Observation and Discussion of Leading Edge Vortex Shedding From Laboratory-Scaled Cross-Flow Hydrokinetic Turbines in Counter-Rotating Configurations

Abstract In 2011, John Dabiri proposed the use of counter-rotating vertical-axis wind turbines to achieve enhanced power output per unit area of a wind farm. Since then, various studies in the wind energy and marine hydrokinetic (MHK) literature have been dedicated to pairs of vertical axis turbines in both co-rotating and counter-rotating configurations, in terms of their power production, wake characterization, and optimal array design. Previous experimental works suggest an enhancement of up to 27.9% in the system power coefficient of pair configurations compared to a single turbine. Additionally, previous numerical studies have indicated that the increased power output is correlated with higher torque on the turbine blades which correspondingly produces a stronger leading edge vortex. This paper presents an extended investigation into a pair of laboratory scaled cross-flow hydrokinetic turbines in counter-rotating configurations. Experiments were conducted to observe, compare, and discuss the leading edge vortex shedding from the turbine blades during their positive torque phase. The turbines operated in a small water flume at the diameter-based Reynolds number of 22,000 with a 0.316 m/s freestream velocity and 4% turbulent intensity. Using a monoscopic particle image velocimetry setup, multiple realizations of the water flow around each blade at their positive torque phase were recorded and phase-averaged. Results show consistent leading vortex shedding at these turbine angles while a correlation between the turbine power performance and the vortex size and strength was observed.

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Limit-cycle oscillations in unsteady flows dominated by intermittent leading-edge vortex shedding

Journal of Fluids and Structures ◽

10.1016/j.jfluidstructs.2015.02.005 ◽

2015 ◽

Vol 55 ◽

pp. 84-105 ◽

Cited By ~ 26

Author(s):

Kiran Ramesh ◽

Joseba Murua ◽

Ashok Gopalarathnam

Keyword(s):

Limit Cycle ◽

Vortex Shedding ◽

Unsteady Flows ◽

Leading Edge ◽

Limit Cycle Oscillations ◽

Leading Edge Vortex ◽

Edge Vortex

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Discrete-vortex method with novel shedding criterion for unsteady aerofoil flows with intermittent leading-edge vortex shedding

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.297 ◽

2014 ◽

Vol 751 ◽

pp. 500-538 ◽

Cited By ~ 119

Author(s):

Kiran Ramesh ◽

Ashok Gopalarathnam ◽

Kenneth Granlund ◽

Michael V. Ol ◽

Jack R. Edwards

Keyword(s):

Vortex Shedding ◽

Leading Edge ◽

Vortex Method ◽

Critical Value ◽

Empirical Parameter ◽

Discrete Vortex ◽

Suction Parameter ◽

Leading Edge Vortex ◽

Edge Vortex ◽

Leading Edges

AbstractUnsteady aerofoil flows are often characterized by leading-edge vortex (LEV) shedding. While experiments and high-order computations have contributed to our understanding of these flows, fast low-order methods are needed for engineering tasks. Classical unsteady aerofoil theories are limited to small amplitudes and attached leading-edge flows. Discrete-vortex methods that model vortex shedding from leading edges assume continuous shedding, valid only for sharp leading edges, or shedding governed by ad-hoc criteria such as a critical angle of attack, valid only for a restricted set of kinematics. We present a criterion for intermittent vortex shedding from rounded leading edges that is governed by a maximum allowable leading-edge suction. We show that, when using unsteady thin aerofoil theory, this leading-edge suction parameter (LESP) is related to the $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}A_0$ term in the Fourier series representing the chordwise variation of bound vorticity. Furthermore, for any aerofoil and Reynolds number, there is a critical value of the LESP, which is independent of the motion kinematics. When the instantaneous LESP value exceeds the critical value, vortex shedding occurs at the leading edge. We have augmented a discrete-time, arbitrary-motion, unsteady thin aerofoil theory with discrete-vortex shedding from the leading edge governed by the instantaneous LESP. Thus, the use of a single empirical parameter, the critical-LESP value, allows us to determine the onset, growth, and termination of LEVs. We show, by comparison with experimental and computational results for several aerofoils, motions and Reynolds numbers, that this computationally inexpensive method is successful in predicting the complex flows and forces resulting from intermittent LEV shedding, thus validating the LESP concept.

