GB2376740A

GB2376740A - Method and device for determining fluid flow velocity

Info

Publication number: GB2376740A
Application number: GB0205922A
Authority: GB
Inventors: Stephen Lindsay Russell
Original assignee: WRC PLC
Current assignee: WRC PLC
Priority date: 2001-03-13
Filing date: 2002-03-13
Publication date: 2002-12-24
Also published as: GB0106115D0; GB0205922D0

Abstract

A method for determining the surface velocity of a flowing liquid by cross correlation, using two modulated collimated light sources 1 and 2 impinging on the liquid surface with a detector system on a parallel optical axis 9. The light arriving at the detector 7 from the two light sources is separated into two channels using synchronous detection techniques and filtered so as to provide two time series which can be digitised and cross correlated to obtain the time taken for the liquid to flow between the 2 light spots on the liquid surface. The parallel axes of light sources and detector enable the system to accommodate to changes in liquid level without the need for moving parts or focusing systems. Detection of surface turbulence by the system is maximised by arranging the optical axis at an oblique angle to the liquid surface and by using a large aperture detector system.

Description

METHOD AND DEVICE FOR DETERMINING FLUID FLOW VELOCITY This invention relates to a method and device for determining fluid flow velocity. The invention particularly relates to the determination of liquid flow in open channels and part-filled pipes. The invention is especially applicable to aggressive or highly fouling materials such as wastewaters as the measurement can be made without any direct contact between the flowing material and the sensing system.

Liquid flow in open channels is most frequently determined by means of hydraulic structures such as flumes or weirs, and this would be the method of choice for new works or where it is cost-effective to retro-fit such a structure. There are situations where hydraulic structures cannot be used, or where they have severe disadvantages, for example where there is not sufficient hydraulic head available, high civil engineering costs, or in the case of notch-type weirs, suspended material may block a part of the weir causing unacceptable errors. Structures are also inappropriate for temporary open channel flow measurements such as hydraulic surveys of sewer networks.

Most of the alternatives to the use of flow structures are based on velocity-area methods, where the geometry of the channel or pipe is known, the depth and flow velocity at some point in the flow are measured and the volumetric flow calculated using a theoretical or empirical model to calculate the average flow velocity from the velocity measurement available.

The most popular velocity-area technique for temporary measurement of wastewaters is the'Doppler mouse'which measures flow velocity using ultrasonic doppler and the depth using a pressure transducer. The velocity and depth devices are built into a transducer head which is fitted to the floor of the channel and thus immersed in the flowing liquid. The disadvantages of this technique are that it is prone to fouling by silting or ragging of the'mouse', and that the location of the velocity measurement within the bulk of the flow is uncertain, causing errors in the estimation of average flow velocity. Time-of-flight ultrasonic methods are used with transducers embedded in the walls of the channel, but this technique is restricted in practice by its high cost.

Recently a microwave Doppler system,'FLODAR', has been developed by MarshMcBirney where the surface velocity is determined using back-scattered microwaves.

This has solved the problems of sensor fouling, but there remains some uncertainty over where the microwave returns originate, it requires significant surface ripples to obtain backscattered microwaves and involves high cost.

The technique of ciûss-correlation was applied to open channel flow in the 1970s at Bradford University working with WRc (Kaghazchi B, Distance and velocity measurement using cross-correlation techniques. PhD Thesis Bradford University, December 1979). Cross-correlation has been developed commercially for other applications such as measuring the surface velocity of strip steel in rolling mills and road speed measurement as described in US Patent number 5020903. The open channel flow application was not developed further at the time because of the relatively high cost of the digital processing needed and because it required far more electrical power than the Doppler mouse technique. It is now possible to produce a low cost instrument with substantial processing power and diode lasers as low power collimated light sources. The use of lasers has enabled a different configuration to be used which operates over a large range of liquid levels.

