Mechanical Engineering - Theses
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ItemAir-Sea Interaction: Influence of Airflow Separation and Small-Scale Waves on the Drag over Wind-Waves( 2023-05)The study aims to deepen the understanding of turbulent flows above waves, particularly investigating the interaction of wave structure to the airflow separation and its influence on the drag above wind-waves. A series of experimental studies were performed in this research to examine the behaviour of airflow separation above wind-waves, evaluate the important wave components and spectrum in relation to the wave boundary layer, and investigate the role of small-scale ripples on wave turbulence. The drag coefficient Cd is an essential parameter in air-sea momentum transfer. However, its relationship with wind speed U10 has remained scattered and uncertain for over 30 years. Multiple factors, such as wave steepness, wave breaking, gustiness of the wind, non-linear wind-wave interactions, and wave directionality, can affect Cd, leading to deviations from the predictions of climate models. Furthermore, recent field observations by Powell et al. (2003) and Jarosz et al. (2007) have documented drag coefficient saturation and reduction behaviour during hurricanes. The literature suggests that deviations from predicted drag coefficients, especially at high wind speeds, may be related to airflow separation. The airflow separation in the leeward side of wave crests is often observed and may be linked to the occurrence of small-scale waves. Surface tension has been found to cause the formation of ripples at the leeward side of crests, and the interaction of these small-scale ripples with larger waves is important due to their impact on radiation stress and viscosity. The small-scale waves in the gravity-capillary regime are the main contributor to ocean roughness and affect air momentum transfer. However, a systematic investigation into the influence of gravity-capillary waves on the atmospheric boundary layer was lacking until the Direct Numerical Study (DNS) was conducted by Druzhinin et al. (2019). Therefore, this thesis aims to fill a gap in the current understanding of the role of gravity-capillary waves in the atmospheric boundary layer, particularly in laboratory settings where relatively limited research has been conducted on this topic. The experimental investigations are divided into three main parts: Particle Image Velocimetry (PIV) measurement of airflow above wind-waves, highly resolved wave surface measurement in both spatial and temporal domains, and systematic PIV measurement of solid wavy-wall in a wind tunnel. The PIV measurements in the wave flume employed a Large Field of View (LFOV) setup designed to capture large-scale turbulent motions associated with surface-wave topography, such as airflow separation, while characterizing mean velocity, surface drag, and Reynolds stresses over wind-waves. Additionally, the High Magnification Field of View (HMFOV) setup was focused on investigating airflow separation closely at the leeward side of the crests, as well as other potential parameters affecting airflow separation and drag. This thesis also evaluated the idea of wall similarity on rough-wall boundary layers from Castro (2007) to the flow characteristics above wind-waves, which can be seen as moving roughness. Next is an experimental study that uses a laser-based technique to measure the temporally resolved surface elevation of wind-generated gravity-capillary waves. The aim is to study the structural and physical properties of the wind-wave field by capturing the fine details of waves, especially gravity-capillary waves, at different wind speeds and wave scales. This knowledge is critical to accurately reconstruct wind-waves as solid roughness. Surface elevations were obtained using a dynamic threshold algorithm, and the spatial and temporal spectra were analyzed and compared to the linear wave theory. The study also investigated decomposed waves of different scales, particularly small-scale waves. The PIV experiment at the wave flume showed some degree of similarity between wind-waves and solid walls in the fully-rough regime, which is in line with the findings of Geva and Shemer (2022) over wind waves. In addition, small-scale waves occurring on the leeward side of the dominant waves were found to be closely associated with airflow separation. The results from the laser surface experiment provided valuable insights into the structure of wind-waves and gravity-capillary waves, leading to final PIV experiments on turbulence flow above a solid wall in wind tunnel studies. The practicality of wind tunnel testing with a solid wall allowed for a systematic investigation of the relationship between selected parameters, in this case, gravity-capillary waves/ripples and airflow separation to drag. The wind-waves roughness was reconstructed based on the wind-waves generated in the wave flume, resulting in two types of solid wavy-wall: one with the dominant wavelength alone and the other with the addition of gravity-capillary waves. Our study shows that the small-scale waves influence the separation characteristics, which leads to contradictory behaviour between the two solid wavy-wall. The small-scale ripples play a crucial role in maintaining a momentum deficit region as the Reynolds number increases. This leads to a decrease in the drag coefficient with increasing wind speed and may explain the drop in sea drag above the ocean during extreme wind conditions. These findings emphasize the significant impact of small-scale waves on drag above wind-waves and highlight the critical role of airflow separation in air-sea interaction at certain wave ages.