OIST-Keio Showcase Talk Series Vol. 10: Diverse Approaches to Fluid Dynamics

This talk introduces various compact continuous reactors for transitioning chemical and food manufacturing from batch to continuous processes. We explore Oscillatory Baffled Reactors that enhance mixing via vortex generation and discuss liquid-liquid slug flow dynamics through experiments and CFD. Regarding food processing, we highlight static mixers achieving emulsification without moving parts, alongside an in-line rheological measurement technique specifically addressing the non-Newtonian behavior of many food fluids. These technologies offer versatile solutions for process intensification in modern continuous manufacturing, aiming for enhanced efficiency and consistent product quality.

Friction drag in turbulent flow is much larger than that of the laminar flow at the same Reynolds number, and its reduction is of crucial importance for improving the energy efficiency of industrial products and mitigating environmental burden. My talk will begin with our early attempts on feedback control for turbulent friction drag reduction, then I present our recent attempts on predetermined control using streamwise traveling waves. I will also briefly introduce our ongoing attempts for flow control design based on machine learning.

The development of micro/nanofluidics has led to a growing emphasis on the study of transport phenomena in 100-1000 nm spaces, where surface effects become dominant. However, fluid flows and mass transport in such small spaces are unknown due to the lack of measurement methods. Recently, we have developed a defocusing nanoparticle tracking velocimetry (nanoPTV) with a 10 nm spatial resolution for study on 100-1000 nm spaces. This talk introduces our recent results on the development of defocusing nanoPTV and its application to study on the behavior of nanoparticles suspended in a liquid flowing in micro/nanochannels.

We have released the first analysis dataset for the Venus atmosphere, in which horizontal winds derived by ultra-violet images observed by the Venus climate orbiter “Akatsuki” were assimilated. In the analysis, super-rotation and cold collar consistent with observations has been reproduced. We will introduce recent results of Venusian general circulation model (AFES-Venus; Atmospheric GCM for the Earth Simulator for Venus) and the data assimilation system for the Venus atmosphere (ALEDAS-V; AFES-LETKF (Local Ensemble Transform Kalman Filter) data assimilation system for Venus).

From a humble kitchen blender to the vast galactic disks, rotating turbulent flows exhibit remarkable diversity. They are also replete with puzzling features. A notable example is the energy spectrum of a widely studied rotating flow—the turbulent Taylor-Couette (TC) flow. Previous studies have shown that, unlike other canonical turbulent flows, it does not obey Kolmogorov’s universal power law or any other power law. Here, we report measurements of the energy spectra in turbulent TC flow from unique “flying-wire” experiments where a rotating probe sweeps through the flow. In contrast with previous studies, which focused primarily on spectral power laws, we analyze spectral data collapse. Through this broader approach, we show that, contrary to the prevailing understanding, the spectral structure of small scales in turbulent TC flow is in excellent accord with the potent paradigm of Kolmogorov’s small-scale universality.

References:
1. C. Butcher, et al, Okinawa Institute of Science and Technology – Taylor–Couette (OIST-TC): a new experimental set-up to study turbulent Taylor–Couette flow. Flow. 2024.
2. J. Barros, C. Butcher, and P. Chakraborty, Universality in the small scales of turbulent Taylor-Couette flow. Science Advances 2025.

Brenner proposed that, in compressible fluids, the velocity entering Newton’s law of viscosity and the no-slip condition should be the volume velocity rather than the mass velocity appearing in the continuity equation. The compatibility of such formulations with angular-momentum balance and the second law requires careful analysis.
Here, mass and linear momentum are allowed to be transported by distinct velocity fields. When angular-momentum balance and nonnegative entropy production are imposed, restrictions on the stress and energy flux follow. Under these conditions, Brenner’s equations reduce to the classical Navier–Stokes–Fourier system. The equations obtained here retain distinct mass and momentum velocities while satisfying the balance laws and thermodynamic constraints.
Shock-structure simulations for argon at Mach numbers between 1.55 and 9 show improved agreement with experiment at moderate Mach numbers and stable profiles without artificial diffusion in high-order discretizations.

Turbulent flows containing modest amounts of long-chained polymers have remained an intriguing area of research since the discovery of turbulent drag reduction. Here, we perform direct numerical simulations of statistically stationary, homogeneous, and isotropic turbulent flows of dilute solutions of polymers at various Reynolds and Deborah numbers. At large Re, we present evidence that there is a range of scales over which the energy spectra and the structure functions show new elastic scaling consistent with recent experimental results, while at small Re, we uncover an hidden intermittent behaviour.

Viscoelastic flows in microstructured microfluidic environments exhibit a hierarchy of instabilities arising from the coupling of elastic stresses with geometric heterogeneity. In this talk, I will highlight recent work from our unit on nonlinear flow dynamics in cross-slots, micropost arrays, and porous-media analogs (with extensions to canopy-like geometries), identifying routes from steady flow to symmetry breaking, self-sustained oscillations, synchronized collective motion, metachronal-like waves, and ultimately broadband chaotic states characteristic of elastic turbulence. Leveraging selective laser-induced etching (SLE) to fabricate robust 3D glass microfluidic devices capable of sustaining high flow rates and strong elastic stresses, and combining micro-particle image velocimetry (micro-PIV) with flow-induced birefringence (FIB) to resolve velocity and stress fields, we systematically vary confinement, obstacle spacing, and geometric disorder to reveal how structure reshapes instability thresholds and spatiotemporal coherence in viscoelastic microflows.