Poul S. Larsen

Statistical collision model for Monte Carlo simulation of polyatomic gas mixture

Abstract A formal derivation of the direct simulation technique of solution to the Boltzmann equation for a mixture of polyatomic gases is presented. A phenomenological model for binary collisions in a gas mixture having continuous internal energy is then developed. The model is based on the relaxation concept applied to individual collisions and interpreted statistically in a manner suitable to Monte Carlo simulation of rarefied flows. The model has a high degree of flexibility and requires little more computing time per collision than the hard-sphere model of monatomic gas. Sample calculations of a rotationally relaxing gas are compared to loaded-sphere and rough-sphere results.

Comparative ecophysiology of active zoobenthic filter feeding, essence of current knowledge

The present contribution gives an overview of current knowledge of a comprehensive and steadily growing research field. The first section deals with water pumping and particle retention mechanisms in ciliary and muscular filter feeders. The second section examines the biological filter pumps in order to assess adaptation to the environment. Filter-feeding benthic invertebrates have evolved filter pumps to solve common basic problems. This has led to a large degree of similarity between otherwise distant standing species, which makes comparative studies interesting and important. The present review of zoobenthic filter feeding aims at accentuating such recognition.

FILTER‐FEEDING IN MARINE MACRO‐INVERTEBRATES: PUMP CHARACTERISTICS, MODELLING AND ENERGY COST

11. Sponges . . . . . . . . . . ( I ) Functional morphology: filter mechanism ahd pump design (2) Energy cost and pump model 111. Polychaetes . . . . . . . . . ( I ) Sabella penicillus . . . . . . . (2) Chaetopterus variopedatus . . . . . . ( 3 ) Nereas diversicolor . . . . . . . IV. Bivalves . . . . . . . . . . ( I ) Water pumping and particle retention (2) Pump characteristics, energetics and modelling (3) Interpretation problems . . . . . . V. Ascidians . . . . . . . . . . . . . .

Water pumping and analysis of flow in burrowing zoobenthos: an overview

Burrowing animals maintain contact with the water above the sediment by pumping water through a tube system and therefore measurements of water pumping rate of burrowing animals is of crucial importance for the study of many processes both within and above the sea floor. This review deals with the measuring of water pumping and the analysis of flow generated by burrowing deposit- and filter-feeding zoobenthos in order to determine the type of pump and mechanisms involved, flow rate, pump pressure, and pumping power. The practical use of fluid mechanical principles is examined, and it is stressed that not only the pump pressure that a burrowing animal can apply is of interest for assessing the energy cost of pumping, but also the distribution of excess pressure along its burrow is of importance for assessing the seepage flow of oxygen-rich water into the sediment surrounding the burrow because this bioirrigation exerts a considerable effect on the chemistry and microbiology of sediments. Dense populations of burrowing filter-feeding zoobenthos also interact with the water above the sediment interface and this is reflected in the development of phytoplankton concentration profiles above the filter-feeding animals. In stagnant situations the near-bottom water may be depleted of food particles, depending on the population filtration rate and the intensity of the biomixing induced by the filtering activity. But moderate currents and the biomixing can presumably generate enough turbulence to facilitate mixing of water layers at the sea bed with the layers above where food particle concentrations are relatively higher. Following a brief summary of types of burrowing benthic animals, common methods for measuring pumping rates are described along with examples. For estimating the required pump pressure, biofluid mechanical theory for flow in tube–pump systems is summarised (elaborated in Appendix A). Specific examples are given to illustrate general principles and to give an idea of typical values of flow rate, pressure drop and power involved. Finally, some flow effects generated by burrowing animals in and above the sediment are described.

Fluid motion and particle retention in the gill of Mytilus edulis: video recordings and numerical modelling

Particle trajectories of 6.4μm Latex spheres were recorded by video, both near an isolated blue mussel, Mytilus edulis, gill filament and, in place of an intact interfilamentary canal, in a model canal of width 200, 100 or 70μm, formed by a transparent plate positioned next to a gill filament. Each arrangement was placed in a 2 x 10 x 10 cm test vessel filled with seawater. Serotonin (nerve-transmitter) stimulation was used to activate lateral cilia and to either lock latero-frontal cirri at the end of an active stroke (10-5M), or to activate them (10-6M), yielding lateral cilia beat frequencies of 19 and 16 Hz, respectively. With latero-frontal cirri locked, image analysis of particle tracks gave maximum velocities of ca. 2.9±0.2mm s-1 close to the tips of lateral cilia, for both isolated filament and model canal cases. Experimental velocity profiles along the 200-μm wide model canal were recorded and used as good approximations to the fluid velocity because of the low Reynolds number. A two-dimensional steady model was proposed for the gill pump, assumed to only comprise lateral cilia. This model was solved numerically for the experimental model, canal in the vessel and the results showed satisfactory agreement with experimental volocity profiles from particle tracks. The numerical approach was also applied to a model of a single interfilamentary canal in the vessel. The resulting mean velocity in the canal was 1.70 mm s-1, but the resistance to flow in the model was less than that in an intact mussel gill. Video graphs of particle tracks indicated that active latero-frontal cirri play a role in the transfer of particles from through current to frontal current, probably by means of a strong interaction through the motion of intervening fluid rather than through a direct physical contact. M. edulis specimens used in the present study were collected in 1990 at Helsingør and in 1991 at Kerteminde, Denmark.

