Probe for measuring fluid flow and a flow measuring system

Abstract

Probes and systems for measuring flow are provided. In one arrangement, a pitot tube extends parallel to a probe axis and defines a first probe opening at a distal end. A shroud extends axially beyond the distal end to inhibit oblique flow into the first probe opening. The shroud defines a flow passage between the shroud and the pitot tube. The shroud comprises a distal surface defining one or more second probe openings. The distal surface has an outer portion facing at least partly in a radially outward direction. The one or more second probe openings are at least partially defined in the outer portion.

Description

[0001]PROBE FOR MEASURING FLUID FLOW AND A FLOW MEASURING SYSTEM The present disclosure relates to probes for measuring fluid flow, particularly for applications involving unsteady flow. Many aerodynamic flows are highly three-dimensional. Velocity components may be obtained from laser techniques or hot wire anemometry, but these methods do not measure total pressure, which is often desirable for characterizing performance, especially for internal flows. Calibrated multi-hole probes are known that can obtain velocity and pressure measurements. In “steady” probes, pneumatic tubing connects the probe head to a transducer, which only records averaged pressures. Such steady multi-hole probes have various advantages. They are relatively simple to construct, low cost, and easy to operate. They can be made at small-scale (∼1 mm diameter) to minimize spatial errors, and enable measurement access (e.g., for intrarow traverses in multistage turbomachines). These probes can also be made sufficiently robust to operate in harsh environments. Such steady probes are often used when there is significant unsteadiness in the flow. For example, the probes may be used in the measurement of turbomachinery performance, which often relies on stationary-frame multi-hole probe traverses downstream of rotating blade rows. Other uses involving significant unsteadiness include measurements of shedding wakes. In such unsteady flows, the pneumatic-averaged flow indicated by the steady probes may differ from the true time average. It is an object of the disclosure to provide alternative and / or improved probes and associated systems. According to an aspect of the invention, there is provided a probe for measuring fluid flow, comprising: a pitot tube extending distally parallel to a probe axis and defining a first probe opening at a distal end of the pitot tube; and a shroud radially surrounding and extending axially beyond the distal end of the pitot tube to inhibit oblique flow of fluid into the first probe opening, the shroud being spaced apart from the pitot tube to define a flow passage between the shroud and the pitot tube, wherein: the shroud comprises a distal surface defining one or more second probe openings, the distal surface having an outer portion facing at least partly in a radially outward direction and wherein the one or more second probe openings are at least partially defined in the outer portion. Thus an arrangement is provided in which a pitot tube is shielded from oblique flows by an axially protruding shroud. The shroud reduces variations in yaw of flow incident on the first probe opening defined by the pitot tube. The shroud reduces errors in measurements of total pressure obtained via the first probe opening in the presence of unsteady flow involving large length scale fluctuations in flow direction in the vicinity of probe. Integrating the second probe openings into the shroud provides a spatially compact and robust way of allowing the probe to also provide information about flow direction and / or velocity. Positioning the second probe openings so that they face at least partially in a radially outwards direction allows the second probe openings to capture information about flow direction and / or velocity of oblique flows with minimal influence from the probe in a spatially compact structure. In an embodiment, the one or more second probe conduits comprises a plurality of the second probe conduits and the shroud comprises a corresponding plurality of azimuthally separated, elongate protrusions respectively containing the second probe conduits, each protrusion protruding radially and extending parallel to the probe axis. Providing the second probe conduits in protrusions in this manner facilitates construction and minimises a cross-sectional area of the probe obstructing flow. According to an aspect of the invention, there is provided a probe for measuring fluid flow, comprising: an elongate body extending along a probe axis and having a distal end defining a concave distal surface, wherein: the elongate body defines a first probe opening and one or more second probe openings; the elongate body defines a first probe conduit configured to provide a fluidic connection between the first probe opening and a first sensor; the elongate body defines one or more second probe conduits corresponding to the one or more second probe openings, each second probe conduit being configured to provide a fluidic connection between the second probe opening corresponding to that second probe conduit and a respective second sensor; each of the first probe opening and the one or more second probe openings has an edge forming a closed loop defining an opening plane and normals to at least two of the opening planes are aligned obliquely; and at least the first probe opening is defined in the concave distal surface. Thus an arrangement is provided in which a first probe opening, which can act as a pitot tube, is shielded from oblique flows by an axially protruding portion of the elongate body. The axially protruding portion reduces variations in yaw of flow incident on the first probe opening, thereby reducing errors in measurements of total pressure obtained via the first probe opening in the presence of unsteady flow involving large length scale fluctuations in flow direction in the vicinity of probe. At the same time, integrating the second probe openings into the elongate body provides a spatially compact and robust way of allowing the probe to also provide information about flow direction and / or velocity. Integrating the first and second probe openings into the same body facilitates manufacture and / or provision of a radially compact structure. In an embodiment, the one or more second probe openings are defined in the concave distal surface. This configuration facilitates radial compactness and minimises disruption to the flow being measured. In an embodiment, the one or more second probe conduits comprises a plurality of the second probe conduits and the elongate body comprises a corresponding plurality of azimuthally