|
Denny's Paradox
In biology, Denny's paradox refers to the apparent impossibility of surface-dwelling animals such as the water strider generating enough propulsive force to move. It is named after biologist Mark Denny, and relates to animal locomotion on the surface layer of water. If capillary waves are assumed to generate the momentum transfer to the water, the animal's legs must move faster than the phase speed c_m of the waves, given by :c_m=\left(\frac\right)^, where g is the acceleration due to gravity, \sigma is the strength of surface tension, and \rho the density of water. For standard conditions, this works out to be about 0.23 m/s. In fact, water striders' legs move at speeds much less than this and, according to this physical picture, cannot move. Writing in the Journal of Fluid Mechanics, David Hu and John Bush state that Denny's paradox "rested on two flawed assumptions. First, water striders' motion was assumed to rely on the generation of capillary waves, since the pro ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gerris Lacustris 03
''Gerris'' is a bug genus in the family Gerridae (water striders). Species The genus ''Gerris'' contains 43 species in 3 subgenera: Subgenus ''Gerris'' Subgenus ''Gerriselloides'' * ''Gerris asper'' (Fieber, 1860) * '' Gerris brachynotus'' Horváth, 1907 * '' Gerris kiritshenkoi'' Kanyukova, 1979 * '' Gerris lateralis'' Schummel, 1832 Subgenus ''Macrogerris'' * ''Gerris cui'' Esaki, 1925 * ''Gerris gracilicornis'' Horváth, 1879 * ''Gerris insularis'' Motschulsky, 1866 * '' Gerris issikii'' Miyamoto, 1961 * '' Gerris lundbladi'' Andersen & Chen, 1993 * '' Gerris tigrinus'' Brown, 1949 * '' Gerris yezoensis'' Miyamoto, 1958 Mating system Species of genus ''Gerris'' demonstrate a high degree of female control over most aspects of mating and there is significant evidence that supports antagonistic coevolution, and the convenience polyandry hypothesis of multiple matings. There is evidence to suggest that the post-copulatory guarding exhibited by ''Gerris buenoi'' - and other s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Water Strider
The Gerridae are a family of insects in the order Hemiptera, commonly known as water striders, water skeeters, water scooters, water bugs, pond skaters, water skippers, or water skimmers. Consistent with the classification of the Gerridae as true bugs (i.e., suborder Heteroptera), gerrids have mouthparts evolved for piercing and sucking, and distinguish themselves by having the unusual ability to walk on water, making them pleuston (surface-living) animals. They are anatomically built to transfer their weight to be able to run on top of the water's surface. As a result, one could likely find water striders present in any pond, river, or lake. Over 1,700 species of gerrids have been described, 10% of them being marine. While 90% of the Gerridae are freshwater bugs, the oceanic '' Halobates'' makes the family quite exceptional among insects. The genus ''Halobates'' was first heavily studied between 1822 and 1883 when Buchanan-White collected several different species during the C ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mark Denny
Mark W. Denny (born 1951) is a professor of biology at Stanford University. His research on the intertidal zone of wave-swept shores has led to increased understanding of this habitat. His most publicized research is his work on locomotion of water striders, which led to the coining of the term "Denny's paradox" to explain a discrepancy between physics and previous understanding of how surface-dwelling animals such as these insects move. In 2008 he examined greyhounds, thoroughbred horses and human athletes trying to find their maximum running speed. He predicted the fastest possible time for men's 100 metres The 100 metres, or 100-meter dash, is a sprint race in track and field competitions. The shortest common outdoor running distance, the dash is one of the most popular and prestigious events in the sport of athletics. It has been conteste ... will be 9.48 seconds. Books Denny is the author of several books, including: *''Biology and the Mechanics of Wave-Swept Sh ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Animal Locomotion On The Surface Layer Of Water
Animal locomotion on the surface layer of water is the study of animal locomotion in the case of small animals that live on the surface layer of water, relying on surface tension to stay afloat. There are two types of animal locomotion on water, determined by the ratio of the animal's weight to the water's surface tension: those whose weight is supported by the surface tension at rest, and can therefore easily remain on the water's surface without much exertion, and those whose weight is not supported by the water's surface tension at rest, and must therefore exert additional motion in a direction parallel to the water's surface in order to remain above it. A creature such as the basilisk lizard, often dubbed the 'Jesus lizard', has a weight which is larger than the surface tension can support, and is widely known for running across the surface of water. Another example, the western grebe, performs a mating ritual that includes running across the surface of water. Surface living ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Capillary Waves
A capillary wave is a wave traveling along the phase boundary of a fluid, whose dynamics and phase velocity are dominated by the effects of surface tension. Capillary waves are common in nature, and are often referred to as ripples. The wavelength of capillary waves on water is typically less than a few centimeters, with a phase speed in excess of 0.2–0.3 meter/second. A longer wavelength on a fluid interface will result in gravity–capillary waves which are influenced by both the effects of surface tension and gravity, as well as by fluid inertia. Ordinary gravity waves have a still longer wavelength. When generated by light wind in open water, a nautical name for them is cat's paw waves. Light breezes which stir up such small ripples are also sometimes referred to as cat's paws. On the open ocean, much larger ocean surface waves (seas and swells) may result from coalescence of smaller wind-caused ripple-waves. Dispersion relation The dispersion relation describes the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Phase Speed
