Vortex Induced Vibration For Aquatic Clean Energy Research Articles

The Influence of Different Mass Ratios on Vortex-Induced Vibration Energy Extraction of Four Cylinder Arrays

Currently, there is a growing demand for renewable energy harnessed from fluid dynamics within the oil and gas industry. The surge in demand has propelled electricity to become a vital and irreplaceable form of universal energy worldwide. Vortex-Induced Vibrations (VIV) energy harvesting shows great potential as a technology for capturing energy from flowing bodies of water. The purpose of this research is to investigate the numerical aspect of VIV in rigid circular cylinders with the intention of capturing renewable energy from the sea. The investigation employs a Vortex-Induced Vibration Aquatic Clean Energy (VIVACE) converter to analyze the vibration characteristics of densely packed cylinders featuring varying mass ratios (m*) at both minimum and maximum values. Another purpose of the study is to investigate the effect that m* has on the performance of a VIV converter that is comprised of four cylinders positioned in a staggered pattern. For the purpose of analyzing power conversion in the VIV energy converter model across a wide range of mass ratios (from 2.36 to 12.96), simulations are carried out with a Reynolds number of 82000. The findings indicate that the highest converted power reaches a peak of 7.48 W with a mass ratio of 2.36, whereas a greater mass ratio of 12.96 results in only 4.33. The study highlights the substantial influence of mass ratios on the extraction of power output from VIV. The results essentially offer crucial information about the optimum mass ratio in closed four cylinder arrays to design VIV energy harvesting to produce clean and renewable energy sources.

Numerical Simulation on VIV Energy Harvesting of Four Cylinders in Close Staggered Formation with Different Mass Ratios

Nowadays, the demand for marine renewable and clean energy from fluid flow in the oil and gas industry has made electricity the most sought-after and indispensable source of uncontrollable power worldwide. Vortex-Induced Vibrations (VIV) energy harvesting is a promising technology in harnessing energy from flowing water bodies. This study focuses on numerically investigating the VIV of rigid circular cylinders as a sustainable energy source, utilizing a Vortex-Induced Vibration Aquatic Clean Energy (VIVACE) converter to harvest energy from the ocean. Specifically, the research explores the vibration behavior of closely arranged cylinders with different mass ratios, both at low and high values. The study aims to understand the effects of mass ratios on the VIV converter’s performance with four cylinders in close staggered formation. The power conversion of the VIV energy converter model with varying mass ratios (ranging from 2.36 to 12.96) is thoroughly examined, with simulations conducted at a Reynolds number of 82000. The results demonstrate that the maximum converted power peaks at 7.48 W for a mass ratio of 2.36, whereas a higher mass ratio of 12.96 only yields 4.33 W. This emphasizes the significant impact of lower mass ratios in enhancing the power generation from VIV. Overall, the findings of this research provide essential insights to optimize the layout of VIVACE converters in a close staggered array, facilitating the efficient harvesting of energy from flowing water bodies for sustainable and clean energy resources.

Vortex-induced vibration characteristics of three cylinders with isosceles-triangle arrangements at low Reynolds number

The geometric factors of multi-cylinders are one of the factors affecting the efficient energy capture of vortex-induced vibration aquatic clean energy. Based on the computational fluid dynamics numerical simulation method and overset mesh technology, an efficient dynamic model of VIV (vortex-induced vibration) of two-dimensional elastic-supported cylinders was established. The influence of different vertex angles on the VIV characteristics in the isosceles-triangle arrangement was investigated. The results show that the upstream cylinder is less affected by the vertex-angle, and the peak value moves to the left in the α-A*y curve of the downstream cylinders. As α increases, the upstream cylinder moves in an 8 shape and moves in the negative direction of the flow direction, while the downstream cylinders move in an O shape and gradually become narrow. The vortex shedding mode of downstream cylinders is mostly the P + S mode, and its amplitude is related to the position of the upstream cylinder's wake vortex effect. The lift and drag coefficients of the downstream cylinders are affected by α, while the dominant frequency of vibration is independent of the change in α and is greatly affected by Re. With the increase in Re, the positive correlation between the lift coefficient and transverse displacement gradually weakens.

