Passive Turbulence Control Research Articles

Sensitivity to Zone Covering of the Map of Passive Turbulence Control to Flow-Induced Motions for a Circular Cylinder at 30,000 ≤ Re ≤ 120,000

Passive turbulence control (PTC) in the form of two straight roughness strips with variable width, and thickness about equal to the boundary layer thickness, is used to modify the flow-induced motions (FIM) of a rigid circular cylinder. The cylinder is supported by two end springs and the flow is in the TrSL3, high-lift, regime. The PTC-to-FIM Map, developed in the previous work, revealed zones of weak suppression (WS), strong suppression (SS), hard galloping (HG), and soft galloping (SG). In this paper, the sensitivity of the PTC-to-FIM map to: (a) the width of PTC covering, (b) PTC covering a single or multiple zones, and (c) PTC being straight or staggered is studied experimentally. Experiments are conducted in the low turbulence free surface water channel of the University of Michigan, Ann Arbor, MI. Fixed parameters are: cylinder diameter D = 8.89 cm, m* = 1.725, spring stiffness K = 763 N/m, aspect ratio l/D = 10.29, and damping ratio ζ = 0.019. Variable parameters are circumferential PTC location αPTC∈ (0–180 deg), Reynolds number Re ∈ (30,000–120,000), flow velocity U∈ (0.36–1.45 m/s). Measured quantities are amplitude ratio A/D, frequency ratio fosc/fn,w, and synchronization range. As long as the roughness distribution is limited to remain within a zone, the width of the strips does not affect the FIM response. When multiple zones are covered, the strong suppression zone dominates the FIM.

Numerical Study on the Effect of Tandem Spacing on Flow-Induced Motions of Two Cylinders With Passive Turbulence Control

The effect of tandem spacing on the flow-induced motions (FIM) of two circular cylinders with passive turbulence control is investigated using two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes equations with the Spalart–Allmaras turbulence model. Results are compared to experiments in the range of Reynolds number of 30,000 < Re < 100,000. The center-to-center spacing between the two cylinders is varied from 2 to 6 diameters. Simulation results predict well all the ranges of FIM including vortex-induced vibrations (VIV) and galloping and match well with experimental measurements. For the upstream cylinder, the amplitude and frequency responses are not considerably influenced by the downstream cylinder when the spacing is greater than 2D. For the downstream cylinder, a rising amplitude trend in the VIV upper-branch can be observed in all the cases as is typical of flows in the TrSL3 flow regime (transition in shear layer 3; 2 × 104 < Re < 3 × 105). The galloping branch merges with the VIV upper-branch for spacing greater than three-dimensional (3D). Vortex structures show significant variation in different flow regimes in accordance with experimental observations. High-resolution postprocessing shows that the interaction between the wakes of cylinders results in various types of FIM.

Computational and Experimental Assessment of Turbulence Stimulation on Flow Induced Motion of a Circular Cylinder

Vortex-induced vibrations (VIVs) are highly nonlinear and it is hard to approach the problem analytically or computationally. Experimental investigation is therefore essential to address the problem and reveal some physical aspects of VIV. Although computational fluid dynamics (CFDs) offers powerful methods to generate solutions, it cannot replace experiments as yet. When used as a supplement to experiments, however, CFD can be an invaluable tool to explore some underlying issues associated with such complicated flows that could otherwise be impossible or very expensive to visualize or measure experimentally. In this paper, VIVs and galloping of a cylinder with selectively distributed surface roughness—termed passive turbulence control (PTC)—are investigated experimentally and computationally. The computational approach is first validated with benchmark experiments on smooth cylinders available in the literature. Then, experiments conducted in the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan are replicated computationally to visualize the flow and understand the effects of thickness and width of roughness strips placed selectively on the cylinder. The major outcomes of this work are: (a) Thicker PTC initiates earlier galloping but wider PTC does not have a major impact on the response of the cylinder and (b) The amplitude response is restricted in VIV due to the dead fluid zone attached to the cylinder, which is not observed in galloping.

Performance prediction of horizontal hydrokinetic energy converter using multiple-cylinder synergy in flow induced motion

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.

