Author ORCID Identifier

0009-0006-5883-9986

Biosketch

Dr. Ganesh K. T. earned his PhD in 2025 from School of Mechanical Engineering, SASTRA Deemed to be University, where he pursued his experimental study on how surface roughness influences the instability and transitional characteristics of laminar, separated shear layer. He is currently working as a Postdoctoral Fellow in Wind Aerodynamics Lab, Amrita Vishwa Vidyapeetham, continuing his research in computational and experimental aerodynamic studies. Dr. Ganesh is skilled in working on both experimental and computational aerodynamic projects. To be mentioned, he has developed instrumentation such as 3D automated traverse setup, optics-based roughness measuring device, and economical hotwire soldering unit. In addition, he has also developed incompressible CFD solver and multiblock grid generator code using Fortran-90 programming language. Dr. Ganesh has a diverse professional background, having worked at top Indian institutions like IIT Kanpur and IIT Madras. He also spent over two years teaching as an Assistant Professor. His research is well-regarded in the field, with several papers published in international journals such as MDPI Symmetry and the Proceedings of the iMechE (Aerospace & Mechanical Engineering). Throughout his career, he has focused on understanding complex flow patterns, making him a versatile expert in both aerospace theory and practical engineering.

Date of Award

17-8-2025

Document Type

Thesis

School

School of Mechanical Engineering

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

Dr.K.Anand

Keywords

Separated Shear Layer, Laminar-to-Turbulent Transition, Boundary Layer Measurements, Flow Instability

Abstract

The present experimental work investigates the effects of irregularly roughened surfaces and imposed streamwise pressure gradients on the topological, transitional, and instability characteristics of the laminar separated shear layer (SSL). Experiments are conducted in the low-speed wind tunnel for 𝑅𝑒𝑑=30000, based on the thickness of the airfoil model, t, and freestream turbulence intensity, fst = 0.8%. The airfoil model used in this study features a semicircular leading edge followed by a constant-thickness flat portion and a pitchable trailing-edge flap. Test conditions include two rough surfaces: sandblasted (SB) and sand-deposited (SD), and flap deflections: 𝛽=βˆ’30Β° and 𝛽=+30Β°, imposing adverse (APG) and favorable (FPG) pressure gradients on the model surface, respectively. The base case is a smooth (S) surface with 𝛽=0Β° zero-imposed pressure gradient (ZPG). Surface pressure distribution and streamwise velocity data are acquired along the midspan of the model using electronic pressure scanner and single-wire hotwire probe coupled with the constant temperature anemometer, respectively. xv Results indicate that the time-mean laminar separation bubble and SSL are significantly affected by SD roughness and imposed APG, where the former produces 31% short bubble and the latter yields 51% thinner bubble, compared to the base case. Either of the rough surfaces drag the mean trajectory of the core vorticity close to the wall, acting like a passive flow control device. Imposing APG significantly decelerates the evolution of SSL and the subsequent transition is delayed, resulting in longer bubble compared to the base case. Furthermore, the former causes the SSL to evolve close to the wall, leading to enhanced damping effects due to the wall. On the contrary, and vice-versa of APG cases is valid when FPG is imposed on the evolving SSL. Regardless of the modifications imposed, the SSL is convectively unstable exhibiting exponential disturbance amplification in linear instability regime, followed by shear layer roll-up and nonlinear breakdown to turbulence near reattachment. Active shear layer identified based on the magnitude of spanwise vorticity reveals Kelvin cat’s eye pattern, implying the 2D disturbance waves in convectively unstable shear layer. For all test cases investigated at 𝑅𝑒𝑑=0.3Γ—105 and fst = 0.8%, the disturbances are amplified primarily due to the Kelvin-Helmholtz-like inviscid instability mechanism. In addition, instability due to viscous Tollmien-Schlichting waves is observed in the SSL formed over the SB rough surface, regardless of the pressure gradients imposed. xvi The present research contributes to the fundamental understanding of time-mean transitional behavior in separated-reattaching flows influenced by three-dimensional irregular rough surfaces. The insights from this study may help characterize the instability and transition mechanisms in separated-reattaching flows over deformed leading edges of in-service turbine blades.

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