Effects Of Small-scale Turbulence Research Articles

Particle inertial effects on radar Doppler spectra simulation

Abstract. Radar Doppler spectra observations provide a wealth of information about cloud and precipitation microphysics and dynamics. The interpretation of these measurements depends on our ability to simulate these observations accurately using a forward model. The effect of small-scale turbulence on the radar Doppler spectra shape has been traditionally treated by implementing the convolution process on the hydrometeor reflectivity spectrum and environmental turbulence. This approach assumes that all the particles in the radar sampling volume respond the same to turbulent-scale velocity fluctuations and neglects the particle inertial effect. Here, we investigate the inertial effects of liquid-phase particles on the forward modeled radar Doppler spectra. A physics-based simulation (PBS) is developed to demonstrate that big droplets, with large inertia, are unable to follow the rapid change of the velocity field in a turbulent environment. These findings are incorporated into a new radar Doppler spectra simulator. Comparison between the traditional and newly formulated radar Doppler spectra simulators indicates that the conventional simulator leads to an unrealistic broadening of the spectrum, especially in a strong turbulent environment. This study provides clear evidence to illustrate the droplet inertial effect on radar Doppler spectrum and develops a physics-based simulator framework to accurately emulate the Doppler spectrum for a given droplet size distribution (DSD) in a turbulence field. The proposed simulator has various potential applications for the cloud and precipitation studies, and it provides a valuable tool to decode the cloud microphysical and dynamical properties from Doppler radar observation.

LES Investigation of Kerosene Spray Flame Emission Characteristics in a Staged Combustor

ABSTRACT Staged combustion technology is widely applied in modern low-emission energy and power systems. This study employs large eddy simulations (LES) to investigate the flame dynamics and emission characteristics in a staged two-phase partially premixed model combustor. To improve the suitability of the thickened flame model (TFM) for turbulent flames in the thin-reaction-zone regime, a new model for efficiency function is formulated such that the effects of small-scale turbulence on flame propagation are taken into account. The enhanced LES-TFM approach is then integrated with an efficient NOx model, in which an additional NOx transport equation is solved with pre-tabulated NOx formation source terms. Five LES runs are performed to investigate the evolution of flame dynamics and emission characteristics with staging ratios at the scaled cruise operating conditions, in which the optimal staging ratio is critical for NOx reduction. Results show that the trend of flame morphology and NOx emission predictions with staging ratios are consistent with the experimental results. The flame zones in the staged model combustor shift from a pilot-stage-only flame to a pilot-primary-stage flame, then to a primary-stage-only flame with decreasing fuel staging ratio. Due to the redistribution of the equivalence ratio and the NOx emission characteristics trade-off between the two-stage flows, there is an optimal staging ratio for NOx reduction in the staged model combustor. The proposed LES-TFM-NOx approach, which has both the benefits of efficient flame dynamics modeling with the global reaction mechanism and the advantages of prediction accuracy with the detailed mechanism for NOx emission, demonstrates its advantages in simulating flame and emission characteristics in low-emission combustion devices.

Effects of small-scale turbulence on two species of Dinophysis

Dinoflagellate species of Dinophysis, in particular D. acuminata and D. acuta, produce lipophilic toxins that pose a threat to human health when concentrated in shellfish and jeopardize shellfish exploitations in western Europe. In northwestern Iberia, D. acuminata has a long growing season, from spring to early autumn, and populations develop as soon as shallow stratification forms when the upwelling season begins. In contrast, D. acuta blooms in late summer, when the depth of the pycnocline is maximal and upwelling pulses are moderate. In situ observations on the hydrodynamic regimes during the two windows of opportunity for Dinophysis species led us to hypothesize that D. acuta should be more sensitive to turbulence than D. acuminata.To test this hypothesis, we studied the response of D. acuminata and D. acuta to three realistic turbulence levels low (LT), ε ≈ 10−6 m2 s-3; medium (MT), ε ≈ 10-5 m2 s-3 and high (HT), ε ≈ 10-4 m2 s-3generated by Turbogen, a highly reproducible, computer-controlled system. Cells of both species exposed to LT and MT grew at rates similar to the controls. Marked differences were found in the response to HT: D. acuminata grew slowly after an initial lag phase, whereas D. acuta cell numbers declined. Results from this study support the hypothesis that turbulence may play a role in shaping the spatio-temporal distribution of individual species of Dinophysis. We also hypothesize that, in addition to cell disturbance affecting division, sustained high shear generated by microturbulence may cause a decline in Dinophysis numbers due to decreased densities of ciliate prey.

