Aerospace Science and Technonlogy Dictionary V Section

V-22 aircraft: A Bell/Boeing developed tilt rotor aircraft, nicknamed "Osprey", and designed for light multiservice use. Used for Osprey aircraft.
V-band: A frequency band used in radar extending approximately from 46 to 56 gigacycles per second. See frequency band.
V/STOL aircraft: A hybrid form of heavier-than-air aircraft that is capable, by virtue of one or more horizontal rotors or units acting as rotors, of taking off, hovering, and landing as, or in a fashion similar to, a helicopter, and once aloft, and moving forward, capable, by means of a mechanical conversion of one sort or another, of flying as a fixed-wing aircraft, especially in its higher speed ranges. Used for convertaplanes and steep gradient aircraft.
vacancies (crystal defects): Vacent sites in a crystal structure due to the absence of an atom or ion from its normal structural position.
vacua: Sometimes used as the plural of vacuum.
vacuum: 1. A given space filled with gas at pressures below atmospheric pressure. Various approximate ranges are:
low vacuum, torr 760 to 25;
medium vacuum, torr 25 to 10E-3;
high vacuum, torr 10E-3 to 10E-6;
very high vacuum, torr 10E-6 to 10E-9;
ultrahigh vacuum, torr 10E-9 and below.
2. In reference to satellite orbital parameters, without consideration of the perturbing effects of an atmosphere, as in vacuum perigee, vacuum apogee.
vacuum gage: An instrument for measuring pressure below atmospheric pressure. Some of the more common types of vacuum gages listed in order of descending pressure range of use are:
(a) Manometer, usually consists of a column of liquid supported by the pressure to be measured, the determination of which is a matter of measuring the column height.
(b) Thermal conductivity gage, consisting of a heated surface. The heat transported by the gas molecules from the surface is related to gas pressure. The heat transfer is reflected in changes in surface temperature (or in the heating power required to maintain constant temperature).
Various types of thermal conductivity gages are distinguished according to the method of indicating the surface temperature. The most common types are Pirani gage and thermocouple gage.
(c) Knudsen gage, which measures pressure in terms of the net rate of transfer of momentum by molecules between two surface maintained at different temperatures and separated by a distance smaller than the mean free path of the gas molecule. Also called radiometer vacuum gage.
(d) McLeod gage, in which a known volume of gas, at the pressure to be measured, is compressed by the movement of a liquid column to a much smaller known volume, at which the resulting higher pressure is measured.
(e) Ionization gage, comprising a means of ionizing the gas molecules and a means of correlating the number and type of ions produced with the pressure of the gas.
Various types of ionization gages are distinguished according to the method of producing the ionization. The common types are hot-cathode ionization gage, cold-cathode ionization gage, radioactive ionization gage.
vacuum pump: A device which sets up a flow of gas in a vacuum system. Some of the more common types are mechanical pump, vapor or diffusion pump, cryopump.
vacuum system: A chamber, or chambers, having walls capable of withstanding atmospheric pressure and having an opening through which the gas can be removed through a pipe or manifold to a pumping system. The pumping system may or may not be considered as part of the vacuum system.
A complete vacuum system contains all necessary pumps, gages, valves, work-holding fixtures, and other components necessary to carry out some particular process; such a system is referred to in England as vacuum plant.
vacuum tube: An electron tube evacuated to such a degree that its electrical characteristics are essentially unaffected by the presence of residual gas or vapor.
valence: numbers representing the combining or displacing power of an atom, number of electrons lost, gained or shared by an atom in a compound, number of hydrogen atoms with which an atom will combine, or which it will displace. The valance of an element is the ratio of the atomic weight to the equivalent weight.
Valsalva exercise: The procedure of raising the pressure in the nasapharynx by forcible expiration with the mouth closed and nostrils pinched, in order to clear the eustachian tubes. Used for valsalva maneuver.
valsalva maneuver: The procedure of raising the pressure in the naspharynx by forcible expiration with the mouth closed and nostrils pinched, in order to clear the eustachian tubes.
Van Allen belt: (or Van Allen radiation belt)(For James A. Van Allen, 1915 - .) The zone of high-intensity particulate radiation surrounding the earth beginning at altitudes of approximately 1000 kilometers.
The radiation of the Van Allen belt is composed of protons and electrons temporarily trapped in the earth's magnetic field. The intensity of radiation varies with the distance from the earth.
Van Allen radiation belt: = Van Allen belt.
Van de Graaff accelerators: Electrostaic machines in which an electrical charge is carried into the high voltage terminal by a belt made of insulating materials moving at high speed. The particles are then accelerated along a discharge path through a vacuum tube by the potential difference between the insulated terminal and the gorunded end of the accelerator.
