scholarly journals TIME DEPENDENT FLUCTUATIONS IN LONGSHORE CURRENTS

1976 ◽  
Vol 1 (15) ◽  
pp. 37 ◽  
Author(s):  
Guy A. Meadows
Keyword(s):  
Flow Field ◽  
Current Velocity ◽  
Surf Zone ◽  
Field Investigation ◽  
Edge Wave ◽  
Spectral Signature ◽  
Time Averaging ◽  
Longshore Current ◽  
Major Power ◽  
Longshore Currents

During constant sea state conditions, longshore current velocities were monitored continuously for fifteen minute periods separated by fifteen minute periods separated by fifteen minute intervals. Three ducted impellor flowmeters were placed at equally spaced vertical positions through the water column. Sequential measurements were made with similar vertical current meter arrays at different locations across the surf zone. Simultaneous measurements of wave height, period and celerity were made at stations placed at equal intervals from the outer surf zone to the beach. The fifteen minute continuous records were subjected to spectral analysis. This analysis showed that the major power associated with fluctuations in the longshore current velocity field occurs in two major frequency bands. A significant spectral peak was coincident with the breaker period of the incident wave field, 4.2 seconds and, another dominant signature occurred at 78.8 seconds. Attenuation with depth of both the steady and fluctuating components of the longshore current flow field was relatively small. The maximum observed velocities for each station and each vertical current meter position varied from 90 to 150 percent above the observed mean longshore current velocity. However, at each station, variation of the means with depth was not appreciable and thus supports the results from time and space averaged theories of vertical uniformity in longshore currents, away from the boundary layer. Results from the field investigation of Wood and Meadows (1975) indicated that the steady state components are dominated by the fluctuating portions of the flow field. Therefore, time averaging of conservation equations in longshore current theory is a physically inappropriate procedure. In order to evaluate the magnitude of the unsteady components, a close examination of surf zone dynamics was made. The most obvious contribution to unsteadiness in longshore currents arises from the longshore component of the maximum horizontal particle velocity. However, the magnitude of observed current fluctuations is too large to be completely accounted for by this component. Spectral peaks at longer periods appear to be related to modes of edge wave phenomena. The long period spectral signature of 78.8 seconds is in direct agreement with the calculated period for a zero mode edge wave in shallow water.

scholarly journals Analisa Model Matematik Arus Sejajar Pantai Pada Groin Seri Permeable

2018 ◽  
Vol 19 (2) ◽  
pp. 113
Author(s):  
Adi Surya Pria Pranata ◽  
Novi Andhi Setyo Purwono ◽  
Ary Sismiani
Keyword(s):  
Current Velocity ◽  
Physical Modeling ◽  
Surf Zone ◽  
Rip Current ◽  
Longshore Current ◽  
Long Distance ◽  
Direction Of Movement ◽  
The Mathematical Model ◽  
Additional Reduction

This research is the mathematical model has been made with two groins, permeable and impermeable groins, with the variation is in length and distance; 50 meters, 100 meters, 150 meters and 200 meters. A modelling has been done by using the data of physical modeling study by Chen., et al, 2003, and used as input to the simulation of mathematical models using 2D Boussinesq software.Results of the research showed the influence of variation model long distance between the groins, the reduction of longshore current velocity along the coast with an average 59.21% reduction for the double permeable groin with 50 meters long, 76.02% for 100 meters, 79.50% for 150 meters, 80.49% for 200 meters. The reduction of longshore current velocity along the coast are 57.42% for an impermeable groin groin double with 50 meters, 84.61% for 100 meters, 150% for 88.89 meters, and 89.91% for 200 meters. The distance variation between one groin to the other groin has a result that a longer permeable or impermeable groin affects the reduction longshore current velocity along the coast of the Surf Zone with the addition of longshore current velocity reduction occurs along the coast up to 20%. Reduction of longshore current velocity along the coast by the permeable and impermeable groin Permeable and impermeable groins has not been significantly occured in the long-term groin area exceeding Surf zone with an average additional reduction up to 5%. The longshore current velocity approaching the speed of currents along the coast for the existing conditions at a distance up to 2.4 times of length of groins. And the distance between the groins are too close if we compare to the long groins causing the current direction of movement towards the sea (rip Current). 

