Swimming freely near the ground leads to flow-mediated equilibrium altitudes

2019 ◽  
Vol 875 ◽  
Author(s):  
Melike Kurt ◽  
Jackson Cochran-Carney ◽  
Qiang Zhong ◽  
Amin Mivehchi ◽  
Daniel B. Quinn ◽  
...  
Keyword(s):  
Stable Equilibrium ◽  
Initial Conditions ◽  
Control Strategies ◽  
Leading Edge ◽  
Ground Effect ◽  
Solid Boundary ◽  
Mass Force ◽  
Reduced Frequency ◽  
Circulatory Force ◽  
Stream Direction

Experiments and computations are presented for a foil pitching about its leading edge near a planar, solid boundary. The foil is examined when it is constrained in space and when it is unconstrained or freely swimming in the cross-stream direction. It was found that the foil has stable equilibrium altitudes: the time-averaged lift is zero at certain altitudes and acts to return the foil to these equilibria. These stable equilibrium altitudes exist for both constrained and freely swimming foils and are independent of the initial conditions of the foil. In all cases, the equilibrium altitudes move farther from the ground when the Strouhal number is increased or the reduced frequency is decreased. Potential flow simulations predict the equilibrium altitudes to within 3 %–11 %, indicating that the equilibrium altitudes are primarily due to inviscid mechanisms. In fact, it is determined that stable equilibrium altitudes arise from an interplay among three time-averaged forces: a negative jet deflection circulatory force, a positive quasistatic circulatory force and a negative added mass force. At equilibrium, the foil exhibits a deflected wake and experiences a thrust enhancement of 4 %–17 % with no penalty in efficiency as compared to a pitching foil far from the ground. These newfound lateral stability characteristics suggest that unsteady ground effect may play a role in the control strategies of near-boundary fish and fish-inspired robots.

Unsteady propulsion near a solid boundary

2014 ◽  
Vol 742 ◽  
pp. 152-170 ◽  
Author(s):  
Daniel B. Quinn ◽  
Keith W. Moored ◽  
Peter A. Dewey ◽  
Alexander J. Smits
Keyword(s):  
Equilibrium Point ◽  
Lift Force ◽  
Stable Equilibrium ◽  
Ground Effect ◽  
Solid Boundary ◽  
Stable Equilibrium Point ◽  
Computational Results ◽  
Propulsive Efficiency ◽  
Vortex Streets ◽  
The Mean

AbstractExperimental and computational results are presented on an aerofoil undergoing pitch oscillations in ground effect, that is, close to a solid boundary. The time-averaged thrust is found to increase monotonically as the mean position of the aerofoil approaches the boundary while the propulsive efficiency stays relatively constant, showing that ground effect can enhance thrust at little extra cost for a pitching aerofoil. Vortices shed into the wake form pairs rather than vortex streets, so that in the mean a momentum jet is formed that angles away from the boundary. The time-averaged lift production is found to have two distinct regimes. When the pitching aerofoil is between 0.4 and 1 chord lengths from the ground, the lift force pulls the aerofoil towards the ground. In contrast, for wall proximities between 0.25 and 0.4 chord lengths, the lift force pushes the aerofoil away from the ground. Between these two regimes there is a stable equilibrium point where the time-averaged lift is zero and thrust is enhanced by approximately 40 %.

scholarly journals Effects of flexibility on the aerodynamic performance of flapping wings

2011 ◽  
Vol 689 ◽  
pp. 32-74 ◽  
Author(s):  
C.-K. Kang ◽  
H. Aono ◽  
C. E. S. Cesnik ◽  
W. Shyy
Keyword(s):  
Reynolds Number ◽  
Density Ratio ◽  
Flapping Wings ◽  
Mass Force ◽  
Propulsive Force ◽  
Propulsive Efficiency ◽  
Reduced Frequency ◽  
Moving Body ◽  
Order Of Magnitude ◽  
Direct Guidance

