Dimensionless Wavelength Research Articles

Hydrodynamic characteristics of Oscillating Water Column caisson breakwaters

This study uses the finite difference software, FLOW-3D, to develop a numerical model to simulate the hydrodynamic characteristics for Oscillating Water Column (OWC) caisson breakwaters. The numerical model is verified by a flume channel experiment for the interaction mechanism between air and water. Through the analyses with different incident wave parameters of the OWC caisson breakwaters, the following parameters are evaluated as energy capture efficiency indices: the amplitude magnification (a/a0), the air velocity at the orifice (Vori), the air oscillation pressure (Hp), the hydrodynamic efficiency (ηh), the energy produced by air for a period (Eair), and the average power produced by the air (Pair). The results show that the optimal dimensionless wavelength ratio (L/lc’) for the optimal hydrodynamic efficiency (ηh) is different from the optimal dimensionless wavelength ratio (L/lc’) for the average power produced by the air (Pair), the amplitude magnification (a/a0), the air velocity at the orifice (Vori), and the air oscillation pressure (Hp). For power generation purpose, this study suggests that the relationship curve between the dimensionless wavelength ratio (L/lc’) and the average power produced by air (Pair) can be used to determine the optimal range for wave energy capture and to design the optimal size for the OWC caisson breakwaters. Under the incident wave conditions of maximum average power produced by air, the lateral force and the base moment has enlargement phenomenon.

Surface manifestation of internal waves emitted by submerged localized stratified turbulence

The internal waves (IWs) radiated by the turbulent wake of a sphere of diameter $D$ towed at speed $U$ are investigated using three-dimensional fully nonlinear simulations performed in a linearly stratified Boussinesq fluid with buoyancy frequency $N$. The study focuses on a broad range of wave characteristics in the far field of the turbulent wave source, specifically at the sea surface (as modelled by a free-slip rigid lid) where the IWs reflect. Six simulations are performed at Reynolds number $Re\equiv UD/{\it\nu}\in \{5\times 10^{3},10^{5}\}$ and Froude number $Fr\equiv 2U/(ND)\in \{4,16,64\}$, where ${\it\nu}$ is viscosity. The wave-emitting wake is located at a fixed distance of $9D$ below the surface. As the wake evolves for up to $O(300)$ units of buoyancy time scale $1/N$, IW characteristics, such as horizontal wavelength ${\it\lambda}_{H}$ and wave period $T$, are sampled at the sea surface via wavelet transforms of horizontal divergence signals. The statistics of amplitudes and orientations of IW-induced surface strains are also reported. The mean dimensionless observable wavelength $\overline{{\it\lambda}}_{H}/D$ at the sea surface decays in time as $(Nt)^{-1}$, which is due to the waves’ dispersion. This observation is in agreement with a linear propagation model that is independent of the wake $Re$ and $Fr$. This agreement further suggests that the most energetic waves impacting the surface originate from the early-time wake that is adjusting to buoyancy. The most energetic dimensionless wavelength $\hat{{\it\lambda}}_{H}/D$ is found to scale as $Fr^{1/3}$ and decrease with $Re$, which causes the arrival time (in $Nt$ units) of the strongest waves at the surface to scale as $Fr^{-1/3}$ and increase with $Re$. This wavelength $\hat{{\it\lambda}}_{H}$ is also found to correlate with the vertical Taylor scale of the wake turbulence. IW-driven phenomena at the surface that are of interest to an observer, such as the local enrichment of surfactant and the transport of ocean surface tracers, are also examined. The local enrichment ratio of surface scalar scales linearly with the steepness of IWs that reach the surface, and the ratio often exceeds a possible visibility threshold. The Lagrangian drifts of ocean tracers, which are linked to the nonlinear interaction between incident and reflecting IW packets, create a local divergence in lateral mass transport right above the wake centreline, an effect that intensifies strongly with increasing $Fr$. The findings of this study may serve as a platform to investigate the generation and surface manifestation of IWs radiated by other canonical submerged stratified turbulent flows.

