Error Sources in Wave-Based Remote Sensing and Free-Surface Synthetic Schlieren
Doctoral thesis
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https://hdl.handle.net/11250/3134904Utgivelsesdato
2024Metadata
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Sammendrag
Physical processes at and under the water surface of oceans play a key role in the Earth’s climate. For the models that are used to make predictions of temperature, gas contents, humidity, etc., we rely on data from laboratory experiments and field observations. This thesis presents numerical studies of error sources in two measurement techniques commonly used in the context of waves at the air-water interface. Findings include conditions under which results obtained from these methods become unreliable as well as mitigation strategies.
In laboratory studies, it is often desirable to obtain the surface topography in an area of interest. An appropriate technique in this context is the Free Surface Synthetic Schlieren (FS-SS) method. It utilizes the apparent distortions of a pattern seen through the surface due to refraction at the air-water interface, to infer the free-surface topography.
In the context of field observations of currents, remote sensing techniques are of appreciable interest due to their ability to cover large areas at low cost compared to in-situ methods. The second technique investigated in this work uses the wavenumber-frequency spectrum obtained from, e.g., airborne video footage, to measure Doppler shifts in the waves’ frequencies due to a background current, from which the current can be inferred.
Both methods rely on certain assumptions or approximations that may not always be applicable. In order to test their limits, numerical schemes were developed that create ideal input data, which, when analyzed with the respective method, allow the isolation of error sources.
Systematic errors in the measured surface gradients obtained using the FSSS method were investigated for two configurations. In the standard configuration, the camera is placed above the water surface and a pattern visualizing the distortions is placed underwater, typically at the bottom. In the “flipped”configuration, the camera is placed underneath the transparent water container and the pattern above the free surface. The errors found for the standard geometry stay within a few percent for typical setups, whereas the flipped configuration shows errors that can be much larger, exceeding 50%. With proper adjustment of the setup, these can be reduced to below 10%, retaining the usability of this configuration.
For the investigation of biases in the effective Doppler shift extraction from wave elevation data, a framework was developed to generate synthetic surface wave “videos”, where the background current can be both vertically and horizontally sheared. These were subsequently analyzed using the muchused Normalized Scalar Product method. The key findings of this part of the study include that both spectral leakage and horizontal shear can cause significant errors in the current measurements. Their severity was found to depend strongly on a range of parameters, most notably the wave spectrum shape in terms of angular spread and peakedness, and the current variation and direction. Knowledge of how the respective biases emerge enables the detection and mitigation of these errors, which are discussed in this thesis as well.
Består av
Paper 1: Bullee, Pim; Weichert, Stefan; Nore, Astrid; Li, Leon; Ellingsen, Simen Ådnøy, Hearst, R. Jason. The influence of grid-generated turbulent flows on the gas transfer rate across an air-water interface. This paper is submitted for publication and is therefore not included.Paper 2: Weichert, Stefan; Smeltzer, Benjamin Keeler; Ellingsen, Simen Ådnøy. Biases From Spectral Leakage in Remote Sensing of Near-Surface Currents. IEEE Transactions on Geoscience and Remote Sensing 2023 ;Volum 61. s. – Copyright © 2023 IEEE. Available at: http://dx.doi.org/10.1109/TGRS.2023.3321305
Paper 3: Weichert, Stefan; Smeltzer, Benjamin Keeler; Ellingsen, Simen Ådnøy. The effect of horizontal shear in extracting near-surface currents from wave data. This paper is submitted for publication and is therefore not included.