Author ORCID Identifier

https://orcid.org/0000-0002-3457-3806

Date Available

4-11-2023

Year of Publication

2023

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Mechanical Engineering

Advisor

Sean C.C. Bailey

Abstract

This study presents two approaches to investigate the surface-layer structure during the morning transition using uncrewed aircraft systems. The first approach employs three uncrewed aircraft systems- each equipped with a single multi-hole probe- simultaneously measuring horizontal transects were partnered with a fourth measuring vertical profiles during two consecutive mornings as part of the 2017 Collaboration Leading Operational Unmanned Aerial System Development for Meteorology and Atmospheric Physics (CLOUDMAP) measurement campaign near Stillwater, Oklahoma, U.S.A. Data were analyzed to extract time-dependent single-point statistics of kinematic and thermodynamic variables from the uncrewed aircraft systems. In addition, an approach is presented by which multi-point spatial statistics in the form of auto- and cross-correlations could be calculated from the measurements. The second approach employs two fixed-wing uncrewed aircraft systems simultaneously flying horizontal transects with a third rotorcraft uncrewed aircraft system for vertical profiling during a limited deployment at the University of Kentucky North Research Farm (UKNRF), Lexington, U.S.A. The first fixed-wing aircraft is equipped with a custom-built multi-hole-probe-based vorticity probe. The configuration of the vorticity probe allows the estimation of the velocity and small-scale velocity gradients. These gradients are employed to estimate the dissipation rate and vorticity fields which can be used for identifying and characterizing the atmospheric boundary layer structure. The second fixed-wing aircraft is equipped with a single multi-hole probe used to resolve the advection velocity of coherent structures which can be used to approximate the streamwise spatial flow field using frozen Taylor’s hypothesis. The results from the first approach during CLOUDMAP campaign reflect differences in the evolution of spatial statistics with altitude for each of the two days at scales smaller than 500 m, despite very similar synoptic conditions. Conditional averaging was also applied to identify the structure of sweep and ejection motions and results revealed similarities to observations from canonical wall-bounded flow. Whereas, the results from the UKNRF campaign reveal the ability of the vorticity probe to estimate the dissipation rate flow field even though the spatial separation between the probes is much larger than Kolmogorov length scale. The mean dissipation rate estimated from the vorticity probe showed good agreement with the dissipation rate estimated based on Kolmogorov theory from the streamwise power spectrum. Additionally, the instantaneous dissipation rate flow field had signatures, represented by intermittent intense regions, associated with coherent structures which align with similar signatures in the flow fields of the vorticity and virtual temperature fluctuations. The observed structures have spatial length scales in the order of 200 m. Additionally, the spatial extent of the detected coherent structures, as well as the results from the conditional averaging of sweep and ejection events, imply that these regions could correspond to either low-momentum streaks, that evolve to form the legs of hairpin vortices, or the hairpin vortices legs. Finally, the observed average flow structure of different atmospheric quantities during the three flights has characteristics associated with coherent structures similar to what was observed previously in the atmospheric surface layer and in canonical wall-bounded flows.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2023.048

Funding Information

This study was supported by US National Science Foundation Award No. CNS-1932105.

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