Turbulent boundary layers under complex spatial and temporal pressure gradient histories

A flat-plate turbulent boundary layer (TBL) is experimentally subjected to steady and unsteady favorable-adverse pressure gradients (FAPGs). FAPG is defined in this work as a sequence of streamwise pressure gradients in the order of favorable followed by adverse, similar in sequence to the pressure gradients over the suction side of an airfoil. The pressure gradient imposition on the flat plate is accomplished through a wind tunnel mechanism that deforms a false ceiling section into an inverted convex bump. The ceiling is statically deformed at different curvatures to generate a family of steady FAPGs and is dynamically deformed at different temporal rates to generate unsteady FAPGs of different timescales. In the strongest FAPG case, the acceleration parameter, $K$, varies spatially from 6 \times 10^-6 to -4.8 \times 10^-6. For the fastest FAPG imposition, the reduced frequency, $k$ = 4.38. The adverse pressure gradient (APG) region of this configuration is the focus of this thesis, where the TBL’s response is recorded using particle image velocimetry in the streamwise—wall-normal plane. The flow statistics and the organization and dynamics of turbulent structures are analyzed. The steady cases reveal that the boundary layer's response to the APG is significantly altered by its memory of the upstream favorable pressure gradient (FPG). For 15 of the 22 cases studied, an internal boundary layer forms within the TBL. This layer typically occupies $20\%$ of the boundary layer thickness and dominates the flow physics. The peaks in turbulent production, strength and population of vortices, and spectral energy content of the flow are contained within the layer. The outer layer, on the other hand, develops in the APG region without considerable changes to the state dictated by the upstream FPG. These trends are in striking contrast to APG TBLs that originate from a zero pressure gradient (ZPG) region, where the outer/wake region is known to dominate TBL response. The observed changes are quantified across the family of FAPGs studied. The implications of the fundamentally different APG effects are discussed, and interesting questions are laid out to guide future work. The effects of unsteadiness on the TBL's response to the spatial FAPGs are studied for the case where $k = 4.38$. The time-varying TBL is compared to the 22 steady TBLs noted above, at discrete time instances where the FAPG strengths (or the ceiling curvatures) instantaneously match. Significant non-quasi-steady effects are observed in the statistics, the organization of vortices, the turbulent spectra, and space-time proper orthogonal decomposition (ST-POD) structures. The coupled FPG-APG effects exhibited by the steady TBLs appear less coupled when the same FAPG magnitudes are applied rapidly in time. In particular, the upstream FPG effect manifests more mildly and the APG effect is realized more strongly. It is suggested, with supporting arguments, that the unsteadiness embeds different spatio-temporal pressure gradient histories in the turbulent structures, leading to the non-quasi-steady effects observed. In studying the ST-POD structures, a new method is developed to quantify spatio-temporal changes underwent by structures in a TBL.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Aerospace Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Sundarajan Parthasarathy, Aadhy

Contributors dc:contributor

• Saxton-Fox, Theresa
• Chamorro, Leonardo
• Villafane Roca, Laura
• Ansell, Phillip

• Turbulent Boundary Layers
• Pressure Gradients
• Unsteadiness
• History Effects

Statement dc:rights

• Copyright 2023 Aadhy Sundarajan Parthasarathy

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/121485

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F121485