Abstract: Ultrasound imaging velocimetry (UIV) refers to the technique wherein ultrasound images are analysed with 2D cross-correlation techniques developed originally in the framework of particle image velocimetry. Applying UIV to opaque, particle-laden multiphase flows have long been considered to be an attractive prospect. In this study, we demonstrate how fundamental differences in acoustical imaging, as compared to optical imaging, manifest themselves in the 2D cross-correlation analysis. A chief point of departure from conventional particle image velocimetry is the strong variation in the intensity profile of the acoustic wavefield, primarily caused by the attenuation of ultrasonic waves in particle-laden flows. Attenuation necessitates using a larger ensemble of correlation planes to obtain satisfactory time-averaged velocity profiles. For a given combination of imaging and flow conditions, attenuation sets upper limits on volume fraction, penetration depth, as well as temporal resolutions that may be accessed confidently. This behaviour is demonstrated in two experimental datasets and is also supported by a modified cross-correlation theory. The modification is brought about by incorporating a lumped model of ultrasonic backscattering in suspensions into existing spatial cross-correlation analysis. The two experimental datasets correspond to two distinct particle-laden pipe flows: (1) a neutrally buoyant non-Brownian suspension in a laboratory-scale flow facility, wherein particle sizes are comparable to the ultrasonic wavelength and (2) a non-Newtonian slurry in an industrial-scale flow facility, wherein particle sizes are much smaller than the ultrasonic wavelength. We illustrate how and to what extent correlation averaging can counteract the adversity caused by attenuation. The work herein offers a template for one to evaluate the performance of UIV in particle-laden flows. Graphical abstract: [Figure not available: see fulltext.].

Abstract: We discuss empirical techniques to extract quantitative particle volume fraction profiles in particle-laden flows using an ultrasound transducer. A key step involves probing several uniform suspensions with varying bulk volume fractions from which two key volume fraction dependent calibration parameters are identified: the peak backscatter amplitude (acoustic energy backscattered by the initial layer of the suspension) and the amplitude attenuation rate (rate at which the acoustic energy decays with depth owing to scattering losses). These properties can then be used to reconstruct spatially varying particle volume fraction profiles. Such an empirical approach allows circumventing detailed theoretical models which characterize the interaction between ultrasound and suspensions, which are not universally applicable. We assess the reconstruction techniques via synthetic volume fraction profiles and a known particle-laden suspension immobilized in a gel. While qualitative trends can be easily picked up, the following factors compromise the quantitative accuracy: (1) initial reconstruction errors made in the near-wall regions can propagate and grow along the reconstruction direction, (2) multiple scattering can create artefacts which may affect the reconstruction, and (3) the accuracy of the reconstruction is very sensitive to the goodness of the calibration. Despite these issues, application of the technique to particle-laden pipe flows shows the presence of a core with reduced particle volume fractions in laminar flows, whose prominence reduces as the flow becomes turbulent. This observation is associated with inertia-induced radial migration of particles away from the pipe axis and is observed in flows with bulk volume fractions as high as 0.08. Even transitional flows with low levels of intermittency are not devoid of this depleted core. In conclusion, ultrasonic particle volume fraction profiling can play a key complementary role to ultrasound-based velocimetry in studying the internal features of particle-laden flows. Graphic abstract: [Figure not available: see fulltext.]

While laboratory experiments, numerical simulations as well as field tests have underlined the influence of noise barriers in dispersing vehicular emissions and reducing downwind peak concentrations, these pollutants still remain in the atmosphere. Artificial pollutant sinks (for example, particle capturing or toxic gas treating devices) installed on top of noise barriers can further alleviate this problem by eliminating the pollutants passing through it. However, it is not known how the installation of a semi-permeable pollutant sink affects the aerodynamics of the pollutants’ flow. By finding an optimal position and orientation for these sinks, the mass of the pollutants reaching the sink inlet can be maximized. Scaled down water tunnel experiments have been used to investigate the effectiveness of installing such a pollutant sink, of fixed dimensions, on top of a noise barrier adjacent to a highway. It is found that installing a sink is more beneficial on top of shorter barriers and that vertically elevating the sink, only slightly, can enhance its pollutant capturing performance. Using a sink in a ‘highway canyon’ (two noise barriers placed symmetrically with respect to the highway) must be done cautiously as there are several flow regimes observed, which are sensitive not only to the canyon aspect ratio (ratio between canyon width and height), but also to the presence/absence of the sink. The results here not only demonstrate the effectiveness of installing pollutant sinks on noise barriers, but also provide ballpark estimates on the optimal placement, orientation and performance of these devices, prior to field tests or even large-scale installation.