55,854 research outputs found
Groundwater in fractured bedrock environments: managing catchment and subsurface resources – an introduction
Full text linkHard rocks, including crystalline igneous, metamorphic and strongly cemented sedimentary and carbonate rocks, cover about 50% of the Earth's land surface (Singhal & Gupta 2010). Globally, the volume of groundwater contained in hard rock aquifers is not well constrained (Comte et al. 2012) but locally they can be important aquifers (MacDonald et al. 2012), albeit with low groundwater storage and poor primary porosity and permeability. Groundwater flow in these hard rocks is commonly observed to be associated with water-bearing discontinuities, such as fractures, joints and faults (Mazurek 2000; Berkowitz 2002; Font-Capo et al. 2012), and in the weathered regolith (Wright 1992; Chilton & Foster 1995; Deyassa et al. 2014). Structural elements such as fault zones also strongly govern the behaviour of these systems (Forster & Evans 1991; López & Smith 1995; Bense et al. 2013). The nature, abundance, orientation and connectivity of these water-bearing features are largely governed by the history and nature of structural deformation of the bedrock, and commonly impose strong anisotropic flow and transport parameters on these bedrock aquifers (Hsieh et al. 1985; Bour & Davy 1997; Mortimer et al. 2011). Weathering processes furthermore lead to an alteration of bedrock composition and associated aquifer properties resulting in enhanced fracture connectivity and an overall vertical stratification/zonation of bulk aquifer properties, ranging from highly altered shallow regolith horizons to more competent sparsely fractured bedrock at depth (Dewandel et al. 2006; Krásný & Sharp 2007; Lachassagne et al. 2011)
A Dynamic Subfilter-scale Stress Model for Large Eddy Simulations Based on Physical Flow Scales
No full textWe propose a new definition of the length scale in an eddy-viscosity model for large-eddy simulations (LES). This formulation extends and generalizes a previous proposal [Piomelli, Rouhi and Geurts, Proc. ETMM10, 2014], in which the LES length scale was expressed in terms of the integral length-scale of turbulence determined by the flow characteristics and explicitly decoupled from the simulation grid; this approach was named Integral Length-Scale Approximation (ILSA). As in the original ILSA, the model coefficient was determined by the user, and required to maintain a desired contribution of the unresolved, subfilter scales (SFS) to the global transport. We propose a local formulation (local ILSA) in which the model coefficient is local in space, allowing a precise control over SFS activity as a function of location. This new formulation preserves the properties of the global model; application to channel flow and backward-facing step verifies its features and accuracy
Large-eddy simulation of a separated flow with a sub-filter scale model based on the integral length-scale
No full textA new sub-filter scale model for large-eddy simulations, which uses a length-scale proportional to the integral scale of the turbulence instead of the grid resolution to parametrize the modelled stresses, will be assessed in the prediction of the flow of a boundary-layer over a rough surface, which includes separation and reattachment
Near Wall PIV-Measurements on the Windward Slope of a Hill
No full textThe turbulent flow over periodic hills was measured near to the wall, using planar Particle-Image-Velocimetry (PIV) at high spatial resolution. Our focus is on the near wall turbulence structure on the windward slope of the hill. For large-eddy simulation (LES) we suspect that, if this was not predicted accurately, it affects the prediction of the velocity profiles over the hill crest which in turn will affect the recirculation length downstream of the hill. Regarding the time averaged velocities, we were able to resolve the linear viscous region of the boundary layer. The velocity distribution and also the Reynolds stress does not comply with the law of the wall as it is valid for a turbulent boundary layer at equilibrium
Energy dissipation and flux laws for unsteady turbulence
No full textDirect Numerical Simulations of spatially periodic unsteady turbulence show that the high Reynolds number scalings of the instantaneous energy dissipation rate and interscale energy flux at intermediate wavenumbers are qualitatively different from the well-known cornerstone scalings of equilibrium turbulence where and are time-dependent rms velocity and integral length-scales. Instead, they both scale as where and are length and velocity scales characterizing initial/overall unsteady turbulence conditions
Contribution of geostatistics in mapping subsoil temperature evolution in urban areas