separated, elongate protrusions respectively containing the second probe conduits, each protrusion protruding radially and extending parallel to the probe axis. Providing the second probe conduits in protrusions in this manner may facilitate construction and provide additional freedom for configuring the second probe conduits, as well as allowing the second probe openings to face at least partly in a radially outward direction. In an embodiment, at least a portion of the distal surface is locally curved in directions within a plane passing through the probe axis. This may facilitate provision of second probe openings in the distal surface while maintaining a relatively sharp angle at a leading edge of the elongate body. Thus, the curvature may be such that an average angle between normals to the distal surface and the probe axis decreases as a function of distance away from the leading edge of the elongate body. Embodiments of the disclosure will be further described by way of example only with reference to the accompanying drawings. Figure 1 is a heat map depicting a snapshot in time of variations in kinetic energy behind an object in a simulated flow of a fluid, demonstrating yaw oscillations in a measurement plane. Figures 2-5 schematically depict different flow angles onto a distal portion of a standard multi-hole probe and associated comparison between a pressure ^^measured by the central hole of the probe and the total flow pressure ^^. Figures 6 is a perspective view of an example probe for measuring fluid flow. Figures 7 and 8 are schematic front and side views of a probe of the type shown in Figure 6. Figure 9 schematically depicts a flow measuring system comprising a probe, sensors, and a data processing system. Figures 10 and 11 are schematic front and side view of an example probe with four probe openings in a shroud having a distal surface with a less curved outer portion. Figures 12-14 are schematic front views of different example distributions of probe openings in a shroud. Figures 15 and 16 are schematic front and perspective views of a probe in which the shroud comprises probe conduits formed in protrusions. Figures 17 and 18 are schematic front and perspective views of a variation on the probe of Figures 15 and 16 having three protrusions at evenly spaced angles. Figures 19 and 20 are schematic front and perspective views of a further variation on the probe of Figures 15 and 16 having only two of the protrusions, spaced apart by 90 degrees. Figures 21-23 are schematic perspective, front and side views of a distal portion of an example probe having an elongate body with probe openings formed in a frustoconical, concave distal surface. Figure 24-26 are schematic perspective, front and side views of a variation on the probe of Figures 21-23 in which an axially aligned probe opening is larger than other probe openings. Figure 27-29 are schematic perspective, front and side views of a further variation on the probe of Figures 21-23 in which the distal surface is more smoothly curved. Figures 30-32 are schematic end, perspective and side sectional views of a further variation on the probe of Figures 21-23 in which the elongate body comprises probe conduits formed in protrusions. Figure 33 is an enlarged version of the side view of the probe of Figure 8 depicting example dimensions and positioning of second probe conduits in the shroud. Figure 34 is an enlarged version of the side view of the probe of Figure 23 depicting example dimensions and positioning of second probe conduits. The present disclosure relates to probes for measuring fluid flow. Measuring flow is challenging where the flow is turbulent. Larger scale flow fluctuations may be particularly problematic because they can affect the pressure field on all sides of a probe and cause large errors if the probe is not suitably configured to mitigate such errors. As mentioned in the introductory part of the description, steady multi-hole probes are widely used because of their various constructional and cost advantages. However, known multi-hole probes are vulnerable to errors when used in unsteady flow. Figure 1 depicts an example situation where measurements from such a probe would involve significant errors. Figure 1 is a heat map depicting a snapshot in time of variations in kinetic energy behind an object 2 in a simulated flow of a fluid. The simulation was generated by performing an unsteady Reynolds-averaged Navier-Stokes calculation. The heat map shows significant large scale flow fluctuations in a wake of the object 2. The fluctuations would cause oscillations as a function of time of an angle of the flow onto a probe positioned in a measurement plane (depicted by the vertical broken line). Figures 2-5 schematically depict flow 5 onto a probe head 4 of a standard multi- hole probe at different times. The example depicted is a five-hole probe (5HP) in which the probe head 4 comprises a central hole 6C and four peripheral holes (with example top and bottom peripheral holes labelled 6T and 6B) respectively connected to conduits 7C, 7T, 7B leading to pressure transducers. Figure 2 depicts flow 5 at an initial time that is approximately parallel to an axis of the probe head 4. At a subsequent time, the flow 5 is obliquely upwards onto the probe head 4, as depicted in Figure 3. At a subsequent time, the flow 5 again becomes axial, as depicted in Figure 4. At a subsequent time, the flow 5 is obliquely downwards onto the probe head 4, as depicted in Figure 5. At time points when the flow 5 is axial, as exemplified in Figures 2 and 4, a pressure ^^measured by the central hole will be approximately equal to the total pressure ^^(stagnation + static) that it is desired to measure (^^≈ ^^). However, at time points when the flow is oblique, as exemplified in Figures 3 and 5, the pressure ^^will be less than ^^. Averaged over time, the pressure ^^will thus be lower than ^^when large scale fluctuations exist. The peripheral holes allow further information to be obtained about the direction of flow. For example, at the time points of Figures 2 and 4 the pressures measured via the peripheral holes (e.g., holes 6T and 6B) will be approximately equal, indicating that the flow onto the probe head 4 is axial. At the time points of Figures 3 and 5, pressures measured via the lower peripheral hole 6B may be different from pressures measured by the upper peripheral hole 6T, thereby indicating oblique flow onto the probe head 4. The peripheral holes thus provide useful information about the flow but do not address the challenge of accurately measuring total pressure. Averaged over time the pressures measured by the lower and upper peripheral holes 6B, 6T will still be lower than the total pressure for the same reason that time averaged pressures