Passively enhanced VIV responses of side-by-side cylinders at moderate Reynolds number

Stationary upstream cylinders were used to enhance the response of vortex-induced vibration (VIV) of side-by-side (SBS) cylinders at Reynolds number Re = 1000. A modified direct-forcing immersed boundary (DFIB) with a large eddy simulation (LES) technique was employed to get the numerical solution. The VIV responses and VIVACE (vortex-induced vibration for aquatic clean energy) efficiency were investigated for various reduced velocities (UR∗) at a constant mass-damping (m∗ζ). Furthermore, the influence of diameter (d∗) and spacing (s∗) of stationary upstream cylinders on the enhancement of amplitude response and VIVACE efficiency were analysed. The optimal upstream cylinder diameter and spacing were d∗ = 0.4 - 0.6 and s∗⩽ 0.4. Within these ranges, the enhancement of amplitude response and efficiency was observed for UR∗⩾ 5.0, while the decrease in amplitude response and efficiency for UR∗ = 4.0 – 4.5 was minimized.

Vortex-induced vibration of two circular cylinders in a side-by-side arrangement at moderate Reynolds number: A numerical study

Vortex-induced vibration (VIV) of two transversely vibrating cylinders in a side-by-side (SBS) arrangement is numerically investigated using a combination of direct-forcing immersed boundary and large eddy simulation techniques. The VIV responses of vibrating SBS cylinders at two reduced velocities (UR* = 4.0 and 6.0) are studied for a range of gap ratio 1.0 ≤g*≤ 3.0. Moreover, the influence of mass ratio, damping ratio, and Reynolds number in the amplitude response and efficiency of VIVACE (Vortex-Induced Vibration for Aquatic Clean Energy) from vibrating SBS cylinders are investigated at moderate Reynolds numbers (Re = 1000 and 10 000). The optimal gap ratio for UR* = 4.0 is in the range of 1.0 ≤g*≤ 1.2. Larger than this range, the VIV responses are close to single-cylinder responses. At UR* = 6.0, all gap ratios show lower responses than a single-cylinder case. The vibrating SBS cylinder with a larger damping ratio results in higher maximum VIVACE efficiency with a narrower UR* range for significant efficiency. With almost the same amplitude response, the SBS cylinders with a lower mass ratio result in lower VIVACE efficiency. Using the same mass-damping parameters, it appears that a low mass ratio could be desirable to increase the UR* range of significant VIVACE efficiency and pick the proper damping ratio to reach a high value of maximum VIVACE efficiency. The effect of flow conditions on the amplitude response and VIVACE efficiency of vibrating SBS cylinders with the same VIV parameters is not significant.

Numerical predictions of vibration responses and flow energy conversion efficiency of side-by-side cylinders at moderate Reynolds number

Vortex-induced vibration (VIV) of two circular cylinders in a side-by-side (SBS) configuration was modelled using three-dimensional numerical simulation at Reynolds number Re = 1,000. The VIV parameter values used for the study were mass ratio, m∗ = 10.0, damping ratio, ζ = 0.01, and a range of reduced velocities, UR∗ = 2.0 to 10.0. Various VIV responses were investigated, such as amplitude, frequency, and response correlation for gap ratios 1.0 ⩽g∗⩽ 3.0. Moreover, the efficiency of flow energy conversion was determined to assess the potential of this configuration for VIVACE (Vortex-Induced Vibration Aquatic Clean Energy). For comparison, the VIV responses and efficiency of the single-cylinder case were also presented with identical variables. It was found that the optimal gap ratio between the SBS cylinders was in the interval of 1.0 ⩽g∗⩽ 1.2. A significant increase in efficiency occurred in the initial excitation region between 4.0 ⩽UR∗⩽ 5.0. Above this range, the efficiency was less than that of a single cylinder. In the case of SBS cylinders, the soft lock-in appeared to start early. The anti-phase correlation led to a significant increase in the vibration response as well as efficiency. The increase in maximum efficiency was more than twofold.