Suppression of vortex-induced vibrations of rigid circular cylinder on springs by localized surface roughness at 3×104≤Re≤1.2×105

Suppression of vortex-induced vibrations (VIV) of an elastically mounted circular cylinder in a steady flow is studied experimentally using localized surface roughness, called Passive Turbulence Control (PTC). PTC consists of two roughness strips with thickness on the order of the boundary layer thickness and placed parallel to the cylinder axis symmetrically with respect to the flow. The range of Reynolds number (3×104≤Re≤1.2×105) considered covers primarily the Transition to Shear Layer3, high-lift, flow regime. For the smooth cylinder, the broad synchronization range and higher peak amplitude in the upper branch and unstable oscillations in the lower branch are characteristic of VIV at higher Reynolds numbers and distinctive from those of lower Reynolds numbers. From the PTC-cylinder results with systematic variation of the PTC location, one strong-suppression zone and two weak-suppression zones are identified based on the location of the PTC. These zones are part of the Map that established the relation between PTC and Flow-Induced Motions in previous work. In the strong suppression zone, reduction of 30–80% is achieved while in the weak suppression zones reduction of less than 30% is achieved. Broad field-of-view flow visualization shows that smooth cylinder vortex structures change with PTC and between zones.

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.

URANS vs. experiments of flow induced motions of multiple circular cylinders with passive turbulence control

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.

Flow induced motion and energy harvesting of bluff bodies with different cross sections

The flow induced motion (FIM) and energy conversion of cylinders with different cross sections are investigated using two-dimensional unsteady Reynolds-Averaged Navier–Stokes simulations in the Reynolds number range of 10,000<Re<130,000. The model for energy harvesting in FIM is established and verified by experimental measurements. For the PTC-cylinder (circular cylinder with passive turbulence control), square cylinder, Q-trapezoid I (quasi-trapezoid cylinder with the long edge as the windward side), and triangular prism, energy can be obviously harvested when Re>30,000. The initial and upper branches of vortex induced vibration (VIV), transition from VIV to galloping, and galloping branch are clearly observed in the amplitude and frequency responses. The FIM responses of PTC-cylinder and Q-trapezoid I are stronger than the other cylinders. The maximum amplitude of 3.5D is achieved and 16 vortices are captured in one cycle in the fully-developed galloping branch. The optimum regime for energy harvesting is the VIV upper branch. And the PTC-cylinder and Q-trapezoid I have better performance on energy harvesting in FIM than other cylinders. The maximum energy efficiencies of 45.7% and 37.9% are achieved for Q-trapezoid I and PTC-cylinder respectively. Contrarily, the vibration of Q-trapezoid II (quasi-trapezoid cylinder with the short edge as the windward side) displays a quite different character with low amplitude and high frequency, and the vortex pattern is a constant 2S in the test Re range.

RANS Simulation Versus Experiments of Flow Induced Motion of Circular Cylinder With Passive Turbulence Control at 35,000 &lt; RE &lt; 130,000

Two-dimensional (2D) Unsteady Reynolds-Averaged Navier–Stokes equations (URANS) equations with the Spalart–Allmaras turbulence model are used to simulate the flow and body kinematics of the transverse motion of spring-mounted circular cylinder. The flow is in the high-lift TrSL3 regime of a Reynolds number in the range 35,000 &lt; Re &lt; 130,000. Passive turbulence control (PTC) in the form of selectively distributed surface roughness is used to alter the cylinder flow induced motion (FIM). Simulation is performed using a solver based on the open source Computational Fluid Dynamics (CFD) tool OpenFOAM, which solves continuum mechanics problems with a finite-volume discretization method. Roughness parameters of PTC are chosen based on tests conducted in the Marine Renewable Energy Lab (MRELab) of the University of Michigan. The numerical tool is first tested on smooth cylinder in vortex-induced vibration (VIV) and results are compared with available experimental measurements and URANS simulations. For the cylinder with PTC cases, the sandpaper grit on the cylinder wall is modeled as a rough-wall boundary condition. Two sets of cases with different system parameters (spring, damping) are simulated and the results are compared with experimental data measured in the MRELab. The amplitude ratio curve shows clearly three different branches, including the VIV initial and upper branches, and a galloping branch. The numerical branches are similar to those observed experimentally. Frequency ratio, vortex patterns, transitional behavior, and lift are also predicted well for PTC cylinders at such high Reynolds numbers.

