Shear dispersion from near‐inertial internal Poincaré waves in large lakes

In this work, we study mixed layer lateral dispersion that is enhanced by near-inertial internal Poincaré waves in the offshore region of a large stratified lake, Lake Michigan. We examine the hypothesis that the vertical shear created by near-inertial internal Poincaré waves is not only an energy source for vertical mixing in the thermocline and mixed layer, but also enhances horizontal dispersion via an unsteady shear flow dispersion mechanism. Complex empirical orthogonal function analysis reveals that the dominant shear structure is observed to mirror the thermal structure, with the location of maximum shear gradually lowered as the mixed layer deepens. This changing structure of shear and vertical mixing produces different characteristics in shear flow dispersion between the early and later stratified periods. The estimated depth-averaged surface layer vertical turbulent diffusivity grows from 10-5 m2s-1 to 10-3 m2s-1 over the stratified period, and the associated lateral dispersion coefficients are estimated as 0.1 – 40 m2s-1. The Poincaré waves are found to enhance greatly lateral dispersion for times less than the inertial period following release. In contrast, sub-inertial shear is the dominant mechanism responsible for shear dispersion for times greater than the inertial period. A simple approximation of the dispersion coefficient for lateral dispersion is developed, which scales as the product of surface current velocity (or wind friction velocity) and mixed layer depth. The calculated dispersion coefficients agree well with Okubo’s diffusion diagram for times up to a week, which suggests that unsteady shear dispersion is a plausible mechanism to explain observed dispersion rates in the mixed layer for early times after release.

Something to write home(work) about: An analysis of writing exercises in fluid mechanics textbooks

Something to Write Home(work) About: An Analysis of Writing Exercises in Fluid Mechanics TextbooksAs assessments of learning outcomes are increasingly emphasized through accreditationrequirements (e.g., via ABET) and other quality assurance initiatives, written communication isone area that engineering instructors often find challenging to incorporate and assess. This isparticularly true in large core courses at the sophomore and junior levels. In this project, ananalysis of writing-based problems in fluid mechanics textbooks attempts to locate theavailability of activities that would allow students to practice writing, to learn through writing,and to use writing to relate course content to broader applications and contexts.This study is part of a larger ongoing project to understand and expand the incorporation ofwriting in large-lecture engineering courses, including investigation of faculty perspectives andtextbook assignments and assessments. This snapshot of writing-across-engineering will theninform efforts to create a range of writing activities (and assessment methods), mapped to ABETlearning outcomes, that instructors can incorporate in large engineering classes. Our objective isto examine widely-used textbooks in thermodynamics, materials, circuits, statics, and dynamics.In the preliminary analysis reported in this paper, we study problem sets from five popular fluidmechanics textbooks to find problems requiring more than merely numerical or calculatedanswers. As these writing-based problems are identified, we will categorize them in terms of thetype of prompt they represent, such as asking for explanation of a solution, application of aconcept to real-world examples, or problem-solving that requires description of processes.Once the number and types of writing problems in these textbooks has been analyzed andorganized, we will further explore how well the texts support actually using these exercises inclass. While writing-based prompts might be present, for example, instructors might not havethe resources, expertise, and/or support needed to incorporate them in their classes. Morespecifically, examination of solution sets and instructor’s guides will reveal how much thetextbooks equip instructors to assign and then assess the writing prompts that do exist. Finally,the extant writing assignments will be mapped to ABET learning outcomes to see how promptscan potentially be used to address key learning outcomes, e.g., in relation to the studentoutcomes in ABET Criterion 3.The result of this analysis will be an understanding of how well popular fluid mechanicstextbook assignments guide students in writing, and how well the textbooks equip instructors tomake use of those assignments. We will use this data to identify areas in which more writingassignments and assessment training would be useful for the teaching of engineering. This paperwill likely be of particular interest to faculty and staff interested in using writing to support avariety of technical, professional, and global learning outcomes in core engineering courses.

Species‐specific effects of subdaily temperature fluctuations on consumption, growth and stress responses in two physiologically similar fish species.