Flow around a Rectangular Cylinder Placed in a Channel with a High Blockage Ratio under a Subcritical Reynolds Number

With the depletion of fossil energy sources, clean energy has become a growing concern for scholars. Vortex-Induced Vibration Aquatic Clean Energy (VIVACE), a device that uses water flow energy to generate electricity, has attracted much attention for its broad applicability and other advantages. Particle Image Velocimetry (PIV) experiments were conducted to improve the efficiency of the VIVACE device in low-velocity areas. The present study investigated the effects of the Blockage ratio (Br), Reynolds number (Re = ρU0D/μ), and Aspect ratio (Ar = B/D, width-to-height) of rectangular cylinders on flow characteristics. The influence of the Ar, Br, and Re on the flow field structure was systematically analyzed in terms of the time-averaged flow field, Reynolds shear stress, space–time correlation, vorticity field, and water pressure characteristics. The vorticity field was deconstructed by Proper Orthogonal Decomposition (POD). The results show that the first two orders of POD modal energy accounted for 75% of the total energy, indicating that the first two modes can be used to identify the large-scale vortex structure. The main water pressure frequency and vortex shedding frequency (f) had a high degree of consistency. Thus, vortex shedding was the main cause of wall water pressure fluctuations. Given the blockage effect, the shear layer’s development spanwise was restricted. Moreover, the blockage effect increased the local flow velocity and accelerated the vortex shedding. The dimensionless time-averaged flow velocity U/U0 increased to 1.5, and the frequency of vortex shedding increased by approximately 25% when the Br increased from 0.067 to 0.25. The frequency increased by 25% when the Ar decreased from 0.5 to 0.2. The experimental results also provide a new idea for optimizing the VIVACE device.

Prediction of Dynamic Responses of Flow-Induced Vibration Using Deep Learning

Flow-induced vibration (FIV) is a phenomenon in which the flow passing through a structure exerts periodic forces on the structure. Most studies on FIVs focus on suppressing this phenomenon. However, the Marine Renewable Energy Laboratory (MRELab) at the University of Michigan, USA, has developed a technology called the vortex-induced vibration for aquatic clean energy (VIVACE) converters that reinforces FIV and converts the energy in tidal currents to electrical energy. This study introduces the experimental data of the VIVACE converter and the associated method using deep neural networks (DNNs) to predict the dynamic responses of the converter. The DNN was trained and verified with experimental data from the MRELab, and the findings show that the amplitudes and frequencies of a single cylinder in the FIV predicted by the DNN under various test conditions were in good agreement with the experimental data. Finally, based on both the predicted and experimental data, the optimal power envelope of the VIVACE converter was generated as a function of the flow speed. The predictions using DNNs are expected to be more accurate as they can be trained with more experimental data in the future and will help to substantially reduce the number of experiments on FIVs.

Hydrokinetic Energy Conversion by Flow-Induced Oscillation of Two Tandem Cylinders of Different Stiffness

Abstract The vortex-induced vibration for aquatic clean energy (VIVACE) converter harnesses hydrokinetic energy by enhancing flow-induced oscillations (FIOs) of elastically supported rigid cylinders in a river, tide, or ocean current. The harnessing power depends on the intensity of the oscillation, which is a consequence of the flow–structure interaction. The inflow condition for the downstream (second) cylinder is slowed down and perturbed by the upstream (first) cylinder, due to the shielding effect. Therefore, the optimal structural parameters, i.e., stiffness and damping ratio, for the second cylinder may be different from the first cylinder, in terms of energy harnessing. To improve the performance of the VIVACE converter, a series of experiments are conducted in a recirculating water channel, with various stiffness combinations of two cylinders in tandem. Results show that the stiffness of the second cylinder, K2, does not affect the energy harnessing power in vortex-induced vibration (VIV) occurring at low speeds, because the oscillation of the downstream cylinder in this velocity range is completely dominated by the wake of the upstream cylinder. K2 has a great influence on the harnessing power at higher velocities in the transition region from VIV to galloping and in galloping. Changing K2 onsets and enhances galloping at lower flow velocity and harnesses up to 110% more energy than the case of K1 = K2.

