Changes between Version 265 and Version 266 of WikiStart


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Timestamp:
Feb 10, 2017, 1:20:37 PM (4 years ago)
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peakall3je
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    7070Two ultrasonic systems are used to measure velocity.
    7171
    72 '''Ultrasonic Velocimetry Profiling (UVP)''' Ultrasonic velocimetry profiling (UVP) is a technique that measures a single component of velocity at up to several hundred points along a line. A transducer sends out an ultrasonic pulse, and then gates the return signal into a series of spatial bins. The individual transducers can be multiplexed in order to provide pseudo-velocity fields. The transducers are linked via a multiplexer with a delay of 15 ms, so the two-dimensional velocity field is not instantaneous, however velocity fields can be collected at 3-4 Hz. A UVP-duo was used, with two different frequencies of transducers. An array of ten 4 MHz UVP probes (with 10 m long cables) was used for collecting downstream velocity profiles. Initial test experiments used probes positioned at heights of (centre point of each probe): 7, 16, 26, 56, 86, 116, 146, 176, 206 and 236 mm. The actual experiments used probes are positioned at heights (centre point of each probe) of 10, 25, 50, 75, 100, 150, 200, 250, 350 and 450 mm from the base of the channel. These probes are positioned in a custom made plastic holder, in turn connected to a bar strapped to the channel top. Initially, the probes are positioned on the channel centreline, 80 mm downstream of the apex of bend 2, looking upstream. An array of ten 2 MHz UVP probes (with 4 m long cables) is used to examine the nature of secondary flow at the second bend apex. This involves drilling holes in the apex of bend 2 and inserting the UVP probes. The probes are positioned at heights of 45, 90, 135, 180, 225, 270, 315, 360, 404, and 450 mm from the base. 2 MHz probes are required for the cross-section measurements since the measurement range needs to be much larger (60 cm) than is required for the axial velocity measurements.
     72'''Ultrasonic Velocimetry Profiling (UVP)''' Ultrasonic velocimetry profiling (UVP) is a technique that measures a single component of velocity at up to several hundred points along a line. A transducer sends out an ultrasonic pulse, and then gates the return signal into a series of spatial bins. The individual transducers can be multiplexed in order to provide pseudo-velocity fields. The transducers are linked via a multiplexer with a delay of 15 ms, so the two-dimensional velocity field is not instantaneous, however velocity fields can be collected at 3-4 Hz. A UVP-duo was used, with two different frequencies of transducers. An array of ten 4 MHz UVP probes (with 10 m long cables) was used for collecting downstream velocity profiles. Initial test experiments used probes positioned at heights of (centre point of each probe): 7, 16, 26, 56, 86, 116, 146, 176, 206 and 236 mm. The actual experiments used probes positioned at heights (centre point of each probe) of 10, 25, 50, 75, 100, 150, 200, 250, 350 and 450 mm from the base of the channel. These probes are positioned in a custom made plastic holder, in turn connected to a bar strapped to the channel top. Initially, the probes are positioned on the channel centreline, 80 mm downstream of the apex of bend 2, looking upstream. An array of ten 2 MHz UVP probes (with 4 m long cables) is used to examine the nature of secondary flow at the second bend apex. This involves drilling holes in the apex of bend 2 and inserting the UVP probes. The probes are positioned at heights of 45, 90, 135, 180, 225, 270, 315, 360, 404, and 450 mm from the base. 2 MHz probes are required for the cross-section measurements since the measurement range needs to be much larger (60 cm) than is required for the axial velocity measurements.
    7373
    7474'''Profiling Acoustic Doppler Velocimetry (ADV)''' Early experiments used 3 co-mounted Nortek Vectrino Profiler Acoustic Doppler Velocimetry (ADV) probes ADVs for mapping three-dimensional flow velocities, whilst later experiments including the main experiments used 2 ADVs to minimize side lobe interference, and because of the loss of one probe. The ADVs were run at 100Hz in all experiments, both in the initial tests and in the actual experiments. The ADV probes measure three component velocities over a depth range of around 30 mm (up to 34 mm), with this zone starting 40 mm below the probe head. Bottom tracking by the instruments enables this depth to be precisely known and controlled, provided sufficient seeding is present in the flow. ADV collection was fully synchronised with the traverse through sending of the required voltage offset signal to them, thus enabling individual velocity files to be collected for each probe at each traverse. Movement of these probes on the traverse in the y and z planes allows detailed vertical profiles and flow mapping to be undertaken. Initial test experiments used the 3 probes, 1 was a stem probe with additional shielding, and 2 were flexible probes. The basal probe was numbered 1, middle 2, and upper probe 3, and were at heights of 7.2, 10 and 13 cm respectively. Lateral offset in the x-direction was 8.5 cm between probes 1 (most downstream probe) and 2, and 7.5 cm between probes 2 and 3 (most upstream). The traverse positions for each cross-section were based on probe 2 being positioned directly above the cross-section. These 3 probes were co-mounted on the traverse in order to collectively measure over a height of approximately 6 cm. ADV dwell times varied between 30 and 60 seconds.