AbstractEffects of chordwise, spanwise, and isotropic flexibility on the force generation and propulsive efficiency of flapping wings are elucidated. For a moving body immersed in viscous fluid, different types of forces, as a function of the Reynolds number, reduced frequency (k), and Strouhal number (St), acting on the moving body are identified based on a scaling argument. In particular, at the Reynolds number regime of $O(1{0}^{3} \ensuremath{-} 1{0}^{4} )$ and the reduced frequency of $O(1)$, the added mass force, related to the acceleration of the wing, is important. Based on the order of magnitude and energy balance arguments, a relationship between the propulsive force and the maximum relative wing-tip deformation parameter ($\gamma $) is established. The parameter depends on the density ratio, St, k, natural and flapping frequency ratio, and flapping amplitude. The lift generation, and the propulsive efficiency can be deduced by the same scaling procedures. It seems that the maximum propulsive force is obtained when flapping near the resonance, whereas the optimal propulsive efficiency is reached when flapping at about half of the natural frequency; both are supported by the reported studies. The established scaling relationships can offer direct guidance for micro air vehicle design and performance analysis.

Vortex formation on surging aerofoils with application to reverse flow modelling

2018 ◽  
Vol 859 ◽  
pp. 59-88 ◽  
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.

scholarly journals Hydromechanics of swimming propulsion. Part 2. Some optimum shape problems

1971 ◽  
Vol 46 (3) ◽  
pp. 521-544 ◽  
Author(s):  
T. Yao-Tsu Wu
Keyword(s):  
Energy Loss ◽  
Degrees Of Freedom ◽  
Leading Edge ◽  
Present Theory ◽  
Optimum Shape ◽  
Recent Theory ◽  
Flexible Plate ◽  
Reduced Frequency ◽  
Isoperimetric Constraints ◽  
Optimum Solution

The optimum shape problems considered in this part are for those profiles of a two-dimensional flexible plate in time-harmonic motion that will minimize the energy loss under the condition of fixed thrust and possibly also under other isoperimetric constraints. First, the optimum movement of a rigid plate is completely determined; it is necessary first to reduce the original singular quadratic form representing the energy loss to a regular one of a lower order, which is then tractable by usual variational methods. A favourable range of the reduced frequency is found in which the thrust contribution coming from the leading-edge suction is as small as possible under the prescribed conditions, outside of which this contribution becomes so large as to be hard to realize in practice without stalling. This optimum solution is compared with the recent theory of Lighthill (1970); these independently arrived-at conclusions are found to be virtually in agreement.The present theory is further applied to predict the movement of a porpoise tail of large aspect-ratio and is found in satisfactory agreement with the experimental measurements. A qualitative discussion of the wing movement in flapping flight of birds is also given on the basis of optimum efficiency.The optimum shape of a flexible plate is analysed for the most general case of infinite degrees of freedom. It is shown that the solution can be determined to a certain extent, but the exact shape is not always uniquely determinate.

Effect of amplitude and mean angle-of-attack on the boundary layer of an oscillating aerofoil

2008 ◽  
Vol 112 (1138) ◽  
pp. 705-713 ◽  
Author(s):  
M. R. Soltani ◽  
A. Bakhshalipour
Keyword(s):  
Boundary Layer ◽  
Transition Point ◽  
Angle Of Attack ◽  
Wind Tunnel Test ◽  
Leading Edge ◽  
Stream Velocity ◽  
Velocity Amplitude ◽  
Surface Pressure Distribution ◽  
Reduced Frequency ◽  
Hot Film

Abstract Extensive experiments were conducted to study the effect of various parameters on the surface pressure distribution and transition point of an aerofoil section used in a wind turbine blade. In this paper details of the variation of transition point on the aforementioned aerofoil are presented. The aerofoil spanned the wind-tunnel test section and was oscillated sinusoidally in pitch about the quarter chord. The imposed variables of the experiments were free stream velocity, amplitude of motion, mean angle-of-attack, and oscillation frequency. The spatial-temporal progressions of the leading-edge transition point and the state of the unsteady boundary-layer were measured using eight closely-spaced, hot-film sensors (HFS). The measurements show that: (i) Reduced frequency has a pronounced effect on the variations of the transition point. (ii) There exists a hysteresis loop in the dynamic transition location and its shape varies with the reduced frequency and mean angle-of-attack.

