A hydrokinetic energy converter using Flow Induced Oscillations (FIOs) of a one-degree-of-freedom cylinder-oscillator, with nonlinear adaptive damping and linear spring stiffness, is introduced and ...studied experimentally. Comparison to a linear-oscillator in FIO shows that this new converter, with velocity-proportional damping coefficient, is more effective in galloping, where both flow and cylinder speeds are higher. It also impacts VIV, since the converter is no longer restricted by fixed damping, which results either in ceasing motion due to excessive damping, or in low harnessed energy due to insufficient damping. The impact is most profound in the VIV to galloping transition where adaptive damping prevents shutting down of hydrokinetic energy conversion. Damping-to-velocity rate, linear spring-stiffness, and flow-velocity are the experimental parameters with Reynolds number 30,000 ≤ Re ≤ 120,000. Experimental results for amplitude response, frequency response, energy harvesting, efficiency and instantaneous energy of the converter are presented and discussed. The main conclusions are: (1) The nonlinear, adaptive, velocity-proportional damping coefficient increases the harnessed power. (2) The operational range of flow velocities increases. (3) At lower flow speeds, the adaptive damping stabilizes the unstable oscillations typically occurring in this region. (4) At higher flow speeds, adaptive damping results in higher harnessed power than constant damping, thus, better emulating passively a corresponding, natural, active motion by fish. (5) Increase of 51%–95% in converted power by the nonlinear oscillator compared to linear oscillator has been measured. (6) The adaptive damping converter reaches a plateau in harnessed efficiency at high flow velocity (fully developed galloping).
A nonlinear oscillator, using a cubic-spring restoring function with high-deformation stiffening, is introduced and studied experimentally to improve passively the harnessed marine hydrokinetic power ...using Flow Induced Vibrations (FIVs) of a cylinder. In this research, the FIV of a single, rigid, circular cylinder on elastic end-supports is tested for Reynolds number 30,000 ≤ Re ≤ 120,000. Damping, cubic stiffness, and flow-velocity are used as parameters. Selective roughness is applied to enhance FIV and increase the hydrokinetic energy converted by the oscillator. The second generation of the digital, virtual spring-damping system Vck, developed in the Marine Renewable Energy Laboratory (MRELab), enables embedded computer-controlled change of the functions and values of viscous damping and spring stiffness. Cubic modeling of the oscillator stiffness in parametric form is thus realized and tested. Experimental results for amplitude response, frequency response, energy harvesting, efficiency and instantaneous energy of the converter are presented and discussed. All experiments are conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) The cubic stiffness function is an effective way to raise the harnessed efficiency over a wider range of flow velocities. (2) At lower flow speed (upper and lower VIV branches), the harnessed power increases as the nonlinearity increases. A strongly nonlinear system exhibits a 100% increase in harnessed energy in this region. (3) At a higher flow speed (galloping), the cubic nonlinearity benefits the harnessed power in two ways because the natural frequency of the oscillator in water (fn,water) depends on the amplitude of oscillation. At low harness damping, the amplitude increases resulting in higher fn,water thus enhancing the harnessed power. At high harness damping, the harnessed power increases regardless of fn,water.
•The paper studies the nonlinear cubic stiffness, alternating lift energy converter based on FIV.•The experiments are based on the second generation of virtual spring-damping system Vck.•Each nonlinear cubic stiffness function has its own merits in power harnessing.•The optimally harnessed power envelope and four performance zones are established based on FIV.
Flow Induced Motions (FIMs) of a single, rigid, circular cylinder with end-springs are investigated for Reynolds number 30,000 ≤ Re ≤ 120,000 with mass ratio, damping, and stiffness as parameters. ...Selective roughness is applied to enhance FIM and increase the hydrokinetic energy captured by the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter at higher Reynolds numbers. The second generation of virtual spring-damping system Vck, recently developed in the Marine Renewable Energy Laboratory (MRELab), enables embedded computer-controlled change of viscous-damping and spring-stiffness for fast and precise oscillator modeling. Experimental results for amplitude response, frequency response, energy harvesting, and efficiency are presented and discussed. All experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) The oscillator can harness energy from flows as slow as 0.3946 m/s with no upper limit. (2) Increasing the spring stiffness, shifts the VIV synchronization range to higher flow velocities, resulting in reduced gap between VIV and galloping, where the harnessed power drops. (3) In galloping, the harnessed power increases with the mass ratio. (4) Local optima in energy conversion efficiency appear at the beginning of the VIV upper branch and at the beginning of galloping. (5) Local optima in power appear at the end VIV upper branch and at the beginning of galloping.
