GB2325753A - Control of fluid flows from manifolds and side weirs - Google Patents

Abstract

An elongate fluid flow channel is provided, which can be closed or open, the channel having a plurality of transverse apertures through each of which a predetermined portion of the longitudinal fluid flow is adapted to pass, wherein the longitudinal channel is provided with an inwardly extending abutment means adjacent to and downstream of each transverse aperture. One embodiment is shown in Figure 6. There is also provided an elongate fluid flow channel having a plurality of transverse apertures through each of which a predetermined portion of the longitudinal fluid flow is adapted to pass, wherein each of said apertures is so shaped as to have a notch of an included angle which is more acute at a lowermost, in use, zone thereof, than is the included angle at an upper, in use, zone thereof, (figures 8 to 15, not shown). Corresponding methods of utilising the elongate fluid flow channels are also disclosed. The fluid flow channels are particularly suitable for use with distributor units for the treatment of effluents, in which the fluid flow channels comprise arms which distribute effluent over a treatment bed.

Description

CONTROL OF FLUID FLOWS FROM MANIFOLDS AND SIDE WEIRS The present invention relates to the control of branch flows issuing from a main channel through one or a series of transverse apertures each in the form of a branch pipe, orifice, or additionally in the case of open channels, side weirs, or any combinations thereof.

The invention will be described, for convenience, with reference to side weirs, but other apertures are envisaged. The invention may be addressed to either i) a fully enclosed channel in the form of a pipe or a duct such as a manifold arrangement distributing a single or multiple phase flow or ii) a single or two phase liquid flow, possibly with solid suspensions along an open channel.

Distribution of flows from a duct through fully enclosed channels and thereafter being discharged through adjustable side gates such as used for drying grain are known as are apparatus for the distribution of flows through inlet manifolds of combustion engines. The distribution of sewage flows over a treatment bed, through open channel flows, is another field of interest and will be described, but the invention is not intended to be limited thereto and, indeed, has wide applicability in fluid systems.

It is often the case that an even distribution of flows is required from the transverse apertures.

That is, for any given total flow input into the channel this flow is uniformly divided between the total number of transverse aperture thus an equal flow from each transverse apertures is required regardless of its position. There are two well known effects which confound this.

The first is that flow passing any one transverse apertures in a channel of constant cross sectional area increases its static pressure downstream of the aperture. See Frazer on the behaviour of side weirs in prismatic recctangular channels (p. 306 7). The second is that the flow downstream of the aperture is subject to secondary flows manifesting themselves as cross currents.

The effect of a sideways discharge from a longitudinal channel with constant cross sectional dimension over its length and through a transverse aperture induces diffusion in the main longitudinal flow which causes an increase in the downstream static pressure. The process of diffusion induces secondary flows in the form of cross currents, these currents, being superimposed on the channel flow downstream of the first aperture, affect the discharge from the next aperture and so forth. Each subsequent passage of the main flow past an aperture causes further diffusion and determines a new order of secondary flow patterns. It will also be recognised that the nature of this condition is further confounded by the precise form and surface roughness of each channel, the form and area of each aperture in relationship to the channel cross section and the spacing interval between each aperture.

The physical characteristics of the fluid medium or media also influence the discharge from each aperture, in that, although not exclusively for the liquid phase, density, viscosity and in open channel flows, surface tension all influence the nature of any one or a combination thereof of i) the progression of induced secondary flows ii) friction losses and iii) the discharge from each transverse aperture.

Various proposals have been employed to overcome these problems In closed channels a common approach to obtain a prescribed discharge from each aperture has been to fit a regulating device such as a valve or to set the area of each transverse aperture. Setting the position of each valve or area of the aperture, however is a time consuming process and rarely achieves an even distribution as the setting of any one particular valve or aperture area influences the discharge from all other upstream or downstream apertures. Furthermore, the positions set are unique for any one main channel flow.

It should also be recognised that the strength of the induced secondary flows can cause yet further problems in the prediction of friction losses, as well as any subsequent flow losses which may be induced by further downstream components such as bends or diffusers for example. Secondary flows can also seriously interfere with the accuracy of flow measurement whatever style of meter is employed (saving the use of a positive displacement mechanism).

