Flow curvature effects on the parallel velocity shear driven instability: MHD simulations

2019 ◽  
Vol 174 (7-8) ◽  
pp. 691-696
Author(s):  
D. Lin ◽  
W. Scales ◽  
S. Sen
Keyword(s):  
Velocity Shear ◽  
Curvature Effects ◽  
Mhd Simulations ◽  
Flow Curvature

Influence of the Blade-Spoke Connection Point on the Aerodynamic Performance of Darrieus Wind Turbines

2016 ◽  
Author(s):  
Alessandro Bianchini ◽  
Francesco Balduzzi ◽  
Giovanni Ferrara ◽  
Lorenzo Ferrari
Keyword(s):  
Wind Turbines ◽  
Design Parameter ◽  
Flow Direction ◽  
Power Coefficient ◽  
Speed Ratio ◽  
Common Solution ◽  
Curvature Effects ◽  
Flow Curvature ◽  
Low Incidence

The assessment of robust CFD techniques is casting new light on the aerodynamics of airfoils rotating around an axis orthogonal to flow direction, with particular reference to flow curvature effects and stall mechanisms. In particular, Darrieus wind turbines’ designers are taking profit from these new discovers to improve the aerodynamic design of the rotors, in view of an increase of the overall efficiency and a reduction of the structural stresses on the blades. A controversial design parameter for Darrieus turbines, especially in case of small-size rotors, is represented by the location of the blade-spoke connection along the chord. The most common solution is indeed to place the connection at approximately airfoil’s quarter chord, i.e. where the pressure center is commonly located for low incidence angles. In some cases, however, the blade is connected at middle chord due to symmetry or aesthetic reasons. In some small turbines, innovative designs have even disregarded this parameter. Even if one can argue that the blade connection point is about to have some aerodynamic effects on the turbine’s performance, the real impact of this important design parameter is often not fully understood. The present study makes use of extensive CFD simulations on a literature case study, using a NACA 0021 airfoil, to assess the influence of the blade-spoke connection point. In particular, the differences in terms of power coefficient curve of the turbine, optimal tip-speed ratio, torque profiles and stresses on the connection are analyzed and discussed. Detailed flow analyses are also shown for azimuthal positions of particular interest. Results on the selected case study showed that the middle-chord blade-spoke connection point seems to guarantee a higher performance of the rotor, even if additional solicitation is applied to the connection itself. It is further shown that the same performance can indeed be obtained with the airfoil attached at quarter chord and properly pitched. By doing so, the stresses are contained and the performance is maximized.

Blade Design Criteria to Compensate the Flow Curvature Effects in H-Darrieus Wind Turbines

2014 ◽  
Author(s):  
Francesco Balduzzi ◽  
Alessandro Bianchini ◽  
Riccardo Maleci ◽  
Giovanni Ferrara ◽  
Lorenzo Ferrari
Keyword(s):  
Wind Turbines ◽  
Scale Effects ◽  
Medium Size ◽  
Design Criteria ◽  
Wind Speeds ◽  
Curvature Effects ◽  
Average Wind ◽  
Energy Scenario ◽  
Flow Curvature ◽  
Real Flow

Darrieus wind turbines are experiencing a renewed interest in the wind energy scenario, in particular whenever small and medium-size installations are considered. In these contexts, the average wind speeds are generally quite low due to scale effects and therefore the most exploited design choices for the turbines are the H-shape configuration, as the entire blade can take advantage of the maximum rotational radius, and high chord to radius ratios, in order to ensure suitable Reynolds numbers on the airfoils. By doing so, the aerodynamic effects induced by the motion of the airfoils in a curved flowpath become more evident and the airfoils themselves have to be designed to compensate these phenomena if conventional design tools based on the BEM theory are used. In this study, fully unsteady 2D simulations were exploited to analyze a three-bladed H-Darrieus wind turbine in order to define the real flow structure and its effects on the turbine performance; in detail, the influence of both the virtual camber and the virtual incidence were investigated. CFD results were supported by experimental data collected on full-scale models reproducing two different airfoil mountings. Finally, the proper design criteria to compensate these phenomena are proposed and their benefits on a conventional simulation with a BEM approach are discussed.

Heat Flux Measurements in Homogeneous Curved Shear Flow

1999 ◽  
Vol 121 (1) ◽  
pp. 190-194 ◽  
Author(s):  
A. G. L. Holloway ◽  
S. A. Ebrahimi-Sabet
Keyword(s):  
Heat Flux ◽  
Shear Flow ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Turbulent Shear ◽  
Turbulent Heat ◽  
Curvature Effects ◽  
Turbulent Shear Stress ◽  
Uniform Shear ◽  
Flow Curvature

Turbulent heat fluxes were measured far downstream of a fine heating wire stretched spanwise across a curved, uniform shear flow. The turbulence was approximately homogeneous and the overheat small enough to be passive. Strong destabilizing and stabilizing curvature effects were produced by directing the shear toward the center of curvature and away from the center of curvature, respectively. The dimensionless turbulent shear stress was strongly affected by the flow curvature, but the dimensionless components of the turbulent heat flux were found to be relatively insensitive.

