2 research outputs found
CFD study of hydrodynamic signal perception by fish using the lateral line system
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2009The lateral line system on fish has been found to aid in schooling behavior, courtship
communication, active and passive hydrodynamic imaging, and prey detection. The
most widely used artificial prey stimulus has been the vibrating sphere, which some
fish are able to detect even when the signal velocities to its lateral line are orders
of magnitude smaller than background current velocities. It is not clear how the
fish are able to extract this signal. This thesis uses a series of computational fluid
dynamic (CFD) simulations, matched with recent experiments, to quantify the effects
of 3D fish body parts on the received dipole signals, and to determine signal detection
abilities of the lateral line system in background flow conditions.
An approximation is developed for the dipole induced, oscillatory, boundary layer
velocity profile over the surface of a fish. An analytic solution is developed for the case
when the surface is a wall, and is accurate at points of maximal surface tangential
velocity. Results indicate that the flow outside a thin viscous layer remains potential
in nature, and that body parts, such as fins, do not significantly affect the received
dipole signal in still water conditions. In addition, the canal lateral line system of the
sculpin is shown to be over 100 times more sensitive than the superficial lateral line
system to high frequency dipole stimuli.
Analytical models were developed for the Mottled Sculpin canal and superficial
neuromast motions, in response to hydrodynamic signals. When the background flow
was laminar, the neuromast motions induced by the stimulus signal at threshold had
a spectral peak larger than spectral peaks resulting from the background flow induced
motions. When the turbulence level increased, the resulting induced neuromast motions
had dominant low frequency oscillations. For fish using the signal encoding
mechanisms of phase-locking or spike rate increasing, signal masking should occur
Error and uncertainty in estimates of Reynolds stress using ADCP in an energetic ocean state
Submitted in partial fulfillment of the requirements for the degree of Master of Science in Oceanographic Engineering at the Massachusetts Institute of Technology
and the Woods Hole Oceanographic Institution September 2005The challenge of estimating the Reynolds stress in an energetic ocean environment derives from the turbulence process overlapping in frequency, or in wavenumber, with
the wave process. It was surmised that they would not overlap in the combined wavenumber-frequency spectrum, due to each process having a different dispersion relationship. The turbulence process is thought to obey a linear dispersion relationship, as the turbulent flow is advected with the mean current (Taylor's frozen turbulence
approximation). However, the Acoustic Doppler Current Profiler (ADCP) looks at radial wavenumbers and frequencies, and finds overlap. Another approach is to exploit the physical differences of each process, namely that the wave induced velocities are correlated over much larger distances than the turbulence induced velocities. This method was explored for current meters by Shaw and Trowbridge. Upon adapting the method for the ADCP, it is found that the resulting Reynolds stress estimates are
of the correct order of magnitude, but somewhat noisy. The work of this thesis is to uncover the source of that noise, and to quantify the performance limits of estimating the Reynolds Stress when using ADCP measurements that are contaminated with strong wave-induced velocities. To that end, the space-time correlations of the error, turbulence, and wave processes are developed and then utilized to find the extent
to which the environmental and internal processing parameters contribute to this error. It is found that the wave-induced velocities, even when filtered, introduce error variances which are of similar magnitude to that of the Reynolds stresses.This thesis has been funded by the NDSEG Fellowship Program (National Defense
Science and Engineering Graduate Fellowship), in association with ASEE (American
Society for Engineering Education)