13,576 research outputs found
On the estimation of swimming and flying forces from wake measurements
The transfer of momentum from an animal to fluid in
its wake is fundamental to many swimming and flying
modes of locomotion. Hence, properties of the wake are
commonly studied in experiments to infer the magnitude
and direction of locomotive forces. The determination of
which wake properties are necessary and sufficient to
empirically deduce swimming and flying forces is
currently made ad hoc. This paper systematically
addresses the question of the minimum number of wake
properties whose combination is sufficient to determine
swimming and flying forces from wake measurements. In
particular, it is confirmed that the spatial velocity
distribution (i.e. the velocity field) in the wake is by itself insufficient to determine swimming and flying forces, and must be combined with the fluid pressure distribution. Importantly, it is also shown that the spatial distribution of rotation and shear (i.e. the vorticity field) in the wake is by itself insufficient to determine swimming and flying forces, and must be combined with a parameter that is analogous to the fluid pressure. The measurement of this parameter in the wake is shown to be identical to a calculation of the added-mass contribution from fluid surrounding vortices in the wake, and proceeds identically to a measurement of the added-mass traditionally associated with fluid surrounding solid bodies. It is demonstrated that the velocity/pressure perspective is equivalent to the vorticity/vortex-added-mass approach in the equations of motion. A model is developed to approximate the contribution of wake vortex added-mass to locomotive forces, given a combination of velocity and vorticity field measurements in the wake. A dimensionless parameter, the wake vortex ratio (denoted Wa), is introduced to predict the types of wake flows for which the contribution of forces due to wake vortex added-mass will become non-negligible. Previous wake analyses are reexamined in light of this parameter to infer the existence and importance of wake vortex added-mass in those cases. In the process, it is demonstrated that the commonly used time-averaged force estimates based on wake
measurements are not sufficient to prove that an animal is
generating the locomotive forces necessary to sustain flight
or maintain neutral buoyancy
An overview of a Lagrangian method for analysis of animal wake dynamics
The fluid dynamic analysis of animal wakes is becoming increasingly popular in studies of animal swimming and flying, due in part to the development of quantitative flow visualization techniques such as digital particle imaging velocimetry (DPIV). In most studies, quasi-steady flow is assumed and the flow analysis is based on velocity and/or vorticity fields measured at a single time instant during the stroke cycle. The assumption of quasi-steady flow leads to neglect of unsteady (time-dependent) wake vortex added-mass effects, which can contribute significantly to the instantaneous locomotive forces. In this paper we review a Lagrangian approach recently introduced to determine unsteady wake vortex structure by tracking the trajectories of individual fluid particles in the flow, rather than by analyzing the velocity/vorticity fields at fixed locations and single instants in time as in the Eulerian perspective. Once the momentum of the wake vortex and its added mass are determined, the corresponding unsteady locomotive forces can be quantified. Unlike previous studies that estimated the time-averaged forces over the stroke cycle, this approach enables study of how instantaneous locomotive forces evolve over time. The utility of this method for analyses of DPIV velocity measurements is explored, with the goal of demonstrating its applicability to data that are typically available to investigators studying animal swimming and flying. The methods are equally applicable to computational fluid dynamics studies where velocity field calculations are available
Note on the induced Lagrangian drift and added-mass of a vortex
Darwin (1953) introduced a simple heuristic that relates the Lagrangian fluid drift induced by a solid body propagating in irrotational flow to its virtual- or added-mass. The force required to accelerate the solid body must also overcome this added-mass. An extension of Darwin's (1953) method to the case of vortices propagating in a real fluid is described here. Experiments are conducted to demonstrate the existence of an added-mass effect during uni-directional vortex motion, which is analogous to the effect of solid bodies in potential flow. The definition of the vortex added-mass coefficient is modified from the solid body case to account for entrainment of ambient fluid by the vortex. This modified coefficient for propagating vortices is shown to be equal in magnitude to the classical coefficient for a solid body of equivalent boundary geometry. An implication of these results is that the vortex added-mass concept can be used as a surrogate for the velocity potential, in order to facilitate calculations of the pressure contribution to forces required to set fluid into unsteady vortical motion. Application of these results to unsteady wake analyses and fluid–structure interactions such as vortex-induced vibrations is suggested
Conditional stability of particle alignment in finite-Reynolds-number channel flow
Finite-size neutrally buoyant particles in a channel flow are known to
accumulate at specific equilibrium positions or spots in the channel
cross-section if the flow inertia is finite at the particle scale. Experiments
in different conduit geometries have shown that while reaching equilibrium
locations, particles tend also to align regularly in the streamwise direction.
