scholarly journals TURBULENT CURRENTS IN THE PRESENCE OF WAVES

1970 ◽  
Vol 1 (12) ◽  
pp. 141
Author(s):  
Helge Lundgren
Keyword(s):  
Boundary Layer ◽  
Wave Propagation ◽  
Velocity Profile ◽  
Current Velocity ◽  
Friction Velocity ◽  
Approximate Theory ◽  
Current Direction ◽  
Constant Depth ◽  
Wave Boundary ◽  
A Current

This paper presents an approximate theory for the reduction of the velocity of a current due to the presence of sinusoidal waves. For a given slope, S, in water of constant depth, d, the current velocity profile is U(z) = U^ (2.5'ln — - A) (1) t zo as a function of the height, z, above the bed. Eq. 1 is valid only above the thin wave boundary layer near the bed, the roughness of which is k = 30 z . Uf is the current friction velocity defined by p Ul = y d S = T (2) f ' cw CW Values of A can be found from: Fig. 2 where Aj applies when the direction of wave propagation is parallel to the current direction, and Fig. 3 where A2 applies when the direction of wave propagation is perpendicular to the current direction, cf. Notation in Sec. 2. The theory is based upon a number of assumptions (see Sec. 4).

scholarly journals THB INFLUENCE OF WAVES OH CURRENT PROFILES

1986 ◽  
Vol 1 (20) ◽  
pp. 7 ◽  
Author(s):  
Felicity C. Coffey ◽  
Peter Nielsen
Keyword(s):  
Boundary Layer ◽  
Friction Velocity ◽  
Dimensionless Parameter ◽  
Velocity Amplitude ◽  
Steady Current ◽  
Vertical Scale ◽  
Flow Components ◽  
Current Reduction ◽  
Wave Boundary ◽  
The Waves

A simple model is presented for steady current profiles in the presence of waves. The current reduction and apparent roughness increase caused by the waves are shown to depend mainly on one dimensionless parameter u*/u"*, i.e. the ratio between the friction velocity amplitude due to the waves and the time averaged friction velocity. The model recognises the need to apply different eddy viscosities to different flow components. Also, the thickness of the wave influenced layer near the bed is comceptually separated from the vertical scale of the wave boundary layer.

The Effects of Surface Roughness on the Mean Velocity Profile in a Turbulent Boundary Layer

2002 ◽  
Vol 124 (3) ◽  
pp. 664-670 ◽  
Author(s):  
Donald J. Bergstrom ◽  
Nathan A. Kotey ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Friction Velocity ◽  
Outer Region ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Outer Flow ◽  
The Mean

Experimental measurements of the mean velocity profile in a canonical turbulent boundary layer are obtained for four different surface roughness conditions, as well as a smooth wall, at moderate Reynolds numbers in a wind tunnel. The mean streamwise velocity component is fitted to a correlation which allows both the strength of the wake, Π, and friction velocity, Uτ, to vary. The results show that the type of surface roughness affects the mean defect profile in the outer region of the turbulent boundary layer, as well as determining the value of the skin friction. The defect profiles normalized by the friction velocity were approximately independent of Reynolds number, while those normalized using the free stream velocity were not. The fact that the outer flow is significantly affected by the specific roughness characteristics at the wall implies that rough wall boundary layers are more complex than the wall similarity hypothesis would allow.

Friction velocity and power law velocity profile in smooth and rough shallow open channel flows

2002 ◽  
Vol 29 (2) ◽  
pp. 256-266 ◽  
Author(s):  
R Balachandar ◽  
D Blakely ◽  
J Bugg
Keyword(s):  
Boundary Layer ◽  
Velocity Profile ◽  
Froude Number ◽  
Power Law ◽  
Friction Velocity ◽  
Open Channel ◽  
Rough Surfaces ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Log Law

