Boundary-layer turbulence

Efforts at RE-TE-G also go toward characterizing and quantifying the turbulent boundary layer developed over geophysical-scale topographies and heterogeneous canopies. Particular attention is placed on atmospheric boundary layers (ABL) developed over land and sea. Laboratory and field experiments are currently underway to tackle these fundamental problems.


Laboratory Research

Laboratory research takes place in both the Talbot wind tunnel and refractive-index matching (RIM) flumes. Experiments in both settings focus on wall turbulence under geometrically-organized and complex roughness using high spatial- and temporal-resolution planar, stereoscopic, and volumetric particle image velocimetry (PIV).

Flow over geophysical-like topographies

Large-scale roughness occupying a large portion of the boundary layer thickness governs many aspects of atmospheric flows over complex terrain and river flows. To gain insight into this problem, our lab studies the flow over simplified geophysical-like topographies given by 2D and 3D wavy wall. We study the developing and developed flow using high-spatial and temporal resolution PIV in refractive index matching channel.

A: Developed boundary layer flow over 2D and 3D wavy walls.

The turbulent boundary layer is allowed to develop over the roughness for a large length allowing it to approach the fully developed regime. We focus on the effects of roughness complexity (2D vs. 3D) and Reynolds number on the turbulence and its structure. Figure 1 shows a photograph of the flume and Figure 2 illustrates instantaneous and time-average velocity fields at two Reynolds numbers.

Figure 1: Setup of the geometrically-organized and complex roughness in the RIM facility.

Figure 2: Instantaneous and time-average velocity field over wavy walls from 2D PIV.

B: Developing turbulent boundary layer over 2D and 3D wavy walls.

To simulate the case of offshore winds entering mountainous or hilly terrains or river flows passing bed forms, we study the boundary layer development over large-scale roughness. Figure 3 shows a schematic of the experimental setup studying the development of the flow, the channel, and the wavy walls. An instantaneous velocity vector field and the modified ratio of sweeps to ejections are shown in figures 4.

Figure 3: Schematics of the experimental setup for the turbulence development over 2D and 3D wavy walls.

Figure 4: (Top) Instantaneous field within the first wavelength. The modified ratio of sweep to ejection events at the high Re in the developing region: (middle) 2D wall, (bottom) 3D wall.

Flow around compact elements

One of outstanding cases related to the flow over geophysical topographies is that from compact objects such as tabs, hills, single building, or roughness spots. We are performing systematic oratory particle image velocimetry in a refractive-index matching flume to uncover the flow and vortex dynamics around these structures. Efforts are particularly placed on quantifying the mixing and ways to control large-scale flow dynamics.

Figure 5: (top) basic experimental setup showing the 4 tabs; (bottom) 3D distribution of the streamwise, vertical and spanwise velocity components.

Figure 6: Primary structures induced by a trapezoidal tab; (left) streamwise counter-rotating vortex pair CVP; (right) instantaneous field showing a CVP and hairpin vortices.

Impact of Height Heterogeneity on Canopy Turbulence

The interaction of the flow with forests, urban canopies, and river vegetation governs many atmospheric and ecological processes. While the majority of previous efforts to understand this interaction have focused on the case of homogeneous canopies, we examine the effect of canopy height heterogeneity on the mean flow, turbulence statistics, and turbulence structure. Experimental investigation using high-resolution PIV and refractive-index-matching techniques allows us to obtain flow field measurements within the canopy. Figure 7 shows a schematic highlighting height heterogeneity, an example of a mean flow distribution, and instantaneous fields in homogeneous and heterogeneous canopies sharing the same density and bulk flow. Our results highlight the presence of topology-induced periodic behavior in the mean flow as well as an alternation of the obstructed shear layer formed on top of the canopy with enhanced turbulent stress and turbulent kinetic energy due to height heterogeneity. The penetration of the shear layer within the heterogeneous canopy was further enhanced in contrast to the homogeneous case as highlighted in the representative instantaneous fluctuation fields (shown in figure 7).

Figure 7: Flow over and within heterogeneous canopies. (Top) Schematics; (middle) sample mean flow; (bottom) instantaneous field comparison for homogeneous and heterogeneous canopies sharing the same density and bulk flow.

Field Research

A novel technique to quantify the atmospheric boundary-layer turbulence and scalars over complex topography is being developed. The technique utilizes coordinated fleets of unmanned aerial vehicles with advanced autonomous control and navigation systems (Fig. 8). This approach combines the high data quality of a meteorological tower with the flexibility of other deployable measurement techniques such as LiDAR and SODAR.

Figure 8: Schematic of the coordinated fleets of unmanned aerial vehicles (left) and a photograph of a unit during preliminary testing at UIUC.

 

Our research group aims at providing fundamental insights on the role of turbulence in basic and applied problems of high interest, which can be divided in the following sub-areas:

i) structure of the boundary layer over complex topographies;

ii) wind & hydrokinetic energy technologies,

iii) scalar transport over urban and natural environments,

iv) flow-structure interaction; and

v) instrumentation for turbulence measurements.

We have developed a comprehensive research on these topics that are going to be sustained and expanded in the future. Our versatile experimental approach combines a set of state-of-the-art experimental techniques, including particle image velocimetry (PIV), computer vision, and our recently developed 3D particle tracking velocimetry (PTV). This framework allows us to study fluid dynamics from Eulerian and Lagrangian frame of references

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