Communications: directive radio wave systems and devices (e.g. – Radar for meteorological use
Reexamination Certificate
2001-05-17
2002-11-12
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar for meteorological use
C342S104000, C342S107000, C342S109000, C342S110000, C342S115000, C342S118000, C342S134000, C342S135000, C342S136000, C342S137000, C342S139000, C342S147000, C342S192000, C342S195000, C342S196000
Reexamination Certificate
active
06480142
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the detection of weather disturbances and precursors to weather disturbances, and more particularly to the detection of clear air turbulence, microbursts, aircraft wake vortices, gust fronts, thunderstorms, updrafts, downdrafts, convective flows, tornadoes, hurricanes, and cyclones.
2. Description of the Prior Art
Clear air turbulence, microbursts, aircraft wake vortices, gust fronts, thunderstorms, updrafts, downdrafts, and convective flows all present severe hazards to aircraft. Effects on aircraft encountering any of these hazards range from severe buffeting to the ultimate catastrophe. Thunderstorms, tornadoes, hurricanes, and cyclones have been extremely destructive. Though these severe storms may be detected and tracked once they are formed, detecting precursors to their formation have not been implemented. Consequently, warnings of the initial onslaught of these storms cannot be given and loss of life may result at their initiation.
The prior art utilizes two basic techniques to detect hazardous atmospheric flows: Doppler radar and the Radio Acoustic Sounding System (RASS). Doppler radar measures the average radial motion of scatterers in a volume formed by a radar range gate and the antenna beam. Processed samples of the radar return yield the Doppler spectrum of the radar backscatter in each range gate. The zeroth moment of the signal spectrum is a measure of echo strength, the first moment a measure of the mean radial velocity, and the second moment a measure of the Doppler width. Doppler radar backscatter from hydrometeors (rain droplets) and dust are utilized to determine various atmospheric conditions. Doppler radars of the prior art, though useful for the determination of existing weather conditions, are not able to detect the clear air turbulences which are hazardous to aircraft in flight.
RASS emits overlapping radar and acoustic beams from a common ground-based location. Because the air's index of refraction is a function of density, the acoustic beam, which consists of a spatial pattern of condensations and rarefactions, produces corresponding refractive index variations in situ. Radar waves reflect from these index variations and reflections are strongest when the acoustic wavelength is one-half the radar wavelength. Radar reflections focus onto the radar antenna due to the parallel alignment of the radar and acoustic wavefronts. The Doppler shift of the radar reflections corresponds to the speed of sound.
A vertically pointing RASS has been utilized to measure the vertical temperature profile of the atmosphere by measuring the speed of sound as a function of altitude. Since the speed of sound has a known relation to temperature, a virtual temperature profile of the atmosphere may be deduced from the data. Due to atmospheric attenuation of the sound beam and very small radar reflections from clear air, RASS can measure sound speed, wind velocity, and clear air turbulence only within restricted bounds.
A horizontally pointing RASS has been used to detect, track, and measure the strength of wake vortices of landing aircraft. Segments of the RASS acoustic wave speed up or slow down by varying amounts as they pass through a vortex. As a result, the Doppler spectrum of the acoustically reflected radar signal is a mapping of the vortex's line-of-sight velocity distribution. Vortex circulation (strength) may be deduced from the Doppler spectrum.
A fluid flow is laminar when the velocity and direction of flow particles do not change with time. Laminar flow is favored by flows with a low Reynolds number, a dimensionless quantity defined as R=LU/&ggr;, where L is a characteristic length of the flow, U is a characteristic speed, and &ggr; is the fluid kinematic viscosity. A laminar flow with a high Reynolds number is unstable, so that a slight disturbance changes the flow to turbulent. A turbulent flow consists of overlapping eddies with varying characteristic lengths and velocity scales. First order eddies comparable to the size of the flow appear first. First order eddies generate second order eddies which are smaller and draw their energy from the first. Second order eddies generate third order eddies, and so on, creating a hierarchy of eddies. The smallest eddies have high local velocity gradients which cause them to dissipate under the influence of viscosity and their kinetic energy to be converted into heat.
An eddy may be described as a swirling flow with approximately circular streamlines. Streamline velocities increase linearly from zero at the center to a maximum value, after which they decrease inversely with radius. The eddy core is defined as the region inside the maximum velocity streamline, that fluctuates due to turbulence. Eddy overall size varies from 10 to 30 times the mean core size.
FIG. 1A
shows the process by which large eddies produce smaller eddies. A kink induced in an eddy's core by turbulence causes the core to twist into a figure 8. Opposing core flows cause the core to split, culminating in the creation of two eddies with opposite rotation.
Small scale turbulence is essentially homogeneous and isotropic, and may be mathematically described by Fourier eigenfunctions (modes) indexed by wave numbers k, which are related to eddy core size. Mode wave number space may be divided into three ranges, each having its own wave number distribution: (i) low wave numbers (production range); (ii) intermediate wave numbers (inertial range), and (iii) high wave numbers (dissipation range). Eddy wave numbers in the production range are distributed as k
4
and in the inertial range as k
−5/3
. Hence, wave numbers on the boundary between the production and inertial ranges have the highest density.
Atmospheric flows typically have high Reynolds numbers and are always turbulent. The size of the largest eddies in the normally turbulent atmosphere are tens of meters while the smallest are several centimeters. Clear Air Turbulence (CAT) refers to a highly turbulent clear air flow that occurs about 10 km above the earth's surface. Theoretical considerations and observations of the free atmosphere indicate that CAT is mainly a manifestation of stably stratified shear flow instability, generally referred to as Kelvin-Helmholtz instability or KHI. Onset of KHI over an atmospheric layer of depth &Dgr;z is determined by the layer Richardson number which is inversely proportional to the square of the vector wind change over &Dgr;z. A necessary condition for KHI is a Richardson number less than 0.25.
CAT turbulence is severe enough to perturb the motion of aircraft flying through it, causing injuries to passengers and cabin attendants as well as structural damage to aircraft. CAT can be neither seen nor avoided by pilots without prior knowledge of its location. The largest eddies in CAT may be several kilometers, corresponding to the flow size. Flow velocities typically range from 100 to 200 kts (50 to 100 m/s).
The two well-known mechanisms by which kinetic energy is converted into sound are, first, by forcing a mass in a fixed region of space to fluctuate, as with a loudspeaker diaphragm embedded in a very large baffle, and second, by forcing momentum in a fixed region of space to fluctuate, which occurs when a solid object vibrates after being struck. The first is more efficient than the second.
Localized fluctuations in turbulent flows produce pressure variations that propagate away from their source and, if an observer is present, will be recognized as sound. Examples include the roar produced by high winds, the noise emitted by jet aircraft exhaust flows, and the whooshing sound produced by aircraft wake vortices. The mechanism by which sound is generated aerodynamically is fundamental because fluctuating shearing motions are converted into fluctuating longitudinal motions. The conversion efficiency is much smaller than either of the above mechanisms.
It has been shown that there is an exact mathematical analogy between density fluctuations in
Gregory Bernarr E.
Levine Seymour
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