Clint L. Dancey

The role of impulse on the initiation of particle movement under turbulent flow conditions.

Fundamental to our understanding of erosional and transport phenomena in earth-surface dynamics and engineering is knowledge of the conditions under which sediment motion will begin when subjected to turbulent flow. The onset criterion currently in use emphasizes the time-averaged boundary shear stress and therefore is incapable of accounting for the fluctuating forces encountered in turbulent flows. We have validated through laboratory experiments and analytical formulation of the problem a criterion based upon the impulse imparted to a sediment grain. We demonstrate that in addition to the magnitude of the instantaneous turbulent forces applied on a sediment grain, the duration of these turbulent forces is also important in determining the sediment grains threshold of motion, and that their product, or impulse, is better suited for specifying such conditions.

Role of instantaneous force magnitude and duration on particle entrainment

[1] A new criterion for the onset of entrainment of coarse sediment grains is presented here. It is hypothesized that not only the magnitude, but also the duration of energetic near bed turbulent events is relevant in predicting grain removal from the bed surface. It is therefore proposed that the product of force and its duration, or impulse, is a more appropriate and universal criterion for identifying conditions for particle dislodgement. This conjecture is investigated utilizing two theoretical models, representative of two modes of entrainment: saltation and rolling. In these models, instantaneous, highly fluctuating turbulent forces are simulated as short‐lived pulses of characteristic magnitude and duration, which transfer adequate fluid momentum to the particle, to trigger its entrainment. The analytical solution of the respective equations of motion is employed in deriving representations of threshold conditions in terms of the impulse characteristics. It is shown that hydrodynamic forces of sufficiently high magnitude are capable of entraining a particle only when they last long enough so that their impulse exceeds a critical value. To illustrate further the validity of the critical impulse concept, as well as extend and generalize its application to different entrainment levels of an individual grain, a novel experimental setup is utilized. This setup facilitates observations of angular displacement of a steel mobile particle in air due to electromagnetic pulses of different magnitude and duration. The experimentally obtained conditions for partial or complete entrainment support the concept of a critical impulse.

Our First Online Offering of Introduction to Thermal-Fluid Engineering

In this paper the researchers reflect on the use of various communication technologies from the first online offering of our introductory thermodynamics course. The asynchronous (i.e., forums) and synchronous communication technologies such as Centra™ were employed for instruction and explication of useful feedback and self-explanation to promote students’ collaboration. The instructors outline the types of questions used in these guided activities that challenged students to search for multiple ways to demonstrate their conceptual understanding of very fundamental physical notions. The discussion in this paper outlines ways to improve forum questions, instructor’ feedback, and the frequency of the feedback to improve students’ metacognitive strategies in learning and the application of the course material. The observational data are also examined to note if there were any differences in forum contributions online versus students’ contributions in a face-to-face class. This paper provides a platform for research about learning and evaluation of instruction in abstract engineering courses in an online environment. The study is significant and of interest to faculty and administrators who have taught courses in traditional classrooms and who are now considering online teaching to increase access to engineering education.© 2011 ASME