scholarly journals Hovering flight in hummingbird hawkmoths: kinematics, wake dynamics, and aerodynamic power

2021 ◽  
Author(s):  
Kajsa Warfvinge ◽  
L. Christoffer Johansson ◽  
Anders Hedenström
Keyword(s):  
Experimental Techniques ◽  
Vortex Rings ◽  
Model Parameters ◽  
Near Wake ◽  
Hovering Flight ◽  
Measurement Volume ◽  
Vortex Interactions ◽  
Plane Normal ◽  
Standard Models ◽  
Far Wake

Hovering insects are divided into two categories: “normal” hoverers that moves the wing symmetrically in a horizontal stroke plane, and those with an inclined stroke plane. Normal hoverers have been suggested to support their weight during both down- and upstroke, shedding vortex rings each half stroke. Insects with an inclined stroke plane should, according to theory, produce flight forces only during downstroke, and only generate one set of vortices. The type of hovering is thus linked to the power required to hover. Previous efforts to characterize the wake of hovering insects have used low-resolution experimental techniques or simulated the flow using CFD, and so it remains to be determined if insect wakes can be represented by any of the suggested models. Here, we used tomographic PIV, with a horizontal measurement volume placed below the animals, to show that the wake shed by hovering hawkmoths are best be described as a series of bilateral, stacked vortex “rings”. While the upstroke is aerodynamically active, despite an inclined stroke plane, it produces weaker vortices than the downstroke. In addition, compared to the near wake, the far wake lacks structure and is less concentrated. Both near and far wakes are clearly affected by vortex interactions, suggesting caution is required when interpreting wake topologies. We also estimated induced power (Pind) from downwash velocities in the wake. Standard models predicted a Pind more than double that from our wake measurements. Our results thus question some model assumptions and we propose a reevaluation of the model parameters.

A Double Birkhoff Wake Oscillator for the Modeling of Vortex-Induced Vibration

2011 ◽  
Author(s):  
R. H. M. Ogink
Keyword(s):  
Free Vibration ◽  
Added Mass ◽  
Mass Force ◽  
Near Wake ◽  
Vibration Measurements ◽  
Vortex Induced Vibration ◽  
Fluid Forces ◽  
Wake Oscillator ◽  
Model Equations ◽  
Far Wake

A double Birkhoff wake oscillator for the modeling of vortex-induced vibration is presented in which the oscillating variables are assumed to be associated with the boundary layer/near wake and the far wake. The fluid forces are assumed to consist of a potential added mass force and a force due to vortex shedding. In the limit of vanishing incoming flow velocity, the model equations reduce to a form similar to the Morison equation. The results of the double wake oscillator have been compared with forced vibration measurements and free vibration measurements over a range of mass and damping ratios. The model is capable of describing the most important trends in both the forced and free vibration experiments. Specifically, the double wake oscillator is able to model both the upper and lower branch of free vibration.

Flow Unsteadiness and Stability Characteristics of Low-Re Flow Past an Inclined Triangular Cylinder

2017 ◽  
Vol 139 (12) ◽  
Author(s):  
Wei Zhang ◽  
Hui Yang ◽  
Hua-Shu Dou ◽  
Zuchao Zhu
Keyword(s):  
Growth Rate ◽  
Sensitivity Analysis ◽  
Unsteady Flow ◽  
Maximum Velocity ◽  
Flow Patterns ◽  
Wake Flow ◽  
Near Wake ◽  
Flow Unsteadiness ◽  
Flow Past ◽  
Far Wake