measured via the central hole 6C will be lower than the total pressure. The lower central-hole pressure (^^< ^^) during oblique flow is deeply problematic for steady probes because they cannot distinguish between steady and fluctuating flow. The fluctuating flow condition is therefore interpreted simply as having lower total pressure. Example probes and systems are described below that address the above challenges. Figures 6-8 depict an example probe 10. The probe 10 comprises pitot tube 12. The pitot tube 12 extends parallel, optionally along, a probe axis 14. The pitot tube 12 has a distal end 16. The pitot tube 10 defines (i.e., comprises) a first probe opening 21 at the distal end 16. The probe 10 further comprises a shroud 18. The shroud 18 may be substantially tubular. The tubular shape may have a substantially constant cross-sectional size and / or shape or the size and / or shape may vary longitudinally (e.g., becoming wider or narrower and / or changing shape). The shroud 18 may be substantially cylindrical. The shroud 18 radially surrounds and extends beyond the distal end 16 of the pitot tube 12. The distal end 16 of the pitot tube 12 is thus axially recessed relative to at least an axially leading edge of the shroud 18. By extending axially beyond the end of the distal end 16 of the pitot tube 12, the shroud 18 inhibits oblique flow of fluid (i.e., flow directed at an oblique angle relative to the probe axis 14) into the first probe opening 21. The shroud 18 is spaced apart from the pitot tube 12, optionally except for a connecting arm 24 for connecting the pitot tube 12 to the shroud 18. The spacing apart of the pitot tube 12 from the shroud 18 defines a flow passage 23 between the shroud 18 and the pitot tube 12. The flow passage 23 may have a substantially cylindrical form (or otherwise correspond to the shape of the shroud 18) except for structure associated with the pitot tube 12 (e.g., the pitot tube 12 itself and the connecting arm 24). The flow passage 23 may have a distal opening at a distal end of the shroud 18. The flow passage 23 may have a proximal opening at an axially opposite end of the shroud 18. The proximal opening may face substantially along the probe axis 14, typically in a substantially downstream direction in use. Alternatively or additionally, the proximal opening may face radially (e.g., being formed in a radially outwardly facing surface of the shroud 18). In some arrangements, a plurality of radially facing proximal openings are provided. Such radially facing proximal openings may be referred to as bleed holes. A longitudinal axis of the shroud 18 may be aligned with the probe axis 14. The pitot tube 12 may thus be aligned centrally within the shroud 18, although this is not essential. In some arrangements, as exemplified in Figures 6-8, the shroud 18 is spaced apart from the pitot tube 12 in all radial directions relative to the probe axis 14 at the distal end 16 of the pitot tube 12. The connecting arm 24 may thereby be offset axially (i.e., positioned proximally relative to) the distal end 16 of the pitot tube 12. In the example shown, the shroud 18 is spaced apart from the pitot tube 12 in all radial directions over an axial range extending from the distal end 16 to where the connecting arm 24 connects to the pitot tube 12. Arranging for the pitot tube 12 to be spaced apart from the shroud 18 in all radial directions in the vicinity of the distal end 16 minimises disruption to flow in the vicinity of the first opening 21, for example by structure such as the connecting arm 24. The shroud 18 comprises a distal surface 30 at an axially distal end of the shroud 18. The probe 10 will typically be oriented so that the distal surface 30 faces partially upstream in a flow to be measured. The distal surface 30 may be positioned distally relative to the first probe opening 21 (i.e., nearer to a leading edge of the shroud 18 than the first probe opening 21). The distal surface 30 may be an upstream-most surface of the shroud 18. The distal surface 30 defines one or more second probe openings 22. One or more, optionally all, of the second probe openings 22 may be positioned distally relative to the first probe opening 21. The distal surface 30 has an outer portion facing at least partly in a radially outward direction. Thus, a vector normal facing away from the distal surface 30 in the outer portion will have a radial component that points away from the probe axis 14. The vector normal may point substantially or exactly in the radial direction. Alternatively, the outer portion may be oriented obliquely relative to the probe axis 14. The oblique orientation may be such that the outer surface faces partly forwards, in a distal direction, and partly radially outwards. The vector normal facing away from the distal surface 30 in the outer portion may point in a direction that is oblique relative to the probe axis 14 and diverges in a radially outward direction. One or more of the second probe openings 22 are at least partially, optionally wholly, defined in the outer portion of the distal surface 30. The probe 10 may be provided as part of a flow measuring system 100, as depicted schematically in Figure 9. The system 100 comprises the probe 10, a first sensor 41 and second sensors 42A-D (which may be collectively referred to as second sensors 42). In the example shown, the system 100 further comprises a third sensor 43. The first, second and third sensors 41, 42, 43 may comprise pressure sensors such as pressure transducers. The pitot tube 12 defines a first probe conduit 31 configured to provide a fluidic connection between the first probe opening 21 and the first sensor 41. The shroud 18 defines one or more second probe conduits 32A-D (which may be collectively referred to as second probe conduits 32). The one or more second probe conduits 32 correspond to the one or more second probe openings 22. Each second probe conduit 32 is configured to provide a fluidic connection between the second probe opening 22 corresponding to that second probe conduit 32 and a respective second sensor 42. The shroud 18 may further define a third probe opening 23 and a third probe conduit 33 configured to provide a fluidic connection between the third probe opening 23 and the third sensor 43. The system 100 further comprise a data processing system 60. The data processing system 60 may be configured to perform computing operations, which may be referred to as steps of a computer-implemented method. The data processing system 60 may be referred to as a computer or comprise a computer. The data processing system 60 may comprise various combinations of computer hardware, including for example CPUs, RAM, SSDs, motherboards, network connections, firmware, software, and / or other