Michael M. Bernitsas

VIVACE (Vortex Induced Vibration Aquatic Clean Energy): A New Concept in Generation of Clean and Renewable Energy From Fluid Flow

Any device aiming to harness the abundant clean and renewable energy from ocean and other water resources must have high energy density, be unobtrusive, have low maintenance, be robust, meet life cycle cost targets, and have a 10-20 year life. The vortex induced vibration aquatic clean energy (VIVACE) converter-invented by Bernitsas and Raghavan, patent pending through the University of Michigan-satisfies those criteria. It converts ocean/river current hydrokinetic energy to a usable form of energy such as electricity using VIV successfully and efficiently for the first time. VIVACE is based on the idea of maximizing rather than spoiling vortex shedding and exploiting rather than suppressing VIV. It introduces optimal damping for energy conversion while maintaining VIV over a broad range of vortex shedding synchronization. VIV occurs over very broad ranges of Reynolds (Re) number Only three transition regions suppress VIV. Thus, even from currents as slow as 0.25 m/s, VIVACE can extract energy with high power conversion ratio making ocean/river current energy a more accessible and economically viable resource. In this paper, the underlying concepts of the VIVACE converter are discussed. The designs of the physical model and laboratory prototype are presented. A mathematical model is developed, and design particulars for a wide range of application scales are calculated. Experimental measurements on the laboratory prototype are reported in the sequel paper and used here for preliminary benchmarking.

Large deformation three-dimensional static analysis of deep water marine risers

The problem of static three-dimensional, nonlinear, large deformation response of a marine riser is formulated within small strain theory and solved numerically. This type of analysis is necessary, for the new generation of drilling and production risers. The mathematical model takes properly into account the effects of internal and external pressure and the complete nonlinear boundary conditions, without linearizing the follower forces. The extensibllity or inextensibility condition is used as the constitutive relation in the tangential direction. Torsion and bending are coupled. The external load and the boundary conditions are deformation dependent. A solution method is developed based on an incremental finite element algorithm, which involves a prediction-correction scheme. In the correction phase deformation dependent quantities are updated. The extensibility or inextensibility condition is used to reduce the degrees of freedom of the system. The numerical results of the developed computer code compare very well with available semi-analytical and numerical solutions. Three numerical applications are used to demonstrate the importance of large deformation, nonlinear and three-dimensional analyses.

Selective Roughness in the Boundary Layer to Suppress Flow-Induced Motions of Circular Cylinder at 30,000<Re<120,000

A passive control means to suppress flow-induced motions (FIM) of a rigid circular cylinder in the TrSL3, high-lift, flow regime is formulated and tested experimentally. The developed method uses passive turbulence control (PTC) consisting of selectively located roughness on the cylinder surface with thickness about equal to the boundary layer thickness. The map of “PTC-to-FIM,” developed in previous work, revealed robust zones of weak suppression, strong suppression, hard galloping, and soft galloping. PTC has been used successfully to enhance FIM for hydrokinetic energy harnessing using the VIVACE Converter. PTC also revealed the potential to suppress FIM to various levels. The map is flow-direction dependent. In this paper, the “PTC-to-FIM” map is used to guide development of FIM suppression devices that are flow-direction independent and hardly affect cylinder geometry. Experiments are conducted in the Low Turbulence Free Surface Water Channel of the University of Michigan on a rigid, horizontal, circular cylinder, suspended on springs. Amplitude and frequency measurements and broad field-of-view visualization reveal complex flow structures and their relation to suppression. Several PTC designs are tested to understand the effect of PTC roughness, location, coverage, and configuration. Gradual modification of PTC parameters, leads to improved suppression and evolution of a design reducing the VIV synchronization range. Over a wide range of high reduced velocities, VIV is fully suppressed. The maximum amplitude occurring near the system’s natural frequency is reduced by about 63% compared to the maximum amplitude of the smooth cylinder.

Effect of Bottom Boundary on VIV for Energy Harnessing at 8×103<Re<1.5×105

The concept of extracting energy from ocean/river currents using vortex induced vibration was introduced at the OMAE2006 Conference. The vortex induced vibration aquatic clean energy (VIVACE) converter, implementing this concept, was designed and model tested; VIV amplitudes of two diameters were achieved for Reynolds numbers around 10 5 even for currents as slow as 1.6 kn. To harness energy using VIV, high damping was added. VIV amplitude of 1.3 diameters was maintained while extracting energy at a rate of P VIVACE = 0.22 × 0.5 × pU 3 DL at 1.6 kn. Strong dependence of VIV on Reynolds number was proven for the first time due to the range of Reynolds numbers achieved at the Low-Turbulence Free Surface Water (LTFSW) Channel of the University of Michigan. In this paper, proximity of VIVACE cylinders in VIV to a bottom boundary is studied in consideration of its impact on VIV, potential loss of harnessable energy, and effect on soft sediments. VIV tests are performed in the LTFSW Channel spanning the following ranges of parameters: Re ∈ [8 × 10 3 ― 1.5 × 10 5 ], m * ∈[1.0―3.14], U∈[0.35―1.15 m/s], L/D ∈[6―36], closest distanee to bottom boundary (G/D)∈[4―0.1], and m * ξ ∈ [0.14―0.26]. Test results show strong impact for gap to diameter ratio of G/D <3 on VIV, amplitude of VIV, range of synchronization, onset of synchronization, frequency of oscillation, hysteresis at the onset of synchronization, and hysteresis at the end of synchronization.