Effects of small-scale turbulence at the air-water interface on microcystis surface scum formation

Cyanobacterial surface scum (here defined as visible Cyanobacteria colonies accumulating at the lake surface) is a harmful phenomenon that negatively affects water quality, human and animal health. Colony-forming Microcystis is one of the most important and ubiquitous genera that can suddenly accumulate at water surfaces. Turbulent water motion, e.g., generated by wind, can vertically disperse this scum layer, which later can re-establish by upward migration of Microcystis colonies. However, the role of wind-generated turbulence in scum formation and development is still poorly understood. Here we present results from a laboratory mesocosm study where we analysed the processes of scum formation and its response to wind-generated turbulence at low wind speed (≤3.6 m s−1). Microcystis colony size and flow velocity at the water surface and in the bulk water were measured using a microscope camera and particle tracking velocimetry. The surface scum formed by aggregation of colonies at the water surface, where they formed loose clusters of increasing size. The presence of large colony aggregations or of a surface film determined the stability of the scum layer. For the largest applied wind speed, most of the aggregations were broken down to sizes <2 mm, which were dispersed to the bulk water. The surface scum recovered quickly from such disturbances after the wind speed decreased. We further observed reduced momentum transfer from wind to water with the growing scum layer. The presence of the scum increased the threshold wind speed for the onset of flow and reduced the flow velocities that were generated above that threshold. This effect was likely caused by the presence of a film of surface-active material at the water surface (surface microlayer), which is related to the presence of Microcystis. Both the small-scale turbulence and surface microlayer might play an important, yet largely unexplored role in Microcystis surface scum development in aquatic ecosystems. Improved understanding of the interplay of both processes will be instrumental for improving current mechanistic models for predicting surface bloom dynamics.

Bifurcation parameters of a reflected shock wave in cylindrical channels of different roughnesses

To investigate the effect of bifurcation on the induction time in cylindrical shock tubes used for chemical kinetic experiments, one should know the parameters of the bifurcation structure of a reflected shock wave. The dynamics and parameters of the shock wave bifurcation, which are caused by reflected shock wave–boundary layer interactions, are studied experimentally in argon, in air, and in a hydrogen–nitrogen mixture for Mach numbers $$M =$$ 1.3–3.5 in a 76-mm-diameter shock tube without any ramp. Measurements were taken at a constant gas density behind the reflected shock wave. Over a wide range of experimental conditions, we studied the axial projection of the oblique shock wave and the pressure distribution in the vicinity of the triple Mach configuration at 50, 150, and 250 mm from the endwall, using side-wall schlieren and pressure measurements. Experiments on a polished shock tube and a shock tube with a surface roughness of 20 $${\upmu }$$ m Ra were carried out. The surface roughness was used for initiating small-scale turbulence in the boundary layer behind the incident shock wave. The effect of small-scale turbulence on the homogenization of the transition zone from the laminar to turbulent boundary layer along the shock tube perimeter was assessed, assuming its influence on a subsequent stabilization of the bifurcation structure size versus incident shock wave Mach number, as well as local flow parameters behind the reflected shock wave. The influence of surface roughness on the bifurcation development and pressure fluctuations near the wall, as well as on the Mach number, at which the bifurcation first develops, was analyzed. It was found that even small additional surface roughness can lead to an overshoot in pressure growth by a factor of two, but it can stabilize the bifurcation structure along the shock tube perimeter.

