Jim Thomson

Measurements of Turbulence at Two Tidal Energy Sites in Puget Sound, WA

Field measurements of turbulence are presented from two sites in Puget Sound, WA, that are proposed for electrical power generation using tidal current turbines. Time series data from multiple acoustic Doppler instruments are analyzed to obtain statistical measures of fluctuations in both the magnitude and direction of the tidal currents. The resulting turbulence intensities (i.e., the turbulent velocity fluctuations normalized by the deterministic tidal currents) are typically 10% at the hub heights (i.e., the relevant depth) of the proposed turbines. Length and time scales of the turbulence are also analyzed. Large-scale, anisotropic eddies dominate the turbulent kinetic energy (TKE) spectra, which may be the result of proximity to headlands at each site. At small scales, an isotropic turbulent cascade is observed and used to estimate the dissipation rate of TKE, which is shown to balance with shear production. Data quality and sampling parameters are discussed, with an emphasis on the removal of Doppler noise from turbulence statistics. The results are relevant to estimating the performance and fatigue of tidal turbines.

Swell and sea in the emerging Arctic Ocean

Ocean surface waves (sea and swell) are generated by winds blowing over a distance (fetch) for a duration of time. In the Arctic Ocean, fetch varies seasonally from essentially zero in winter to hundreds of kilometers in recent summers. Using in situ observations of waves in the central Beaufort Sea, combined with a numerical wave model and satellite sea ice observations, we show that wave energy scales with fetch throughout the seasonal ice cycle. Furthermore, we show that the increased open water of 2012 allowed waves to develop beyond pure wind seas and evolve into swells. The swells remain tied to the available fetch, however, because fetch is a proxy for the basin size in which the wave evolution occurs. Thus, both sea and swell depend on the open water fetch in the Arctic, because the swell is regionally driven. This suggests that further reductions in seasonal ice cover in the future will result in larger waves, which in turn provide a mechanism to break up sea ice and accelerate ice retreat.

Wave Breaking Dissipation Observed with “SWIFT” Drifters

AbstractEnergy dissipation rates during ocean wave breaking are estimated from high-resolution profiles of turbulent velocities collected within 1 m of the surface. The velocity profiles are obtained from a pulse-coherent acoustic Doppler sonar on a wave-following platform, termed a Surface Wave Instrument Float with Tracking (SWIFT), and the dissipation rates are estimated from the structure function of the velocity profiles. The purpose of the SWIFT is to maintain a constant range to the time-varying surface and thereby observe the turbulence in breaking crests (i.e., above the mean still water level). The Lagrangian quality is also useful to prefilter wave orbital motions and mean currents from the velocity measurements, which are limited in magnitude by phase wrapping in the coherent Doppler processing. Field testing and examples from both offshore whitecaps and nearshore surf breaking are presented. Dissipation rates are elevated (up to 10−3 m2 s−3) during strong breaking conditions, which are confir...

Tidal energy resource characterization: methodology and field study in Admiralty Inlet, Puget Sound, WA (USA):

Tidal energy resource characteristics are presented from a multi-year field study in northern Admiralty Inlet, Puget Sound, WA (USA). Measurements were conducted as part of a broader effort to characterize the physical and biological environment at this location ahead of a proposed tidal energy project. The resource is conceptually partitioned into deterministic, meteorological, and turbulent components. Metrics with implications for device performance are used to describe spatial variations in the tidal resource. The performance differences between passive and fixed yaw turbines are evaluated at these locations. Results show operationally significant variations in the tidal resource over length scales less than 100 m, likely driven by large eddies shed from a nearby headland. Finite-record length observations of tidal currents are shown to be acceptable for estimating device performance, but unsuitable for direct investigation of design loads.

Quantifying turbulence for tidal power applications

Using newly collected data from a tidal power site in Puget Sound, WA, metrics for turbulence quantification are assessed and discussed. Of particular interest is the robustness of the “turbulent intensity,” defined as the ratio of velocity standard deviation to velocity mean. Simultaneously, the quality of raw ping Acoustic Doppler Current Profiler (ADCP) data for turbulence studies is evaluated against Acoustic Doppler Velocimeter (ADV) data at a point. Removal of Doppler noise from the raw ping data is shown to be a crucial step in turbulence quantification. Excluding periods of slack tide, the corrected turbulent intensity estimates at a height of 4.6 m above the seabed are 10% and 11% from the ADCP and ADV, respectively. Estimates of the turbulent dissipation rate are more variable, from 10-3 to 10-1 W/m3. An example analysis of coherent Turbulent Kinetic Energy (TKE) is presented.

