Physical Review Fluids

Recent efforts to study discrete large-amplitude gust encounters have often focused on one of three canonical problems: transverse, vortex, or streamwise gust encounters. A selection of this work is highlighted, concentrating on two-dimensional problems where the length scale of the gust flow is similar to the wing chord, and the amplitude of the gust flow is similar to the freestream. Current and outstanding questions are outlined for future work, including the role of low-order models and the limitations of using canonical problems to represent real-world gust encounters.

Wall-bounded turbulence is sometimes described as consisting of hierarchies of similar structures. This is embodied in the Attached Eddy Model, and at high Reynolds numbers this model predicts a logarithmic variation of the turbulence intensity with distance from the wall. The high Reynolds number experiments described here confirm this behavior, but they also reveal new aspects such as the slower than expected rise in the near-wall peak, and the unexpected appearance of an outer layer peak. Turbulence remains an elusive phenomenon.

Technical communication is a critical component of scientific life but is an uncomfortable area for many. Fortunately, integrating regular communication training and practice into a research lifestyle is not difficult. Activities, exercises, and practices to help both individuals and research groups improve their technical communication skills are presented.

A perspective highlights the recent progress in our understanding of how the interplay between viscous, capillary, and frictional forces shape the interactions of multiphase flow with deformable granular media, with particular attention to the central role of wettability. Beyond their intrinsic interest as processes that give rise to spectacular pattern formation, these coupled phenomena in granular media can control continental-scale fluxes, such as methane venting from the seafloor, and geohazards, such as earthquakes and landslides.

Advanced GPU-optimized algorithms coupled with a multiresolution short-time sampling paradigm can make extreme-scale turbulence simulations focused on small-scale physics much more accessible. One example is in the study of scaling exponents of the locally averaged energy dissipation rate at high Reynolds number.

Understanding how turbulence leads to the enhanced irreversible transport of heat and other scalars in density-stratified fluids is a fundamental research challenge in geophysical and environmental fluid dynamics, although there are still leading-order open questions. One useful approach to addressing these questions is to consider carefully chosen idealized flow geometries. It is then possible to analyze in detail the energetic pathways and the subtle interplay between various characteristic time and length scales. Such analyses can lead to insight into at least some of the apparently mysterious emergent properties of turbulent stratified mixing.

The influence of suspended particles on Taylor-Couette (TC) flow transitions is a new direction in the fluid mechanics of suspensions. The microstructure and properties of suspensions are reviewed to support discussion of this and other bulk behavior. Tracer particle tracks in a 10% solid suspension in TC
 flow in the circular Couette flow, spiral vortex flow, and Taylor vortex flow (left to right, respectively) show some of the flows examined.

Hydrodynamic interactions in multiphase flows result in rich multiscale physics, such as clustering and pseudoturbulence, with important practical implications. Alternative theoretical formulations that overcome current challenges, and particle-resolved direct numerical simulations to build accurate closure models for unclosed terms that arise in these statistical theories, are outlined as promising future research directions. The study provides a perspective on the importance of integrating theoretical, modeling, computational, and experimental efforts at different scales.

Turbulence deep within the interior of stars bears many qualitative similarities with turbulence in the ocean and atmosphere on Earth. In both contexts, various forms of convective instabilities and shear instabilities govern the transport of heat, momentum (angular momentum) and chemical species. However, an important distinction between stellar fluids and geophysical fluids is the value of the Prandtl number, which is of order unity in oceanic and atmospheric flows, and asymptotically small in stars. As a result, many well-known scaling laws for turbulent transport in geophysical flows must be revisited at low Prandtl number, as reviewed in this paper.