Effects of small-scale turbulence on NOx formation in premixed flame fronts

Turbulent lean premixed combustion plays a predominant role in reducing NOx emission due to the low flame temperatures. For turbulent premixed flames located in the thin-reaction-zones regime, small-scale eddies could penetrate into the preheat zone of the flames and enhance the mixing process. In this study, the effects of small-scale turbulence on NOx formation in premixed flame fronts are investigated through the incorporation of turbulence induced diffusion in the preheat zone of one-dimensional premixed flames for methane/air, simulated with the 53-species GRI-Mech 3.0 mechanism at both atmospheric pressure and engine conditions with different turbulence intensities. It is found that the NO generated in flame fronts deceases with increased intensity of small-scale turbulence and the effect is more pronounced at high pressures, where the turbulence-induced diffusion in the preheat zone can reduce the NOx formation in flame fronts by more than 40%. In addition, a flamelet-based approach that accounts for the flame thickening effects has been formulated to simulate NOx formation in turbulent lean premixed combustion. In this approach, the species NO is transported and solved in a simulation with its chemical source term modeled by combining its formation in flame fronts and its integrated formation rate in the post-flame zone from 1-D premixed flames with detailed chemical kinetics and turbulence induced diffusion.

The Effects of Small-Scale Turbulence on Air–Sea Heat Flux

Abstract The air–sea exchange of heat is mainly controlled by the molecular diffusive layer adjacent to the surface. With an order of magnitude difference between the kinematic viscosity and thermal diffusivity of water, the thermal sublayer is embedded within its momentum analog: the viscous sublayer. Therefore, the surface heat exchange rates are greatly influenced by the surface kinematics and dynamics; in particular, small-scale phenomena, such as near-surface turbulence, have the greatest potential to affect the surface fluxes. Surface renewal theory was developed to parameterize the details of the turbulent transfer through the molecular sublayers. The theory assumes that turbulent eddies continuously replace surface water parcels with bulk fluid, which is not in equilibrium with the atmosphere and therefore is able to transfer heat. The so-called controlled-flux technique gives direct measurements of the mean surface lifetime of such surface renewal events. In this paper, the authors present results from field experiments, along with a review of surface renewal theory, and show that previous estimates of air–sea scalar fluxes using the controlled-flux technique may be erroneous if the probability density function (PDF) of surface renewal time scales is different from the routinely assumed exponential distribution. The authors show good agreement between measured and estimated heat fluxes using a surface renewal PDF that follows a χ distribution. Finally, over the range of forcing conditions in these field experiments, a clear relationship between direct surface turbulence measurements and the mean surface renewal time scale is established. The relationship is not dependent on the turbulence generation mechanism. The authors suggest that direct surface turbulence measurements may lead to improved estimates of scalar air–sea fluxes.

Study on effects of small-scale turbulence on flow patterns around rectangular cylinder

Study on effects of small-scale turbulence on flow patterns around rectangular cylinder

A numerical investigation of the impact of turbulence on the feeding rates of Oithona davisae

Individual based numerical simulations of the copepod, Oithona davisae, feeding on motile prey, Oxyrrhis marina, under variable turbulent conditions are performed. These simulations correspond to laboratory observations conducted by Saiz et al. [Saiz, E., Calbet, A., and Broglio, E., 2003. Effects of small-scale turbulence on copepods: the case of Oithona Davisae. Limnol. Oceanogr., 48:1304–1311.]. The flow field in the simulation is reconstructed by a kinematic simulation whose characteristic scales are derived from the grid mesh and the dissipation rates of the laboratory experiments. The kinematic simulation provides a simplified model, which while not fully realistic, captures the basic relevant feature of turbulence. A hop and sink swimming behaviour is prescribed for O. davisae, while O. marina moves along helical paths with random changes of directions. Three possible effects are tested: the existence of a time threshold in the duration of the contacts between predator and prey, a progressive reduction of the perceptive distance with increasing turbulence level and an abrupt reduction in feeding of O. davisae when the flow speed, in relation to the copepod position, is higher than a prescribed threshold. This last approach introduces an intermittency in the feeding which depends on the variations of velocity both in space and time within the numerical box. The introduction of the time threshold causes a dome-shaped relationship between the simulated enhancement factor and the dissipation rate, while with the other two effects, a monotonic decrease in the enhancement factor is observed, with values reasonably close to the ones observed in the laboratory experiment. In all the cases, the use of realistic values of biological parameters (e.g. swimming behaviour) reproduces response curves in the range of the observations.