Flow Curvature Effects for VAWT: a Review of Virtual Airfoil Transformations and Implementation in XFOIL

2016 ◽  
Author(s):  
Sander van der Horst ◽  
Jelmer E. van de Wiel ◽  
Carlos Simao Ferreira ◽  
Nestór Ramos García
Keyword(s):  
Curvature Effects ◽  
Flow Curvature

Blade Design Criteria to Compensate the Flow Curvature Effects in H-Darrieus Wind Turbines

2014 ◽  
Vol 137 (1) ◽  
Author(s):  
Francesco Balduzzi ◽  
Alessandro Bianchini ◽  
Riccardo Maleci ◽  
Giovanni Ferrara ◽  
Lorenzo Ferrari
Keyword(s):  
Wind Turbines ◽  
Scale Effects ◽  
Medium Size ◽  
Design Criteria ◽  
Wind Speeds ◽  
Curvature Effects ◽  
Average Wind ◽  
Energy Scenario ◽  
Flow Curvature ◽  
Real Flow

Darrieus wind turbines are experiencing a renewed interest in the wind energy scenario, in particular, whenever small and medium-size installations are considered. In these contexts, the average wind speeds are generally quite low due to scale effects and therefore the most exploited design choices for the turbines are the H-shape configuration, as the entire blade can take advantage of the maximum rotational radius, and high chord to radius ratios, in order to ensure suitable Reynolds numbers on the airfoils. By doing so, the aerodynamic effects induced by the motion of the airfoils in a curved flowpath become more evident and the airfoils themselves have to be designed to compensate these phenomena if conventional design tools based on the blade element momentum (BEM) theory are used. In this study, fully unsteady 2D simulations were exploited to analyze a three-bladed H-Darrieus wind turbine in order to define the real flow structure and its effects on the turbine performance; in detail, the influence of both the virtual camber and the virtual incidence were investigated. Computational fluid dynamics (CFD) results were supported by experimental data collected on full-scale models reproducing two different airfoil mountings. Finally, the proper design criteria to compensate these phenomena are proposed and their benefits on a conventional simulation with a BEM approach are discussed.

Rayleigh–Taylor modes in the presence of velocity shear and vortices

1998 ◽  
Vol 59 (4) ◽  
pp. 737-750 ◽  
Author(s):  
N. CHAKRABARTI ◽  
K. H. SPATSCHEK
Keyword(s):  
Linear Instability ◽  
Dynamical Behaviour ◽  
The Other ◽  
Velocity Shear ◽  
Viscous Damping ◽  
Long Wavelength ◽  
Oscillatory State ◽  
Long Time ◽  
Flow Curvature ◽  
Real Physical Process

Two-field models for Rayleigh–Taylor modes are investigated. The changes due to external velocity shear (without flow curvature) are reviewed, and the influences of the various terms in the models are discussed. It is shown that, in principle, velocity shear in combination with dissipation leads to the suppression of linear Rayleigh–Taylor modes in the long-time limit. The long-wavelength modes first seem to be damped; however, later they show an algebraic growth in time, before ultimately the exponential viscous damping wins. In general, the amplitudes become very large, and therefore the often-quoted stability of Rayleigh–Taylor modes in the presence of velocity shear is more a mathematical artefact than a real physical process. Vortices, on the other hand, can lead (together with velocity shear) to a quite different dynamical behaviour. Because of a locking of the wave vectors, pronounced oscillations appear. This effect is demonstrated by a simple model calculation. When vortices and velocity shear are generated from linear instability, the resulting oscillatory state finally becomes unstable with respect to Rayleigh–Taylor modes on a long time scale (‘secondary instability’).

Flow Curvature Effects on Darrieus Turbine Blade Aerodynamics

1980 ◽  
Vol 4 (2) ◽  
pp. 49-55 ◽  
Author(s):  
P. G. Migliore ◽  
W. P. Wolfe ◽  
J. B. Fanucci
Keyword(s):  
Turbine Blade ◽  
Curvature Effects ◽  
Flow Curvature

Flow Curvature Effect on Dynamic Behaviour of a Novel Vertical Axis Tidal Current Turbine: Numerical and Experimental Analysis

2005 ◽  
Author(s):  
Domenico Coiro ◽  
Fabrizio Nicolosi ◽  
Agostino De Marco ◽  
Stefano Melone ◽  
Francesco Montella
Keyword(s):  
Vertical Axis ◽  
Test Results ◽  
Vortex Model ◽  
Curvature Effects ◽  
Flow Curvature ◽  
Dynamic Performances ◽  
Tidal Current Turbine ◽  
Work Done ◽  
Current Turbine ◽  
Design Construction

This paper presents a summary of the work done by the authors regarding the design, construction and test of vertical axis hydro turbines to exploit tidal currents. Double Multiple Streamtube (DMS) model and Vortex model have been used to predict turbines performances either with fixed blades or with self-acting variable pitch blades. Within the DMS model, VAWT and VAWT_dyn codes have been developed to analyze steady and dynamic performances; within Vortex model, VAT-VOR3D code has been developed. Theoretical analysis and numerical predicted performances have been compared and validated with experimental test results on both model and real scale turbines. A comparison between DMS and Vortex model results has been presented. Moreover, the recent activities in terms of numerical investigations on the flow curvature effects are presented.

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