Fluctuations in water temperature can have important physiological consequences for fishes. Effects of daily thermal cycles are well studied and can be beneficial, increasing prey consumption and growth rates when mean and maximum temperatures of the fluctuations are at or below the species’ optimum temperature. While less studied, subdaily temperature fluctuations are also common in many aquatic habitats and can be caused by both natural and anthropogenic processes. We performed laboratory experiments to examine how two fish species (yellow perch, Perca flavescens, and walleye, Sander vitreus) with similar thermal preferences respond to chronic exposure to subdaily temperature variability. We selected temperature treatments that reflected observed thermal variation after examining water temperature data from multiple aquatic systems. We then separately exposed yellow perch and walleye to a stable 23 °C treatment and 12-h cycles of 23 ± 2 °C or 23 ± 4 °C for 45 days. Adult yellow perch exposed to fluctuations of 23 ± 4 °C over 12 h expressed higher consumption, growth and food conversion efficiency than fish experiencing stable 23 °C. Temperature fluctuations, though, resulted in mortalities and the development of skin ulcers in yellow perch that did not occur under stable temperatures. In contrast, the same 12-h temperature fluctuations did not result in mortalities or stress responses in juvenile walleye. Moreover, unlike yellow perch, growth rates of walleye were lower under 12-h temperature fluctuations compared with the stable 23 °C treatment. Our results indicate that species with similar thermal preferences can respond differently to the same subdaily temperature fluctuations.

Diets and growth potential of early stage larval yellow perch and alewife in a nearshore region of southeastern Lake Michigan

Transition from endogenous to exogenous feeding is thought to be a critical period for many fish larvae, when prey availability (type, size, and density) and ambient physical conditions (e.g., temperature, water clarity) can strongly influence survival. In Lake Michigan, two important fish species, yellow perch (Perca flavescens) and alewife (Alosa pseudoharengus), hatch and, presumably, begin exogenously feeding in the nearshore zone, an area characterized by short-term variation in environmental conditions. During 2010-2011, we examined environmental conditions and spatial and temporal distributions of larval yellow perch, larval alewife, and their potential prey in a nearshore region of southeastern Lake Michigan. To consider implications of environmental conditions on larval fish habitat quality, we quantified diet contents of young larval yellow perch and alewife and modeled bioenergetic growth rate potential (an index of habitat quality) under observed and predicted prey consumption scenarios. As expected, in this dynamic nearshore zone temperatures, light levels, zooplankton prey availability, and resulting growth rate potential were highly variable. Many larval fish digestive tracts were empty, suggesting that starvation may affect cohort survival. Among early-feeding larval fish, relatively small diet items were common, with larval alewives consuming diatoms and larval yellow perch consuming veligers of invasive dreissenid mussels. Though the mechanisms underlying such prey consumption and the consequences of ingesting these prey items remain largely unexplored, our results suggest dreissenid mussel veligers present early-feeding larvae with a relatively abundant prey source that may partially offset the apparent low consumption of other prey sources within Lake Michigan’s nearshore region.

Thermal habitat quality of aquatic organisms near power plant discharges: potential exacerbating effects of climate warming

Water temperature strongly affects aquatic ectotherms, as even slight temperature changes can have dramatic effects on physiological rates. Water bodies receiving industrial thermal discharges can undergo dramatic spatial and temporal changes in water temperature. To quantify effects on aquatic ectotherms, thermal habitat quality (bioenergetic growth rate potential; GRP) for zebra mussel, Dreissena polymorpha (Pallas), rusty crayfish, Orconectes rusticus (Girard), walleye, Sander vitreus (Mitchill) and smallmouth bass, Micropterus dolomieu (Lacepède) was estimated near two power plant thermal discharges on the Ohio River, USA, from 2010 to 2012 using bioenergetics models. These results were then compared with GRP under increased base temperatures representing climate warming. Growth rate potential for all species was low near the discharges during summer and highest in winter, with increasing prey consumption minimising the negative effects of increased temperatures. In their immediate vicinity, thermal discharges had a more adverse effect on GRP than plausible climate warming but primarily affected GRP over a small spatial area, particularly within 400 m downstream from the power plants. Examining thermal habitat suitability will become increasingly important as rising energy demand and climate change collectively affect aquatic organisms and their habitats.