elements known in the art that allow the computer hardware to perform the required computing operations. The required computing operations may be defined by one or more computer programs. The one or more computer programs may be provided in the form of media, optionally non-transitory media, storing computer readable instructions. When the computer readable instructions are read by the computer, the computer performs the required method steps. The computer may consist of a self-contained unit, such as a general-purpose desktop computer, laptop, tablet, etc. Alternatively, the computer may consist of a distributed computing system having plural different computers connected to each other via a network such as the internet or an intranet. The data processing system 60 is configured to receive input from the first sensor 41, one or more second sensors 42A-D, and / or the third sensor 43. The data processing system 100 may be configured to use input from the first sensor 41 to determine a total pressure (e.g., the sum of stagnation pressure and static pressure) and / or the stagnation pressure (where a separate static pressure measurement is made, for example using the third sensor 43). The data processing system 100 may be configured to use input from the one or more second sensors 42 to determine a direction and / or velocity of the flow. For example, by calibrating the probe 10 in a steady wind tunnel, pressures measured by second sensors 42 corresponding to diametrically opposite second probe openings 22 can be compared to calculate the flow angle onto the probe 10. The data processing system 100 may be configured to use input from the third sensor 43 to determine information about a static pressure. Examples of different configurations of second probe openings 22 are depicted in Figures 6-8 and 10-14. These examples all involve the shroud 18 having a plurality of the second probe openings 22. Providing a plurality of the second probe openings 22 rather than a single second probe opening 22 facilitates measurements of flow direction relative to different planes. However, it is also possible to configure the shroud 18 to have only a single second probe opening 22. Providing a single second probe opening 22 may facilitate construction and / or compactness and / or lower probe 10 cost. Positioning the second probe openings so that they face at least partially in a radially outwards direction allows the second probe openings to capture information about flow direction and / or velocity of oblique flows with minimal influence from the probe in a spatially compact structure. The spatial compactness achievable by this approach is exemplified in Figure 33, which is an enlarged version of the side view of the example probe shown in Figure 8. The configuration depicted is an example of a class of embodiment in which the second probe conduit 32 (where only one second probe conduit 32 is present), or each of the second probe conduits 32 (where a plurality of second probe conduits 32 are present, as in the example depicted), for at least a portion 60 of a length of the second probe conduit 32 that starts from the respective second probe opening 22, has an average radial width D and is separated from the flow passage 23 by a distance ^^, where ^^< 2^. The radial width is a width of the second probe conduit 32 along the radial direction relative to the probe axis 14. Typically, the portion 60 of the second probe conduit 32 will be substantially cylindrical, in which case the radial width will be equal to the diameter of the circular cross-section of the second probe conduit 32. The average radial width (e.g., diameter) is averaged over the length of the portion 60 of the second probe conduit 32. Typically, the cross-section of the second probe conduit 32 will be constant for the portion 60 of the second probe conduit 32 (e.g., defining a cylinder), in which case the average radial width will be equal to the radial width at any point along the portion 60. Thus, the separation between the second probe conduit 32 and the flow passage 23 may be of a similar order of size as the diameter of the second probe conduit 32 (i.e., relatively small). Optionally, ^^< 1.9^, optionally ^^< 1.8^, optionally ^^< 1.7^, optionally ^^< 1.6^, optionally ^^< 1.5^, optionally ^^< 1.4^, optionally ^^< 1.3^, optionally ^^< 1.2^, optionally ^^< 1.1^, optionally ^^< 1.0^. In some implementations, as exemplified in Figure 33, the second probe conduit 32 is separated from a nearest radially outermost surface 62 of the shroud by a distance ^^, where ^^< 2^. Thus, the separation between the second probe conduit 32 and the radial exterior of the probe may be of a similar order of size as the diameter of the second probe conduit 32 (i.e., relatively small). Optionally, ^^< 1.9^, optionally ^^< 1.8^, optionally ^^< 1.7^, optionally ^^< 1.6^, optionally ^^< 1.5^, optionally ^^< 1.4^, optionally ^^< 1.3^, optionally ^^< 1.2^, optionally ^^< 1.1^, optionally ^^< 1.0^. The spatial compactness is particularly promoted where the one or more second probe openings 32 are the only openings in the distal surface 30 of the shroud 18, as exemplified in Figures 6-8, 10-20 and 33. In such implementations, all openings in the distal surface 30 are thus defined at least partially in the outer portion of the distal surface 30 of the shroud 18, which promotes radial compactness. In embodiments where a plurality of the second probe openings 22 is provided, two or more of the second probe openings are positioned at the same radius relative to the probe axis. This may facilitate interpretation of measurements and / or improve measurement accuracy. In some arrangements, centres (e.g., corresponding to where axes of respective second probe conduits pass through the second probe openings) of two or more of the second probe openings 22 may be azimuthally separated by less than 180 degrees. Providing second probe openings 22 with this configuration may allow flow direction to be measured in at least two different planes. All of the examples shown in Figures 6-8 and 10-14 include some second probe openings 22 that are azimuthally separated by less than 180 degrees. In some examples, centres of two or more of the second probe openings 22 are azimuthally separated by substantially 360 / N degrees, where N is an integer equal to 2 or higher. In the examples of Figures 6-8, 10 and 11, centres of the second probe openings 22 are separated by 90 degrees (360 / 4). Figure 13 shows an example in which centres of the second probe openings 22 are separated by 120 degrees (360 / 3). Figure 14 shows an example in which centres of the second probe openings 22 are separated by 45 degrees (360 / 8). The second probe openings 22 may be spaced apart evenly along a circular path