Experiments were conducted in open channel flows under near-critical mobility conditions to quantify local evolution of the sediment bed induced by emergent and submerged rectangular vertical porous walls in yawed conditions. The goal was to design minimally invasive hydraulic structures, or vanes, redirecting sediment mass flux to be coupled with intake pipes in dam bypass systems for sustainable sediment redistribution. Theoretical scaling for the maximum scour depth derived from the phenomenological theory of turbulence was extended to our specific emergent vane geometry under varying structure porosity, angle, and size. Most of our attention was devoted to quantifying the asymmetry of the induced bathymetric effects and developing an appropriate metric to identify the most effective vane configuration to steer sedimentation along a desired direction. The orientation angle was shown to significantly affect the geometry and volume of the scour and deposit, the spanwise shift of the deposit volume, and the velocity profiles in the wake, for both emergent and submerged vane configurations. The optimal streamwise and spanwise spacings in yawed, submerged, porous vane arrays were determined based on the spatial evolution of the wake flow, ensuring minimal sheltering effects and maximal lateral directionality of the sediment deposit. We here provide supporting evidence that wind and marine hydrokinetic turbine wake models can be extended to the case of porous planar structures to capture the key physical mechanisms governing sediment deposits. (C) 2022 American Society of Civil Engineers.

Characterizing wind farm flow fields at high temporal and spatial resolutions is critical prerequisite for the optimal design and operation of utility-scale wind farms and for reducing the levelized cost of energy. However, due to the large disparity of underlying scales, measurements or simulations alone cannot provide high resolution wind fields, which are informed by and account for the effect of both large scale (i.e. hour, day, month and year) and small scale (i.e. second and minute) site-specific variations in the atmosphere. We explore the feasibility of integrating field measurements and high-fidelity large-eddy simulation (LES) to characterize the wind field in a utility-scale wind farm while accounting for flow phenomena across multiple temporal scales. Specifically, we employ field measurements to characterize the monthly wind speed and wind direction distributions and investigate the wind characteristics in turbine wakes. It was found that the probability density function (PDF) of the wind speed in turbine wakes can be reasonably represented using the Weibull distribution but with shape factors smaller than those not in the wake. LES of the wind farm under statistically steady inflow is subsequently carried out for one wind direction. The LES predictions are compared with the measured data conditionally averaged based on the wind speed, wind direction and the root-mean-square of wind speed fluctuations over time intervals of 30 min. Good agreement is obtained for both mean wind speed and turbulence intensity. The present work shows the possibility of integrating field measurements and high-fidelity simulations for improved characterization of the site-specific wind fields in utility-scale wind farms.

