Abstract
Recent studies have sought to use Microsoft Kinect sensors to measure water surface shape in steady flows or transient flow processes. They have typically employed a white colourant, usually titanium dioxide (TiO2), in order to make the surface opaque and visible to the infrared-based sensors. However, the ability of Kinect Version 1 (KV1) and Kinect Version 2 (KV2) sensors to measure the deformation of ostensibly smooth reflective surfaces has never been compared, with most previous studies using a V1 sensor with no justification. Furthermore, the TiO2 has so far been used liberally and indeterminately, with no consideration as to the type of TiO2 to use, the optimal proportion to use or the effect it may have on the very fluid properties being measured. This paper examines the use of anatase TiO2 with two generations of the Microsoft Kinect sensor. Assessing their performance for an ideal flat surface, it is shown that surface data obtained using the V2 sensor is substantially more reliable. Further, the minimum quantity of colourant to enable reliable surface recognition is discovered (0.01% by mass). A stability test shows that the colourant has a strong tendency to settle over time, meaning the fluid must remain well mixed, having serious implications for studies with low Reynolds number or transient processes such as dam breaks. Furthermore, the effect of TiO2 concentration on fluid properties is examined. It is shown that previous studies using concentrations in excess of 1% may have significantly affected the viscosity and surface tension, and thus the surface behaviour being measured. It is therefore recommended that future studies employ the V2 sensor with an anatase TiO2 concentration of 0.01%, and that the effects of TiO2 on the fluid properties are properly quantified before any TiO2-Kinect-derived dataset can be of practical use, for example, in validation of numerical models or in physical models of hydrodynamic processes.
Highlights
The dynamic pattern on the free surface of open channel flows varies according to the flow rate and boundary conditions
More recent work has clarified somewhat the link by showing that water surface fluctuations in shallow flows can be associated with the underlying velocity field and turbulence, which can in turn be related to the flow conditions, boundary conditions and hydraulic processes [2,3,4,5,6,7]
This study examined the use of titanium dioxide with two generations of the Microsoft Kinect sensor, Kinect Version 1 (KV1) and Kinect Version 2 (KV2), in order to evaluate their performance against an ideal flat surface
Summary
The dynamic pattern on the free surface of open channel flows varies according to the flow rate and boundary conditions. Previous research has found that turbulence generated near the bottom of a channel by the bursting phenomenon is transferred towards the water surface [1]. More recent work has clarified somewhat the link by showing that water surface fluctuations in shallow flows can be associated with the underlying velocity field and turbulence, which can in turn be related to the flow conditions, boundary conditions and hydraulic processes [2,3,4,5,6,7]. There is still a lack of detailed explanation regarding the link between free-surface features and the underlying flow conditions [1,8,9,10]. Investigating the relationship between the underlying flow and the free surface is crucial as it has the potential to enable remote, nonintrusive measurement of flow processes.
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