We consider the genesis and dynamics of interfacial instability in gas-liquid flows, using as a model the two-dimensional channel flow of a thin falling film sheared by counter-current gas. The methodology is linear stability theory (Orr-Sommerfeld analysis) together with direct numerical simulation of the two-phase flow in the case of nonlinear disturbances. We investigate the influence of three main flow parameters (density contrast between liquid and gas, film thickness, pressure drop applied to drive the gas stream) on the interfacial dynamics. Energy budget analyses based on the Orr-Sommerfeld theory reveal various coexisting unstable modes (interfacial, shear, internal) in the case of high density contrasts, which results in mode coalescence and mode competition, but only one dynamically relevant unstable internal mode for low density contrast. The same linear stability approach provides a quantitative prediction for the onset of (partial) liquid flow reversal in terms of the gas and liquid flow rates. A study of absolute and convective instability for low density contrast shows that the system is absolutely unstable for all but two narrow regions of the investigated parameter space. Direct numerical simulations of the same system (low density contrast) show that linear theory holds up remarkably well upon the onset of large-amplitude waves as well as the existence of weakly nonlinear waves. In comparison, for high density contrasts corresponding more closely to an air-watertype system, although the linear stability theory is successful at determining the most-dominant features in the interfacial wave dynamics at early-to-intermediate times, the short waves selected by the linear theory undergo secondary instability and the wave train is no longer regular but rather exhibits chaotic dynamics and eventually, wave overturning.
Administrative agencies in the United States have developed highly formalized and complex processes for public participation in rulemaking, especially in areas of social regulation such as the environment and workplace safety and health. This case study considers the significance of participation in formal rulemaking processes by connecting the quality of participation to the strategic possibilities in litigation between private interests and regulatory agencies. Specifically, the strategic possibilities of the leading interest groups engaged in the Occupational Safety and Health Administration's major “Lockout/Tagout” rulemaking illustrate how legal resources are created through the development of evidence and claims in hearings. Written and oral presentations, apparently aimed directly at persuading the agency, indirectly affect agency deliberations by increasing the possibility that courts will constrain agency decisionmaking, thus creating opportunities for negotiated alternatives. The case ultimately serves as a paradigmatic example of how bargaining arises at the micro level of policy systems that are infused with broader legal structures.
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