Co-oscillation of a fjord basin’s circulation with decade-long oceanographic, hydrologic, and climatic regimes: Puget Sound 1916-'87 [abstract]

EXTRACT (SEE PDF FOR FULL ABSTRACT): Oceanographic, hydrologic, and climatic data collected during 1916-'87 in Puget Sound's Main Basin (~200 m x 5 km x 100 km) and approaches oscillate at low frequency between two regimes (I, II). The oscillation accounts for a large fraction of the interannual variability (41-75%) and the zero crossings between regimes span approximately a decade. ... The transition between regimes is accompanied by substantial changes in the horizontal pressure and density fields between the Pacific coast and the mixing zones leading to the Basin, as well as within the Basin itself.

Vortices, sound and flames - a damaging combination

The interaction between vortices, sound and combustion can lead to self-excited oscillations of such large amplitudes that structural damage is done. These occur because any small unsteadiness in the rate of combustion is a source of sound, generating pressure and velocity fluctuations. However, the velocity fluctuations perturb the flame, thereby altering the instantaneous rate of heat release. Instability is then possible because while acoustic waves perturb the combustion, the unsteady combustion generates yet more sound! Combustion oscillations can occur in afterburners and at idle in conventional aeroengine combustors. Lean premixed, prevapourized technology has tremendous potential to reduce NOx emissions, but is proving highly susceptible to self-excited oscillations. An overview of the physics of the interaction between vortices, sound and flames is presented, and illustrated by examples of instability in generic premixed ducted flames and in aeroengine combustors. The potential for both passive and active control is discussed.

Prediction of sound emission from open turbulent premixed flames

The sound emission from open turbulent flames is dictated by the two-point spatial correlation of rate of change of fluctuating heat release rate and this correlation has not been investigated directly in the past studies. Turbulent premixed flame data from DNS and laser diagnostics are analyzed to study this correlation function and the two-point spatial correlation of the fluctuating heat release rate. This shows that the correlation functions have simple Gaussian forms whose integral length scale is related to the laminar flame thickness and amplitude depends on the spatial distribution of the time-mean rate of heat release. These results and RANS-CFD solution of open turbulent premixed flames are post-processed to obtain the far field SPL, which agrees well with measured values. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

Absorption of sound in cellular foams

To further enhance the sound absorption of metal foams via combining the high sound absorption and good heat conductivity of the cellular foam metals, the use and acoustic modeling of these materials are reviewed. The predictions made by three viscous models developed by the authors for the propagation of sound through open-cell metal foams are compared with an experiment both for the metal foams and for the polymer substrates used to manufacture the foam. All models are valid in the limit of low Reynold's number which is valid for the typical cell dimensions found in metal foams provided the amplitude of the waves is below 160 dB. The first model considers the drag experienced by acoustic waves as they propagate passing rigid cylinders parallel to their axes, the second considers the propagation normal to their axes, and the third considers the propagation passing the spherical joints. All three are combined together to give a general model of the acoustic behavior of the foams. In particular, the sound absorption is found to be significant and well predicted by the combined model. In addition, a post-processing technique is described for the experiment used to extract the fundamental wave propagation characteristics of the material.

Shifts in the estuarine demersal fish community after a fishery closure in Puget Sound, Washington

Puget Sound is one of the largest and most ecologically significant estuaries in the United States, but the status and trends of many of its biological components are not well known. We analyzed a 21-year time series of data from standardized bottom trawl sampling at a single study area to provide the first assessment of population trends of Puget Sound groundfishes after the closure of bottom trawl fisheries. The expected increase in abundance was observed for only 3 of 14 species after this closure, and catch rates of most (10) of the abundant species declined through time. Many of these changes were stepwise (abrupt) rather than gradual, and many stocks exhibited changes in catch rate during the 3-year period from 1997 through 2000. No detectable change was recorded for either temperature or surface salinity over the entire sampling period. The abrupt density reductions that were observed likely do not reflect changes in demographic rates but may instead represent distributional shifts within Puget Sound.