This paper examines the mean velocity profiles in shallow, turbulent open channel flows. Velocity measurements were carried out in flows over smooth and rough beds using a laser-Doppler anemometer. One set of profiles, composed of 29 velocity distributions, was obtained in flows over a polished smooth aluminum plate. Three sets of profiles were obtained in flows over rough surfaces. The rough surfaces were formed by two sizes of sand grains and a wire mesh. The flow conditions over the rough surface are in the transitional roughness state. The measurements were obtained along the centerline of the flume at three different Froude numbers (Fr ~ 0.3, 0.8, 1.0). The lowest Froude number was selected to obtain data in the range of most other open channel testing programs and to represent a low subcritical Froude number. For each surface, the Reynolds number based on the boundary layer momentum thickness was varied from about 600 to 3000. In view of the recent questions concerning the applicability of the log-law and the debate regarding log-law versus power law, the turbulent inner region of the boundary layer is inspected. The fit of one type of power law for shallow flows over a smooth surface is considered. The appropriateness of extending this law to flows over rough surfaces is also examined. Alternate methods for determining the friction velocity of flows over smooth and rough surfaces are considered and compared with standard methods currently in use.Key words: power law, open channel flow, velocity profile, surface roughness.

scholarly journals ASPECTS OF WAVE CURRENT BOUNDARY LAYER FLOWS

1984 ◽  
Vol 1 (19) ◽  
pp. 150 ◽  
Author(s):  
Felicity C. Coffey ◽  
Peter Nielsen
Keyword(s):  
Boundary Layer ◽  
Eddy Viscosity ◽  
Friction Velocity ◽  
Field Measurements ◽  
Laboratory Measurements ◽  
Boundary Layer Flows ◽  
Steady Current ◽  
Bed Roughness ◽  
Flow Components ◽  
A Current

Field measurements of steady current profiles under the influence of waves are described, including a technique for obtaining an extra independant estimate of the friction velocity. Field and laboratory measurements are analysed for the effect on apparent bed roughness by superimposing waves on a current. Finally the applicability of the eddy viscosity concept to combined flows is examined. The conclusion is that in general, different eddy viscosities must be applied to different flow components.

scholarly journals Simulation Experiments on the Migration of Gammarus Zaddachi and Gammarus Chevreuxi

1975 ◽  
Vol 45 (1) ◽  
pp. 20-38 ◽  
Author(s):  
H.B. Girisch ◽  
H.G. Dennert
Keyword(s):  
Environmental Factors ◽  
Population Density ◽  
Current Velocity ◽  
Tidal Cycle ◽  
Current Direction ◽  
Simulation Experiments ◽  
Running Water ◽  
Migratory Activity ◽  
Gammarus Zaddachi ◽  
A Current

The migratory activity of Gammarus zaddachi and Gammarus chevreuxi is investigated in a current chamber, in which a tidal cycle can be simulated. In fresh running water the number of animals drifting with the current is of about the same magnitude as the number of animals actively swimming against the current. A diurnal periodicity is recorded in the migratory activity. The migratory activity is influenced by the population density and the food supply. In the current chamber a tidal cycle can be simulated by varying the factors current velocity, current direction, salinity, and temperature. The combination of a decrease in current velocity, followed by a slow current in the opposite direction, an increase in the salinity, and a rise in the temperature of the medium causes a significant increase in the activity of both species investigated. An increase in only one of the environmental factors mentioned above can also produce an increase in the migratory activity, but less pronounced than when all the factors coordinate in simulating a complete tidal cycle. Both G. zaddachi and G. chevreuxi react in a similar way on the simulation of a tidal cycle. In both species no great differences have been found between juveniles and adults with regard to the migratory activity. The results of the simulation experiments are discussed in connection with the migration cycles of G. zaddachi and G. chevreuxi as recorded in the field.

Importance of hydrodynamic lift to crinoid autecology, or, could crinoids function as kites?

