Summertime winds and direct cyclonic circulation: observations from Lake Geneva

International audienceRecords of wind, air temperature and air pressure from nine stations, situated along the shoreline of Lake Geneva, Switzerland, were analyzed for the summer period May to September. At all stations the consistent appearance of significant spectral peaks and changes in wind direction at the diurnal frequency indicates the importance of lake-land breezes. It is shown that the surrounding topography has a strong modifying effect (temporal and spatial) on the lake-land breeze. Superimposed on this cyclic wind pattern, short episodes of strong winds with long fetch over parts of Lake Geneva are regularly observed. Both of these winds exert a spatially variable wind stress over the lake surface on the same time scale. Typical examples of the expected lake's response are presented, among them the seasonally persistent gyre in the central part of the lake. Evidence is provided that this dominant circulation is part of a direct cyclonic circulation, generated by the curl of the diurnal wind field. It is concluded that the mean circulation is caused by these winds and affected by the topography of the surrounding land

Near-field flow structure of a confined wall jet on flat and concave rough walls

Experimental results are presented of the mean flow and turbulence characteristics in the near field of a plane wall jet issuing from a nozzle onto flat and concave walls consisting of fixed sand beds. This is a flow configuration of interest for sediment erosion, also referred to as scouring. The measurements were made with an acoustic profiler that gives access to the three components of the instantaneous velocities. For the flat-wall flow, it is shown that the outer-layer spatial growth rate and the maxima of the Reynolds stresses approach the values accepted for the far field of a wall jet at a downstream distance x/b0 ≈ 8. These maxima are only about half the values of a plane free jet. This reduction in Reynolds stresses is also observed in the shear-layer region, x/b0 11, the maximum Reynolds shear stress approaches the value of a plane free jet. This change in Reynolds stresses is related to the mean vertical velocity that is negative for x/b0 < 8 and positive further downstream. The evolution of the inner region of the wall jet is found to be in good agreement with a previous model that explicitly includes the roughness length. On the concave wall, the mean flow and the Reynolds stresses are drastically changed by the adverse pressure gradient and especially by the development of Görtler vortices. On the downslope side of the scour hole, the flow is nearly separating with the wall shear stress tending to zero, whereas on the upslope side, the wall-friction coefficient is increased by a factor of about two by Görtler vortices. These vortices extend well into the outer layer and, just above the wall, cause a substantial increase in Reynolds shear stres

Double-average methodology applied to turbulent gravel-bed river flows

River hydrodynamicsTurbulent open channel flow and transport phenomen

Turbulent transport in the outer region of rough-wall open-channel flows : the contribution of large coherent shear stress structures (LC3S)

Author Posting. © Cambridge University Press, 2007. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 574 (2007): 465-493, doi:10.1017/S0022112006004216.Acoustic Doppler velocity profiler (ADVP) measurements of instantaneous three-dimensional velocity profiles over the entire turbulent boundary layer height, δ, of rough-bed open-channel flows at moderate Reynolds numbers show the presence of large scale coherent shear stress structures (called LC3S herein) in the zones of uniformly retarded streamwise momentum. LC3S events over streamwise distances of several boundary layer thicknesses dominate the mean shear dynamics. Polymodal histograms of short streamwise velocity samples confirm the subdivision of uniform streamwise momentum into three zones also observed by Adrian et al. (J. Fluid Mech., vol. 422, 2000, p. 1). The mean streamwise dimension of the zones varies between 1δ and 2.5δ. In the intermediate region (0.2<z/δ<0.75), the contribution of conditionally sampled u'w' events to the mean vertical turbulent kinetic energy (TKE) flux as a function of threshold level H is found to be generated by LC3S events above a critical threshold level Hmax for which the ascendant net momentum flux between LC3S of ejection and sweep types is maximal. The vertical profile of Hmax is nearly constant over the intermediate region, with a value of 5 independent of the flow conditions. Very good agreement is found for all flow conditions including the free-stream shear flows studied in Adrian et al. (2000). If normalized by the squared bed friction velocity, the ascendant net momentum flux containing 90% of the mean TKE flux is equal to 20% of the shear stress due to bed friction. In the intermediate region this value is nearly constant for all flow conditions investigated herein. It can be deduced that free-surface turbulence in open-channel flows originates from processes driven by LC3S, associated with the zonal organization of streamwise momentum. The good agreement with mean quadrant distribution results in the literature implies that LC3S identified in this study are common features in the outer region of shear flows.The study was supported by the Swiss National Foundation for Scientific Research for the experimental part (grant 2100 050739) and the French National Center for Scientific Research (CNRS) for the data analysis and interpretation

Direct effects of dominant winds on residence and travel times in the wide and open lacustrine embayment: Vidy Bay (Lake Geneva, Switzerland)

Numerical simulations were carried out to determine the residence (or flushing) time of water in Vidy Bay (north shore of Lake Geneva) for different meteorological conditions. A hydrodynamic model (Delft3D-FLOW) was applied to simulate the flow field in the embayment during 2010 and January 2011. Using these results, particle tracking was applied to estimate transport of wastewater effluent discharged into the embayment. The model predictions compared well with published field measurements of dissolved species (as given by electrical conductivity profiles) within the wastewater. The pelagic boundary of the embayment was defined by the largest within-bay gyre. Based on this definition, particle tracking was used to quantify the residence time under dominant wind conditions. Similarly, particle tracking was used to determine the travel time (i.e., time to exit the embayment) for each of Vidy Bay's three inflows (stream, stormwater and wastewater effluent). Although the wind field over the lake is variable, current patterns in the embayment can be simulated using the hydrodynamic model forced by a spatially uniform wind field. For a given wind speed, the main factor influencing residence and travel times is the wind angle. The presence of gyres leads to high mean residence times with large variability. As the wind direction becomes more aligned with the shoreline (i.e., with increasing westerly or easterly components), longshore currents dominate. These disrupt gyre formation and markedly reduce the mean and variability of embayment residence time. The numerical model was utilized to assess the potential for plume movement (in plan) from above the wastewater effluent outfall towards one of Lausanne's drinking water intakes. In the most direct pathway, westward longshore currents can move water from the embayment to the water column above the intake location