xevice and method for regulating pressure The invention relates to a device for regulating a pressure decrease in the flow direction ofa fluid flow. The invention also relates to a method for regulating a pressure decrease in the flow direction of a fluid flow by means of such a device. According to the prior art, a pressure decrease in the flow direction of a fluid flow is obtained by guiding the fluid flow through a constriction / valve. In the constriction/valve the flowspeed increases and the pressure decreases. This however results in relatively great shear forces in the fluid, which can be undesirable or impermissible, for instance in the case of a dispersion or a, thixotropic or dilatant, non-Newtonian fluid. Erosion resulting from the higher flow speeds at the position of the constriction / valve can also form a problem. Pressure decrease by means of a valve is for instance applied in oil extraction. In order to increase oil production a water flow is therein injected into the ground close to an oil well by means of an injector. The injected water flow pushes oil present in the ground to the oil well. At the point of injection, for instance at a depth of 400 metres, the pressure in the water flow must generally be decreased. This is because the pressure in the approaching water flowis generally too high there, among other reasons because of the high static pressure, for instance 40 bar in the case of injection at a depth of 400 metres. Pressure decrease in the water flow is then obtained by guiding the water flow through a valve built into the injector. It has now been found that the pushing action of a dispersion of polymer particles in water on oil present in the ground is greater than that of water alone. However, if such a dispersionis guided through the valve, the polymer particles experience very great shear forces, whereby they fracture / disintegrate, which is highly undesirable. The present invention now provides a solution to the stated problems, and has many other options for use in addition to the stated use in oil extraction. The invention provides a device for regulating a pressure decrease in the flow direction of a fluid flow, characterized in that the device comprises a chamber, which chamber is elongate and has along at least the greater part of its length an at least substantially round cross section, which chamber is configured and suitable for having at least a part of the fluid flow flow therethrough, which chamber is provided with at least one first inlet configured and suitable for having at least a first part of the fluid flow flow therethrough into the chamber,and which chamber is also provided with at least one outlet configured and suitable for having at least a part of the fluid flow flow therethrough out of the chamber. The invention also provides a method for regulating a pressure decrease in the flow direction of a fluid flow by means of such a device, characterized in that the method comprises of making at least a part of the fluid flow flow through the chamber such that a first vortex extending along atleast the greater part of the chamber is brought about in the at least one part of the fluid flow flowing through the chamber. The first vortex provides for an absorption of energy which is distributed over the chamber and thus for a controlled and gradual pressure decrease. The invention can be especiallyadvantageously applied in the case that relatively great shear forces and/or flow accelerations are undesirable or impermissible. The invention is further elucidated hereinbelow on the basis of exemplary embodiments. In the drawings: figure 1 shows a cross-section of an exemplary employment of a device according to the invention; - figure 2 shows an enlarged first portion of that cross-section; - figure 3 shows an enlarged second portion of that cross-section; - figure 4 shows a perspective view of a first part of the device; - figure 5 shows a perspective view of a second part of the device; figure 6 shows a perspective view and cross-sections of an alternative embodiment of a first insert according to the invention; and figure 7 shows a cross-section of a part of an alternative embodiment of a device according to the invention comprising an alternative embodiment of a second insert according to the invention. The exemplary embodiment of a device (1) according to the invention shown in figures 1-5 is here an injector for injecting a liquid flow (2) deep into the ground. The liquid is here a dispersion of polymer particles in water. According to the invention, the device (1) comprises an elongate chamber (3), having a conical shape here, with a wider first outer end(12) which is directed downward in the situation of use, and a narrower second outer end (13) which is directed away from the first outer end (12) and is directed upward in the situation of use. The device (1) also comprises a number of inflow openings (8) in the vicinity of the second outer end (13) of the chamber (3), and a number of outflow openings (10) in the vicinity of the first outer end (12) of the chamber (3). The chamber (1) is providedat the first outer end (12) thereof with a first inlet (4) and an outlet (5) which is placed centrally relative to the chamber (3) and is here formed by a first insert (14). The device (1) also comprises a channel (9) which connects the inflow openings (8) to the first inlet (4) and extends adjacently of the chamber (3) placed eccentrically in the device (1). The chamber (3) is also provided at the second outer end (13) thereof with a second inlet (11) which isplaced centrally relative to the chamber (3) and is here formed by a second insert (15). The device (1) here also comprises a second chamber (7) which forms a connection between the outlet (5) and the outflow openings (10). According to a method according to the invention, a liquid flow (2) supplied by means of aconduit (not shown) is made to flow through the inflow openings (8) into the device (1) with a higher intemal pressure (Pi) and to flow through the outflow openings (10) out of the device (1) again with a lower intemal pressure (P2). The pressure drop over the device (1) is obtained largely by making the liquid flow (2) flow through the chamber (3) and therein bringing about a first vortex (6) extending along the length of the chamber (3). The firstvortex (6) provides for an absorption of energy which is distributed over the chamber (3) and thus for a controlled and gradual pressure decrease. The polymer particles thus experience only relatively small shear forces, and will thus not fracture / disintegrate into smaller particles, or hardly so. A first part (2a) of the liquid flow (2) is here made to flow through the first inlet (4) largely tangentially into the chamber (3) at the first outer end (12) thereof. The first vortex (6) extending along the length of the chamber (3) is thus brought about. At the second outer end (13) of the chamber (3) the liquid flow `reverses' and then flows, roughly axially, back therethrough to the outlet (5) at the first outer end (12) thereof. A second part (2b) of the liquid flow (2) is made to flow, here axially, into the chamber (3) through the second inlet (11), and then to flow therethrough, roughly axially, to the outlet (5). The pressure drop over the device (1) is determined by, among other factors, the mutual ratio of the second part (2b) of the liquid flow (2), which flows through the second inlet (11) roughly axially into the chamber (3), and the first part (2a) of the liquid flow (2), which flows through the first inlet (4) largely tangentially into the chamber (3): the greater the secondpart (2b), the smaller the pressure drop. Said mutual ratio is determined by, among other factors, the flow resistances of the first inlet (4) and the second inlet (11). The pressure drop can thus for instance be changed by adjusting the flow resistance of the second inlet (11). This can for instance be done by exchanging the second insert (15) for a different one with a different flow resistance, or by closing the second inlet (11) completely. The pressure dropcan for instance also be changed by adjusting the flow resistance of the outlet (5), for instance by exchanging the first insert (14) for a different one with a different flow resistance. Because the given example relates to an injector for injecting a liquid flow (2) deep into the ground, it is necessary that the diameter of the device (1) is relatively small along its wholelength in order to be able to fit in a downhole drill pipe, and that in the situation of use the inflow openings (8) are situated on the upper side of the device (1) and the outflow openings (10) on the lower side thereof, and that the wall thicknesses are sufficiently great, this in respect of the high pressures that occur. The device (1) here meets all these requirements. This is achieved, among other ways, by means of the channel (9) which connects the inflowopenings (8) to the first inlet (4) and which extends in the immediate vicinity of the chamber (3) and parallel to the longitudinal direction of the chamber (3), and by the eccentric placement of the chamber (3). Because in the given example use is made of a dispersion of polymer particles in water, theconcentration of the polymer particles in the water can begin to vary over the chamber (3) as a result of the centrifugal forces occurring in the chamber (3) and the force of gravity, all this depending on the size and the specific weight of the polymer particles. In the second chamber (7), which forms the connection between the outlet (5) and the outflow openings (10), the liquid (2) flowing from the outlet (5) is homogenized again, in so far as this is necessary. Because the narrower second outer end (13) of the chamber (3) is directed upward in the situation of use, dirt which may be present, such as sand, will not accumulate therein. Figure 6 shows an altemative embodiment of the second insert (15') which forms an alternative embodiment of the second inlet (11'). The second inlet (11') is now embodied such that the second part (2b') of the liquid flow (2) flowing therethrough forms a second vortex (6") therein. The second part (2b') of the liquid flow (2) then flows, at least largely,tangentially into the chamber (3). The rotation direction of the second vortex (6") is here the same as the rotation direction of the first vortex (6), whereby the occurring shear forces in the second part (2b') flowing into the chamber (3) are relatively small as compared to the device (1) shown in figures 1-5. Figure 7 shows an alternative embodiment of a device according to the invention comprising an alternative embodiment of the first insert (14'), which here forms an altemative embodiment of the outlet (5'). The chamber (3) is now also provided in the vicinity of the first outer end (12) thereof with a third inlet (4") for having a third part (2a") of the liquid flow (2) flow therethrough into the outlet (5'). The third inlet (4") is embodied and placedrelative to the outlet (5') such that the third part (2a") of the liquid flow (2) flows, at least largely, tangentially into the outlet (5') and forms therein a third vortex (6"). The rotation direction of the third vortex (6"') in the outlet (5') is here the same as the rotation direction of the first vortex (6') in the chamber (3). The axial component of the flow direction of the third part (2a") of the liquid flow (2) flowing into the outlet (5') is here however opposite tothe flow direction of the liquid flow (2') flowing from the chamber (3) through the outlet (5'), and thus causes an increase of the pressure drop over the device. More generally, it is the case that the connection between pressure drop (`effore) and flow rate (`flow') is determined mainly by the shapes, dimensions and relative placements ofchamber(s), inlets, outlets, inflow openings and outflow openings. A desired operating characteristic of the device can for instance be further controlled by setting or varying inflow or outflow directions (axial / tangential, 'with the flow' / `against the flow', 'with the rotation' / `against the rotation') or setting or varying flow resistances of inlets or outlets, discretely (for instance by exchanging inserts or by partially or wholly closing them) or continuously (by means of suitable constructions / controls). In addition to controlled and gradual pressure decrease, relatively small occurring shear forces in the Huid flow and relatively little erosion owing to the relatively low flow speeds and flow accelerations, the invention has still more advantages relative to pressure decrease by means of a constriction / valve according to the prior art, these being: no valve (spring) necessary; no, or few, moving parts; an open structure with no, or little, chance of blockage; and - easily scalable / controllable in respect of pressure drop and flow rate. In other embodiments a vortex can also be brought about, or be brought about in part, bymeans of means provided for this purpose, for instance fixed or rotatable blades disposed in the chamber or for instance in an inlet. It will be apparent that the invention is not limited to the given exemplary embodiments but that diverse variants and combinations obvious to a skilled person are possible within thescope of the invention.

Reference symbols used ¹ device 2 fluid flow 2a,2a' first part of 2 (flowing through 4,4') 2b,2b' second part of 2 (flowing through 11,11') 2a" third part of 2 (flowing through 4") ³ chamber ^4,4' first inlet 4" third inlet 5,5' outlet 6,6' first vortex (in 3) 6" second vortex (in 11') 6— third vortex (in 5') 7 second chamber 8 inflow opening (of 1) 9 channel (between 8 and 4) 10 outflow opening (of 1) 11,11' second inlet 12 first outer end (of 3) 13 second outer end (of 3) 14,14' first insert (forms 5,5') 15,15' second insert (forms 11,11') A first part (of 1) B second part (of 1) Pi pressure incoming fluid flow _P2 pressure outgoing fluid flow