Van de Graaff generator: An electrostatic generator which employs a system of conveyor belt and spray points to charge an insulated electrode to a high potential.
Van der Waal equation: The best known of the many laws which have been proposed to describe the thermodynamic behavior of real gases and their departures from the ideal gas laws. It states:

where a and b are constants dependent upon the gas; p is the pressure of the gas; v is its specific volume (measured in units of the specific volume of the gas at normal temperature and pressure); R is the universal gas constant; and T is the Kelvin temperature.
vane: 1. A thin and more-or-less flat object intended to align itself with a stream or flow in a manner similar to that of the common weathercock, as: (a) a device that project ahead of an aircraft to sense gusts or other actions of the air so as to create impulses or signals that are transmitted to the control system to stabilize the aircraft; (b) a fixed or movable surface used to control or give stability to a rocket. See control vane.
2. A blade or paddle-like object, often fashioned like an airfoil and usually one of several, that rotates about an axis, either being moved by a flow or creating a flow itself, such as the blade of a turbine, of a fan, of a rotary pump or air compressor, etc. See impeller vane.
3. Any of certain stationary blades, plates, or the like that serve to guide or direct a flow, or to create a special kind of flow, as: (a) any of the blades in the nozzle ring of a gas-turbine engine; (b) any of the plates or slatlike objects that guide the flow in a wind tunnel; (c) a plate or fence projecting from a wing to prevent spanwise flow. See contravane. See airfoil, note.
vapor: A gas whose temperature is below its critical temperature, so that it can be condensed to the liquid or solid state by increase of pressure alone.
vapor barrier clothing: Impermeable garments used with respirators as life support systems in toxic environments (caustic chemicals, etc.).
vapor phase epitaxy: A crystal growth process whereby an element or a compound is deposited as a thin layer on a slice of substrate single crystal material by the vapor phase technique.
vapor phase lubrication: The use of an organic liquid that is vaporized into a flowing air stream directed to sliding surfaces where lubrication is needed. The organic vapor reacts at the concentrated contact sliding area generating a lubricous deposit. This deposit has been characterized as a thin polymeric film that can provide effective lubrication temperatures greater than 400 degrees Celsius.
vapor pressure: 1. The pressure exerted by the molecules of a given vapor. For a pure confined vapor, it is that vapor's pressure on the walls of its containing vessel; and for a vapor mixed with other vapors or gases, it is that vapor's contribution to the total pressure (i.e., its partial pressure). Also called vapor tension.
In meteorology, vapor pressure is used almost exclusively to denote the partial pressure of water vapor in the atmosphere. See saturation vapor pressure, equilibrium vapor pressure.
2. The sum of the partial pressures of all the vapors in a system.
vapor tension: 1. The maximum possible vapor pressure that can be exerted, at a given temperature, by a system composed of a plane surface of a liquid or solid substance in contact with the substance's vapor. Compare equilibrium vapor pressure, saturation vapor pressure.
2. = vapor pressure (obsolescent).
vapor thorax: A condition characterized by the existence of large water-vapor bubbles in the intrapleural space between the lungs and the chest wall, occurring when an unprotected person (or animal) is exposed to ambient pressures less than 47 millimeters of mercury and water at body temperature vaporizes from the liquid state.
vapor trail: = condensation trail.
vapors: Gases whose temperatures are below their critical temperatures, so that they can be condensed to the liquid or solid state by increase of pressure alone.
Variability of Solar Irradiance and Gravity Oscillations: Helioseismology instrument aboard SOHO which analyzes variations in electromagnetic radiation from the Sun.
variable cycle: Pertaining to a computer in which succeeding sequences are started by the completion of the previous sequence rather than at predetermined intervals. See asynchronous computer.
variable stream control engines: Advanced, moderate bypass-ratio turbofan configurations that use duct burner thrust augmentation and coannular nozzles for jet noise reduction.
variable-area exhaust nozzle: On a jet engine, an exhaust nozzle of which the exhaust exit opening can be varied in area by means of some mechanical device, permitting variation in the jet velocity. Compare fixed-area exhaust nozzle.
variance: A measure of the precision of a measurement based on summming the squares of the deviations from individual determinations from the average and dividing by the degrees of freedom.
variance: (symbol ²)
In statistics, a measure of variability (or spread); the mean-square deviation from the mean, that is , the mean of the squares of the differences between individual values of x and the mean value .

where E denotes expected value. The positive square root of the variance is called the standard deviation.
variate: = random variable.
variation: The angle between the magnetic and geographical meridians at any place, expressed in degrees east or west to indicate the direction of magnetic north from true north. Called magnetic variation when a specificity is needed to prevent possible ambiguity. Also called magnetic declination.
