The three-dimensional evolution of a plane mixing layer. Part 1: The Kelvin-Helmholtz roll-up

The Kelvin Helmholtz roll up of three dimensional, temporally evolving, plane mixing layers were simulated numerically. All simulations were begun from a few low wavenumber disturbances, usually derived from linear stability theory, in addition to the mean velocity profile. The spanwise disturbance wavelength was taken to be less than or equal to the streamwise wavelength associated with the Kelvin Helmholtz roll up. A standard set of clean structures develop in most of the simulations. The spanwise vorticity rolls up into a corrugated spanwise roller, with vortex stretching creating strong spanwise vorticity in a cup shaped region at the vends of the roller. Predominantly streamwise rib vortices develop in the braid region between the rollers. For sufficiently strong initial three dimensional disturbances, these ribs collapse into compact axisymmetric vortices. The rib vortex lines connect to neighboring ribs and are kinked in the opposite direction of the roller vortex lines. Because of this, these two sets of vortex lines remain distinct. For certain initial conditions, persistent ribs do not develop. In such cases the development of significant three dimensionality is delayed. When the initial three dimensional disturbance energy is about equal to, or less than, the two dimensional fundamental disturbance energy, the evolution of the three dimensional disturbance is nearly linear (with respect to the mean and the two dimensional disturbances), at least until the first Kelvin Helmholtz roll up is completed

Coherent structures in a simulated turbulent mixing layer

A direct numerical simulation of a plane turbulent mixing layer has been performed. The simulation was initialized using two turbulent velocity fields obtained from direct numerical simulation of a turbulent boundary layer at momentum thickness Reynolds number 300 (Spalart, 1988). The mixing layer is allowed to evolve long enough for self-similar linear growth to occur, with the visual thickness Reynolds number reaching 14,000. The simulated flow is examined for evidence of the coherent structures expected in a mixing layer (rollers and rib vortices). Before the onset of self-similar growth, such structures are present with properties similar to the corresponding laminar or transitional structures. In the self-similar growth regime, however, only the rollers are present with no indication of rib vortices and no indication of conventional pairing. This results in a reduction of mixing and layer growth

The three-dimensional evolution of a plane mixing layer. Part 2: Pairing and transition to turbulence

The evolution of three-dimensional temporally evolving plane mixing layers through as many as three pairings was simulated numerically. Initial conditions for all simulations consisted of a few low-wavenumber disturbances, usually derived from linear stability theory, in addition to the mean velocity. Three-dimensional perturbations were used with amplitudes ranging from infinitesimal to large enough to trigger a rapid transition to turbulence. Pairing is found both to inhibit the growth of infinitesimal three-dimensional disturbances and to trigger the transition to turbulence in highly three dimensional flows. The mechanisms responsible for the growth of three-dimensionality as well as the initial phases of the transition to turbulence are described. The transition to turbulence is accompanied by the formation of thin sheets of span wise vorticity, which undergo a secondary roll up. Transition also produces an increase in the degree of scalar mixing, in agreement with experimental observations of mixing transition. Simulations were also conducted to investigate changes in span wise length scale that may occur in response to the change in stream wise length scale during a pairing. The linear mechanism for this process was found to be very slow, requiring roughly three pairings to complete a doubling of the span wise scale. Stronger three-dimensionality can produce more rapid scale changes but is also likely to trigger transition to turbulence. No evidence was found for a change from an organized array of rib vortices at one span wise scale to a similar array at a larger span wise scale

Technical Challenges to Reducing Subsonic Transport Drag

No abstract availabl

Kentucky Law Survey: Administrative Law

This article provides a survey of administrative law in the Commonwealth of Kentucky, including discussions of de novo review and the delegation doctrine

Direct simulation of a self-similar plane wake

Direct simulations of two time-developing turbulent wakes have been performed. Initial conditions for the simulations were obtained from two realizations of a direct simulation of a turbulent boundary layer at momentum thickness Reynolds number 670. In addition, extra two dimensional disturbances were added in one of the cases to mimic two dimensional forcing. The unforced wake is allowed to evolve long enough to attain self similarity. The mass-flux Reynolds number (equivalent to the momentum thickness Reynolds number in spatially developing wakes) is 2000, which is high enough for a short k(exp -5/3) range to be evident in the streamwise one dimensional velocity spectrum. Several turbulence statistics have been computed by averaging in space and over the self-similar period in time. The growth rate in the unforced flow is low compared to experiments, but when this growth-rate difference is accounted for, the statistics of the unforced case are in reasonable agreement with experiments. However, the forced case is significantly different. The growth rate, turbulence Reynolds number, and turbulence intensities are as much as ten times larger in the forced case. In addition, the forced flow exhibits large-scale structures similar to those observed in transitional wakes, while the unforced flow does not