Influence of hard marine fouling on energy harvesting from Vortex-Induced Vibrations of a single-cylinder

Marine fouling changes the flow regime around sub-sea structures, such as Vortex-Induced Vibration Aquatic Clean Energy (VIVACE) converters. The present experimental study investigates the effects of marine fouling on mechanical power harvesting performance from the Vortex-Induced Vibration (VIV) of single circular cylinders. The towing tank VIV tests are conducted on elastically mounted rigid cylinders. Hard artificial marine fouling with aggregated spatial distribution is considered. Effects of coverage ratio and spatial distributions are investigated.The test results show that, on the whole, marine fouling suppresses the VIV oscillations. Over the upper branch, lower branch and the de-synchronization zone of the VIV response, energy transfer ratio (η) with the biofouled cylinders stay around 15–36% below that from the corresponding clean cylinder. It is also found that the coverage percentage of the fouling and the fouling irregularity negatively affect the VIV hydro energy transfer ratio. The fouling irregularity effects, however, are less significant than those from the coverage percentage. The results of the study suggest that biofouling growth may considerably degrade the long-term energy extraction performance of a VIVACE device.

Numerical investigation on interactive FIO of two-tandem cylinders for hydrokinetic energy harnessing

Flow induced oscillations (FIO) of two rough, rigid, tandem-cylinders on springs are investigated for hydrokinetic power conversion at Reynolds number 30,000 ≤ Re ≤ 120,000. Passive turbulence control (PTC) in the form of roughness strips is applied to enhance FIO and increase the power harness efficiency of the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) converter. Numerical simulations are performed using two-dimensional, Unsteady Reynolds-Averaged Navier-Stokes equations with the Spalart-Allmaras turbulence model. The center-to-center spacing ratio d/D of the two cylinders is set as 2.0 or 2.57 with mass ratio m* = 1.343, damping ratio ζ = 0.26, and stiffness K = 1,200N/m. Amplitude response, frequency response, interaction, energy harvesting, and conversion efficiency are presented and discussed. Results are compared to experimental data for validation and reveal how the interaction of two tandem cylinders may enhance the harnessed power. CFD provides complementary valuable information on visualization of wake and vortex structures. The main conclusions are: (1) In the VIV region at Re = 60,000, the amplitude response, frequency response, harnessed power, and power conversion efficiency of the upstream cylinder is the same for the two spacing ratios tested. Due to the shedding effect, the motion of the downstream cylinder for spacing ratio d/D = 2.0 is more severely suppressed than spacing ratio d/D = 2.57, which reduces the harnessed power and conversion efficiency for the downstream cylinder. (2) In the galloping region at Re = 110,000, due to the different timing of impingement of the shed vortices on the downstream cylinder, the upstream cylinder harnesses more power and has higher energy conversion efficiency for spacing ratio d/D = 2.0 than d/D = 2.57.

