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Article ; Online: Optimal blade pitch control for enhanced vertical-axis wind turbine performance.

Le Fouest, Sébastien / Mulleners, Karen

2024 Volume 15, Issue 1, Page(s) 2770

Abstract: Vertical-axis wind turbines are great candidates to enable wind power extraction in urban and off-shore applications. Currently, concerns around turbine efficiency and structural integrity limit their industrial deployment. Flow control can mitigate ... ...

Abstract	Vertical-axis wind turbines are great candidates to enable wind power extraction in urban and off-shore applications. Currently, concerns around turbine efficiency and structural integrity limit their industrial deployment. Flow control can mitigate these concerns. Here, we experimentally demonstrate the potential of individual blade pitching as a control strategy and explain the flow physics that yields the performance enhancement. We perform automated experiments using a scaled-down turbine model coupled to a genetic algorithm optimiser to identify optimal pitching kinematics at on- and off-design operating conditions. We obtain two sets of optimal pitch profiles that achieve a three-fold increase in power coefficient at both operating conditions compared to the non-actuated turbine and a 77% reduction in structure-threatening load fluctuations at off-design conditions. Based on flow field measurements, we uncover how blade pitching manipulates the flow structures to enhance performance. Our results can aid vertical-axis wind turbines increase their much-needed contribution to our energy needs.
Language	English
Publishing date	2024-03-30
Publishing country	England
Document type	Journal Article
ZDB-ID	2553671-0
ISSN	2041-1723 ; 2041-1723
ISSN (online)	2041-1723
ISSN	2041-1723
DOI	10.1038/s41467-024-46988-0
Database	MEDical Literature Analysis and Retrieval System OnLINE

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Article ; Online: Phenomenology and scaling of optimal flapping wing kinematics.

Gehrke, Alexander / Mulleners, Karen

Bioinspiration & biomimetics

2021 Volume 16, Issue 2

Abstract: Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping wing flight are dominated by large-scale vortical structures ... ...

Abstract	Biological flapping wing fliers operate efficiently and robustly in a wide range of flight conditions and are a great source of inspiration to engineers. The unsteady aerodynamics of flapping wing flight are dominated by large-scale vortical structures that augment the aerodynamic performance but are sensitive to minor changes in the wing actuation. We experimentally optimise the pitch angle kinematics of a flapping wing system in hover to maximise the stroke average lift and hovering efficiency with the help of an evolutionary algorithm and
MeSH term(s)	Animals ; Biomechanical Phenomena ; Flight, Animal ; Models, Biological ; Wings, Animal
Language	English
Publishing date	2021-01-29
Publishing country	England
Document type	Journal Article ; Research Support, Non-U.S. Gov't
ZDB-ID	2235670-8
ISSN	1748-3190 ; 1748-3182
ISSN (online)	1748-3190
ISSN	1748-3182
DOI	10.1088/1748-3190/abd012
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Article ; Online: To tread or not to tread: comparison between water treading and conventional flapping wing kinematics.

Krishna, Swathi / Gehrke, Alexander / Mulleners, Karen

Bioinspiration & biomimetics

2022 Volume 17, Issue 6

Abstract: Hovering insects are limited by their physiology and need to rotate their wings at the end of each back-and-forth motion to keep the wing's leading edge ahead of its trailing edge. The wing rotation at the end of each half-stroke pushes the leading edge ... ...

Abstract	Hovering insects are limited by their physiology and need to rotate their wings at the end of each back-and-forth motion to keep the wing's leading edge ahead of its trailing edge. The wing rotation at the end of each half-stroke pushes the leading edge vortex away from the wing which leads to a loss in the lift. Unlike biological fliers, human-engineered flapping wing micro air vehicles have different design limitations. They can be designed to avoid the end of stroke wing rotation and use so-called water-treading flapping kinematics. Flapping wings using conventional flapping kinematics have a designated leading and trailing edge. In the water-treading mode, the role of the leading and trailing edges are continuously alternated throughout the stroke. Here, we compare velocity field and force measurements for a rectangular flapping wing conducting normal hovering and water-treading kinematics to study the difference in fluid dynamic performance between the two types of flapping kinematics. We show that for similar power consumption, the water-treading mode produces more lift than the conventional hovering mode and is 50% more efficient for symmetric pitching kinematics. In the water-treading mode, the leading edge vortex from the previous stroke is not pushed away but is captured and keeps the newly formed leading edge vortex closer to the wing, leading to a more rapid increase of the lift coefficient which is sustained for longer. This makes the water-treading mode a promising alternative for human-engineered flapping wing vehicles.
MeSH term(s)	Animals ; Humans ; Flight, Animal/physiology ; Biomechanical Phenomena/physiology ; Water ; Biomimetics ; Models, Biological ; Wings, Animal/physiology ; Stroke
Chemical Substances	Water (059QF0KO0R)
Language	English
Publishing date	2022-11-03
Publishing country	England
Document type	Journal Article ; Research Support, Non-U.S. Gov't
ZDB-ID	2235670-8
ISSN	1748-3190 ; 1748-3182
ISSN (online)	1748-3190
ISSN	1748-3182
DOI	10.1088/1748-3190/ac9a1b
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Article ; Online: Aeroelastic characterisation of a bio-inspired flapping membrane wing.