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Flow Field Measurement of Laboratory-Scaled Cross-Flow Hydrokinetic Turbines: Part I—The Near-Wake of a Single Turbine

Journal of Marine Science and Engineering ◽

10.3390/jmse9050489 ◽

2021 ◽

Vol 9 (5) ◽

pp. 489

Author(s):

Minh N. Doan ◽

Yuriko Kai ◽

Takuya Kawata ◽

Shinnosuke Obi

Keyword(s):

Flow Field ◽

Cross Flow ◽

Average Diameter ◽

Vertical Axis ◽

Power Production ◽

Particle Image Velocimetry Technique ◽

Freestream Velocity ◽

Hydrokinetic Turbines ◽

Marine Energy ◽

Recent Developments

Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2×105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.

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Modulation of the Leading-Edge Vortex Shedding Rate in Discrete-Vortex Methods

10.2514/6.2022-2416 ◽

2022 ◽

Author(s):

Alfonso Martínez ◽

Guosheng He ◽

Karen Mulleners ◽

Kiran Kumar Ramesh

Keyword(s):

Vortex Shedding ◽

Leading Edge ◽

Vortex Methods ◽

Discrete Vortex ◽

Leading Edge Vortex ◽

Edge Vortex

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The aerodynamics of hovering insect flight. VI. Lift and power requirements

Philosophical Transactions of the Royal Society of London Series B Biological Sciences ◽

10.1098/rstb.1984.0054 ◽

1984 ◽

Vol 305 (1122) ◽

pp. 145-181 ◽

Cited By ~ 234

Keyword(s):

Vortex Shedding ◽

Power Output ◽

Flight Muscle ◽

Insect Flight ◽

Separation Bubble ◽

Leading Edge ◽

Mechanical Power ◽

Mechanical Power Output ◽

Edge Vortex ◽

Edge Separation

The lift and power requirements for hovering insect flight are estimated by combining the morphological and kinematic data from papers II and III with the aerodynamic analyses of papers IV and V. The lift calculations are used to evaluate the importance in hovering of two distinct types of aerodynamic mechanisms: (i) the usual quasi-steady mechanism, where the circulation for lift is primarily determined by translation of the wing, and (ii) rotational mechanisms, where the circulation is largely governed by wing rotation at either end of the wingbeat. Power estimates are compared with the available measurements of metabolic rate during hovering to investigate the role of elastic energy storage, the maximum mechanical power output of the flight muscles, and the muscle efficiency. The quasi-steady mechanism proves inadequate for the lift requirements of hover-flies using an inclined stroke plane, and for a ladybird beetle and a crane-fly hovering with a horizontal stroke plane. Observed angles of attack rule out lift enhancement by unsteady modifications to the quasi-steady mechanism, such as delayed stall, but the rotational lift mechanisms proposed in paper IV seem consistent with the kinematics. The rotational mechanisms rely on concentrated vortex shedding from the leading edge during rotation, with attachment of that vorticity as a leading edge separation bubble during the subsequent half-stroke. Strong leading edge vortex shedding should result from delayed pronation for the hover-fly, a near fling and partial fling for the ladybird, and profile flexion for the crane-fly (the flex mechanism). The kinematics for the other insects hovering with a horizontal stroke plane are basically the same as for the anomalous crane-fly, and the quasi-steady mechanism cannot be accepted for them while rejecting it for the crane-fly. All of these insects flex their wings in a similar manner during rotation, and could use the flex mechanism for lift generation. The implication is that most, if not all, hovering animals do not rely on quasi-steady aerodynamics, but use rotational lift mechanisms instead. It is not possible to reconcile the power estimates with the commonly accepted values of both the mechanochemical efficiency of insect flight muscle (about 25%) and its maximum mechanical power output (about 20 W N -1 of muscle). Maximum efficiencies of 12-29% could be obtained only if there is no elastic storage of the kinetic energy of the flapping wings, but this would require more than twice the accepted value for maximum mechanical power output. The available evidence suggests that substantial elastic storage does occur, and that the maximum mechanical power output is close to the accepted value. If so, then the efficiency of both fibrillar and non-fibrillar flight muscle is likely to be only 5-9%.

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