Effects of wavelength and number of bends on the performance of zigzag demisters with drainage channels

In this paper, the effects of number of bends and bend wavelength on droplet removal efficiency and pressure drop of a zigzag demister with drainage channels have been numerically investigated. A combination of Eulerian-Lagrangian method with eddy interaction model (EIM) has been used for the simulation of the droplet dispersion in turbulent gas flow. The droplet collection efficiency has been calculated and compared with available experimental data to show the validity of the simulation. The Reynolds stress transport model (RSTM) was applied to model the turbulent airflow. The results show that applying more bends in the demister reduces the filtration size of the demister but it can increase the pressure drop significantly. On the other hand, the demisters with small dimensionless bend wavelength have more pressure losses and higher droplet removal efficiencies.

Three-dimensional effects on flag flapping dynamics

We examine three-dimensional (3D) effects on the flapping dynamics of a flag, modelled as a thin membrane, in uniform fluid inflow. We consider periodic spanwise variations of length $S$ (ignoring edge effects), so that the 3D effects are characterized by the dimensionless spanwise wavelength ${\it\gamma}=S/L$, where $L$ is the chord length. We perform linear stability analysis (LSA) to show increase in stability with ${\it\gamma}$, with the purely 2D mode being the most unstable. To confirm the LSA and to study nonlinear responses of 3D flapping, we obtain direct numerical simulations, up to Reynolds number 1000 based on $L$, coupling solvers for the Navier–Stokes equations and that for a thin membrane structure undergoing arbitrarily large displacement. For nonlinear flapping evolution, we identify and characterize the effect of ${\it\gamma}$ on the distinct flag motions and wake vortex structures, corresponding to spanwise standing wave (SW) and travelling wave (TW) modes, in the absence and presence of cross-flow respectively. For both SW and TW, the response is characterized by an initial instability growth phase (I), followed by a nonlinear development phase (II) consisting of multiple unstable 3D modes, and tending, in long time, towards a quasi-steady limit-cycle response (III) dominated by a single (most unstable) mode. Phase I follows closely the predictions of LSA for initial instability and growth rates, with the latter increased for TW due to suppression of restoring forces by the cross-flow. Phase II is characterized by multiple competing flapping modes with energy cascading towards the more unstable mode(s). The wake is characterized by interwoven (SW) and oblique continuous (TW) shed vortices. For phase III, the persistent single dominant mode for SW is the (most unstable) 2D flag displacement with a continuous parallel wake structure; and for TW, the fundamental oblique travelling-wave flag displacement corresponding to the given ${\it\gamma}$ with continuous oblique shedding. The transition to phase III occurs slower for greater ${\it\gamma}$. For the total forces, drag decreases for both SW and TW with decreasing ${\it\gamma}$, while lift is negligible in phase I and II and comparable in magnitude to drag in phase III for any ${\it\gamma}$.

Probing [formula omitted] cosmology with sterile neutrinos via measurements of scale-dependent growth rate of structure

In this paper, we constrain the dimensionless Compton wavelength parameter B0 of f(R) gravity as well as the mass of sterile neutrino by using the cosmic microwave background observations, the baryon acoustic oscillation surveys, and the linear growth rate measurements. Since both the f(R) model and the sterile neutrino generally predict scale-dependent growth rates, we utilize the growth rate data measured in different wavenumber bins with the theoretical growth rate approximatively scale-independent in each bin. The employed growth rate data come from the peculiar velocity measurements at z=0 in five wavenumber bins, and the redshift space distortions measurements at z=0.25 and z=0.37 in one wavenumber bin. By constraining the f(R) model alone, we get a tight 95% upper limit of log10⁡B0<−4.1. This result is slightly weakened to log10⁡B0<−3.8 (at 2σ level) once we simultaneously constrain the f(R) model and the sterile neutrino mass, due to the degeneracy between the parameters of the two. For the massive sterile neutrino parameters, we get the effective sterile neutrino mass mν,sterileeff<0.62 eV (2σ) and the effective number of relativistic species Neff<3.90 (2σ) in the f(R) model. As a comparison, we also obtain mν,sterileeff<0.56 eV (2σ) and Neff<3.92 (2σ) in the standard ΛCDM model.

