Theory and practical implementations for wake-free propulsion systems are proposed and proven with computational fluid dynamic modeling. Introduced earlier, the concept of active hydrodynamic metamaterials is advanced by introducing magnetohydrodynamic metamaterials, structures with custom-designed volumetric distribution of Lorentz forces acting on a conducting fluid. Distributions of volume forces leading to wake-free, laminar flows are designed using multivariate optimization. Theoretical indications are presented that such flows can be sustained at arbitrarily high Reynolds numbers. Moreover, it is shown that in the limit Re≫10^{2}, a fixed volume force distribution may lead to a forced laminar flow across a wide range of Re numbers, without the need to reconfigure the force-generating metamaterial. Power requirements for such a device are studied as a function of the fluid conductivity. Implications to the design of distributed propulsion systems underwater and in space are discussed.

Wake reduction is a crucial link in the chain leading to undetectable watercraft. Here, we explore a volumetric approach to controlling the wake in a stationary flow past cylindrical and spherical objects. In this approach, these objects are coupled with rigid, fluid-permeable structures prescribed by a macroscopic design approach where all solid boundaries are parameterized and modeled explicitly. Local, gradient-based optimization is employed which permits topological changes in the manifold describing the composite solid component(s) while still allowing the use of adjoint optimization methods. This formalism works below small Reynolds number (Re) turbulent flow (Re ≈ 100-10,000) when simulated using small Reynolds-averaged Navier-Stokes (RANS) models. The output of this topology optimization yields geometries that can be fabricated immediately using fused deposition modeling (FDM). Our prototypes have been verified in an experimental water tunnel facility, where the use of Particle Image Velocimetry (PIV) described the velocity profile. Comparisons with our computational models show excellent agreement for the spherical shapes and reasonable match for cylindrical shapes, with well-understood sources of error. Two important figures of merit are considered: domain-wide wake (DWW) and maximum local wake (MLW), metrics of the flow field disturbance whose definitions are described.

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It is long understood that many students do not take advantage of faculty assistance outside of class. In an attempt to improve the use of office hours, faculty have made efforts to schedule times that are most convenient to students and are most likely to have high attendance; before homework assignments are due or examinations are being held. Despite these efforts, students rarely take advantage of this support service. As a first attempt to improve student engagement, the number of office hours held by teaching assistants (TAs) was increased, expecting that students would feel more comfortable asking for help from TAs rather than faculty. However, office hour attendance was no better for TAs than for faculty. Yet, exam performance continued to indicate that many students could benefit from help outside the classroom. In an effort to better understand this trend, a survey was conducted to examine reasons why students choose not to attend office hours. In particular, we were looking for the effect of social norms, student’s perception of their understanding of the material and their need for extra help, as well as the use of other resources such as on-line solutions to homework problems and cooperative learning with other students. This survey was conducted in six classes (300 students) comprising our engineering science core curriculum, including: Statics, Mechanics of Materials, Dynamics, Thermodynamics, Fluid Mechanics and Heat Transfer. Results indicated that of all the factors tested, the only ones that positively correlated to low office hour attendance were (1) students felt they understood the material well enough and did not need extra help, (2) students procrastinated and therefore did not have time to seek help before homework was due, and (3) students who spent less overall time studying outside of class attended fewer office hours. The data did not support our initial premise that students who attended more office hours performed better. Further study is warranted to explore behaviors that enhance student performance. It is expected that results from these studies will provide information to improve students’ efficient use of time outside the classroom.

Providing example problems that stimulate curiosity and cultivate intuition among students is a prominent responsibility for physics educators. A concept that can be particularly challenging for students to grasp and educators to convey is the relationship between frictional and normal forces in rolling motion. A system that may be a useful tool to exhibit a rich interplay between these forces is a rolling eccentric disk. An eccentric disk has a non-constant normal force and therefore has four distinct phases of motion: oscillations about a stable equilibrium, roll without slip, roll with slip, and hop. Thus, this paper examines the eccentric disk rolling down an incline as a valuable teaching medium and explores its peculiar behavior. The rolling system is analytically modeled using an augmented Lagrangian formulation, solved with numerical integration, and experimentally realized. Then, the results are presented and discussed in detail.