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- Sometimes used as the plural of 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 10-3;
high vacuum, torr 10-3 to 10-6;
very high vacuum, torr 10-6 to 10-9;
ultrahigh vacuum, torr 10-9
- 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
- (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
gage and thermocouple
- (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
- 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.
- 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
- Van Allen belt, 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
- 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 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
behavior of real gases and their departures from the ideal gas laws. It
states: [p + (a/v2)](v - b) = RT, 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.
- 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
- 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
- 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.
- 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 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
- 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
- 2. = vapor
- 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.
- 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
- 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
- variance (symbol σ2)
- 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 μ.
σ = E[(x - μ)2] = E(x2) - μ2
where E denotes expected value. The
positive square root σ of the variance is called the standard
- variate = random variable.
- 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
- 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.
- An instrument for comparing magnetic forces,
especially of the earth's magnetic
- A two-electrode semiconductor
device having a voltage- dependent nonlinear resistance.
- A frequency
band used in radar extending approximately from 46 to 56 gigacycles per
second. See frequency
- 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, and
a3 along the coordinate axes X, Y, and Z,
respectively; e.g., A = a1 + a2 + a3. 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, tensor, Helmholtz
- 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 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.
- 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
- 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 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 mf, after all propellant has been used, to the
initial mass mo: vehicle mass ratio = mf/mo
- The inverse ratio mo/mf , is sometimes called mass ratio
- Vel, Velr
- International Astronomical Union abbreviations for Vela. See constellation.
- Vela (abbr Vel, Velr)
- See constellation.
- 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 head
- 1. = velocity
- 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
- (a) Phase velocity. The velocity of propagation of surfaces of constant
- 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
pressure. Also called velocity head.
- velocity space
- The subspace of phase space
whose coordinates are the velocities in each of the three directions of
- velocity transducer
- A transducer
which generates an output proportional to imparted velocities.
- International Astronomical Union abbreviation for Vela. See constellation.
- 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.
- Pertaining to the belly, or the underside of a vehicle, as ventral
- 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.
- See planet, table.
- vernal equinox
- 1. That point of intersection of the ecliptic and
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.
- 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
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.
- As a function of, as temperature versus time.
- 1. The highest point of a trajectory
or other curve, as the vertexes of a great
circle, the points nearest the poles.
- 2. = node,
- 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
- 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 scanning
- See scanning.
- 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
- very-high-speed motion-picture
- Picture taking at a frequency range from 500 to 10,000 pictures per
- very low frequency (abbr VLF)
- See frequency
- 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.
- VHF (abbr) = very high frequency
- See frequency
- 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.
- 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.
- Any frequency
used in transmission images, as by television.
- video signal = target signal.
- Virgo (abbr Vir, Virg)
- See constellation.
- A television pickup tube utilizing a photoconductor as the sensing
element. In conjunction with a telescope this is known as a vidicon
- 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.
- Vir, Virg
- International Astronomical Union abbreviations for Virgo. See constellation.
- 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 g* = g = V2/a - 2ΩnV , where g is the magnitude of the acceleration of
gravity; V is the parcel speed; a is the earth's radius; and Ωn 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
- 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
- virtual stress = Reynolds stresses.
- 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
- 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 manometer = decrement gage.
- 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
stresses. Compare inviscid
- viscous force
- The force per unit volume or per unit mass arising from the action of
tangential stresses in a
fluid. This force may then be introduced as a term in the equations
- 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
- 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
- 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
- visible binaries
- See binary
- visible horizon
- See horizon.
- visible radiation
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
- The term is without reference to the variable response of the human eye
in its reception of radiation.
- 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 magnitude (symbol mv)
- The apparent
magnitude of a star or other celestial
body measured by visual observation. See photovisual
- 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
- 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
contrast of an observer; to be distinguished from the night visual range.
Also called daytime visual range.
- vitrifying tendency
- Tendency of the crystalline phase of a ceramic to
transform into an amorphous or glassy phase when subjected to aging or
- VLF (abbr)
- See frequency
- 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.
- 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.
- 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
- 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.
- 1. Any flow
- 2. Specifically a flow with closed streamlines
or the idealized case in which all vorticity is concentrated in a vortex
- 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 line
- A curve tangent at every point of a field to the vorticity
vector at that point.
- vortex ring
- A closed vortex
- 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.
- A vector
measure of local rotation in a
fluid flow, defined mathematically as the curl of the velocity vector,
Ω = X V where
Ω 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.
- Vul, Vulp
- International Astronomical Union abbreviations for Vulpecula. See
- Vulpecula (abbr Vul, Vulp)
- See constellation.