Improved prediction of laminar leading edge separation

Research was conducted to provide a definite criterion for the prediction of the bubble burst on airfoils typical of those used for fighter wings. The approach taken was to correlate existing airfoil bubble burst data using various parameters at the laminar separation point. The method due to Weber was modified to provide a continuous analytic solution for the velocity distribution around the airfoil leading edge. Coupling the modified Weber method with the Stratford laminar separation prediction method leads to a universal chart giving the conditions at separation as a function of stagnation location and leading edge radius. Application of the combined method to available two-dimensional airfoil data resulted in an empirical criterion presenting the limiting local velocity gradient at separation as a function of the boundary layer momentum thickness at separation for bubble burst. The correlation leads as well to the qualitative explanation of two types of laminar stall: thin airfoil and leading edge. The validity of the correlation is demonstrated by predicting the lift coefficient and angle of attack for stall on airfoils with leading edge or trailing edge flaps

The Role of Capital in Financial Institutions

This paper examines the role of capital in financial institutions. As the introductory article to a conference on the role of capital management in banking and insurance, it describes the authors' views of why capital is important, how market-generated capital requirements' differ from regulatory requirements and the form that regulatory requirements should take. It also examines the historical trends in bank capital, problems in measuring capital and some possible unintended consequences of capital requirements. According to the authors, the point of departure for all modern research on capital structure is the Modigliani-Miller (M&M, 1958) proposition that in a frictionless world of full information and complete markets, a firm s capital structure cannot affect its value. The authors suggest however, that financial institutions lack any plausible rationale in the frictionless world of M&M. Most of the past research on financial institutions has begun with a set of assumed imperfections, such as taxes, costs of financial distress, transactions costs, asymmetric information and regulation. Miller argues (1995) that these imperfections may not be important enough to overturn the M&M Proposition. Most of the other papers presented at this conference on capital take the view that the deviations from M&M s frictionless world are important, so that financial institutions may be able to enhance their market values by taking on an optimal amount of leverage. The authors highlight these positions in this article. The authors next examine why markets require' financial institutions to hold capital. They define this capital requirement' as the capital ratio that maximizes the value of the bank in the absence of regulatory capital requirements and all the regulatory mechanisms that are used to enforce them, but in the presence of the rest of the regulatory structure that protects the safety and soundness of banks. While the requirement differs for each bank, it is the ratio toward which each bank would tend to move in the long run in the absence of regulatory capital requirements. The authors then introduce imperfections into the frictionless world of M&M taxes and the costs of financial distress, transactions costs and asymmetric information problems and the regulatory safety net. The authors analysis suggests that departures from the frictionless M&M world may help explain market capital requirements for banks. Tax considerations tend to reduce market capital requirements , the expected costs of financial distress tend to raise these requirements , and transactions costs and asymmetric information problems may either increase or reduce the capital held in equilibrium. The federal safety net shields bank creditors from the full consequences of bank risk taking and thus tends to reduce market capital requirements . The paper then summarizes the historical evolution of bank capital ratios in the United States and the reasons regulators require financial institutions to hold capital. They suggest that regulatory capital requirements are blunt standards that respond only minimally to perceived differences in risk rather than the continuous prices and quantity limits set by uninsured creditors in response to changing perceptions of the risk of individual banks. The authors suggest an ideal system for setting capital standards but agree that it would be prohibitively expensive, if not impossible. Regulators lack precise estimates of social costs and benefits to tailor a capital requirement for each bank, and they cannot easily revise the requirements continuously as conditions change. The authors continue with suggestions for measuring regulatory capital more effectively. They suggest that a simple risk-based capital ratio is a relatively blunt tool for controlling bank risk-taking. The capital in the numerator may not always control bank moral hazard incentive; it is difficult to measure, and its measured value may be subject to manipulation by gains trading . The risk exposure in the denominator is also difficult to measure, corresponds only weakly to actual risk and may be subject to significant manipulation. These imprecisions worsen the social tradeoff between the externalities from bank failures and the quantity of bank intermediation. To keep bank risk to a tolerable level, capital standards must be higher on average than they otherwise would be if the capital ratios could be set more precisely, raising bank costs and reducing the amount of intermediation in the economy in the long run. Since actual capital standards are, at best, an approximation to the ideal, the authors argue that it should not be surprising that they may have had some unintended effects. They examine two unintended effects on bank portfolio risk or credit allocative inefficiencies. These two are the explosive growth of securitization and the so-called credit crunch by U.S. banks in the early 1990s. The authors show that capital requirements may give incentives for some banks to increase their risks of failure. Inaccuracies in setting capital requirements distort relative prices and may create allocative inefficiencies that divert financial resources from their most productive uses. During the 1980s, capital requirements may have created artificial incentives for banks to take off-balance sheet risk, and changes in capital requirements in the 1990s may have contributed to a credit crunch.