surrounding the probe axis 14, as in the examples of Figures 6- 8, 10, 11, 13 and 14. This may facilitate interpretation of measurements and / or improve measurement accuracy. In other examples, the spacing may be irregular, such as is shown in Figure 12. Allowing for irregular spacing may allow measurements of flow direction in multiple planes using fewer second probe openings 22 and corresponding second probe conduits 32 and / or second sensors 42. More generally, increasing the number of second probe openings 22 may typically improve measurement accuracy. Decreasing the number of second probe openings 22 may typically reducing probe complexity and / or cost and / or improve compactness. In some embodiments, the plurality of second probe openings 22 comprises one or more pairs of the second probe openings 22, with each pair comprising second probe openings 22 that positioned azimuthally opposite to each other. This is exemplified in Figures 6-8, 10, 11, 13 and 14. Referring to Figure 10 for example, the uppermost and lowermost second probe openings 22 form one such pair and the leftmost and rightmost second probe openings 22 provide another such pair. Providing such azimuthally opposite pairs may improve measurement accuracy in respect of flow directions in a plane containing the second probe openings 22 of the pair and the probe axis 14. In some embodiments, two or more of the second probe openings 22 each have an edge forming a closed loop defining an opening plane. The opening plane is the plane that comprises most or all of the closed loop. The opening plane thus defines an orientation of the respective second probe opening 22. An angle between the opening plane and the probe axis 14 is the same for two or more of the second probe openings 22. For example, the angle between the opening plane and the probe axis 14 may be same for the second probe openings 22 in one or more of the pairs of second probe openings 22 mentioned above. As shown in Figures 6-8, in some embodiments at least a portion of the distal surface 30, optionally including at least a portion of the outer portion of the distal surface 30, is locally curved in directions within a plane passing through the probe axis 14. Thus, lines following the distal surface 30 that are also in the plane passing through the probe axis 14 are curved rather than straight in regions that are locally curved in the defined manner. Alternatively or additionally, as shown in Figures 10 and 11, in some embodiments at least a portion of the distal surface 30, optionally including at least a portion of the outer portion of the distal surface 30, is locally straight directly adjacent to one or more of the second probe openings 22 in directions within a plane passing through the probe axis 14. Thus, lines following the distal surface 30 that are also in the plane passing through the probe axis 14 are straight rather than curved. Optionally, at least a portion of the distal surface 30, optionally including at least a portion of the outer portion of the distal surface 30, is frustoconical. As depicted in Figures 6, 8 and 11, the probe 10 may additionally comprise a third opening 23 for measuring static pressure. The third opening 23 may, for example, be formed in a laterally outer surface of the shroud 18. The third opening 23 may be oriented so as not to face into a flow to be measured. The third opening 23 may, for example, face directly outwards in a radial direction relative to the probe axis 14. The third opening 23 may be fluidically connected to a third sensor 43 (see Figure 9) via a third probe conduit 33. Figures 15 and 16 depict an alternative configuration in which the second probe conduits 32 are formed by elongate protrusions 34. The shroud 18 may thus comprise a plurality of protrusions 34 that corresponds to a plurality of the second probe conduits 32. Each second probe conduit 32 is formed in a respective protrusion 34. The protrusions 34 contain the second probe conduits 32. Each protrusion 34 protrudes radially and extends parallel to the probe axis 14. The protrusions 34 are azimuthally separated and elongate. In the examples of Figures 15 to 18, axes of the protrusions are evenly spaced azimuthally, by 90 degrees in the example of Figures 15 and 16 and by 120 degrees in the example of Figures 17 and 18. In the example of Figures 19 and 20, only two protrusions 34 are provided, with respective axes spaced by 90 degrees. In configurations having such protrusions 34, distal ends of the protrusions 34 define spaced apart sub-portions of the outer portion of the distal surface 30. In terms of the functionality provided by the second probe openings 22, the example of Figures 15 and 16 corresponds to that of Figures 6-8, the example of Figures 17 and 18 corresponds to that of Figure 13, and the example of Figures 19 and 20 corresponds to that of Figure 12. Figures 21 to 32 depict examples of a probe 10 according to a further class of embodiment. The probe 10 comprises an elongate body 50. The elongate body 50 extends along a probe axis 14. The elongate body 50 has a distal end that defines a concave distal surface 52. Figures 21-23, Figures 24-26, Figures 27-29, and Figures 30-32 respectively define four different example configurations. The elongate body 50 defines a first probe opening 21 and one or more second probe openings 22. At least the first probe opening 21 is defined in the concave distal surface 52. In the examples of Figures 21-23, Figures 24-26, and Figures 27-29, the one or more second probe openings 22 are also defined in the concave distal surface 52. In other arrangements, as exemplified in Figures 30-32, the one or more second probe openings 22 are defined in radial protrusions 34 instead of in the concave distal surface 52. The elongate body 50 defines a first probe conduit 31 configured to provide a fluidic connection between the first probe opening 21 and a first sensor 41. The elongate body 50 defines one or more second probe conduits 32 corresponding to the one or more second probe openings 22. Each second probe conduit 32 is configured to provide a fluidic connection between the second probe opening 22 corresponding to that second probe conduit 32 and a respective second sensor 42. Each of the first probe opening 21 and the one or more second probe openings 22 have an edge forming a closed loop defining an opening plane. The opening plane may, for example, be the plane that contains the largest portion or all of the closed loop. Normals to (i.e., vectors that are perpendicular to) at least two of the opening planes are aligned obliquely relatively to each other, as depicted schematically by the arrows adjacent to first and second probe openings 21, 22 in Figure 23. The opening plane corresponding to the first