The present study investigates the two-dimensional flow past an inclined triangular cylinder at Re = 100. Numerical simulation is performed to explore the effect of cylinder inclination on the aerodynamic quantities, unsteady flow patterns, time-averaged flow characteristics, and flow unsteadiness. We also provide the first global linear stability analysis and sensitivity analysis on the targeted physical problem for the potential application of flow control. The objective of this work is to quantitatively identify the effect of cylinder inclination on the characteristic quantities and unsteady flow patterns, with emphasis on the flow unsteadiness and instability. Numerical results reveal that the flow unsteadiness is generally more pronounced for the base-facing-like cylinders (α → 60 deg) where separation occurs at the front corners. The inclined cylinder reduces the velocity deficiency in the near-wake, and the reduction in far-wake is the most notable for the α = 30 deg cylinder. The transverse distributions of several quantities are shifted toward the negative y-direction, such as the maximum velocity deficiency and maximum/minimum velocity fluctuation. Finally, the global stability and sensitivity analysis show that the spatial structures of perturbed velocities are quite similar for α ≤ 30 deg and the temporal growth rate of perturbation is sensitive to the near-wake flow, while for α ≥ 40 deg there are remarkable transverse expansion and streamwise elongation of the perturbed velocities, and the growth rate is sensitive to the far-wake flow.

Small-scale structure in colliding off-axis vortex rings

1994 ◽  
Vol 259 ◽  
pp. 281-290 ◽  
Author(s):  
G. B. Smith ◽  
T. Wei
Keyword(s):  
Large Scale ◽  
Laser Induced Fluorescence ◽  
Vortex Rings ◽  
Small Scale ◽  
Vortex Interactions ◽  
Equal Strength ◽  
Small Scales ◽  
Nonlinear Amplification ◽  
Visualization Techniques ◽  
Insight Into

Off-axis collisions of equal-strength vortex rings were experimentally examined. Two equal-strength vortices were generated which moved toward each other along parallel, but offset, trajectories. Two colour laser-induced fluorescence visualization techniques were used to observe these phenomena and gain insight into their importance in vortex interactions. The most prominent features of this interaction were rapid growth and rotation of the rings and formation of evenly spaced ringlets around the cores of the original rings. Large-scale motions are described using simple vortex induction arguments. The small scales are caused by nonlinear amplification of instabilities during the asymmetric interaction.

scholarly journals Two-scale dynamics of flow past a partial cross-stream array of tidal turbines

2013 ◽  
Vol 730 ◽  
pp. 220-244 ◽  
Author(s):  
Takafumi Nishino ◽  
Richard H. J. Willden
Keyword(s):  
Analytical Model ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Wake Region ◽  
Near Wake ◽  
New Model ◽  
Tidal Turbines ◽  
Type Design ◽  
Flow Past ◽  
Far Wake

AbstractThe characteristics of flow past a partial cross-stream array of (idealized) tidal turbines are investigated both analytically and computationally to understand the mechanisms that determine the limiting performance of partial tidal fences. A two-scale analytical partial tidal fence model reported earlier is further extended by better accounting for the effect of array-scale flow expansion on device-scale dynamics, so that the new model is applicable to short fences (consisting of a small number of devices) as well as to long fences. The new model explains theoretically general trends of the limiting performance of partial tidal fences. The new model is then compared to three-dimensional Reynolds-averaged Navier–Stokes (RANS) computations of flow past an array of various numbers (up to 40) of actuator disks. On the whole, the analytical model agrees well with the RANS computations, suggesting that the two-scale dynamics described in the analytical model predominantly determines the fence performance in the RANS computations as well. The comparison also suggests that the limiting performance of short partial fences depends on how much of device far-wake mixing takes place within the array near-wake region. This factor, however, depends on the structures of the wake and therefore on the type/design of devices to be arrayed.

The wake from a cylinder subjected to amplitude-modulated excitation

1993 ◽  
Vol 247 ◽  
pp. 79-110 ◽  
Author(s):  
M. Nakano ◽  
D. Rockwell
Keyword(s):  
Reynolds Number ◽  
Modulation Frequency ◽  
Low Reynolds Number ◽  
Vortex Formation ◽  
Near Wake ◽  
Proper Selection ◽  
Wake Structure ◽  
Far Wake ◽  
Stream Direction ◽  
Selection Of

Controlled, amplitude-modulated excitation of a cylinder at low Reynolds number (Re equals; 136) in the cross-stream direction generates several states of response of the near wake including: a locked-in wake structure, which is periodic at the modulation frequency; a period-doubled wake structure, which is periodic at a frequency half the modulation frequency; and a destabilized structure of the wake, which is periodic at the modulation frequency, but involves substantial phase modulations of the vortex formation relative to the cylinder displacement. The occurrence of each of these states depends upon the dimensionless modulation frequency, as well as the nominal frequency and amplitude of excitation. Transition through states of increasing disorder can be attained by either decreasing the modulation frequency or increasing the amplitude of excitation at a constant value of nominal frequency. These states of response in the near wake are crucial in determining whether the far wake is highly organized or incoherent. Both of these extremes are attainable by proper selection of the parameters of excitation.

