1,721,062 research outputs found
Wave Boundary Layer over a Rough Bottom at Moderate Reynolds Numbers
No full textWe make use of the partial slip boundary condition, commonly introduced to investigate steady flows, to evaluate the oscillatory flow generated close to a rough sandy bottom by propagating surface waves. Moderate values of the Reynolds number are considered such that the flow regime is laminar. However, an eddy viscosity is introduced to take into account the momentum transfer induced by the small vortices shed by the roughness/grain elements and the effect that these vortices have on the mixing processes. Because of the oscillatory forcing, the slip velocity at the bottom turns out to be characterized by both an amplitude and a phase that are evaluated by fitting the model results to experimental measurements. Moreover, a reasonable form of the 'eddy viscosity' is chosen on the basis of physical arguments. A comparison between the model predictions and the experimental measurements supports the proposed approach
River Dunes and Tidal Sand Waves: Are They Generated by the Same Physical Mechanism?
No full textMany subaerial and subaqueous morphological patterns are repetitive both in space and time, and a characteristic wavelength L and a characteristic period T can be associated with the bottom forms, along with a migration speed cr=L/T. In this contribution we focus our attention on river dunes, which form under steady flows, and on tidal sand waves, which form under oscillatory flows. Now, it is widely accepted that these bedforms arise because of the instability of the plane interface between the riverbed/seabed, made up of cohesionless sand, and the water flowing over it. However, the physical mechanisms commonly suggested to explain the growth of these two bottom forms differ. In the present contribution, the theories of formation of river dunes and sand waves, based on modal stability analysis, are revisited, and it is shown that the different physical mechanisms proposed to explain the formation of river dunes and sand waves exhibit more similarity than was believed until today
DNS del flusso oscillante su una parete scabra
No full textNel presente lavoro si affronta lo studio del campo di moto di un fluido che oscilla
al di sopra di una parete scabra. Sono state effettuate delle simulazioni numeriche dirette delle
equazioni di Navier-Stokes con l’ausilio della tecnica dei “contorni immersi” (Fadlun et al., 2001)
per imporre le condizioni al contorno sulla parete. Il metodo numerico è stato ampiamente testato
e sono stati effettuati confronti con risultati sperimentali. I risultati delle simulazioni numeriche
effettuate hanno consentito un’indagine approfondita del campo di moto e in particolare della
dinamica delle strutture vorticose coerenti. E’ stata inoltre analizzata la distribuzione spaziale e
temporale delle tensioni scambiate tra il fluido e la parete. Mantenendo fissa la geometria della
parete sono state effettuate simulazioni per diversi valori del numero di Reynolds.In this paper the flow field generated by a longitudinal pressure gradient which
oscillates periodically above a rough wall has been studied. Direct numerical simulations of the
Navier-Stokes equation have been performed for different values of the Reynolds number, keeping
the geometry of the wall fixed. In order to force the boundary condition on the rough wall, the
“immersed boundaries” tecnique (Fadlun et al., 2001) has been adopted. The numerical method
has been tested and the results have been compared with experimental data. The obtained results
concern the flow field and the coherent vortex structures. Moreover the spatial and the temporal
distribution of the stresses on the bed is described
Direct numerical simulation of an oscillatory boundary layer close to a rough wall
No full textIn the present contribution, the boundary layer generated close to a rough wall by an oscillatory, uniform pressure gradient is studied. The flow simulates the boundary layer generated at the sea bottom by a monochromatic propagating wave. The problem is tackled by numerical means and detailed information on flow dynamics is obtained. In particular, the evolution of vorticity is considered and the coherent vortex structures which are formed within the boundary layer are identified. The force exerted by the fluid on the bed and on the roughness elements is computed along with its pressure and viscous components
Mass transport at the bottom of propagating surface waves over a rippled bottom