Energetics of oscillating lifting surfaces by the use of integral conservation laws

1994 ◽  
Vol 266 ◽  
pp. 347-370 ◽  
Author(s):  
Ali R. Ahmadi ◽  
Sheila E. Windnall
Keyword(s):  
Conservation Laws ◽  
Mechanical Energy ◽  
Leading Edge ◽  
Two Dimensions ◽  
Three Dimensions ◽  
Total Power ◽  
Lifting Surface ◽  
Reduced Frequency ◽  
Total Thrust ◽  
Lifting Surfaces

The energetics of oscillating lifting surfaces in two and three dimensions is calculated by the use of integral conservation laws in inviscid incompressible flow for general and harmonic transverse oscillations. Wing deformations are prescribed as a function of time and total thrust is calculated from the momentum theorem, and energy loss rate due to vortex shedding in the wake is calculated from the principle of conservation of mechanical energy. Total power required to maintain the oscillations and hydro-dynamic efficiency are also determined. In two dimensions, the results are obtained in closed form. In three dimensions, the distribution of vorticity on the lifting surface is also required as input to the calculations. Therefore, unsteady lifting-surface theory must be used as well. The analysis is applicable to oscillating lifting surfaces of arbitrary planform, aspect ratio and reduced frequency and does not require calculation of the leading-edge thrust.

Inertia–Gravity Waves Generated within a Dipole Vortex

2007 ◽  
Vol 64 (12) ◽  
pp. 4417-4431 ◽  
Author(s):  
Chris Snyder ◽  
David J. Muraki ◽  
Riwal Plougonven ◽  
Fuqing Zhang
Keyword(s):  
Gravity Waves ◽  
Initial Conditions ◽  
Initial Period ◽  
Potential Temperature ◽  
Leading Edge ◽  
Vertical Shear ◽  
Coriolis Parameter ◽  
Dipole Vortex ◽  
The Waves ◽  
Inertia Gravity Waves

Abstract Vortex dipoles provide a simple representation of localized atmospheric jets. Numerical simulations of a synoptic-scale dipole in surface potential temperature are considered in a rotating, stratified fluid with approximately uniform potential vorticity. Following an initial period of adjustment, the dipole propagates along a slightly curved trajectory at a nearly steady rate and with a nearly fixed structure for more than 50 days. Downstream from the jet maximum, the flow also contains smaller-scale, upward-propagating inertia–gravity waves that are embedded within and stationary relative to the dipole. The waves form elongated bows along the leading edge of the dipole. Consistent with propagation in horizontal deformation and vertical shear, the waves’ horizontal scale shrinks and the vertical slope varies as they approach the leading stagnation point in the dipole’s flow. Because the waves persist for tens of days despite explicit dissipation in the numerical model that would otherwise damp the waves on a time scale of a few hours, they must be inherent features of the dipole itself, rather than remnants of imbalances in the initial conditions. The wave amplitude varies with the strength of the dipole, with waves becoming obvious once the maximum vertical vorticity in the dipole is roughly half the Coriolis parameter. Possible mechanisms for the wave generation are spontaneous wave emission and the instability of the underlying balanced dipole.

Influence of mean loading on noise generated by the interaction of gusts with a flat-plate cascade: upstream radiation

1997 ◽  
Vol 347 ◽  
pp. 315-346 ◽  
Author(s):  
N. PEAKE ◽  
E. J. KERSCHEN
Keyword(s):  
Incidence Angle ◽  
Outer Region ◽  
Leading Edge ◽  
Mean Flow ◽  
Flat Plates ◽  
Flow Distortion ◽  
Reduced Frequency ◽  
Flow Interactions ◽  
Source Mechanisms ◽  
Harmonic Components

The sound generated by the interaction between convected vortical and entropic disturbances and a blade row is a significant component of the total noise emitted by a modern aeroengine, and the blade geometry has an important effect on this process. As a first step in the development of a general prediction scheme, we model in this paper just the action of the blade mean loading by treating the blades as flat plates aligned at a non-zero incidence angle, δ, to the oncoming stream, and consider harmonic components of the incident field with reduced frequency k. We then use asymptotic analysis in the realistic limit k[Gt ]1, δ[Lt ]1 with kδ=O(1) to make a consistent asymptotic expansion of the compressible Euler equations. The flow is seen to consist of inner regions around each leading edge, in which sound is generated by the local gust–airfoil and gust–flow interactions, and an outer region in which both the incident gust is distorted according to rapid distortion theory and the out-going sound is refracted by the non-uniform mean flow. The complicated multiple interactions between the sound and the cascade are included to the appropriate asymptotic order, and analytical expressions for the forward radiation are derived. It is seen that even a relatively small value of δ can have a significant effect, thanks to both the O(δk1/2) change in the amplitudes and the O(kδ) change in the phases of the various radiation components, corresponding to the additional source mechanisms associated with the flow distortion around each leading edge and the effects of propagation through the non-uniform flow, respectively. Further work will extend this analysis to include the effects of camber and thickness.