A Fourier-Based Method for the Distribution of Breaking Crests from Video Observations

A Fourier-based method is presented to process video observations of water waves and calculate the speed distribution of breaking crest lengths. The method has increased efficiency and robust statistics compared with conventional algorithms that assemble distributions from tracking individual crests in the time domain. The method is tested using field observations (video images of whitecaps) of fetch-limited breaking waves during case studies with low (6.7 m s 21 ), moderate (8.5 m s 21 ), and high (12.6 m s 21 ) wind speeds. The method gives distributions consistent with conventional algorithms, including breaking rates that are consistent with direct observations. Results are applied to obtain remote estimates of the energy dissipation associated with wave breaking.

A cost-effective RNA sequencing protocol for large-scale gene expression studies

RNA sequencing has increasingly become an indispensable tool for biological research. While sequencing costs have fallen dramatically in recent years, the current cost of RNA sequencing, nonetheless, remains a barrier to even more widespread adoption. Here, we present a simple RNA sequencing protocol with substantially reduced costs. This protocol uses as little as 10 ng of total RNA, allows multiplex sequencing of up to 96 samples per lane, and is strand specific. Extensive validation using human embryonic stem cells showed high consistency between technical replicates at various multiplexing levels.

A vessel noise budget for Admiralty Inlet, Puget Sound, Washington (USA)

One calendar year of Automatic Identification System (AIS) ship-traffic data was paired with hydrophone recordings to assess ambient noise in northern Admiralty Inlet, Puget Sound, WA (USA) and to quantify the contribution of vessel traffic. The study region included inland waters of the Salish Sea within a 20 km radius of the hydrophone deployment site. Spectra and hourly, daily, and monthly ambient noise statistics for unweighted broadband (0.02-30 kHz) and marine mammal, or M-weighted, sound pressure levels showed variability driven largely by vessel traffic. Over the calendar year, 1363 unique AIS transmitting vessels were recorded, with at least one AIS transmitting vessel present in the study area 90% of the time. A vessel noise budget was calculated for all vessels equipped with AIS transponders. Cargo ships were the largest contributor to the vessel noise budget, followed by tugs and passenger vessels. A simple model to predict received levels at the site based on an incoherent summation of noise from different vessels resulted in a cumulative probability density function of broadband sound pressure levels that shows good agreement with 85% of the temporal data.

Wave Breaking Dissipation in a Young Wind Sea

Coupled in situ and remote sensing measurements of young, strongly forced wind waves are applied to assess the role of breaking in an evolving wave field. In situ measurements of turbulent energy dissipation from wave-following Surface Wave Instrument Float with Tracking (SWIFT) drifters and a tethered acoustic Doppler sonar system are consistent with wave evolution and wind input (as estimated using the radiative transfer equation). The Phillips breaking crest distribution L(c) is calculated using stabilized shipboard video recordings andtheFourier-basedmethodofThomsonandJessup,with minor modifications. TheresultingL(c)areunimodal distributionscenteredaroundhalfofthephasespeedofthedominantwaves,consistentwithseveralrecentstudies. Breaking rates from L(c) increase with slope, similar to in situ dissipation. However, comparison of the breaking rateestimatesfromtheshipboardvideorecordingswiththeSWIFTvideorecordingsshowthatthebreakingrateis likely underestimated in the shipboard video when wave conditions are calmer and breaking crests are small. The breaking strength parameter b is calculated by comparison of the fifth moment of L(c) with the measured dissipation rates. Neglecting recordings with inconsistent breaking rates, the resulting bdata do not display any clear trends and are in the range of other reported values. The L(c) distributions are compared with the Phillips equilibrium range prediction and previous laboratory and field studies, leading to the identification of several inconsistencies.

Limits to the predictability of tidal current energy

The predictability of tidal currents in the context of hydrokinetic power generation are assessed using current data from a series of surveys in Admiralty Inlet, Puget Sound, Washington, USA. Both current speed and kinetic power density are shown to be well-described by harmonic analysis. Three challenges to predictability are identified. First, non-sinusoidal fluctuations over time scales on the order of hours are observed but cannot be replicated by conventional harmonic analysis. Second, turbulent fluctuations over time scales on the order of seconds are relatively large and inherently unpredictable. Third, for this site, predictions may not be extrapolated more than 100 m from the location of measurement. While none of these issues are insurmountable, they contribute to a degree of unpredictability for tidal hydrokinetic power.