probe opening 31 may be substantially perpendicular to the probe axis 14. Thus, a normal vector (indicated by the arrow pointing away from the first probe opening 31 depicted in Figure 23) is parallel to the probe axis 14. The first probe opening 31 may thus operate in the same way as the first probe opening 31 of the pitot tube 12 described above with reference to Figures 6-20. The concave distal surface 52 may be configured such that an average angle 53 between a normal to the concave distal surface 52 and the probe axis 14 is in the range of about 60 degrees to about 80 degrees, optionally in the range of about 70 degrees to about 80 degrees, optionally about 75 degrees. This configuration has been found to provide a desirable balance between manufacturability, axial compactness, and performance. The average angle 53 may be averaged as a function of position over the surface area of the concave distal surface 52. The angle 53 may be substantially constant (as shown in Figures 21-26 and 30-32) or may vary as a function of axial position (as shown in Figures 27-29). Figures 27-29 depict an arrangement in which the at least a portion of the distal surface 52 is locally curved in directions within a plane passing through the probe axis 14. This may facilitate provision of second probe openings in the distal surface while maintaining a sharp angle at a leading edge of the elongate body. Thus, the curvature may be such that an average angle between normals to the distal surface and the probe axis decreases as a function of distance away from the leading edge of the elongate body. An axially leading edge 51 of the elongate body 50 (indicated in the side views of Figures 23, 26 and 29) defines a closed loop surrounding the probe axis 14. A radially outer surface of the elongate body 50 may comprise a portion having the same cross- sectional size and / or shape as the closed loop, from the axially leading edge 51 to at least a position along the probe axis 14 that is further from the axially leading edge 51 than the first probe opening 21. In some configurations, the elongate body 50 has rotational symmetry of at least order 2 about the probe axis 14. As exemplified in Figures 21-29, the closed loop may, for example, be circular and the portion of the radially outer surface having the same cross-sectional shape may be cylindrical. The first probe opening 21 may be aligned with the probe axis 14. In the examples shown in Figures 21-29 the elongate body 50 is cylindrical along the entire axial length of the elongate body 50, but this is not essential. Positioning the first probe opening 21 in the concave distal surface 52 inhibits oblique flow into the first probe opening 21, thus reducing losses in accuracy due to time averaging errors when measuring total pressure via the first probe opening 21. Positioning the first probe opening 21 in the concave distal surface 52 can thus have a similar effect to the axial offsetting of the first probe opening 21 relative to the shroud 18 described above with reference to Figures 6-8 and 10-20. The axially leading edge 51 of the elongate body 50 serves to axially recess the first opening 21, and thereby protect the first opening 21 from oblique flows, in the same way as the shroud 18 axially recesses and protects the first probe opening 21 of the pitot tube 10 in the examples of Figures 6-8 and 10-20. This functionality may be particularly effective where the first probe opening 21 in the distal surface 52 is aligned with the probe axis 14 and / or where the first probe opening 21 is positioned deeper into the concave distal surface 52 than any of the second probe openings 22. The second probe openings 22 may also be configured to operate in an analogous way to the second probe openings 22 described above with reference to Figures 6-8 and 10-20. Figures 30-32 depict a variation on the arrangement of Figures 21-23 in which the one or more second probe conduits 32 comprises a plurality of the second probe conduits 32 and the elongate body 50 comprises a corresponding plurality of azimuthally separated, elongate protrusions 34. The elongate protrusions 34 respectively contain the second probe conduits 32. Each protrusion 34 protrudes radially and extends parallel to the probe axis 14. The protrusions 34 may take any of the forms described above for the protrusions 34 with reference to Figures 15-20. The spatial compactness achievable by this approach is exemplified in Figure 34, which is an enlarged version of the side view of the example probe shown in Figure 26. The configuration depicted is an example of a class of embodiment in which the second probe conduit 32 (where only one second probe conduit 32 is present), or each of the second probe conduits 32 (where a plurality of second probe conduits 32 are present, as in the example depicted), for at least a portion 60 of a length of the second probe conduit 32 that starts from the respective second probe opening 22, has an average radial width D and is separated from the first probe conduit 31 by a distance ^^, where ^^< 2^. The radial width is a width of the second probe conduit 32 along the radial direction relative to the probe axis 14. Typically, the portion 60 of the second probe conduit 32 will be substantially cylindrical, in which case the radial width will be equal to the diameter of the circular cross-section of the second probe conduit 32. The average radial width (e.g., diameter) is averaged over the length of the portion 60 of the second probe conduit 32. Typically, the cross-section of the second probe conduit 32 will be constant for the portion 60 of the second probe conduit 32 (e.g., defining a cylinder), in which case the average radial width will be equal to the radial width at any point along the portion 60. Thus, the separation between the second probe conduit 32 and the first flow conduit 31 may be of a similar order of size as the diameter of the second probe conduit 32 (i.e., relatively small). Optionally, ^^< 1.9^, optionally ^^< 1.8^, optionally ^^< 1.7^, optionally ^^< 1.6^, optionally ^^< 1.5^, optionally ^^< 1.4^, optionally ^^< 1.3^, optionally ^^< 1.2^, optionally ^^< 1.1^, optionally ^^< 1.0^. In some implementations, as exemplified in Figure 33, the second probe conduit 32 is separated from a nearest radially outermost surface 62 of the shroud by a distance ^^, where ^^< 2^. Thus, the separation between the second probe conduit 32 and the radial exterior of the probe may be of a similar order of size as the diameter of the second probe conduit 32 (i.e., relatively small). Optionally, ^^< 1.9^, optionally ^^< 1.8^, optionally ^^< 1.7^, optionally ^^< 1.6^, optionally ^^< 1.5^, optionally ^^< 1.4^, optionally ^^< 1.3^, optionally ^^< 1.2^, optionally ^^< 1.1^, optionally ^^< 1.0^.

