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The continued drive towards increased zones of laminar flow on aircraft flying surfaces requires the research community to improve its ability to understand and predict factors causing instability leading to transition and turbulent flow. One of the factors that arise in trying to develop laminar wings is surface finish and the influence played by various types of surface imperfections. It turns out that small protuberances, or depressions, on the surface can interact with either sound waves, or existing instability waves, to generate additional instability disturbances through the receptivity process. There are many possible ways of tackling this problem and predicting the disturbances created by the interaction of, say, a sound wave and a small surface bump. Colleagues who work with DNS methods assure me that this problem can be tackled effectively by DNS, but as we are mainly considering very small acoustic excitation so that linearised equations are appropriate. High order methods commonly employed can deal with very weak perturbations with adequate accuracy. Until these solutions are available for the parameters of the current project we cannot gauge the value of the method. It is also possible to further simplify the computing by linearising the boundary conditions on the excrescence to plate values on the bounding surface. This is certainly permissible for very shallow imperfections but may not be suitable for larger ones. Triple deck methods have also successfully solved these problems see [1].

One of the possible forcing mechanisms for inducing transition from laminar to turbulent flow is freestream turbulence. Among the different transition mechanisms, freestream turbulence influences the cross-flow instability which occurs on swept wings. In order to understand better the physical interaction between freestream turbulence and boundary layer, this work analyses how the turbulence is distorted approaching a leading edge and whether it is amplified approaching a swept leading edge. The paper contains a brief literature review of both the two-dimensional stagnation point and three-dimensional attachment-line flows. This reveals that freestream turbulence amplification has been investigated for two-dimensional leading edge flows, but there is a lack of research on the swept leading edge. In addition, the paper presents some experimental results on a circular cylinder at various sweep angles. The results show that the phenomenon observed at the two-dimensional stagnation point also occurs at a swept attachment-line, although some differences are identified.

An experimental investigation was carried out in a low-turbulence wind tunnel to study the early development of artificially initiated turbulent spots in a laminar boundary layer over a flat plate. The reproducibility of the experiments allowed us to observe fine structural details that have not been observed previously. Initial velocity disturbances quickly developed into hairpin-like structures that multiplied downstream, which increased the width, length and height of the incipient turbulent spots. Only those disturbances that were greater than a threshold value developed into turbulent spots while the others decayed. The rate of development was also affected by the duration of the initial disturbances. We found that the behaviour of turbulence generation within a turbulent spot is similar to the burst events in the turbulent boundary layer, where ejection events are followed by sweep events.

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 at plate model. The freestream 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�m < jhj < 750�m (equivalent to 0:025 < jhj=��B < 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 non-linear 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 � 150 �m (jhj=��B � 0:126). The departure from the linear behaviour is, however, dependent on the geometry of the surface imperfection. For cavities of signi�cant depth, the non-linear 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.

Cultured human myotubes offer a unique model to distinguish between primary and environmental factors in the aetiology of insulin resistance in human skeletal muscle. The objective of this review was to summarize our and other group studies on insulin resistance in human myotubes established from lean, obese and type 2 diabetes (T2D) subjects. Overall, studies of human myotubes established from lean, obese and T2D subjects clearly show that part of the diabetic phenotype observed in vivo is preserved in diabetic myotubes. Diabetic myotubes express a primary coordinated impairment of lipid oxidation, oxidative phosphorylation (OXPHOS) and insulin-stimulated glucose metabolism. Currently, both the responsible molecular mechanisms as well as the extent to which these alterations depend on genetic and/or epigenetic alterations have yet to be identified. Based on the data, it is hypothesized that the impaired insulin-mediated glucose metabolism, impaired OXPHOS and reduced lipid oxidation observed in diabetic myotubes are caused by the reduced peroxisome proliferator-activated receptor gamma coactivator-1α (PGC1α) expression.

A decrease in skeletal muscle lipolysis and hormone sensitive-lipase (HSL) expression has been linked to insulin resistance in obesity. The purpose of this study was to identify potential intrinsic defects in lipid turnover and lipolysis in myotubes established from obese and type 2 diabetic subjects. Lipid trafficking and lipolysis were measured by pulse-chase assay with radiolabeled substrates in myotubes from non-obese/non-diabetic (lean), obese/non-diabetic (obese) and obese/diabetic (T2D) subjects. Lipolytic protein content and level of Akt phosphorylation were measured by Western blot. HSL was overexpressed by adenovirus-mediated gene delivery. Myotubes established from obese and T2D subjects had lower lipolysis (-30-40%) when compared to lean, using oleic acid as precursor. Similar observations were also seen for labelled glycerol. Incorporation of oleic acid into diacylglycerol (DAG) and free fatty acid (FFA) level was lower in T2D myotubes, and acetate incorporation into FFA and complex lipids was also lower in obese and/or T2D subjects. Both protein expression of HSL (but not ATGL) and changes in DAG during lipolysis were markedly lower in cells from obese and T2D when compared to lean subjects. Insulin-stimulated glycogen synthesis (-60%) and Akt phosphorylation (-90%) were lower in myotubes from T2D, however, overexpression of HSL in T2D myotubes did not rescue the diabetic phenotype. In conclusion, intrinsic defects in lipolysis and HSL expression co-exist with reduced insulin action in myotubes from obese T2D subjects. Despite reductions in intramyocellular lipolysis and HSL expression, overexpression of HSL did not rescue defects in insulin action in skeletal myotubes from obese T2D subjects.

The paper describes research work that has been carried out to reduce the viscous drag of transport aircraft wings by controlling the turbulent contamination on the attachment line. Laminar leading edges will reduce the boundary layer thickness downstream and thus reduce drag. This scenario was postulated by M. Gaster in a paper presented to the AIAA Seattle conference in 2008. The effect of maintaining a laminar attachment line on the overall flow on a wing has been modeled and drag reductions calculated. These values are compared with wind tunnel measurements of viscous drag of a wing with both laminar and turbulent attachment lines.

N EAR-STALL three-dimensional separation patterns, which are also referred to as “stall cells,” can appear on the suction side of certain airfoils. An early report on this phenomenon was provided by Gregory et al. [1]. Detailed oilflow visualizations of stall cells were obtained by, e.g., Winkelmann and Barlow [2], Bippes et al. [3], and Schewe [4]. Yon and Katz [5] sampled the wall pressure in the separated flow region and observed large-amplitude but lowfrequency fluctuations at the beginning of the range of the angle of attack over which the stall cells occurred. Outside this range, the amplitude of the fluctuations was much lower and the low-frequency signal was absent. Broeren and Bragg [6] investigated several airfoils with different stalling characteristics. Stall for airfoils with trailingedge separation was characterized by the appearance of stall cells. The stall cells that were observed for angles of attack at or above stall were generally steady. Stall for airfoils with leading-edge separation (i.e., thin-airfoil stall) displayed low-frequency oscillations near stall but no stall cells. Stall cells can appear suddenly and, therefore, have a profound effect on the performance, maneuverability, and safety of aircraft. An improved understanding of the criteria determining the appearance of stall cellswouldmake flight at near-stall conditionsmore predictable, and thus safer. In addition, the mechanism responsible for the formation of stall cells may be exploited for high angle-of-attack flow control applications. Weihs and Katz [7] proposed a physical mechanism for the appearance of stall cells based on the Crow instability [8]. At stall, in the time mean, a spanwise vortex can be associatedwith the recirculating flow region on the suction side of the airfoil. Because this vortex is located close to the airfoil surface, a reflected image of the vortex with the opposite sense of rotation can be imagined. As shown by Crow [8], this steady vortex pair is unstable with respect to a symmetric mode. The growth of the symmetric mode may result in the breakup of the steady vortex and the formation of stall cells. Rodriguez and Theofilis [9] argued that stall cells are the consequence of a global instability. The linear superposition of the two-dimensional flow around a stalled airfoil with the leading stationary three-dimensional eigenmode resulted in skin-friction lines that were strongly reminiscent of stall cells. In the present Note, an explanation for the appearance of stall cells based on lifting-line theory is offered and a simple formula for estimating the stall cell spacing is derived.