scholarly journals CURRENTS IN THE SURF ZONE

2000 ◽  
Vol 1 (2) ◽  
pp. 3 ◽  
Author(s):  
D. L. Inman ◽  
W. H. Quinn
Keyword(s):  
Standard Deviation ◽  
Friction Coefficient ◽  
Velocity Distribution ◽  
Surf Zone ◽  
Longshore Current ◽  
Mean Velocity ◽  
Single Observation ◽  
Bottom Currents ◽  
Longshore Currents ◽  
The Mean

Surface and bottom currents in the surf zone were measured at 15 equally spaced points along two straight beaches with approximately parallel bottom contours. The measurements showed that offshore currents predominate over onshore currents at the bottom, while at the surface there is a slight predominance in the onshore direction. With regard to the longshore component, it was found that surface and bottom currents have a similar velocity distribution. The variability of the longshore component as measured by its standard deviation is equal to or larger than the mean longshore velocity. This wide variation in longshore currents indicates the impracticability of estimating the mean velocity from a single observation of longshore current. It was found that the momentum approach to the prediction of longshore currents by Putnam, Munk and Traylor (1949) leads to useful forecasts provided the beach friction coefficient k is permitted to vary with the longshore velocity, V. The indicated relation is k~v^(-3/2).

Nearshore mixing and dispersion

1994 ◽  
Vol 445 (1925) ◽  
pp. 561-576 ◽  
Keyword(s):  
Surf Zone ◽  
Three Dimensional ◽  
Dispersion Effect ◽  
Longshore Current ◽  
Turbulent Length Scale ◽  
Interaction Terms ◽  
Longshore Currents ◽  
Order Of Magnitude ◽  
Lateral Mixing ◽  
The One

Longshore currents have in the past been analysed assuming that the lateral mixing could be attributed to turbulent processes. It is found, however, that the mixing that can be justified by assuming an eddy viscosity v t = l√k where l is the turbulent length scale, k the turbulent kinetic energy, combined with reasonable estimates for l and k is at least an order of magnitude smaller than required to explain the measured cross-shore variations of longshore currents. In this paper, it is shown that the nonlinear interaction terms between cross-and longshore currents represent a dispersive mechanism that has an effect similar to the required mixing. The mechanism is a generalization of the one-dimensional dispersion effect in a pipe discovered by Taylor (1954) and the three-dimensional dispersion in ocean currents on the continental shelf found by Fischer (1978). Numerical results are given for the dispersion effect, for the ensuing cross-shore variation of the longshore current and for the vertical profiles of the longshore currents inside as well as outside the surf zone. It is found that the dispersion effect is at least an order of magnitude larger than the turbulent mixing and that the characteristics of the results are in agreement with the sparse experimental data material available.

scholarly journals FIELD EXPERIMENTS ON LONGSHORE SAND TRANSPORT IN THE SURF ZONE

1982 ◽  
Vol 1 (18) ◽  
pp. 61 ◽  
Author(s):  
Nicholas C. Kraus ◽  
Masahiko Isobe ◽  
Hajime Igarashi ◽  
Tamio O. Sasaki ◽  
Kiyoshi Horikawa
Keyword(s):  
Current Velocity ◽  
Field Experiments ◽  
Surf Zone ◽  
Transport Rate ◽  
Sand Transport ◽  
Breaking Wave ◽  
Longshore Current ◽  
Mixing Depth ◽  
Sand Transport Rate ◽  
Advection Velocity

Eight fluorescent sand tracer experiments were performed in energetic surf zones on natural beaches and on beaches near structures to measure the short-term longshore sand transport rate. Tracer of up to four distinct colors was injected on a line crossing the surf zone to investigate the on-offshore distributions of the longshore sand adveetion velocity and transport rate. The tracer advection velocity, v , and the depth of mixing into the bed, b, were determined from large numbers of cores taken in situ throughout the sampling area. The sand advection velocity and mixing depth were not constant across the surf zone, but usually exhibited a maximum either toward the shoreline or toward the breaker line, or in both regions. The local breaking wave height, H. , and horizontal current velocity in the surf zone (yielding an average longshore current velocity V) were also measured. The data were interpreted with simple dimensional arguments to give the following results: b = 0.027 H,, v = 0.011 V, and the volumetric transport rate Q = 0.024 H V. Agreement was also found between the measured total longshore sand transport rate and a predictive expression due to Bagnold involving the breaking wave power and average longshore current velocity. Although the results appear reasonable and consistent, a problem remains concerning the apparent decrease in tracer advection speed alongshore recorded in most experiments at the longer sampling times.

