1,721,062 research outputs found
An analytical description of the energy balance in turbulent, round, free jets
No full textThis brief contribution provides a quantification of the terms of the turbulent kinetic energy transport equation for a round steady turbulent free jet. The analysis is based on the assumption of flow self-similarity, and it is performed by means of a simple analytical asymptotic analysis. The results are in good agreement with the experimental findings of Panchapakesan and Lumley [J. Fluid Mech. 246, 197-223 (1993)] and with the large eddy simulations of Bogey and Bailly [J. Fluid Mech. 627, 129-160 (2009)], hence providing a theoretical interpretation of such findings
Wave-forced dynamics in the nearshore river mouths, and swash zones
Full text linkThe role of wave forcing on the main hydro‐morphological dynamics evolving in the shallow waters of the nearshore and at river mouths is analyzed. Focus is mainly on the cross‐shore dynamics that evolve over mildly sloping barred, dissipative sandy beaches from the storm up to the yearly timescale, at most. Local and non‐local mechanisms as well as connections across three main inter‐related subsystems of the nearshore – the region of generation and evolution of nearshore bars, river mouths and the swash zone – are analyzed. The beach slope is a major controlling parameter for all nearshore dynamics. A local mechanism that must be properly described for a suitable representation of wave‐forced dynamics of all such three subsystems is the proper correlation between orbital velocity and sediment concentration in the bottom boundary layer; while specific dynamics are the wave–current interaction and bar generation at river mouths and the sediment presuspension at the swash zone. Fundamental non‐local mechanisms are both infragravity (IG) waves and large‐scale horizontal vortices (i.e. with vertical axes), both influencing the hydrodynamics, the sediment transport and the seabed morphology across the whole nearshore. Major connections across the three subsystems are the upriver propagation of IG waves generated by breaking sea waves and swash–swash interactions, the interplay between the swash zone and along‐river‐flank sediment transport and the evolution of nearshore sandbars
A new process-based, wave-resolving, 2DH circulation model for the evolution of natural sand bars: The role of nearbed dynamics and suspended sediment transport
No full textWe study the migration of natural sand bars that evolve in the nearshore by means of a new process-based waveresolving, 2DH circulation model. In order to perform reliable and accurate computations, the robust Nonlinear Shallow Water Equations (NSWEs) hydro-morphodynamic solver of Brocchini et al. (2001) and Postacchini et al. (2012) implements a detailed description of the Bottom Boundary Layer (BBL) dynamics and a new predictor for the Suspended Sediment Transport (SST) based on the solution of a Depth-Averaged Advection-Diffusion Equation (DAADE) for the sediment concentration. The robustness and accuracy of the enhanced model are validated against literature theoretical, experimental, and numerical results, all comparisons highlighting good performances and clarifying the role of both BBL and SST contributions, the former one having a larger positive influence than the latter one on the results. Both original and enhanced models are, then, used to predict the evolution of the sand bar system that characterizes the nearshore of Senigallia (AN). The analysis leverages the field observations collected at such a site by means of the Sena Gallica Speculator video-monitoring system. Modeling of the storm-forced sand bar migration patterns reveals that: 1) the enhanced model can adequately reproduce the seaward migration of the sand bars of the system; 2) the process of shoreline retreat in coincidence with the generation of a new-born bar is well described; 3) inclusion of the BBL improves quantitative prediction of the bar crest migration; 4) the SST, beyond improving the prediction of the bar crest location, induces some smoothing of the bar profile, in line with the literature findings of SST being a stabilizing factor for the bar emergence
Horizontal mixing of quasi-uniform, straight, compound channel flows
No full textThe generation and evolution of large-scale vortices with vertical axis (macro-vortices) in a straight compound channel under quasi-uniform flow conditions is investigated.
