GB2199409A

GB2199409A - Improvements relating to the detection of turbulence and/or shock waves

Info

Publication number: GB2199409A
Application number: GB07942115A
Authority: GB
Inventors: Dr James Clark
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-12-06
Filing date: 1979-12-06
Publication date: 1988-07-06
Also published as: GB2199409B

Abstract

An electromagnetic field detector is used to detect either the magnetic or electric field component of an electromagnetic field generated by turbulence and/or shock waves. The detector preferably takes the form of a magnetic field detector 4 comprising a pole piece 6 and a pick up coil 8. The detector can be used for detecting stall conditions over an aircraft wing section and can also be incorporated into an electromagnetic surveillance system which operates predominantly in the frequency range 10 Hz to 100 KHz. In this frequency range most of the energy is in the form of 'surface waves'. By analysing the electromagnetic field object signatures can be developed which can later be used to identify unknown objects. By triangulation techniques utilising two spaced detectors (Fig. 4) the positions of detected objects may be determined. Detection of over the horizon and low flying aircraft and of submerged submarines is possible. <IMAGE>

Description

IMPROVEMENTS RELATING To THE DETECTION OF TURBULENCE AND/OR SHOCK WAVES The present invention relates to a method of detecting turbulence and/or shock waves in fluid flows.

One application of turbulence detection is to detect incipient stall conaitions in an aircraft. Known methods of turbulence detection do not respond quickly enough to ensure that remedial action is taken quickly enough.

It is known that part of the energy in turbulent fluid flows and/or shock waves is converted into electromagnetic radiation in regions of high molecular

(AIAA Journal Vol 13, No 10, Oct 1975, pp 1382-1384 and Electromagnetic Fields generated by turbulent air flow and shock waves Delft University, Report WTHD 68, Jan 1975).

The present invention provides a method of detecting turbulence and/or shock waves, wherein an electromagnetic field detector is used to detect either the electric or magnetic field component of an electromagnetic field generated by turbulence and/or shock waves.

In a preferred embodiment of the present invention the detector is embedded in an aerodynamic surface, such as an aircraft wing section, and is used to detect incipient stall conditions. However, the detector can be incorporated into a probe assembly for more general use.

The invention further provides an aircraft having at least one turbulence detector embedded in an aerodynamic surface, wherein the turbulence detector comprises an electromagnetic field detector adapted to detect either the electric or magnetic field component of an electromagnetic field generated by turbulent flow over the aerodynamic surface.

In a further embodiment of the present invention the detector is incorporated into an electromagnetic surveillance system. Known surveillance systems, particularly radar systems, are unable to detect and recognise aircraft which are flying near to the surface of the earth. Furthermore, during ionospheric disturbances, such systems are unable to detect objects which are positioned 'over the horizon'. A further problem associated with known electromagnetic surveillance systems is that they are usually active systems and therefore betray their location while radiating to the target object.

According to the present invention there is also provided a method of detecting moving objects which generate turbulence andyor shock waves, wherein an electromagnetic field detector is used to detect either the electric or magnetic field component of an electromagnetic field generated by the turbulence and/or shock waves.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a turbulence detector comprising a magnetic field detector.

Figure 2 is a schematic diagram of a turbulence detector embedded in an aerodynamic surface.

Figure 3 is a block diagram showing a turbulence detector incorporated into an electromagnetic surveillance system.

Figure 4 is a block diagram showing a modified electromagnetic surveillance system.

Referring to Figure 1 a magnetic field detector 4, used to detect the magnetic field component of an electromagnetic wave, comprises a cylindrical pole piece 6 of, for example, iron ferrite, and a pick-up coil 8 of, for example insulated copper wire.

Electromagnetic waves generated by turbulent fluid flows and/or shock waves, as described in the aforementioned documents, induce voltages in the pick up coil 8 which are amplified by a pre-amplifier 10 and a main amplifier (not shown). The main amplifier may be connected to suitable recording and/or controlling means (not shown).

In a further embodiment of the present invention, shown in Figure 2, detector 4 and associated pre-amplifier 10 are embedded in an insert 12 formed of, for example, a plastics material, which is mounted flush with an aerodynamic surface 14.

A typical application of the above embodiment is that of detecting incipient stall conditions over an aircraft wing section. The output at the pre-amplifier 10 can be fed, via the main amplifier, either to suitable recording instruments or to an aircraft flight control computer. In the latter case the flight control computer would adjust one or more flight parameters in order to eliminate the turbulent flow and hence the risk of a stall condition arising.

If it is required to determine the point of airflow separation from the wing surface, ie the stall point, several detectors can be mounted on the wing section, in line with the airflow. Furthermore modified versions of the present invention may be used either, by aircraft in flight or by ground staff on the runway, to detect clear air turbulence. In the latter case a detector on the ground detects the electromagnetic radiation from the aircraft.