Experimental investigation of Reynolds number and spring stiffness effects on vortex induced vibrations of a rigid circular cylinder

Abstract The Vortex Induced Vibration for Aquatic Clean Energy (VIVACE) is a breakthrough technology, which harvests the hydrokinetic energy of ocean currents through enhancing Vortex Induced Vibration (VIV). In this paper, the VIV of an elastically mounted circular cylinder in a towing tank water channel is studied experimentally to investigate the effect of natural frequency, by using five different springs in a wide range of stiffness (125 N/m × 104 × 104) with high mass-damping parameter (m* ζ = 0 .11). Results demonstrated the strong dependency of VIV on natural frequency and lock-in observed in a broad range of spring stiffness. Moreover, the studied Re range showed the gradual transition between two distinct turbulence shear layers from previously published works at Re ≈ 4,000 to Re ≈ 100,000. The amplitude ratio and range of synchronization in the upper branch increases by spring stiffness. The lower branch of response disappeared due to the high mass-damping parameter in conducted experiments. These observations suggested to employ an adjustable natural frequency system to have an optimum energy harvesting in VIV-based ocean energy converters in an expanded range of operation.

Nonlinear Piecewise Restoring Force in Hydrokinetic Power Conversion Using Flow-Induced Vibrations of Two Tandem Cylinders

Flow-induced vibrations (FIVs) of two tandem, rigid, circular cylinders with piecewise continuous restoring force are investigated for Reynolds number 24,000 ≤ Re ≤ 120,000 with damping, and restoring force function as parameters. Selective roughness is applied to enhance FIV and increase the hydrokinetic energy captured by the vortex-induced vibration for aquatic clean energy (VIVACE) converter. Experimental results for amplitude response, frequency response, interactions between cylinders, 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 as follows: (1) the nonlinear-spring converter can harness energy from flows as slow as 0.33 m/s with no upper limit; (2) the nonlinear-spring converter has better performance at initial galloping than its linear-spring counterpart; (3) the FIV response is predominantly periodic for all nonlinear spring functions used; (4) the influence from the upstream cylinder is becoming more dominant as damping increases; (5) optimal power harnessing is achieved by changing the linear viscous damping and tandem spacing L/D; (6) close spacing ratio L/D = 1.57 has a positive impact on the harnessed power in VIV to galloping transition; and (7) the interactions between two cylinders have a positive impact on the upstream cylinder regardless of the spacing and harness damping.

Nonlinear piecewise restoring force in hydrokinetic power conversion using flow induced motions of single cylinder

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.

Effect of mass-ratio, damping, and stiffness on optimal hydrokinetic energy conversion of a single, rough cylinder in flow induced motions

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.

Numerical simulation and experimental validation for energy harvesting of single-cylinder VIVACE converter with passive turbulence control

Flow Induced Motions (FIM) of a single, rigid, circular cylinder on end-springs are investigated for Reynolds number 30,000 < Re < 110,000. Passive Turbulence Control (PTC) in the form of roughness strips is applied to enhance FIM and increase the efficiency of the VIVACE (Vortex Induced Vibration for Aquatic Clean Energy) converter in harnessing marine hydrokinetic energy. Numerical simulations are performed using a solver for two-dimensional Unsteady Reynolds-Averaged Navier-Stokes equations, which is developed based on OpenFOAM using a finite-volume discretization method. The results are in excellent agreement with experiments conducted in the Marine Renewable Energy Laboratory (MRELab) at the University of Michigan. Amplitude and frequency are predicted correctly in the initial and upper VIV branches, in VIV-to-galloping transition, and in galloping. The 2S (S = Single) vortex pattern is observed in the initial VIV branch and the 2P + 2S (P = Pair) in the upper branch for 30,000 ≤ Re ≤ 80,000. Transition from VIV to galloping initiates at Re = 90,000 with a P + 2S + P + S pattern. Maximum amplitude of 3.5D (D = Diameters) is achieved and more than seven vortices per cycle are observed in fully developed galloping. With PTC, the VIVACE converter can harness hydrokinetic energy from currents or tides over the entire range of FIM synchronization. Energy conversion efficiency reaches 37% in simulations and 28% in experiments.

