Vortex development on pitching plates with lunate and truncate planforms

AbstractThe three-dimensional flow field and instantaneous forces are measured on pitching rectangular, lunate and truncate planforms of aspect-ratio four. The leading-edge vortex on the rectangular planform is compressed as it grows, and subsequently forms an arch-shaped vortex. For the lunate and truncate planforms, which both have identical spanwise leading-edge curvature but differ in planform area, outboard-directed convection of vorticity, rather than vortex stretching, mitigates arch-vortex formation. The vortical near wake that is formed by the planforms with spanwise leading-edge curvature is found to be strongly correlated with a favourable lift-to-drag ratio during the force-relaxation phase.

Download Full-text

3D Effects in the Wake Vortex Formation of an Elliptic Flat Plate in Hover

Volume 1: Advances in Aerospace Technology ◽

10.1115/imece2017-71692 ◽

2017 ◽

Author(s):

Vivek Nair ◽

Siddarth Chintamani ◽

B. H. Dennis

Keyword(s):

Flat Plate ◽

Vortex Formation ◽

Three Dimensional ◽

Leading Edge ◽

Leading Edge Vortex ◽

Elliptic Geometry ◽

3D Effects ◽

Edge Vortex ◽

Formation Number ◽

Vorticity Transport

A Numerical Analysis is conducted to investigate the Leading Edge Vortex (LEV) dynamics of an elliptic flat plate undergoing 2 dimensional symmetric flapping motion in hover. The plate is modeled with an aspect ratio of 3 and a flapping trajectory resulting in Reynolds number 225 is studied. The leading edge vortex stability is analyzed as a function of the non dimensional formation number and a vorticity transport analysis is carried to understand the flux budgets present. The LEV formation number is found to be 2.6. The results of vorticity analysis show the highly three dimensional nature of the LEV growth for an elliptic geometry.

Download Full-text

An aeroelastic instability provides a possible basis for the transition from gliding to flapping flight

Journal of The Royal Society Interface ◽

10.1098/rsif.2012.0940 ◽

2013 ◽

Vol 10 (80) ◽

pp. 20120940 ◽

Cited By ~ 20

Author(s):

Oscar M. Curet ◽

Sharon M. Swartz ◽

Kenneth S. Breuer

Keyword(s):

Lift Coefficient ◽

Leading Edge ◽

Flapping Flight ◽

Fluid Forces ◽

Low Wind Speed ◽

Stiffness Properties ◽

Edge Vortex ◽

Lifting Surfaces ◽

Drag Ratio ◽

Lift To Drag Ratio

The morphology, kinematics and stiffness properties of lifting surfaces play a key role in the aerodynamic performance of vertebrate flight. These surfaces, as a result of their flexible nature, may move both actively, owing to muscle contraction, and passively, in reaction to fluid forces. However, the nature and implications of this fluid–structure interaction are not well understood. Here, we study passive flight (flight with no active wing actuation) and explore a physical mechanism that leads to the emergence of a natural flapping motion. We model a vertebrate wing with a compliant shoulder and the ability to camber with an idealized physical model consisting of a cantilevered flat plate with a hinged trailing flap. We find that at low wind speed the wing is stationary, but at a critical speed the wing spontaneously flaps. The lift coefficient is significantly enhanced once the wing starts to oscillate, although this increase in lift generation is accompanied by an increase in drag. Flow visualization suggests that a strong leading edge vortex attached to the wing during downstroke is the primary mechanism responsible for the enhanced lift. The flapping instability we observe suggests a possible scenario for an evolutionary transition from gliding to powered flapping flight in animals that possess compliant wings capable of passive camber. Although the flapping state is accompanied by a lower lift-to-drag ratio, the increased lifting capability it confers might have enabled increased body mass, improved foraging performance and/or flight at lower speeds, any of which might have been selectively advantageous.