In the previously described embodiments of the present invention the detector has been used to detect the magnetic field component of the electromagnetic field; however, by using a suitable charge-detecting electrode and electrometer type amplifier, the detector can be used to detect the electric field component of the electromagnetic field. The amplifier would have a typical input impedance of 1 x 1 ohms and a sub-pico ampere bias current which are necessary to measure the very low signal levels. In order to reduce the response of such a high input impedance circuit to mechanical vibration, it would be necessary to support the entire length of the electrode with an insulated support having a low triboelectric value.

Referring to Figure 3 an electromagnetic sensor 4 and associated pre-amplifier 10 are incorporated into an electromagnetic surveillance system. The pre-amplifier 10 is connected, via a junction box 16 and a main amplifier 18, to a spectrum analyser 20 and strip chart recorder 22. A d.c. power supply 24 is connected via the junction box 16 to the pre-amplifier 10 and the main amplifier 18.

In operation, the detector is initially used to detect the presence, location and type of various known objects. The data relating to the known objects is either recorded by the strip chart recorder 22 or stored by a computer memory. At a later stage unknown objects are identified by comparing the data received from them with the previously stored data relating to the known objects.

The limiting factor for magnetic sensor is the thermal noise in the various electronic components. This problem can be overcome by operating at cryogenic temperatures, ie liquid helium, with

type device comprising a superconducting pick up coil 8 (Fig 1) and a Josephson type semiconductor junction. In a

levice described in Proceedings IEEE, Vol 61, No 1, Jan 73, pp 20-27, Magnetic flux levels down to 1 x 10 Tesla have been accurately measured.

Referring now to Figure , two magnetic detectors 26 and 28, equipped with automatic servo-controls to keep them pointed in the direction of maximum signal strength, are connected to a computer 30. The distance 32 between the detectors 26 and 28 is either accurately known or is continually measured by a laser beam connecting the two sensors.

The computer 30 is connected to read out equipment 34 through a spectrum analyser 38. Analysed data from the read out equipment

can be fed back to the computer 30 for storage. The computer 30 is further connected to a correlator 42 and read out equipment 44. The correlator 42, operating on the principle of cross correlation, can be used to separate signals coming from unknown objects from a high level of background noise.

Data from the read out equipment 44 can also be fed back to the computer 30 for storage. Knowing the distance 32 and using triangulation it is possible, using this modified system, to locate the position of an unknown object.

It has been found that the major frequency components of the radiated electromagnetic fields lie in the frequency range of 10 Hz to 50 Hz, the largest amplitudes being in the ranqe of

10 t to 100 RHz. This places the major frequency componentsin the VLF and LF radio bands. At such frequencies it is well known that most of the transmitted energy of the electromagnetic wave is in the form of "surface waves" which follow the surface of the earth.

It is seen therefore that the method used in this electromagnetic surveillance system is particularly useful during ionospheric disturbances, for detecting aircraft which are flying over the horizon or alternatively for detecting aircraft which are flying near to the surface of the earth.

the equipment outlined in Figure 4 May also be used to detect the movement of submerged submarines at large distances by monitoring the frequency range of 10 Hz. to 100 kHz.

Claims

1. A method of detecting turbulence and/or shock waves, wherein an electromagnetic field detector is used to detect either The electric or magnetic field component of an electromagnetic field generated by turbulence and/or shock waves.

2. A method according to claim 1, wherein the generated field has frequency components in the V.L.F and L.F. radio frequency bands.

3. A method according to claim 1 or 2, wherein at least one detector is embedded in an aerodynamic surface.

4. A method according to any of claims 1 to 3, wherein the generated field has frequency components below and in the y.L.F.

Sand L.F. radio frequency bands.

4. A method according to claim 3, wherein the detector is embedded in a plastics material which is set into the aerodynamic surface.

5. A method according to any of claims 1 to 4, wherein the detector comprises a pole piece and a pick up coil surrounding the pole piece.

6. A method according to any of claims 1 to 4, wherein the detector comprises a charge detecting electrode and an electrometer type amplifier.

7. An aircraft having at least one turbulence detector embedded in an aerodynamic surface, wherein in the turbulence detector comprises an electromagnetic field detector adapted to detect either the electric or magnetic field component of an electromagnetic field generated by turbulent flow over the aerodynamic surface.

8. A method of detecting moving

which generate turbulence andjor shock waves; wherein an electromagnetic field detector is used to detect either the electric or magnetic field component of an electromagnetic field generated by the turbulence and/or shock waves.

9. A method according to claim 8, wherein the electromagnetic field received fran the object is ; analysed to produce an object signature.

10. A method according to claim 9, wherein the object signature of a known object is stored and used to identify unknown objects.

11. A method according to any of claims 8 to 10, wherein the

generated field has frequency componentsln the V.L.F and L.F. radio frequency bands.

Amendments to the claims have been filed as follows CLAIMS:

1. A method of detecting moving objects which generate turbulence and/or shock waves, wherein an electromagnetic field detector is used to detect either the electric or magnetic field component of an electromagnetic field generated by the turbulence andyor shock waves.

2. A method according to claim 1, wherein the electromagnetic field received from the object is analysed to produce an object signature.

3. A method according to claim 2, wherein the object signature of a known object is stored and used to identify unknown objects.