AN EXPERIMENTAL STUDY ON FLAME PROPAGATION IN CORNSTARCH DUST CLOUDS

Following the quantitative determination of dust cloud parameters, this study investigates the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160 × 160 mm square cross section, and gives particular attention to the effect of small scale turbulence and small turbulence intensity on flame characteristics. Dust suspensions in air were produced using an improved apparatus ensuring more uniform distribution and repeatable dust concentrations in the testing duct. The dispersion-induced turbulence was measured by means of a particle image velocimetry (PIV) system, and dust concentrations were estimated by direct weighing method. This quantitative assessment made it possible to correlate observed flame behaviors with the parameters of the dust cloud. Upward propagating dust flames, from both closed/open bottom end to open/closed top end of the duct, were visualized by direct light and shadow photography. From the observation of propagation regimes and the measurements of flame velocity, a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition was determined to be 10 cm/s. Laminar flames propagated with oscillations from the closed bottom end to the open top end of the testing duct, while the turbulent flames accelerated continuously. Both laminar and turbulent flames propagated with steady velocity from the open bottom end to the closed top end of the duct. The measured propagation velocity of laminar flames appeared to be in the range of 0.45–0.56 m/s, and it was consistent with the measurements reported in the literature. In the present experimental study, the influence of dust concentration on flame propagation was also examined, and the flame propagation velocity was found weakly sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of the dust combustion process.

Effects of small-scale turbulence on Phaeocystis globosa (Prymnesiophyceae) growth and life cycle

The response of Phaeocystis globosa to small-scale turbulence was studied in 5 l microcosms. Turbulence was generated by oscillating grids. The effect of small-scale turbulence was examined under 3 turbulence levels representative of the P. globosa natural environment, and in non-turbulent control cultures. Single cell numbers, nitrogen concentrations and colony formation (number and diameter) were followed over 13 days in each experimental culture. Small-scale turbulence decreased single cell growth and also influenced colony formation. More colonies were formed when turbulence increased to a given threshold, but above this turbulence level, fewer and smaller colonies were observed in P. globosa cultures. The ecological significance of these results, particularly, the potential influence of small-scale turbulence on competition mechanisms between P. globosa and diatoms are finally discussed and suggested as a key factor to understand phytoplankton successions in the Eastern English Channel.

Effects of small-scale turbulence on the growth of two diatoms of different size in a phosphorus-limited medium

The effect of turbulence on the nutrient flux towards osmotrophic cells is predicted to be size dependent. This should translate into growth. We experimentally followed and modelled the growth of two marine diatoms of different size ( Thalassiosira pseudonana, 6 μm in diameter and Coscinodiscus sp., ca. 109 μm in diameter) under still water and turbulent conditions, using a shaker table. Experiments were done with phosphorus-limited cultures and lasted for ca. 5 days. Turbulence enhanced the growth of Coscinodiscus sp. in agreement with theory but not the growth of T. pseudonana, which was actually slightly lower under turbulence. At the end of the experiments there were about 1.7 times as many Coscinodiscus sp. cells in the turbulent treatment than in the still treatment, while for T. pseudonana almost the same cell concentration was found in both conditions. In addition, the Coscinodiscus sp. cells growing under still conditions presented a higher specific alkaline phosphatase activity than those growing in turbulence which indicates a higher need for phosphorus in the still cultures. A simple dynamic model, based on Michaelis–Menten nutrient uptake kinetics, needed nearly no optimisation other than using observed initial conditions of phosphate and cell concentrations. The model showed how an increased nutrient flux towards the cells translates non-linearly into cell growth, most likely by affecting the half-saturation constant ( K M). However, since Coscinodiscus sp. experienced significant mortality and cells partially settled to the bottom of the containers, unequivocal support for the size-dependent effect of turbulence on nutrient uptake will require further experiments and more sophisticated modelling. The mechanisms to connect an increased nutrient flux towards cells with population growth and whether this process is size dependent are important in parameterizing the effects of turbulence on marine plankton in coupled physical–biological models.

