PIV algorithm. Previous studies have applied uncertainty analysis (Moffat 1988) and established that error in the velocity gradient can be expressed in terms of the random velocity error (Atkinson et al 2010;Earl et al 2013):…”

Section: Measurement Uncertaintymentioning

confidence: 99%

Cylinder wakes in shallow oscillatory flow: the coastal island wake problem

et al. 2019

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Topographic complexity on continental shelves is the catalyst that transforms the barotropic tide into the secondary and residual circulations that dominate vertical and cross-shelf mixing processes. Island wakes are one such example that are observed to significantly influence the transport and distribution of biological and physical scalars. Despite the importance of island wakes, to date, no sufficient, mechanistic description of the physical processes governing their development exists for the general case of unsteady tidal forcing. Controlled laboratory experiments are necessary for the understanding of this complex flow phenomena. Three-dimensional velocity field measurements of cylinder wakes in shallow-water, oscillatory flow are conducted across a parameter space that is typical of tidal flow around shallow islands. Previous studies investigated the wake form dependance on KC = U 0 T /D, where KC is the Keulegan-Carpenter number, D is the island diameter, U 0 the tidal velocity amplitude and T the tidal period, and the stability parameter S = c f D/h where h is the water depth and c f is the bottom boundary friction coefficient. In this study we demonstrate that when the influence of bottom friction is confined to a Stokes boundary layer the parameter S = δ + /KC where δ + = δ/h and δ = 2π 2ν/ω is the wavelength of the Stokes bottom boundary layer. Three classes of wake form are observed for decreasing wake stability: (1) Steady Bubble for S 0.1; (2) Unsteady Bubble for 0.06 S 0.1; and (3) Vortex Shedding for S 0.06. Transitions in wake form and wake stability are shown to depend on the magnitude and temporal evolution of the wake return flow. Scaling laws are developed to allow upscaling of the laboratory results to island wakes. Vertical and lateral transport depend on three parameters: 1) the flow aspect ratio h/D, where h is the flow depth and D the cylinder (island) diameter; 2) the amplitude of tidal motion relative to the island size, given by KC; and 3) the relative influence of bottom friction to the flow depth given by δ + . A model of wake upwelling based on Ekman pumping from the bottom boundary layer demonstrates that upwelling in the near wake region of an island scales with U 0 (h/D)KC 1/6 and is independent of the wake form. Finally, we demonstrate an intrinsic link between the dynamical eddy scales, predicted by the Ekman pumping model, and the island wake form and stability. †

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Section: Measurement Uncertaintymentioning

confidence: 99%

Cylinder wakes in shallow oscillatory flow: the coastal island wake problem

et al. 2019

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Importance of Refractive Index Matching of Fluids for PIV and PLIF Measurements in Buoyant Jets

Mishra

Philip

2021

Lecture Notes in Mechanical Engineering

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Three-dimensionality of shallow island wakes

2019

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Island wakes are thought to play a significant role in the vertical and 6 cross shelf mixing processes in strong tidally forced coastal regions. This paper de-7 scribes a comprehensive laboratory study of shallow water wakes behind islands of 8 circular cross section forced by a sinusoidal tidal flow. The wake structure and ver-9 tical circulation are determined through novel three-dimensional particle imaging 10 velocimetry measurements. Four archetypal wake forms (symmetric, asymmetric, 11 unsteady bubble, and vortex shedding) are observed. Through examination of the 12 vertical structure of each of these wake forms, we demonstrate the dependence 13 of vertical transport in island wakes on three key parameters: i) the tidal excur-14 sion relative to the island size, ii) the bottom boundary layer thickness relative 15 to the flow depth and iii) the aspect ratio of the island size to the flow depth. 16 The importance of secondary vortices in island upwelling is highlighted by local 17 peaks in vertical velocity that exceed 40% of the peak external tidal velocity. This 18 study fundamentally changes the view of island wake upwelling from a weak 'tea 19 cup'-like recirculation process to one where primary and secondary flow structures 20 vigorously stir the water column over the full depth. This has fundamental impli-21 cations for the fate of passive biological tracers and the time scales that determine 22 productivity in topographically-complex continental shelf regions. Vorticity 25 1 Introduction 26The wakes of islands and headlands play an important role in lateral and vertical 27 mixing processes, with implications for the transport of nutrients, plankton, sedi-28 ment and pollutants. The vertical transport (upwelling) of nutrient rich water into 29 the surface layer (that has higher light intensity) has important ecological implica-30 tions due to the resulting increased primary productivity. Numerous studies have 31 identified the importance of complex flow features such as eddies and fronts (shear 32 zones) for aggregating sediment and plankton [35, 15, 23]. This can, in turn, ef-33 fect the distribution of benthic and higher order pelagic organisms that graze 34 along these fronts and wakes, taking advantage of lower-trophic-level aggregations 35 [34, 16, 17]. Despite the perceived ecological importance of island wakes, they 36 have received only intermittent research attention over the past several decades, 37 and to date no clear mechanistic description of the flow has clearly supported the 38 hypothesised ecological importance. 39 1.1 Variation of island wake form 40 One of the most interesting features of island wakes is the insensitivity of the 41 large scale flow features to the Reynolds number, Re = U D/ν, where U is the 42 characteristic flow speed, D the characteristic island diameter and ν the molecular 43 kinematic viscosity (or eddy viscosity for turbulent flow [10]). For flow around a 44 cylinder in the laboratory a Kármán vortex street is observed for Re 40, which is 45 beautif...

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Tomographic Piv Measurements of Turbulent Fountains With Refraction Index Matching

Cited by 3 publications

References 0 publications

Cylinder wakes in shallow oscillatory flow: the coastal island wake problem

Cylinder wakes in shallow oscillatory flow: the coastal island wake problem

Importance of Refractive Index Matching of Fluids for PIV and PLIF Measurements in Buoyant Jets

Three-dimensionality of shallow island wakes

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