The phase velocity of a wave is the rate at which the wave propagates in any medium. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, the crest) will appear to travel at the phase velocity. The phase velocity is given in terms of the wavelength (lambda) and time period as :v_\mathrm = \frac. Equivalently, in terms of the wave's angular frequency , which specifies angular change per unit of time, and wavenumber (or angular wave number) , which represent the angular change per unit of space, :v_\mathrm = \frac. To gain some basic intuition for this equation, we consider a propagating (cosine) wave . We want to see how fast a particular phase of the wave travels. For example, we can choose , the phase of the first crest. This implies , and so . Formally, we let the phase and see immediately that and . So, it immediately follows that : \frac = -\frac \frac = \frac ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Standard Gravity
The standard acceleration due to gravity (or standard acceleration of free fall), sometimes abbreviated as standard gravity, usually denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is defined by standard as . This value was established by the 3rd CGPM (1901, CR 70) and used to define the standard weight of an object as the product of its mass and this nominal acceleration. The acceleration of a body near the surface of the Earth is due to the combined effects of gravity and centrifugal acceleration from the rotation of the Earth (but the latter is small enough to be negligible for most purposes); the total (the apparent gravity) is about 0.5% greater at the poles than at the Equator. Although the symbol is sometimes used for standard gravity, (without a suffix) can also mean the local acceleration due to local gravity and centrifugal acceleration, which varies depending on one's position on Earth (see Earth's ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Surface Tension
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to float on a water surface without becoming even partly submerged. At liquid–air interfaces, surface tension results from the greater attraction of liquid molecules to each other (due to cohesion) than to the molecules in the air (due to adhesion). There are two primary mechanisms in play. One is an inward force on the surface molecules causing the liquid to contract. Second is a tangential force parallel to the surface of the liquid. This ''tangential'' force is generally referred to as the surface tension. The net effect is the liquid behaves as if its surface were covered with a stretched elastic membrane. But this analogy must not be taken too far as the tension in an elastic membrane is dependent on the amount of deformation of the m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically, density is defined as mass divided by volume: : \rho = \frac where ''ρ'' is the density, ''m'' is the mass, and ''V'' is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more specifically called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure. To simplify comparisons of density across different s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a solvent). It is vital for all known forms of life, despite not providing food, energy or organic micronutrients. Its chemical formula, H2O, indicates that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°. "Water" is also the name of the liquid state of H2O at standard temperature and pressure. A number of natural states of water exist. It forms precipitation in the form of rain and aerosols in the form of fog. Clouds consist of suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor. Water co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Journal Of Fluid Mechanics
The ''Journal of Fluid Mechanics'' is a peer-reviewed scientific journal in the field of fluid mechanics. It publishes original work on theoretical, computational, and experimental aspects of the subject. The journal is published by Cambridge University Press and retains a strong association with the University of Cambridge, in particular the Department of Applied Mathematics and Theoretical Physics (DAMTP). Until January 2020, volumes were published twice a month in a single-column B5 format, but the publication is now online-only with the same frequency. The journal was established in 1956 by George Batchelor, who remained the editor-in-chief for some forty years. He started out as the sole editor, but later a team of associate editors provided assistance in arranging the review of articles. John W. Miles is the author who has most papers (117 times) appeared in this journal. Editors The following people have been editor (later, editor in chief) of the ''Journal of Fluid Me ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Wave Drag
In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (resting) value at some frequency. When the entire waveform moves in one direction, it is said to be a ''traveling wave''; by contrast, a pair of superimposed periodic waves traveling in opposite directions makes a ''standing wave''. In a standing wave, the amplitude of vibration has nulls at some positions where the wave amplitude appears smaller or even zero. Waves are often described by a ''wave equation'' (standing wave field of two opposite waves) or a one-way wave equation for single wave propagation in a defined direction. Two types of waves are most commonly studied in classical physics. In a ''mechanical wave'', stress and strain fields oscillate about a mechanical equilibrium. A mechanical wave is a local deformation (strain) in s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
_nymph.jpg "Click for more on -> Water Strider")
.gif "Click for more on -> Phase Speed")
.svg "Click for more on -> Water")
