Control of large-scale flow phenomena

Flow over a shark denticle that can be used to obtain effective boundary conditions

The control of turbulent flows has the potential to significantly increase the efficiency of energy-related applications in process industries (e.g. pulp, paper), transport (ship, planes, pipelines) and energy sector (wind turbines, wave energy devices). One way to control the bulk flow in practical settings is via the texture of the solid surfaces. Moreover, in most applications, the bulk fluid flow needs to be manipulated to reach the best compromise of multiple objectives related to heat transfer, friction drag, pressure drag, wear and fouling.

Today, we lack versatile methods to engineer surfaces that are sufficiently complex to achieve multiple objectives. Therefore, one of our goals is to develop a framework for designing surface textures given set of criteria related to friction and heat transfer. We have built the core tools that translate a complex surface texture into a constitutive equation that can be applied to a smooth surface. This tool, combined with detailed computations and experiments, allows us to systematically characterize how complex anisotropic surface textures interact with turbulent flows.

Control of transport phenomena into porous media

Sketch of a gas over porous material for drying.

The interaction of free (unobstructed) flows with porous materials is another research area of ours. The understanding of how particles (ions, proteins, cells, nutritious) are transported from surrounding fluid into a porous medium is crucial in as diverse fields as groundwater contamination and tissue engineering. We are interested in multiphase/multicomponent transport between free fluids and porous media, which is relevant, for example, for describing drying process in materials.

More specifically, two related applications are bone tissue engineering and proton exchange membrane fuel cells. In the former,  the transport of ions, proteins into porous scaffolds is essential to stimulate cell growth and it is essential to characterize how the outside free flow carries molecules into the porous medium. In the latter, the drying and condensation processes inside materials are very sensitive to both the features of outside flow and to the pore geometry of the material. We want to determine the properties of porous materials and the features of outside flow to allow for efficient evaporation/condensation.

Control small-scale fluid-surface interaction

Numerical simulation of gas in a cavity entrapped under liquid.

We are also exploring – both computationally and experimentally – how anisotropic surface texture can be used to control dynamic wetting of surfaces as well as overlying particles suspended in the fluid. Yet another interest of ours is on textured surfaces infused with a lubricant (gas or liquid) that is different form the overlying flowing fluid.  To increase the stability of these surfaces, we need to understand how the three-phase (solid-lubricant-free fluid) contact line moves along the solid texture, which in turn demands new multi-scale models that combine atomistic and continuum description of complex surfaces.

Data-driven and modal techniques

In our research, we develop and use data-driven methods for analysing and controlling nonlinear fluid flows. These methods are rooted in statistical mechanics, control theory and dynamical systems. The method of Koopman mode decomposition (KMD) uses the spectral properties of the linear Koopman operator (related to Perron-Frobenius/Kolmogorov operators) to extract physically meaningful information from nonlinear systems. In our research, we want to understand the physical significance of KMD and the circumstances for which the method may be of practical use in fluid mechanics; for example, understanding how KMD relates to transient flow phenomena and noisy oscillatory flows. Our research also incorporates tools from control theory and dynamical systems  to analyze and control flows. In particular, our tools have been used for delaying the onset of transition to turbulence on a flat plate in both wind-tunnel tests and in-flight experiments.