Numerical analysis of unsteady compressible flow in the hose of Bubble Curtain Technology

Abstract

The offshore wind industry is rapidly advancing, expected to play a crucial role in renewable energy production, with a growing emphasis on addressing associated environmental concerns, particularly the impact of underwater noise generated during the installation of wind turbines. In response, bubble curtain technology (BCT) has emerged as a potential solution. While the BCT operational concept has been demonstrated in practice, a detailed investigation related to fluid dynamics, geometric characteristics, and operational parameters has yet to be conducted. It's important to note that this need for investigation is specifically applicable to large BCT systems. This research aims to enhance the understanding of unsteady compressible flow inside BCT hoses and examine the impact of operational parameters on the pneumatic aspects of the bubble curtain system along the hose length.

This thesis systematically defined its fundamental objectives, beginning with the development of a 1D unsteady model for compressible flow in pipeline systems. This model, which served as the basis for subsequent studies, was verified against the existing literature, revealing a 2-4% difference in flow rates and pressure responses. Subsequently, the scope was broadened to include nozzle configurations, which were gradually included in the pipeline model. The model evolved through configurations with one, three, and five nozzles, where larger diameters increased dampening on transient pressure fluctuations. As the research progressed, the final model, incorporating five nozzles, was used as the basis for the scaled Bubble Curtain Technology (BCT) model.

A sensitivity analysis for this study was carried out utilizing parameters from existing research. The sensitivity study specifically emphasized the influence of geometric (hose and nozzle diameters, hose length, nozzle spacing) and operational factors (discharge coefficient, water depth, air flow rate) on the flow dynamic of Bubble Curtain Technology (BCT). The main findings from this research included reducing backflow with smaller diameters and lowering reverse flow with greater discharge coefficients and airflow rates. Changing the hose length and nozzle spacing proved effective for adjusting the required flow rates. The investigation also found that nozzle diameter and discharge coefficient had a considerable impact on nozzle flow rates, with a 2-3% increase over reference values at the maximum value range. Other geometric and operational parameters in the tested ranges had a relatively lower influence on the nozzle flow rates or generated pressure variations.

The scaled BCT unsteady compressible flow dynamics model presented in this thesis is still in its early stages of development, but it can serve as a basis for the development of full-scale pneumatic models that can enhance BCT and lessen the environmental impact of offshore wind farm operations.