Wind Waves in the Arctic Seas: Envisat and AltiKa Data Analysis

Wind-generated waves in the Kara, Laptev, and East-Siberian Seas are investigated using altimeter data from Envisat RA-2 and SARAL-AltiKa. Only isolated ice-free zones had been selected for analysis. Wind seas can be treated as pure wind-generated waves without any contamination by ambient swell. Such zones were identified using ice concentration data from microwave radiometers. Altimeter data, both significant wave height (SWH) and wind speed, for these areas were further obtained for the period 2002–2012 using Envisat RA-2 measurements, and for 2013 using SARAL-AltiKa. Dependencies of dimensionless SWH and wavelength on dimensionless wave generation spatial scale are compared to known empirical dependencies for fetch-limited wind wave development. We further check sensitivity of Ka- and Ku-band and discuss new possibilities that AltiKa's higher resolution can open.

Impact of pressure, salt concentration, and temperature on the convective dissolution of carbon dioxide in aqueous solutions.

The convective dissolution of carbon dioxide (CO2) in salted water is theoretically studied to determine how parameters such as CO2 pressure, salt concentration, and temperature impact the short-time characteristics of the buoyancy-driven instability. On the basis of a parameter-free dimensionless model, we perform a linear stability analysis of the time-dependent concentration profiles of CO2 diffusing into the aqueous solution. We explicit the procedure to transform the predicted dimensionless growth rate and wavelength of the convective pattern into dimensional ones for typical laboratory-scale experiments in conditions close to room temperature and atmospheric pressure. This allows to investigate the implicit influence of the experimental parameters on the characteristic length and time scales of the instability. We predict that increasing CO2 pressure, or decreasing salt concentration or temperature destabilizes the system with regard to convection, leading to a faster dissolution of CO2 into salted water.

Forcing of a bottom-mounted circular cylinder by steep regular water waves at finite depth

AbstractForcing by steep regular water waves on a vertical circular cylinder at finite depth was investigated numerically by solving the two-phase incompressible Navier–Stokes equations. Consistently with potential flow theory, boundary layer effects were neglected at the sea bed and at the cylinder surface, but the strong nonlinear motion of the free surface was included. The numerical model was verified and validated by grid convergence and by comparison to relevant experimental measurements. First-order convergence towards an analytical solution was demonstrated and an excellent agreement with the experimental data was found. Time-domain computations of the normalized inline force history on the cylinder were analysed as a function of dimensionless wave height, water depth and wavelength. Here the dependence on depth was weak, while an increase in wavelength or wave height both lead to the formation of secondary load cycles. Special attention was paid to this secondary load cycle and the flow features that cause it. By visual observation and a simplified analytical model it was shown that the secondary load cycle was caused by the strong nonlinear motion of the free surface which drives a return flow at the back of the cylinder following the passage of the wave crest. The numerical computations were further analysed in the frequency domain. For a representative example, the secondary load cycle was found to be associated with frequencies above the fifth- and sixth-harmonic force component. For the third-harmonic force, a good agreement with the perturbation theories of Faltinsen, Newman &amp; Vinje (J. Fluid Mech., vol. 289, 1995, pp. 179–198) and Malenica &amp; Molin (J. Fluid Mech., vol. 302, 1995, pp. 203–229) was found. It was shown that the third-harmonic forces were estimated well by a Morison force formulation in deep water but start to deviate at decreasing depth.

Influence of oval-shaped tube on falling film flow characteristics on horizontal tube bundle

Falling liquid film flow on a horizontal tube is an important process which plays an important role in heat transfer in heat exchangers. The aim of the present study is to investigate experimentally the effect of the oval-shaped tube on flow characteristics of liquid falling film. The effects of the tube type, Reynolds number, and intertube spacing on the falling film flow modes, dimensionless wavelength, and film thickness for a horizontal tube bundle were studied. Compared with the circular tube, the transitional Reynolds number of the shaped tube is smaller for the oval-shaped tube. The falling wavelength increases with the decrease of Reynolds number and firstly increases then tends towards stability with increasing of intertube spacing. The falling wavelength of the oval-shaped tube is smaller than that of the circular tube. The minimum value of the film thickness appears approximately at the angular position of 135° for the oval-shaped tube, while the angular position of the circular is 105°. Finally, these conclusions obtained in this article provide a valuable reference to the application of the shaped tube in the falling film evaporator for the seawater desalination system.