An exact study of charge-spin separation, pairing fluctuations and pseudogaps in four-site Hubbard clusters

An exact study of charge-spin separation, pairing fluctuations and pseudogaps is carried out by combining the analytical eigenvalues of the four-site Hubbard clusters with the grand canonical and canonical ensemble approaches in a multidimensional parameter space of temperature (T), magnetic field (h), on-site interaction (U) and chemical potential. Our results, near the average number of electrons =3, strongly suggest the existence of a critical parameter U_{c}(T) for the localization of electrons and a particle-hole binding (positive) gap at U>U_{c}(T), with a zero temperature quantum critical point, U_{c}(0)=4.584. For U<U_{c}(T), particle-particle pair binding is found with a (positive) pairing gap. The ground state degeneracy is lifted at U>U_c(T) and the cluster becomes a Mott-Hubbard like insulator due to the presence of energy gaps at all (allowed) integer numbers of electrons. In contrast, for U< U_c(T), we find an electron pair binding instability at finite temperature near =3, which manifests a possible pairing mechanism, a precursor to superconductivity in small clusters. In addition, the resulting phase diagram consisting of charge and spin pseudogaps, antiferromagnetic correlations, hole pairing with competing hole-rich (=2), hole-poor (=4) and magnetic (=3) regions in the ensemble of clusters near 1/8 filling closely resembles the phase diagrams and inhomogeneous phase separation recently found in the family of doped high T_c cuprates.Comment: 10 pages, 7 figure

Current driven magnetization dynamics in helical spin density waves

A mechanism is proposed for manipulating the magnetic state of a helical spin density wave using a current. In this paper, we show that a current through a bulk system with a helical spin density wave induces a spin transfer torque, giving rise to a rotation of the order parameter.The use of spin transfer torque to manipulate the magnetization in bulk systems does not suffer from the obstacles seen for magnetization reversal using interface spin transfer torque in multilayered systems. We demonstrate the effect by a quantitative calculation of the current induced magnetization dynamics of Erbium. Finally we propose a setup for experimental verification.Comment: In the previous version of this paper was a small numerical mistake made when evaluating equation 3 and 9. The number of digits given in the calculation of the torque current tensor is reduced to better represent the accuracy of the calculation. A slightly modified paper have been published in Phys. Rev. Lett. 96, 256601 (2006) 4 pages 3 figure

Morphological instabilities of a thin film on a Penrose lattice: a Monte Carlo study

We computed by a Monte Carlo method the thermal relaxation of a polycrystalline thin film deposited on a Penrose lattice. The thin film was modelled by a 2 dimensional array of elementary domains, which have each a given height. During the Monte Carlo process, the height of each of these elementary domains is allowed to change as well as their crystallographic orientation. After equilibrium is reached at a given numerical temperature, all elementary domains have changed their orientation into the same one and small islands appear, preferentially on the domains of the Penrose lattice located in the center of heptagons. This method is a new numerical approach to study the influence of the substrate and its defects on the islanding process of polycrystalline films.Comment: 9 pages,5 figure

Transcriptome profiling reveals significant changes in the gastric muscularis externa with obesity that partially overlap those that occur with idiopathic gastroparesis

BACKGROUND: Gastric emptying is impaired in patients with gastroparesis whereas it is either unchanged or accelerated in obese individuals. The goal of the current study was to identify changes in gene expression in the stomach muscularis that may be contributing to altered gastric motility in idiopathic gastroparesis and obesity. METHODS: Quantitative real time RT-PCR and whole transcriptome sequencing were used to compare the transcriptomes of lean individuals, obese individuals and either lean or obese individuals with idiopathic gastroparesis. RESULTS: Obesity leads to an increase in mRNAs associated with muscle contractility whereas idiopathic gastroparesis leads to a decrease in mRNAs associated with PDGF BB signaling. Both obesity and idiopathic gastroparesis were also associated with similar alterations in pathways associated with inflammation. CONCLUSIONS: Our findings show that obesity and idiopathic gastroparesis result in overlapping but distinct changes in the gastric muscularis transcriptome. Increased expression of mRNAs encoding smooth muscle contractile proteins may be contributing to the increased gastric motility observed in obese subjects, whereas decreased PDGF BB signaling may be contributing to the impaired motility seen in subjects with idiopathic gastroparesis