NONLINEAR STABILITY AND SIMULATION OF TWO-LINE SHIP TOWING AND MOORING

The horizontal plane motions — surge, sway, yaw — of a ship towed by two tug boats, or moored to two terminals, are modelled by the nonlinear third order maneuvering equations without memory. Nylon lines, chains, or steel cables are used as towing/mooring lines. Excitation consists of time independent current, wind an mean wave drift forces. The resulting Nonlinear Time Varying (NTV) model is studied by simulation, local linear, and global nonlinear stability analyses. Two-Line Towing/Mooring (TLT/M) systems exhibit, in general, three equilibria. Linear analysis reveals that equilibria may be attractor or repellor nodal or focus points. Near first equilibrium, it is shown that of the five Routh-Hurwitz criteria only two can be active in TLT/M systems of conventional ship hulls. Subsequent parametric study reveals the complexity of stability boundaries which may exhibit fold or cusp singularities. Global analysis is used to find attractor limit cycles and chaotic behavior. A barge with or without stabilizing skegs, a marinetype cargo ship, and a tanker with and without propeller which have different stability characteristics are used in nonlinear numerical simulations to verify all theoretical conclusions.

Reduction/Suppression of VIV of Circular Cylinders Through Roughness Distribution at 8×103 < Re < 1.5×105

Vortex Induced Vibration (VIV) of a circular cylinder in a steady flow is reduced using distributed surface roughness. VIV reduction is needed in numerous applications where VIV is destructive. Roughness is distributed to the surface of the cylinder in the form of sandpaper strips to achieve three goals: (1) Trip separation in a controlled manner so that some uncertainties are removed and the flow becomes more predictable. (2) Reduce spanwise correlation, which is strongly linked to VIV. (3) Select roughness grit size to achieve the first goal without energizing too much the boundary layer, which would induce higher vorticity and circulation, and consequently lift. Our experiments show that it is possible to reduce VIV amplitude and synchronization range. More tests are needed to achieve full suppression. Our experiments are conducted in the TrSL2 and TrSL3 flow regimes.Copyright

Development of Proper Models of Hybrid Systems

A method to improve the ability of design engineers to generate proper dynamic models of systems from sets ofcomponent models is developed. This two stage method, called Eigenvalue MODA, is a new model deduction procedure for developing proper models of hybrid systems. The first stage in Eigenvalue MODA consists of using a previously published Model Order Deduction Algorithm (MODA) to systematically increment the complexity of each component model in the system. The first stage continues until a set of Critical System Eigenvalues (CSE) has been defined. During the second stage, the complexity component models is incremented based on the convergence of the CSE. The second stage continues until each CSE has converged to within a user specified tolerance. Component models may be represented by first-order (state space) or second-order equations and may be modal expansion or finite segment models. An example shows that the deduction of the proper system model depends on the interactions between the component model representations and the model deduction method. Eigenvalue MODA is a model deduction method that facilitates the generation of models of sufficient accuracy with physically meaningful parameters and states. This makes the models useful for system design and, as such, Eigenvalue MODA would be a useful tool in an automated modeling environment for design engineers.

Structural model correlation using large admissible perturbations incognate space

A nonlinear perturbation method is developed to solve the problem of correlating a finite element model to a structure for which an incomplete set of natural frequencies and mode shapes and/or some static deflections have been measured.

Admissible large perturbations in structural redesign

A new algorithm for structural redesign by perturbation is developed. It allows for large changes between baseline and objective structures and can satisfy both modal and static displacement objectives. It is faster and more accurate than previously developed perturbation resizing methods because it advances incrementally to the objective structure using large admissible perturbations. The computer code developed is used as postprocessor to general or special purpose finite element codes, to improve upon the design of structures with unacceptable modal and/or static displacement response. Depending on the relation betweem the number of redesign goals, allowable structural changes, and admissibility constraints, the desired redesign may be feasible - either locally optimal or unique - or may not exist. In the latter case, a minimum-error inadmissible design is produced. Several numerical examples are used to study the effects of definition and relaxation of the redesign admissible domain and demonstrate the accuracy of the new redesign algorithm. An offshore tower with repeated eigen-values and 192 degrees of freedom is also redesigned subject to frequency, static displacement goals, and admissibility constraints. 22 refs.

Stability of single point mooring systems

A mathematical model for the horizontal plane slow motions of ships connected to a mooring terminal through a single elastic line is developed based on the ship manoeuvring equations and a non-linear elastic mooring line model. Slowly varying excitations from current, wind and drift forces are included. Local perturbation analysis about the critical points of the corresponding autonomous system reveals the asymptotic stability characteristics of the mooring system. Time simulations using a tanker and a barge confirm the theoretically predicted long term behaviour and show the short term system behaviour. The mooring system may exhibit instability in sway, yaw and surge or reach a limit cyclic depending on the properties of the critical points, vessel type, propeller, initial conditions, current and wind.