Brenden P. Epps

Unified Rotor Lifting Line Theory

A unified lifting line method for the design and analysis of axial flow propellers and turbines is presented. The method incorporates significant improvements to the classical lifting line methods for propeller design to extend the method to the design of turbines. In addition, lifting line analysis methods are developed to extend the usefulness of the lifting line model to allow generation of performance curves for off-design analysis. The result is a fast computational methodology for the design and analysis of propellers or turbines that can be used in preliminary design and parametric studies. Design and analysis validation cases are presented and compared with experimental data.

A Method for Propeller Blade Optimization and Cavitation Inception Mitigation

Propeller blade design for fast ships is often driven by cavitation constraints. A tradeoff exists, in which larger chord lengths and section thicknesses typically improve cavitation performance but result in lower efficiency. Typically, chord lengths are optimized for the design condition (ship endurance speed) with some specified margin to prevent cavitation off-design (at maximum ship speed). Cavitation performance at the maximum speed is considered postfacto, and blade shape often needs to be modified for cavitation considerations in high-speed operation. This article presents an improved method for blade shape optimization. The present method simultaneously considers the cavitation performance at the endurance speed design point and a maximum speed off-design point, and blade chord lengths and thicknesses are set to prevent cavitation at both operational conditions. During the present design optimization routine, the on-design load distribution is optimized, and the off-design performance is determined such that the chord lengths can be set to a minimum that still prevents cavitation at both the on- and off-design conditions. A case study is presented, considering the notional design of a propeller for the U.S. Navy DDG51 destroyer-class ship. Propellers designed using standard chord/thickness optimization procedures are compared with those designed using the present procedures. Cavitation performance is compared for the two design methods.

Design and Analysis of Trochoidal Propulsors Using Nonlinear Programming Optimization Techniques

Flapping foil propulsors may increase the propulsive efficiency of large shipping vessels. This paper presents the design of a notional propulsor for an inland waterway vessel. Calculations for the trochoidal propulsor are performed using a lumped, two-dimensional model, which includes the influence of the free vortex wake, as well as approximations to account for three-dimensional effects. The numerical model is used to optimize the foil pitch function in order to achieve the highest efficiency for given geometric and operational parameters. Foil-to-foil interactions are studied for multiple-foil propulsors to determine effects of blade number on hydrodynamic efficiency. Considerations of packaging options for a trochoidal propulsor are explored. The results presented herein can also be applied to large- and mid-sized ships, such as international shipping vessels, fishing vessels, cruise liners, and military ships.Copyright

Vortex Sheet Strength in the Sears, Küssner, Theodorsen, and Wagner Aerodynamics Problems

The seminal aerodynamics literature provides analytic predictions of the loads due to sinusoidal gusts (Sears and von Karman), sharp-edged transverse gusts (Kussner), sinusoidal motions (Theodorsen...

On the Rotor Lifting Line Wake Model

This article comments on the wake model used in moderately-loaded rotor lifting line theory for the preliminary design of propellers and horizontal-axis turbines. Mathematical analysis of the classical wake model reveals an inconsistency between the induced velocities numerically computed by the model versus those theoretically predicted by the model. An improved wake model is presented, which better agrees with theory than previous models and thus improves the numerical consistency and robustness of rotor lifting line design algorithms. The present wake model analytically relates the pitch of the trailing vortices to the pitch of the total inflow computed at the lifting line control points. For conciseness, the article focuses on the propeller case, although both propeller and horizontal-axis turbine examples are presented.

Next-Generation Hydrokinetic Power Take-Off via a Novel Variable-Stroke Hydraulic System

Hydrokinetic power generation has the potential to supply nearly ten percent of the United States annual energy demand. However, the hydrokinetic generation has lagged behind other renewable energy technologies, and many engineering challenges remain. Here, we consider the impacts of using a hydraulic power transfer system for hydrokinetic power generation. The incorporation of hydraulic power transfer into hydrokinetic systems has the potential to increase durability, reduce required maintenance, and increase power-to-weight ratio, all of which would lower the overall levelized cost of energy (LCOE). In the proposed system, patented low friction, variable-stroke hydraulic pump and motor pairs would allow energy to be harvested efficiently throughout the full range of water velocities in either tidal or riverine flows and with any type of rotary prime mover. A full system characterization is provided, along with a calculation of expected LCOE and a considered analysis of the applicability of hydraulic PTO systems as a way to advance commercial hydrokinetic power generation.Copyright

Rheological properties of corn stover slurries during fermentation by Clostridium thermocellum

BackgroundMilling 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.ResultsFermented 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.ConclusionsWhereas 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.

On the Interfoil 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.