Writing to learn engineering: Identifying effective fechniques for the integration of written communication into engineering classes and curricula (NSF RIGEE project)

The inclusion of writing-based exercises in technical courses has multiple learning benefits to students. Writing exercises not only serve to improve students’ written communication skills (i.e., “learn to write”), but can also be leveraged to develop critical thinking skills and promote deeper understanding of technical concepts (i.e., “write to learn”). Nevertheless, while writing-intensive assignments are relatively common in upper-level technical courses, especially in the form of laboratory and project reports, writing is often absent in the larger, required core courses that are taken by large numbers of engineering students. This is a missed opportunity to both enhance student learning of technical content as well as missed chance for students to have more writing practice. This NSF RIGEE project aims to investigate, support, and promote the inclusion of writing in technical courses, particularly introductory and core courses. Analysis of an engineering instructor survey carried out as part of the project revealed concerns about assessment and feedback on students’ written work. Additionally, writing instructors were interested in the creation of guides designed to aid instructors in the creation and tailoring of writing prompts for use in their existing technical courses. This paper introduces preliminary resources we have created in response to these stated needs, in order to help instructors develop, implement, and assess writing assignments in their courses. Current resources include a decision tree to help instructors create writing assignments within their classrooms and assessment rubrics that can easily be adapted to specific writing assignment needs. Resources will continue to be developed during the remainder of the project, culminating in a writing website geared towards instructors.

Adventures in paragraph writing: The development and refinement of scalable and effective writing exercises for large-enrollment engineering courses

Adventures in paragraph writing: the development and refinement of scalable and effective writing exercises for large enrollment engineering coursesThe ability to communicate effectively is a highly desirable attribute for today’s graduatingengineers. Additionally, the inclusion of communication components in technical courses hasbeen shown to enhance learning of technical content and can be leveraged to satisfy non-technical learning outcomes. However, the incorporation of such components in undergraduateengineering curricula remains challenging due to resource limitations, credit hour crunches, andother issues. This paper presents the design considerations and preliminary results from ourongoing work to create an effective, transferrable, low-overhead approach to paragraph writingexercises suitable for inclusion in any large engineering course. Key considerations in thedevelopment of these exercises include: identification of the motivations and learning outcomesfor each exercise; development and tailoring of writing prompts (questions) appropriate for theseoutcomes; and the development and implementation of an assessment and feedback strategy,including resource-efficient grading rubrics and techniques.Results are reported from the application of the paragraph writing exercise in a large civilengineering undergraduate fluid mechanics course (120 students; approximately 15assignments). A primary focus of this first application centered on two key components thatmust be refined in order for the exercise to be effective and transferrable: (1) the selection ofwriting prompts, and (2) assessment and feedback. Analysis of student paragraphs highlights theimportance of the writing prompts in the success of the exercise, indicating that specific wordchoice, question focus, and supplemental instruction greatly affected the level of writing studentssubmitted. Some writing prompts were selected to address and enhance technical content in thecourse, while other writing prompts were developed to broaden student awareness of engineeringin societal, environmental, and global contexts. In addition to developing productive writingprompts, the assessment and feedback strategies were evaluated using student surveys andfeedback. While minimal marking and holistic rubric assessment methods proved effective froma grading resource standpoint, students were frustrated by the lack of feedback associated withthese techniques and uncomfortable with the holistic grading rubric. Data from student surveyspoint to the importance of giving meaningful feedback to students, and providing them withopportunities to revise their written submissions. Student surveys also highlighted an unforeseenobstacle to the exercise: student resistance to writing in technical courses. We provide severalsuggestions for overcoming student resistance, as well as improved assessment and feedbackstrategies that better meet student needs while still not over-burdening instructors and teachingassistants.