The angle between the magnetic and grid meridians is called grid variation or grivation.
variation of latitude: A small periodic change in the astronomical latitude of points on the earth, due to wandering of the poles.
variometer: An instrument for comparing magnetic forces, especially of the earth's magnetic field.
varistor: A two-electrode semiconductor device having a voltage- dependent nonlinear resistance.
VATOL aircraft: Vertical attitude takeoff and landing aircraft. Used for vertical attitude takeoff-landing aircraft and XBQM-180A aircraft.
vector: Any quantity, such as force, velocity, or acceleration, which has both magnitude and direction at each point in space, as opposed to a scalar which has magnitude only. Such a quantity may be represented geometrically by an arrow of length proportional to its magnitude, pointing in the assigned direction.
A unit vector is a vector of unit length; in particular, the three unit vectors along the positive X-, Y-, Z-axes of rectangular Cartesian coordinates are denoted, respectively, by i, j, and k. Any vector A can be represented in terms of its components, a₁ , a₂ , a₃ along the coordinate axes X, Y, and Z, respectively; e.g., A = a₁ i + a₂ j + a₃ k. A vector drawn from a fixed origin to a given point (X, Y, Z) is called a position vector and is usually symbolized by r; in rectangular Cartesian coordinates,
r = xi + yj + zk
Equations written in vector form are valid in any coordinate system. Mathematically, a vector is a single-row or -column array of functions obeying certain laws of transformation. See scalar product, vector product, tensor, Helmholtz theorem.
vector product: A vector whose magnitude is equal to the product of the magnitudes of any two given vectors and the sine of the angle between their positive directions. Also called cross product, outer product. See scalar product. For two vectors A and B, the vector product is often written A X B (read A cross B), and defines a vector perpendicular to both A and B and so directed that a right-hand rotation about A X B through an angle of not more than 180 degrees carries A into B. The magnitude of A X B is equal to twice the area of the triangle of which A and B are coterminous sides. IF the vector product is zero, one of the vectors is zero or else the two are parallel. When A and B are written in terms of their components along the X-, Y-, and Z-axes of the rectangular Cartesian coordinates, i.e.,
A = a₁i + a₂j + a₃k
B = b₁i + b₂j + b₃k
then the vector product is the determinant
vector quantity: = vector.
vector resolution: The representation of a given vector as the sum of vectors in given directions.
vector steering: A steering method for rockets and spacecraft wherein one or more thrust chambers are gimbal mounted so that the direction of the thrust force (thrust vector) may be tilted in relation to the center of gravity of the vehicle to produce a turning movement.
vegetative index: Linear combinations of spectral band responses in digital count, reflectance factor, or voltage to determine the vigor, greenness and/or biomass of the vegetation. Observations can be made by satelliteborne, aircraftborne, truck mounted, or hand held spectrometers.
vehicle: Specifically, a structure, machine, or device, such as an aircraft or rocket, designed to carry a burden through air or space; more restrictively, a rocket vehicle.
This word has acquired its specific meaning owing to the need for a term to embrace aircraft, rockets, and all other flying craft, and has more currency than other words used in this meaning. See launch vehicle.
vehicle control system: A system, incorporating control surfaces or other devices, which adjusts and maintains the altitude and heading, and sometimes speed, of a vehicle in accordance with signals received from a guidance system.
The essential difference between a control system and a guidance system is that the control system points the vehicle and the guidance system gives the commands which tell the control system where to point. However, the control system maintains theinstantaneous orientation of the vehicle without specific commands from the guidance system.
vehicle mass ratio: The ratio of the final mass of a vehicle m_f, after all propellant has been used, to the initial mass m_o:

vehicle mass ratio = m_f / m_o
The inverse ratio m_o / m_f is sometimes called mass ratio also.
Vel, Velr: International Astronomical Union abbreviations for Vela. See constellation.
Vela: (abbr Vel, Velr)
See constellation.
velocimeter: A continuous-wave reflection Doppler system used to measure the radial velocity of an object.
velocity: (symbol V )
1. = speed. See note.
2. A vector quantity equal to speed in a given direction.
In sense 1, velocity is often used synonymously with speed, as in the velocity of the airplane, but in such contexts speed is properly the preferred term; except in the compound airspeed, velocity is preferred to speed in reference to motion of air or other fluid.
velocity coupling: The response of the burning propellant surface to the local velocity which would include both mean flow as well as acoustic velocity (both being parallel to the burning surface).
velocity head: 1. = velocity pressure.
2. The unit energy of a fluid stream owing to its motion.
velocity microphone: A microphone in which the electric output substantially corresponds to the instantaneous particle velocity in the impressed sound wave.
velocity of escape: The initial speed an object, particularly a molecule of gas, must have at the surface of a celestial body to overcome the gravitational pull and proceed out into space without returning to the celestial body. Also called escape velocity, escape speed.
The velocity of escape determines a body's ability to retain an atmosphere. The velocity of escape on the surface of the earth is nearly 7 miles per second, neglecting air resistance.
velocity of light: (symbol c)
= speed of light.
velocity of propagation: Rate of flow of electromagnetic radiation, including:
(a) Phase velocity. The velocity of propagation of surfaces of constant phase.
Strictly, this definition is applicable only to space periodic fields of infinite length.
(b) Group velocity. The velocity of propagation of electromagnetic radiant energy in a nondispersive or normally dispersive medium.
For a complex waveform, group velocity refers to the velocity of propagation of the beats between the component frequencies of the waveform.
(c) Signal velocity. The velocity of propagation of a signal.
In a nondispersive or normally dispersive medium, signal and group velocity are the same. For pure CW (continuous-wave) systems, utilizing no modulation, phase velocity is applicable. For systems utilizing modulated CW, signal velocity is applicable.
velocity of sound: = speed of sound.
velocity pressure: The difference between dynamic (or total) pressure and static pressure. Also called velocity head.
velocity space: Mathematical space where each point corresponds not to a certain location in reality, but to a certain velocity. Distribution functions typically involve mixes of both position and velocity spaces. (See distribution function.) Contrast with "position space" where each point corresponds to a given location.
velocity space: The subspace of phase space whose coordinates are the velocities in each of the three directions of ordinary space.
velocity transducer: A transducer which generates an output proportional to imparted velocities.
Velr: International Astronomical Union abbreviation for Vela. See constellation.
Venera 10 satellite: One in a series of Soviet spacecraft to probe the environment near and on the planet Venus.
Venera 11 satellite: One in a series of Soviet spacecraft to probe the environment near and on the planet Venus.
Venera 12 satellite: One in a series of Soviet spacecraft to probe the environment near and on the planet Venus.
Venera 9 satellite: One in a series of Soviet Spacecraft to probe the environment near and on the planet Venus.
Venera satellites: The Venera satellites were a series of probes (fly-bys and landers) sent by the Soviet Union to the planet Venus. Several Venera satellites carried high-energy astrophysics detectors.
ventilation: Biologically, the aeration of the lungs and blood by breathing; the inhalation and exhalation of air in the process of respiration.
ventilation garment: A lightweight, specially designed garment that is integrated with the pressure suit for providing adequate evaporation and heat dissipation from the surface of the body, by circulating dry air through the porous material.
ventral: Pertaining to the belly, or the underside of a vehicle, as ventral camera.
vents: Vents are openings in the Earth's crust from which molten rock and volcanic gases escape onto the ground or into the atmosphere. Vents may consist of a single circular-shaped structure, a large elongate fissure and fracture, or a tiny ground crack. The release of volcanic gases and the eruption of molten rock will result in an assortment of constructional features ranging from enormous shield volcanoes to small rootless hornitos.
VentureStar launch vehicle: Reusable single-stage-to-orbit launch vehicle employing linear aerospike engines, and having a payload capacity roughly equivalent to that of the Space Shuttle; developed in coordination with the X-33 advanced technology demonstrator vehicle.
Venturi tube: A short tube of smaller diameter in the middle than at the ends. When a fluid flows through such a tube, the pressure decreases as the diameter becomes smaller, the amount of the decrease being proportional to the speed of flow and the amount of restriction.
Venus: See planet, table.
Venus (planet): Second planet from the sun, a terrestrial planet.
Venus orbiting imaging radar (spacecraft): A spacecraft also known as VOIR whose mission is to obtain synthetic aperture radar (SAR) images of at least 70% of the surface of Venus as well as information on the gravity field of the planet, nature of its inertial composition and dynamics of its atmosphere and interaction with the solar wind.
Venus surface: The surface features and/or composition of the planet Venus.
vermiculite: An aggregate used in lightweight insulating concrete, formed by heating and expanding a micaceous mineral.
vernal equinox: The spring equinox. The term is also used for the point occupied by the Sun at that time, one of the two intersections on the celestial spher, between the ecliptic and the celestial equator. Also known as first point in Aries.
vernal equinox: 1. That point of intersection of the ecliptic and the celestial equator, occupied by the sun as it changes from south to north declination, on or about March 21. Also called March equinox, first point of Aries.