A scaling for vortex formation on swept and unswept pitching wings

2017 ◽  
Vol 832 ◽  
pp. 697-720 ◽  
Author(s):  
Kyohei Onoue ◽  
Kenneth S. Breuer
Keyword(s):  
Control Volume ◽  
Vortex Formation ◽  
Fluid Motion ◽  
Three Dimensional ◽  
Leading Edge ◽  
Sweep Angle ◽  
Reduced Frequency ◽  
Vortex Formation Time ◽  
The Stability ◽  
Vorticity Transport

We examine the dynamics of the leading-edge vortex (LEV) on a rapidly pitching plate with the aim of elucidating the underlying flow physics that dictates the stability and circulation of the LEV. A wide variety of flow conditions is considered in the present study by systematically varying the leading-edge sweep angle ($\unicode[STIX]{x1D6EC}=0^{\circ }$, $11.3^{\circ }$, $16.7^{\circ }$) and the reduced frequency ($f^{\ast }=0.064{-}0.151$), while keeping the pitching amplitude and the Reynolds number fixed. Tomographic particle image velocimetry is used to characterise the three-dimensional fluid motion inside the vortex core and its relation to the LEV stability and growth. A series of control volume analyses are performed to quantify the relative importance of the vorticity transport phenomena taking place inside the LEV to the overall vortex development. We show that, near the wing apex where tip effects can be neglected, the vortex develops in a nominally two-dimensional manner, despite the presence of inherently three-dimensional vortex dynamics such as vortex stretching and compression. Furthermore, we demonstrate that the vortex formation time and circulation growth are well-described by the principles of optimal vortex formation number, and that the occurrence of vortex shedding is dictated by the relative energetics of the feeding shear layer and the resulting vortex.

Experimental Aerodynamic Characteristics of a Compound Wing in Ground Effect

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
Saeed Jamei ◽  
Adi Maimun Abdul Malek ◽  
Shuhaimi Mansor ◽  
Nor Azwadi Che Sidik ◽  
Agoes Priyanto
Keyword(s):  
Reynolds Number ◽  
Drag Coefficient ◽  
Center Of Pressure ◽  
Lift Coefficient ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Ground Effect ◽  
Reynolds Numbers ◽  
Aerodynamic Characteristics ◽  
Ground Clearance

Wing configuration is a parameter that affects the performance of wing-in-ground effect (WIG) craft. In this study, the aerodynamic characteristics of a new compound wing were investigated during ground effect. The compound wing was divided into three parts with a rectangular wing in the middle and two reverse taper wings with anhedral angle at the sides. The sectional profile of the wing model is NACA6409. The experiments on the compound wing and the rectangular wing were carried to examine different ground clearances, angles of attack, and Reynolds numbers. The aerodynamic coefficients of the compound wing were compared with those of the rectangular wing, which had an acceptable increase in its lift coefficient at small ground clearances, and its drag coefficient decreased compared to rectangular wing at a wide range of ground clearances, angles of attack, and Reynolds numbers. Furthermore, the lift to drag ratio of the compound wing improved considerably at small ground clearances. However, this improvement decreased at higher ground clearance. The drag polar of the compound wing showed the increment of lift coefficient versus drag coefficient was higher especially at small ground clearances. The Reynolds number had a gradual effect on lift and drag coefficients and also lift to drag of both wings. Generally, the nose down pitching moment of the compound wing was found smaller, but it was greater at high angle of attack and Reynolds number for all ground clearance. The center of pressure was closer to the leading edge of the wing in contrast to the rectangular wing. However, the center of pressure of the compound wing was later to the leading edge at high ground clearance, angle of attack, and Reynolds number.

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