Convective flow during dendritic growth

A review is presented of the major experimental findings obtained from recent ground-based research conducted under the SPAR program. Measurements of dendritic growth at small supercoolings indicate that below approximately 1.5 K a transition occurs from diffusive control to convective control in succinonitrile, a model system chosen for this study. The key theoretical ideas concerning diffusive and convective heat transport during dendritic growth are discussed, and it is shown that a transition in the transport control should occur when the characteristic length for diffusion becomes larger than the characteristic length for convection. The experimental findings and the theoretical ideas discussed suggest that the Fluid Experiment System could provide appropriate experimental diagnostics for flow field visualization and quantification of the fluid dynamical effects presented here

Anisotropic diffusion limited aggregation in three dimensions : universality and nonuniversality

We explore the macroscopic consequences of lattice anisotropy for diffusion limited aggregation (DLA) in three dimensions. Simple cubic and bcc lattice growths are shown to approach universal asymptotic states in a coherent fashion, and the approach is accelerated by the use of noise reduction. These states are strikingly anisotropic dendrites with a rich hierarchy of structure. For growth on an fcc lattice, our data suggest at least two stable fixed points of anisotropy, one matching the bcc case. Hexagonal growths, favoring six planar and two polar directions, appear to approach a line of asymptotic states with continuously tunable polar anisotropy. The more planar of these growths visually resembles real snowflake morphologies. Our simulations use a new and dimension-independent implementation of the DLA model. The algorithm maintains a hierarchy of sphere coverings of the growth, supporting efficient random walks onto the growth by spherical moves. Anisotropy was introduced by restricting growth to certain preferred directions

Flight software development for the isothermal dendritic growth experiment

The Isothermal Dendritic Growth Experiment (IDGE) is a microgravity materials science experiment scheduled to fly in the cargo bay of the shuttle on the United States Microgravity Payload (USMP) carrier. The experiment will be operated by real-time control software which will not only monitor and control onboard experiment hardware, but will also communicate, via downlink data and uplink commands, with the Payload Operations Control Center (POCC) at NASA George C. Marshall Space Flight Center (MSFC). The software development approach being used to implement this system began with software functional requirements specification. This was accomplished using the Yourdon/DeMarco methodology as supplemented by the Ward/Mellor real-time extensions. The requirements specification in combination with software prototyping was then used to generate a detailed design consisting of structure charts, module prologues, and Program Design Language (PDL) specifications. This detailed design will next be used to code the software, followed finally by testing against the functional requirements. The result will be a modular real-time control software system with traceability through every phase of the development process

Spontaneous deterministic side-branching behavior in phase-field simulations of equiaxed dendritic growth

The accepted view on dendritic side-branching is that side-branches grow as the result of selective amplification of thermal noise and that in the absence of such noise dendrites would grow without the development of side-arms. However, recently there has been renewed speculation about dendrites displaying deterministic side-branching [see, e.g., M. E. Glicksman, Metall. Mater. Trans A 43, 391 (2012)]. Generally, numerical models of dendritic growth, such as phase-field simulation, have tended to display behaviour which is commensurate with the former view, in that simulated dendrites do not develop side-branches unless noise is introduced into the simulation. However, here, we present simulations that show that under certain conditions deterministic side-branching may occur. We use a model formulated in the thin interface limit and a range of advanced numerical techniques to minimise the numerical noise introduced into the solution, including a multigrid solver. Spontaneous side-branching seems to be favoured by high undercoolings and by intermediate values of the capillary anisotropy, with the most branched structures being obtained for an anisotropy strength of 0.03. From an analysis of the tangential thermal gradients on the solid-liquid interface, the mechanism for side-branching appears to have some similarities with the deterministic model proposed by Glicksman

Reevaluating Environmental Citizen Suits in Theory and Practice

Citizen suits are frequently cited as an essential legal innovation by virtue of their capacity to provide a backstop to lax or ideologically antagonistic administrations. Drawing on data from fifteen years of litigation under two prominent environmental statutes, we find little evidence that citizen suits effectively serve this role in practice. Instead, we find that limited resources and institutional barriers strictly limit the number of citizen suits filed annually against the federal government under two of the most litigated environmental statutes, the National Environmental Policy Act (NEPA) and the Endangered Species Act (ESA). While our findings do not negate the importance of citizen suits, they expose their limitations and the close alignment that exists between where suits are filed and local politics. Citizen suits mirror local values as they are overwhelmingly filed in jurisdictions in which concerns about the environment are the highest, and they are rare where public concern is lowest. Our empirical findings lead us to reject the conventional model for citizen suits in favor of three alternative models that range from discrete, localized action to continuous lines of litigation over high-profile natural resources that can span decades. We find that the citizen suits under NEPA and the ESA that are covered by our study rarely target state or federal programs that are lax by national standards; instead, they serve a range of goals, large and small, that differ depending on the form of the suit (private enforcement actions, suits to perform nondiscretionary duties, or challenges to legally deficient agency action) and the programs and resources affected. Specifically, we find that citizen suits provide important constraints on agency discretion when environmental statutory mandates and the ideological outlook of presidential administrations diverge. Our findings also negate prominent critiques of citizen suits, most notably claims that citizen suits usurp government authority without the safeguard of political accountability and therefore are in need of vigorous gatekeeping by either executive branch officials or federal judge

