Shock wave mitigation using zig-zag structures and cylindrical obstructions

The present study focuses on the mitigation of shock wave using novel geometric passages in the flow field. The strategy is to produce multiple shock reflections and diffractions in the passage with minimum flow obstruction, which in turn is expected to reduce the shock wave strength at the target location. In the present study the interaction of a plane shock front (generated from a shock tube) with various geometric designs such as, 1) zig-zag geometric passage, 2) staggered cylindrical obstructions and 3) zig-zag passage with cylindrical obstructions have been investigated using computational technique. It is seen from the numerical simulation that, among the various designs, the maximum shock attenuation is produced by the zig-zag passage with cylindrical obstructions which is then followed by zig-zag passage and staggered cylindrical obstructions. A comprehensive investigation on the shock wave reflection and diffraction phenomena happening in the proposed complex passages have also been carried out. In the new zig-zag design, the initial shock wave undergoes shock wave reflection and diffraction process which swaps alternatively as the shock front moves from one turn to the other turn. This cyclic shock reflection and diffraction process helps in diffusing the shock wave energy with practically no obstruction to the flow field. It is found that by combining the shock attenuation ability of zig-zag passage (using shock reflection and diffraction) with the shock attenuation ability of cylindrical blocks (by flow obstruction), a drastic attenuation in shock strength can be achieved with moderate level of flow blocking.     

LAND INEQUALITY AND CONFLICT IN LATIN AMERICA IN THE TWENTIETH CENTURY

Fiji's total debt stands at 65% of GDP. Domestic debt constitutes 55% of GDP. The goal of this paper is to investigate whether military expenditure has contributed to Fiji's exploding debt levels over the period 1970 to 2005. Our empirical analysis, conducted within a cointegration and vector error‐correction framework, suggests that, in the long‐run, military expenditure has had a statistically significant positive impact on both external debt and domestic debt, while income has had a statistically significant positive impact on domestic debt and a statistically significant negative impact on external debt. We explain the reasons behind this finding and draw some policy implications.     

New terrorist threat to India's internal security: the danger from Pakistan's “Karachi Project”

The spectacular commando-style terrorist strike on Mumbai in November 2008 exposed India's lax internal security structure. As nearly all the security apparatus broke down during the long spell of attacks, massive public outrage flared up across the country calling for a firmer government response. Shockingly, India has done little to prevent a recurrence and a new security threat faces the country every single day not merely before but even after the Mumbai attacks. In contrast, Pakistan's Inter Services Intelligence and the Lashkar-e-Taiba have successfully evaded pressures from both India and the international community and continue their terror campaign against India under the “Karachi Project” with the explicit intention to unsettle South Asia. Classified documents indicated that India is at the forefront of a cataclysmic “nuclear terrorism” threat from a “combination of Pakistan-based terrorists and homegrown radicals.” This article analyses the danger posed by the Indo-Pak radical groups targeting India and India's lack of preparedness to the new security threat from the “Karachi Project.”     

An application of yield management for Internet Service Providers

In this paper we study strategies for better utilizing the network capacity of Internet Service Providers (ISPs) when they are faced with stochastic and dynamic arrivals and departures of customers attempting to log‐on or log‐off, respectively. We propose a method in which, depending on the number of modems available, and the arrival and departure rates of different classes of customers, a decision is made whether to accept or reject a log‐on request. The problem is formulated as a continuous time Markov Decision Process for which optimal policies can be readily derived using techniques such as value iteration. This decision maximizes the discounted value to ISPs while improving service levels for higher class customers. The methodology is similar to yield management techniques successfully used in airlines, hotels, etc. However, there are sufficient differences, such as no predefined time horizon or reservations, that make this model interesting to pursue and challenging. This work was completed in collaboration with one of the largest ISPs in Connecticut. The problem is topical, and approaches such as those proposed here are sought by users. © 2001 John Wiley & Sons, Inc., Naval Research Logistics 48:348–362, 2001