Broadly, the present invention makes it possible to determine surface fluid flow velocity by detecting a surface disturbance on the fluid, such as a ripple or turbulence, then measuring the time taken by the ripple to travel between two points. In the most advantageous form of the invention, two sources of collimated radiation are directed at the surface, and the radiation is reflected back at a detector, which analyses the results. The radiation is modulated at different frequencies so that it is possible to determine which source the detected radiation originated from, then a conventional cross-correlation calculation is made to determine flow velocity.

According to one aspect of the invention there is provided a method for determining the surface velocity of a liquid, comprising the steps of: directing two collimated radiation beams onto the fluid surface at an oblique angle thereto, and spaced along the fluid surface a predetermined distance, modulating the beams at different frequencies, detecting radiation received back from the liquid surface, generating signals representative of the two collimated beams received back from the liquid surface, cross correlating the signals representative of the two collimated beams, determining a time delay corresponding the a peak of the cross correlation function, and determining surface fluid flow velocity by dividing said predetermined distance by said time delay.

Advantageously, the collimated beams are substantially parallel. They may, if desired, be arranged at a small angle to one another in order to compensate for problems caused by variations in the distance between the radiation sources and the surface of the fluid. In this case, the angle would typically be less than 10 degrees.

The detector preferably has an optical axis substantially parallel to the axes of the collimated radiation beams.

Preferably, the collimated beams are of rectangular form with the shorter dimension along the direction of the flow velocity to be measured.

Desirably, the collimated beams are produced by lasers with an additional cylindrical lens.

The detector may be, for example, a single radiation detector or a dual radiation detector.

Preferably, the method further includes the step of independently measuring the depth of fluid. This enables the further determination of the average fluid flow velocity and/or the volumetric flow rate. The independent measurement of level may be made, for example, using an ultrasonic pulse-echo device or using a radar ranging device.

According to another aspect of the invention there is provided a device for determining the surface velocity of a fluid, comprising two sources of collimated radiation for directing beams of said collimated radiation onto a liquid surface at an oblique angle thereto and spaced apart by a predetermined distance, means for modulating the beams at different frequencies, a detector for detecting the radiation received back from the liquid and generating a signal corresponding thereto, signal separation means for separating the signal received by the detector into two signals each representative of a respective one of the collimated radiation sources, cross correlation means for cross correlating the two signals generated by the signal generation means and determining a time delay corresponding to the peak of the cross correlation function ; and computing means to determine the surface fluid velocity by dividing said predetermined distance by said time delay.

In accordance with one preferred aspect of the invention there is provided a method for measuring the surface velocity of a liquid without contact, using optical cross correlation, comprising the steps of : directing two collimated radiation beams parallel to each other onto the liquid surface at an oblique angle, modulating the beams at different frequencies, detecting the radiation from the liquid surface using a large

aperture detector system whose optical axis is parallel to that of the collimated radiation source beams, using synchronous detection to separate the signals from the two collimated beams which are then cross correlated and obtaining a measurement of flow velocity atcng a line joining the collimated beam spots by dividing the distance between the spots on the liquid surface by the delay time corresponding to the cross correlation function peak.

The method and device according to the invention have the specific feature that it is possible to making the velocity determination without contact using optical cross correlation. The invention provides a simple, economical and reliable way of measuring surface fluid velocity, average fluid velocity and volumetric flow of liquids.

Reference is now made to the accompanying drawings, in which: Figure 1 is a schematic drawing showing a side elevation of an embodiment of a flow device according to the invention; Figure 2 is a schematic drawing, like Figure 1, showing the limits of the reflection of the radiation from one of the radiation sources of the device according to the invention; Figure 3 is a schematic plan view of radiation spots on the liquid surface; Figure 4 is a schematic drawing showing a side elevation of another embodiment of a device according to the invention having converging collimated radiation beams.

Figure 5 is a schematic diagram showing an embodiment of the signal processing for a device according to the invention; and Figure 6 is a schematic drawing showing a side elevation of another embodiment of a device according to the invention, which includes an optical level measurement system.