The net result is that designs are best estimates as the actual discharge from any one transverse aperture cannot be predicted with accuracy due to the uncertainty of the extent of diffusion and subsequent secondary flows.

A further proposal to overcome these problems is to taper inwardly the walls and thereby reduce the cross sectional area of the channel. This will increase the frictional resistance between successive transverse apertures. However, this presupposes that the resulting static pressure losses are acceptable. Also the magnitude of second flows increase by virtue of the greater downstream velocity and therefore cause an even greater uncertainty as to the actual discharge from subsequent transverse apertures.

By way of example it is a common practice to use a floor mounted main air duct with or without floor mounted lateral air ducts for drying and cooling grain. The application of a suitably sized fan at one end of the main air duct provides forced convection through the grain which is stacked over and along a typical 15 meter length of main duct at a typical depth of 4 metres.

The side wall transverse apertures either leading into lateral channels or directly into the stacked grain can be adjusted from within the main duct. The procedure in setting the apertures is based on the experience of the operator who regularly monitors the moisture content of the grain along the length of the grain stack. As the length of the grain stack extends and the moisture content changes in whatever order along the grain stack then the settings of each aperture must be altered.

Ideally, the operator would wish to control the amount of air issuing from any one aperture in a predictable manner. However, his corrections in positioning the gates are as a rule in error as each resetting compounds the complexities of the already complex flow. The crop is rarely at risk, except when harvesting is forced at a higher grain moisture content than would otherwise be deemed acceptable, such as may be caused by a protracted period of precipitation.

In such a case precise regulation would be required.

A further and more demanding situation is the distribution of flows entering the inlet manifold of a combustion engine. In order to achieve a desired division of flow from any one particular serial transverse aperture relies on particular designs which are peculiarly researched in order to attempt to gain the required distribution of flow from any one aperture.

In open channels resolving the problems of achieving an even distribution from a series of side weirs, such as is required for liquid flows, such as in sewage treatment, can be even more difficult. The increase in static pressure past each aperture, which may be in the form of a notch cut into the side of the channel wall, manifests itself as an increase in depth of flow.

This increase in depth is not uniform across the width of the channel, nor indeed along its remaining length but is a sequence of crest and troughs which can, but not exclusively, be observed as a diamond like pattern. The height of the crests and depths of the troughs vary as a function of the channel cross section, the main channel flow, the size and form of the notch, the relative position of the height of the invert of the notch above the channel bed, and other factors.

A still yet further complication emerges in that the actual discharge from a side weir can be composed of both an upstream and downstream component. This is particularly the case at lower main channel velocities and as a consequence the apparent discharge coefficient varies quite markedly, and can exceed unity at lower Froude Numbers, tending to diminish as the Froude Number increases. It is convenient to express the relationship between main channel velocity and side weir discharge through the use of the Froude Number which is a quotient proportional to the channel velocity and inversely proportional to the square root of the channel depth and flow and is a non dimensional number.

Another convenient relationship is the hydraulic ratio (HR) which is the area of a fluid flow divided by the wetted perimeter at that point of the flow. That is for a rectangular channel of base width w with a fluid depth of d:- HR = wd/2d - w.

One proposal to compensate for these increases in height as flow moves down the channel is to progressively increase the height of the notch inverts above the channel bed. However, this nether quells the secondary flows nor permits the channel to distribute a complete range of flows, since setting progressive increases in height of the inverts of each notch for one particular flow comprises the opportunity for an equal division of any other flow. This is readily understood by considering that the setting of the invert height of each notch is undertaken for consideration of the likely highest flow rates down the main channel. At lower flow rates the magnitude of diffusion diminishes. As flows diminish not only will the magnitude of discharge from the further notches progressively diminish but with lower flows the situation of no flow at the furthest notch and then the penultimate notch etc is apparent.

The invention is based on the realisation that all previous attempts to solve the problem have concentrated on methods which do not suppress diffusing and thus the generation of secondary flows.

According to a first aspect of the present invention, there is provided an elongate fluid flow channel, either closed or open, the channel having a plurality of transverse apertures through each of which a predetermined portion of the longitudinal fluid flow is adapted to pass, wherein the longitudinal channel is provided with an inwardly extending abutment means adjacent to and downstream of each transverse aperture.

