193 research outputs found
The motion of fine sand particles in turbulent open channel shear flows over porous bed conditions
The current study aims to investigate the physical mechanisms controlling fine sediment transport within open channel shear flows over porous beds, with particular emphasis on the role of flow turbulence in particle settling and deposition processes.
Preliminary visualisation experiments used a VHS camera to observe the near-bed motion of sand particles and their behaviour within the surface layer of a rhombically-packed bed of uniform spheres. Measurement of near-bed particle trajectories indicate that turbulent particle fall velocities w's are generally larger than fall velocities measured in still water ws, most notably for finer sand grades. Distinctive modes of particle behaviour observed at the bed interface also suggest that flow-separation eddies, generated within surface interstices, have a primary influence on subsequent particle motion, i.e. deposition or re-entrainment. Similar particle behaviour is also displayed in a natural gravel bed.
A more detailed analysis of sand particle motion in turbulent open channel flow was carried out employing a high-speed camera and particle-tracking technique to record and analyse particle trajectories within different flow regions. The non-dimensional ratio of measured particle fall velocity w's and still water fall velocity ws was used to indicate the relative enhancement of vertical particle motion within the turbulent flow conditions. Experiment-averaged values of this ratio reveal that particle fall velocities are generally enhanced (i.e. w's/ws > 1) in recorded near-bed and intermediate flow regions (z/H 0.5) and hindered (i.e. w's/ws < 1) in a recorded outer flow region (z/H 0.5). The ratio w's/ws also reveals a general tendency to increase with decreasing grain size di. Vertical profiles of the normalised particle fall velocity w's/u* are shown to be analogous to turbulence intensity distributions (u' rms/u* and w'rms/u*), with the highest values of w's/u* occurring in the near-bed region and coinciding approximately with the regions of highest turbulence activity. This clearly implies the existence of turbulence-enhanced particle fall velocities within the flow conditions considered. Application of a quadrant analysis technique reinforces this notion, revealing further similarities between conditioned turbulent fluid fluctuations and particle motions, in particle, the dominance of 'inrush' events (quadrant 4) in the near-bed flow and 'ejection' events (quadrant 2) away from the bed
Understanding the dynamics of unsteady buoyant Jets:an experimental analogue of Vulcanian and Strombolian style eruptions
Explosive volcanic eruptions, which are characterized by the discharge of ash and gas from the vent into the atmosphere, are an example of a naturally occurring buoyant jet. These buoyant jets can significantly impact the surrounding environment; for example, the presence of fine ash particles in the atmosphere can damage aircraft engines, potentially leading to engine failure. Therefore, during an explosive eruption, volcanic ash advisory centers (VAACs) consistently monitor the concentration of ash in the atmosphere using numerical models. These numerical models require the definition of a source term (i.e., source mass eruption rate, plume height and total grain size distribution), which is often obtained from simpler one-dimensional models. One dimensional models derived from well-established theories successfully replicate the dynamics of the initial buoyant jet; however, they assume time-averaged source conditions which are not observed in field-scale vulcanian and strombolian style eruptions. As such, there is a disconnectbetween these well-established theories assuming time averaged source conditions and reality. This disconnect may introduce uncertainties in ash concentration forecasts, potentially resulting in practical implications such as unnecessary airspace closures or flights operating in hazardous conditions. The present contribution utilizes scaled laboratory experiments to quantify theinfluence of source variability on the dynamics of buoyant jets and evaluates potential deviations from time-average assumptions
Understanding the dynamics of unsteady buoyant Jets:an experimental analogue of Vulcanian and Strombolian style eruptions
Explosive volcanic eruptions, which are characterized by the discharge of ash and gas from the vent into the atmosphere, are an example of a naturally occurring buoyant jet. These buoyant jets can significantly impact the surrounding environment; for example, the presence of fine ash particles in the atmosphere can damage aircraft engines, potentially leading to engine failure. Therefore, during an explosive eruption, volcanic ash advisory centers (VAACs) consistently monitor the concentration of ash in the atmosphere using numerical models. These numerical models require the definition of a source term (i.e., source mass eruption rate, plume height and total grain size distribution), which is often obtained from simpler one-dimensional models. One dimensional models derived from well-established theories successfully replicate the dynamics of the initial buoyant jet; however, they assume time-averaged source conditions which are not observed in field-scale vulcanian and strombolian style eruptions. As such, there is a disconnectbetween these well-established theories assuming time averaged source conditions and reality. This disconnect may introduce uncertainties in ash concentration forecasts, potentially resulting in practical implications such as unnecessary airspace closures or flights operating in hazardous conditions. The present contribution utilizes scaled laboratory experiments to quantify theinfluence of source variability on the dynamics of buoyant jets and evaluates potential deviations from time-average assumptions
Measurement of density and porosity profiles within mixed sediment deposits using an electrical resistivity technique
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