Light from collimated radiation sources 1 and 2 is arranged to pass through the lens 3 (which may for example be a Fresnel lens fabricated from acrylic material) without change of direction by means of small holes, machining and polishing of the surface or other means. The light strikes the flowing surface 4 at an oblique angle 5 which is typically in the range 200 - 450 and is ideally substantially 350. The direction of flow may be as indicated by arrow 6 or in the opposite direction. Some of the light

scattered from both collimated radiation sources at the flowing surface is collected by lens 3 and focussed on to detector 7. The assembly of collimated radiation sources 1 and 2, lens 3 and detector 7 is enclosed in a housing 8. For outdoor use an optical window (not shown) can be used to protect the lens 3 and this may have a hood (not shown) to shield it from rain and a motorised wiper (not shown) to keep it clean.

The optical axis 9 of the lens 3 and detector 7 is arranged to be parallel to the beams of the collimated radiation sources 1 and 2. This has the effect that as the level of the flowing liquid varies vertically, some of the light scattered from the area of interaction between the collimated radiation source beams and the flowing surface will still arrive at the detector to provide signals for the cross correlation measurement. For example, with the system mounted some 0.5 metres above the highest level of the liquid, measurements can be made with the liquid level as much as 1 metre below this.

This depth of field is not possible with other cross correlation configurations without the use of moving parts.

The performance of the system improves as the aperture of the detector lens is increased. This is very noticeable at flow velocities below 0.3 metres per second where there is very little turbulence on the liquid surface for cross correlation to operate.

Referring to figure 2, a disturbance on the surface 4 can reflect into any part of the aperture 10 and be detected. The large aperture therefore increases the probability that turbulence will be detected as it passes the collimated radiation source spots. The dimensions of the lens 3 are typically a length 10 of 300 millimetres by 100 millimetres for the ranges up to 2 metres used in wastewater applications.

The shape and size of the collimated radiation source spots on the flowing surface affects the performance of the system. Cross correlation requires a disturbance on the flow surface to pass from the first spot to the second in a form which is optically recognisable. The greater the spacing between the spots, the more likely it is that the disturbance will have decayed or changed unrecognisably. However, as the spots are moved closer, the percentage accuracy of the measurement of distance between the spots decreases.

Figure 3 shows the dimensions of the collimated radiation source spots at the liquid surface. Referring to figure 3, the optimum distance 11 between the spots 12 and 13 is in the range 40-60 millimetres for wastewater flows which are typically 0.2-2

metres/second flow velocity. The length 14 of the spots along the flow direction 15 is in the range 2-5 millimetres. For longer lengths, the cross correlation peak becomes broader, for shorter lengths, the number of cross correlation events is reduced. The width 16 of the spots across the direction of flow 15 can be in the range 5-15 millimetres. As the width increases, the probability of a disturbance at the first spot 12 crossing the second spot 33 increases, and the number of events detected will also increase with increasing width. The maximum width is limited by the channel width, since there is a velocity profile across the surface in the direction 16 and as the measurement width increases, the cross correlation peak will broaden. There is also an issue of light intensity: the signal strength falls as the collimated radiation source light is spread over increasing areas and more power will be needed for wider spots. A good working arrangement uses a spacing of 50 millimetres, with spots of 3 millimetres by 10 millimetres. Laser diodes of 1 milliwatt power are suitable collimated radiation sources with some modification of the standard optics for example additional cylindrical lenses to produce the rectangular beam profile. If the collimated radiation source is plane polarised there is benefit in arranging the E vector to be parallel to the sample surface to minimise the radiation transmitted into the sample.

In a typical measurement situation, the flow velocity will decrease as the liquid depth in the channel falls. At low velocities, say below 0.4 metres per second, it is an advantage to have the 2 light spots closer together than is suitable for higher velocities. The arrangement of figure 4 takes advantage of the change in velocity with falling level to optimise the spacing of the spots. The collimated beams 1 and 2 are arranged to converge slowly, so that with the sample 4 at the higher level the spot spacing will be greater, for example 80 millimetres, whilst when the level has fallen the spots are closer, for example 30 millimetres.