Effects of Small-Scale Turbulence on Bacteria: A Matter of Size

We examined the influence of small-scale turbulence and its associated shear on bacterioplankton abundance and cell size. We incubated natural microbial assemblages and bacteria-only fractions and subjected them to treatments with turbulence and additions of mineral nutrients and/or organic carbon. Bacterial abundance was not affected directly by turbulence in bacteria-only incubations. In natural microbial assemblage incubations, bacterial concentrations were higher under turbulence than in still-water controls when nutrients were added. In general, in the turbulence treatments bacteria increased significantly in size, mainly due to elongation of cells. The addition of inorganic nutrients had a negative effect on bacterial size, but a significantly positive effect on abundance independently of other factors such as turbulence and the presence of predators. Flagellate grazing did not trigger an increase in bacterial size as a grazing resistance response in unmixed containers. With the addition of organic carbon, bacteria elongated and partly settled to the bottom of the containers, in both the turbulent and still treatment, but bacterial abundance did not further increase. Furthermore, bacteria aggregated in the turbulence treatments after the second day of incubation even in the absence of other components of the microbial community. We found that turbulence and the associated shear increase bacterial size and change bacterial morphology, at least under certain nutrient conditions. This might be due to a physiological response (enhanced growth rate and/or unbalanced growth) or due to the selection of opportunistic strains when organic carbon is in excess compared to mineral nutrients. We suggest that shear associated with turbulent flow enhances the DOM flux to bacteria directly as well as indirectly through enhanced grazing activity and photosynthetic release. The formation of bacterial aggregates and filaments under turbulence might give selective advantage to bacteria in terms of nutrient uptake and grazing resistance.

Effects of Turbulent Mixing and Horizontal Shear on Double-Diffusive Interleaving in the Central and Western Equatorial Pacific

Abstract The effects of small-scale turbulence and the horizontal shear on double-diffusive interleaving are treated in the framework of a linear stability problem in the equatorial β-plane approximation. The model predicts the decrease of the vertical wavenumber (mmax) and growth rate (λmax) for the fastest-growing intrusion with the increase in the turbulent diffusivity and predicts increased λmax and decreased mmax with the horizontal shear of the mean zonal flow. The effects of turbulent mixing are analyzed at different values of the Prandtl number, and the criteria of instability are obtained analytically for some simple cases. Special attention is focused on the influence of the horizontal shear on instability, as with the f-plane models of interleaving. The CTD profiling data obtained from Korean and Russian research vessels are used to prove the model. A closely spaced CTD section across the equator at 176°16′W shows several intrusive layers with high lateral coherence throughout the section, whic...

Small-scale turbulence affects the division rate and morphology of two red-tide dinoflagellates

The effects of small-scale turbulence on two species of dinoflagellates were examined in cultures where the turbulent forces came randomly from all directions and were intermittent both spatially and temporally; much like small-scale turbulence in the ocean. With Lingulodinium polyedrum (Stein) Dodge (syn. Gonyaulax polyedra), division rate increased linearly (from ∼0.35 to 0.5 per day) and the mean cross-sectional area (CSA) decreased linearly (from ∼1100 to 750 μm 2) as a function of the logarithmic increase in turbulence energy dissipation rate ( ε). These effects were noted when ε values increased between ∼10 −8 and 10 −4 m 2 s −3. However, when ε increased to ∼10 −3 m 2 s −3, division rate sharply decreased and mean CSA increased. Over the same range of ε, Alexandrium catenella (Wheedon and Kofoid) Balech had its division rate decrease linearly (from ∼0.6 to 0.45 per day) and its CSA increase linearly (from ∼560 to 650 μm 2) as a function of the logarithmic increase in ε. Even at the highest ε examined (∼10 −3 m 2 s −3), which may be unrealistically high for their ambits, both L. polyedra and A. catenella still had fairly high division rates, ∼0.2 and 0.45 per day, respectively. Turbulence strongly affected chain formation in A. catenella. In non-turbulent cultures, the mode was single cells (80–90% of the population), but at ε of ∼10 −5 to 10 −4 m 2 s −3, the mode was 8 cells per chain. At the highest ε (∼10 −3 m 2 s −3), the mode decreased to 4 cells per chain. The vertical distributions of A. catenella populations in relation to hydrographic flow fields were studied in the summers of 1997 and 1998 in East Sound, Washington, USA (latitude 48°39′N, 122°53′W). In both summers, high concentrations of A. catenella were found as a subsurface bloom in a narrow depth interval (∼2 m), where both current shear and turbulence intensity were at a minimum. Other researchers have shown that A. catenella orients its swimming in shear flows, and that swimming speed increases with chain length. These responses, when combined with our observations, support a hypothesis that A. catenella actively concentrates at depths with low turbulence and shear.

