ISBN 9783843958240

978-3-8439-5824-0, Reihe Ingenieurwissenschaften

Tristan Römer
Experimental investigation on boundary-layer streaks induced by free-stream turbulence and their influence on roughness-induced transition

132 Seiten, Dissertation Universität Stuttgart (2026), Softcover, A5

Zusammenfassung / Abstract

This thesis provides a systematic experimental investigation of how free-stream turbulence (FST) influences roughness-induced transition. Homogeneous FST is a fundamental prerequisite, in which roughness elements are subjected to ‘naturally’ meandering boundary-layer streaks.

Accordingly, the first part analyses the influence of turbulence-grid configurations on the induced FST and resulting Klebanoff modes in a laminar water channel. Unreported sensitivities are identified: grid parameters, orientation order, and minor grid imperfections can lock streak spacing to the mesh width or alter natural streak meandering. Consequently, key grid requirements are characterised to establish a ‘natural’ and reproducible FST environment for subsequent investigations.

Building on this validated FST environment, the second part examines how FST alters roughness-induced transition by exposing cylindrical elements with aspect ratios 𝜂 = 1, 2, 3 to three FST levels (Tu ≈ 0.05%, 1.15%, 1.55%). While FST consistently promotes earlier transition, its influence on the underlying instability mechanisms depends strongly on aspect ratio. For thick cylinders (𝜂 = 2, 3), the global instability remains remarkably robust: the dominant varicose mode shows virtually no sensitivity to FST. In contrast, the thin cylinder (𝜂 = 1) exhibits extreme sensitivity: moderate FST suppresses the sinuous mode predicted by global stability theory at supercritical Reynolds numbers.

These findings provide a clear, experimentally grounded explanation for why instabilities predicted by global stability theory often fail to materialise in real, turbulence-contaminated environments, establishing FST as a decisive factor governing how roughness-induced instabilities manifest.