Spatial structure of internal Poincaré waves in Lake Michigan

In this paper we examine the characteristics of near-inertial internal Poincaré waves in Lake Michigan (USA) as discerned from field experiments and hydrodynamic simulations. The focus is on the determination of the lateral and vertical structure of the waves. Observations of near-inertial internal wave properties are presented from two field experiments in southern Lake Michigan conducted during the years 2009 and 2010 at Michigan City (IN, USA) and Muskegon (MI, USA), respectively. Spectra of thermocline displacements and baroclinic velocities show that kinetic and potential baroclinic energy is dominated by near-inertial internal Poincaré waves. Vertical structure discerned from empirical orthogonal function analysis shows that this energy is predominantly vertical mode 1. Idealized hydrodynamic simulations using stratifications from early summer (June), mid-summer (July) and fall (September) identify the basin-scale internal Poincaré wave structure as a combination of single- and two-basin cells, similar to those identified in Lake Erie by Schwab, with near-surface velocities largest in the center of the northern and southern basins. Near-inertial bottom kinetic energy is seen to have roughly constant magnitude over large swathes across the basin, with higher magnitude in the shallower areas like the Mid-lake Plateau, as compared with the deep northern and southern basins. The near-bottom near-inertial kinetic energy when mapped appears similar to the bottom topography map. The wave-induced vertical shear across thermocline is concentrated along the longitudinal axis of the lake basin, and both near-bottom velocities and thermocline shear are reasonably explained by a simple conceptual model of the expected transverse variability.

A Year of Internal Poincaré Waves in Southern Lake Michigan

A unique set of full year, deep water observations from the middle of Lake Michigan’s southern basin are analyzed to quantify the seasonal variability of the dominant near-inertial internal Poincaré wave. At this mid-lake location, the Poincaré wave is seen to describe more than 80% of the observed surface current variability for much of the year, with characteristic near-inertial frequency and clockwise-rotating velocities. The dominance of the near-inertial seiche on the flow decreases with depth. The wave persists during the “stratified period,” roughly May through late December, and is supported by as few as 1–2 degrees of thermal stratification over 150 m; only after complete water column mixing does the wave go dormant for January through April. The strongest Poincaré wave activity is seen to correspond to the period of strongest summer thermal stratification (August), in spite of the relatively weak winds at this time. A simple inertial slab model optimized with linear friction is shown to capture the seasonal variability of the near-inertial energy at this location reasonably well. The vertical structure of the wave shows good agreement with that calculated with a standard normal modes formulation, which is in turn used to characterize the potential shear and mixing caused by the wave. Late-spring and summer events of elevated Poincaré wave activity are shown to generate sufficiently strong shear with persistent periods of sub-1 Richardson numbers within the thermocline, suggesting that the near-inertial seiche is likely generating thermocline instabilities in the lake’s interior.

Cross-shelf thermal structure in Lake Michigan during the stratified periods

Results from a field experiment in southern Lake Michigan are used to quantify the cross-shelf nearshore variability in Great Lakes temperatures during the stratified season. The experiment was conducted along the Indiana coast of southern Lake Michigan, with temperature and velocity moorings arranged in a cross-shelf transect that extended to approximately 20 km from shore (40 m depth). The field site is noteworthy because of its location at the end of a major axis of an elliptical Great Lake, the relatively mild bathymetric slope, and local shoreline orientation that is perpendicular relative to the dominant summer winds. Measurements demonstrate that the location of the thermocline-bottom intersection is highly variable, causing a wide zone of extreme thermal variability in the nearshore region with time scales of variability ranging from hours to months. Near-inertial internal Poincaré waves are shown to cause large thermocline excursions but primarily only during periods of elevated activity. Several full upwelling events were observed, but in general, they were brief, lasting only 1–2 days, and had very limited spatial extent (2.5 km or less). Nonetheless, the offshore extent of the upwelling front was shown to be reasonably estimated with a simple estimate of the cross-shelf transport caused by alongshore wind events. A persistent feature that determined the zone of elevated thermal variability (the thermocline-shelf intersection point) was the strongly tilted thermocline, which resulted in the thermocline being located very close to shore. No evidence was found to support the hypothesis that internal Kelvin waves affect thermal variability at the study location.