No full textThe sea surface can be described by means of the superposition of many sinusoidal functions. However, quite often the amplitude of each component turns out to be much smaller than its wavelength, and any component evolves independently of the others. Hence, it is common to investigate the dynamics of a simple monochromatic surface wave. Hereinafter, the flow generated by a monochromatic surface wave within the bottom boundary layer over a rippled sea bed is determined by means of the numerical integration of vorticity and continuity equations. The forcing term that drives the fluid motion within the boundary layer is evaluated assuming that the steepness of the monochromatic surface wave is much smaller than one and considering the first term of the Stokes expansion. Even though the irrotational flow that forces the viscous rotational flow near the sea bottom is symmetric with respect to the ripple crests, Blondeaux and Vittori ["A route to chaos in an oscillatory flow: Feigenbaum scenario," Phys. Fluids A 3(11), 2492-2495 (1991a)] showed that the symmetry of the flow field is broken when the Reynolds number becomes larger than a threshold value R(delta,t1 )that depends on the geometrical characteristics of the ripples. The results of Blondeaux and Vittori ["A route to chaos in an oscillatory flow: Feigenbaum scenario," Phys. Fluids A 3(11), 2492-2495 (1991a)] suggest that, when the Reynolds number is larger than R-delta,R-t1 but not too far from it, a steady current is also generated. Hereinafter, the steady velocity component is determined as a function of the ripple characteristics
Simulazione numerica del flusso oscillante su una parete scabra
No full textNell’articolo è affrontato lo studio del flusso oscillante su una parete coperta da una scabrezza
regolare. La scabrezza considerata è costituita da semisfere disposte su una parete piana secondo una
matrice esagonale. Le equazioni che reggono il moto del fluido sono state risolte numericamente su
una griglia cartesiana. Le condizioni al contorno sulla parete sono state imposte utilizzando la tecnica
dei contorni immersi (Fadlun et al., 2000). Il metodo numerico è stato validato riproducendo i risultati
sperimentali di Keiller & Sleath, (1976). E’ analizzato l’andamento temporale delle strutture vorticose
e delle forze indotte sulla parete dalle oscillazioni del fluido. Il lavoro numerico condotto contribuisce
a chiarire alcuni aspetti dei lavori sperimentali presenti in letteratura (Keiller & Sleath, 1976; Sleath,
1987; Jensen et al., 1989) e risulta essere un potente strumento di indagine nello studio di questo tipo
di flusso
Revisiting the momentary stability analysis of the Stokes boundary layer
No full textThe stability of the boundary layer generated by the harmonic oscillations of a plate in its own plane in a fluid otherwise at rest (Stokes boundary layer) is investigated by considering the time development of perturbations of small amplitude and introducing a momentary criterion of instability. The temporal scale of the perturbations is assumed to be much smaller than the period of the plate oscillations because transition takes place at values of the Reynolds number much larger than one. The results confirm that the Stokes boundary layer is linearly unstable when the Reynolds number is larger than a first critical value equal to which is almost coincident with that determined by Von Kerczek & Davis (J. Fluid Mech. vol. 62 issue 4 1974 753-773) for a Stokes boundary layer in a fluid domain which is bounded by a second fixed plate at a distance from the oscillating one. For values of the Reynolds number close to the instability is restricted to phases close to the inversion of the plate velocity. When the Reynolds number becomes larger than a second threshold value close to the instability rapidly pervades a large part of the cycle. However only when the Reynolds number becomes larger than a third critical value equal to is the instability present during the whole cycle. Heuristically these three critical values of the Reynolds number can be associated with the transition from the laminar regime to the disturbed laminar the intermittently turbulent and fully turbulent regimes
Direct numerical simulation of an oscillatory boundary layer close to a rough wall
No full textIn the present contribution, the boundary layer generated close to a rough wall by an oscillatory, uniform pressure gradient is studied. The flow simulates the boundary layer generated at the sea bottom by a monochromatic propagating wave. The problem is tackled by numerical means and detailed information on flow dynamics is obtained. In particular, the evolution of vorticity is considered and the coherent vortex structures which are formed within the boundary layer are identified. The force exerted by the fluid on the bed and on the roughness elements is computed along with its pressure and viscous components
Sea waves and mass transport on a 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
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