Preferably each abutment means extends longitudinally substantially parallel to an opposite side of the channel as far as a next aperture, whereby the width of the channel is reduced progressively, aperture by aperture.

The abutment means may extend into the longitudinal flow path by such a distance as to reduce the hydraulic radius of the fluid flow by 0.5 to 12% as measured upstream and downstream of the aperture.

Preferably the abutment means are adapted to reduce the hydraulic radius by 2.5 to 6% optionally by 3.5 to 4%.

Each abutment means may be provided with an upstream extension at or adjacent its edge remote from the channel wall.

One or all of the abutment means may have an angled or curved vertical cross section whereby it protrudes further into the channel towards the base thereof.

The channel may have substantially generally parallel side walls, the width of the channel being progressively decreased by the extent of each abutment means.

The abutment means may extend into the fluid flow in equal amounts along the channel.

Alternatively the extent of each successive abutment means may decrease along the channel.

The channel may have, in other embodiments, generally non-parallel side walls, either convergent or divergent.

The channel may be narrower at one end than at the other. This may be achieved by tapering the channel or by providing the channel with a jogged or contracted profile, for example by connecting together progressively narrower sections.

The channel may have a progressively downward sloping or stepped base to aid in overcoming friction losses.

According to a second aspect of the present invention there is provided a method of distributing a fluid flow in predetermined proportions from side apertures spaced along the length of a fluid carrying channel, wherein the method comprises providing fluid at or against one end of the channel, and providing abutment means adjacent to and downstream of one or more side apertures whereby diffusion flows in the longitudinal fluid flow are suppressed.

Preferably the abutment means may extend into the longitudinal flow path by such a distance as to reduce the hydraulic radius of the flow by 0.5 to 12% as measured upstream and downstream of the aperture.

Preferably the abutment means are adapted to reduce the hydraulic radius by 2.5 to 6% optionally by 3.5 to 4%.

In one embodiment the predetermined portion discharged from each side aperture is equal to that discharged from the other apertures.

In an alternative the predetermined portion discharged from any one side aperture may be greater than that of one adjacent aperture and less than that of the other adjacent aperture.

In this case, the portion of flow discharged through each aperture may increase progressively along the longitudinal flow path.

Alternatively, the portion of flow discharged through each aperture may decrease progressively along the longitudinal flow path.

In another embodiment, the spacing between successive apertures may so vary as to distribute fluid substantially evenly over an area traversed by said channel when moving rotationally or translocatably.

According to a third aspect of the present invention there is provided an elongate fluid flow channel, either closed or open, the channel having a plurality of transverse apertures through each of which a predetermined portion of the longitudinal fluid flow is adapted to pass, wherein each of said apertures is so shaped as to have a notch of an included angle which is more acute at a lowermost, in use, zone thereof than is the included angle at an upper, in use, zone thereof.

Preferably, the aperture has an included angle at an uppermost, in use, zone which is more acute than is the included angle at a lower, in use, zone thereof. This shape is designed to "de- tune" the notch, thus reducing the effect of excess flow.

The aperture may be so shaped as to have a lowermost, in use, acutely angled notch zone, an uppermost, in use, acutely angled notch zone, and a median zone wherein the included angles forming the notch are less acute than those at the uppermost and lowermost zones of the aperture.

The included angle of any aperture may increase progressively from an uppermost and/or a lowermost zone thereof to the median zone of the aperture.

The aperture may comprise a regular or irregular rhomboidal form.

The progressive increase of the included angle may be linear or may increase as a function of the height above a lower invert of the aperture.

According to a fourth aspect of the present invention there is provided a method of distributing a fluid flow in predetermined proportions from side apertures spaced along the length of a fluid carrying channel, wherein the method comprises providing a plurality of side apertures as described in the third aspect above, each spaced one from another by a predetermined distance along the length of the channel.