Eulerian spectral closures for isotropic turbulence using a time-ordered fluctuation-dissipation relation

Procedures for time-ordering the covariance function, as given in a previous paper (K. Kiyani and W.D. McComb Phys. Rev. E 70, 066303 (2004)), are extended and used to show that the response function associated at second order with the Kraichnan-Wyld perturbation series can be determined by a local (in wavenumber) energy balance. These time-ordering procedures also allow the two-time formulation to be reduced to time-independent form by means of exponential approximations and it is verified that the response equation does not have an infra-red divergence at infinite Reynolds number. Lastly, single-time Markovianised closure equations (stated in the previous paper above) are derived and shown to be compatible with the Kolmogorov distribution without the need to introduce an ad hoc constant.Comment: 12 page

Low temperature shape relaxation of 2-d islands by edge diffusion

We present a precise microscopic description of the limiting step for low temperature shape relaxation of two dimensional islands in which activated diffusion of particles along the boundary is the only mechanism of transport allowed. In particular, we are able to explain why the system is driven irreversibly towards equilibrium. Based on this description, we present a scheme for calculating the duration of the limiting step at each stage of the relaxation process. Finally, we calculate numerically the total relaxation time as predicted by our results and compare it with simulations of the relaxation process.Comment: 11 pages, 5 figures, to appear in Phys. Rev.

The experience of cognitive behavioural therapy in depressed adolescents who are fatigued

Objective: Fatigue is a common and debilitating symptom of major depressive disorder (MDD). Cognitive behavioural therapy (CBT) is a recommended psychological treatment for adolescents with moderate to severe depression. This study explored the experience of CBT in fatigued adolescents with MDD. Design: A qualitative study was conducted using existing data from the qualitative arm of a large randomized control trial, the IMPACT study. Methods: Data were obtained from semi-structured interviews conducted after therapy. Participants were 18 adolescents (aged 13–18 years) who reached the clinical threshold for fatigue on diagnostic assessment before starting treatment. The data were analysed using thematic framework analysis. Results: Three themes and seven sub-themes were developed. Adolescents appeared to find taking part in initial sessions, engaging in ongoing sessions and completing homework challenging. Perceiving the therapist as genuine seemed to provide a sense of safety which enabled adolescents to open up in sessions. When the therapist was not perceived as genuine, adolescents appeared to find CBT less helpful. The structure of CBT appeared to enable treatment goals to be set, and facilitated an increase in meaningful activity. Ensuring that tasks were perceived as manageable and goals as achievable seemed important for participation. Cognitive restructuring appeared useful, although some adolescents tended to engage in distraction from thoughts as an alternative strategy. Conclusions: This study provides an initial insight into how fatigued adolescents with MDD experience CBT. Further research is required to establish whether the themes are pervasive and relatedly, how best to treat depression in fatigued adolescents receiving CBT. Practitioner points: Fatigued adolescents with depression found engaging in CBT sessions and therapeutic homework demanding. Establishing a collaborative therapeutic relationship, where the therapist was perceived as genuine, appeared helpful for participation. The structured approach to therapy, combined with flexibility, was experienced as helpful. Adolescents who perceived the pace of sessions to be manageable and therapeutic goals as achievable seemed to find CBT helpful overall. These findings provide insight into how fatigued adolescents with depression experience CBT and highlight the importance of being aware of fatigue and adapting therapy accordingly

Electric Conductivity of the Zero-gap Semiconducting State in Alpha-(BEDT-TTF)2I3 Salt

The electric conductivity which reveals the zero gap semiconducting (ZGS) state has been investigated as the function of temperature $T$ and life time $\tau$ in order to understand the ZGS state in quarter-filled $\alpha$-(BEDT-TTF)$_2$I$_3$ salt with four sites in the unit cell. By treating $\tau$ as a parameter and making use of the one-loop approximation, it is found that the conductivity is proportional to $T$ and $\tau$ for $k_B\gg\hbar/\tau$ and independent of $T$ and $\tau$ for $k_B T\ll\hbar/\tau$. Further the conductivity being independent of $T$ in the ZGS state is examined in terms of Born approximation for the impurity cattering.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp