Session T45: General Fluid Dynamics: Drag Reduction and Multi-Physics Phenomena

Salinity reduces bubbly drag reduction

We investigate bubbly drag reduction in the context of air lubrication for maritime applications. To do so, we utilize our Twente Turbulent Taylor–Couette facility to study the effect of naturally-present salts in the ocean on the efficacy of bubbly drag reduction for Re = O(10^6). Whereas the drag reduction is up to 40% for 4% of air in fresh water, we find that the most common salts in the ocean dramatically mitigate the drag reduction. Using high-speed imaging, we find that the bubble size is reduced because bubble coalescence is inhibited due to the presence of the salt. The smaller bubbles have different Weber and Stokes numbers, becoming less deformable and more mobile, respectively, i.e., more tracer-like, reducing their effect on the flow.   

A Novel Sprayable Slippery Poylmer Coating for Drag Reduction

Reducing hydrodynamic friction drag on marine vehicles can lead to significant environmental and economic benefits. Lubricant-infused surfaces (LIS) and superhydrophobic surfaces (SHS) have received significant attention as promising candidates for achieving this goal. However, these conventional low-friction surfaces tend to lose their effectiveness when exposed to external forces, including flow-induced shear forces. This is because when LIS and SHS are subjected to high-speed flows, they can lose their entrapped lubricant and air plastron, respectively. Improving the robustness of entrapped fluids against external shearing flows is essential to achieve sustainable drag reduction in real turbulent environments.

In this study, we fabricated a scalable slippery low-friction surface containing lubricant-infused micro-spherical cavities using the spraying breath figure method. The unique surface topography effectively retains the infused lubricant even under harsh flow conditions. As a result, the new scalable slippery surface achieved a significant drag reduction rate in turbulent flow conditions, similar to those experienced by typical container ships cruising on the ocean.    

Analytical modeling of flow through circular geometries for optimizing superhydrophobic and liquid-infused surfaces

Superhydrophobic (SHS) and liquid-infused surfaces (LIS) have attracted significant interest for their potential to reduce drag and repel aqueous liquids, which is utilized in diverse engineering applications. However, understanding the intricate flow behaviour along such heterogenous surfaces is challenging.  Circular textured surfaces, particularly, hold significance. Typically, they are represented as axially traversed tubes or annuli, featuring no-slip walls scattered with rotationally symmetric finite-shear regions. These regions represent viscous interaction zones with a second fluid. We provided analytical equations that capture the flow field and effective slip length for such circular geometries. The applicability of these equations extends to Newtonian fluids with arbitrary viscosity ratios between the main and lubricating fluid.

Based on these analytical models, we develop design principles and guidelines for SHS and LIS to optimize sliding effects. By employing this approach, comprehensive assessments and efficient optimizations of slippery circular surfaces become possible, paving the way for enhanced energy efficiency. The research offers insights into significant energy savings and improved fluid transport performance, thus contributing to the advancement of sustainable fluid engineering systems.   

Influence of early crystal growth on frost formation.

Frost grows out of the phase change of water vapour when it desublimates into ice on a cold substrate. It naturally occurs in a very wide range of phenomena and weather conditions and potentially hinders manmade structures and means of transportation, it is therefore important to understand its governing mechanisms to prevent unwanted consequences. A complete understanding of frost growth is still lacking and knowledge on the early development of frost layers will help us understand their properties, such as the porosity and thermal conductivity. We therefore studied frost growth experimentally on cold substrates in the crystal growth phase where crystal seeds initiate the frost formation. The temporal evolution of crystal growth was monitored in function of substrate temperature and vapour supersaturation. We subsequently characterized its formation and compared to theoretical predictions.   

Large-Scale Turbulent Pressure Fluctuations Revealed by Ned Kahn's Artwork

The exhibits of the American environmental artist Ned Kahn incorporate the ephemeral beauty of nature such as wind and water flow. Amongst his artworks are the kinetic facades, which are composed of thousands of suspended aluminum panels, often covering the entire facade of buildings from parking to Museum. These hinged panels flap freely in the wind and form certain meso-scale dynamic patterns on the wall. We, as research scholars in fluid mechanics, got interested in the underlying physical mechanisms at play. Could we learn something about the wind turbulent fluctuations from simple visualization of Ned Kahn’s realisations?

To do so, we implemented a laboratory scale model composed of a one-dimensional chain of pendulums immersed in a wind tunnel. Using Fourier analysis of the pendulum motions, we showed that the advection speed of turbulent pressure fluctuations can be revealed, thanks to a mechanism similar to a wind-induced liquid surface deformation below the wave onset first introduced by Phillips in 1957.   

On the distinct drag and wake of flexible plates with a single perforation

The effect of single perforations and their location on the drag and wake of flexible plates was explored through wind tunnel experiments and numerical simulations. Plates were subjected to uniform flows with low turbulence, and a fixed-size square perforation was placed at various locations along the plate center, resulting in a low porosity ratio (γ ≈ 0.028). A high-frequency, high-resolution load cell captured instantaneous aerodynamic force experienced by the plates, while particle tracking velocimetry (PTV) and particle image velocimetry (PIV) measured plate deformation and wake flow for different Reynolds and Cauchy numbers. PIV results showed that perforated plate wakes exhibited distinct jet-like structures through the square perforation, significantly affecting aerodynamic forces and plate deformation. The jet centerline velocity distribution with respect to downwind distance was influenced by the incoming flow and perforation location. These velocity profiles normalized using effective incoming velocity and corrected perforation half-width revealed their dependence on these factors. A simple first-order formulation was developed to predict the change in drag for flexible plates with various perforation locations and a wide range of incoming velocities. Numerical simulations across a broader range of the Cauchy number supported this formulation, confirming the proposed incoming effective velocities model and separating the effects of Cauchy and Reynolds numbers. These findings may inform the design of flexible structures, defining effective porosity, and establishing a foundation for modeling the complex interaction between flow and low-porosity structures.