2. Tat instant the sun reaches the point of zero declination when crossing the celestial equator from south to north.
Verneuil process: Method of single-crystal growth in which powder is dropped through an oxy-hydrogen flame, falling molten on crystal seed.
vernier: A scale or control used for fine adjustment to obtain a more precise reading of an instrument or closer adjustment of any equipment.
vernier engine: A rocket engine of small thrust used primarily to obtain a fine adjustment in the velocity and trajectory of a rocket vehicle just after the thrust cutoff of the last sustainer engine, and used secondarily to add thrust to a booster or sustainer engine. Also called vernier rocket.
vernier rocket: = vernier engine.
versus: As a function of, as temperature versus time.
vertex: 1. The highest point of a trajectory or other curve, as the vertexes of a great circle, the points nearest the poles.
2. = node, sense 3.
vertical circle: A great circle of the celestial sphere, through the zenith and nadir. Vertical circles are perpendicular to the horizon.
The prime vertical circle or prime vertical passes through the east and west points of the horizon. The principal vertical circle passes through the north and south points of the horizon and coincides with the celestial meridian.
vertical gyro: A two-degree-of-freedom gyro with provision for maintaining its spin axis vertical. In this gyro, output signals are produced by gimbal angular displacements which correspond to components of the angular displacements of the base about two orthogonal axes.
vertical junction solar cells: Solar cells made from wafers on which narrow grooves are formed using a preferential KOH etch. The grooved region is radiation tolerant.
vertical motion simulators: Vibration machines which produce mechanical oscillations parallel to the vertical axis.
vertical orientation: The attitude of an object in reference to a plane which is parallel to the direction of gravity (determined with a plumbline).
vertical scanning: See scanning.
vertigo: The sensation that the outer world is revolving about the patient ( objective vertigo ) or that he himself is moving in space ( subjective vertigo ).
The word frequently is used erroneously as a synonym for dizziness or giddiness to indicate an unpleasant sensation of disturbed relations to surrounding objects in space.
very high frequency: (abbr VHF)
See frequency band.
Very Large Array (VLA): A synthetic aperture radio telescope, consisting of 27 parabolic antennas each of which is 25 meters in diameter. The system when connected together is capable of arcsecond resolution with high sensitivity resulting in the world's most powerful radio telescope. Operated by the National Radio Astronomy Observatory, it is located in Socorro, New Mexico.
very large scale integration: A very complex integrated circuit, which contains ten thousand or more individual devices, such as basic logic gates and transistors, placed on a single semiconductor chip. Used for VLSI.
very large transport aircraft: Aircraft capable of a maximum takeoff weight greater than 400 metric tons (881,600 lbs) or having a seating capacity greater than 660.
very long base interferometry: The simultaneous observation of radio sources by two radio telescopes spaced very far apart to enhance angular resolution. The signals are recorded on magnetic tapes and combined electronically on a computer. Used for VLBI.
Very Long Baseline Array (VLBA): A transcontinental radio telescope, being developed by the National Radio Astronomy Observatory, to consist of ten dedicated and automated 25-meter (82 foot) diameter antennas distributed from Hawaii to St. Croix, Virgin Islands.
very low frequency: (abbr VLF)
See frequency band.
very-high-speed motion-picture photography: Picture taking at a frequency range from 500 to 10,000 pictures per second.
vestigial sideband: (abbr VSB)
The transmitted portion of the sideband which has been largely suppressed by a transducer having a gradual cutoff in the neighborhood of the carrier frequency, the other sideband being transmitted without much suppression.
veterinary medicine: The branch of medical practice dealing with the treatment of diseases and injuries of animals.
VGR1: Voyager 1 spacecraft.
VGR2: Voyager 2 spacecraft.
VHF (abbr): = very high frequency
See frequency band.
VHSIC (circuits): Chips being developed by a DOD program to provide high speed MIL spec VLSI device for use in military systems. Used for very high speed integrated circuits.
vibration: 1. Motion due to a continuous change in the magnitude of a given force which reversed its direction with time.
Vibration is generally interpreted as the cyclical (symmetrical or nonsymmetrical) fluctuations in the rate at which an object accelerates. In longitudinal vibration the direction of motion of the particles is the same as the direction of advance of the vibratory motion; in transverse vibration it is perpendicular to the direction of advance.