Loads and propulsive efficiency of a flexible airfoil performing sinusoidal deformations

This paper presents the application of state-space airloads theory to a flexible airfoil performing sinusoidal deformations at high Reynolds numbers. Given the two-dimensional motion of a flexible airfoil, we derived the closed forms for the propulsive force, lift force, generalized forces of pitching and bending as functions of reduced frequency k, dimensionless wavelength z, and dimensionless amplitude A/(2b). We also calculate the power required to perform this sinusoidal deformation and the propulsive efficiency. Our results show a positive, time-averaged propulsive force for all k>k0=π/z, which is when the wave speed is greater than the moving speed. At k=k0, which is when the moving speed reaches the wave speed, the motion reaches a steady-state with all forces being zero. When k<k0, the system is the case of energy extraction in which the drag force (negative propulsive force) and wake are causing the airfoil to vibrate. For the propulsive case, the propulsive efficiency decreases from 1.0 to 0.5 as k goes to ∞, or k0 goes to 0. If there were no wake, the propulsive force would be zero at wavelengths of z=0.569 and z=1.3 for all k, and local optimum at z=0.82. Though these extrema of propulsive force with wavelength are smoothed out by the wake effect, one can still see around z=1.3 (k=2.4) the slope transitions of all three powers in Fig. 9. When k<2.4, the cost for high propulsion become more expensive as more power input is used by wake, thus less efficiency.

Discrimination of bed form scales using robust spline filters and wavelet transforms: Methods and application to synthetic signals and bed forms of the Río Paraná, Argentina

There is no standard nomenclature and procedure to systematically identify the scale and magnitude of bed forms such as bars, dunes, and ripples that are commonly present in many sedimentary environments. This paper proposes a standardization of the nomenclature and symbolic representation of bed forms and details the combined application of robust spline filters and continuous wavelet transforms to discriminate these morphodynamic features, allowing the quantitative recognition of bed form hierarchies. Herein the proposed methodology for bed form discrimination is first applied to synthetic bed form profiles, which are sampled at a Nyquist ratio interval of 2.5–50 and a signal‐to‐noise ratio interval of 1–20 and subsequently applied to a detailed 3‐D bed topography from the Río Paraná, Argentina, which exhibits large‐scale dunes with superimposed, smaller bed forms. After discriminating the synthetic bed form signals into three‐bed form hierarchies that represent bars, dunes, and ripples, the accuracy of the methodology is quantified by estimating the reproducibility, the cross correlation, and the standard deviation ratio of the actual and retrieved signals. For the case of the field measurements, the proposed method is used to discriminate small and large dunes and subsequently obtain and statistically analyze the common morphological descriptors such as wavelength, slope, and amplitude of both stoss and lee sides of these different size bed forms. Analysis of the synthetic signals demonstrates that the Morlet wavelet function is the most efficient in retrieving smaller periodicities such as ripples and smaller dunes and that the proposed methodology effectively discriminates waves of different periods for Nyquist ratios higher than 25 and signal‐to‐noise ratios higher than 5. The analysis of bed forms in the Río Paraná reveals that, in most cases, a Gamma probability distribution, with a positive skewness, best describes the dimensionless wavelength and amplitude for both the lee and stoss sides of large dunes. For the case of smaller superimposed dunes, the dimensionless wavelength shows a discrete behavior that is governed by the sampling frequency of the data, and the dimensionless amplitude better fits the Gamma probability distribution, again with a positive skewness. This paper thus provides a robust methodology for systematically identifying the scales and magnitudes of bed forms in a range of environments.

Experiments on evolution characteristics of momentum wakes generated by the towed sphere with a horizontal jet

