scholarly journals VERTICAL VARIATION OP UNDERTOW IN THE SURF ZONE

1988 ◽  
Vol 1 (21) ◽  
pp. 33 ◽  
Author(s):  
Akio Okayasu ◽  
Tomoya Shibayama ◽  
Kiyoshi Horikawa
Keyword(s):  
Laboratory Experiments ◽  
Eddy Viscosity ◽  
Surf Zone ◽  
Viscosity Coefficient ◽  
Laser Doppler Velocimeter ◽  
Vertical Variation ◽  
Two Component ◽  
The Mean ◽  
Eddy Viscosity Coefficient ◽  
The Vertical Distribution

In order to establish a model of the vertical distribution of the undertow, laboratory experiments were performed on uniform slopes of 1/20 and 1/30. The turbulent velocity in the surf zone including the area close to the bottom was measured by using a two-component laser doppler velocimeter. The distributions of the mean Reynolds stress and the mean eddy viscosity coefficient were calculated. Based on the experimental results, a model to predict the vertical profile of the undertow was presented.

scholarly journals TURBULENCE GENERATED BY WAVE BREAKING ON BEACH

1982 ◽  
Vol 1 (18) ◽  
pp. 1 ◽  
Author(s):  
T. Sakai ◽  
Y. Inada ◽  
I. Sandanbata
Keyword(s):  
Vertical Distribution ◽  
Turbulence Intensity ◽  
Wave Breaking ◽  
Surf Zone ◽  
Turbulent Wake ◽  
Wave Period ◽  
Laser Doppler Velocimeter ◽  
Two Component ◽  
Hot Film ◽  
The Vertical Distribution

Horizontal and vertical velocities are measured with a hot-film anemometer (HFA) and a two-component laser-doppler velocimeter(LDV) in surf zones on uniform slopes of about 1/30 in two wave tanks. The turbulence generated by wave breaking is detected from the records. Following three aspects of the turbulence are discussed : (1) the distribution of the turbulence intensity in the surf zone, (2) the variation of the vertical distribution of the turbulence during one wave period and (3) the variation of the Reynolds stress during one wave period. It is found that the pattern of the distribution of the turbulence in the surf zone depends on the breaker type. A model is proposed, by extending the turbulent wake theory, to explain the variation of the vertical distribution of the turbulence during one wave period.

Direct numerical simulation of turbulence–mean field interactions in a stably stratified fluid

2002 ◽  
Vol 455 ◽  
pp. 213-242 ◽  
Author(s):  
M. GALMICHE ◽  
O. THUAL ◽  
P. BONNETON
Keyword(s):  
Turbulent Flows ◽  
Eddy Viscosity ◽  
Viscosity Coefficient ◽  
Density Field ◽  
Mean Flow ◽  
Flow Perturbation ◽  
Turbulent Field ◽  
Buoyancy Forces ◽  
The Mean ◽  
Eddy Viscosity Coefficient

Freely decaying turbulent flows in a stably stratified fluid are simulated with a pseudo-spectral numerical code solving the fully nonlinear Navier–Stokes equations under the Boussinesq approximation with periodic boundary conditions. The flow is decomposed into a turbulent field and a horizontal mean flow ū(z, t) defined as the average of the horizontal velocity component in a horizontal plane at height z and time t. Similarly, the density field is decomposed into a turbulent field and a (stable) mean density profile ρ(z, t) defined as the average of the density field in a horizontal plane at height z and time t. Attention is paid to the effect of the turbulent velocity field on an initial z-periodic horizontal mean flow (Simulation A) or an initial z-periodic perturbation of the mean density profile (Simulation B). Both A and B are performed under conditions of moderate and strong stratification and are compared to the non-stratified simulations.Simulation A shows that the turbulence–mean flow interaction is strongly affected by the buoyancy forces. In the absence of a stratification, the mean flow perturbation decays rapidly due to the turbulent diffusion of momentum. When a moderate stratification is applied, the mean flow perturbation decays much more slowly whereas it oscillates and grows with time when the stratification is strong. These results are interpreted by defining a time-dependent eddy viscosity. Whereas the eddy viscosity coefficient has positive values in the non-stratified simulation, it is affected by the buoyancy forces and decreases after a period of order N−1. For large times, the eddy diffusivity oscillates and its time-averaged value over a few turnover timescales is positive but small when the stratification is moderate, and roughly zero when the stratification is strong. These results are interpreted by defining a time-dependent eddy viscosity. Whereas the eddy viscosity coefficient has positive values in the non-stratified simulation, it is affected by the buoyancy forces and decreases after a period of order N−1 in the stratified simulations (where N is the Brunt–Väisälä frequency associated with the background linear stratification). At large time, we find that the eddy viscosity remains roughly zero when the stratification is moderate, whereas it oscillates but remains persistently negative in the strongly stratified case, which causes the horizontal mean flow to accelerate.We conclude that the presence of a stable stratification strongly affects the temporal behaviour of the mean quantities ū and ρ in turbulent flows and partly explains the formation of horizontal layers in stratified geofluids such as oceans and atmospheres.

Measurements of the Velocity Field Downstream of an Impeller

1996 ◽  
Vol 118 (3) ◽  
pp. 602-610 ◽  
Author(s):  
Per Petersson ◽  
Magnus Larson ◽  
Lennart Jo¨nsson
Keyword(s):  
Velocity Field ◽  
Momentum Flux ◽  
Laser Doppler Velocimeter ◽  
Self Similarity ◽  
Mean Velocity ◽  
Measured Velocity ◽  
Swirling Jets ◽  
Axial Velocity Profile ◽  
Two Component ◽  
The Mean

The velocity field downstream of a model impeller operating in water was measured using a two-component laser doppler velocimeter. The investigation focussed on the spatial development of the mean velocity in the axial, radial, and circumferential direction, although simultaneous measurements were performed of the velocity unsteadiness from which turbulence characteristics were inferred. The measurements extended up to 12 impeller diameters downstream of the blades displaying the properties of the generated swirling jet both in the zone of flow establishment and the zone of established flow. The division between these zones was made based on similarity of the mean axial velocity profile. Integral properties of the flow such as volume and momentum flux were computed from the measured velocity profiles. The transverse spreading of the impeller jet and its development towards self-similarity were examined and compared with non-swirling jets and swirling jets generated by other means.

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