Embodiments of the present invention will now be more particularly described way of example, and with reference to the accompanying drawings, in which: Fig. 1 shows graphically the output measured at each notch for differing rates of longitudinal flow according to the prior art; Fig.2 shows in elevation and plan liquid flows in the region of an aperture in an arrangement according to the prior art; Fig. 3 shows an end elevation and plan view the surface perturbation conditions caused by a side aperture, as in Figure 2; Fig.4 shows schematically the continuing perturbation conditions along the longitudinal flow beyond a side aperture; Fig.5 illustrates the region of diffusion causing the perturbation; Fig. 6 shows an embodiment of the invention which is adapted to obviate the diffusion effect shown in the previous figures; Fig. 7 shows in more detail an abutment adjacent an aperture; Fig. 8 shows one form of hybrid aperture suitable for use with the invention; Fig. 9 shows another form of hybrid aperture suitable for use with the invention; Fig. 10 shows a third form of hybrid aperture suitable for use with the invention; and Figs. 11 to 15 each show a further alternative form of hybrid aperture suitable for use with the invention.

In a sewage treatment apparatus, utilising open channels to distribute liquid flow evenly over a treatment bed, the channels for a circular distribution bed are made to fit the bed and therefore the length of each channel arm can typically be from 3 - 16 metres, although longer channels may be needed for larger beds. The cross section of the arm needs to be as small as possible since the weight of the arm and the sewage flow is supported, at least in part, by the headwords. This brings added complications to the fluid engineering. The cross section of the channel is typically in the order of 140mm width by 70mm in depth. The side apertures currently used are "V" notches, which are set into the side wall of the channel at a spacing which will result in the event distribution of flows over the entire bed provided there is an equal division of flows from each notch.

Considering notch geometry, the discharge from a "V" notch is governed by an equation with an index of 2.5. To avoid any confusion in this context a notch is actually a side weir, or as more generically described herein - an aperture. The relevant equation is: Q = Constants x (depth of flow in the channel - the height of the notch invert over the channel bed) 2.5 Some idea as to the sensitivity of the notch can be gained by considering a change in the depth of flow at the notch by plus or minus inim. At say, a depth of flow in the channel of 60 mm the result for an invert height of 40mm of plus lmm is a 13 % increase in notch discharge, for minus 1mm a 12% reduction in discharge.

Flow rates change continually in part following diurnal water distribution pattern but delayed by the time it takes for the collected sewage flow to enter the sewage works. Of course there is also the additional load from storm flows.

Staying with the notch geometry, there is proposed a compound notch which will be slanted in an upstream manner and one example is shaped as shown in Fig. 8. Other variants are shown in Figs. 9 to 15. The slant is not shown here. The elegance of this geometry is that in the higher reaches of the notch the exponent is greatly reduced.

The discharge equation reduces to: Q = Constants x (Depth of Flow - Height of Invert)0 7 At a plus or minus lmm change in depth the result is: plus lmm an increase of 3.5% in notch discharge and minus lmm a reduction of 3.5% in notch discharge. Compare this to the equation for the "V" notch, given above.

By observing both upstream and downstream components of flow around a "V" notch side weir, it was noted that an aspect not previously reported is that the flow from the "V" notch weir comprises both an upstream and reverse flow downstream component.

In Frazer on the behaviour of side weirs in prismatic rectangular channels (p.306 307) there is demonstrated the phenomenon of an increase in the static head from upstream to downstream, and that other flow profiles can exist. However, these other profiles are dependent on the Froude Number and it will be assumed that the Froude Number will always be less than unity, thus the other profiles are of no interest, at least not in this application.

The Froude Number is the ratio of inertial forces divided by gravitational forces.

Froude Number = Velocity/(depth of flow x gravity)05 Understanding the nature of the flow structures is of fundamental importance since it determines the path of the following example.

A length of reduced channel of approximately 700mm was required to still the surface. On entry into the channel at the highest flows the surface was churning in a chaotic manner revealing the intensity and scale of the emergency turbulent structures. The magnitude of these perturbations was of the order of plus or minus 2 to 3mm which adds inaccuracy to height measurement in this region.

After the surface had stilled, a travelling wave persisted down the remainder of the channel.

At the moment there seems no reason to investigate this since i) the rms value of a travelling wave is zero and (ii) as yet there has not been observed any inlet condition likely to cause a periodic disturbance with the potential of causing a standing wave.

As previously explained, at Froude Number greater than 0.1 there is a bundle of streamlines (this is known as a stream tube) along the near wall which at the highest channel flow rates are accelerated by the gravitational pull through the notch. The surface draw down commences about 30mm from the upstream edge of the notch and the width of the stream tube is about 4 to 5mm (Fig.2).

Moving downstream, just past the notch edge the surface profile swells up across the entire width of the channel. The appearance of this swelling for a backward facing notch is shown in Fig 3 where it can be seen there is a crest propagating diagonally away from the downstream edge of the notch, with the greatest increase in surface height occurring beyond the downstream notch wall and the least increase at the outer wall. The surface settles to a near constant height after about 150mm and the width of the crest at the highest flow rates is in the order of 30mm.

It is believed the crest also tapers towards the outside wall but this observation may need to be verified.

Note that the small wavelets propagating upstream away from the crest are of no significance except they are pronouncing the existence of a region of stalled flow.

The flow structure described above existed for all notch geometries tried and at this stage there was no reason to believe the overall form will change whatever the notch.

Downstream of the notch the surface was now very disturbed revealing a complex pattern of surface structures as shown in plan view in Fig 4. Of these, however, the most important point here is that they reveal the existence of strong secondary flows as a result of discharge through the side weir. The magnitude of the changes in surface height gradually diminishes over the remainder of the longitudinal flume channel but their presence persists down its entire length. Superimposed on this are the travelling waves as observed in the interim channel section.

It can thus be seen that the greater the discharge from a notch then the greater will be both the rise in height past the notch and the strength of the secondary flow currents. This will apply to any notch geometry, although it is possible that as the slant of the notch becomes more shallow and the width of the notch decreases, the strength of secondary flows will diminish.

In the above example the following parameters were observed:i) In measuring the surface height the depth was recorded when the pointer came into contact with the surface for about 50% of the elapsed time period.

ii) In measuring the discharge from the notch a short connecting tube, with a modified funnel and flexible seal, was discharged into a separate container in which water was collected for around 30 seconds before weighing.

The following format was used in collecting data: Depth of flows, accuracy of measurement plus or minus 0.25mm using 0.1mum resolution with vernier gauge.

Positions from head works 160mm Centre of channel, accuracy here was plus minus 2mm 1010mm Centre of channel 2260mm Centre of channel, this position only applies to the last set.

Notch discharge flows are of no great significance at the moment saving that at the highest channel flow rates the nappe twists by about 45 degrees, its upper edge remaining intact and the lower edge showing signs of breaking up after about 100mm from the notch face.

It is also worthwhile to note that at low Froude Numbers the nappe comprises both an upstream and reverse flow downstream component of channel flow, whereas at higher Froude Numbers the only source is upstream. Indeed at the highest Froude Numbers, it can be seen by seeding the surface of the near upstream notch wall with droplets of potassium permanganate, surprisingly they are not swept through the notch but are accelerated down the surface contraction and are momentarily stalled as they climb the crest before proceeding downstream.

It is important to recognise in the above that as both the notch area and Froude Number increases the effects described above become more severe.

The dominant effect is the diffusion of channel flow downstream of the notch. Referring to Fig 5, it can be seen that in the downstream plane nearest the notch wall that the sheath of channel flow which would have passed into this region in the absence of a notch has been discharged through the notch. Hence, fluid in this region has suffered the greatest depletion of kinetic energy (velocity head). In the stream tube adjacent to this, some of its kinetic energy is diverted into this stagnant region where it diffuses increasing the static head and causing the surface level to rise progressively until it reaches equilibrium at the centre of the crest. It can thus be seen that in the next stream tube pair the same process occurs, however, this time the transfer of energy is smaller, thus the resultant increase in height is less. This process continues across the entire width of the channel with the equilibrium point moving further and further downstream as the low energy regions of flow progressively move downstream. Hence a diagonal crest is created.

This effect induces strong secondary flow currents moving from the outer wall to the inner wall (notch side). Hence it also explains the reason why this structure persists downstream with the peak of the swell zigzagging down the channel as the process revers from one wall to the other. This can be seen in Fig.4.