Surface protrusion based mechanisms of augmenting energy extraction from vibrating cylinders at Reynolds number 3 × 103–3 × 104

The objective of the current experimental work is to investigate the effectiveness of surface protrusion type modifications to a circular cylinder in augmenting fluid induced motions at 3 × 103 &amp;lt; Re &amp;lt; 3 × 104 and falls within the TrSL2 (Transition in Shear Layer) Reynolds number regime. The current experiments build on the previous successful efforts to intentionally enhance oscillations by Bernitsas and group at the University of Michigan, for use in their Fluid Induced Motion based energy harvester Vortex induced vibration for aquatic clean energy (VIVACE) at higher Reynolds number (2–4 × 104 &amp;lt; Re &amp;lt; 1–2 × 105) that falls in the TrSL3 regime. Surface protrusions tested in the current work include three different sandpaper strips of widely varying roughness; and smooth strips that only had a thickness with no embedded roughness. Amplified vortex induced vibrations and galloping oscillations were observed while using cylinder configurations with all strips. The grit size of the sandpapers used also seemed to have an effect on the vibration amplitudes. Different positions of the strip (with respect to the frontal stagnation point) were tested and proved to be of crucial importance as the response showed a drastic variation depending on the position. The variation in response of the cylinder to roughness and strip-position that were observed in the current experiments are different from the experiments performed at TrSL3 regime; thereby suggesting a strong Reynolds number effect. The power harnessing potential of VIV based devices with different surface-protrusion configurations was evaluated based on the experimental results. There appears to be a very conspicuous difference in potential of the energy available especially while executing galloping oscillations. Experiments were also repeated with springs of different stiffnesses which proved to have an effect on the incited galloping oscillations depending on the type of strip employed.

Underwater compressed air energy storage improved through Vortex Hydro Energy

A power generating energy storage system is presented. The proposed self-powered energy storage technology (UWCAES–VHE) is a hybrid of Underwater Compressed Air Energy Storage (UWCAES) and the Vortex Induced Vibration Aquatic Clean Energy (VIVACE) converter invented by Bernitsas and Raghavan [1] to harness Vortex Hydro Energy (VHE). The present technology significantly improves the roundtrip efficiency of conventional UWCAES. Through this hybridization, the energy conversion efficiency of the VIVACE converters performing as the accumulator–converters of the UWCAES–VHE is expected to be higher than that of the conventional VIVACE converters. It is further demonstrated that the round trip efficiency of the UWCAES–VHE and the VHE conversion efficiency of the VIVACE are linearly related. UWCAES–VHE and conventional UWCAES are quantitatively compared.

Lumped parameter models of vortex induced vibration with application to the design of aquatic energy harvester

In the present study, a lumped parameter model for vortex-induced vibrations is analysed. In this work, the vortex-induced vibrations of an elastically mounted rigid cylinder are able to move in-line and transverse to the flow with equal mass ratio and natural frequencies. A simplified lumped mass model is proposed to study the two degree of freedom (dof) structural oscillator. A classical van der Pol equation along with acceleration coupling, models the near wake dynamics describing the fluctuating nature of vortex shedding. The model dynamics is investigated analytically and the results are compared for moderate mass ratios. The results predicted using this model show a good agreement with the experimental data. The dependence of stream-wise displacement on mass and damping is explored. The cause of cross-flow displacement magnification due to freedom to move in stream-wise direction is also explored using the proposed model. Apart from these two degrees of freedom, the cylinder can also undergo rotation about its centre of mass. The effect of freedom to move in this rotational degree of freedom is exploited to our advantage by applying it to the VIVACE (Vortex induced vibration aquatic clean energy) design which was originally proposed by Bernitsas et al. (2008). The original design was not reported to be the optimal one and the set-up was shown to work only for a given flow velocity. But, the flow environment keeps changing and hence there is a need to bring in robustness and optimize the proposed design. The values of optimized spring stiffness have been found using the lumped mass model. The design is made robust by exploiting the rotational mode. This mode is triggered by varying the overhang lengths in accordance with the varying flow velocity in order to strike resonance for a certain flow regime.

2-D URANS vs. experiments of flow induced motions of two circular cylinders in tandem with passive turbulence control for 30,000

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.