Legal Adaptive Capacity : How Program Goals and Processes Shape Federal Land Adaption to Climate Change

The degree to which statutory goals are pliable is likely to significantly affect the ability of an agency with regulatory or management responsibilities to achieve those objectives in the face of novel challenges or changing circumstances. This Article explores this dynamic by comparing the degree of \u27ive provided by the goals of the regimes governing management of the five types of federal public lands in responding to the challenges posed by climate change. A comparative analysis of federal land adaptation to climate change demonstrates that a management regime\u27s legal adaptive capacity is influenced not only by procedural flexibility, but also by the flexibility the agency has in defining and pursuing a program\u27s substantive goals. Though a few scholars have explored the concept of adaptive capacity as it applies to law, most focus on the impact of procedural discretion on the ability to manage change. Counterintuitively, the land regimes most closely tied to resource preservation goals have generally lagged behind those with mixed conservation-commodity development mandates in preparing for climate change. Accordingly, this Article suggests ways to enhance the substantive legal adaptive capacity of land management agencies to promote ecological health in the face of climate change, and evaluates tradeoffs implicated when policymakers choose more appropriate levels of such adaptive capacity. More generally, the Article considers how effectively accommodating change may actually require legal constraints on when or how an agency may exercise that flexibility

Functional Government in 3-D

The creation of new administrative agencies and the realignment of existing governmental authority are commonplace and high-stakes events, as illustrated by the recent creation of the Department of Homeland Security after 9/11 and of new financial regulatory agencies after the global recession of 2009. Scholars and policymakers have not devoted sufficient attention to this subject, failing to clearly identify the different dimensions along which government authority may be structured or to consider the relationships among them. Analysis of these institutional design issues typically also gives short shrift to whether authority should be allocated differently based on agency function. These failures have contributed to reorganization efforts that have proven ill-suited to achieving policymakers’ goals due to mismatches between the perceived defects of existing structures and the allocations of authority chosen to replace them. This Article introduces a framework for assessing how governmental authority may be structured along three dimensions: centralization, overlap, and coordination. Using examples from diverse policy areas including national security, financial markets, and environmental protection, it demonstrates how differentiating among these dimensions and among particular governmental functions better illuminates the advantages and disadvantages of available structural options. Though recognizing that the optimal allocation of authority is inexorably context-specific, the Article concludes with preliminary observations about how certain allocations of authority are likely to better promote important social policy goals than others

A Collective Action Perspective on Ceiling Preemption By Federal Environmental Regulation: The Case of Global Climate Change

In an era of regulatory skepticism, proponents of regulation in general and environmental regulation in particular face a number of new political and legal hurdles, particularly at the federal level. Frustrated with federal inaction or weak federal regulation, it is increasingly common for states and local governments to adopt environmental laws that seek to provide greater environmental protection. The critical question is when federal environmental law provides a ceiling, preempting such state regulatory programs. In this article, which is part of a forthcoming symposium on federal preemption in the Northwestern Law Review, Professors Glicksman and Levy develop a framework for evaluating ceiling preemption issues in the environmental law arena and then apply it to the specific problem of state regulation of greenhouse gas (GHG) emissions that contribute to global climate change. Traditional preemption doctrine emphasizes the purpose of federal laws, which is relevant to all three major forms of preemption: express, occupation of the field, and conflict preemption (including state laws that are an obstacle to the accomplishment of the object and purpose of federal law). Few environmental statutes expressly preempt more stringent state laws. The values of federalism, political process theory, and textualism support a presumption against implied preemption that should only be overcome if there is a direct conflict between federal and state law or state regulation would compromise a clear and primary purpose of a federal statute. The question then becomes which environmental purposes might provide sufficient justification to overcome the presumption and support ceiling preemption. Federal environmental law emerged largely as a response to various collective action problems that tended to prevent states, acting individually, from adopting appropriate environmental laws. Such concerns included negative externalities, the need for resource pooling, the race to the bottom phenomenon, the rationalization of standards (uniformity), and the so-called NIMBY (not in my back yard) problem. In considering the problem of ceiling preemption, then, it makes sense to ask whether the collective action problem in question is implicated by state regulation that is more protective of the environment than federal regulation. In general terms, negative externalities, resource pooling issues, and the race to the bottom can be expected to cause underregulation by states, and thus do not support ceiling preemption. In contrast, uniformity and NIMBY concerns would tend to support ceiling preemption. Using this basic framework, the article offers a detailed analysis of state regulation of GHG emissions, which industry has argued is preempted by federal law, including the Clean Air Act. This analysis suggests that the primary purposes of the Act, which include controlling negative externalities, taking advantage of resource pooling, and preventing a race to the bottom, do not generally support ceiling preemption. Although resource pooling might support ceiling preemption as a means of strengthening the bargaining position of the Unites States vis-à-vis other countries, this purpose was not a significant concern when the Act was adopted. The need for uniformity supports limited ceiling preemption for mobile sources of emissions (i.e., cars and trucks), which are subject to express preemption provisions. Uniformity concerns are less significant for stationary sources, and there is little evidence to support broad ceiling preemption on this basis. Finally, NIMBY concerns were not an important factor in the adoption of the Clean Air Act. The article also considers the implications of the framework for preemption under a cap and trade program, which is the likely form that any federal regulatory program for GHG emissions would take