Oscillatory Flow Regimes Research Articles

Measurements of primary productivity and nitrogenase activity of coral reef algae in a chamber incorporating oscillatory flow

Diffusive limitation of C and nutrient uptake can be an important factor regulating metabolic processes in aquatic plants. Relationships between the flow regime and both primary productivity and nitrogenase activity (ethylene production) of coral reef algal components were studied via a productivity chamber that simulates oscillatory flow. Net primary productivity of algal turfs was increased significantly by flow speeds (steady flow equivalent) up to 16 cm s−1. The mode of water flow also was important; primary productivity of algal turfs increased by 21% under an oscillatory flow regime as compared to vortex flow created by stirring. Acanthophora spicifera exhibited increased primary productivity at flow speeds up to 10 cm s−1, while primary productivity of another rhodophyte, Coelothrix irregularis, increased up to 4 cm s−1. Rates of nitrogenase activity exhibited by algal turfs also were affected significantly by flow speed. These data indicate that diffusive limitation is important to varying degrees in reef algal turfs and macroalgae and suggest that primary productivity and rates of nitrogenase activity depend on ambient flow speeds and boundary‐layer dynamics.

Bedforms and depositional sedimentary structures of a barred nearshore system, eastern Long Island, New York

The depositional sedimentary structures and textures of a single-barred nearshore system on the Atlantic coast of eastern Long Island, New York, were studied along seven shore-normal transects. Data along these transects consisted of textural analysis of 160 sediment samples, temporal bedform observations, and 42 can cores for the analysis of sedimentary structures. Six sedimentary subenvironments were observed, based on distinct combinations of sediment color and texture, bedforms, physical, and biogenic sedimentary structures, and benthic infaunal communities. The shoreface environment is divided into the upper shoreface, the longshore trough, and the longshore bar. The divisions of the inner shelf environment are the shoreface-inner shelf transition, the offshore, and the coarse-grained deposit. The first five subenvironments are arranged in bands parallel to the shoreline, whereas the coarse-grained deposit occurs in patches across the inner shelf. The location of fair-weather wave base, coinciding with a reduction in slope (3.0–0.3°) from the shoreface to the inner shelf, is characterized by the cessation of debris surge in the troughs of ripples, the formation of a “rust layer” of microorganisms over the bedform surface, and a sediment color change caused by an increase in organic detritus. The sequence of bedforms and physical sedimentary structures observed in this system fits well with existing wave-generated (oscillatory) flow regime models. These models explain the observed sequences as a response to the degree of asymmetric flow created by shoaling waves. Distribution of biogenic structures and assemblages of infaunal organisms is influenced by the distance landward or seaward of fair-weather wave base. The overall relationships of this nearshore system can then be summarized as a hypothetical prograding stratigraphic sequence. The entire sequence is underlain by organic-rich, bioturbated, offshore deposits. Overlying the offshore is the planar-laminated sediments of the transition. Grading upward from the transition are the cleaner, planar-laminated, seaward slope deposits of the longshore bar. Above this, is a distinct erosional surface indicating the base of the massive to cross-laminated coarse sediments of the longshore trough. Capping the sequence are the cross- to planar-laminated, clean sands of the upper shoreface and foreshore.

Rotating cylinder in a flowing viscous incompressible fluid

New results are obtained in the problem of flow of a viscous fluid past a rotating cylinder by numerical solution of the Navier—Stokes equations for Reynolds numbers 10 ⩽ Re ⩽ 100. The drag and lift have been calculated. The oscillatory flow regime in the wake behind a fixed cylinder is investigated for Re = 80. The Strouhal number for auto-oscillations is 0.16.

Flow regime and bedforms in a ridge and runnel system, S.E. Spain

During landward migration, ridge and runnel systems are subjected to asymmetric oscillatory and/or unidirectional flow regimes, depending on the stage of development reached by these systems. In the early stages of evolution, when the ridge is situated in the upper shoreface, the whole system is subjected to asymmetric oscillatory flow. The runnel is under lower flow regime conditions and the ridge may be under upper or lower flow regime according to water depth and wave energy. Later, when the ridge has migrated to a position on the foreshore, the runnel is largely under a unidirectional lower flow regime while the ridge itself is under oscillatory upper flow regime. When the ridge welds to berm, it is largely emergent and exposed to high-tide swash action under upper flow regime conditions. The runnel is eventually filled with sand and transformed into a low-lying area. All these types grade laterally into each other. One or more ridge and runnel systems can occur at the same time. Wave energy, tide level and position of the ridge control the variations in the characteristics of the ridge and also the position of the zones of bedforms found at the upper shoreface.

Velocity Profiles near a Vertical Ice Surface Melting into Fresh Water

An experimental determination is made of the velocity profiles which result from the free convective melting of a vertical ice sheet into fresh water at temperatures in the range from 2.0°C to 7.0°C. The results suggest that upward flows exist for water temperatures below 4.7°C. Entirely downward flowing boundary layers are suggested for temperatures above 7.0°C. For intermediate temperatures, an oscillatory dual flow regime is indicated.

A study of detonation in methane/air clouds

In the present paper, we study the problem of detonation in unconfined, gaseous mixtures of methane/oxygen/nitrogen. A numerical simulation approach is employed in which we use a one-dimensional (spherical symmetry), time-dependent computer model to simulate the coupled compressible fluid dynamics-chemical kinetics processes which occur upon direct initiation of detonation. We establish the magnitude of explosive yield of tetryl required to initiate detonation in mixtures of CH 4 + 2O 2 with varying degrees of nitrogen dilution, up to and including stoichiometric CH 4 air . The numerical simulations illustrate the features of direct initiation observed in many experimental investigations, e.g. shock-wave breakaway followed by detonation reestablishment via a quasi-steady, oscillatory flow regime which occurs before the establishment of a steadily propagating spherical detonation. Our results compare well with recent experimental data obtained by Bull et al. (1976) over the range of tetryl masses studied by them. We find that tetryl explosive masses in excess of 10 7 grams would be required to initiate detonation in an unconfined, stoichiometric CH 4 air mixture.

DISPERSIVE TRANSPORT IN RIVER AND TIDAL FLOWS

Analytical results are presented which describe the mechanisms of longitudinal dispersive mass transport in rectangular channels of finite and infinite widths for both unidirectional (river) and oscillatory (tidal) flow regimes. Emphasis is placed upon the discussion of results and the characteristics of longitudinal dispersive mass transport revealed by the analytical treatment. Expressions presented for the dispersion coefficient were obtained from solutions to four sets of boundary value problems for the velocity and concentration variation components u" and c". Examination of these expressions reveals that in oscillatory flow the dispersive mass transport is described by a type of resonant interaction between the period of oscillation and the time scales of vertical and lateral mixing. The analysis also shows that for oscillatory flow regimes the effect of lateral shear becomes negligible for very wide channels and the three dimensional solution collapses to the two dimensional case in which vertical shear and mixing effects dominate. It is shown analytically that this is not the case in unidirectional flows. For this case the lateral shear and mixing effects dominate the corresponding vertical effects and dispersive mass transport increases without bound with increasing channel widths.