1,721,025 research outputs found
An experimental setup to investigate non-Newtonian fluid flow in variable aperture channels
Full text linkNon-Newtonian fluid flow in porous and fractured media is of considerable technical and environmental interest. Here, the flow of a non-Newtonian fluid in a variable aperture fracture is studied theoretically, experimentally and numerically. We consider a shear-thinning power-law fluid with flow behavior index n. The natural logarithm of the fracture aperture is a two-dimensional, spatially homogeneous and correlated Gaussian random field. An experimental device has been conceived and realized to allow the validation of the theory, and several tests are conducted with Newtonian and shear-thinning fluids and different combinations of parameters to validate the model. For Newtonian fluids, experimental results match quite well the theoretical predictions, mostly with a slight overestimation. For non-Newtonian fluids, the discrepancy between experiments and theory is larger, with an underestimation of the experimental flow rate. We bear in mind the high shear-rates involved in the experiments, covering a large range where simple models seldom are effective in reproducing the process, and possible interferences like slip at the wall. For all test conditions, the comparison between analytical and numerical model is fairly good
Velocity and density measurements in forced fountains with negative buoyancy
Full text linkIn fluid mechanics, fountains take place when a source fluid is driven by its own momentum into a surrounding ambient fluid, and it is counterbalanced by buoyancy. These phenomena are largely encountered in nature and human activities. Despite the numerous studies on the subject, few experimental data are available about the internal structure of turbulent fountains. Here, we present a set of laboratory experiments with the aim to (i) get direct velocity and density measurements of fountains in a controlled environment and (ii) obtain insights about the basic physics of the phenomenon. The results concern the characteristics of the mean and turbulent flow: we report the analysis of the turbulent kinetic energy, the velocity skewness, and the Reynolds stresses, including a quadrant analysis of the fluctuating velocities. For some tests, the correlation between density and vertical velocity is investigated for both mean and fluctuating values. We have quantified the momentum transport, which is mainly out-downward at the nozzle axis with peaks at the mean rise height, where also maximum levels of the buoyancy and mass fluxes are present. The ability of acoustic Doppler current profilers to identify the rise height of the fountain and to measure the velocity field is also discussed
Pipeline Systems
No full textPipeline circuits are the most common application of Hydraulics and Fluid Mechanics in civil and industrial engineering. The liquid is conveyed into pipes of various diameters and roughness, usually in the presence of special components such as valves, curves, elbows, fittings. In most cases the problems can be solved by applying the energy balance in terms of Bernoulli’s extended theorem with additional energy losses. Energy losses belong to the category of concentrated and distributed losses, both proportional to the velocity head (at least in turbulent conditions) with a coefficient that depends on the type of the special component, on the roughness of the duct and on the Reynolds number. In particular, the distributed energy losses are calculated with the Darcy formula where the friction factor is calculated with the Moody chart or, in case of turbulent flow, with the Colebrook–White equation, an equation in implicit form of the friction factor, the relative roughness and the number of Reynolds The Colebrook–White equation in the general case has no analytical solution, and a numerical procedure is required, if necessary with iterations
Supplementary material to Longo, S., Di Federico, V., Chiapponi, L., Archetti, R.. Experimental verification of power-law non-Newtonian axisymmetric porous gravity currents. Journal of Fluid Mechanics, DOI: 10.1017/jfm.2013.389
No full textSupplementary material to Longo, S., Di Federico, V., Chiapponi, L., Archetti, R.. Experimental verification of power-law non-Newtonian axisymmetric porous gravity currents. Journal of Fluid Mechanics, DOI: 10.1017/jfm.2013.38
Velocity and stresses of partially-reflected water waves in the presence of opposing wind
No full textIt is known that long waves (or swell) ruffled by an opposing wind tends to be dissipated, while shorter waves develop on top of them and travel in the wind direction. However, the long waves are mostly considered progressive and the effect of reflection is neglected. We present an experiment study of mechanically generated regular waves in partially-reflective conditions and observed how they interact with wind an opposing. We look at how different reflection conditions and spatial variability affect the water flow and the stresses within it. By breaking down the signal through a triple decomposition, we analyse the velocity components and compare the wave-induced Reynolds stresses with a theoretical model which takes into account partial reflection conditions (Addona et al., 2018). This model helps us understand the spatial variability of the wave-induced stresses and avoid misinterpretations of the experimental results, proving the role the key role of reflection. A quadrant analysis of the fluctuating velocities is performed to study the direction of momentum transfer, which always seems to be from the interface to the water below. The work provides novel experimental data of the flow field of partially-reflected water waves in the presence of wind
Hydraulic Transients
No full textThe flow regime in hydraulic plants is often unsteady with frequent variations of flow rate due to opening/closing valves, pumps, turbines. The inertia of the fluid must be included in computations, and the induced variations of pressure can be so high to excite the fluid compressibility. According to the dominant aspect, a broad classification of unsteady phenomena is in (i) mass oscillations and (ii) elastic oscillations, although both phenomena are generally present at the same time, but with well different time scales
Balances of Linear and Angular Momentum
No full textSeveral practical problems of Fluid Dynamics can be solved with the use of the linear momentum balance equation, mostly in integral form. We select an appropriate control volume and evaluate all the forces acting on it, separating the surface forces (acting on the outer surface of the control volume), and the volume forces (acting on the fluid particles contained in the volume). For ideal fluids (zero viscosity), only pressure is present and the surface forces are normal to the external surface. The volume forces are the weight, the local inertia (in unsteady flows) and the apparent forces (e.g., centrifugal, Coriolis, Euler), defined in non-inertial control volumes
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