•The paper experimentally studies the mass ratios, spring stiffness, and damping on power harness of a single circular cylinder with passive turbulence control.•The experiments are base on the Vck, which enables embedded computer-controlled change of viscous-damping and stiffness.•The research exhibits local optima in energy conversion efficiency at the beginning of the VIV upper branch and at the beginning of galloping.•The amplitude and frequency response, power harness, and efficiency, are presented and discussed based on stiffness, damping and mass ratios.•All the experiments are conducted in the TrSL3 (20,000 < Re < 300,000) flow regime.
•Multiple cylinders in FIM work synergistically for hydrokinetic power harnessing.•Peak power conversion efficiency was estimated at 88.6% of the Betz limit.•Power-to-volume density reached 874.7W/m3 ...at flow speed of 1.45m/s.•VIVACE can efficiently harness energy from flows as slow as 0.8m/s, with no upper limit.
Horizontal hydrokinetic energy can be harnessed using Steady Lift Technology (SLT) like turbines or Alternating Lift Technology (ALT) like the VIVACE Converter. Tidal/current turbines with low mechanical losses typically achieve about 30% peak power efficiency, which is equivalent to 50.6% power efficiency over the Betz limit at flow speed nearly 3.0m/s. The majority of flows worldwide are slower than 1.0–1.5m/s. Turbines also require large in-flow spacing resulting in farms of low power-to-volume density. Alternating-lift overcomes these challenges. The purpose of this study is to show that the ALT Converter is a three-dimensional energy absorber that efficiently works in river/ocean currents as slow as 1.0–1.5m/s a range of velocities presently inaccessible to watermills and turbines. This novel converter utilizes flow-induced motions (FIM), which are potentially destructive phenomena for structures, enhances them, and converts hydrokinetic energy to electricity. It was invented in the Marine Renewable Energy Lab (MRELab) and patented through the University of Michigan. MRELab has been studying the effect of passive turbulence control (PTC) to enhance FIMs and to expand their synchronization range for energy harnessing. This study shows that multiple cylinders in proximity can synergistically work and harness more energy than the same number of a single cylinder in isolation. Estimation based on experiments, shows that a 4 PTC-cylinder Converter can achieve 88.6% peak efficiency of the Betz limit at flow speed slower than 1.0m/s and power-to-volume density of 875W/m3 at 1.45m/s. Thus, the Converter can efficiently harness energy from rivers and ocean current as slow as 0.8–1.5m/s, with no upper limit in flow velocity.
Flow Induced Motions (FIMs) of rigid circular cylinders, and particularly VIV (Vortex Induced Vibrations) and galloping, are induced by alternating lift. The VIVACE (VIV for Aquatic Clean Energy) ...Converter uses single or multiple cylinders, in tandem, on elastic end-supports, in synergistic FIM, to convert MHK energy to electricity. Selectively distributed surface roughness is applied to enhance FIM and increase efficiency. In this paper, two cylinders are used in tandem with center-to-center spacing of 1.57, 2.0 and 2.57 diameters, harnessing damping ratio 0.00<ζ < 0.24, for Reynolds number 30,000 ≤ Re ≤ 120,000. The virtual spring-damping system Vck in the Marine Renewable Energy Laboratory (MRELab) enables embedded computer-controlled change of viscous-damping and spring-stiffness for fast and mathematically correct oscillator realization, without including the hydrodynamic force in the closed control loop. Experimental results for oscillatory response, energy harvesting, and efficiency are presented and the envelope of optimal power is derived. All the experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) For the tested cylinder spacing, two cylinders harness power is between 2.56 and 13.49 times the power of a single cylinder, the efficiency of two cylinders is between 2.0 and 6.68 of a single cylinder. (2) The MHK power harnessed by the upstream cylinder is increased by up to 100%, affected by the downstream cylinder. (3) The MHK power harnessed by the downstream cylinder and its FIM are affected to a lesser extent by the interaction. (4) VIVACE can harness energy from flows as slow as 0.4 m/s with no upper limit in flow velocity. (5) Close spacing and high spring stiffness yield highest harnessed power. (6) The optimal harnessed power shifts to softer springs as spacing increases.
•The effects of tandem spacing, spring stiffness, and damping on power harness by two circular cylinders with passive turbulence control are studied experimentally.•The Vck based oscillator enables embedded computer-controlled change of viscous-damping and spring-stiffness for precise oscillator modeling and fast parametric testing.•Amplitude response, frequency response, harnessed power, and efficiency are presented vs. flow velocity with spring stiffness, damping, and spacing as parameters.•In the galloping range, two cylinders in synergistic flow induced motion can produce more power than the same cylinders in isolation.•All the experiments were conducted in the TrSL3 (20,000<Re<300,000) flow regime.
