Large-eddy simulation of large-scale structures in long channel flow

We investigate statistics of large-scale structures from large-eddy simulation (LES) of turbulent channel flow at friction Reynolds numbers Re_τ = 2K and 200K (where K denotes 1000). In order to capture the behaviour of large-scale structures properly, the channel length is chosen to be 96 times the channel half-height. In agreement with experiments, these large-scale structures are found to give rise to an apparent amplitude modulation of the underlying small-scale fluctuations. This effect is explained in terms of the phase relationship between the large- and small-scale activity. The shape of the dominant large-scale structure is investigated by conditional averages based on the large-scale velocity, determined using a filter width equal to the channel half-height. The conditioned field demonstrates coherence on a scale of several times the filter width, and the small-scale–large-scale relative phase difference increases away from the wall, passing through π/2 in the overlap region of the mean velocity before approaching π further from the wall. We also found that, near the wall, the convection velocity of the large scales departs slightly, but unequivocally, from the mean velocity

On Inefficiency of Markowitz-Style Investment Strategies When Drawdown is Important

The focal point of this paper is the issue of "drawdown" which arises in recursive betting scenarios and related applications in the stock market. Roughly speaking, drawdown is understood to mean drops in wealth over time from peaks to subsequent lows. Motivated by the fact that this issue is of paramount concern to conservative investors, we dispense with the classical variance as the risk metric and work with drawdown and mean return as the risk-reward pair. In this setting, the main results in this paper address the so-called "efficiency" of linear time-invariant (LTI) investment feedback strategies which correspond to Markowitz-style schemes in the finance literature. Our analysis begins with the following principle which is widely used in finance: Given two investment opportunities, if one of them has higher risk and lower return, it will be deemed to be inefficient or strictly dominated and generally rejected in the marketplace. In this framework, with risk-reward pair as described above, our main result is that classical Markowitz-style strategies are inefficient. To establish this, we use a new investment strategy which involves a time-varying linear feedback block K(k), called the drawdown modulator. Using this instead of the original LTI feedback block K in the Markowitz scheme, the desired domination is obtained. As a bonus, it is also seen that the modulator assures a worst-case level of drawdown protection with probability one.Comment: This paper has been published in Proceedings of 56th IEEE Conference on Decision and Control (CDC) 201

The M Word: The Rise and Fall of Interracial Coalitions On Fathers And Welfare Reform

Buoyed by the success of the 1996 Personal Responsibility and Work Opportunities Reconciliation Act (PRWORA), whose time limits and work requirements played a large role in the reduction of the welfare rolls, conservative advocates of welfare reform are now moving to ensure that our welfare system reflects traditional family values as well. Responding to this sentiment, the Bush Administration is encouraging states to use TANF to support marriage promotion efforts and the Administration's 2002 budget includes $100 million in support of demonstration projects to promote marriage (source). By contrast, the$60 million President Bush had committed to support efforts to promote responsible fatherhood, not restricted to marriage, has been pared back to $20 million, along with cutbacks in other domestic initiatives that are needed to pay for the "war against terrorism."

Hamiltonian formulation of SL(3) Ur-KdV equation

We give a unified view of the relation between the $SL(2)$ KdV, the mKdV, and the Ur-KdV equations through the Fr\'{e}chet derivatives and their inverses. For this we introduce a new procedure of obtaining the Ur-KdV equation, where we require that it has no non-local operators. We extend this method to the $SL(3)$ KdV equation, i.e., Boussinesq(Bsq) equation and obtain the hamiltonian structure of Ur-Bsq equationin a simple form. In particular, we explicitly construct the hamiltonian operator of the Ur-Bsq system which defines the poisson structure of the system, through the Fr\'{e}chet derivative and its inverse.Comment: 12 pages, KHTP-93-03 SNUTP-93-2

Just Get Me to the Church: Assessing Policies to Promote Marriage among Fragile Families

This article examines alternative approaches to encourage family formation among fragile families, including higher cash benefits, more liberal acceptance of welfare applications, more effective child support enforcement, and efforts to increase education and employment of low-income parents. We examine these approaches by refining and expanding previous work on a generalized logit model of the mothers’ actual family formation outcomes, in a hierarchy that includes father absence, father involvement, cohabitation, and marriage. Refinements involve measurements of family formation that make our results more comparable to other studies and new controls for previous fertility with the father of the focal child and with another partner (multiple partner fertility). We estimate these models using interim data from the Fragile Families and Child Well-Being 12 month follow-up Survey. The results indicate that, unlike their effects on mature families, cash benefits increase the odds of family formation (short of marriage) among fragile families and effective child support enforcement increases the odds of marriage. However, the father’s employment status outweighs the effects of these traditional income security policies on family formation, because it affects outcomes all along the hierarchy, including marriage, and its effects are larger. Unlike previous research, our data on previous fertility enables us to separate the effects of previous children in common from multiple partner fertility on family formation. Both significantly affect family formation (though in opposite directions), but even after including these variables, blacks, who are more likely to bring children from previous unions into a new union, have substantially lower odds of cohabitation and marriage than non-Hispanic whites.