1992 ◽  
Vol 66 (4) ◽  
pp. 658-665 ◽  
Author(s):  
Tomasz K. Baumiller
Keyword(s):  
Current Velocity ◽  
Current Direction ◽  
Optimum Conditions ◽  
Stalked Crinoids ◽  
A Current

The importance of hydrodynamic lift to the autecology of stalked crinoids was evaluated by comparing the maximum hydrodynamic lift experienced by the crinoid crown to the weight of the crinoid in water. Results of the analysis suggest that even under optimum conditions a current velocity of over 20 cm/s would be required to produce enough lift to overcome the animals' weight in water. This implies that under normal conditions crinoids could not function as kites, using the stalk as a tether, unless they possessed some means of reducing their densities. Lift, however, may play a role in the reorientation of crinoids in response to current direction changes, and also in elevating the crown above the substrate following dislodgement or crawling.

scholarly journals THE INCREASE OP BED SHEAR IN A CURRENT DUE TO WAVE MOTION

1966 ◽  
Vol 1 (10) ◽  
pp. 42 ◽  
Author(s):  
E.W. Bijker
Keyword(s):  
Wave Propagation ◽  
Velocity Profile ◽  
Wave Motion ◽  
Indirect Method ◽  
Uniform Flow ◽  
Hydraulics Laboratory ◽  
A Current ◽  
Complicated Nature

As early as 1948 Exnstein [lj suggested that the approach to the calculation of the sand transportation by waves could be similar to that for uniform flow. Tests conducted by author proved that for a current, with waves being propagated m a direction perpendicular or almost perpendicular to this current, the sand transportation is a function of the intensity of bed shear in the direction of the current. Therefore, an attempt has been made to study the increase of the bed shear of a current due to wave motion. The results of this study are presented in this paper. This problem has also been studied by Jonsson and Lundgren !?l >L5] t>ut they have assumed that current and wave-propagation occurs m the same direction. As for normal beach conditions, the angle between current and wave crests is between 0° and 20°, tests have been executed in the Delft Hydraulics Laboratory for angles of 0° and 15°. For an angle of 0° between the wave crests and the direction of the current, results have been presented by Bijker in the proceedings of the seminars at the I.A.H.R. conference which was held m Leningrad in 1965. Because it is not feasible to measure the bed shear directly an indirect method had to be chosen. Determination of the bed shear by means of the velocity profile m the vicinity of the bed is not feasible m this case as the combined velocity profile is of a rather complicated nature. The bed shear is therefore determined by means of the energy slope.

Rem Study of Current Effect on the Structure of Clean Si(111) Surface

1990 ◽  
Vol 48 (1) ◽  
pp. 306-307
Author(s):  
A. Yamanaka ◽  
H. Ohse ◽  
K. Yagi
Keyword(s):  
The Other ◽  
Current Direction ◽  
Clean Surface ◽  
Current Effect ◽  
Atomic Steps ◽  
Step Bunching ◽  
Terrace Width ◽  
Step Down ◽  
A Current

Recently current effects on clean and metal adsorbate surfaces have attracted much attention not only because of interesting phenomena but also because of practically importance in treatingclean and metal adsorbate surfaces [1-6]. In the former case, metals deposited migrate on the deposit depending on the current direction and a patch of the deposit expands on the clean surface [1]. The migration is closely related to the adsorbate structures and substrate structures including their anisotropy [2,7]. In the latter case, configurations of surface atomic steps depends on the current direction. In the case of Si(001) surface equally spaced array of monatom high steps along the [110] direction produces the 2x1 and 1x2 terraces. However, a relative terrace width of the two domain depends on the current direction; a step-up current widen terraces on which dimers are parallel to the current, while a step-down current widen the other terraces [3]. On (111) surface, a step-down current produces step bunching at temperatures between 1250-1350°C, while a step-up current produces step bunching at temperatures between 1050-1250°C [5].In the present paper, our REM observations on a current induced step bunching, started independently, are described.Our results are summarized as follows.(1) Above around 1000°C a step-up current induces step bunching. The phenomenon reverses around 1200 C; a step-down current induces step bunching. The observations agree with the previous reports [5].

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