Roughness-induced Transient Growth

Roughness-induced Transient Growth
Author: Nicholas Allen Denissen
Publisher:
Total Pages:
Release: 2012
Genre:
ISBN:

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This dissertation analyzes the effect of periodic roughness elements on the stability of a flat plate boundary layer. Receptivity data is extracted from direct numerical simulations and experimental data and the results are compared to theoretical predictions. This analysis shows that flow in the immediate vicinity of roughness elements is non-linear; however, the evolution of roughness-induced perturbations is a linear phenomena. New techniques are developed to calculate receptivity information for cases where direct numerical simulations are not yet possible. Additionally, the stability behavior of the roughness wake is analyzed. New instability modes are found, and the effect of boundary layer complexity, perturbation amplitude and other factors are examined. It is shown that the wake is much less stable than optimal perturbation theory predicts, and highlights the importance of receptivity studies. The implication of these results on transition-to-turbulence is discussed, and future work is proposed.

Hypersonic Measurements of Roughness-Induced Transient Growth

Hypersonic Measurements of Roughness-Induced Transient Growth
Author: Nicole Susanne Sharp
Publisher:
Total Pages:
Release: 2015
Genre:
ISBN:

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The effects of surface roughness on boundary-layer disturbance growth and laminar-to-turbulent transition are not well understood, especially in hypersonic boundary layers. The transient growth mechanism that produces algebraic growth of streamwise streaks may play a key role in roughness-induced transition but has not previously been deliberately observed in hypersonic flow. To make such measurements, the present work studies the boundary layer of a 5° half-angle smooth cone paired with a slightly blunted nose tip and a ring of 18 periodically-spaced cube-like discrete roughness elements 1-mm tall by 1.78-mm wide by 1.78-mm long. The roughness element height is approximately equal to the boundary-layer thickness. Measurements are made in the low-disturbance Texas A&M Mach 6 Quiet Tunnel. No transition to turbulence is observed for freestream unit Reynolds numbers between 7.5 x 106 m−1 and 9.8 x 106 m−1. Pitot measurements reveal azimuthally-alternating high- and low-speed streaks growing downstream of the roughness. Large unsteadiness is measured in the roughness wake but decays downstream. The streamwise evolution of the steady and unsteady disturbance energy is consistent with low-speed observations of transient growth in the mid-wake region behind periodically-spaced cylindrical roughness elements. This experiment contains the first quantitative measurements of roughness-induced transient growth in a high-speed boundary layer. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152647

Advances In Computation, Modeling And Control Of Transitional And Turbulent Flows

Advances In Computation, Modeling And Control Of Transitional And Turbulent Flows
Author: Tapan K Sengupta
Publisher: World Scientific
Total Pages: 551
Release: 2015-12-01
Genre: Science
ISBN: 9814635170

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The role of high performance computing in current research on transitional and turbulent flows is undoubtedly very important. This review volume provides a good platform for leading experts and researchers in various fields of fluid mechanics dealing with transitional and turbulent flows to synergistically exchange ideas and present the state of the art in the fields.Contributed by eminent researchers, the book chapters feature keynote lectures, panel discussions and the best invited contributed papers.

IUTAM Symposium on One Hundred Years of Boundary Layer Research

IUTAM Symposium on One Hundred Years of Boundary Layer Research
Author: Hans-Joachim Heinemann
Publisher: Springer Science & Business Media
Total Pages: 507
Release: 2006-12-20
Genre: Technology & Engineering
ISBN: 1402041500

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This book collects peer-reviewed lectures of the IUTAM Symposium on the 100th anniversary of Boundary Layer research. No other reference of this calibre, on this topic, is likely to be published for the next decade. Covers classification, definition and mathematics of boundary layers; instability of boundary layers and transition; boundary layers control; turbulent boundary layers; numerical treatment and boundary layer modelling; special effects in boundary layers.

04-2527 - 04-2554

04-2527 - 04-2554
Author:
Publisher:
Total Pages: 442
Release: 2004
Genre: Fluid dynamics
ISBN:

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Distributed Roughness Receptivity in a Flat Plate Boundary Layer

Distributed Roughness Receptivity in a Flat Plate Boundary Layer
Author: Matthew Scott Kuester
Publisher:
Total Pages:
Release: 2015
Genre:
ISBN:

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Surface roughness can affect boundary layer transition by acting as a receptivity mechanism for transient growth. Several experiments have investigated transient growth created by discrete roughness elements; however, very few experiments have studied transient growth initiated by distributed surface roughness. Some of the work in this field predicts a "shielding" effect, where smaller distributed roughness displaces the boundary layer away from the wall and shields larger roughness peaks from the incoming boundary layer. This dissertation describes an experiment specifically designed to study the shielding effect. Three roughness configurations, a deterministic distributed roughness patch, a slanted rectangle, and the combination of the two, were manufactured using rapid prototyping and installed flush with the wall in a flat plate boundary layer. The main objective was to compare the wakes of the discrete roughness and the combined roughness to examine if the distributed roughness shields the discrete roughness. Naphthalene flow visualization and hotwire anemometry were used to characterize the boundary layer in the wakes of the different roughness configurations. For roughness Reynolds numbers (Re[subscript k]) between 113 and 230, the distributed roughness initiated small amplitude disturbances that underwent transient growth. The discrete roughness element created a pair of high- and low-speed streaks in the boundary layer at a sub-critical Reynolds number (Re[subscript k]) = 151). At a higher Reynolds number (Re[subscript k]) = 220), the discrete element created a turbulent wedge 15 boundary layer thicknesses downstream. When the distributed roughness was added around the discrete roughness, the wake amplitude decreased at the sub-critical Reynolds number, and transition was delayed by two boundary layer thicknesses at the higher Reynolds number. The distributed roughness redirects energy from longer spanwise wavelength modes to shorter spanwise wavelength modes. The presence of the distributed roughness also decreased the growth rate of secondary instabilities in the roughness wake. This dissertation documents the first detailed measurements of transient growth over streamwise-extended distributed roughness and demonstrates that the shielding effect has the potential to delay roughness-induced transition. The results from this experiment lay the ground work for future studies of roughness receptivity and transient growth. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152606