An impulse framework for hydrodynamic force analysis : fish propulsion, water entry of spheres, and marine propellers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.This thesis presents an impulse framework for analyzing the hydrodynamic forces on bodies in flow. This general theoretical framework is widely applicable, and it is used to address the hydrodynamics of fish propulsion, water entry of spheres, and the offdesign performance of marine propellers. These seemingly-unrelated physics problems share a key common thread: The forces on these fish, spheres, and propellers can be modeled as the sum of the reaction to the rate of change of (1) the pressure impulse required to set up the potential flow about the body, and (2) the vortex impulse required to create the vortical structures in the wake of the body. Fish generate propulsive forces by creating and manipulating large-scale vortical structures using their body and tail. High-speed particle image velocimetry experiments show that a fish generates two vortex rings during a C-turn maneuver and that the change in momentum of the fish balances the change in pressure impulse plus the vortex impulse of these rings. When a sphere plunges into a basin of water and creates a sub-surface air cavity in place of a vortical wake, the vortex impulse is zero, and the force on the sphere is given by the pressure impulse component. Using data from high-speed imaging experiments, a semi-empirical numerical simulation is developed herein; this numerical model shows how the presence of the cavity alters the unsteady pressure force on the sphere and modulates the dynamics of the impact event. During steady propeller operation, the pressure impulse is constant, and the loads on the propeller are given by the vortex impulse component. To analyze these loads, a computational design and analysis tool is presented; this code suite is based on propeller lifting line theory, which is shown to be a special case of the general impulse framework of this thesis. A marine propeller is designed, built, and tested over a range of off-design operating conditions. Experimental results match the predicted performance curve for this propeller, which provides important validation data for the numerical method presented herein. 3 Bringing this thesis full circle, the unsteady startup of the propellor is addressed, which is analogous to the impulsive maneuvering of the swimming fish. As in the fish maneuvering problem, the propellor generates a ring-like vortical wake, and it is shown herein how the vortex impulse of these rings provides thrust for the propellor. With the perspective of the impulse framework developed in this thesis, the results of these tandem experimental investigations and numerical simulations provide deeper insight into classical fluid-dynamics theory and modern experimental hydrodynamics.by Brenden P. Epps.Ph.D

Design of spacecraft for exploration of the Moon and Mars

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 123-125).In this thesis, I develop the conceptual design of the spacecraft required for human-Lunar and human-Mars exploration. The requirements for these vehicles are derived in the context of the NASA Concept Exploration & Refinement project. Similarly, the concepts generated are intended to operate within the transportation architecture developed during this project. Therefore, this thesis serves as a vehicle-level design exercise. Four vehicle architecture options are synthesized by combining system concepts in a logical fashion. These four options are evaluated on several performance criteria, and one vehicle design concept is selected for detailed modeling. In addition, I investigate the conceptual design of the airlock system, as a system-level design exercise. This research project culminated in a set of vehicle concept designs and design recommendations for NASA.by Brenden P. Epps.S.M

Propeller blade stress estimates using lifting line theory

OpenProp, an open-source computational tool for the design and analysis of propellers and horizontal-axis turbines, is extended to provide estimates of normal stresses in the blades for both on- and off-design operating conditions. The numerical model is based on propeller lifting theory, and the present implementation of the code includes an analysis capability to estimate the off-design performance of the propeller or turbine and to make blade stress predictions. As an example, we present the design and performance of a two-bladed propeller. Experimental measurements of the propeller performance over a wide range of off-design operating conditions agree with performance predictions. Estimates of the blade stress are given for on-design and off-design operating states of the propeller.United States. Office of Naval Research (N000140810080)United States. National Oceanic and Atmospheric Administration (NOAA NSG NA060AR4170019)Project Ocean (Robert Damus

On the inter-foil spacing and phase lag of tandem flapping foil propulsors

The aim of this article is to provide a theoretical basis upon which to advance and deploy novel tandem flapping foil systems for efficient marine propulsion. We put forth three key insights into tandem flapping foil hydrodynamics related to their choreography, propulsive efficiency, and unsteady loading. In particular, we propose that the performance of the aft foil depends on a new nondimensional number, s/Utau, which is the inter-foil separation s normalized by the distance that the freestream U advects in one flapping period tau. Additionally, we show how unsteady loading can be mitigated through choice of phase lag

Unsteady forces on spheres during free-surface water entry

We present a study of the forces during free-surface water entry of spheres of varying masses, diameters, and surface treatments. Previous studies have shown that the formation of a subsurface air cavity by a falling sphere is conditional upon impact speed and surface treatment. This study focuses on the forces experienced by the sphere in both cavity-forming and non-cavity-forming cases. Unsteady force estimates require accurate determination of the deceleration for both high and low mass ratios, especially as inertial and hydrodynamic effects approach equality. Using high-speed imaging, high-speed particle image velocimetry, and numerical simulation, we examine the nature of the forces in each case. The effect of mass ratio is shown, where a lighter sphere undergoes larger decelerations and more dramatic trajectory changes. In the non-cavity-forming cases, the forces are modulated by the growth and shedding of a strong, ring-like vortex structure. In the cavity-forming cases, little vorticity is shed by the sphere, and the forces are modulated by the unsteady pressure required for the opening and closing of the air cavity. A data-driven boundary-element-type method is developed to accurately describe the unsteady forces using cavity shape data from experiments.United States. Office of Naval Research (Laboratory Initiative Grant N00014-06-1-0445

Rheological properties of corn stover slurries during fermentation by Clostridium thermocellum

Abstract Background Milling during fermentation, termed cotreatment, has recently been proposed as an alternative to thermochemical pretreatment as a means to increase the accessibility of lignocellulosic biomass to biological attack. A central premise of this approach is that partial solubilization of biomass changes the slurry’s physical properties such that milling becomes more impactful and more feasible. A key uncertainty is the energy required to mill partially fermented biomass. To inform both of these issues, we report rheological characterization of small-particle, corn stover slurries undergoing fermentation by Clostridium thermocellum. Results Fermented and unfermented corn stover slurries were found to be shear-thinning and well described by a power law model with an exponent of 0.10. Plastic viscosity of a slurry, initially at 16 wt.% insoluble solids, decreased as a result of fermentation by a factor of 2000, with the first eightfold reduction occurring in the first 10% of carbohydrate conversion. Large amplitude oscillatory shear experiments revealed only minor changes to the slurry’s rheological fingerprint as a result of fermentation, with the notable change being a reduction in the critical strain amplitude needed for the onset of nonlinearity. All slurries were found to be elastoviscoplastic, with the elastic/viscous crossover at roughly 100% strain amplitude. Conclusions Whereas prior biomass rheology studies have involved pretreated feedstocks and solubilization mediated by fungal cellulase, we report results for feedstocks with no pretreatment other than autoclaving and for solubilization mediated by C. thermocellum. As observed in prior studies, C. thermocellum fermentation results in a dramatic decrease in viscosity. The magnitude of this decrease, however, is much larger starting with unpretreated feedstock than previously reported for pretreated feedstocks. LAOS measurements provide a detailed picture of the rheological fingerprint of the material. Viscosity measurements confirm the hypothesis that the physical character of corn stover slurries changes dramatically during fermentation by C. thermocellum, and indicate that the energy expended on overcoming slurry viscosity will be far less for partially fermented corn stover than for unfermented corn stover