Observations of the vertical structure of turbulent oscillatory boundary layers above fixed roughness beds using a prototype wideband coherent Doppler profiler: 1. The oscillatory component of the flow

2012 ◽  
Vol 117 (C3) ◽  
pp. n/a-n/a ◽  
Author(s):  
Alex E. Hay ◽  
Len Zedel ◽  
Richard Cheel ◽  
Jeremy Dillon
Keyword(s):  
Boundary Layers ◽  
Vertical Structure ◽  
Oscillatory Component ◽  
Oscillatory Boundary Layers

Coherent structures in oscillatory boundary layers

2003 ◽  
Vol 474 ◽  
pp. 1-33 ◽  
Author(s):  
PAOLA COSTAMAGNA ◽  
GIOVANNA VITTORI ◽  
PAOLO BLONDEAUX
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Vortex Structures ◽  
Small Scale ◽  
Computational Domain ◽  
Steady Flows ◽  
Low Speed ◽  
Sequence Of Events ◽  
Low Speed Streaks ◽  
Oscillatory Boundary Layers

The dynamics of the vortex structures appearing in an oscillatory boundary layer (Stokes boundary layer), when the flow departs from the laminar regime, is investigated by means of flow visualizations and a quantitative analysis of the velocity and vorticity fields. The data are obtained by means of direct numerical simulations of the Navier–Stokes and continuity equations. The wall is flat but characterized by small imperfections. The analysis is aimed at identifying points in common and differences between wall turbulence in unsteady flows and the well-investigated turbulence structure in the steady case. As in Jimenez & Moin (1991), the goal is to isolate the basic flow unit and to study its morphology and dynamics. Therefore, the computational domain is kept as small as possible.The elementary process which maintains turbulence in oscillatory boundary layers is found to be similar to that of steady flows. Indeed, when turbulence is generated, a sequence of events similar to those observed in steady boundary layers is observed. However, these events do not occur randomly in time but with a repetition time scale which is about half the period of fluid oscillations. At the end of the accelerating phases of the cycle, low-speed streaks appear close to the wall. During the early part of the decelerating phases the strength of the low-speed streaks grows. Then the streaks twist, oscillate and eventually break, originating small-scale vortices. Far from the wall, the analysis of the vorticity field has revealed the existence of a sequence of streamwise vortices of alternating circulation pumping low-speed fluid far from the wall as suggested by Sendstad & Moin (1992) for steady flows. The vortex structures observed far from the wall disappear when too small a computational domain is used, even though turbulence is self-sustaining. The present results suggest that the streak instability mechanism is the dominant mechanism generating and maintaining turbulence; no evidence of the well-known parent vortex structures spawning offspring vortices is found. Although wall imperfections are necessary to trigger transition to turbulence, the characteristics of the coherent vortex structures, for example the spacing of the low-speed streaks, are found to be independent of wall imperfections.

The nature of rough-wall oscillatory boundary layers

2001 ◽  
Vol 39 (6) ◽  
pp. 655-666 ◽  
Author(s):  
R.V. Krstic ◽  
H.J.S. Fernando
Keyword(s):  
Boundary Layers ◽  
Rough Wall ◽  
Oscillatory Boundary Layers

Forces on particles in oscillatory boundary layers

2002 ◽  
Vol 468 ◽  
pp. 327-347 ◽  
Author(s):  
PAUL F. FISCHER ◽  
GARY K. LEAF ◽  
JUAN M. RESTREPO
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Boundary Layers ◽  
Lift Force ◽  
Drag Forces ◽  
Buoyancy Forces ◽  
Lift Forces ◽  
Lift And Drag ◽  
Standing Question ◽  
Oscillatory Boundary Layers

The lift and drag forces on an isolated particle resulting from an oscillating wall- bounded flow, are approximated using direct numerical simulation and extrapolation techniques. We also confirm the existence of anomalies in the lift force, which arise from the interaction of the vortical field with the particle. Anomalies can also occur for computational reasons and these are discussed as well.This study was motivated by a long-standing question about the importance of lift forces in the dynamics of sediments in oceanic settings. To answer this question we use the numerically generated data as well as extrapolations to compute the ratio of the lift to buoyancy forces on a particle. This analysis suggests that for particles and oceanic conditions typical of the nearshore, the lift force can play a role in the dynamics of sedimentary beds.

Modification of the damping function in the k–ε model to analyse oscillatory boundary layers

2007 ◽  
Vol 34 (2) ◽  
pp. 320-326 ◽  
Author(s):  
Ahmad Sana ◽  
Abdul Razzaq Ghumman ◽  
Hitoshi Tanaka
Keyword(s):  
Boundary Layers ◽  
Damping Function ◽  
Oscillatory Boundary Layers

Turbulence in Oscillatory Boundary Layers

1987 ◽  
pp. 556-567 ◽  
Author(s):  
B. M. Sumer ◽  
B. L. Jensen ◽  
J. Fredsøe
Keyword(s):  
Boundary Layers ◽  
Oscillatory Boundary Layers
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