The model of the towed sphere with a L-shape nozzle jet was used to generate both drag and jet momentum wakes due to the body and cooling discharge water of the underwater vehicle, the results of experiments were presented for the evolution characteristics of such combined momentum wakes in a stratified fluid. It is shown that for J D/ J C D, the evolution characteristics for the combined momentum wakes are mainly attributed to the drag momentum wake generated by the towed sphere, and the far wake becomes quasi two-dimensional (2D) in the form of a Karman-like dipolar vortex street, where J D is the drag momentum flux due to the towed sphere, J is the jet momentum flux due to the L-shape nozzle jet and C D is the drag coefficient of the sphere. For J D/ J C D, the evolution characteristics for the combined momentum wakes are strongly dependent of the ( Rej , Frj ) combinations, and the far wake can become quasi 2D in the form of an anti-Karman-like dipolar vortex street being mainly attributed to the jet momentum wake under some ( Rej , Frj ) combinations, but there may be no occurrence of large-scale coherent structures, where Rej and Frj are Reynolds and Froude numbers duo to the L-shape nozzle jet respectively. The conditions under different Reynolds and Froude numbers ( Rej , Frj ) combinations where the anti-Karman-like dipolar vortex streets can be formed by such combined momentum wakes were determined by use of data based on the present series experiments, and the dependence of the dimensionless formation time and inverse dimensionless average wavelength of the quasi 2D dipolar vortex streets on Frj for different values of Rej was obtained, results indicate that these parameters are independent of Rej and approximately follow some power exponent laws on Frj .

Numerical Study of Non-Newtonian Liquid Sheet Primary Breakup

Dynamic atomization processes of non-Newtonian liquid were investigated by the method of numerical simulation. The full transient process of the primary instability, deformation, and fragmentation of free surface were simulated numerically by solving the Navier-Stokes equations using an algorithm based on the finite volume method. The tracking of the free surface was achieved using the volume of fluid (VOF) technique and the geometric reconstruction was based on the technique of Piecewise-Linear Interface Construction (PLIC). The continuum surface force (CSF) method was used to model surface tension. In this paper, atomization characteristics of non-Newtonian liquid were analyzed detailedly. The distribution of dimensionless potential length against time was obtained, and the dimensionless wavelength of primary waves was investigated.

Experiments on quasi-two-dimensional dipolar vortex streets generated by a moving momentum source in a stratified fluid

Experiments were carried out in a stratified fluid with a momentum source modeled by a horizontally moving jet. The generation mechanism was investigated and the evolution characteristics were analyzed for the quasi-two-dimensional dipolar vortex streets generated by such a moving momentum source in the stratified fluid. The conditions of different Reynold and Froude number (Re, Fr) combinations under which the dipolar vortex street can be generated by moving momentum source were determined by use of data based on the present series of experiments. Moreover, the dependence of the dimensionless formation time and inverse dimensionless average wavelength of the dipolar vortex street on Fr for different values of Re was obtained. The results show that these parameters are independent of Re and approximately follow some exponential laws of Fr.

Micro-/nanoscaled irreversible Otto engine cycle with friction loss and boundary effects and its performance characteristics

An irreversible cycle model of the micro-/nanoscaled Otto engine cycle with internal friction loss is established. The general expressions of the work output and efficiency of the cycle are calculated based on the finite system thermodynamic theory, in which the quantum boundary effect of gas particles as working substance and the mechanical Casimir effect of gas system are considered. It is found that, for a micro-/nanoscaled Otto cycle devices, the work output W and efficiency η of the cycle can be expressed as the functions of the temperature ratio τ of the two heat reservoirs, the volume ratio rV and the surface area ratio rA of the two isochoric processes, the dimensionless thermal wavelength λ and other parameters of cycle, while for a macroscaled Otto cycle devices, the work output W0 and efficiency η0 of the cycle are independent of the surface area ratio rA and the dimensionless thermal wavelength λ. Further, the influence of boundary of cycle on the performance characteristics of the micro-/nanoscaled Otto cycle are analyzed in detail by introducing the output ratio W/W0 and efficiency ratio η/η0. The results present the general performance characteristics of a micro-/nanoscaled Otto cycle and may serve as the basis for the design of a realistic Otto cycle device in micro-/nanoscale.

Experimental study of heat transfer and flow characteristics of liquid falling film on a horizontal fluted tube

The aim of the present study is to investigate experimentally the effect of the fluted surface tube on the heat transfer and flow characteristics of liquid falling film. Experiments have indicated that, when a liquid falling film falls on a horizontal fluted surface tube, the transition starts at low Reynolds number than that of the plain tube. The value of the film thickness has been slightly decreased by decreasing the fluted pitch. A reduction of the film thickness was observed at about 9% for tube number 4, which has lower pitch, at Reynolds number of 485. A clear reduction of the dimensionless wavelength, λ*, has occurred at low fluted pitch tube. The use of enhanced surfaces can provide heat transfer coefficients higher values than those obtained from plain tube. Heat transfer enhancement was noticed due to the use of fluted tube surface. An improvement of the Nusselt number reached about 45% for tube 4. However, the low values of the fluted pitch increased the heat transfer enhancement than that of the high values.