Download Full-text

Vortex formation on surging aerofoils with application to reverse flow modelling

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.800 ◽

2018 ◽

Vol 859 ◽

pp. 59-88 ◽

Cited By ~ 8

Author(s):

Philip B. Kirk ◽

Anya R. Jones

Keyword(s):

Surface Pressure ◽

Reverse Flow ◽

Vortex Formation ◽

Three Dimensional ◽

Tangential Velocity ◽

Leading Edge ◽

Field Measurements ◽

Reduced Frequency ◽

Advance Ratio ◽

Convection Speed

The leading-edge vortex (LEV) is a powerful unsteady flow structure that can result in significant unsteady loads on lifting blades and wings. Using force, surface pressure and flow field measurements, this work represents an experimental campaign to characterize LEV behaviour in sinusoidally surging flows with widely varying amplitudes and frequencies. Additional tests were conducted in reverse flow surge, with kinematics similar to the tangential velocity profile seen by a blade element in recent high-advance-ratio rotor experiments. General results demonstrate the variability of LEV convection properties with reduced frequency, which greatly affected the average lift-to-drag ratio in a cycle. Analysis of surface pressure measurements suggests that LEV convection speed is a function only of the local instantaneous flow velocity. In the rotor-comparison tests, LEVs formed in reverse flow surge were found to convect more quickly than the corresponding reverse flow LEVs that form on a high-advance-ratio rotor, demonstrating that rotary motion has a stabilizing effect on LEVs. The reverse flow surging LEVs were also found to be of comparable strength to those observed on the high-advance-ratio rotor, leading to the conclusion that a surging-wing simplification might provide a suitable basis for low-order models of much more complex three-dimensional flows.

Download Full-text

Video: Leading edge vortex formation in aquatic swimming

10.1103/aps.dfd.2014.gfm.v0071 ◽

2014 ◽

Author(s):

Mohsen Daghooghi ◽

Richard G. Bottom ◽

Iman Borazjani

Keyword(s):

Vortex Formation ◽

Leading Edge ◽

Leading Edge Vortex ◽

Edge Vortex

Download Full-text

Kinematics of diving Atlantic puffins (Fratercula arctica L.):evidence for an active upstroke

Journal of Experimental Biology ◽

10.1242/jeb.205.3.371 ◽

2002 ◽

Vol 205 (3) ◽

pp. 371-378

Author(s):

L. Christoffer Johansson ◽

Björn S. Wetterholm Aldrin

Keyword(s):

Three Dimensional ◽

Fratercula Arctica ◽

Induced Drag ◽

Video Images ◽

Hand Bones ◽

On Line ◽

Oscillating Motion ◽

Drag Ratio ◽

Wing Tip ◽

Lift To Drag Ratio

SUMMARY To examine the propulsion mechanism of diving Atlantic puffins (Fratercula arctica), their three-dimensional kinematics was investigated by digital analysis of sequential video images of dorsal and lateral views. During the dives of this wing-propelled bird, the wings are partly folded, with the handwings directed backwards. The wings go through an oscillating motion in which the joint between the radius-ulna and the hand bones leads the motion, with the wing tip following. There is a large rotary motion of the wings during the stroke, with the wings being pronated at the beginning of the downstroke and supinated at the end of the downstroke/beginning of the upstroke. Calculated instantaneous velocities and accelerations of the bodies of the birds show that, during the downstroke, the birds accelerate upwards and forwards. During the upstroke, the birds accelerate downwards and, in some sequences analysed, also forwards, but in most cases the birds decelerate. In all the upstrokes analysed, the forward/backward acceleration shows the same pattern, with a reduced deceleration or even a forward acceleration during ‘mid’ upstroke indicating the production of a forward force, thrust. Our results show that the Atlantic puffin can use an active upstroke during diving, in contradiction to previous data. Furthermore, we suggest that the partly folded wings of diving puffins might act as efficient aft-swept wingtips, reducing the induced drag and increasing the lift-to-drag ratio. A movie is available on-line.