Acoustic forcing of oblique wave resonance in the far wake

1993 ◽  
Vol 256 ◽  
pp. 315-341 ◽  
Author(s):  
C. H. K. Williamson ◽  
A. Prasad
Keyword(s):  
Free Stream ◽  
Higher Order ◽  
Two Dimensional ◽  
Near Wake ◽  
Peak Response ◽  
Oblique Waves ◽  
Oblique Wave ◽  
Wave Resonance ◽  
Acoustic Forcing ◽  
Far Wake

In this paper, we investigate to what extent the far-wake ‘signature’ of the near-wake vortex dynamics of a nominally two-dimensional bluff body is affected by the character of the free-stream noise. We confirm the existence of an oblique wave resonance (at frequency, fK–fT), which is caused by nonlinear ‘quadratic’ interactions between primary oblique shedding waves (fK) and secondary two-dimensional waves (fT), which are amplified from free-stream disturbances. In this work, oblique wave resonance is induced by acoustic forcing of two-dimensional waves. The use of acoustic forcing reveals a set of higher-order oblique wave resonances corresponding to frequencies (fK–nfT), where n is an integer. We find from visualization that, even when the secondary two-dimensional waves have the same frequency as the oblique waves, it is the oblique waves that are preferentially amplified. Oblique wave angles up to 74° have been observed. The response of the wake to a large range of forcing frequencies shows a broad region of peak response, centred around F = (fT/fK) = 0.55, and is in reasonable agreement with predictions from linear stability analysis. A similar broad response is found for each of the higher-order oblique wave modes. Simple equations for the oblique waves yield approximate conditions for maximum wake response, with a frequency for peak response given by Fmax = 1/2n = 1/2, 1/4, 1/6,…, and an oblique wave angle given by θmax = 2θK, where θK is the angle of oblique vortex shedding. An increase in forcing amplitude has the effect of bringing the nonlinear wave interactions, leading to oblique wave resonance, further upstream. Paradoxically, the effect of an increase in amplitude (A) of the two-dimensional wave forcing is to further amplify the oblique waves in preference to the two-dimensional waves and, under some conditions, to inhibit the appearance of prominent two-dimensional waves where they would otherwise appear. With a variation in forcing amplitude, the amplitude of oblique wave response is found to be closely proportional to A½. In summary, this investigation confirms the surprising result that it is only through the existence of noise in the free stream that the far wake is ‘connected’ to the near wake.

Flow structure in the frequency-modulated wake of a cylinder

1994 ◽  
Vol 266 ◽  
pp. 93-119 ◽  
Author(s):  
M. Nakano ◽  
D. Rockwell
Keyword(s):  
Flow Structure ◽  
Modulation Frequency ◽  
Excitation Frequency ◽  
Frequency Deviation ◽  
Near Wake ◽  
Spectral Content ◽  
Wake Structure ◽  
Vortical Structures ◽  
Far Wake ◽  
Sinusoidal Excitation

A cylinder is subjected to frequency-modulated (FM) excitation and the structure of its wake is characterized in terms of the modulation frequency and the frequency deviation. It is possible to destabilize or restabilize the degree of organization of the vortical structures in the near wake and thereby substantially manipulate the spectral content, relative to the case of purely sinusoidal excitation. These processes of destabilization and restabilization are attainable by varying the frequency deviation while holding the modulation frequency constant or vice versa. A phase-locked periodicity of the nearwake response is attainable at the period of the modulation frequency, as well as at double its period. This phase-locked periodicity, or lack of it, is related to the degree of organization of the wake. The structure of the far wake is strongly dependent upon the nature of the near wake modification. Either coherent or destabilized wake structure can be induced in the far wake, at a given value of nominal excitation frequency, by employing appropriate FM excitation.

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