In aerodynamics, the Crow instability, or V.C.I. vortex crow instability, is an inviscid line-vortex instability, named after its discoverer S. C. Crow. The effect of the Crow instability can often be observed in the skies behind large aircraft, when the wingtip vortices interact with contrails from the engines, producing visible distortions in the shape of the contrail.
The Crow instability is a vortex pair instability, and typically goes through several stages:
The wings of airplanes in flight produce at least one pair of trailing vortices. These vortices are a major source of wake turbulence as they persist for a significant period of time after the airplane has passed. If the decay of trailing vortices were due solely to viscous effects in the core of each vortex, decay would be so slow that they would persist for hundreds of miles behind the airplane. In fact, these vortices only persist for tens of miles. The additional cause of the collapse of these vortices is large-scale instabilities such as Crow instability. [1]
When a fluid flows around an object, the fluid exerts a force on the object. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it is defined to act perpendicular to the flow and therefore can act in any direction.
Contrails or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. They are composed primarily of water, in the form of ice crystals. The combination of water vapor in aircraft engine exhaust and the low ambient temperatures at high altitudes causes the trails' formation. Impurities in the engine exhaust from the fuel, including sulfur compounds provide some of the particles that serve as cloud condensation nuclei for water droplet growth in the exhaust. If water droplets form, they can freeze to form ice particles that compose a contrail. Their formation can also be triggered by changes in air pressure in wingtip vortices, or in the air over the entire wing surface. Contrails, and other clouds caused directly by human activity, are called homogenitus.
Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial, elastic, and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classified into two fields: static aeroelasticity dealing with the static or steady state response of an elastic body to a fluid flow, and dynamic aeroelasticity dealing with the body's dynamic response.
In fluid dynamics, a vortex is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in the wake of a boat, and the winds surrounding a tropical cyclone, tornado or dust devil.
The 1940 Tacoma Narrows Bridge, the first bridge at this location, was a suspension bridge in the U.S. state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It opened to traffic on July 1, 1940, and dramatically collapsed into Puget Sound on November 7 of the same year. The bridge's collapse has been described as "spectacular" and in subsequent decades "has attracted the attention of engineers, physicists, and mathematicians". Throughout its short existence, it was the world's third-longest suspension bridge by main span, behind the Golden Gate Bridge and the George Washington Bridge.
Lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, in aerodynamics, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or a lifting body redirecting air to cause lift and also in cars with airfoil wings that redirect air to cause a downforce. It is symbolized as , and the lift-induced drag coefficient as .
Wake turbulence is a disturbance in the atmosphere that forms behind an aircraft as it passes through the air. It includes several components, the most significant of which are wingtip vortices and jet-wash, the rapidly moving gases expelled from a jet engine.
In fluid dynamics, a Kármán vortex street is a repeating pattern of swirling vortices, caused by a process known as vortex shedding, which is responsible for the unsteady separation of flow of a fluid around blunt bodies.
Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift. The name is a misnomer because the cores of the vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes named trailing or lift-induced vortices because they also occur at points other than at the wing tips. Indeed, vorticity is trailed at any point on the wing where the lift varies span-wise ; it eventually rolls up into large vortices near the wingtip, at the edge of flap devices, or at other abrupt changes in wing planform.
In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the relative motion of any object, moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or between a fluid and solid surface. Drag forces tend to decrease fluid velocity relative to the solid object in the fluid's path.
The Gurney flap is a small tab projecting from the trailing edge of a wing. Typically it is set at a right angle to the pressure-side surface of the airfoil and projects 1% to 2% of the wing chord. This trailing edge device can improve the performance of a simple airfoil to nearly the same level as a complex high-performance design.
The superconductor–insulator transition is an example of a quantum phase transition, whereupon tuning some parameter in the Hamiltonian, a dramatic change in the behavior of the electrons occurs. The nature of how this transition occurs is disputed, and many studies seek to understand how the order parameter, , changes. Here is the amplitude of the order parameter, and is the phase. Most theories involve either the destruction of the amplitude of the order parameter - by a reduction in the density of states at the Fermi surface, or by destruction of the phase coherence; which results from the proliferation of vortices.
The Kutta–Joukowski theorem is a fundamental theorem in aerodynamics used for the calculation of lift of an airfoil translating in a uniform fluid at a constant speed so large that the flow seen in the body-fixed frame is steady and unseparated. The theorem relates the lift generated by an airfoil to the speed of the airfoil through the fluid, the density of the fluid and the circulation around the airfoil. The circulation is defined as the line integral around a closed loop enclosing the airfoil of the component of the velocity of the fluid tangent to the loop. It is named after Martin Kutta and Nikolai Zhukovsky who first developed its key ideas in the early 20th century. Kutta–Joukowski theorem is an inviscid theory, but it is a good approximation for real viscous flow in typical aerodynamic applications.
The horseshoe vortex model is a simplified representation of the vortex system present in the flow of air around a wing. This vortex system is modelled by the bound vortex and two trailing vortices, therefore having a shape vaguely reminiscent of a horseshoe. A starting vortex is shed as the wing begins to move through the fluid. This vortex dissipates under the action of viscosity, as do the trailing vortices which eventually dissipate far behind the aircraft.
In fluid dynamics, the starting vortex is a vortex which forms in the air adjacent to the trailing edge of an airfoil as it is accelerated from rest. It leaves the airfoil, and remains (nearly) stationary in the flow. It eventually decays through the action of viscosity, but before doing so it contributes significantly to wake turbulence.
The Vortex lattice method, (VLM), is a numerical method used in computational fluid dynamics, mainly in the early stages of aircraft design and in aerodynamic education at university level. The VLM models the lifting surfaces, such as a wing, of an aircraft as an infinitely thin sheet of discrete vortices to compute lift and induced drag. The influence of the thickness and viscosity is neglected.
In fluid dynamics, a Tollmien–Schlichting wave is a streamwise unstable wave which arises in a bounded shear flow. It is one of the more common methods by which a laminar bounded shear flow transitions to turbulence. The waves are initiated when some disturbance interacts with leading edge roughness in a process known as receptivity. These waves are slowly amplified as they move downstream until they may eventually grow large enough that nonlinearities take over and the flow transitions to turbulence.
In fluid dynamics, the process of a laminar flow becoming turbulent is known as laminar–turbulent transition. The main parameter characterizing transition is the Reynolds number.
Turbulent flows are complex multi-scale and chaotic motions that need to be classified into more elementary components, referred to coherent turbulent structures. Such a structure must have temporal coherence, i.e. it must persist in its form for long enough periods that the methods of time-averaged statistics can be applied. Coherent structures are typically studied on very large scales, but can be broken down into more elementary structures with coherent properties of their own, such examples include hairpin vortices. Hairpins and coherent structures have been studied and noticed in data since the 1930s, and have been since cited in thousands of scientific papers and reviews.
A whistle is a device that makes sound from air blown from one end forced through a small opening at the opposite end. They are shaped in a way that allows air to oscillate inside of a chamber in an unstable way. The physical theory of the sound-making process is an example of the application of fluid dynamics or hydrodynamics and aerodynamics. The principles relevant to whistle operation also have applications in other areas, such as fluid flow measurement.
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