Riming, the process whereby ice crystals get coated by impacting supercooled liquid droplets, is one of the dominant processes leading to precipitation in mixed-phase clouds. The present numerical study aims at providing insight on how turbulence affects the riming of ice crystals, which we model as very small, narrow oblate ellipsoids. By neglecting the effect of fluid inertia on the motion of the crystals and droplets, and using direct numerical simulations of the Navier–Stokes equations in a moderately turbulent regime, with a kinetic energy dissipation, ε, in the range 1 cm² s^-3 ≤  ε ≤ 256 cm² s^-3, we determine the collision rate between disk-shaped ice crystals and very small liquid water droplets. Whereas differential settling plays the dominant role in determining the collision rate at small turbulence intensity, the role of turbulence becomes more important as ε increases, an effect that can be partly attributed to the role of inertia. The difference in the settling velocity of crystals and droplets induces a strong asymmetry in the probability of collision between the faces of the ellipsoids. Collisions occur with a large probability on the rim of the ellipsoids, a phenomenon that can be explained by kinematic considerations.

Canopies such as seagrass meadows alter their environment by impacting the flow and turbulent structures. However, unlike well-studied homogenous canopies in which fully developed mixed layers develop, most aquatic canopies display patchiness, e.g. gaps/clearings that alter the flow. We aim to answer the question: how will gaps and patchiness affect the flow characteristics in a canopy system?

We conducted experiments in a recirculating flume with model vegetation. The canopy is interrupted by a spanwise homogeneous gap, and momentum and turbulent statistics were observed in the gap and in the wake. Observations suggest that there is a critical length of canopies over which the canopy will fully develop the flow (mean/turbulent), and downstream characteristics will be identical regardless of the upstream gap. But below this threshold, the turbulence downstream is affected by the upstream gaps. Changing the gap lengths brings about two major phenomena. First, it affects the decay of the mixing layer in the gap before it re-enters the downstream canopy segment and impacts local momentum fluxes. Second, depending on packing density of the fragments, we may observe canopy segments behaving independently of one another, interacting with one another, or essentially uninterrupted flow.

Understanding scalar dispersion in an urban canopy is crucial in predicting the spread of hazardous materials released in cities. We present analysis of velocity and scalar concentration data acquired using magnetic resonance imaging techniques in simple urban canopies comprising a regular array of cubical buildings with a single central tall building. Comparisons are made between two wind directions, either aligned with the streets or skewed at 45 degrees.  A dominant feature in both orientations is the separated near wake of the tall building. Contaminant is advected upwards in the tall building wake then turns downstream forming a plume above the cubical buildings. Streamtube analysis around the contaminant release area shows that turbulent dispersion dominates in the plume. Geometrical dispersion is much more important within the canopy of short buildings.  The contaminant dispersion is highly dependent on the size and distance between separation bubbles behind the short buildings as well as the orientation of the bulk flow relative to the building canyons.

Details of understanding non-linear time-periodic wall flows are of interest to engineering applications in biomedical and environmental flows. Non-linearity in wall-bounded time-periodic flows, due to higher frequency contribution to the velocity and/or acceleration in the outer layer, significantly alters the wall turbulence. The most striking example is the formation of a net current in the opposite direction of the wave propagation. Here, we present the results of direct numerical simulations of a flow over a smooth wall, driven by a Stokes Second-order Wave (SOSW). Channel flow driven by SOSW has a skewed velocity but symmetric acceleration in the outer region. This allows us investigate the role of velocity skewness to the wall turbulence. Our results suggest that the velocity skewness in the outer region leads to a skewness in the friction velocity and the viscous length scale. The difference of viscous length scale, between the crest and the trough of the wall shear stress, creates a wall-normal offset in Reynolds shear stress during the positive and negative flow. This gives a net Reynolds shear stress,  if integrated over a cycle,  and creates a net flow. How net flow characteristics changes with respect to Reynolds number is further discussed in this presentation.

As a result of recent redevelopment by construction of renewal buildings in Tokyo, many high-rise-building clusters tend to be distributed widely and sparsely in city. The roughness aspects of urban surface become heterogeneous. In order to examine wind impact on buildings and their surrounding spaces in view of wind resistance design or environmental evaluation, it is important to grasp the turbulent characteristics of approaching urban wind by clarifying coherent flow structures. This study applies LES based on BCM (Building Cube Method) to flow field for several 10km square area of Tokyo.  This numerical model is formulated on the very fine Cartesian mesh system utilizing the IBM. The relation between characteristics of velocity profile and information about roughness configuration of actual urban districts is provided on the basis of the LES results. Also, the observational data obtained by Doppler Lidar is used for the numerical validation. Then, focusing on the complicated geometry of urban surface, the unsteady characteristics of turbulence structures over actual urban canopy are analyzed and elucidated. Accordingly various kinds of coherent turbulent structures including unsteadiness and gust are provided based on their classification of generating mechanism.

Turbulent dynamics play an important role in nutrient and mass transfer in corals communities. At the colony scale, transport depends on the formation of the boundary layer at the branch-water interface. To understand the effects of the turbulent dynamics over an individual coral colony, three-dimensional simulations were performed by implementing the immersed boundary method in the large eddy simulation framework for Reynolds numbers between 5,000 and 20,000 for Pocillopora meandrina. We performed detailed calculations of the logarithmic mean velocity profile over the coral surface and quantified the vertical mixing and momentum transport by estimating the Reynold stresses above the top surface of the coral. Additionally, the turbulent kinetic energy budget for the flow was estimated in order to understand the balance of turbulence production and dissipation over the top surface of the colony. The calculation and estimation of these turbulence parameters for a single coral colony furthers our understanding of mass and energy transfer mechanisms in corals. This may help in designing more effective interventions for current challenges faced by corals, like ocean warming and bleaching events.