Furthermore, it should also be noted that some of the static energy loss to velocity head as the crest falls augments the strength of the secondary flows. Thus it can be seen there is a see-saw action of depletion and then increase between velocity head and static head across the width of the channel caused by the gradually varied diffusion process across the width of the channel as a result of discharge through a side weir.

Having unravelled the composition of flows surrounding the notch enables a further approach to be employed.

Clearly, the problem of increasing height as channel flow passes the notch is further complicated by the strong secondary currents which will increase in intensity with further notch passes. Hence, in addition to the idea of de-tuning the notch geometry using a Gamma notch as shown in Fig 8, it seems that removal of the effects of diffusion could be achieved by completely removing the stagnant region of flow. This is achieved with a channel section as shown in Fig 6. This arrangement also carries a weight advantage in that the channel would be step wise tapered and from the fluid engineering point of view a constant Froude Number is achieved past each notch.

The above approach is intended to obtain an even distribution of side flows, whatever the input flow. The Gamma or hybrid notches serve to diminish inaccuracies in setting up the weir plate, and serve to equalise outflow, whether the inflow is high, median or low.

Flow from a notch is a function of the flow in the main channel and the depth of flow over the notch invert.

For low flows, if an acute angle is chosen then the discharge from any one particular notch will be less sensitive to changes in the depth of flow at the notch, and hence upstream notches will be less likely to affect downstream discharges.

Consider the effect of a plus or minus lmm change in height for simple V notches at different angles.

Notch angle Notch Flow Height over percentage change in notch invert discharge for a change in height over the invert.

plus lmm minus lmm 120 degrees 0.05L/sec 13.41mm 20% -18% 60 degrees 0.05L/sec 20.81mm 12% -12% 20 degrees 0.05L/sec 33.44mm 8% - 7% 10 degrees 0.05L/sec 44.26mm 6% -6% At low flows the main problem is the respective height of each notch. Clearly setting up the channel so that all of the notch inverts are at the same level may be a problem. Hence considering the above then a sharply acute angle diminishes the sensitivity of any particular notch discharge as a result of the channel being not level.

In essence at low flows the sharper the angle the better.

At high flows the problems are complicated by the side weir effect wherein the height of flow increases beyond each notch.

Consider a notch flow of 0.35 Litres/sec: Notch angle Notch Flow Height over percentage change in notch invert discharge for a change in height over the invert.

plus lmm minus lmm v notch 60 degrees 0.35L

The apparatus and method of the invention are particularly suitable for use with distributor units for the treatment of effluent. The distributor units may be rotary distributor units in which fluid flow channels project as arms from a central rotor. However, the distributor units may also comprise travelling distributor units in which a central body reciprocates back and forth along a longitudinal path, fluid flow channels comprising arms projecting transversely from the central body.

Particularly with the so-called travelling distributor, the arms may be designed to provide even curtain flow of effluent across a treatment bed at a predetermined "design flow". Above this flow rate, a standing wave may develop and the discharge from the first two to three notches may reduce. To offset this, weirs may be cut into the wall of a side tank onto which the arms are mounted, such as to discharge flow onto arm wings progressively as the total flow exceeds the design flow.

To achieve a consistent flow across the treatment bed, the notch shape may need to be changed part way along an arm and, since notches may be cut on both sides of the arm, the change point may differ on either side (to achieve a smoother transition).

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims (30)

1. An elongate fluid flow channel either closed or open, the channel having a plurality of transverse apertures through each of which a predetermined portion of the longitudinal fluid flow is adapted to pass, wherein the longitudinal channel is provided with an inwardly extending abutment means adjacent to and downstream of each transverse aperture.

2. A flow channel as claimed in claim 1, in which each abutment means extends longitudinally substantially parallel to an opposite side of the channel as far as a next aperture, whereby the width of the channel is reduced progressively, aperture by aperture.

3. A flow channel as claimed in Claim 1 or Claim 2, in which the abutment means extends into the longitudinal flow path by such a distance as to reduce the hydraulic radius of the fluid flow by 0.5 to 12% as measured upstream and downstream of the aperture.

4. A flow channel as claimed in any one of the preceding claims, in which each abutment means is provided with an upstream extension at or adjacent its edge remote from the channel wall.