2. The motion of an oscillating body during one complete cycle; two oscillations.
vibration isolator: A resilient support that tends to isolate a system from steady-state excitation. Also called isolator.
vibration mode: In a system undergoing vibration, a characteristic pattern assumed by the system in which the motion of every particle is simple harmonic with the same frequency. Used for mode of vibration.
video: Pertaining to the picture signals in a television system or to the information-carrying signals which are eventually presented on the cathode-ray tubes of a radar.
video disks: Disks, usually the size of long-playing stereo records, which store video data. The data is recorded by one of two techniques: the capacitance method, in which the disk has spiral grooves and is read by a contact stylus, and the optical method, which uses lasers in both the recording and playback of the data.
video landmark acquisition and tracking: Shuttle era system for earth-feature identification, acquisition, and tracking.
video observations: A detection technique using a video camera coupled with an image intensifier to record meteors. The positional accuracy is almost as high as that of photographic observations . The faintest meteor magnitudes are comparable to visual or telescopic observations depending on the used lens. Meteor shower activity as well as radiant positions can be determined. Multiple-station video observations allow the determination of meteoroid orbits.
video signal: = target signal.
video signals: Signals with a bandwidth of over 20 kilohertz.
videofrequency: Any frequency used in transmission images, as by television.
vidicon: A television pickup tube utilizing a photoconductor as the sensing element. In conjunction with a telescope this is known as a vidicon telescope.
view effects: Effects of change in angular size of field of view upon receptors of radiation.
view factor: The fraction of the total energy emitted by one surface that is directly incident on another surface. Also called geometric factor, configuration factor, shape factor.
Viking spacecraft: A collective term for the composite Viking orbiter-lander space vehicle.
Vir, Virg: International Astronomical Union abbreviations for Virgo. See constellation.
virga: Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.
VIRGO: See Variability of Solar Irradiance and Gravity Oscillations
Virgo: (abbr Vir, Virg)
See constellation.
Virgo galactic cluster: A cluster of galaxies nearest to the Milky Way Galaxy, centered in the constellation Virgo and about 16 million light-years from Earth. Used for Virgo star cluster.
virtual gravity: The force of gravity on an atmospheric parcel, reduced by centrifugal force due to the motion of the parcel relative to the earth. The virtual gravity g* is

where g is the magnitude of the acceleration of gravity; V is the parcel speed; a is the earth's radius; and _n is the component of the earth's angular velocity vector normal to the motion of the parcel.
For reasonable atmospheric values, the correction terms are of the order of 0.01 percent of the magnitude of gravity. The identity of g* and g is implied by the assumption of hydrostatic equilibrium.
virtual height: The apparent height of an ionized atmospheric layer determined from the time interval between the transmitted signal and the ionospheric echo at vertical incidence, assuming that the velocity of propagation is the velocity of light in a vacuum over the entire path. See ionospheric recorder. Compare scale height.
virtual image: An image that cannot be shown on a surface but is visible, as in a mirror.
virtual mass: The actual mass of a body, plus its apparent additional mass.
virtual reality: An artificial reality that projects the user into a three dimensional space generated by the computer.
virtual stress: = Reynolds stresses.
viscoelastic damping: The absorption of oscillatory motions by materials which are viscous while exhibiting certain elastic properties.
viscoelasticity: Property of materials that strain under stress partly elastically and partly viscously, that is, whose strain is partly dependent on time and magnitude of stress. Used for viscoelastic flow.
viscosity: That molecular property of a fluid which enables it to support tangential stresses for a finite time and thus to resist deformation; the ratio of shear stress divided by shearing strain. See viscosity coefficient.
viscosity coefficient: The ratio of the shearing component of stress to the velocity gradient in a fluid where the stress acts across a plane perpendicular to the direction of the velocity gradient. Also called viscosity. See also dynamic viscosity, kinematic viscosity, eddy viscosity.
viscosity manometer: = decrement gage.
viscous: Pertaining to viscosity, as a viscous fluid.
viscous damping: The dissipation of energy that occurs when a particle in a vibrating system is resisted by a force that has a magnitude proportional to the magnitude of the velocity of the particle and direction opposite to the direction of the particle.
viscous flow: The flow of fluid through a duct under conditions such that the mean free path is very small in comparison with the smallest dimension of a transverse section of the duct.
This flow may be either laminar or turbulent.
viscous fluid: A fluid whose molecular viscosity is sufficiently large to make the viscous forces a significant part of the total force field in the fluid. See Navier-Stokes equations, viscous stresses. Compare inviscid fluid.
viscous force: The force per unit volume or per unit mass arising from the action of tangential stresses in a moving viscous fluid. This force may then be introduced as a term in the equations of motion.
viscous relaxation: The process whereby topographic features become subdued over time due to the flow of the surrounding geologic material.
viscous stresses: The components of the stress tensor when the pressure, i.e., the mean of the three normal stresses, has been subtracted out from each of the normal stresses. See Reynolds stresses.
visibility meter: The general term for instruments used to make direct measurements of visual range in the atmosphere or of the physical characteristics of the atmosphere which determine the visual range.
