Robert W. Whittlesey

abstract:

Dytiscus marginalis simultaneously uses its hind legs to propel itself through the water. Previous work has suggested that use of synchronized leg motions, such as that used by D. marginalis, allows it to swim with higher hydrodynamic efficiency than similarly sized insects that alternate its legs during swimming. A model is developed based on the generation of vortices in the wake to calculate the relative efficiency of synchronized-leg-swimming kinematics compared to alternating-leg-swimming kinematics. The model agrees well with measured values of swimming speeds during steady state and predicts an overall hydrodynamic swimming efficiency of 18% for synchronized-leg-swimming. Additionally, synchronized-leg swimming is calculated to be 39% more hydrodynamically efficient than alternating-leg-swimming kinematics, thus verifying previous suggestions of greater hydrodynamic efficiency in D. marginalis based on swimming observation.

abstract:

It has been previously suggested that the generation of coherent vortical structures in the near-wake of a self-propelled vehicle can improve its propulsive efficiency by manipulating the local pressure field and entrainment kinematics. This paper investigates these unsteady mechanisms analytically and in experiments. A self-propelled underwater vehicle is designed with the capability to operate using either steady-jet propulsion or a pulsed-jet mode that features the roll-up of large-scale vortex rings in the near-wake. The flow field is characterized by using a combination of planar laser-induced fluorescence, laser Doppler velocimetry and digital particle-image velocimetry. These tools enable measurement of vortex dynamics and entrainment during propulsion. The concept of vortex added-mass is used to deduce the local pressure field at the jet exit as a function of the shape and motion of the forming vortex rings. The propulsive efficiency of the vehicle is computed with the aid of towing experiments to quantify hydrodynamic drag. Finally, the overall vehicle efficiency is determined by monitoring the electrical power consumed by the vehicle in steady and unsteady propulsion modes. This measurement identifies conditions under which the power required to create flow unsteadiness is offset by the improved vehicle efficiency. The experiments demonstrate that substantial increases in propulsive efficiency, over 50% greater than the performance of the steady-jet mode, can be achieved by using vortex formation to manipulate the near-wake properties. At higher vehicle speeds, the enhanced performance is sufficient to offset the energy cost of generating flow unsteadiness. An analytical model explains this enhanced performance in terms of the vortex added-mass and entrainment. The results suggest a potential mechanism to further enhance the performance of existing engineered propulsion systems. In addition, the analytical methods described here can be extended to examine more complex propulsion systems such as those of swimming and flying animals, for whom vortex formation is inevitable.

abstract:

Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the high power coefficient (mechanical power output divided by the power of the free-stream air through the turbine cross-sectional area) of an isolated turbine. However when in close proximity to neighbouring turbines, HAWTs suffer from a reduced power coefficient. In contrast, previous research on vertical axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience only small decreases (or even increases) in an individual turbine's power coefficient when placed in close proximity to neighbours, thus yielding much higher power outputs for a given area of land. A potential flow model of inter-VAWT interactions is developed to investigate the effect of changes in VAWT spatial arrangement on the array performance coefficient, which compares the expected average power coefficient of turbines in an array to a spatially-isolated turbine. A geometric arrangement based on the configuration of shed vortices in the wake of schooling fish is shown to significantly increase the array performance coefficient based upon an array of 16x16 wind turbines. Results suggest increases in power output of over one order of magnitude for a given area of land as compared to HAWTs.