One overarching objective of this program of study was the accumulation of objective, scientifically valid information relating to auditory performance of bald and golden eagles that may be used to guide the development of acoustic alerting/deterrence technologies intended to discourage encroachment into wind energy air spaces. To that end, analyses aimed at the characterization of sensitivity to sound in bald and golden eagles, along with findings in the supra-threshold, dynamic frequency spaces related to response latencies and amplitudes, leads us to conclude that bald, and golden eagles navigate the same basic working auditory space, as in other known and thus far characterized members of the diurnal raptor family. Specifically, bald and golden eagles, along with other raptor species within the group, operate in an auditory space characterized by a frequency band at least four octaves wide and centered on 2 kHz, with an upper frequency limit between 6 and 10 kHz at 80 dB SPL and a lower frequency limit that almost certainly extends below 0.2 kHz. Consequently, we recommend that signal designers use these data as a guideline in efforts to design effective and efficient acoustic alerting/deterrent systems. It is important to note that signal energy broadcast outside of this frequency band at moderate levels will not contribute to the efficacy of a deterrent but will add an unnecessary fraction to the overall acoustic pollution budget. The importance of this consideration is heightened by contemporaneous concerns related to the transmission of noise broadcast by wind energy farms. In addition, based on analyses of data acquired from red-tailed hawks using the same experimental paradigm and data acquisition system, we conclude that auditory function in the red-tailed hawk is sufficiently like that observed in bald and golden eagles to permit its use as a surrogate species. Response waveforms, threshold-frequency curves, and input-output characteristics match those of eagles closely. It should be noted however, that differences in sensitivity and slightly extended high-frequency limits of hearing should be taken into account when extrapolating findings from one species to the others. Although the inclusion of behavioral tests of red-tailed hawks to acoustic stimuli was beyond the scope of this investigation, future efforts to assess response parameters like signal-type preference and habituation rate will further elucidate their suitability to serve as eagle surrogates in behavioral studies; nonetheless, the species in question are well matched with respect to basic auditory performance. A second essential objective of this program of study was the acoustic characterization of a subset of calls comprising the vocal repertoires of bald and golden eagles that may be used to supplement auditory performance findings in the effort to guide the development of acoustic alerting signals. With regard to that objective, the vocal repertoires of both bald and golden eagle species are rich and varied. While similar in spectrographic structure, distinctive differences are also clear. Generally, golden eagles produce some calls with shorter durations, and similar “sounding” calls exhibit distinctively different spectrographic patterns than those of bald eagles. Both species produce calls that contain a wide variety of nonlinear elements that operate to enhance the rich and varied nature of commonly observed vocal products. Comparison of the average power spectra of commonly observed bald and golden eagle calls with threshold-frequency curves leads to the conclusion that call energies fall within the frequency bounds of hearing. Further, the acoustic energy of calls considered in this report tend to fall into overlapping, but different frequency ranges of the acoustic sensitivity curve. This condition may encourage signal designers to vary the frequency content of acoustic deterrence signals in the field. Finally, preliminary observations relating to the tendencies and proclivities of bald eagles to attend to the acoustic landscape lead to the conclusion that eagles monitor their immediate sound environment assiduously. Individuals respond to a variety of natural and synthetic sound signals reliably and, perhaps most relevant in the context of the engineering of acoustic alerting/deterrence technologies, habituation to most sounds considered in this effort was minimal. These preliminary results, while calling for extended behavioral testing, are promising and set the stage for the exportation of behavioral studies into real world scenarios.

As part of an initial effort to determine if acoustic signals can be used to discourage eagles from entering wind turbine facility airspaces and thereby reduce morbidity/mortality collision rates, behavioral responses of bald eagles (Haliaeetus leucocephalus) to a battery of both natural and synthetic acoustic stimuli of varying spectral complexities were studied. Each signal was directed randomly to one of two loudspeakers in a sequence of ~10 trials, and the stimulus order was randomized for each bird. A variant of an observer-based psychoacoustic protocol was implemented, and judges were instructed to report the absence or presence of a response, response strength, and other distinctive response attributes. In pilot studies, subjects responded to ~74% of trials across all stimuli. Responsivity was greater to spectrally complex stimuli (~80% vs 59%), and greater responsivity was observed to natural stimuli than to synthetic stimuli (~82% vs 69%). Responsive subjects oriented correctly in the direction of the signal source in ~74% of trials. A significant difference in overall responsivity was not observed across stimulus sets, although habituation was observed across repeated trials when responses to all stimulus types were combined. The relevance of findings in relation to the design of deterrence/alerting protocols will be discussed. [Work supported by DOE Grant No. DE-EE0007881.]

Collision with wind turbines is a conservation concern for eagles with population abundance implications. The development of acoustic alerting technologies to deter eagles from entering hazardous air spaces is a potentially significant mitigation strategy to diminish associated morbidity and mortality risks. As a prelude to the engineering of deterrence technologies, auditory function was assessed in bald eagles (Haliaeetus leucocephalus), as well as in red-tailed hawks (Buteo jamaicensis). Auditory brainstem responses (ABRs) to a comprehensive battery of clicks and tone bursts varying in level and frequency were acquired to evaluate response thresholds, as well as suprathreshold response characteristics of wave I of the ABR, which represents the compound potential of the VIII cranial nerve. Sensitivity curves exhibited an asymmetric convex shape similar to those of other avian species, response latencies decreased exponentially with increasing stimulus level and response amplitudes grew with level in an orderly manner. Both species were responsive to a frequency band at least four octaves wide, with a most sensitive frequency of 2 kHz, and a high-frequency limit of approximately 5.7 kHz in bald eagles and 8 kHz in red-tailed hawks. Findings reported here provide a framework within which acoustic alerting signals might be developed.