Visibility meters may be classified according to the quantities that they measure. Telephotometers and transmissometers measure the transmissivity or alternatively, the extinction coefficient of the atmosphere. Nephelometers measure the scattering function of the atmospheric suspensoids. A third category of visibility meters makes use of an artificial haze of variable density which is used to obscure a marker at a fixed distance from the meter.
visible binaries: See binary star.
visible horizon: See horizon.
visible infrared spin scan radiometer: A radiometer used for satellite sounding of the atmosphere.
visible radiation: Electromagnetic radiation lying within the wavelength interval to which the human eye is sensitive, the spectral interval from approximately 0.4 to 0.7 micron (4000 to 7000 angstroms).
The term is without reference to the variable response of the human eye in its reception of radiation.
visible spectrum: The range of wavelengths of visible radiation; display or graph of the intensity of visible radiation emitted or absorbed by a material as a function of wavelength or some related parameter.
visible spectrum: That portion of the electromagnetic spectrum occupied by the wavelengths of visible radiation, roughly 4000 to 7000 angstroms. This portion of the electromagnetic spectrum is bounded on the short-wavelength end by ultraviolet radiation, and on the long-wavelength end by infrared radiation.
visual acuity: Keeness of perception and sharpness of vision.
visual fields: The entire area that can be seen when the eye is focused directly forward, including that which is seen with peripheral vision.
visual magnitude: (symbol m_v)
The apparent magnitude of a star or other celestial body measured by visual observation. See photovisual magnitude, color index.
visual observation: Monitoring meteor activity by the naked eye, it is the least accurate method of detection. Large numbers of observations permit statistically significant results. Visual observations are used to monitor major meteor showers, sporadic activity and minor showers.
visual photometer: See photometer.
visual photometry: A subjective approach to the problem of photometry, wherein the human eye is used as the sensing element; to be distinguished from photoelectric photometry.
visual range: The distance, under daylight conditions, at which the apparent contrast between a specified type of target and its background becomes just equal to the threshold contrast of an observer; to be distinguished from the night visual range. Also called daytime visual range.
vitrification: Formation of a glassy or noncrystalline material.
vitrifying tendency: Tendency of the crystalline phase of a ceramic to transform into an amorphous or glassy phase when subjected to aging or temperature cycling.
VLBI: Very Long Baseline Interferometry.
VLF (abbr): See frequency band.
voice control: Using the voice to activate devices which respond or operate by means of speech recognition. SN (device operation by voice).
void fraction: The fraction of the frontal area of a reactor that is open to airflow. Also called free-flow area.
Vol, Voln: International Astronomical Union abbreviations for Volans. See constellation.
Volans: (abbr Vol, Voln)
See constellation.
volcanic ash: Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 2 mm (0.1 inch) in diameter, which is slightly larger than the size of a pinhead. Volcanic ash is not the same as the soft fluffy ash that results from burning wood, leaves, or paper. It is hard, does not dissolve in water, and can be extremely small-ash particles less than 0.025 mm (1/1,000th of an inch) in diameter are common. Ash is extremely abrasive, similar to finely crushed window glass, mildly corrosive, and electrically conductive, especially when wet.
volcanic block: A volcanic block is a solid rock fragment greater than 64 mm in diameter that was ejected from a volcano during an explosive eruption. Blocks commonly consist of solidified pieces of old lava flows that were part of a volcano's cone.
volcanic bombs: Volcanic bombs are lava fragments that were ejected while viscous (partially molten) and larger than 64 mm in diameter. Many acquire rounded aerodynamic shapes during their travel through the air. Volcanic bombs include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and "cow-dung" bombs.
volcanic gas: Volcanic gas is released from magma as it rises toward the surface and during eruptions. Just as when the cap is removed from a pressurized bottle of soda and dissolved gas forms bubbles, magmas become bubbly as they rise to the surface and depressurize. The most common gas released by magma is steam (H2O), followed by CO2 (carbon dioxide), SO2 (sulfur dioxide), (HCl) hydrogen chloride and other compounds. The ratios and abundances of these gases have been used to anticipate if and when a volcano might erupt.
volcanism: The eruption of molten material or gas-driven solid fragments onto the surface of a planetary body. On icy moons the phenomenon is sometimes called cryovolcanism to distinguish it from silicate or sulfur volcanism.
volcanoes: Naturally occuring vents or fissures at the Earth's surface through which erupt molten, solid, and gaseous materials.
Voln: International Astronomical Union abbreviation for Volans. See constellation.
volt: (abbr V, v)
The unit of electric potential difference and electromotive force, equal to the difference of electric potential between two points of a conductor carrying a constant current of 1 ampere when the power dissipated between these points equals 1 watt.
voltage controlled oscillators: An oscillator whose frequency of oscillation can be varied by changing an applied voltage. Used for VCO.
volume level: In an electric circuit, the level, as measured on a standard volume indicator, of a complex wave such as produced by speech or music. Often shortened to volume.