Robert W. Whittlesey
home research publications robert press etc	Publications R W Whittlesey (2011). Wake-based unsteady modeling of the aquatic beetle Dytiscus marginalis. Journal of Theoretical Biology, 291, pp 14-21 doi: 10.1016/j.jtbi.2011.08.005. [pdf] abstract: Dytiscus marginalis simultaneously uses its hind legs to propel itself through the water. Previous work has suggested that use of synchronized leg motions, such as that used by D. marginalis, allows it to swim with higher hydrodynamic efficiency than similarly sized insects that alternate its legs during swimming. A model is developed based on the generation of vortices in the wake to calculate the relative efficiency of synchronized-leg-swimming kinematics compared to alternating-leg-swimming kinematics. The model agrees well with measured values of swimming speeds during steady state and predicts an overall hydrodynamic swimming efficiency of 18% for synchronized-leg-swimming. Additionally, synchronized-leg swimming is calculated to be 39% more hydrodynamically efficient than alternating-leg-swimming kinematics, thus verifying previous suggestions of greater hydrodynamic efficiency in D. marginalis based on swimming observation. L A Ruiz, R W Whittlesey, J O Dabiri (2011). Vortex-enhanced propulsion. Journal of Fluid Mechanics, 668, pp 5-32 doi: 10.1017/S0022112010004908. [pdf] abstract: It has been previously suggested that the generation of coherent vortical structures in the near-wake of a self-propelled vehicle can improve its propulsive efficiency by manipulating the local pressure field and entrainment kinematics. This paper investigates these unsteady mechanisms analytically and in experiments. A self-propelled underwater vehicle is designed with the capability to operate using either steady-jet propulsion or a pulsed-jet mode that features the roll-up of large-scale vortex rings in the near-wake. The flow field is characterized by using a combination of planar laser-induced fluorescence, laser Doppler velocimetry and digital particle-image velocimetry. These tools enable measurement of vortex dynamics and entrainment during propulsion. The concept of vortex added-mass is used to deduce the local pressure field at the jet exit as a function of the shape and motion of the forming vortex rings. The propulsive efficiency of the vehicle is computed with the aid of towing experiments to quantify hydrodynamic drag. Finally, the overall vehicle efficiency is determined by monitoring the electrical power consumed by the vehicle in steady and unsteady propulsion modes. This measurement identifies conditions under which the power required to create flow unsteadiness is offset by the improved vehicle efficiency. The experiments demonstrate that substantial increases in propulsive efficiency, over 50% greater than the performance of the steady-jet mode, can be achieved by using vortex formation to manipulate the near-wake properties. At higher vehicle speeds, the enhanced performance is sufficient to offset the energy cost of generating flow unsteadiness. An analytical model explains this enhanced performance in terms of the vortex added-mass and entrainment. The results suggest a potential mechanism to further enhance the performance of existing engineered propulsion systems. In addition, the analytical methods described here can be extended to examine more complex propulsion systems such as those of swimming and flying animals, for whom vortex formation is inevitable. R W Whittlesey, S Liska, J O Dabiri (2010). Fish schooling as the basis for vertical axis wind turbine farm design. Biosinspiration and Biomimetics, 5, 035005 doi: 10.1088/1748-3182/5/3/035005. [pdf] [arXiv:1002.2250] abstract: Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the high power coefficient (mechanical power output divided by the power of the free-stream air through the turbine cross-sectional area) of an isolated turbine. However when in close proximity to neighbouring turbines, HAWTs suffer from a reduced power coefficient. In contrast, previous research on vertical axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience only small decreases (or even increases) in an individual turbine's power coefficient when placed in close proximity to neighbours, thus yielding much higher power outputs for a given area of land. A potential flow model of inter-VAWT interactions is developed to investigate the effect of changes in VAWT spatial arrangement on the array performance coefficient, which compares the expected average power coefficient of turbines in an array to a spatially-isolated turbine. A geometric arrangement based on the configuration of shed vortices in the wake of schooling fish is shown to significantly increase the array performance coefficient based upon an array of 16x16 wind turbines. Results suggest increases in power output of over one order of magnitude for a given area of land as compared to HAWTs. Conferences R W Whittlesey, J O Dabiri, "Passively pulsed propulsion of aquatic vehicles," 63rd Annual Meeting of the APS Division of Fluid Dynamics, Long Beach, California, November 2010 abstract: Recent work by Ruiz has shown that pulsed-jet propulsion for aquatic vehicles, similar to that used by sea jellies, salps, and squid, requires a significant decrease in energy input (~30%). These results were obtained despite mechanical inefficiencies in the system to generate the pulsed flow. Thus an approach to generate the pulsed flow using a passive means has been explored. This approach uses collapsible tubing in a pressurized chamber to generate oscillatory, unsteady flow through the outlet. Current results will be presented along with a look toward future developments. R W Whittlesey, S Liska, J O Dabiri, "Fish Schooling as a Basis for Vertical Axis Wind Turbine Farm Design," 4th Southern California Symposium on Flow Physics, UC: Irvine, Irvine, California, April 2010 R W Whittlesey, S Liska, J O Dabiri, "Innovative concepts: Fish Schooling as a Basis for Vertical Axis Wind Turbine Farm Design," New Energy Husum's 1st World Summit on Small Wind Turbines, Husum, Germany, March 2010 R W Whittlesey, J O Dabiri, "Fish schooling as the basis for wind farm design," 62nd Annual Meeting of the APS Division of Fluid Dynamics, Minneapolis, Minnesota, November 2009 A Thiam, R W Whittlesey, C E Wark, D R Williams, "Corner Separation and the onset of stall in an axial compressor," AIAA's 38th Fluid Dynamics Conference, Seattle, Washington, June 2008 [pdf] last updated october 3rd, 2011
contact information: 1200 e california blvd mc 301-46 pasadena, ca 91125 robertw at caltech dot edu
this site is copyright robert whittlesey, 2008-2011.

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