Claims

CLAIMS 1. A probe for measuring fluid flow, comprising: a pitot tube extending distally parallel to a probe axis and defining a first probe opening at a distal end of the pitot tube; and a shroud [JC1]radially surrounding and extending axially beyond the distal end of the pitot tube to inhibit oblique flow of fluid into the first probe opening, the shroud being spaced apart from the pitot tube to define a flow passage between the shroud and the pitot tube, wherein: the shroud comprises a distal surface defining one or more second probe openings, the distal surface having an outer portion facing at least partly in a radially outward direction and wherein the one or more second probe openings are at least partially defined in the outer portion.

2. The probe of claim 1, wherein the outer portion is oriented obliquely relative to the probe axis.

3. The probe of claim 1 or 2, wherein the pitot tube defines a first probe conduit configured to provide a fluidic connection between the first probe opening and a first sensor.

4. The probe of any preceding claim, wherein the shroud defines one or more second probe conduits corresponding to the one or more second probe openings, each second probe conduit being configured to provide a fluidic connection between the second probe opening corresponding to that second probe conduit and a respective second sensor.

5. The probe of any preceding claim, wherein the second probe conduit, or each of the second probe conduits, for at least a portion of a length of the second probe conduit that starts from the respective second probe opening, has an average radial width D and is separated from the flow passage by a distance ^^, where ^^< 2^.

6. The probe of claim 5, wherein the second probe conduit having the average radialwidth D, for at least the portion of the length of the second probe conduit that starts from the respective second probe opening, is separated from a nearest radially outermost surface of the shroud by a distance ^^, where ^^< 2^.

7. The probe of any preceding claim, wherein the one or more second probe openings are the only openings in the distal surface of the shroud. 8 The probe of any of claims 4-7, wherein the one or more second probe conduits comprises a plurality of the second probe conduits and the shroud comprises a corresponding plurality of azimuthally separated, elongate protrusions respectively containing the second probe conduits, each protrusion protruding radially and extending parallel to the probe axis.