Effects of turbulence on plankton: an overview of experimental evidence and some theoretical considerations

From the literature we obtained experimental data on the effects of small-scale turbulence on plankton at the organism scale. Normalized rates in response to turbulence were calculated in the present study for parameters related to growth, ingestion and energy expenditure. Growth rates are, in general, negatively affected by turbulence. Nevertheless, the data are highly biased towards a specific group of organisms, dinoflagellates, which could have peculiar physiological impediments under turbulence. Ingestion rates seem to be increased by turbulence, especially at low and intermediate levels. The few data available on energy expenditure indicate increases under turbulent conditions. Although many experiments were conducted at very high levels of turbulence with respect to oceanic conditions, in most studies the Kolmogorov size microscale remained larger than the size of the planktonic organisms. Thus, organisms responded to turbulence or turbulence-derived stimuli below the Kolmogorov lengthscale. The ecological relevance of interactions between turbulence and other size-related parameters, such as reactive distances, mean free paths and Batchelor microscales, are mentioned mainly in terms of predation, nutrient uptake and competition. Specifically, mean free paths seem relevant to determine encounter rates for organisms that show concentrations similar to those typical for large protozoa and algae (both within these organisms and between them and their potential predators). Characteristic time scales for different planktonic organisms are also explored in relation to the corresponding Kolmogorov time microscale. All planktonic organisms will experience fluctuations in the nutrient or food-particle fields over their lifetime. If other characteristic times are considered, such as the time between particle capture or nutrient uptake events, fluctuations may or may not be experienced over those times depending on the feeding mode and the turbulence intensity.

Effect of small-scale turbulence on feeding rates of larval cod and haddock in stratified water on Georges Bank

A set of vertically stratified MOCNESS tows made on the southern flank of Georges Bank in spring 1981 and 1983 was analyzed to examine the relationship between larval cod and haddock feeding success and turbulent dissipation in a stratified water column. Observed feeding ratios (mean no. prey larval gut−1) for three size classes of larvae were compared with estimated ingestion rates using the Rothschild and Osborn (Journal of Plankton Research, 10, 1988, 465–474) predator-prey encounter rate model. Simulation of contact rates requires parameter estimates of larval fish and their prey cruising speeds, density of prey, and turbulent velocity of the water column. Turbulent dissipation was estimated from a formulation by James (Estuarine and Coastal Marine Science, 5, 1977, 339–353) incorporating both a wind a tidal component. Larval ingestion rates were based on swallowing probabilities derived from calm-water laboratory observations.Model-predicted turbulence profiles generally showed that dissipation rates were low to moderate (10−11−10−7 W kg−1). Turbulence was minimal at or below the pycnocline (≈ 25 m) with higher values(1–2 orders of magnitude) near the surface due to wind mixing and at depth due to shear in the tidal current near bottom. In a stratified water column during the day, first-feeding larvae (5–6 mm) were located mostly within or above the pycnocline coincident with their copepod prey (nauplii and copepodites). The 7–8 mm larvae were most abundant within the pycnocline, whereas the 9–10 mm larvae were found within and below the pycnocline. Feeding ratios were relatively low in early morning following darkness when the wind speed was low, but increased by a factor of 2–13 by noon and evening when the wind speed doubled. Comparison of depth-specific feeding ratios with estimated ingestion rates, derived from turbulence-affected contact rates, generally were reasonable after allowing for an average gut evacuation time (4 h), and in many cases the observed and estimated values had similar profiles. However, differences in vertical profiles may be attributed to differential digestion time, pursuit behavior affected by high turbulence, vertical migration of the larger larvae, an optimum light level for feeding, smaller-scale prey patchiness, and the gross estimates of turbulence.Response-surface estimation of averaged feeding ratios as a function of averaged prey density (0–50 m) with a minimum water-column turbulence value predicted that 5–6 mm larvae have a maximum feeding response at the highest prey densities (> 30 prey 1−1) and lower turbulence estimates (<10−10 W kg−1). The 7–8 mm and 9–10 mm larvae also have a maximum feeding response at high prey densities and low turbulence, but it extends to lower prey densities (> 10 prey 1−1) as turbulence increases to intermidiate levels, clearly showing an interaction effect. In general, maximum feeding ratios occur at low to intermediate levels of turbulence where average prey density is greater than 10–20 prey 1−1.

Preferential concentration of marine particles in isotropic turbulence

The effect of small-scale turbulence on marine and aquatic particle transport has traditionally been to act as a means for creating homogeneous distributions. However, previous numerical simulations of heavy particle transport in turbulent flows have shown that particles are preferentially concentrated by turbulence and that effects of preferential concentration are most pronounced for particle parameters comparable to the Kolmogorov scales. Therefore, the focus of the present work is examination of the preferential concentration of marine particles. Application of Kolmogorov scaling indicates that effects of preferential concentration may be important for marine particles with diameters of order 1 mm in the upper mixed layer. Numerical simulations of unstratified isotropic turbulence are then used to support the notion that preferential concentration of particles possessing material characteristics representative of those encountered in marine environments can occur. In the simulations, particles of order 1 mm diameter are idealized as rigid spheres with a density ratio of 1.005. Simulation results demonstrate preferential concentration with peak particle number densities ranging from 10 to 60 times the global mean value. Implications of preferential concentration are also discussed, together with the limitations of the approach employed in the present study.

The effects of small-scale turbulence on the ingestion rate of fathead minnow (Pimephales promelas) larvae

Small-scale turbulence increases encounter rates between planktonic predators and their prey. It has been hypothesized to lead to an increase in feeding by fish larvae, yet this has rarely been tested experimentally. We investigated the effects of turbulence on feeding by fathead minnow (Pimephales promelas) larvae to test the null hypothesis of no effect of turbulence on ingestion. In one experiment, larvae were exposed to four turbulent velocities from 0 to 2 cm∙s−1 at a single prey concentration. Data analysis led to a rejection of the null hypothesis. Moreover these data suggested a significant nonlinear response of ingestion to increasing turbulence. In a second experiment, we varied turbulence and prey concentrations (15, 50, and 500 prey/L) simultaneously. We rejected the null hypothesis of no effect of turbulence for all prey concentrations. Results differed among prey concentrations. We observed a negative linear response for the two lower prey concentrations, but a domed response at 500 prey/L. We conclude that turbulence does influence ingestion, but not as suggested by an extension of encounter models. Thus, turbulence may not always be beneficial in the field.

Turbulence Effects on the Design and Performance of Low-Drag Keels

Achievement of laminar boundary-layer flow over the keel of sailing yachts offers great potential for increased boat speed resulting from the reduction in viscous drag. The intensity and the spectral energy distribution of small-scale turbulence in the upper layer of the ocean can have a large effect on the extent of laminar flow and, hence, the drag reduction. A detailed summary and interpretation is provided of measured turbulent intensities and length scales in the upper ocean. An analysis is given of the effect of small-scale turbulence on boundary-layer transition and profile drag for two laminar-flow foils at zero and moderate leeway angles. Using the modified en-transition criterion it is shown that the level of small-scale oceanic turbulence is an important parameter in the design of a laminar-flow foil. At high turbulence levels, the foil with a more forward location of the minimum-pressure point generates less drag than the foil with a more aft location of the minimum-pressure point at the same flow conditions.