This second of three parts of the history of the St. Anthony Falls Laboratory (SAFL) at the University of Minnesota documents the transition to increased emphasis on environmental research. SAFL methodology includes laboratory experimentation and field observations, physical model studies and numerical simulations, and stochastic data analysis. Much of the research at SAFL has been connected to hydraulic structures, renewable energy, protection of the environment, and geophysical fluid dynamics. From its beginning in 1938, SAFL has been an interdisciplinary science and engineering research and educational facility with a strong grounding in fluid mechanics. Starting in the 1960s and 1970s, when awareness and legislation of environmental impacts of human activities grew dramatically, SAFL's research expanded significantly into areas connecting fluid mechanics with the chemistry and biology of aquatic environments, and into geophysical (earth-surface) processes. Environmental research at SAFL began with water resources engineering and riverine sediment transport. After developing and applying techniques of physical model studies for hydraulic structures and high-speed marine propulsion, SAFL researchers developed numerical flow and water quality simulation models for the protection of aquatic environments. Studies on the influence of fluid flow on pollutant transport and the growth and behavior of organisms were initiated. Geophysical processes became a centerpiece of SAFL research with the creation of the National Center for Earthsurface Dynamics (NCED), which got a home at SAFL in 2002. Fluid flow in the human body has been studied in co-operation with medical professionals. Field-scale experimentation was added for environmental and geophysical studies. Sophisticated experimental facilities and data acquisition and simulation tools have been developed by SAFL researchers. Examples of environmental research and design studies that have been conducted at SAFL since its opening in 1938 will be presented in nine major research categories: urban storm water runoff and water quality; environmental transport and mixing; water quality dynamics and modeling; global climate change effects; protection of fish and fish habitat; eco-and bio-fluid mechanics; watershed eco-hydrology and the Outdoor Stream Lab; sediment transport, earth surface dynamics, and the NCED legacy; and innovations in instrumentation and data acquisition. Examples will showcase the evolution and significance of environmental research at SAFL. The outlook for environmental research at SAFL and its connection to renewable energy will be presented in Part 3 of the presentation.

According to the U.S. Fish and Wildlife Service (2018), fatalities associated with wind turbine collisions have been reported for more than 200 bird species. Furthermore, based on statistical models of industry growth it has been suggested that as many as 1.4 million bird fatalities/year could be realized if the Department of Energy (DOE) wind energy goals are achieved; i.e., wind energy supplying 20% of total U.S. energy needs by 2030. Although passerine bird fatalities are most commonly reported, raptors that hunt by day, including bald and golden eagles, are the second most frequent casualties of turbine collisions. To address this concern, deterrence protocols designed to discourage eagles from encroaching into wind energy facility air spaces and thereby constrain the degree of risk to which birds are exposed are under investigation. As part of an effort to guide development of acoustic deterrence protocols, we report that the responsive frequency range of golden eagles is similar to that reported for bald eagles; upper and lower frequency limits of hearing are approximately 6.0 and 0.3 kHz, respectively. Suprathreshold response profiles measured in golden eagles exhibit standard features that will be compared with those of bald eagles. [Work supported by DOE grant #DE-EE0007881.]

