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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Turb. Bound. Layers (Effects of multi-scale rough patches on hydrodynamics and scalar dispersion in turbulent boundary layers)

Teaser

Summary

The objective of this research is to experimentally investigate the effects of irregular multi-scale wall roughness on turbulent boundary layers. It is widely understood that wall roughness has a significant effect on turbulent boundary layers, however the majority of the existing research on this complex mechanism has been limited to homogeneous distributions of roughness covering the entire surface. Current models fall short for the majority of real-world flows that develop over surfaces with natural irregularities such as flows over ship hulls containing barnacles or atmospheric flows over cities or forests. The results of this project further the fundamental understanding of turbulent boundary layer flows, which have practical relevance to environmental, transportation, and energy engineering.

In this project, a range of experiments was carried out to investigate the flow around custom-manufactured patches of wall roughness placed within a turbulent boundary layer. Wall roughness elements were designed using fractal patterns in order to isolate and parameterize the multi-scale effects of the roughness geometry. Measurements were performed in a wind tunnel and in a water channel at the University of Southampton, using a force balance to directly measure the wall friction, particle image velocimetry to measure the local flow structure, and planar-laser-induced fluorescence to measure the local dispersion for a variety of different patch topologies.

Work performed

\"The project was divided in two main work packages, one focused on the measurements of the drag and flow structure around complex topologies in a wind tunnel and the second on dispersion measurements in a water tunnel.

For WP1, experiments were conducted in the 3\'x2\' boundary layer wind tunnel at the University of Southampton. To prepare for these experiments, multiple fractal-inspired rough patches were designed in a parametric way, in order to study the effects of frontal and planform solidity, then 3D printed. Calibration of the wind flow and the custom-built drag balance were also completed in advance of the wind tunnel testing. The experiments were then completed to measure the drag force and flow patterns over a set of roughness patches in order to determine the sensitivity of drag versus these select parameters. Measurements of the detailed flow structure were conducted using particle image velocimetry (PIV) using multiple 16MP cameras to capture a large range of scales that will be expected in these flows, in both a streamwise plane and a cross-section of the wake. This phase was completed in 2016.

For WP2, experiments were conducted in a water tunnel focussing on studying the effect of the roughness patches on scalar dispersion. In order to mount the roughness patches in the water tunnel a new false floor was designed and built, which also concealed a ground-level point source dye injector. Simultaneous measurements of the velocity and scalar field were performed using PIV and planar laser-induced fluorescence (PLIF). The objectives of these experiments were to compare the time-averaged properties of the scalar fluxes with existing models and to investigate how the scalar interacts with the wake of the roughness patch and whether it becomes entrained. This phase was completed in 2017.

So far these results have lead to the following publications, and we plan to continue analysing the rich data resulting from these experiments to produce further outputs:

- Vanderwel, C., Placidi, M., & Ganapathisubramani, B. (2017). Wind resource assessment in heterogeneous terrain. Philosophical Transactions of The Royal Society A: Mathematical, Physical and Engineering Sciences. DOI: 10.1098/rsta.2016.0109
- Vanderwel, C. and Ganapathisubramani, B. (2017). Turbulent boundary layers over multiscale rough patches\"\", 10th International Symposium on Turbulence and Shear Flow Phenomena TSFP10, Chicago, USA, July 6-9, 2017.
- another in preparation for Boundary-Layer Meteorology\"

Final results

The innovative aspects of this work included (1) studying the effect of multiscale rough patches on the drag and structure of a boundary layer, and (2) completing time-resolved simultaneous concentration and velocity measurements of the dispersion near a complex surface.

Results from this project were published in journals and presented at several conferences including the Turbulence and Shear Flow Phenomena Symposium in Melbourne (2015), the European Turbulence Consortium in Delft (2015), a workshop in Sheffield, UK (2015), the American Physical Society Fluid Mechanics Conference in Boston (2016), and the Turbulence and Shear Flow Phenomena Symposium in Chicago (2017). Presenting this study at the prestigious international Symposium on Turbulence and Shear Flow Phenomena in Chicago 2017 was one of the highlights that created significant impact. The talk was well-attended by experts in the field from around the globe. Networking at this event has contributed to developing a new joint NSF-EPSRC research project with Johns-Hopkins University in the US (where I am a Co-I). This has also lead to plans for collaborations with the group at the University of Sydney, Australia, to further study ground-level pollution and the results of WP2.

Through this project I have also been invited as a guest speaker at many universities including McGill University, Canada (2015), Imperial College London, UK (2015, 2016), Karlsruhe Institut Technolgie, Germany (2016), Sheffield University (2016), University of Surrey (2017), and as an expert in wind tunnel experiments for the opening event of a new wind tunnel at the Technology Institute of Eindhoven, Netherlands (2017). In addition to the scientific advances derived from this project, this fellowship also substantially supported my career development and I have since secured a permanent position as a Lecturer at the University of Southampton.