Aline Cotel

Do Brown Trout Choose Locations with Reduced Turbulence

Abstract The physical habitat requirements of cover, depth, and current speed for brown trout Salmo trutta are associated with high shear zones in stream flows, which in turn result in high turbulence. Observations were made on current speeds and turbulence intensity (TI) in a sand-bed trout stream. Exemplary transects showed that current speeds ranged from 0 to 60 cm/s and that TI ranged from 0 to 0.7. Turbulence intensity was inversely related to current speed. Brown trout were usually found in the lower 5 cm of the stream, where shear forces result in high turbulence. Locations occupied by brown trout had lower TI than similar locations without brown trout but higher TI than is typical of an average stream.

Development, testing and demonstration of a portable submersible miniature particle imaging velocimetry device

A portable underwater particle image velocimetry (PIV) device has been developed, tested and demonstrated. The underwater PIV uses a 532 nm battery-powered 90 mW continuous laser. The laser beam is pulsed via a camera-synchronized chopper wheel. Images were recorded using a 1 megapixel black and white 10-bit CCD battery-powered camera controlled via a PCMCIA frame grabber card connected to a laptop computer. The system was validated against a standard laboratory PIV for average velocities up to 15 cm s −1 downstream from a 1.6 cm circular cylinder. The average vorticities calculated between the two systems were similar with a maximum difference of 3.6%. The average velocities were also similar with the largest difference occurring at the slowest flow recorded (difference of 0.5 cm s −1 ), resulting in a 9.4% difference. The maximum eddy size was comparable between the two systems with an average error of 4%. The system was field tested in the Huron River, Michigan downstream from a 1.2 cm diameter submerged limb. Mean velocities and standard deviations were comparable to acoustic Doppler velocimeter data. This paper presents the first published subsurface PIV data from a fluvial environment, demonstrating potential applications for a number of ecological and geomorphological studies.

Effects of Turbulence on Fish Swimming in Aquaculture

The role of turbulence in aquaculture facilities is a multi-faceted, largely unexplored, and potentially important topic in understanding the energetics and behavior of rearing fishes. Here, we review some common principles of turbulent flow and discuss methods to measure and describe them. Flows that display chaotic and wide fluctuations in velocity can repel fishes, while flows that have a component of predictability can attract fishes. We reveal how fish in turbulence can save energy by using two distinct, though not mutually exclusive mechanisms; flow refuging (exploiting regions of reduced flow) and vortex capture (harnessing the energy of discrete vortices). We summarize the energetics of fish holding station in turbulent flows around a cylinder from recent work. Turbulent flows can also create instabilities that negatively affect fishes, such as reducing critical swimming speed and increasing oxygen consumption. Our aim is to discuss aspects of turbulence from key lab and field experiments which may prove productive if applied to aquaculture systems.

The Challenge of Understanding and Quantifying Fish Responses to Turbulence-Dominated Physical Environments

The natural habitats of fishes are characterized by water movements driven by a multitude of physical processes of either natural or human origin. The resultant unsteadiness is exacerbated when flow interacts with surfaces, such as the bottom and banks, and protruding objects, such as corals, boulders, and woody debris. There is growing interest in the impacts on performance and behavior of fishes swimming in “turbulent flows”. The ability of fishes to stabilize body posture and their swimming trajectories is thought to be important in determining species distributions and densities, and hence resultant assemblages in various habitats. Understanding impacts of turbulence and vorticity on fishes is important as human practices modify water movements, and as turbulence-generating structures ranging from hardening shorelines to control erosion, through designing fish deterrents, to the design of fish passageways become common. Collaboration between engineers and biologists is essential in order to generate adequate and sustainable solutions. Previous work on fish responses to turbulent perturbations is discussed and new theoretical concepts/framework are proposed to quantify fish-eddy interactions.

Assessing possible effects of fish-culture systems on fish swimming: the role of stability in turbulent flows

Fish are cultured in ponds, recirculating systems, raceways, and cages. Turbulence is associated with one or more of mechanisms to facilitate food accessibility, maintain adequate levels of oxygen, remove carbon dioxide, urinary and fecal wastes, as well as from locomotion of fishes themselves. Turbulence has been shown to have positive and negative effects on fish swimming, feeding, and energetics, usually with negative impacts at very low and at high levels, and least effects and sometimes positive effects at intermediate levels. Differences in responses of fishes with varying levels of turbulence are related to the size of eddies relative to the size of a fish (larvae, juveniles, and adults). Impacts on locomotor functions are associated with eddy diameters of the order of 0.5–1L, where L is the total length of a fish. Negative locomotor impacts of turbulence are associated with eddies challenging stability, while positive effects promote drafting and station holding with reduced locomotor motions. Deployment of control surfaces increases with the level of turbulence up to a threshold where control is overwhelmed. The design of culture facilities is expected to affect levels of turbulence and may be engineered to provide optimal levels facilitating high growth.

Temperature and velocity measurements of a rising thermal plume

The three-dimensional velocity and temperature fields surrounding an isolated thermal plume in a fluid with temperature-dependent viscosity are measured using Particle-Image Velocimetry and thermochromatic liquid crystals, respectively. The experimental conditions are relevant to a plume rising through the mantle. It is shown that while the velocity and the isotherm surrounding the plume can be used to visualize the plume, they do not reveal the finer details of its structure. However, by computing the Finite-Time Lyapunov Exponent fields from the velocity measurements, the material lines of the flow can be found, which clearly identify the shape of the plume head and characterize the behavior of the flow along the plume stem. It is shown that the vast majority of the material in the plume head has undergone significant stretching and originates from a wide region very low in the fluid domain, which is proposed as a contributing factor to the small-scale isotopic variability observed in ocean-island basalt regions. Lastly, the Finite-Time Lyapunov Exponent fields are used to calculate the steady state rise velocity of the thermal plume, which is found to scale linearly with the Rayleigh number, in contrast to some previous work. The possible cause and the significance of these conflicting results are discussed, and it is suggested that the scaling relationship may be affected by the temperature-dependence of the fluid viscosity in the current work.

A review of recent developments on turbulent entrainment in stratified flows

Stratified interfaces are present in many geophysical flow situations, and transport across such an interface is an essential factor for correctly evaluating the physical processes taking place at many spatial and temporal scales in such flows. In order to accurately evaluate vertical and lateral transport occurring when a turbulent flow impinges on a stratified interface, the turbulent entrainment and vorticity generation mechanisms near the interface must be understood and quantified. Laboratory experiments were performed for three flow configurations: a vertical thermal, a sloping gravity current and a vertical turbulent jet with various tilt angles and precession speeds. All three flows impinged on an interface separating a two-layer stably stratified environment. The entrainment rate is quantified for each flow using laser-induced fluorescence and compared to predictions of Cotel and Breidenthal (1997 Appl. Sci. Res. 57 349–66). The possible applications of transport across stratified interfaces include the contribution of hydrothermal plumes to the global ocean energy budget, turbidity currents on the ocean floor, the design of lake de-stratification systems, modeling gas leaks from storage reservoirs, weather forecasting and global climate change.