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"Atkin, Chris"
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Acoustic excitation of Tollmien–Schlichting waves due to localised surface roughness
Experiments on the receptivity of two-dimensional boundary layers to acoustic disturbances from two-dimensional roughness strips were performed in a low-turbulence wind tunnel on a flat plate model. The free stream was subjected to a plane acoustic wave so that a Stokes layer (SL) was created on the plate, thus generating a Tollmien–Schlichting (T–S) wave through the receptivity process. An improved technique to measure the T–S component is described based on a retracting two-dimensional roughness, which allowed for phase-locked measurements at the acoustic wave frequency to be made. This improved technique enables both protuberances and cavities to be explored in the range $30~\\unicode[STIX]{x03BC}\\text{m}<|h|<750~\\unicode[STIX]{x03BC}\\text{m}$ (equivalent to $0.025<|h|/\\unicode[STIX]{x1D6FF}_{B}^{\\ast }<0.630$ in relative roughness height to the local unperturbed Blasius boundary layer displacement thickness). These depths are designed to cover both the predicted linear and nonlinear response of the T–S excitation. Experimentally, cavities had not previously been explored. Results show that a linear regime is identifiable for both positive and negative roughness heights up to ${\\approx}150~\\unicode[STIX]{x03BC}\\text{m}$ ($|h|/\\unicode[STIX]{x1D6FF}_{B}^{\\ast }\\approx 0.126$). The departure from the linear behaviour is, however, dependent on the geometry of the surface imperfection. For cavities of significant depth, the nonlinear behaviour is found to be milder than in the case of protuberances – this is attributed to the flow physics in the near field of the surface features. Nonetheless, results for positive heights agree well with previous theoretical work which predicted a linear disturbance response for small-height perturbations.
Journal Article
Aerofoil wake-induced transition characteristics on a flat-plate boundary layer
This paper presents an experimental investigation of the characteristics of laminar– turbulent transition occurring on a flat-plate boundary layer due to the interaction with a non-impinging aerofoil wake. Previous studies have tended to focus on transition induced by free-stream turbulence or by the wake of a circular cylinder, both of which exhibit different forcing characteristics to the present experimental arrangement. A tripped NACA 0014 aerofoil was used to generate a fully turbulent wake, upstream of and at various heights above a laminar, flat-plate boundary layer, in the UK National Low-turbulence Wind Tunnel at City, University of London. Hot-wire measurements conducted in the pre-transitional region reveal the wall-normal and spanwise structure of the disturbances induced within the boundary layer and the rate of growth of disturbance energy. Disturbance profiles generally (but not uniquely) follow the non-modal distribution obtained from transient growth theory, but energy growth rates are mainly exponential rather than algebraic. Energy spectra demonstrate the existence of mixed transitional features (both natural and bypass) in the boundary layer. Two-point spatial correlations reveal the presence of a streaky structure, but with spanwise scale much larger than the boundary layer thickness, in contrast to the trends seen in free-stream turbulence-induced bypass transition and cylinder wake-induced transition. The gap between aerofoil and flat plate affects both the evolution of non-modal disturbance profile and the appearance of the streaky structure; the spacing of the streaks was also found to scale with the vertical gap between aerofoil and flat plate. Overall, the combination of observed characteristics is quite different from the forced transition mechanisms previously reported in the literature.
Journal Article
Destabilisation and modification of Tollmien–Schlichting disturbances by a three-dimensional surface indentation
We consider the influence of a smooth three-dimensional (3-D) indentation on the instability of an incompressible boundary layer by linear and nonlinear analyses. The numerical work was complemented by an experimental study to investigate indentations of approximately
$11\\unicode[STIX]{x1D6FF}_{99}$
and
$22\\unicode[STIX]{x1D6FF}_{99}$
width at depths of 45 %, 52 % and 60 % of
$\\unicode[STIX]{x1D6FF}_{99}$
, where
$\\unicode[STIX]{x1D6FF}_{99}$
indicates 99% boundary layer thickness. For these indentations a separation bubble confined within the indentation arises. Upstream of the indentation, spanwise-uniform Tollmien–Schlichting (TS) waves are assumed to exist, with the objective to investigate how the 3-D surface indentation modifies the 2-D TS disturbance. Numerical corroboration against experimental data reveals good quantitative agreement. Comparing the structure of the 3-D separation bubble to that created by a purely 2-D indentation, there are a number of topological changes particularly in the case of the widest indentation; more rapid amplification and modification of the upstream TS waves along the symmetry plane of the indentation is observed. For the shortest indentations, beyond a certain depth there are then no distinct topological changes of the separation bubbles and hence on flow instability. The destabilising mechanism is found to be due to the confined separation bubble and is attributed to the inflectional instability of the separated shear layer. Finally for the widest width indentation investigated (
$22\\unicode[STIX]{x1D6FF}_{99}$
), results of the linear analysis are compared with direct numerical simulations. A comparison with the traditional criteria of using
$N$
-factors to assess instability of properly 3-D disturbances reveals that a general indication of flow destabilisation and development of strongly nonlinear behaviour is indicated as
$N=6$
values are attained. However
$N$
-factors, based on linear models, can only be used to provide indications and severity of the destabilisation, since the process of disturbance breakdown to turbulence is inherently nonlinear and dependent on the magnitude and scope of the initial forcing.
Journal Article
Supporting Vulnerable Children in the Early Years
This book provides early years practitioners with clear strategies for supporting vulnerable children. Tackling topics such as inequality, poverty and the attainment gap, experienced practitioners from the field set out practical advice for ensuring all children are given a positive foundation for their future lives.
eBook
Education and Minorities
The contributors explore the experience of learners from minority groups and the education policy response of authorities, drawing on the international research in the USA, Finland, Rwanda, India, South Africa, Hungary, China and the UK. They explore the purpose of education for minority groups and in particular the place of human, social and identity capital in policy and practice. Each chapter contains a summary of the key points and issues within each chapter to enable easy navigation, key contemporary questions to encourage active engagement with the material and an annotated list of suggested reading to support further exploration.
eBook
