ORCID Number
0009-0001-4514-541X
Date of Award
Summer 2026
Embargo Period
9-1-2027
Access Type
Thesis - Open Access
Degree Name
Doctor of Philosophy in Aerospace Engineering
Department
Aerospace Engineering
Committee Chair
Ebenezer Gnanamanickam
Committee Chair Email
gnanamae@erau.edu
First Committee Member
R.R. Mankbadi
First Committee Member Email
mankbadr@erau.edu
Second Committee Member
Richard Prazenica
Second Committee Member Email
prazenir@erau.edu
Third Committee Member
Mark Ricklick
Third Committee Member Email
ridlickm@erau.edu
Fourth Committee Member
Birce Dikici
Fourth Committee Member Email
dikicib@erau.edu
College Dean
James W. Gregory
Abstract
This dissertation investigated the interaction between flexible micropillar arrays and a zero-pressure-gradient turbulent boundary layer in a boundary-layer facility at a nominal friction Reynolds number of Reτ ≈ 2400. The flexible roughness consisted of elastomeric micropillar arrays with systematically varied micropillar height, hp, aspect ratio, AR = hp/dp, Young's modulus, Ep, spanwise spacing, Δyp, and roughness density. The study first characterized the boundary-layer response over small micropillar arrays measuring 2δ × 2δ. It then examined the coupled velocity and micropillar response over selected arrays with different roughness densities using simultaneous velocity and micropillar-motion measurements. Finally, the sparsest configuration was extended to a long array with a streamwise fetch of approximately 11δ and a spanwise width of approximately 5δ.
The flow modifications produced by the small arrays were confined primarily to the near-wall region and exhibited two consistent features, a near-wall velocity deficit and an outward shift of the inner turbulence-intensity peak. The magnitude of this peak depended strongly on the micropillar parameters, particularly roughness density. Dense arrays increased the small-scale energy near the inner peak, whereas the sparsest array reduced it. An empirical parameter incorporating micropillar protrusion, frontal blockage, and roughness density provided a useful framework for organizing the measured near-wall spectral response. Simultaneous velocity and micropillar-motion measurements showed that the micropillar response was dominated by structural resonance. However, it also exhibited a coherent large-scale response associated with the flow, as evidenced by direct velocity and micropillar correlations and amplitude-modulation analysis. The presence of the micropillar arrays also altered the organization of near-wall large-scale events, with these effects becoming more pronounced as roughness density increased.
Information-theoretic analysis showed that the outer-region large-scale velocity provided the strongest and most consistent source of unique predictive information with respect to subsequent large-scale streamwise micropillar displacement. In the inverse direction, micropillar motion did not provide measurable unique predictive information about the future outer-region velocity. However, the large-scale component of the micropillar response did provide unique predictive information about the future large-scale velocity in the inner region, reflecting the relative position of the velocity sensor with respect to the micropillar. Increasing roughness density weakened these velocity and micropillar associations.
Over the long sparse array, the overall micropillar-displacement statistics changed only weakly with streamwise distance. In contrast, the velocity deficit became stronger and extended farther from the wall. More pronounced downstream evolution was observed in the event-conditioned and spatially distributed micropillar response. Correlations between the center micropillar and its spanwise neighbors decreased along the array fetch, particularly for the large-scale spanwise response. Likewise, response-signature analysis indicated that the event-conditioned streamwise response became increasingly distributed in the spanwise direction. These findings indicated that streamwise development over the long array was governed primarily by changes in the lateral organization of the large-scale forcing rather than by changes in the mean micropillar response.
Scholarly Commons Citation
Deshpande, Pratik Suhas, "Coupled Interactions between Flexible Roughness and Wall Turbulence" (2026). Doctoral Dissertations and Master's Theses. 1020.
https://commons.erau.edu/edt/1020