Full text linkUrban settlements influence ground temperature in the shallow layers, this phenomenon called subsurface urban heat island effect at city scale. Heat losses from an individual building generate a bulb-shaped volume of subsurface temperatures higher than in surroundings. Superposing the thermal effect of multiple buildings on the subsoil leads to uncertainties in defining the dimension and shape of the temperature volume. This is due by geological and hydrogeological natural variability and by the practical impossibility of quantifying all heat sources and sinks affecting an area. Therefore, the use of geostatistics can be useful to predict ground temperature space and time variability in an urban area, by the analysis of available information. The improved quantification of ground temperature by geostatistical techniques, with respect to the application of only deterministic models, can advantage many engineering topics, such as insulation design, heat
recovery and shallow geothermal systems. This work presents a geostatistical simulation of ground temperature variability in 4D space in a delimited area. An urban zone, with measurements of groundwater temperature and borehole temperature profiles, was selected. Groundwater temperature measurements, at fixed depth, were used to calculate the time-varying vertical temperature profile at the point, knowing climate, geological and geothermal information. Then, geostatistical simulation was performed to obtain a set of equally probable images of ground temperature in 4D space, conditioned by boundary, GIS-based, information (land cover and population density). Finally, profiles of borehole temperature data were applied to verify the quality of the most appropriate map solutions. The final aim of the
work was to provide a robust quantification and representation of the subsurface urban heat island effect in the study area.This outcome will give the possibility to feed the design of underground spaces and geostructures with many possible coherent distributions of subsoil temperature. The uncertainty of the results of numerical models can then be calculated, defining a risk assessment for each project, useful to overcome possible critical conditions. The results of the work can be repeated and improved for larger and more complex urban areas, and can be useful for further ground temperature assessment on wider urban settlements
Direct numerical simulation of turbulent Couette-Poiseuille flow with zero skin friction
No full textThe near-wall scaling of mean velocity U(y) is addressed for the case of zero skin friction on one wall of a fully turbulent channel flow. The present DNS results can be added to the evidence in support of the conjecture that U is proportional to √yw in the region just above the wall at which the mean shear dU/dy = 0
Real-space Manifestations of Bottlenecks in Turbulence Spectra
No full textAn energy-spectrum bottleneck, a bump in the turbulence spectrum between the inertial and dissipation ranges, is shown to occur in the non-turbulent, one-dimensional, hyperviscous Burgers equation and found to be the Fourier-space signature of oscillations in the real-space velocity, which are explained by boundary-layer-expansion techniques. Pseudospectral simulations are used to show that such oscillations occur in velocity correlation functions in one- and three-dimensional hyperviscous hydrodynamical equations that display genuine turbulence
Braid Entropy of Faraday Waves driven 2D Turbulence
No full textWe report new experimental results that use tools from braid theory to characterize two-dimensional turbulent flows driven by Faraday waves. The average topological length of the material fluid lines is found to grow exponentially with time. It allows us to compute the braid’s topological entropy SBraid. We show that SBraid increases as the square root of the turbulence kinetic energy E ~ u^2, where u^2 is the horizontal velocity variance . At long times, the PDFs of Lbraid are positively skewed and present strong exponential tails
Mean flow generation by Görtler Vortices in a rotating annulus with librating side walls
No full textLongitudinal libration of the cylinder side walls of a rotating annulus in the supercritical regime induces a centrifugally unstable Stokes boundary layer which generates Görtler vortices only in a portion of a libration cycle. We show for the first time that these vortices propagate into the fluid bulk and generate an azimuthal mean flow which is retrograde (prograde) over the outer (inner) cylinder side wall. Direct numerical simulations (DNS) are carried out and Reynolds-averaged equations and kinetic energy budget of mean and fluctuating flow are used as diagnostic equations to discuss the generation mechanism and scaling behavior of the azimuthal mean flow in the fluid bulk
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