Injury and mortality statistics suggest that bald eagles entering the air space of wind energy facilities face considerable risk. To mitigate the hazard, acoustic deterrent systems designed to discourage entry into such hazardous air spaces are under consideration. In this study, the acoustic properties of a collection of call types within the eagle vocal repertoire are reported as a first step in a larger program of study designed to assess the deterrent capacity of the bird’s natural vocal utterances. To that end, calls were recorded from bald eagles housed in the Raptor Center at the University of Minnesota. Based on preliminary acoustic analyses, at least five calls were identified and are referred to here as peal/scream, chatter/cackle, snort, squeal and grunt. With the exception of the low frequency grunt, calls were uniformly high pitched, tonal in nature, exhibited harmonic spectral structure, and they were generally complex, exhibiting distinct nonlinear characteristics. In this presentation, the spectrotemporal properties of each call type will be described with the goal of generating an acoustic repository to enable the consistent classification of calls to be tested for their potential as deterrence signals. [This work was supported by Department of Energy grant #DE-EE0007881.]

Previous data (Nelson et al., ASA 2017) indicated that healthy human adult participants experienced few symptoms from re-created wind turbine sound and infrasound emissions. That report included more than fifty subjects ages 21–73 years who attended to audible and infrasound signals generated from a wind turbine, recorded at 300 meters and re-created in a laboratory. Stimuli consisted of modulated and unmodulated audible turbine sound at 50 dB SPL, as well as natural and peak-enhanced turbine infrasound at an overall level of approximately 85 dB SPL (peaks up to 100 dB SPL). Participants were tested for their postural stability, detection, and ratings of audible and infrasound emissions randomly presented in one-minute exposure intervals in the laboratory. Very few and minor adverse effects had been noted to date, mostly ear fullness or pressure. Healthy participants showed no evidence of any change in postural sway in the presence of infrasound for the group tested. We have recently begun testing of participants who either a) live near turbines and complain of adverse effects, or b) who have symptoms of dizziness/imbalance as reported to their ENT specialist. Results from postural sway, sound quality judgments, and pre- and post-exposure symptoms will be reported from some of these participants. [Work supported by Xcel Energy RDF14.]

Three-dimensional jet flows at high Reynolds (Re) numbers, namely over a million, have a significant importance in hydraulic engineering. Despite their importance, most of the previous investigations have been mainly focused only on jet flows with orders of magnitude lower Re numbers. We present the results of an experimental campaign and a high fidelity large-eddy simulation (LES) to study a jet flow with Re approximate to 1.71 x 10(6) in a large-scale flume. Flow measurements are carried out using a pitot tube apparatus and the Virtual Flow Simulator (VFS-Geophysics) model is employed to simulate the flow field. The measured velocity field of the jet is used to evaluate the LES results. The presented experimental data for the cross-sectional velocity distributions at various distances from the jet source provide an unprecedented dataset for model validation at high Re numbers.

A multi-disciplinary group of researchers at the University of Minnesota Center for Applied and Translational Sensory Science (catss.umn.edu) are designing and pilot testing the perceptual effects of infrasound, in collaboration with Eminent Technologies (rotarywoofer.com). An infrasound generator simulates the acoustic signature of audible sound and infrasound generated by wind turbines in the field. With the support of Xcel Energy, the team of engineers, otologists, hearing scientists, and balance experts are evaluating the effects of infrasound only, acoustic turbine sound, and combined infrasound and acoustic sound. We are testing listeners’ quiet detection, masked detection, discrimination, and rating of signals. Pilot results will be described. Together we hope to test the range of perceptual responses to turbine-generated infrasound and audible sound. [Work supported by Xcel Energy RD4-12 to Jeffrey Marr, St. Anthony Falls Laboratory.]

A field experiment was carried out to study the unsteady behavior of an instrumented full-scale 2.5MW wind turbine under neutral conditions. The analysis focused on the structure of the instantaneous turbine power and strain at its foundation. A meteorological tower located 1.6 rotor diameters upstream of the turbine was used to characterize the turbulent flow. Mean velocity and temperature were steady during the 1h period selected. The results suggest that the turbine power and foundation strain are modulated by atmospheric turbulence in a complex way. The spectral characteristics of both quantities exhibited three distinctive regions. Within the first region, defined by subrotor length scales, the turbine power was insensitive to the flow turbulence. In the intermediate region, with length scales up to those on the order of the atmospheric boundary layer thickness, the spectral contents of the power fluctuations phi(P) and flow phi(U) exhibit a non-linear relationship of the form phi(P)=G(f)phi(U), where G(f)(approximate to)f(-2) is a transfer/damping function. In the third region, dominated by the very large scales of motions, the power fluctuations are found to be directly influenced by the flow. The strain also showed three regions, similar to the power fluctuations. However, it follows the structure of the inertial subrange of the turbulence at subrotor scales. Intermittent gusts were able to induce intermittent behavior on the turbine power. Finally, the flow and power correlation showed that the velocity at the hub height is the best descriptor of the flow turbulence within the rotor area.Copyright (c) 2014 John Wiley & Sons, Ltd.

