Shark Biomimetics: The Role of Denticles and Riblets on the Turbulent Boundary Layer

Shark skin has fascinated biologists, engineers, and physicists for decades due to its highly intricate drag-reducing structure, which has motivated a plethora of research into bio-inspired hydrodynamically efficient surfaces. Throughout this thesis the effect of shark scales on the boundary layer is investigated, with a particular focus on the role of riblets in combination with denticles. In addition to examining flows over shark scales studies are also presented investigating the behaviour of Reynolds Averaged Navier-Stokes (RANS) models close to solid boundaries, and the scaling and driving mechanisms of secondary flows over ribletted surfaces. Extensive numerical and analytical studies are carried out to determine the sensitivity of eleven turbulence closures to the near-wall grid resolution, and their consistency with asymptotic solutions. Results inform the choice of turbulence models adopted for simulations of wall bounded flows, particularly where numerical errors must be minimised. Secondary flows over longitudinal riblets are found to be driven by Reynolds stress anisotropy, consistent with Prandtl’s second type of secondary flow. The strength of the vorticity field is heavily dependent on the inner-scaled riblet spacing s+ where two distinct regimes arise; a viscous regime where vorticity production is balanced by molecular viscous diffusion, and an inertial regime where an effective turbulent viscosity balances anisotropic production. The transition between these regimes occurs when riblet tips protrude into the buffer layer and cause increased turbulent mixing (s+ ≈ 30), such that vorticity reaches its maximum before reducing as s+ increases further. Riblets in combination with shark scales do not operate as they do when applied to smooth walls. Experimental and numerical studies reveal that riblets act to reduce pressure drag acting on roughness elements, rather than the viscous forces typically associated with longitudinal riblets. The mechanisms leading to this behaviour are driven by the ability of riblets to restrict spanwise motion and maintain streamwise-aligned near-wall flow. By doing so riblets protect downstream denticles from high momentum impinging fluid, and reduce high magnitude swirl generated at the exposed denticle edges, which can otherwise lead to increased turbulent production and enhanced momentum transfer through the roughness sub-layer. These mechanisms lead to a significantly more efficient rough surface than smooth denticles, although do not necessarily lead to an overall reduced drag compared to a flat plate. These studies conclude that riblets have evolved as a mechanism to reduce or eliminate the skin friction increase due to the presence of scales. The combination of scales and riblets appears to be relatively hydrodynamically efficient in terms of skin-friction drag, whilst also acting to maintain boundary layer attachment and providing the other advantages associated with scales such as anti-fouling, abrasion resistance, and defence against parasites

Journalism and power: the importance of the institution

The journalist John Lloyd has written extensively about the relationship between journalism and power. It’s at the heart of a book I am writing, so I asked John to send me a summary of his views that I had heard him express at a conference a couple of years ago. The idea that interested me sounds obvious, but is not often set out. Journalism is powerful mainly because it organises itself into professional institutions. This gives the individual journalist, or act of journalism, support and resource. If you lose those institutions (for example if we all turned into bloggers!) do you lose the journalism? Here’s John’s view in full that he was kind enough to send me and that I will quote in part in the book

Goal Measures for psychotherapy:A systematic review of self-report, idiographic instruments

Routine outcome monitoring can support client progress in psychotherapy and provide evidence on population‐level outcomes. However, measures have been almost exclusively nomothetic. Idiographic tools provide a complementary approach, combining individually set outcomes with standardized progress ratings. Evidence suggests that goal‐focused idiographic measures may particularly facilitate client progress, and this systematic review aimed to identify and critically evaluate such measures, as used in psychotherapy. In total, 104 texts were eligible for inclusion in the review, with nine measures identified. These took the form of multidimensional tools, brief rating forms, and goal attainment scaling. Psychometric and clinical evidence suggests that these measures may be appropriate tools for supporting client progress, but there is insufficient evidence to validate their use for population‐level evaluation

The alcohol improvement programme: evaluation of an initiative to address alcohol-related health harm in England

Aims: The evaluation aimed to assess the impact of The Alcohol Improvement Programme (AIP). This was a UK Department of Health initiative (April 2008–March 2011) aiming to contribute to the reduction of alcohol-related harm as measured by a reduction in the rate of increase in alcohol-related hospital admissions (ARHAs). Methods: The evaluation (March 2010–September 2011) used a mix of qualitative and quantitative methods to assess the impact of the AIP on ARHAs, to describe and assess the process of implementation, and to identify elements of the programme which might serve as a ‘legacy’ for the future. Results: There was no evidence that the AIP had an impact on reducing the rise in the rate of ARHAs. The AIP was successfully delivered, increased the priority given to alcohol-related harm on local policy agendas and strengthened the infrastructure for the delivery of interventions. Conclusion: Although there was no measurable short-term impact on the rise in the rate of ARHAs, the AIP helped to set up a strategic response and a delivery infrastructure as a first, necessary step in working towards that goal. There are a number of valuable elements in the AIP which should be retained and repackaged to fit into new policy contexts

The coupled dynamics of internal waves and hairpin vortices in stratified plane Poiseuille flow

A simulation of stably stratified plane Poiseuille flow at a moderate Reynolds number (Formula Presented) and Richardson number (Formula Presented) is presented. For the first time, the dynamics in the channel core are shown to be described as a series of internal waves that approximately obey a linear wave dispersion relationship. For a given streamwise wavenumber Formula Presented there are two internal wave solutions, a dominant low frequency mode and a weaker-amplitude high-frequency mode, respectively corresponding to ‘backward’ and ‘forward’ propagating internal waves relative to the mean flow. Analysis of linearised equations shows that the dominant low-frequency mode appears to arise due to a particularly sensitive response of the mean flow profiles to incoherent forcing. Instantaneous visualisations reveal that hairpin vortices dominate the outer region of the channel flow, neighbouring the buoyancy dominated channel core. These hairpins are fundamentally different from those observed in canonical unstratified boundary layer flows, as they arise via quasi-linear local processes far from the wall, governed by background shear. Outer region ejection events are common and can be induced by high amplitude waves. Ejected hairpins are transported into the channel core, in turn ‘ringing’ the prevailing strong buoyancy gradient and thus generating high-amplitude internal waves, high dissipation and wave breaking, induced by spanwise vortex stretching and baroclinic vorticity generation. Such spontaneous and sustained generation of quasi-linear internal waves by wall-bounded sheared turbulence may provide novel idealised solutions for, and insight into, large-scale turbulent mixing in a wide range of environmental and industrial flows

Influence of Fertilizer Treatments on Lint Yields and the Elemental Analyses of Cotton Leaves Sampled at Different Stages of Development

Agronom