Ongoing research topics
Visualization of the change in vertical turbulent structure with increasing stratification (left to right).
Change in horizontal turbulent structure with stratification.
When the air is warmer than the underlying surface, loosely-formed stratified layers develop in the lowest portion of the atmosphere. While the flow is not entirely stable, it is less unstable owing to the stratification. These conditions are persistent during nighttime, throughout winter in polar regions, and during summer over oceans.
We are studying changes to the wind and air temperature in response to the stratification. The results have important implications for dispersion and mixing in applications ranging from air pollution to micrometeorology. Our efforts thus far have predominately used large-eddy simulations.
Sample study: Evidence of mixed scaling for mean profile similarity in the stable atmospheric surface layer
Visualization of the change in vertical turbulent structure with increasing convection (left to right).
Change in horizontal turbulent structure with convection.
In contrast to stable stratification, convection occurs during the daytime when the Earth’s surface is warmer than the air. In these conditions, buoyancy generates convective plumes and other vertical motions that have a significant impact on basic atmospheric processes like dispersion. We are using large-eddy simulations to improve upon traditional understandings of how the turbulent motions vary within different regions of the convective atmosphere, and in particular how the flow changes from the near-surface region to higher altitudes.
Sample study: On the departure from Monin-Obukhov surface similarity and transition to the convective mixed layer
Tree canopy in a farm (left) and a simplified representation in a wind tunnel (right).
Trees and crops create canopies that significantly alter the surface wind patterns, changing how heat and moisture are exchanged between the land and atmosphere. We are using wind tunnel experiments to better understand the impact of canopies on wind and how it can be more accurately represented in atmospheric models for weather and climate.
Sample study: Velocity asymmetry and turbulent transport closure in smooth- and rough-wall boundary layers
Sample video of turbulent eddies measured in the atmospheric boundary layer using snow.
There is still much to be learned about boundary layers before even considering the added complexity common to the atmosphere and other geophysical flows. Studying the simplified “canonical” turbulent boundary layer is a pet project of ours that we hope to maintain over many years.
Our research on this topic seeks to identify an organisation and pattern to coherent features (i.e. eddies) within the flow. The long-term goal of the research is to use these insights for improving how the effects of these eddies are estimated in simplified models of turbulent for practical applications in nature.
Sample study: Self-similar geometries within the inertial subrange of scales in boundary layer turbulence
Past research topics
Animation of 60 micron dust particles released above the surface of an increasingly steep hill.
You may be surprised to learn that a common source of phosphorus in the Amazon is dust originating from the Saharan Desert. While recent works have observed the transatlantic journey made by the dust, it is still a mystery how the dust is lifted from the Earth’s surface to the free atmosphere. In particular, global models underestimate the presence of coarse atmospheric dust.
We used large-eddy simulations and Lagrangian particle tracking to demonstrate that even a gentle hill can induce flow features which help the dust to avoid deposition and reach several hundred meters in altitude.
Sample study: Gentle topography increases vertical transport of coarse dust by orders of magnitude
Visualization of the change in vertical turbulent structure with increasing convection (left to right).
In a channel with flowing water such as a river, a swift current can carry sand, rocks, and other sediment downstream. This transport of transport determines how rivers slowly reshape over time and how nutrient-rich sediment is moved downstream to be deposited in deltas. We used laboratory flume and in situ river measurements to devise a model for how fast bedforms such as ripples and dunes move the sediment downstream.
Sample study: A mixed length scale model for migrating fluvial bedforms
Tree canopy in a farm (left) and a simplified representation in a wind tunnel (right).
Many road and highway signs have complex shapes which lead to similarly complex aerodynamic interactions with the wind. We helped to investigate the aerodynamic response under turbulent winds for a specific sign geometry used at rural intersections in the US. The study included laboratory experiments, field measurements, and numerical simulations. The weight distribution of the sign, the design of the structural supports, and resonance from vortex shedding and buffeting all contributed to a strong vibrational responses.
Sample study: Aerodynamics of highway sign structures: from laboratory tests and field monitoring to structural design guidelines
Lidar capturing measurements downwind of a utility-scale turbine.
In the wake of a wind turbine, a thin layer of air separates the wake core from the ambient flow outside the wake. This layer is strongly unstable, and the instability causes the wake to “meander” downwind of the turbine. The meandering leads to changing wind loads on downwind turbines in a wind farm that can fatigue the structure. We used a comparison of wind tunnel and field measurements to identify the signature of the meandering behavior.
Sample study: The spectral signature of wind turbine wake meandering: a wind tunnel and field-scale study