Gehrke, Alexander / Richeux, Jules / Uksul, Esra / Mulleners, Karen

Bioinspiration & biomimetics

2022 Volume 17, Issue 6

Abstract: Natural fliers like bats exploit the complex fluid-structure interaction between their flexible membrane wings and the air with great ease. Yet, replicating and scaling the balance between the structural and fluid-dynamical parameters of unsteady ... ...

Abstract	Natural fliers like bats exploit the complex fluid-structure interaction between their flexible membrane wings and the air with great ease. Yet, replicating and scaling the balance between the structural and fluid-dynamical parameters of unsteady membrane wings for engineering applications remains challenging. In this study, we introduce a novel bio-inspired membrane wing design and systematically investigate the fluid-structure interactions of flapping membrane wings. The membrane wing can passively camber, and its leading and trailing edges rotate with respect to the stroke plane. We find optimal combinations of the membrane properties and flapping kinematics that out-perform their rigid counterparts both in terms of increased stroke-average lift and efficiency, but the improvements are not persistent over the entire input parameter space. The lift and efficiency optima occur at different angles of attack and effective membrane stiffnesses which we characterise with the aeroelastic number. At optimal aeroelastic numbers, the membrane has a moderate camber between 15% and 20% and its leading and trailing edges align favourably with the flow. Higher camber at lower aeroelastic numbers leads to reduced aerodynamic performance due to negative angles of attack at the leading edge and an over-rotation of the trailing edge. Most of the performance gain of the membrane wings with respect to rigid wings is achieved in the second half of the stroke when the wing is decelerating. The stroke-maximum camber is reached around mid-stroke but is sustained during most of the remainder of the stroke which leads to an increase in lift and a reduction in power. Our results show that combining the effect of variable stiffness and angle of attack variation can significantly enhance the aerodynamic performance of membrane wings and has the potential to improve the control capabilities of micro air vehicles.
MeSH term(s)	Animals ; Biomechanical Phenomena ; Flight, Animal ; Models, Biological ; Rotation ; Wings, Animal
Language	English
Publishing date	2022-09-13
Publishing country	England
Document type	Journal Article ; Research Support, Non-U.S. Gov't
ZDB-ID	2235670-8
ISSN	1748-3190 ; 1748-3182
ISSN (online)	1748-3190
ISSN	1748-3182
DOI	10.1088/1748-3190/ac8632
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Book ; Online: To tread or not to tread

Krishna, Swathi / Mulleners, Karen

comparison between water treading and conventional flapping wing kinematics

2022

Abstract: Hovering insects are limited by their physiology and need to rotate their wings at the end of each back and forth motion to keep the wing's leading edge ahead of its trailing edge. The wing rotation at the end of each half-stroke pushes the leading edge ... ...

Abstract	Hovering insects are limited by their physiology and need to rotate their wings at the end of each back and forth motion to keep the wing's leading edge ahead of its trailing edge. The wing rotation at the end of each half-stroke pushes the leading edge vortex away from the wing which leads to a loss in the lift. Unlike biological fliers, human-engineered flapping wing micro air vehicles have different design limitations. They can be designed to avoid the end of stroke wing rotation and use so-called water-treading flapping kinematics. Flapping wings using conventional flapping kinematics have a designated leading and trailing edge. In the water-treading mode, the role of the leading and trailing edges are continuously alternated throughout the stroke. Here, we compare velocity field and force measurements for a rectangular flapping wing conducting normal hovering and water-treading kinematics to study the difference in fluid dynamic performance between the two types of flapping kinematics. We show that for similar power consumption, the water-treading mode produces more lift than the conventional hovering mode and is 50% more efficient for symmetric pitching kinematics. This makes the water-treading mode a promising alternative for human-engineered flapping wing vehicles.
Keywords	Physics - Fluid Dynamics
Subject code	629
Publishing date	2022-07-28
Publishing country	us
Document type	Book ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article: The dynamic stall dilemma for vertical-axis wind turbines

Le Fouest, Sébastien / Mulleners, Karen

Renewable energy. 2022 Oct., v. 198

2022

Abstract: Vertical-axis wind turbines (VAWT) are excellent candidates to complement traditional wind turbines and increase the total wind energy capacity. Development of VAWT has been hampered by their low efficiency and structural unreliability, which are related ...

Abstract	Vertical-axis wind turbines (VAWT) are excellent candidates to complement traditional wind turbines and increase the total wind energy capacity. Development of VAWT has been hampered by their low efficiency and structural unreliability, which are related to the occurrence of dynamic stall. Dynamic stall consists of the formation, growth, and shedding of large-scale vortices, followed by massive flow separation. The vortex shedding is detrimental to the turbine's efficiency and causes significant load fluctuations that jeopardise the turbine's structural integrity. We present a comprehensive experimental characterisation of dynamic stall on a VAWT blade including time-resolved load and velocity field measurements. Particular attention is dedicated to the dilemma faced by VAWT to either operate at lower tip-speed ratios to maximise their peak aerodynamic performance but experience dynamic stall, or to avoid dynamic stall at the cost of reducing their peak performance. Based on the results, we map turbine operating conditions to one of three regimes: deep stall, light stall, and no stall. The light stall regime offers VAWT the best compromise in the dynamic stall dilemma as it yields positive tangential forces during the upwind and downwind rotation and reduces load transients by 75% compared to the deep stall regime.
Keywords	aerodynamics ; wind ; wind power
Language	English
Dates of publication	2022-10
Size	p. 505-520.
Publishing place	Elsevier Ltd
Document type	Article
ZDB-ID	2001449-1
ISSN	1879-0682 ; 0960-1481
ISSN (online)	1879-0682
ISSN	0960-1481
DOI	10.1016/j.renene.2022.07.071
Database	NAL-Catalogue (AGRICOLA)

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Article ; Online: Identification of the trade-off between speed and efficiency in undulatory swimming using a bio-inspired robot.

Anastasiadis, Alexandros / Paez, Laura / Melo, Kamilo / Tytell, Eric D / Ijspeert, Auke J / Mulleners, Karen

Scientific reports

2023 Volume 13, Issue 1, Page(s) 15032

Abstract: Anguilliform swimmers, like eels or lampreys, are highly efficient swimmers. Key to understanding their performances is the relationship between the body's kinematics and resulting swimming speed and efficiency. But, we cannot prescribe kinematics to ... ...

Abstract	Anguilliform swimmers, like eels or lampreys, are highly efficient swimmers. Key to understanding their performances is the relationship between the body's kinematics and resulting swimming speed and efficiency. But, we cannot prescribe kinematics to living fish, and it is challenging to measure their power consumption. Here, we characterise the swimming speed and cost of transport of a free-swimming undulatory bio-inspired robot as we vary its kinematic parameters, including joint amplitude, body wavelength, and frequency. We identify a trade-off between speed and efficiency. Speed, in terms of stride length, increases for increasing maximum tail angle, described by the newly proposed specific tail amplitude and reaches a maximum value around the specific tail amplitude of unity. Efficiency, in terms of the cost of transport, is affected by the whole-body motion. Cost of transport decreases for increasing travelling wave-like kinematics, and lower specific tail amplitudes. Our results suggest that live eels tend to choose efficiency over speed and provide insights into the key characteristics affecting undulatory swimming performance.
MeSH term(s)	Animals ; Robotics ; Swimming ; Eels ; Lampreys ; Motion
Language	English
Publishing date	2023-09-12
Publishing country	England
Document type	Journal Article ; Research Support, U.S. Gov't, Non-P.H.S. ; Research Support, Non-U.S. Gov't
ZDB-ID	2615211-3
ISSN	2045-2322 ; 2045-2322
ISSN (online)	2045-2322
ISSN	2045-2322
DOI	10.1038/s41598-023-41074-9
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Book ; Online: All you need is time to generalise the Goman-Khrabrov dynamic stall model

Ayancik, Fatma / Mulleners, Karen

2021

Abstract: Dynamic stall on airfoils negatively impacts their aerodynamic performance and can lead to structural damage. Accurate prediction and modelling of the dynamic stall loads are crucial for a more robust design of wings and blades that operate under ... ...