Key Points This paper summarizes the data collected during StreamLab06 and StreamLab08 exp. Exp. designed to advance res. at the interface of phys./chem./biol. processes All datasets are archived on the Nced data repository

We conducted a series of flume experiments to investigate the response of self-formed gravel-bed channels to floods of varying magnitude and duration. Floods were generated by increasing the discharge into a channel created in sand- and gravel-sized sediment with a median grain size of 2mm. Flooding increased the Shields stress along the channel perimeter, causing bank erosion and rapid channel widening. The sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed, but was rather transported downstream, a process likely facilitated by transient fining of the bed surface. At the end of each experiment, bank sediments were no longer in motion, partial bed load transport characterized the flat-bed portion of the channel, and the Shields stress approached a constant value of 0.056, about 1.2 times the critical Shields stress for incipient motion. Furthermore, the discharge was entirely accommodated by flow within the channel: the creation of a stable channel entirely eliminated overbank flows. We speculate that similar processes may occur in nature, but only where bank sediments are non-cohesive and where channel-narrowing processes cannot counteract bank erosion during overbank flows. We also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates, suggesting that simple methods may be appropriate for predicting width increases in channels with non-cohesive, unvegetated banks, even during overbank flows. Last, we present a model for predicting the equilibrium width and depth of a stable gravel-bed channel with a known channel-forming Shields stress.

[1] In the paper “Experimental evidence for statistical scaling and intermittency in sediment transport rates” by A. Singh et al. (Journal of Geophysical Research, 114, F01025, doi:10.1029/2007JF000963, 2009), we performed a multiscale analysis of bed load sediment transport series collected in a large-scale experimental flume at the St. Anthony Falls Laboratory at the University of Minnesota and quantified a systematic dependence of the statistics of sediment transport rates on the time scale (sampling interval). We characterized this timescale dependence mathematically via the multiscaling formalism and provided a physical interpretation. We also pointed out that our results are consistent with the more limited field observations of Bunte and Abt [2005] in that in both cases the mean bed load sediment transport rate was found to decrease with sampling time at low transport conditions and to increase with sampling time at high transport conditions. [2] The results of our analysis and the comparison between laboratory and field observations remain intact. However, we would like to report two small corrections: one related to the method of sampling of Bunte and Abt (they used bed load traps and not Helley-Smith samplers) and also in the cause of their large transport events (as associated with the passage of several large particles and not with an increased rate resulting from the crests of bedforms). We thank K. Bunte and S. Abt for their careful reading of our paper and for communicating these corrections to us. The following two sentences should be replaced to reflect these corrections: [3] 1. Original version: A similar trend was discovered by Bunte and Abt [2005], who studied the effects of sampling interval on bed load transport rates using Helley-Smith samplers deployed in a mixed gravel-cobble bed stream of a size comparable to ours experiments. [4] Corrected version: A similar trend was discovered by Bunte and Abt [2005], who studied the effects of sampling interval on bed load transport rates using bed load traps deployed in a mixed gravel-cobble bed stream. [5] 2. Original version: Bunte and Abt [2005] attribute the higher-discharge trend to the effect of large but infrequent transport events associated with the crests of bed forms: [6] Corrected version: Bunte and Abt [2005] attribute the higher-discharge trend to the effect of large but infrequent transport events associated with the passage of several large particles: