scholarly journals Wake structure and wingbeat kinematics of a house-martin Delichon urbica

2007 ◽  
Vol 4 (15) ◽  
pp. 659-668 ◽  
Author(s):  
M Rosén ◽  
G.R Spedding ◽  
A Hedenström
Keyword(s):  
Reynolds Numbers ◽  
Laminar Flows ◽  
Frequency Model ◽  
Wake Structure ◽  
Global Circulation ◽  
Weight Support ◽  
House Martin ◽  
Lifting Surfaces ◽  
Thrush Nightingale ◽  
Slender Wing

The wingbeat kinematics and wake structure of a trained house martin in free, steady flight in a wind tunnel have been studied over a range of flight speeds, and compared and contrasted with similar measurements for a thrush nightingale and a pair of robins. The house martin has a higher aspect ratio (more slender) wing, and is a more obviously agile and aerobatic flyer, catching insects on the wing. The wingbeat is notable for the presence at higher flight speeds of a characteristic pause in the upstroke. The essential characteristics of the wing motions can be reconstructed with a simple two-frequency model derived from Fourier analysis. At slow speeds, the distribution of wake vorticity is more simple than for the other previously measured birds, and the upstroke does not contribute to weight support. The upstroke becomes gradually more significant as the flight speed increases, and although the vortex wake shows a signature of the pause phase, the global circulation measurements are otherwise in good agreement with surprisingly simple aerodynamic models, and with predictions across the different species, implying quite similar aerodynamic performance of the wing sections. The local Reynolds numbers of the wing sections are sufficiently low that the well-known instabilities of attached laminar flows over lifting surfaces, which are known to occur at two to three times this value, may not develop.

A Numerical Study Into the Influence of Leading Edge Tubercles on the Aerodynamic Performance of a Highly Cambered High Lift Airfoil Wing at Different Reynolds Numbers

2020 ◽  
Author(s):  
Amr Abdelrahman ◽  
Amr Emam ◽  
Ihab Adam ◽  
Hamdy Hassan ◽  
Shinichi Ookawara ◽  
...  
Keyword(s):  
Control Method ◽  
Numerical Study ◽  
Leading Edge ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Passive Flow Control ◽  
Passive Flow ◽  
Stall Angle ◽  
Lifting Surfaces ◽  
Lift And Drag

Abstract Through the last two decades, many studies have demonstrated the ability of leading-edge protrusions (tubercles), inspired from the pectoral flippers of the humpback whale, to be an effective passive flow control method for the stall phase of an airfoil in some cases depending on the geometrical features and the flow regime. Nevertheless, there is a little work associated with revealing tubercles performance for the lifting surfaces with a highly cambered cross-section, used in numerous applications. The present work aims to investigate the effect of implementing leading edge tubercles on the performance of an infinite span rectangular wing with the highly cambered S1223 foil at different flow regimes. Two sets; baseline one and a modified with tubercles have been studied at Re = 0.1 × 106, 0.3 × 106 and 1.5 × 106 using computational fluid dynamics with a validated model. The numerical results demonstrated that Tubercles have the ability to entirely alter the flow structure over the airfoil, confining the separation to troughs, hence, softening the stall characteristics. However, the tubercle modification expedites the presence of the stalled flow over the suction side, lowering the stall angle for the three mentioned Reynolds numbers. While, no considerable difference occurs in lift and drag before the stall.

scholarly journals Subcritical transition in channel flows

2002 ◽  
Vol 451 ◽  
pp. 35-97 ◽  
Author(s):  
S. JONATHAN CHAPMAN
Keyword(s):  
Poiseuille Flow ◽  
Stokes Equations ◽  
Domain Of Attraction ◽  
Reynolds Numbers ◽  
Linear Instability ◽  
Laminar Flows ◽  
Navier Stokes ◽  
Plane Poiseuille Flow ◽  
Navier Stokes Equations ◽  
Laminar State

Certain laminar flows are known to be linearly stable at all Reynolds numbers, R, although in practice they always become turbulent for sufficiently large R. Other flows typically become turbulent well before the critical Reynolds number of linear instability. One resolution of these paradoxes is that the domain of attraction for the laminar state shrinks for large R (as Rγ say, with γ < 0), so that small but finite perturbations lead to transition. Trefethen et al. (1993) conjectured that in fact γ <−1. Subsequent numerical experiments by Lundbladh, Henningson & Reddy (1994) indicated that for streamwise initial perturbations γ =−1 and −7/4 for plane Couette and plane Poiseuille flow respectively (using subcritical Reynolds numbers for plane Poiseuille flow), while for oblique initial perturbations γ =−5/4 and −7/4 Here, through a formal asymptotic analysis of the Navier–Stokes equations, it is found that for streamwise initial perturbations γ =−1 and −3/2 for plane Couette and plane Poiseuille flow respectively (factoring out the unstable modes for plane Poiseuille flow), while for oblique initial perturbations γ =−1 and −5/4. Furthermore it is shown why the numerically determined threshold exponents are not the true asymptotic values.

scholarly journals Planar flows past thin multi-blade configurations

1996 ◽  
Vol 324 ◽  
pp. 355-377 ◽  
Author(s):  
F. T. Smith ◽  
S. N. Timoshin
Keyword(s):  
Boundary Layer ◽  
Numerical Solutions ◽  
Reynolds Numbers ◽  
Laminar Flows ◽  
Two Dimensional ◽  
Boundary Layer Equations ◽  
Require Solution ◽  
Lift And Drag ◽  
Planar Flows ◽  
Steady Laminar

Two-dimensional steady laminar flows past multiple thin blades positioned in near or exact sequence are examined for large Reynolds numbers. Symmetric configurations require solution of the boundary-layer equations alone, in parabolic fashion, over the successive blades. Non-symmetric configurations in contrast yield a new global inner–outer interaction in which the boundary layers, the wakes and the potential flow outside have to be determined together, to satisfy pressure-continuity conditions along each successive gap or wake. A robust computational scheme is used to obtain numerical solutions in direct or design mode, followed by analysis. Among other extremes, many-blade analysis shows a double viscous structure downstream with two streamwise length scales operating there. Lift and drag are also considered. Another new global interaction is found further downstream. All the interactions involved seem peculiar to multi-blade flows.

scholarly journals The influence of flight style on the aerodynamic properties of avian wings as fixed lifting surfaces

PeerJ ◽  
2016 ◽  
Vol 4 ◽  
pp. e2495 ◽  
Author(s):  
John J. Lees ◽  
Grigorios Dimitriadis ◽  
Robert L. Nudds
Keyword(s):  
Aerodynamic Drag ◽  
Bird Species ◽  
Reynolds Numbers ◽  
Wing Shape ◽  
Minimum Drag ◽  
Quantitative Studies ◽  
Aerodynamic Properties ◽  
Primary Driver ◽  
Morphological Differences ◽  
Lifting Surfaces

The diversity of wing morphologies in birds reflects their variety of flight styles and the associated aerodynamic and inertial requirements. Although the aerodynamics underlying wing morphology can be informed by aeronautical research, important differences exist between planes and birds. In particular, birds operate at lower, transitional Reynolds numbers than do most aircraft. To date, few quantitative studies have investigated the aerodynamic performance of avian wings as fixed lifting surfaces and none have focused upon the differences between wings from different flight style groups. Dried wings from 10 bird species representing three distinct flight style groups were mounted on a force/torque sensor within a wind tunnel in order to test the hypothesis that wing morphologies associated with different flight styles exhibit different aerodynamic properties. Morphological differences manifested primarily as differences in drag rather than lift. Maximum lift coefficients did not differ between groups, whereas minimum drag coefficients were lowest in undulating flyers (Corvids). The lift to drag ratios were lower than in conventional aerofoils and data from free-flying soaring species; particularly in high frequency, flapping flyers (Anseriformes), which do not rely heavily on glide performance. The results illustrate important aerodynamic differences between the wings of different flight style groups that cannot be explained solely by simple wing-shape measures. Taken at face value, the results also suggest that wing-shape is linked principally to changes in aerodynamic drag, but, of course, it is aerodynamics during flapping and not gliding that is likely to be the primary driver.

scholarly journals Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel

2005 ◽  
Vol 3 (7) ◽  
pp. 263-276 ◽  
Author(s):  
A Hedenström ◽  
M Rosén ◽  
G.R Spedding
Keyword(s):  
Wind Tunnel ◽  
Momentum Flux ◽  
Erithacus Rubecula ◽  
Free Flight ◽  
Wake Structure ◽  
Bird Flight ◽  
Image Velocimetry ◽  
Horizontal Momentum ◽  
Gradual Variation ◽  
Thrush Nightingale

The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments.

scholarly journals Some observations regarding steady laminar flows past bluff bodies

2014 ◽  
Vol 372 (2020) ◽  
pp. 20130353 ◽  
Author(s):  
Bengt Fornberg ◽  
Alan R. Elcrat
Keyword(s):  
Numerical Methods ◽  
Flow Regimes ◽  
Reynolds Numbers ◽  
Laminar Flows ◽  
Bluff Bodies ◽  
The Past ◽  
High Reynolds Numbers ◽  
High Reynolds ◽  
Steady Laminar

Steady laminar flows past simple objects, such as a cylinder or a sphere, have been studied for well over a century. Theoretical, experimental and numerical methods have all contributed fundamentally towards our understanding of the resulting flows. This article focuses on developments during the past few decades, when mostly numerical and asymptotical advances have provided insights also for steady, although unstable, high-Reynolds-numbers flow regimes.

Steady Laminar Flows Between a Ball and a Spherical Cavity

1984 ◽  
Vol 51 (1) ◽  
pp. 6-12
Author(s):  
D. Bellet ◽  
D. P. Ly ◽  
M. Milleret
Keyword(s):  
Finite Element ◽  
Experimental Method ◽  
Newtonian Fluid ◽  
Spherical Cavity ◽  
Laser Doppler Anemometry ◽  
Reynolds Numbers ◽  
Laminar Flows ◽  
Element Calculation ◽  
Spherical Ball ◽  
Steady Laminar

Steady laminar flows of a newtonian fluid in the vicinity of a spherical ball located inside a spherical cavity are studied. Two methods were employed: a method involving a finite element calculation and an experimental method based on measurements of local velocities by means of laser Doppler anemometry using the Bragg cell. The influences of the Reynolds numbers and of the ball positions in the cavity have been analyzed and compared.

The Effect of Inertia in Lubrication Flow Including Entrance and Initial Conditions

1985 ◽  
Vol 52 (4) ◽  
pp. 759-765 ◽  
Author(s):  
J. A. Tichy ◽  
P. Bourgin
Keyword(s):  
Initial Conditions ◽  
Inviscid Flow ◽  
Reynolds Numbers ◽  
Lubrication Theory ◽  
Laminar Flows ◽  
Flow Profile ◽  
Fluid Inertia ◽  
Simple Diffusion ◽  
Closed Form Solutions ◽  
Start Up

This study is concerned with the influence of fluid inertia in thin film steady or transient laminar flows, emphasizing the role of initial conditions and inlet boundary conditions. Most existing studies implicitly omit these effects due to the method of formulation or the type of problem studied. In the present study, linearization for the convective inertia terms is used to transform the problem to a coordinate system which results in a simple diffusion equation. Closed-form solutions are obtained, considering initial or entrance conditions in the classical manner. Predictions are tested against cases for which solutions exist in the literature and against a numerical solution and excellent agreement is found. For simple squeeze films, large discrepancies from lubrication theory are found at short times following the start-up, or at moderate Reynolds numbers. In the steady flow case, fluid is assumed to enter the bearing with a slug flow profile. The inlet pressure is determined by coupling the bearing flow with the upstream inviscid flow. Large discrepancies from lubrication theory are found at Reynolds numbers where the lubrication theory was previously thought to be satisfactory. The load may increase or decrease, depending on the upstream conditions.

scholarly journals Geometrical investigation of microchannel with two trapezoidal blocks subjected to laminar convective flows with and without boiling

2022 ◽  
Vol 3 (1) ◽  
pp. 20-36
Author(s):  
Bruno Costa Feijó ◽  
◽  
Ana Pavlovic ◽  
Luiz Alberto Oliveira Rocha ◽  
Liércio André Isoldi ◽  
...  
Keyword(s):  
Numerical Study ◽  
Thermal Conditions ◽  
Reynolds Numbers ◽  
Laminar Flows ◽  
Cooling Power ◽  
Constructal Design ◽  
Boiling Process ◽  
Multi Phase ◽  
Power Generation Systems ◽  
Optimal Configurations

Microchannels are important devices to improve the heat exchange in several engineering applications as heat, ventilation and air conditioning, microelectronic cooling, power generation systems and others. The present work performs a numerical study of a microchannel with two trapezoidal blocks subjected to laminar flows, aiming to analyze the influence of the boiling process on the geometric configuration of the microchannel. Constructal Design and Exhaustive Search are used for the geometrical evaluation of the blocks. The Mixture multi-phase model and the Lee phase change model were both employed for the numerical simulation of the boiling process. In this study, the influence of the height and higher width of the first block (H11/L11) over the heat transfer rate and pressure drop for different magnitudes of the ratio between the lower width and higher width (L12/L11) was investigated. It is considered water in monophase cases and water/vapor mixture for multiphase flow. Two different Reynolds numbers (ReH = 0.1 and 10.0) were investigated. Results indicated that, for the present thermal conditions, the consideration of boiling flows were not significant for prediction of optimal configurations. Results also showed that in the cases where the boiling process was enabled, the multi-objective performance was higher than in the cases without boiling, especially for ReH = 0.1.

Drag and wake structure of a quasi-dandelion pappus model at low and moderate Reynolds numbers: The effects of filament width

2021 ◽  
Vol 33 (12) ◽  
pp. 121904
Author(s):  
Shiqing Li ◽  
Dingyi Pan ◽  
Jun Li ◽  
Xueming Shao
Keyword(s):  
Reynolds Numbers ◽  
Wake Structure
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