Flow Induced Motions (FIMs) of a single, rigid, circular cylinder with piecewise continuous restoring force are investigated for Reynolds number 24,000≤Re≤120,000 with damping, and different ...piecewise functions as parameters. Selective roughness is applied to enhance FIM and increase the hydrokinetic energy captured by the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter at higher Reynolds numbers. The second generation of virtual spring-damping system Vck, developed in the Marine Renewable Energy Laboratory (MRELab), enables embedded computer-controlled change of viscous-damping and spring-stiffness for fast and precise oscillator modeling. Experimental results for amplitude response, frequency response, energy harvesting, and efficiency are presented and discussed. All experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) The nonlinear piecewise spring Converter can harness energy from flows as slow as 0.275m/s with no upper limit. (2) In galloping, the nonlinear spring Converter has up to 76% better performance than its linear spring counterpart. (3) The FIM response is predominantly periodic for all nonlinear spring functions used. (4) Optimal power harnessing is achieved by changing the nonlinear piecewise spring function and the linear viscous damping. (5) VIVACE exhibits local maxima in power conversion at the end of the upper branch in VIV and the highest velocity reached in galloping. (6) The efficiency optima though are at the beginning of the VIV initial branch and at the beginning of galloping.
•The paper experimentally studies the flow induced motions and the hydrokinetic power conversion capacity of a single, rigid, circular cylinder, on nonlinear piecewise restoring force/spring for various functions and harnessing damping.•In galloping, power harnessing is higher with the nonlinear restoring force than the corresponding linear restoring force. At the onset of galloping the difference is about 76%. As the flow velocity increases this difference reduces to about 15%.•All the experiments are conducted in the TrSL3 (20,000<Re<300,000) flow regime. (24,000<Re<120,000, 0.274m/s<U<1.315m/s).•Regardless of whether the oscillator motion is in the galloping or the VIV region, there is only one predominant frequency.•The research exhibits differences and similarities between the nonlinear and linear systems tested in previous work.
Two-dimensional Unsteady Reynolds-Average Navier–Stokes equations with the Spalart–Allmaras turbulence model are used to simulate the flow induced motions of multiple circular cylinders with passive ...turbulence control (PTC) in steady uniform flow. Four configurations with 1, 2, 3, and 4 cylinders in tandem are simulated and studied at a series of Reynolds numbers in the range of 30000<Re<120000. Simulation results are verified by experimental data measured in the Marine Renewable Energy Laboratory. Good agreement was observed between the values of vorticity, amplitude ratio, and frequency ratio predicted by numerical simulations and experimental measurements. The amplitude and frequency response show the initial and upper branches in vortex induced vibration (VIV), transition from VIV to galloping, and galloping branch for all PTC-cylinders. The maximum amplitude of 2.9 diameters for the first cylinder is achieved at Re=104356 in the numerical results. Compared with the first cylinder, the VIV initial branch starts at higher Re for the downstream cylinders due to the presence of the upstream cylinder(s). 2P and 2P+2S vortex patterns are observed at Re=62049 and Re=90254 for the single PTC-cylinder. Furthermore, the shed vortices of the downstream cylinders are strongly disrupted and modified by the vortices shed from the upstream one in the cases of multiple PTC-cylinders.
•URANS simulations of multi-cylinders with PTC in FIM agree well with experiments.•Simulations predict well FIM response as well as VIV branches and galloping.•Vortex patterns of multiple cylinders in VIV and galloping are predicted accurately.
The flow induced motions (FIM) of two rigid circular cylinders, on end linear-springs, in tandem are studied using two-dimensional Unsteady Reynolds-Averaged Navier-Stokes (2-D URANS) simulations ...verified by experimental data. Passive turbulence control (PTC) is being used in the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan to enhance FIM of cylinders in the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) Converter to increase its efficiency and power density in harnessing marine hydrokinetic energy. Simulation is performed using a solver based on the open source CFD tool OpenFOAM, which solves continuum mechanics problems with a finite-volume discretization method. The simulated Reynolds number range for which experiments were conducted in the MRELab is 30,000<Re<105,000, which falls in the TrSL3 regime (Transition in Shear Layer), where the shear layers are fully saturated and consequently lift is high. The amplitude and frequency results are in excellent agreement with experimental data showing the initial and upper branches in VIV, transition from VIV to galloping, and galloping. Vortex structures are studied using high-resolution imaging from the CFD results showing typical 2S structure in the initial branch and both 2P+2S and 2P in the upper branch of VIV. In the galloping branch, amplitudes of 3.5 diameters are reached before the channel stops are hit.
•2-D URANS code developed for two cylinders in FIM and validated experimentally for Reynolds number up to 105,000.•Selectively distributed surface roughness is the key to the agreement between CFD and experiments.•Integral FIM properties are predicted accurately: amplitude, frequency, VIV branches, galloping.•Local flow properties are predicted accurately: vortex structures, shear layer motion.•Coexistence of VIV and galloping driving mechanisms shown in VIV-to-galloping transition.
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