scholarly journals COMPUTATIONS OF LONGSHORE CURRENTS

1964 ◽  
Vol 1 (9) ◽  
pp. 12
Author(s):  
Tsao-Yi Chiu ◽  
Per Bruun
Keyword(s):  
Wave Height ◽  
Wave Breaking ◽  
Surf Zone ◽  
Wave Spectrum ◽  
Wave Theory ◽  
Breaking Wave ◽  
Height Distribution ◽  
Longshore Current ◽  
Longshore Currents ◽  
Actual Height

This article introduces the longshore current computations based on theories published under the title "Longshore Currents and Longshore Troughs" (Bruun, 1963). Two approaches are used to formulate the longshore current velocities for a beach profile with one bar under the following assumptions: (1) that longshore current is evenly distributed (or a mean can be taken) along the depthj (2) that the solitary wave theory is applicable for waves in the surf zone; (3) that the statistical wave-height distribution for a deep water wave spectrum with a single narrow band of frequencies can be used near the shore, and (4) that the depth over the bar crest, Dcr, equal 0.8Hv/i /o\. Breaking wave height H^Q/^X is designated to be the actual height equal to Hw-j (significant wave height). Diagrams have been constructed for both approaches for beach profiles with one bar, from which longshore current velocities caused by various wave-breaking conditions can be read directly. As for longshore currents along the beach with a multibar system, fifteen diagrams covering a great variety of wave-breaking conditions are provided for obtaining longshore current velocities in different troughs.

scholarly journals TOWARD AN IMPROVED EMPIRICAL FORMULA FOR LONGSHORE SAND TRANSPORT

1988 ◽  
Vol 1 (21) ◽  
pp. 88 ◽  
Author(s):  
Nicholas C. Kraus ◽  
Kathryn J. Gingerich ◽  
Julie Dean Rosati
Keyword(s):  
Oscillatory Flow ◽  
Field Experiments ◽  
Surf Zone ◽  
Transport Rate ◽  
Sand Transport ◽  
Longshore Current ◽  
Wave Energy Dissipation ◽  
Total Transport ◽  
Local Wave ◽  
Correction Terms

This paper presents results of two field experiments performed using portable traps to obtain point measurements of the longshore sand transport rate in the surf zone. The magnitude of the transport rate per unit width of surf zone is found to depend on the product of the local wave height and mean longshore current speed, but correlation is much improved by including two correction terms, one accounting for local wave energy dissipation and the other for the fluctuation in the longshore current. The field transport rates are also found to be compatible with laboratory rates obtained under combined unidirectional and oscillatory flow. Total transport rates previously reported for this experiment program are revised with recently determined sand trapping efficiencies.

scholarly journals Flow Field Investigation in the Exit Region of a Radial Inflow Turbine Using LDV

1994 ◽  
Author(s):  
D. Muthuvel Murugan ◽  
Widen Tabakoff ◽  
Awatef Hamed
Keyword(s):  
Flow Field ◽  
Field Investigation ◽  
Operating Conditions ◽  
Tip Clearance ◽  
Complex Flow ◽  
Tip Clearance Flow ◽  
Exit Region ◽  
Flow Investigation ◽  
Clearance Flow ◽  
Radial Inflow Turbine

Detailed flow investigation in the downstream region of a radial inflow turbine has been performed using a three component Laser Doppler Velocimetry. The flow velocities are measured in the exit region of the turbine at off-design operating conditions. The results are presented as contour and vector plots of mean velocities, flow angles and turbulent stresses. The measured parameters are correlated to the rotor blade rotation to observe any periodic nature of the flow. The measurements reveal a complex flow pattern near the tip region at the rotor exit due to the interaction of the tip clearance flow. The degree of swirl of the flow near the tip region at the rotor exit is observed to be high due to the gross under turning of the flow near the tip region. The effect of the rotor on the exit flow field is observed in the proximity of the rotor exit.

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