FEMA's Integration of Preparedness and Development of Robust Regional Offices

In October 2006, Congress enacted major legislation to reform the function and organization of the Federal Emergency Management Agency (FEMA) in response to the recognized failures in preparation for and response to Hurricane Katrina. The Post-Katrina Emergency Management Reform Act of 2006 (PKEMRA) focused national preparedness responsibilities within FEMA and directed additional resources and responsibilities to FEMA's ten regional offices. Directed by Congress, in October 2008 a National Academy Panel began an independent assessment of FEMA's integration of preparedness functions and progress in development of robust regional offices.Main FindingsOver the past three years, FEMA has taken significant steps in an effort to integrate preparedness and develop more robust regional offices. These efforts, undertaken by both the previous and current Administrations, are documented throughout this report and should be recognized and applauded. However, FEMA has yet to define specific goals and outcomes that would permit it, Congress or the public to determine when preparedness has been fully integrated into all aspects of FEMA's work and whether the development and ongoing operation of robust regional offices has been achieved. In the absence of well-defined, measurable outcome indicators, the National Academy Panel relied upon the assessments of FEMA leaders and staff, documentation provided by FEMA, and a review of secondary sources material to inform its findings and recommendations. Based upon this evidence, the Panel has concluded that, while progress has been made: (1) preparedness is not fully integrated across FEMA, (2) FEMA's regional offices do not yet have the capacity required to ensure the nation is fully prepared, (3) stakeholders are not yet full partners with FEMA in national preparedness, and (4) FEMA has ineffective internal business practices, particularly with regard to human resource management. The Panel made seven recommendations for FEMA:Establish a cross-organizational process, with participation from internal and external stakeholders, to develop a shared understanding of preparedness integrationEstablish a robust set of outcome metrics and standards for preparedness integration, as well as a system to monitor and evaluate progress on an ongoing basisWork to eliminate organizational barriers that are adversely impacting the full integration of preparedness across the agencyContinue to build regional office capacity and monitor implementation consistent with the Administrator's recent policy guidanceUndertake steps to improve the ongoing working relationship between headquarters and the regions in accord with Panel-identified principlesTake steps to improve stakeholder engagement and relationships at all levels in accord with Panel-identified principles; andStrengthen internal business practices, especially in the area of human capital planning

Effects of the components of positive airway pressure on work of breathing during bronchospasm

INTRODUCTION: Partial assist ventilation reduces work of breathing in patients with bronchospasm; however, it is not clear which components of the ventilatory cycle contribute to this process. Theoretically, expiratory positive airway pressure (EPAP), by reducing expiratory breaking, may be as important as inspiratory positive airway pressure (IPAP) in reducing work of breathing during acute bronchospasm. METHOD: We compared the effects of 10 cmH(2)O of IPAP, EPAP, and continuous positive airwaypressure (CPAP) on inspiratory work of breathing and end-expiratory lung volume (EELV) in a canine model of methacholine-induced bronchospasm. RESULTS: Methacholine infusion increased airway resistance and work of breathing. During bronchospasm IPAP and CPAP reduced work of breathing primarily through reductions in transdiaphragmatic pressure per tidal volume (from 69.4 ± 10.8 cmH(2)O/l to 45.6 ± 5.9 cmH(2)O/l and to 36.9 ± 4.6 cmH(2)O/l, respectively; P < 0.05) and in diaphragmatic pressure–time product (from 306 ± 31 to 268 ± 25 and to 224 ± 23, respectively; P < 0.05). Pleural pressure indices of work of breathing were not reduced by IPAP and CPAP. EPAP significantly increased all pleural and transdiaphragmatic work of breathing indices. CPAP and EPAP similarly increased EELV above control by 93 ± 16 ml and 69 ± 12 ml, respectively. The increase in EELV by IPAP of 48 ± 8 ml (P < 0.01) was significantly less than that by CPAP and EPAP. CONCLUSION: The reduction in work of breathing during bronchospasm is primarily induced by the IPAP component, and that for the same reduction in work of breathing by CPAP, EELV increases more

Characterization of Space Shuttle Ascent Debris Aerodynamics Using CFD Methods

An automated Computational Fluid Dynamics process for determining the aerodynamic Characteristics of debris shedding from the Space Shuttle Launch Vehicle during ascent is presented. This process uses Cartesian fully-coupled, six-degree-of-freedom simulations of isolated debris pieces in a Monte Carlo fashion to produce models for the drag and crossrange behavior over a range of debris shapes and shedding scenarios. A validation of the Cartesian methods against ballistic range data for insulating foam debris shapes at flight conditions, as well as validation of the resulting models, are both contained. These models are integrated with the existing shuttle debris transport analysis software to provide an accurate and efficient engineering tool for analyzing debris sources and their potential for damage