Closure to “Time Development of Scour around a Cylinder in Simulated Tidal Currents”

A Discussion Closure to “Time Development of Scour around a Cylinder in Simulated Tidal Currents

Laboratory study on the effects of hydro kinetic turbines on hydrodynamics and sediment dynamics

© 2018 The Authors The need for hydrokinetic turbine wake characterisation and their environmental impact has led to a number of studies. However, a small number of them have taken into account mobile sediment bed effects. The aim of the present work is to study the impact of the presence of a horizontal-axis three-bladed turbine with the flow and a mobile sediment bed. We use a series of laboratory experiments with a scaled modelled turbine installed in a flume with a mobile sandy bed at the bottom. Acoustic instruments were used to monitor flow, suspended sediment and bed behaviour. Results show a velocity decrease of about 50% throughout the water column and no flow recovery after a distance of 15 rotor diameters. Clearly visible ripples in the absence of the model turbine were replaced by horseshoe-shaped scour pit in the near wake region, and a depositional heap in the far wake. Suspended sediment differences were recorded in the streamwise direction with a possible effect of the wake as far as 15 rotor diameters. These results imply potentially important effects on the efficiency of turbine arrays, if the flow were to be lower than expected, on turbine foundations and modify coastal sediment transport

Preliminary measurements of turbulence and environmental parameters in a sub-tropical estuary of Eastern Australia

In natural systems, mixing is driven by turbulence, but current knowledge is very limited in estuarine zones where predictions of contaminant dispersion are often inaccurate. A series of detailed field studies was conducted in a small subtropical creek in eastern Australia. Hydrodynamic, physio-chemical and ecological measurements were conducted simultaneously to assess the complexity of the estuarine zone and the interactions between turbulence and environment. The measurements were typically performed at high frequency over a tidal cycle. The results provide an original data set to complement long-term monitoring and the basis for a more detailed study of mixing in sub-tropical systems. Unlike many long-term observations, velocity and water quality scalars were measured herein with sufficient spatial and temporal resolutions to determine quantities of interest in the study of turbulence, while ecological indicators were sampled systematically and simultaneously. In particular the results yielded contrasted outcomes, and the finding impacts on the selection process for key water quality indicators

Diagnostic Morphology for Martian Groundwater Outflows from Flume Experiments

Channels on Mars have been found in many locations and across all possible scales, indi-cating presence of flowing water in the past. Several hypotheses for the formation of these channels have been proposed, including a role for groundwater. In this study we explored the morphology formed by three types of groundwater systems, all three previous-ly hypothesized as possible Martian scenarios: 1) seep-age from a regional groundwater system [1], 2) seep-age from local precipitation [2] and 3) groundwater release from a pressurized aquifer (varying from sub- to super-lithostatic pressure) [3]. Here we present the results of analogue scale models which were focused on characteristic morphological elements, which we compare to Martian cases

Simultaneous measurements with 3D PIV and Acoustic Doppler Velocity Profiler

Simultaneous velocity measurements were taken using Particle Image Velocimetry (PIV) and an Acoustic Doppler Velocity Profiler (ADVP) in a sharp open-channel bend with an immobile gravel bed. The PIV measures 3D velocity vectors in a vertical plane (~40cm x 20cm) at a frequency of 7.5 Hz, whereas the ADVP measures 3D velocity vectors in a vertical profile with a frequency of 31.25 Hz. The paper reports simultaneous measurements with both instruments positioned in the same location. Both instruments resolve accurately spatial structures of the complex mean flow fields characterized by small velocities of the order of 0.01 ms-1, such as the outer-bank secondary flow cell and the secondary flow cell in the zone of flow separation at the inner bank. PIV measurements of the mean velocities are of better quality near the flow boundaries and the spatial distribution of data allows investigation of the temporal behaviour of secondary flow structures. Power spectra and time-series of quasi-instantaneous velocities demonstrate that the ADVP measures turbulence accurately, whilst PIV measurements of turbulence suffer from the lower temporal resolution and the higher noise levels. The results presented in this paper demonstrate that the combined application of PIV and ADVP allows investigation complex 3D flows in greater detail than is possible from a single instrument.Hydraulic EngineeringCivil Engineering and Geoscience

Mechanisms for pressurized groundwater outflow channels, implications for Mars

Various valleys on Mars show evidence for extensive fluvial activity in the past. The largest valleys on Mars are several tens to hundreds of kilometers wide and are thought to have originated from outflow of pressurized groundwater. However, exact mechanisms of these processes are lacking, which hampers a quantitative interpretation of some of the most impressive morphological features on Mars. Using flume experiments, we studied a range of possible pressurized groundwater outflow mechanisms including artesian seepage, enhanced seepage through fissures and the eruption of a pressurized groundwater reservoir. These experiments focused on the morphological development of such systems. We also analyze the scalability of the laboratory analogues to real-word systems and we study the outflow-channel areas in Lunae Planum and Xanthe Terra north of Valles Marineris. In the experiments, we found that low water injection pressures led to the formation of surface lakes, intermediate pressures led to the formation of subsurface fissures, and high pressure led to the buildup of a pressurized subsurface lake that erupted to the surface. Each of these systems resulted in catastrophic release of water greater than the groundwater discharge, from accumulation in a lake, enhanced seepage through fissures or both. In all experiments, an outflow channel formed, but we observed other morphologies as well that are unique to the mechanism of release. Fissure seepage created small holes and in the case of a subsurface lake eruption, large depressions and fractures were formed. In all cases, the sudden release of water resulted in the deposition of sedimentary lobes due to infiltration of water flowing over downstream areas that were still dry. This mechanism is absent in in the case of slow groundwater outflow. Our study of the surface of Mars reveals a range of morphological features that were not associated with pressurized groundwater outflow before. These features include sedimentary lobes, holes, depressions and cracks. We can now link these morphologies to different mechanisms of outflow. Our results contribute to the hypothesis of the formation of outflow channels by pressurized groundwater on Mars. The experimental insight enables us to quantify the amount of water required for the formation of these channels