5 results
Quasi-two-dimensional properties of a single shallow-water vortex with high initial Reynolds numbers
- D.-G. SEOL, G. H. JIRKA
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- Journal:
- Journal of Fluid Mechanics / Volume 665 / 25 December 2010
- Published online by Cambridge University Press:
- 19 October 2010, pp. 274-299
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The evolution and dynamics of a shallow-water vortex system with high initial Reynolds numbers are investigated experimentally without background rotation. A single vortex is generated by rotating a water mass at the centre of an experimental tank using a bottomless cylinder with internal sectors. The surface velocity field is observed via particle image velocimetry. The experimentally observed vorticity fields indicate that strong shallowness (the ratio of the cylinder diameter to the water depth) and high Reynolds number contribute to the formation of large-scale coherent structures in the form of a tripolar vortex system. The shallow-water vortices with high initial Reynolds numbers experience the transition from turbulent to laminar regimes in their decay process. The proposed first-order vortex decay model predicts that a shallow-water vortex decays as t−1 in the initial turbulent stage and as e−t in the later laminar stage due to horizontal diffusion and bottom friction. The estimated transition time scale from the turbulent to laminar stage increases with initial vortex Reynolds number and with shallowness. By taking the vortex expansion into consideration, the second-order vortex decay model is also presented. The azimuthally ensemble-averaged data elucidate effects of the vortex instabilities and of turbulent energy transfer on the formation of large-scale coherent flow structures. Normal mode analysis of the vortex systems is conducted to study the effect of shallowness and Reynolds number on the generation of two-dimensional large-scale coherent structures. The results show that the perturbation wavenumber of mode 2 is the fastest-growing instability in shallow-water conditions, and its effect depends on initial Reynolds number and shallowness.
Synoptic velocity and pressure fields at the water–sediment interface of streambeds
- M. DETERT, V. NIKORA, G. H. JIRKA
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- Journal:
- Journal of Fluid Mechanics / Volume 660 / 10 October 2010
- Published online by Cambridge University Press:
- 16 August 2010, pp. 55-86
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This paper presents a comprehensive study of the near-bed hydrodynamics at non-moving streambeds based on laboratory experiments in open-channel flows. Pressure and velocity measurements were made with an array of up to 15 miniaturized piezo-resistive pressure sensors within the bed and slightly above it, and a two-dimensional particle-image-velocimetry (PIV) system measuring in streamwise vertical or horizontal planes. Three different types of bed materials were studied covering typical natural streambed conditions. The range of the global Reynolds number covered in the experiments was from 20000 to 200000. This study provides new insights into the flow structure over gravel beds based on the PIV measurements in both streamwise vertical and horizontal planes. In a streamwise vertical plane, large-scale wedge-like flow structures were observed where a zone of faster fluid over-rolled a zone with slower fluid. The resulting shear layer was inclined along the flow at an angle of 10°–25° to the bed, and was populated with clockwise rotating eddies. This mechanism occurred with sufficient frequency and shape to leave an ‘imprint’ in the velocity statistics. Typically, the described flow pattern is formed near the bed and is approximately scaled with the height of the logarithmic layer, although the biggest structures extended over the whole flow depth. In a horizontal near-bed plane, turbulent structures formed a patched ‘chessboard’ pattern with regions of lower and higher velocities that were elongated in the streamwise direction. Their lateral extension was typically two to four times the equivalent sand roughness with lengths up to several water depths. The dimensions of the regions were increasing linearly with the distance from the bed. These findings are consistent with conceptual models originally developed for smooth-wall flows. They also support observations made in rough-bed flume experiments, numerical simulations and natural rivers. Spatial fields of bed-pressure fluctuations were reconstructed by applying Taylor's frozen turbulence hypothesis on time data obtained with an array of pressure sensors. Based on the conditional sampling of velocity patterns associated with pressure-drop events a distinct bed-destabilizing flow-pressure pattern was identified. If a high-speed fluid in the wake of a large-scale wedge-like flow structure reaches the vicinity of the bed, a phenomenon akin to a Bernoulli effect leads to a distinctive low-pressure pattern. The resulting force may exceed the particles' submerged weight and is assumed to be able to give an initial lift to the particle. As a result, the exposed area of a particle is amplified and its angle of repose is reduced, increasing the probability for entrainment.
