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F.T. Charruault
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On the stability of partial air cavities
Effect of surface wettability on drag reduction efficiency
This thesis investigates the dynamics, stability, and drag reduction performance of partial air cavities developing beneath flat plates in turbulent flows, with particular emphasis on the role of surface wettability. The main objective is to elucidate the mechanisms governing air leakages under turbulent flows and to determine whether hydrophobic coatings can improve cavity stability and their overall efficiency.
A combined experimental and modelling framework is developed using experiments performed at the cavitation tunnel of the Delft University of Technology. The performance of an existing non-intrusive force balance system is improved and validated, enabling relative drag measurements with an accuracy of approximately 1% at high Reynolds numbers. A Dot Tracking Algorithm for Free-Surface synthetic Schlieren measurements is developed, making the reconstruction of air cavity interfaces possible.
Cavities developing over hydrophobic coatings are found to be thinner and stable at lower cavitation numbers than cavities over the hydrophilic surface. For a given cavity length, they also produce slightly larger drag reductions. A semi-empirical model provides a physically consistent framework linking cavity shape, pressure, wettability, and Froude number. Hydrophobic coatings exhibit substantially lower leakage rates than the hydrophilic surface, with reductions of up to a factor of three at the highest investigated Froude numbers.
Three leakage regimes are identified: leakages mainly associated with re-entrant jets at relatively low Froude numbers, dominant wave-induced contact-spots at intermediate Froude numbers, and coexistence and interaction of both mechanisms at the highest Froude numbers. Cavity-pressure measurements reveal a transition from frequencies scaling with the characteristic gravity-wave frequency to those scaling with an outer-layer turbulent frequency.
Hydrophobic coatings improve cavity performance by reducing cavity thickness, lowering leakage rates, and decreasing the air flow rate required for cavity maintenance. The results contribute to the physical understanding of partial air cavities and provide guidance for the development of more efficient air lubrication systems for maritime transport. ...
A combined experimental and modelling framework is developed using experiments performed at the cavitation tunnel of the Delft University of Technology. The performance of an existing non-intrusive force balance system is improved and validated, enabling relative drag measurements with an accuracy of approximately 1% at high Reynolds numbers. A Dot Tracking Algorithm for Free-Surface synthetic Schlieren measurements is developed, making the reconstruction of air cavity interfaces possible.
Cavities developing over hydrophobic coatings are found to be thinner and stable at lower cavitation numbers than cavities over the hydrophilic surface. For a given cavity length, they also produce slightly larger drag reductions. A semi-empirical model provides a physically consistent framework linking cavity shape, pressure, wettability, and Froude number. Hydrophobic coatings exhibit substantially lower leakage rates than the hydrophilic surface, with reductions of up to a factor of three at the highest investigated Froude numbers.
Three leakage regimes are identified: leakages mainly associated with re-entrant jets at relatively low Froude numbers, dominant wave-induced contact-spots at intermediate Froude numbers, and coexistence and interaction of both mechanisms at the highest Froude numbers. Cavity-pressure measurements reveal a transition from frequencies scaling with the characteristic gravity-wave frequency to those scaling with an outer-layer turbulent frequency.
Hydrophobic coatings improve cavity performance by reducing cavity thickness, lowering leakage rates, and decreasing the air flow rate required for cavity maintenance. The results contribute to the physical understanding of partial air cavities and provide guidance for the development of more efficient air lubrication systems for maritime transport. ...
This thesis investigates the dynamics, stability, and drag reduction performance of partial air cavities developing beneath flat plates in turbulent flows, with particular emphasis on the role of surface wettability. The main objective is to elucidate the mechanisms governing air leakages under turbulent flows and to determine whether hydrophobic coatings can improve cavity stability and their overall efficiency.
A combined experimental and modelling framework is developed using experiments performed at the cavitation tunnel of the Delft University of Technology. The performance of an existing non-intrusive force balance system is improved and validated, enabling relative drag measurements with an accuracy of approximately 1% at high Reynolds numbers. A Dot Tracking Algorithm for Free-Surface synthetic Schlieren measurements is developed, making the reconstruction of air cavity interfaces possible.
Cavities developing over hydrophobic coatings are found to be thinner and stable at lower cavitation numbers than cavities over the hydrophilic surface. For a given cavity length, they also produce slightly larger drag reductions. A semi-empirical model provides a physically consistent framework linking cavity shape, pressure, wettability, and Froude number. Hydrophobic coatings exhibit substantially lower leakage rates than the hydrophilic surface, with reductions of up to a factor of three at the highest investigated Froude numbers.
Three leakage regimes are identified: leakages mainly associated with re-entrant jets at relatively low Froude numbers, dominant wave-induced contact-spots at intermediate Froude numbers, and coexistence and interaction of both mechanisms at the highest Froude numbers. Cavity-pressure measurements reveal a transition from frequencies scaling with the characteristic gravity-wave frequency to those scaling with an outer-layer turbulent frequency.
Hydrophobic coatings improve cavity performance by reducing cavity thickness, lowering leakage rates, and decreasing the air flow rate required for cavity maintenance. The results contribute to the physical understanding of partial air cavities and provide guidance for the development of more efficient air lubrication systems for maritime transport.
A combined experimental and modelling framework is developed using experiments performed at the cavitation tunnel of the Delft University of Technology. The performance of an existing non-intrusive force balance system is improved and validated, enabling relative drag measurements with an accuracy of approximately 1% at high Reynolds numbers. A Dot Tracking Algorithm for Free-Surface synthetic Schlieren measurements is developed, making the reconstruction of air cavity interfaces possible.
Cavities developing over hydrophobic coatings are found to be thinner and stable at lower cavitation numbers than cavities over the hydrophilic surface. For a given cavity length, they also produce slightly larger drag reductions. A semi-empirical model provides a physically consistent framework linking cavity shape, pressure, wettability, and Froude number. Hydrophobic coatings exhibit substantially lower leakage rates than the hydrophilic surface, with reductions of up to a factor of three at the highest investigated Froude numbers.
Three leakage regimes are identified: leakages mainly associated with re-entrant jets at relatively low Froude numbers, dominant wave-induced contact-spots at intermediate Froude numbers, and coexistence and interaction of both mechanisms at the highest Froude numbers. Cavity-pressure measurements reveal a transition from frequencies scaling with the characteristic gravity-wave frequency to those scaling with an outer-layer turbulent frequency.
Hydrophobic coatings improve cavity performance by reducing cavity thickness, lowering leakage rates, and decreasing the air flow rate required for cavity maintenance. The results contribute to the physical understanding of partial air cavities and provide guidance for the development of more efficient air lubrication systems for maritime transport.
The present study introduces an experimental technique based on a Free Surface-Synthetic Schlieren (FS-SS) method in order to characterize free surfaces subjected to strong deformations. Current synthetic Schlieren methods are based on local image correlation and thus limited to rather weak image deformations, implying that they can only resolve rather large surface wavelengths and limited wave amplitude. The present method is a substantial improvement that allows to measure much stronger image deformations, providing access to shorter surface wavelengths and larger amplitudes (i.e. larger surface curvatures).
...
The present study introduces an experimental technique based on a Free Surface-Synthetic Schlieren (FS-SS) method in order to characterize free surfaces subjected to strong deformations. Current synthetic Schlieren methods are based on local image correlation and thus limited to rather weak image deformations, implying that they can only resolve rather large surface wavelengths and limited wave amplitude. The present method is a substantial improvement that allows to measure much stronger image deformations, providing access to shorter surface wavelengths and larger amplitudes (i.e. larger surface curvatures).