9. The probe of claim 8, wherein distal ends of the protrusions define spaced apart sub-portions of the outer portion of the distal surface.

10. The probe of any preceding claim, wherein the shroud is tubular, optionally substantially cylindrical.

11. A probe for measuring fluid flow, comprising: an elongate body extending along a probe axis and having a distal end defining a concave distal surface, wherein: the elongate body defines a first probe opening and one or more second probe openings; the elongate body defines a first probe conduit configured to provide a fluidic connection between the first probe opening and a first sensor; the elongate body defines one or more second probe conduits corresponding to the one or more second probe openings, each second probe conduit being configured to provide a fluidic connection between the second probe opening corresponding to that second probe conduit and a respective second sensor; each of the first probe opening and the one or more second probe openings has an edge forming a closed loop defining an opening plane and normals to at least two of the opening planes are aligned obliquely; andat least the first probe opening is defined in the concave distal surface.

12. The probe of claim 11, wherein the second probe conduit, or each of the second probe conduits, for at least a portion of a length of the second probe conduit that starts from the respective second probe opening, has an average radial width D and is separated from the first probe conduit by a distance ^^, where ^^< 2^.

13. The probe of claim 12, wherein the second probe conduit having the average radial width D, for at least the portion of the second probe conduit that starts from the respective second probe opening, is separated from a nearest radially outermost surface of the elongate body by a distance ^^, where ^^< 2^.

14. The probe of any of claims 11-13, wherein the opening plane corresponding to the first probe opening is substantially perpendicular to the probe axis.

15. The probe of any of claims 11-14, wherein an axially leading edge of the elongate body defines a closed loop surrounding the probe axis.

16. The probe of claim 15, wherein a radially outer surface of the elongate body comprises a portion having the same cross-sectional size and / or shape as the closed loop from the axially leading edge to at least a position along the probe axis that is further from the axially leading edge than the first probe opening.

17. The probe of claim 16, wherein the closed loop is circular and the portion of the radially outer surface having the same cross-sectional size and / or shape is cylindrical.

18. The probe of any of claims 11 to 17, wherein the elongate body has rotational symmetry of at least order 2 about the probe axis and the first probe opening is aligned with the probe axis.

19. The probe of any of claims 11 to 18, wherein the one or more second probe openings are defined in the concave distal surface.

20. The probe of claim 19, wherein the first probe opening is positioned deeper into the concave distal surface than any of the second probe openings.

21. The probe of any of claims 11 to 18, wherein the one or more second probe conduits comprises a plurality of the second probe conduits and the elongate body comprises a corresponding plurality of azimuthally separated, elongate protrusions respectively containing the second probe conduits, each protrusion protruding radially and extending parallel to the probe axis.

22. The probe of any of claims 11 to 21, wherein an average angle between a normal to the concave distal surface and the probe axis is in the range of about 60 degrees to about 80 degrees.

23. The probe of any preceding claim, wherein two or more of the second probe openings are positioned at the same radius relative to the probe axis.

24. The probe of any preceding claim, wherein centres of two or more of the plurality of second probe openings are azimuthally separated by less than 180 degrees.

25. The probe of claim 24, wherein centres of two or more of the plurality of second ^^^ probe openings are azimuthally separated by substantiallydegrees, where ^ is an integer equal to 2 or higher.

26. The probe of any preceding claim, wherein the one or more second probe openings comprises one or more pairs of the second probe openings, each pair comprising second probe openings positioned azimuthally opposite to each other.

27. The probe of any preceding claim, wherein two or more of the second probe openings each have an edge forming a closed loop defining an opening plane.

28. The probe of claim 27, wherein an angle between the opening plane and the probe axis is the same for two or more of the second probe openings.

29. The probe of any preceding claim, wherein the one or more second probe openings comprises a plurality of second probe openings spaced apart evenly along a circular path surrounding the probe axis.

30. The probe of any preceding claim, wherein at least a portion of the distal surface is locally curved in directions within a plane passing through the probe axis.

31. The probe of any preceding claim, wherein at least a portion of the distal surface is locally straight directly adjacent to one or more of the second probe openings in directions within a plane passing through the probe axis, optionally defining a frustoconical surface.

32. The probe of any preceding claim, further comprising a third probe opening configured to measure a static pressure.

33. The probe of any preceding claim, wherein one or more of the second probe openings are positioned distally relative to the first probe opening.

34. A flow measuring system, comprising: the probe of any preceding claim; and a data processing system configured to receive input from a first sensor fluidically connected to the first probe opening and from one or more second sensors fluidically connected to the one or more second probe openings.

35. The flow measuring system of claim 34, wherein the data processing system is configured to use input from a first sensor to determine a total pressure.

36. The flow measuring system of claim 34 or 35, wherein the data processing system is configured to use input from the one or more second sensors to determine information about flow direction.

37. The flow measuring system of any of claims 34 to 36, wherein the probe further comprises a third probe opening fluidically connected via a third probe conduit to a third sensor, and the data processing system is configured to use input from the third sensor todetermine information about a static pressure.

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