The term volume is also used loosely to signify the magnitude of a sound or audiofrequency wave.
volume scattering function: See scattering function.
volume unit: The unit of volume level as measured by a standard volume indicator. The volume level in volume units is the number of decibels by which the volume level exceeds the reference volume level.
VOR (abbr): = VHF omnirange.
vortex: 1. Any flow possessing vorticity.
2. Specifically a flow with closed streamlines or the idealized case in which all vorticity is concentrated in a vortex filament.
vortex advisory system: Display system which compares measured on-minute-average wind magnitudes and direction with the wind-rose criterion to predict wake vorticity and to indicate to the air traffic controller (with a red or green light) when the interarrival spacings for landings may be reduced to the 3 nautical mile limit.
vortex alleviation: The alteration of airfoil configurations to change the airflow patterns directly behind the wings to eliminate or inhibit the vertical motion which directly affects the aircraft immediately following, during closely spaced landings.
vortex avoidance: Schemes which involve airborne or ground-based equipment to track, monitor, and/or predict vortex behavior which might affect the approach and landing operations.
vortex filament: A line along which an infinite vorticity in a fluid motion is concentrated, the surrounding fluid being free of vorticity.
In an autobarotropic frictionless fluid, a vortex line always consists of the same fluid particles; the vortex filament is, thus, a vortex line and is the limiting case of a vortex tube as the cross-sectional area of the tube shrinks to zero.
vortex filaments: The fine-scale structure of turbulent flow; the small non energy containing eddies convected at mean freestream velocities.
vortex flaps: Leading edge flap designs for highly swept wings, in which the leading edge tabs, which are counter reflected, cause vortices to form on the flap. The trapped vortices cause significantly improved wind flow characteristics.
vortex line: A curve tangent at every point of a field to the vorticity vector at that point.
vortex ring: A closed vortex filament.
vortex shedding: Periodic separation of a fluid flowing past an unstreamlined body.
vortex street: Two parallel rows of alternately placed vortexes along the wake of an obstacle in a fluid of moderate Reynolds number. Also called Karman vortex street, vortex trail vortex train.
Fluid drag can be calculated from the motion of these vortexes, which are stable only for a certain radio of the width of the street to the distance between vortexes along the street.
vortex trail: = vortex street.
vortex train: = vortex street.
vortex tube: The closed surface or tube consisting of the vortex lines passing through every point of a given closed curve.
vortices: In fluids, circulations drawing their energy from flows of much larger scale and brought about by pressure irregularities. Used for eddies, rotational flow, vortex columns, vortex disturbances, vortex flow, and vortex tubes.
vorticity: A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.
vorticity: A vector measure of local rotation in a fluid flow, defined mathematically as the curl of the velocity vector,

where is the vorticity; V is the velocity; and is the del-operator.
The vorticity component normal to a small plane element is the lime of the circulation per unit area as the area of the element approaches zero. The vorticity of a solid rotation is twice the angular velocity vector. In meteorology, the vorticity usually refers to the vertical component of the vorticity as defined above.
vorticity equation: A dynamic equation for the rate of change of the vorticity of a parcel, obtained by taking the curl of the vector equation of motion.
vorticity of the earth: = coriolis parameter.
vorticity-transport hypothesis: The hypothesis that, owing to the existence of pressure fluctuations, vorticity and not momentum is conservative in turbulent eddy flux. This would apply especially if the turbulence were strictly two dimensional.
Voyager 1 spacecraft: A spacecraft launched in the 1977 Voyager mission.
Voyager 1977 mission: The launching of two advanced three-axis attitude stabilized spacecraft for the exploration of Jovian and Saturnian environments including investigation of the gravitational fields, atmospheric dynamics, and magnetospheres of these planets.
Voyager 2 spacecraft: A spacecraft launched in the 1977 Voyager mission.
Vul, Vulp: International Astronomical Union abbreviations for Vulpecula. See constellation.
vulcanian eruption: A vulcanian eruption is a type of explosive eruption that ejects new lava fragments that do not take on a rounded shape during their flight through the air. This may be because the lava is too viscous or already solidified. These moderate-sized explosive eruptions commonly eject a large proportion of volcanic ash and also breadcrust bombs and blocks. Andesitic and dacitic magmas are most often associated with vulcanian eruptions, because their high viscosity (resistance to flow) makes it difficult for the dissolved volcanic gases to escape except under extreme pressure, which leads to explosive behavior.
vulcanizing: A chemical reaction in which the physical properties of a rubber are changed in the direction of decreased plastic flow, less surface tackiness, and increased tensile strength by reacting it with sulfur or other suitable agents.
Vulpecula: (abbr Vul, Vulp)
See constellation.