Print Email Facebook Twitter Underwater Drag Reduction: Air Layer Stability over SuperHydrophobic Surface under Turbulent Conditions Title Underwater Drag Reduction: Air Layer Stability over SuperHydrophobic Surface under Turbulent Conditions: A thesis submitted for the degree of MSc Aerospace Engineering at TU Deflt Author Martinez de la Cruz, Roberto (TU Delft Aerospace Engineering) Degree granting institution Delft University of Technology Programme Aerospace Engineering Date 2019-07-25 Abstract Framed in the current trend of global energetic efficiency, active drag reduction techniques in water vehicles have regained popularity in the last decades thanks to the intense research in the air lubrication approach. Within the last decade, bubbly drag reduction has yielded its place to air layer drag reduction, in which a thin, continuous air layer is produced under the ship’s hull. Reductions in friction drag have shown to be between the 80 and 99% and net energy savings of around 8-12 % have been predicted, which is however probably not enough to compensate the complexity of the system. Nevertheless, by coating the lower part of the hull with a superhydrophobic coating, C. Peifer et al. (2019) showed that air flow requirements to obtain a stable air layer diminished by a factor of three. Thereportpresented here is a first study of the mechanisms that enhance the air layer stability when combined with a superhydrophobic surface. Water impact dynamics and free surface - turbulence interaction are studied experimentally and numerically respectively. Water behaviour upon impact has been investigated through an ascending jet impacting on the underside of four different surfaces with contact angles ranging from 45 to 150 degrees, obtaining the plate forces and top and side views of the water spread on the surface. Interesting results regarding the collapse of the spread area when a modified Weber number is used has been obtained. Two characteristic dewetting mechanisms for surfaces with low and high contact angle have been identified and explained by a simple theoretical model. Furthermore, possible independence of the friction coefficient on the surface wettability characteristic has been found. When plotted against the streamwise Reynolds number all surfaces collapse in the theoretical Blasius laminar friction coefficient, which suggests that once the surface is fully wetted (Wenzel state), the lower dimensional drag seen in (super)hydrophobic surfaces is due solely to their smaller contact area. To the best of our knowledge, these comprise the first experiments in which area and force were measured in a scenario in which viscosity, inertia, surface tension and gravity shape the water flow characteristics. On the other hand, a numerical model based on the classic elastic membrane concept has been employed to represent the deformations due to turbulence pounding in an air-water interface. Although further adjustments are needed, the model has been able to predict the tow tank experiments in C. Peifer et al. (2019) by using the surface-dependent water spread obtained in the water impact experiment. The dependence of the critical air flux to form a stable air layer on the surface wettability characteristics can therefore be predicted. Subject Superhydrophobic SurfacesAir Layer Drag ReductionDrag ReductionMultiphase Flow To reference this document use: http://resolver.tudelft.nl/uuid:b401d3a9-4800-4340-8c78-50a3fbae299b Part of collection Student theses Document type master thesis Rights © 2019 Roberto Martinez de la Cruz Files PDF RobertoMartinezdelaCruz_4 ... _Final.pdf 13.67 MB Close viewer /islandora/object/uuid:b401d3a9-4800-4340-8c78-50a3fbae299b/datastream/OBJ/view