Download Full-text

Initiation of Leading-Edge-Vortex Formation on Finite Wings in Unsteady Flow

53rd AIAA Aerospace Sciences Meeting ◽

10.2514/6.2015-0546 ◽

2015 ◽

Cited By ~ 3

Author(s):

Yoshikazu Hirato ◽

Minao Shen ◽

Sachin Aggarwal ◽

Ashok Gopalarathnam ◽

Jack R. Edwards

Keyword(s):

Unsteady Flow ◽

Vortex Formation ◽

Leading Edge ◽

Leading Edge Vortex ◽

Edge Vortex

Download Full-text

Roughness Sensitivity Considerations for Thick Rotor Blade Airfoils

Journal of Solar Energy Engineering ◽

10.1115/1.1624614 ◽

2003 ◽

Vol 125 (4) ◽

pp. 468-478 ◽

Cited By ~ 93

Author(s):

R. P. J. O. M. van Rooij ◽

W. A. Timmer

Keyword(s):

Turbine Blades ◽

Leading Edge ◽

Lower Surface ◽

Vortex Generators ◽

Wind Turbine Blades ◽

High Lift ◽

Edge Roughness ◽

Proper Design ◽

Drag Ratio ◽

Lift To Drag Ratio

In modern wind turbine blades, airfoils of more than 25% thickness can be found at mid-span and inboard locations. At mid-span, aerodynamic requirements dominate, demanding a high lift-to-drag ratio, moderate to high lift and low roughness sensitivity. Towards the root, structural requirements become more important. In this paper, the performance for the airfoil series DU FFA, S8xx, AH, Risø and NACA are reviewed. For the 25% and 30% thick airfoils, the best performing airfoils can be recognized by a restricted upper-surface thickness and an S-shaped lower surface for aft-loading. Differences in performance of the DU 91-W2-250 (25%), S814 (24%) and Risø-A1-24 (24%) airfoils are small. For a 30% thickness, the DU 97-W-300 meets the requirements best. Reduction of roughness sensitivity can be achieved both by proper design and by application of vortex generators on the upper surface of the airfoil. Maximum lift and lift-to-drag ratio are, in general, enhanced for the rough configuration when vortex generators are used. At inboard locations, 2-D wind tunnel tests do not represent the performance characteristics well because the influence of rotation is not included. The RFOIL code is believed to be capable of approximating the rotational effect. Results from this code indicate that rotational effects dramatically reduce roughness sensitivity effects at inboard locations. In particular, the change in lift characteristics in the case of leading edge roughness for the 35% and 40% thick DU airfoils, DU 00-W-350 and DU 00-W-401, respectively, is remarkable. As a result of the strong reduction of roughness sensitivity, the design for inboard airfoils can primarily focus on high lift and structural demands.

Download Full-text

Effects of leading-edge bevel angle on the aerodynamic forces of a non-slender 50° delta wing

The Aeronautical Journal ◽

10.1017/s0001924000000828 ◽

2005 ◽

Vol 109 (1098) ◽

pp. 403-407 ◽

Cited By ~ 7

Author(s):

J. J. Wang ◽

S. F. Lu

Keyword(s):

Delta Wing ◽

Lift Coefficient ◽

Leading Edge ◽

Relative Thickness ◽

Aerodynamic Performances ◽

Stall Angle ◽

Drag Ratio ◽

Low Speed Wind Tunnel ◽

Lift To Drag Ratio ◽

Blunt Leading Edge

Abstract The aerodynamic performances of a non-slender 50° delta wing with various leading-edge bevels were measured in a low speed wind tunnel. It is found that the delta wing with leading-edge bevelled leeward can improve the maximum lift coefficient and maximum lift to drag ratio, and the stall angle of the wing is also delayed. In comparison with the blunt leading-edge wing, the increment of maximum lift to drag ratio is 200%, 98% and 100% for the wings with relative thickness t/c = 2%, t/c = 6.7% and t/c = 10%, respectively.

Download Full-text