Figure 5 shows the signal processing in schematic form. The oscillators 17 and 18 operate at different frequencies, for example 800 Hz and 1200 Hz, and modulate the intensities of the collimated radiation sources designated as 19 and 20 in Figure 5. The signals falling on the detector 21 are amplified by variable gain amplifier 22. The gain can be varied under the control of the computer 23 and typically a gain range of 40 dBs will be needed to accommodate the range of signal strengths for different liquid flow velocities and ranges. The amplified signal is then split into the 2 channels of the original signals using synchronous detection for example lock-in amplifiers 24 and 25

which use the oscillator signals to select the signals originating from the two collimated radiation sources. The lock-in amplifiers are followed by low pass filters 26 and 27 which have bandwidths of typically 100 Hz and a rapid roll-off of 60 dBs per octave above 100 Hz. The resulting signals are AC coupled to the analog to digital converters 28 and 29 which digitise the two time series from the liquid surface. The coupling is arranged to block DC and may have a 3dB point at for example 1 Hz. The digital to analog converters 28 and 29 may be replaced by a single digital to analog converter multiplexing between the two channels. It is desirable that the hardware allows the signals to be sampled simultaneously, or if there is a delay between sampling the two channels, the delay can be allowed for as an adjustment to the cross correlation delay time.

The computer 23 then performs a cross correlation on the digitised time series from the two channels. The peak of the cross correlation function gives the time taken for the liquid surface to travel between the two collimated radiation source beams at the surface. In practice it is necessary to average as many as 100 cross correlation functions to obtain a well-defined peak and thus an accurate measure of the time delay.

The surface velocity is the separation of the collimated radiation source beams along the liquid surface divided by the time delay.

An alternative scheme would be to digitise the signal from the amplifier 22 directly and to carry out the synchronous detection and low pass filtering digitally.

Referring to figure 1, an alternative arrangement would be to use a split or dual detector in place of the single detector 7. The signals from the two parts of the detector would then have their own separate amplification stage so that referring to figure 4, the variable gain amplifier 22 would be duplicated. This would reduce the cross talk between the channels especially close to the range at which the liquid surface was clearly focussed on the detector surface.

In order to measure flow, a measurement of liquid depth is needed from which, knowing the dimensions and shape of the channel or pipe, the cross sectional area of flowing liquid can be calculated. The volumetric flow is then the product of cross sectional area and the average flow velocity. The cross correlation system described does not provide a liquid depth measurement and a separate level transducer for example an ultrasonic or radar type can be used, mounted on the housing 9 of the

velocity system, or mounted separately. It is also possible to have an optical ranging system to generate level data which could for example utilise one or both of the laser beams. An example is shown in figure 6. Within the housing 9 there is a separate

radiation detector 30 and lens 31 which images one or both of the spots on the liquid radialion'Ue'tet. LVI surface on to the detector. As the liquid level moves vertically the position of the image of the spots moves laterally across the detector 30. The detector is a position sensitive detector, photodiode array or similar device which can be used to indicate where along its length the radiation spot image is located. It is then a matter of geometric triangulation to determine the range to the liquid surface and hence the depth of flowing liquid. It is necessary for a portion of the main velocity system lens 3 to be machined or to include another hole where radiation for the level system can pass through without distortion of the image on the detector 30. Another optical method would be to use the modulation capability of one of the collimated light sources to measure the range from the measurement head to the sample surface by phase shift between the drive to the source and the radiation returned. This has the virtue of requiring no extra optical parts.

The relationship between the average flow velocity and the surface velocity measured by the cross correlation technique depends upon a number of factors.

Typically the average velocity is a factor of 0.84 to 0.90 times the surface velocity, depending on factors such as liquid depth, surface roughness, bends and changes in cross section up and downstream. The variation of the factor due to level is known theoretically and a correction to the factor is made from a lookup table in the computer.