Enhancement of flow-induced motion of rigid circular cylinder on springs by localized surface roughness at 3×104≤Re≤1.2×105

Passive turbulence control (PTC) in the form of selectively distributed surface roughness is applied on a rigid circular cylinder on two end-springs. The cylinder is placed perpendicular to a uniform steady flow and the cylinder response is measured experimentally for 3×104≤Re≤1.2×105 and broad ranges of the main PTC parameters. PTC consists of two roughness strips placed parallel to the cylinder axis and symmetrically to the flow with thickness on the order of the boundary layer thickness. Different flow-induced motion (FIM) is observed depending primarily on the circumferential location of the two strips. FIM enhancement is studied in this paper in the soft galloping and the two hard galloping zones identified in the PTC-to-FIM Map. In galloping, amplitudes of oscillation reach 2.9 times the cylinder diameter limited only by the free-surface and bottom-boundary effects of the experimental facility. The galloping range follows the VIV range thus expanding dramatically the FIM range. Enhancement of FIM is needed to convert more hydrokinetic energy to mechanical and subsequently to electrical energy over broad velocity range. Use of laser broad field-of-view visualization reveals very different vortex structures between VIV and galloping confirming the fundamentally different driving mechanism of these two FIM kinds.

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.

Multicylinder Flow-Induced Motions: Enhancement by Passive Turbulence Control at 28,000&lt;Re&lt;120,000

The VIVACE converter was introduced at OMAE2006 as a single, smooth, circular-cylinder module. The hydrodynamics of VIVACE is being improved continuously to achieve higher density in harnessed hydrokinetic power. Intercylinder spacing and passive turbulence control (PTC) through selectively located roughness are effective tools in enhancement of flow induced motions (FIMs) under high damping for power harnessing. Single cylinders harness energy at high density even in 1 knot currents. For downstream cylinders, questions were raised on energy availability and sustainability of high-amplitude FIM. Through PTC and intercylinder spacing, strongly synergetic FIMs of 2/3/4 cylinders are achieved. Two-cylinder smooth/PTC, and three/four-cylinder PTC systems are tested experimentally. Using the “PTC-to-FIM” map developed in previous work at the Marine Renewable Energy Laboratory (MRELab), PTC is applied and cylinder response is measured for inflow center-to-center distance 2D-5D (D = diameter), transverse center-to-center distance 0.5–1.5 D, Re ε [28,000–120,000], m* ε [1.677–1.690], U ε [0.36–1.45 m/s], aspect ratio l/D = 10.29, and m*ζ ε [0.0283–0.0346]. All experiments are conducted in the low turbulence free surface water (LTFSW) channel of MRELab. Amplitude spectra and broad field-of-view (FOV) visualization help reveal complex flow structures and cylinder interference undergoing VIV, interference/ proximity/wake/soft/hard galloping. FIM amplitudes of 2.2–2.8D are achieved for all cylinders in steady flow for all parameter ranges tested.

Selective Roughness in the Boundary Layer to Suppress Flow-Induced Motions of Circular Cylinder at 30,000&lt;Re&lt;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.

VIV and galloping of single circular cylinder with surface roughness at 3.0×10 4≤ Re≤1.2×10 5

Passive Turbulence Control (PTC) in the form of selectively distributed surface roughness is used to alter Flow Induced Motion (FIM) of a circular cylinder in a steady flow. The objective is to enhance FIM's synchronization range and amplitude, thus maximizing conversion of hydrokinetic energy into mechanical energy by oscillator in vortex-induced vibration and/or galloping. Through additional viscous damping, mechanical energy is converted to electrical harnessing clean and renewable energy from ocean/river currents. High Reynolds numbers ( Re) are required to reach the high-lift TrSL3 (Transition-Shear-Layer-3) flow regime. PTC trips flow separation and energizes the boundary layer, thus inducing higher vorticity and consequently lift. Roughness location, surface coverage, and size are studied using systematic model tests with broad-field laser visualization at 3.0×10 4< Re<1.2×10 5 in the low-turbulence free-surface water-channel of the Marine Renewable Energy Laboratory of the University of Michigan. Test results show that 16° roughness coverage is effective in the range (10°–80°) inducing reduced vortex-induced vibration (VIV), enhanced VIV, or galloping. Range of synchronization may increase or decrease, galloping amplitude of oscillation reaches three diameters; wake structures change dramatically reaching up to ten vortices per cycle. Conversion of hydrokinetic energy to mechanical is enhanced strongly with proper PTC.