We discuss possible similarities and clear differences with free shear layer flows induced by the meeting of shallow streams of different speeds. An experimental investigation based on particle image velocimetry (PIV) measurements of free-surface velocities
forms the basis for an analysis of both the specific features of macro-vortices and of the related mean flow characteristics. Dynamical properties strongly depend on the ratio rh between the main channel flow depth (hmc) and the floodplain depth (hfp), and three flow classes can be identified. ‘Shallow flows’ (rh >3) are dominated by strong shearing and large macro-vortices populating the transition region between the main channel and the floodplains. The mean streamwise velocity induced in intermediate flows’(2<rh <3) is characterized by a dip in the transition region, while it closely resembles that occurring in a rectangular channel in the case of ‘deep flows’ (rh <2). For both
the latter cases the shear in the transition region decreases and the macro-vortices are also generated in the wall boundary layer of the floodplains. The typical size of the quasi-two-dimensional macro-vortices, generated at the transition region, is found to
be independent of the streamwise coordinate. This and the non-monotonic behaviour of the mean streamwise velocity suggest that in straight compound channels the
topographic forcing is so dominant that conceptual models interpreting these flows as free shear layers may largely fail to describe the physics of compound channels flows
Dynamics of a pile-moored fish cages in current and waves: A numerical study
No full textWe propose a numerical study of the dynamics of a double-constrained cylindrical pile-moored fish-farm cage, to be possibly installed at the foundations of dismissed offshore structures. Our model, derived from the net-truss model of Kristiansen and Faltinsen (2012), takes into account the elasticity of the net material, the real size of the net and is applied with and without mesh-grouping approaches, this allowing us to evaluate the range of validity of mesh-grouping techniques. First, cage dynamics have been studied with small-scale tests, in which the numerical model accurately reproduces the behaviour of the cage in both sea currents (velocity in the range 0.5−1m/s) and waves of small amplitude (with height of 1 m and period in the range 4−32 s). Use of the net real size for the computation grid of small-scale tests allows for identification of the main phenomena influencing the load distribution and the volume loss, which are: the dominant sail-shape deformation in the streamwise direction, the flattened deformations in the cross flow direction, and high-frequency oscillations in the vertical direction in long waves. Subsequently, a prototype-scale test is run to simulate the dynamics of a realistic installation of a double-constrained cylindrical pile-moored fish-farm cage in the Adriatic sea (considering both 1m/s sea current and storm sea state characterized by a peak period Tp=4 s and significant wave height Hs=1.5 m), showing that the major loads are a compression load on the pile up to 1 kN and a bending moment at the pile base up to 9.1 kNm. The proposed configuration always showed a small net relative volume loss VL<5%
Sea waves and mass transport on sloping beach
No full textThe steady streaming induced by a sea wave shoaling on a sloping beach and partly reflected at the coastline is determined in the region seaward of the breaker line. Shallow waters and waves of small amplitude are considered. Moreover, the Reynolds number is assumed to be large but still within the laminar regime and the flow domain is split into a bottom boundary layer and a core region. For an incoming wave which is fully absorbed at the coast the solution shows that close to the bottom the steady streaming is onshore directed even though the depth–averaged value represents an offshore directed flow. Moreover, the vertical velocity distribution depends on the ratio between the wave amplitude a and the thickness δ of the bottom boundary layer. For a fully reflected wave, steady recirculation cells are induced, the form and strength of which depend on the ratio a/δ. A complex flow is generated for reflection coefficients falling between 0 and 1.The steady streaming induced by a sea wave shoaling on a sloping beach and partly reflected at the coastline is determined in the region seaward of the breaker line. Shallow waters and waves of small amplitude are considered. Moreover, the Reynolds number is assumed to be large but still within the laminar regime and the flow domain is split into a bottom boundary layer and a core region. For an incoming wave which is fully absorbed at the coast the solution shows that close to the bottom the steady streaming is onshore directed even though the depth-averaged value represents an offshore directed flow. Moreover, the vertical velocity distribution depends on the ratio between the wave amplitude a* and the thickness δ* of the bottom boundary layer. For a fully reflected wave, steady recirculation cells are induced, the form and strength of which depend on the ratio a*/δ*. A complex flow is generated for reflection coefficients falling between 0 and 1
Evolution of the air cavity during a depressurized wave impact. II. The dynamic field
No full textThe present paper on wave-impact events in depressurized environments completes the analysis of Part I by focusing on the dynamical features of the impacts and on the influence of the ambient pressure. Connection is made between the impact regimes typically described in the literature and the stages described in Part I [C. Lugni, M. Miozzi, M. Brocchini, and O. M. Faltinsen, "Evolution of the air cavity during a depressurized wave impact. I. The kinematic flow field," Phys. Fluids 22, 056101 (2010)]. The stages of isotropic/anisotropic compression and expansion of the air cavity are of particular interest. The impact duration at the wall is almost independent of its height above the undisturbed surface level, but its intensity rapidly decreases in the body of the fluid (the peak pressure halves within the first two compression/expansion cycles). The time evolution of the pressure loads on the wall is analyzed by means of the Hilbert transform and an empirical mode decomposition of the signals. This enables identification of the intrinsic mode functions which best fit the original signal during its evolution and quantification of the frequency downshifting which characterize the whole process. The pressure decay, largely governed by air leakage out of the cavity, is found to be very intense during the air cavity closure and the isotropic compression/expansion cycle [stages (1) and (2)]; the decay observed during stage (3), i.e., during the anisotropic compression/expansion cycles, is weaker and independent of the vertical location down the wall. Differences between the observed decay rates and those of a three-dimensional bubble in an infinite fluid are mainly due to the bubble being two-dimensional, being close to the free surface and loosing air. The role of both ullage and vapor pressures on the impact is described, respectively, by means of the Euler and cavitation numbers. The frequency of the bubble oscillation depends on these numbers in a way that is closely similar to that displayed by the bubble area, see Part I. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3409491
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