Effects of wavelength and amplitude of a wavy cylinder in cross-flow at low Reynolds numbers

Three-dimensional numerical simulations of laminar flow around a circular cylinder with sinusoidal variation of cross-section along the spanwise direction, named ‘wavy cylinder’, are performed. A series of wavy cylinders with different combinations of dimensionless wavelength (λ/Dm) and wave amplitude (a/Dm) are studied in detail at a Reynolds number of Re = U∞Dm/ν = 100, where U∞ is the free-stream velocity and Dm is the mean diameter of a wavy cylinder. The results of variation of mean drag coefficient and root mean square (r.m.s.) lift coefficient with dimensionless wavelength show that significant reduction of mean and fluctuating force coefficients occurs at optimal dimensionless wavelengths λ/Dm of around 2.5 and 6 respectively for the different amplitudes studied. Based on the variation of flow structures and force characteristics, the dimensionless wavelength from λ/Dm = 1 to λ/Dm = 10 is classified into three wavelength regimes corresponding to three types of wake structures. The wake structures at the near wake of different wavy cylinders are captured. For all wavy cylinders, the flow separation line varies along the spanwise direction. This leads to the development of a three-dimensional free shear layer with periodic repetition along the spanwise direction. The three-dimensional free shear layer of the wavy cylinder is larger and more stable than that of the circular cylinder, and in some cases the free shear layer even does not roll up into a mature vortex street behind the cylinder. As a result, the mean drag coefficients of some of the typical wavy cylinders are less than that of a corresponding circular cylinder with a maximum drag coefficient reduction up to 18%. The r.m.s. lift coefficients are greatly reduced to practically zero at optimal wavelengths. In the laminar flow regime (60 ≤ Re ≤ 150), the values of optimal wavelength are Reynolds number dependent.

Droplet Velocity Fluctuations in Thermally Stimulated Continuous Liquid Jets

The interesting phenomenon of the breakup of continuous liquid jets into droplets has significant practical applications, including inkjet printing. Regardless of print mode (i.e., ondemand vs. continuous), as applications require increasingly precise droplet control, inherent noise in the breakup process is becoming an area of increasing interest. We examine jitter to infer the underlying velocity fluctuations in thermally stimulated continuous fluid jets. Based upon comparisons of jets having different diameters D, as well as jets stimulated at different frequencies (hence different wavelengths &#x3BB;), we find that the jitter depends significantly upon the dimensionless wavelength &#x3BB;/D, showing a minimum at the natural frequency of the jet (corresponding to &#x3BB;/D = 5) and a marked enhancement over the range 6 &lt; &#x3BB;/D &lt; 8. We present an analysis of the frequency dependence of the jitter in the context of a simple model for growth of radial perturbations on the jet.

Vortex-wake interactions of a flapping foil that models animal swimming and flight

The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (lambda*=4-24) at a constant dimensionless flapping amplitude (A*=1.5) and geometric angle of attack amplitude (A(alpha,geo)=15 degrees ). The corresponding Reynolds number was of the order of 1000. Such values are relevant for animal swimming and flight. We found that a significant leading edge vortex (LEV) was generated by the foil at low dimensionless wavelengths (lambda*<10). The LEV separated from the foil for all dimensionless wavelengths. The relative time (compared with the flapping period) that the unstable LEV stayed above the flapping foil increased for decreasing dimensionless wavelengths. As the dimensionless wavelength decreased, the wake dynamics evolved from a wavy von Kármán-like vortex wake shed along the sinusoidal path of the foil into a wake densely packed with large interacting vortices. We found that strongly interacting vortices could change the wake topology abruptly. This occurred when vortices were close enough to merge or tear each other apart. Our experiments show that relatively small changes in the kinematics of a flapping foil can alter the topology of the vortex wake drastically.

BIOMECHANICS OF SWIMMING AND FLIGHT

While airborne birds routinely defy gravity and aquatic creatures glide with ease through water, man has largely been restricted to land for all but the last century. How birds and fish manoeuvre with such freedom in their fluid environments has always fascinated humans, and more recently it has