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Title:Inner-outer interactions in a rough-wall turbulent boundary layer
Author(s):Pathikonda, Gokul
Director of Research:Christensen, Kenneth T
Doctoral Committee Chair(s):Christensen, Kenneth T
Doctoral Committee Member(s):Pantano-Rubino, Carlos A.; Vanka, Surya Pratap; Best, James
Department / Program:Mechanical Sci & Engineering
Discipline:Theoretical & Applied Mechanics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Boundary layers
Inner-outer interactions
Refractive index matching
Complex roughness
Abstract:The primary goal of the present effort is to enhance the current understanding of inner-outer interactions in rough-wall turbulent boundary layers. These interactions were recently established over smooth-wall turbulent boundary layers as modulating interactions, where the outer layer large scales amplitude and frequency modulate the near-wall small scales. Given that the outer layer dynamics responsible for these modulations are identical in most high-Reynolds-number (Re) rough-wall flows as well, similar modulation interactions are are explored to identify the similarities and differences of these interactions and establish a spatio-temporal description of the same. This is particularly important given the engineering significance of the flows over rough walls. This work was performed as two parts. In the first part, high temporal-resolution boundary layer hot-wire measurements were made in a wind tunnel, that fully resolved all dynamical scales temporally at fixed points in the flow. Flows over smooth and rough walls were investigated, with the latter being a complex topography indicative of a realistic roughness commonly encountered in engineering applications. Single- and two-probe measurements provided a dual perspective on the large scales, and enabled analysis of analytical techniques commonly employed. With these measurements, it was found that the nature of amplitude and frequency modulation occur even over this complex topography, and that the structure is very similar to that observed in smooth-wall flow. Further, the simultaneous two-probe measurements enabled the investigation of predictive models, which interestingly suggested a possibly stronger modulation in rough-wall flow compared to the smooth-wall case. A `quasi-steady, quasi-homogenous' theory previously developed for smooth-wall flow showed promising predictions of the calibrations constants even in rough-wall flow, lending additional support to the mechanisms speculating that the small scales respond in a quasi-steady manner, irrespective of the origin, to the large scales. With these inner-outer interactions established, the second part of the current work aimed to develop a spatio-temporal description of the modulating mechanisms using high frame-rate particle-image velocimetry (PIV). The experiments, performed in a refractive-index-matched flow facility, enabled the measurements very close to the surface without being impeded by the near-wall reflections that are common in smooth- and rough-wall PIV experiments. Following a preliminary demonstration of the relevant physics observed via point measurements, a representative large-scale structure was defined using conditional averaging. The associated changes to the small-scale turbulence close to the wall indicated similar modulation interactions, and provided a spatial tool to investigate the same. Further, the large-scale -small-scale interaction structure lended support to the speculations made on the same using hot-wire measurements in the current work and in the recent literature for smooth- and rough-wall flow. These experiments identify and emphasize the significance of inner-outer interactions over rough-wall flows, and the necessity to accurately model them to enhance the fidelity of any high Re simulations over rough walls.
Issue Date:2017-07-14
Rights Information:Copyright 2017 Gokul Pathikonda
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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