This forms the method for an instant calibration of the system for a quick installation. Higher accuracy is usually achieved by making a series of flow velocity measurements using a handheld point flow velocity probe and calculating the volumetric flow according to the methods given in standards literature (see BS-ISO 748-1997: Measurement of liquid flow in open channels-Velocity-area methods). The factor and its scaling for depth can then be adjusted to give agreement at the flow measured and this adjusted lookup table is then used to calculate the average flow velocity and hence the flow.

It will be appreciated that the invention described above may be modified.

Claims

Claims 1. A method for determining the surface velocity of a liquid, comprising the steps of: directing two collimated radiation beams onto the fluid surface at an oblique angle thereto, and spaced along the fluid surface a predetermined distance, modulating the beams at different frequencies, detecting radiation received back from the liquid surface, generating signals representative of the two collimated beams received back from the liquid surface, cross correlating the signals representative of the two collimated beams, determining a time delay corresponding a peak of the cross correlation function, and determining surface fluid flow velocity by dividing said predetermined distance by said time delay.
2. A method according to claim 2, further comprising using a large aperture detector having an optical axis that is substantially parallel to that of the collimated radiation source beams.
3. A method according to claim 1 or 2, wherein the area of the large aperture is from 3000 to 30000 mm2.
4. A method according to claim 1,2 or 3, comprising the use of synchronous detection to separate the signals from the two collimated beams received back from the fluid surface.
5. A method according to claim 4, in which the synchronous detection is carried out using lock-in amplification.
6. A method according to any preceding claim, in which the collimated beams are of rectangular form with the shorter dimension being arranged along the direction of the flow velocity to be measured.
7. A method according to any preceding claim, in which the collimated beams are produced by lasers with an additional cylindrical lens.

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8. A method according to any preceding claim, in which the detector is a single radiation detector.
9., A method according to any one of claims 1 to 7, in which the detector is a dual radiation detector.
10. A method according to any preceding claim, further comprising independently measuring the depth of fluid.
11. A method according to claim 10, in which the independent measurement of level is made using an ultrasonic pulse-echo device.
12. A method according to claim 10, in which the independent measurement of level is made using a radar ranging device.
13. A method of determining average flow velocity in a channel or part-filled pipe, comprising determining the surface flow velocity by a method according to any one of claims 1 to 9, then using said determined flow velocity to determine average flow velocity by applying a factor dependent on the depth of flow.
14. A method of determining volumetric fluid flow in a part-filled pipe or channel, comprising determining the surface flow velocity by a method according to any one of claims 1 to 9, independently measuring the depth of fluid, and combining the surface flow velocity information with the depth information to produce a volumetric flow measurement.
15. A method according to claim 14, in which the independent measurement of level is made using an ultrasonic pulse-echo device.
16. A method according to claim 15, in which the independent measurement of level is made using a radar ranging device.

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17. A device for determining the surface velocity of a fluid, comprising two sources of collimated radiation for directing beams of said collimated radiation onto a liquid surface at an oblique angle thereto and spaced apart by a predetermined distance, means for modulating the beams at different frequencies, a detector for detecting the radiation received back from the liquid and generating a signal corresponding thereto, signal separation means for separating the signal received by the detector into two signals each representative of a respective one of the collimated radiation sources, cross correlation means for cross correlating the two signals generated by the signal generation means and determining a time delay corresponding to a peak of the cross correlation function ; and computing means to determine the surface fluid velocity by dividing said predetermined distance by said time delay.
18. A device according to claim 17, in which said sources of radiation comprise lasers.
19. A device according to claim 17 or 18, in which said signal separation means comprises two lock-in amplifiers.
20. A device according to claim 17, 18 or 19, further comprising means to measure the depth of the fluid.
21. A method for determining the surface velocity of a liquid substantially as herein described with reference to and as shown in the accompanying drawings.
22. A method of determining average flow velocity in a channel or part-filled pipe substantially as herein described with reference to and as shown in the accompanying drawings.
23. A method of determining volumetric fluid flow in a part-filled pipe or channel substantially as herein described with reference to and as shown in the accompanying drawings.

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24. A device for determining the surface velocity of a liquid substantially as herein described with reference to and as shown in the accompanying drawings.