Mesoscale, cyclonic eddies as larval fish habitat along the southeast United States shelf: A Lagrangian description of the zooplankton community

The Charleston Gyre region is characterized by continuous series of cyclonic eddies that propagate northeastwards before decaying or coalescing with the Gulf Stream south of Cape Hatteras, NC, USA. Over 5 d, chlorophyll-a concentration, zooplankton displacement volume, and zooplankton composition and abundance changed as the eddy moved to the northeast. Surface chlorophyll-a concentration decreased, and zooplankton displacement remained unchanged as the eddy propagated. Zooplankton taxa known to be important dietary constituents of larval fish increased in concentration as the eddy propagated. The concurrent decrease in chlorophyll-a concentration and static zooplankton displacement volume can be explained by initial stimulation of chlorophyll-a concentration by upwelling and nutrient enrichment near the eddy core and to possible grazing as zooplankton with short generation times and large clutch sizes increased in concentration. The zooplankton community did not change significantly within the 5 d that the eddy was tracked, and there was no indication of succession. Mesoscale eddies of the region are dynamic habitats as eddies propagate northeastwards at varying speeds within monthly periods. The abundance of zooplankton important to the diets of larval fish indicates that the region can provide important pelagic nursery habitat for larval fish off the southeast coast of the United States. A month of feeding and growth is more than half the larval duration of most fish spawned over the continental shelf of the southeastern United States in winter.

The zooplankton species composition and abundance in Ogun River, Abeokuta, Ogun State, southwestern Nigeria

Investigation on the species composition and abundance of zooplankton in Ogun River, Abeokuta, Ogun state, Southwestern Nigeria was carried out for a period of seven months (December, 2011-June, 2012), Using hand trawling method, 55 µm mesh size plankton net was trawled horizontally ten times to capture zooplankton. The zooplankton samples were put into properly labeled plastic containers and preserved in 4% formalin solution. A total of sixteen genera from five taxonomic groups were recorded from Ogun River. Cladocera predominated both in terms of species composition and abundance. Five species of Cladocera was recorded consisting of 31.25%. Protozoa and Rotifera recorded four species each consisting of 25%. Copepoda recorded two species consisting of 12.5%, and Ostracoda with one species consisting of 6.25%. The low nature of zooplankton abundance and species composition observed in this study must have been caused by the polluted nature of the water due to the anthropogenic activities carried out around its shores The dominance of Cladocera in this river is similar to findings from River Echara and River Imo, Southeastern Nigeria.

Monthly spatial occurrence of phytoplankton and zooplankton in River Ogun, Abeokuta, Ogun State, southwest Nigeria

The aim of this study was to investigate the monthly spatial occurrence of phytoplankton and zooplankton in River Ogun, Abeokuta, Southwest Nigeria. This was carried out for seven months between December, 2011 and June, 2012 in 4 stations. A total of 41 species of phytoplankton and 16 zooplankton species from 5 classes respectively were recorded. Zooplankton was dominated by Cladocera throughout the study period while phytoplankton was dominated by blue green algae (Cyanophyta or Cyanobacteria). The dominance of Cyanophyta in this river is similar to findings by Sekadende and co-workers, Ogato, Deng and co-worker, and Shakila and co-worker. The dominance of Cladocera in this river is similar to findings by Ude and co-workers, and Ogbuagu and co-worker.

Strong, low frequency (decadal) environmental fluctuations during the 20th century in the North Pacific Ocean, on the Washington coast, and in Puget Sound

At decadal period (10-20 years), dynamic linkage was evident between atmospheric low pressure systems over the North Pacific Ocean and circulation in a Pacific Northwest fjord (Puget Sound). As the Aleutian low pressure center shifts, storms arriving from the North Pacific Ocean deposit varying amounts of precipitation in the mountains draining into the estuarine system; in turn, the fluctuating addition of fresh water changes the density distribution near the fjord basin entrance sill, thereby constraining the fjord's vertical velocity structure. This linkage was examined using time series of 21 environmental parameters from 1899 to 1987. Covariation in the time series was evident because of the strong decadal cycles compared with long-term averages, interannual variability, and seasonal cycles.