Current Energy Conversion : Invented, designed, and model-tested for the VIVACE http://www.vortexhydroenergy.com/ (Vortex Induced Vibration Aquatic Clean Energy) energy converter (patent pending UofM#2973). VIVACE is an ocean/river current energy converter based on the idea of enhancing rather than spoiling vortex shedding, increasing rather than suppressing VIV under high damping, and harnessing rather than mitigating VIV energy. VIV was first observed by Leonardo daVinci in 1504AD in the form of “Aeolian Tones”. Since then, engineers have been trying to suppress VIV which damage aero, civil, mechanical, marine, offshore, nuclear engineering structures. The VIVACE Converter takes this destructive force in nature and utilizes for the benefit of mankind. The VIVACE Converter is designed to be in high damping VIV ? thus extracting energy at high efficiency – over the range of current velocity that is of practical interest: 0.25-2.5m/sec (0.5-5.0knots) [79-80]. Testing of the VIVACE Converter in the Low Turbulence Free Surface Water Channel of Ocean Renewable Energy Laboratory at the University of Michigan for high damping resulted in a power harnessing rate of P VIVACE =0.22p w DLU 3 for current velocity of only 0.840m/sec (1.63 knots) [80-82].
Enhancement of High-Damping, High-Reynolds Vortex Induced Vibration: Low-head low-speed currents (2-3knots) are worldwide available. A major challenge in making ocean/river current energy accessible is that (according to EPRI) turbines and water mills require 6knot currents for energy extraction. There are only six sites with such strong currents in the entire North America. The VIVACE Converter is scalable and flexible and can extract energy from slow currents. This has been made possible with passive turbulence control (patent pending UofM#3737) [207].
Suppression of Vortex Induced Vibration: In the process enhancing VIV for energy harness, we revealed issues about roughness effect on VIV that were not understood before. We used these new discoveries to develop a method to suppress VIV (patent pending, UofM#3757) [208]. Our concept has been proven in the LTFSW Channel. More tests are being conducted currently.
Mooring System Dynamics: Mooring/Towing systems exhibit hazardous slow large-amplitude motions. Trial and error and rules of thumb are used typically in design practice. M/T systems have very rich nonlinear dynamic behavior. Since 1983, Professor Bernitsas and his Ph.D. students have developed an analysis and design methodology based on the horizontal-plane slow-motion nonlinear dynamics of M/T systems. Design graphs (catastrophe sets) are developed, which make it possible to design without trial and error or extensive simulations. They have demonstrated that all rules of thumb are not valid in general and have explained the nature of large-amplitude motions even in the absence of time varying excitation. Recently, they proved theoretically the existence of 64 routes to such large-amplitude motions due to slowly varying wave drift forces and demonstrated 13 by simulation. Some of these phenomena (interaction with Hopf bifurcations) result in motions with amplitudes several orders of magnitude larger than motions due to resonance.
Structural Redesign and Topology/Material Evolution: Since 1985, Professor Bernitsas and his PhD students developed the LargE Admissible Perturbation (LEAP) methodology to relate two structural states, which can be modeled by the same Finite Element (FE) model but described by different values of the design variables.LEAP relates the two states – that may differ by 100%-300% in structural properties and performance – and computes the unknown state based on its specifications without trial and error and with only a single FE analysis that of the known state. LEAP has provided breakthrough solutions to some forms of the challenging structural optimization problems of Redesign (inverse design), Model Correlation, Redundancy, and Reliability. Performance objectives in natural frequencies, static displacement, static stress, and forced response amplitude are achieved simultaneously. Topology redesign for performance is done without trial and error and only 3-5 FE analyses for changes in performance on the order of 3000%. Currently, a topology/material evolution method is being developed based on LEAP to generate novel structures. Design of future fast ships requires structural reduction of 25%. Current topologies/materials of ship structures are nearly optimal and prohibitively heavy.
Riser and Pipeline Mechanics: A decade of research has produced sophisticated static/dynamic, nonlinear, three-dimensional codes for analysis of steel risers. Constraint/contact and substructuring/condensation techniques were used to solve problems on riser bundles and pipelines. In the decade of the 80 s, Professor Bernitsas and his PhD students identified and proved the phenomenon of global Euler buckling of risers in tension, proved the post-bucking unstable behavior of risers, and corrected Willer s buckling theory of long risers/columns. They also generated complete expressions for the inertia forces and moments acting on a small submerged body in 6 d.o.f. motion in three-dimensional unsteady flow in an unbounded ideal fluid. The far field approximation of the body motion by a series of multipoles provides formulas attractive for engineering applications. Thus, the need to solve a hydrodynamic problem for the potential of small bodies is eliminated.
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