Particle capture in the musselMytilus edulis: The role of latero-frontal cirri

Microscope video graphs of particle paths near one-filament-thick mussel gill preparations, stimulated with a nerve transmitter (10−6M serotonin which restores normal ciliary activity), were used to disclose the capture of 6 μm algal cells. Suspended algal cells carried with the water were stopped for a while at the entrance to the interfilament gap by the action of the latero-frontal cirri (Ifc), and transferred to the frontal side of the filament to be transported towards the marginal food groove. The event of transfer took place during approximately a time interval of 1150 to 1/25 s. To gain a better understanding of the capture mechanism and retention efficiency versus particle size, the flow through and around the Ifc was theoretically estimated. Normally beating Ifc create periodic, unsteady, three-dimensional flows at the entrance to the interfilament canal. During the active beat most of the water is deflected to flow around the branching cilia of the Ifc while some of the water is strained by these. Large particles (> 4 μm) are stopped and transferred to the frontal current, whereas smaller particles either follow the flow around the Ifc and escape or they are stopped by the branching cilia.

Effect of secondary flows and turbulence on electrostatic precipitator efficiency

Abstract Experiments on a negative corona wire-plate precipitator have shown that there exist axial rolls of secondary flow superposed an essentially uniform bulk flow. The effect of such rolls on efficiency is studied by an idealized model of a duct precipitator. The model employs approximate velocity distributions of rolls, a constant turbulent diffusivity of particles, and a uniform particle drift velocity due to the electrical field. Results show that increasing roll strength decreases the efficiency, which can take values less than the Deutsch limit for weak diffusion. For zero diffusion, rolls above a certain strength limit the maximum efficiency to values less than unity, hence lead to net leakage for any precipitator length.

Statistical Analysis and Accuracy of PIV Data

In an assessment of PIV measurement accuracy under practical experimental circumstances, grid-generated turbulence in its early decay (x/M=13.6–15.2,Reλ≈25) has been studied. Since a real-time processor was used, it was practical to acquire a statistically significant sample size of 3000 vector maps so that accurate turbulence statistics could be calculated. Results include moments of velocity components and spatial structures in terms of auto-correlations, turbulence scales and spectra. Data fall within limits of standard error estimates. This study demonstrates the need for large sample sizes, particularly for higher order statistics.

Biomixing generated by benthic filter feeders: a diffusion model for near-bottom phytoplankton depletion

Abstract Transient concentration distributions of flagellate cells ( Rhodomonas sp.) previously measured by Riisgard and co-workers in laboratory experiments have been examined to develop a diffusion model for the process of phytoplankton depletion in stagnant seawater above populations of benthic filter feeders, the polychaete Nereis diversicolor and the ascidian Ciona intestinalis , respectively. The model is based on sinks located at inhalant openings and Ficks law with an effective diffusivity that decreases with distance above the bottom due to the biomixing generated by exhalant and inhalant feeding currents. For N. diversicolor , having inhalant and exhalant openings flush with the sediment surface and a moderate exhalant jet velocity of ∼0.01 m s −1 , concentration boundary layer growth is retarded and limited by the low values of diffusivity prevailing at heights greater >∼0.05 m above the bottom. For C. intestinalis , having inhalant and exhalant openings situated ∼0.05–0.1 m above the bottom and a higher and inclined exhalant jet velocity of ∼0.1–0.2 m s −1 , the concentration distributions show a nearly uniform depletion over a layer reaching a thickness of 0.2–0.3 m above the bottom due to high biomixing in this layer. Numerical predictions of concentration distributions reproduce essential features of experiments, and suggest near-bottom values of effective diffusivity of 0.3 x 10 −6 and 150 x 10 −6 m 2 s −1 , for N. diversicolor and C. intestinalis , respectively. It is suggested that the latter value is so large that the induced mixing should be accounted for in modelling benthic concentration boundary layers under flow conditions.

Growth of mussels Mytilus edulis at algal (Rhodomonas salina) concentrations below and above saturation levels for reduced filtration rate

Abstract Average filtration and growth rates of groups of juvenile Mytilus edulis (n=25–45 of 22–35 mm shell length) were measured at different concentrations of an algal cell monoculture in 9 laboratory experiments of duration 14–30 days, 4 experiments below and 5 above the limit of incipient saturation concentration (C sat≈6000–7000 Rhodomonas salina cells ml−1). From a nearly constant filtration rate (F≈30 ml min−1 for a 30 mm shell length) at measured algal concentrations below C sat the steady-state filtration rate decreased approximately as 1/C for increasing algal concentrations (C) above C sat to levels as low as 12–9% of the former value. Corresponding calculated gross ingestion rates (I=F×C) increased linearly below C sat and remained nearly constant above C sat. However, the measured weight-specific growth rates (µ) decreased sharply above C sat from a maximal value of about 9.5% day−1 to about 1.5% day−1. Below C sat on the other hand, measured µ values increased linearly with increasing algal concentration, which was in good agreement with an earlier advanced bioenergetic growth model. The overall functional response of M. edulis resembles a Type I in terms of gross ingestion, but with a rapid decrease instead of a constant above C sat in terms of actual ingestion and growth. The physiological implications of the functional response remain uncertain. The response to increasing food concentration with possible regulation of net ingestion appears only to come into play when C sat is exceeded and then as partial valve closure and reduced filtration and growth rates along with production of pseudofaeces. A survey of naturally occurring phytoplankton biomass in the sea shows that this is generally below C sat except for the short spring bloom periods; hence, mussels generally feed at optimal rates depending on the composition and concentration of biomass exceeding the minimal concentration below which the mussels close their valves and reduce or cease filtering.