Abstract	Dynamic stall on airfoils negatively impacts their aerodynamic performance and can lead to structural damage. Accurate prediction and modelling of the dynamic stall loads are crucial for a more robust design of wings and blades that operate under unsteady conditions susceptible to dynamic stall and for widening the range of operation of these lifting surfaces. Many dynamic stall models rely on empirical parameters that need to be obtained from experimental or numerical data which limits their generalisability. Here, we introduce physically derived times scales to replace the empirical parameters in the Goman-Khrabrov dynamic stall model. The physics-based time constants correspond to the dynamic stall delay and the decay rate of post-stall load fluctuations. The dynamic stall delay is largely independent of the type of the motion, the Reynolds number, and the airfoil geometry and is described as a function of a normalised instantaneous pitch rate. The post-stall decay rate is independent of the motion kinematics and is directly related to the Strouhal number of the post-stall vortex shedding. The general validity of our physics-based time constants is demonstrated using three sets of experimental dynamic stall data covering various airfoil profiles, Reynolds numbers varying from 75'000 to 1'000'000, and sinusoidal and ramp-up pitching motions. The use of physics-based time constants removes the empiricism of the Goman-Khrabrov dynamic stall model and extends its range of application. It also opens new opportunities for closed-loop flow control applications.
Keywords	Physics - Fluid Dynamics
Subject code	629
Publishing date	2021-10-16
Publishing country	us
Document type	Book ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Book ; Online: Timescales of dynamic stall development on a vertical-axis wind turbine blade

Fouest, Sébastien Le / Fernex, Daniel / Mulleners, Karen

2022

Abstract: Vertical-axis wind turbines are great candidates to diversify wind energy technology, but their aerodynamic complexity limits industrial deployment. To improve the efficiency and lifespan of vertical axis wind turbines, we desire data-driven models and ... ...

Abstract	Vertical-axis wind turbines are great candidates to diversify wind energy technology, but their aerodynamic complexity limits industrial deployment. To improve the efficiency and lifespan of vertical axis wind turbines, we desire data-driven models and control strategies that take into account the timing and duration of subsequent events in the unsteady flow development. Here, we aim to characterise the chain of events that leads to dynamic stall on a vertical-axis wind turbine blade and to quantify the influence of the turbine operation conditions on the duration of the individual flow development stages. We present time-resolved flow and unsteady load measurements of a wind turbine model undergoing dynamic stall for a wide range of tip-speed ratios. Proper orthogonal decomposition is used to identify dominant flow structures and to distinguish six characteristic stall stages: the attached flow, shear-layer growth, vortex formation, upwind stall, downwind stall, and flow reattachment stage. The timing and duration of the individual stages are best characterised by the non-dimensional convective time. Dynamic stall stages are also identified based on aerodynamic force measurements. Most of the aerodynamic work is done during the shear-layer growth and the vortex formation stage which underlines the importance of managing dynamic stall on vertical-axis wind turbines.
Keywords	Physics - Fluid Dynamics
Subject code	551 ; 690
Publishing date	2022-10-28
Publishing country	us
Document type	Book ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Book ; Online: Aeroelastic characterisation of a bio-inspired flapping membrane wing

Gehrke, Alexander / Richeux, Jules / Uksul, Esra / Mulleners, Karen

2022

Abstract	Natural fliers like bats exploit the complex fluid-structure interaction between their flexible membrane wings and the air with great ease. Yet, replicating and scaling the balance between the structural and fluid-dynamical parameters of unsteady membrane wings for engineering applications remains challenging. In this study, we introduce a novel bio-inspired membrane wing design and systematically investigate the fluid-structure interactions of flapping membrane wings. The membrane wing can passively camber and its leading and trailing edges rotate with respect to the stroke plane. We find optimal combinations of the membrane properties and flapping kinematics that out-perform their rigid counterparts both in terms of increased stroke-average lift and efficiency but the improvements are not persistent over the entire input parameter space. The lift and efficiency optima occur at different angles of attack and effective membrane stiffnesses which we characterise with the aeroelastic number. At optimal aeroelastic numbers, the membrane has a moderate camber between 15% and 20% and its leading and trailing edges align favourably with the flow. Higher camber at lower aeroelastic numbers leads to reduced aerodynamic performance due to negative angles of attack at the leading edge and an over-rotation of the trailing edge. Most of the performance gain of the membrane wings with respect to rigid wings is achieved in the second half of the stroke when the wing is decelerating. The stroke-maximum camber is reached around mid-stroke but is sustained during most of the remainder of the stroke which leads to an increase in lift and a reduction in power. Our results show that combining the effect of variable stiffness and angle of attack variation can significantly enhance the aerodynamic performance of membrane wings and has the potential to improve the control capabilities of micro air vehicles.
Keywords	Physics - Fluid Dynamics
Subject code	590
Publishing date	2022-03-26
Publishing country	us
Document type	Book ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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