5. A flow channel as claimed in any one of the preceding claims, in which one or all of the abutment means have an angled or curved vertical cross section whereby it protrudes further into the channel towards the base thereof.

6. A flow channel as claimed in any one of the preceding claims, in which the channel has substantially generally parallel side walls, the width of the channel being progressively decreased by the extent of each abutment means.

7. A flow channel as claimed in any one of the preceding claims, having non-parallel side walls, either converged or diverged.

8. A flow channel as claimed in any one of the preceding claims, which is narrower at one end than the other.

9. A flow channel as claimed in Claim 8, in which the narrowing is achieved by tapering the channel.

10. A flow channel as claimed in Claim 8, in which the narrowing is achieved by providing the channel with a jogged or contracted profile.

11. A flow channel as claimed in Claim 10, in which the profile is jogged or contracted by connecting together progressively narrower sections.

12. A flow channel as claimed in any one of the preceding claims, in which the channel has a progressively downward sloping or stepped base e.g. to aid in overcoming friction losses.

13. An elongate fluid flow channel either closed or open, the channel having a plurality of transverse apertures through each of which a predetermined portion of the longitudinal fluid flow is adapted to pass, wherein each of said apertures is so shaped as to have a notch of an included angle which is more acute at a lowermost, in use, zone thereof than is the included angle at an upper, in use, zone thereof.

14. A flow channel as claimed in Claim 13, in which the aperture has an included angle at an uppermost, in use, zone which is more acute than is the included angle at a lower, in use, zone thereof.

15. A flow channel as claimed in Claim 13 or Claim 14, in which the aperture is so shaped as to have a lowermost, in use, acutely angled notch zone, an uppermost, in use, acutely angled notch zone, and a median zone wherein the included angles forming the notch are less acute than those at the uppermost and lowermost zones of the aperture.

16. A flow channel as claimed in any one of Claims 13 to 15, in which the included angle of any aperture increases progressively from an uppermost and/or a lowermost zone thereof to the median zone of the aperture.

17. A flow channel as claimed in any one of Claims 13 to 16 in which the aperture comprises a regular or irregular rhomboidal form.

18. A flow channel as claimed in any one of Claims 13 to 17, in which the progressive increase of the included angle is linear or increases as a function of the height above a lower invert of the aperture.

19. A flow channel constructed and arranged substantially as herein described with reference to the accompanying drawings.

20. A distributor unit for the treatment of effluent having at least one elongate fluid flow channel as claimed in any one of the preceding claims.

21. A method of distributing a fluid flow in predetermined proportions from side apertures spaced along the length of a fluid carrying channel, wherein the method comprises providing fluid at or against one end of the channel, and providing abutment means adjacent to and downstream of one or more side apertures whereby diffusion flows in the longitudinal fluid flow are suppressed.

22. A method as claimed in Claim 21, in which the abutment means extends into the longitudinal flow path by such a distance as to reduce the hydraulic radius of the flow by 0.5 to 12% as measured upstream and downstream of the aperture.

23. A method as claimed in Claim 21 or Claim 22, in which the predetermined portion discharged from each side aperture is equal to that discharged from the other apertures.

24. A method as claimed in Claim 21 or Claim 22, in which the predetermined portion discharged from any one side aperture is greater than that of one adjacent aperture and less than that of the other adjacent aperture.

25. A method as claimed in Claim 24, in which the portion of flow discharged through each aperture increases progressively along the longitudinal flow path.

26. A method as claimed in Claim 24, in which the portion of flow discharged through each aperture decreases progressively along the longitudinal flow path.

27. A method as claimed in any one of Claims 21 to 26, in which the spacing between successive apertures varies so as to distribute fluid substantially evenly over an area traversed by said channel when moving rotationally or translocatably.

28. A method of distributing a fluid flow in predetermined proportions from side apertures spaced along the length of a fluid carrying channel, wherein the method comprises providing a plurality of side apertures as described in the third aspect above, each spaced one from another by a predetermined distance along the length of the channel.

29. A method of distributing fluid flow substantially as herein described with reference to the accompanying drawings.

30. A method of treating effluent, utilising a method of distributing fluid as claimed in any one of Claims 21 to 29.