Experiments on gas transfer at the air–water interface induced by oscillating grid turbulence
- HERLINA, G. H. JIRKA
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- Journal:
- Journal of Fluid Mechanics / Volume 594 / 10 January 2008
- Published online by Cambridge University Press:
- 14 December 2007, pp. 183-208
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The gas transfer process across the air–water interface in a turbulent flow environment, with the turbulence generated in the water phase far away from the surface, was experimentally investigated for varying turbulent Reynolds numbers ReT ranging between 260 and 780. The experiments were performed in a grid-stirred tank using a combined particle image velocimetry – laser induced fluorescence (PIV-LIF) technique, which enables synoptic measurements of two-dimensional velocity and dissolved gas concentration fields. The visualization of the velocity and concentration fields provided direct insight into the gas transfer mechanisms. The high data resolution allowed detailed quantification of the gas concentration distribution (i.e. mean and turbulent fluctuation characteristics) within the thin aqueous boundary layer as well as revealing the near-surface hydrodynamics. The normalized concentration profiles show that as ReT increases, the rate of concentration decay into the bulk becomes slower. Independent benchmark data for the transfer velocity KL were obtained and their normalized values (KLSc0.5/uHT) depend on ReT with exponent −0.25. The spectra of the covariance term c′ w′ indicate that the contribution of c′ w′ is larger in the lower-frequency region for cases with small ReT, whereas for the other cases with higher ReT, the contribution of c′ w′ appears to be larger in the higher-frequency region (small eddies). These interrelated facts suggest that the gas transfer process is controlled by a spectrum of different eddy sizes and the gas transfer at different turbulence levels can be associated with certain dominant eddy sizes. The normalized mean turbulent flux profiles increase from around 0 at the interface to about 1 within a depth of approximately 2δe, where δe is the thickness of the gas boundary layer. The measured turbulent flux (c′ w′) is of the same order as the total flux (j), which shows that the contribution of c′ w′ to the total flux is significant.
Plane turbulent jets in a bounded fluid layer
- T. Dracos, M. Giger, G. H. Jirka
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- Journal:
- Journal of Fluid Mechanics / Volume 241 / August 1992
- Published online by Cambridge University Press:
- 26 April 2006, pp. 587-614
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An experimental investigation of plane turbulent jets in bounded fluid layers is presented. The development of the jet is regular up to a distance from the orifice of approximately twice the depth of the fluid layer. From there on to a distance of about ten times the depth, the flow is dominated by secondary currents. The velocity distribution over a cross-section of the jet becomes three-dimensional and the jet undergoes a constriction in the midplane and a widening near the bounding surfaces. Beyond a distance of approximately ten times the depth of the bounded fluid layer the secondary currents disappear and the jet starts to meander around its centreplane. Large vortical structures develop with axes perpendicular to the bounding surfaces of the fluid layer. With increasing distance the size of these structures increases by pairing. These features of the jet are associated with the development of quasi two-dimensional turbulence. It is shown that the secondary currents and the meandering do not significantly affect the spreading of the jet. The quasi-two-dimensional turbulence, however, developing in the meandering jet, significantly influences the mixing of entrained fluid.
Grid turbulence in shallow flows
- W. S. J. UIJTTEWAAL, G. H. JIRKA
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- Journal:
- Journal of Fluid Mechanics / Volume 489 / 25 July 2003
- Published online by Cambridge University Press:
- 30 July 2003, pp. 325-344
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Results of experiments on decaying turbulence in shallow water flow bounded by a solid bottom and a free surface are presented. The evolution of the turbulence structures generated by a grid, with horizontal mesh dimensions larger than the water depth, is measured using laser Doppler velocimetry and particle image velocimetry (PIV). The vertical confinement of the flow forces the large turbulence structures to move in the horizontal plane, thus attaining strongly two-dimensional features. This two-dimensionality and its consequences for the intensities and length scales of the large structures are analysed. It is shown that the decay of the vortices that are shed from the grid is determined by the characteristic size of the grid elements rather than the grid spacing. Furthermore, during the decay process the merging of vortices is observed in combination with a $-3$ slope in the energy density spectrum of the velocity fluctuations. Using the PIV data, spatial properties like divergence and enstrophy can be derived for the velocity field near the free surface. The distinct effect of water depth that is found in the velocity fluctuations is almost insignificant in the enstrophy decay.