In this paper, the Force Coupling Method was used to numerically investigate
the inertia-induced particle alignment, using square channel geometry. The
method was first shown to be suitable to capture the quasi-steady lift force
that leads to particle cross-streamline migration in channel flow. Then the
particle alignment in the flow direction was investigated by calculating the
particle relative trajectories as a function of flow inertia and of the ratio
between the particle size and channel hydraulic diameter. The flow streamlines
were examined around the freely rotating particles at equilibrium, revealing
stable small-scale vortices between aligned particles. The streamwise
inter-particle spacing between aligned particles at equilibrium was calculated
and compared to available experimental data in square channel flow (Gao {\it et
al.} Microfluidics and Nanofluidics {\bf 21}, 154 (2017)). The new result
highlighted by our numerical simulations is that the inter-particle spacing is
unconditionally stable only for a limited number of aligned particles in a
single train, the threshold number being dependent on the confinement
(particle-to-channel size ratio) and on the Reynolds number. For instance, when
the particle Reynolds number is and the particle-to-channel height
size ratio is , the maximum number of stable aligned particles per
train is equal to 3. This agrees with statistics realized on the experiments of
(Gao {\it et al.} Microfluidics and Nanofluidics {\bf 21}, 154 (2017)).Comment: 13 pages, 13 figure
Kinematic studies of transport across an island wake, with application to the Canary islands
Transport from nutrient-rich coastal upwellings is a key factor influencing
biological activity in surrounding waters and even in the open ocean. The rich
upwelling in the North-Western African coast is known to interact strongly with
the wake of the Canary islands, giving rise to filaments and other mesoscale
structures of increased productivity. Motivated by this scenario, we introduce
a simplified two-dimensional kinematic flow describing the wake of an island in
a stream, and study the conditions under which there is a net transport of
substances across the wake. For small vorticity values in the wake, it acts as
a barrier, but there is a transition when increasing vorticity so that for
values appropriate to the Canary area, it entrains fluid and enhances
cross-wake transport.Comment: 28 pages, 13 figure
Molecular Hydrodynamics: Vortex Formation and Sound Wave Propagation
In the present study, quantitative feasibility tests of the hydrodynamic
description of a two-dimensional fluid at the molecular level are performed,
both with respect to length and time scales. Using high-resolution fluid
velocity data obtained from extensive molecular dynamics simulations, we
computed the transverse and longitudinal components of the velocity field by
the Helmholtz decomposition and compared them with those obtained from the
linearized Navier-Stokes (LNS) equations with time-dependent transport
coefficients. By investigating the vortex dynamics and the sound wave
propagation in terms of these field components, we confirm the validity of the
LNS description for times comparable to or larger than several mean collision
times. The LNS description still reproduces the transverse velocity field
accurately at smaller times, but it fails to predict characteristic patterns of
molecular origin visible in the longitudinal velocity field. Based on these
observations, we validate the main assumptions of the mode-coupling approach.
The assumption that the velocity autocorrelation function can be expressed in
terms of the fluid velocity field and the tagged particle distribution is found
to be remarkably accurate even for times comparable to or smaller than the mean
collision time. This suggests that the hydrodynamic-mode description remains
valid down to the molecular scale
Time resolved tracking of a sound scatterer in a turbulent flow: non-stationary signal analysis and applications
It is known that ultrasound techniques yield non-intrusive measurements of
hydrodynamic flows. For example, the study of the echoes produced by a large
number of particle insonified by pulsed wavetrains has led to a now standard
velocimetry technique. In this paper, we propose to extend the method to the
continuous tracking of one single particle embedded in a complex flow. This
gives a Lagrangian measurement of the fluid motion, which is of importance in
mixing and turbulence studies. The method relies on the ability to resolve in
time the Doppler shift of the sound scattered by the continuously insonfied
particle.
For this signal processing problem two classes of approaches are used:
time-frequency analysis and parametric high resolution methods. In the first
class we consider the spectrogram and reassigned spectrogram, and we apply it
to detect the motion of a small bead settling in a fluid at rest. In more
non-stationary turbulent flows where methods in the second class are more
robust, we have adapted an Approximated Maximum Likelihood technique coupled
with a generalized Kalman filter.Comment: 16 pages 9 figure
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