mirage

Extracting energy from unsteady flows through vortex control

WHOAS at MBLWHOI Library

a service of the MBLWHOI Library | About WHOAS

Show simple item record

dc.contributor.author Streitlien, Knut
dc.date.accessioned 2012-11-19T19:55:20Z
dc.date.available 2012-11-19T19:55:20Z
dc.date.issued 1994-09
dc.identifier.uri http://hdl.handle.net/1912/5563
dc.description Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1994 en_US
dc.description.abstract Vortex control is a new paradigm in fluid mechanics, with applications to propulsion and wake reduction. A heaving and pitching hydrofoil placed in a flow with an array of oncoming vortices can achieve a very high propulsive efficiency and reduced wake signature. The canonical example of flow with regular arrays of vortices is the Karman vortex street, and this is our model for the inflow to the foil. The problem of an oscillating foil placed within a Karman vortex street is investigated with a theoretical model and numerical simulation. The theoretical model is an adaptation of the classical linear theory for unsteady aerofoils. It combines the effects of nonuniform inflow and foil motion to predict the resulting thrust, lift, and moment. The numerical procedure allows for nonlinear interaction between the foil, performing large amplitude oscillations, and the oncoming vortex street. The method is based on two- dimensional potential flow and the theory of functions of a complex variable. Careful formulation of the velocity potential, and closed form expressions for force and moment on a Joukowski foil in the presence of point vortices, permits rapid evaluation of hydrodynamic performance. The theory and simulation results agree in their main conclusion: For optimum performance, the foil should try to intercept the vortices head on, while remaining inside the border of the oncoming vortex street. This mode is associated with a high degree of interaction between oppositely signed vorticity in the combined wake leading to reduced wake signature. The lowest efficiency is predicted when the foil avoids coming close to the vortices, here the combined wake consists of a row of very strong vortices of alternate sign. The theory also indicates that an oscillating foil can recover more of the energy contained in the vortex street than a stationary one, but this has not been confirmed in simulation. The interaction process in the wake is studied in more detail, using a much simplified model; the foil wake is modeled as a uniform shear layer of small but finite thickness, and an oppositely signed vortex is placed next to it to simulate the effect of one of the vortices in the Karman street. The subsequent interaction is simulated with the vortex method, assuming periodic boundary conditions. These simulations show that the shear layer rolls up and partially engulfs the vortex patch when two conditions are satisfied. The vortex must be close to the shear layer, and the circulation about the vortex and a representative segment of the shear layer must balance, such that neither one dominates the problem. In both of these simulations, a fast, O(N), vortex summation method based on multipole expansions is used, with special adaptations to account for the influence of image vortices. en_US
dc.description.sponsorship This work was funded by the MIT Sea Grant College Program under grant number NA90AA-D-SG424 by ARPA, and the Office of Naval Research, under contracts N00014-92-J-1726 and N00014-93-1-0587. en_US
dc.format.mimetype application/pdf
dc.language.iso en_US en_US
dc.publisher Massachusetts Institute of Technology and Woods Hole Oceanographic Institution en_US
dc.relation.ispartofseries WHOI Theses en_US
dc.subject Oscillating wings en_US
dc.subject Conformal mapping en_US
dc.subject Vortex-motion en_US
dc.title Extracting energy from unsteady flows through vortex control en_US
dc.type Thesis en_US
dc.identifier.doi 10.1575/1912/5563


Files in this item

This item appears in the following Collection(s)

Show simple item record

Search WHOAS


Browse

My Account

Statistics