An Efficient Algorithm for Computing High-Quality Paths amid Polygonal Obstacles

We study a path-planning problem amid a set $\mathcal{O}$ of obstacles in $\mathbb{R}^2$, in which we wish to compute a short path between two points while also maintaining a high clearance from $\mathcal{O}$; the clearance of a point is its distance from a nearest obstacle in $\mathcal{O}$. Specifically, the problem asks for a path minimizing the reciprocal of the clearance integrated over the length of the path. We present the first polynomial-time approximation scheme for this problem. Let $n$ be the total number of obstacle vertices and let $\varepsilon \in (0,1]$. Our algorithm computes in time $O(\frac{n^2}{\varepsilon ^2} \log \frac{n}{\varepsilon})$ a path of total cost at most $(1+\varepsilon)$ times the cost of the optimal path.Comment: A preliminary version of this work appear in the Proceedings of the 27th Annual ACM-SIAM Symposium on Discrete Algorithm

Path Clearance

In military scenarios, agents (i.e., troops of soldiers, convoys, and unmanned vehicles) may often have to traverse environments with only a limited intelligence about the locations of adversaries. We study a particular instance of this problem that we refer to as path clearance problem.This article presents a survey of our work on scalable and suitable for real-time use approaches to solving the path clearance problem. In particular, in the first part of the article, we show that the path clearance problem exhibits clear preferences on uncertainty. It turns out that these clear preferences can be used to develop an efficient algorithm called probabilistic planning with clear preferences (PPCP). The algorithm is anytime usable, converges to an optimal solution under certain conditions, and scales well to large-scale problems. We briefly describe the PPCP algorithm and show how it can be used to solve the path clearance problem when no scouts are present. In the second part of the article, we show several strategies for how to use the PPCP algorithm in case multiple scouting unmanned aerial vehicles (UAVs) are available. The experimental analysis shows that planning with PPCP results in a substantially smaller execution cost than when ignoring uncertainty, and employing scouts can decrease this execution cost even further

Oxidizing seal for a turbine tip gas path

The sealing of the gas path in a gas turbine engine at the blade tips is improved by maintaining a minimum clearance between the rotor blade tips and the gas path seal. This is accomplished by taking advantage of an increase in volume during controlled oxidation of certain intermetallic compounds which have high melting points. The increase in volume closes the clearance subsequent to a rub between the blades and the seal. Thus, these compounds re-form the tip seal surface to assure continued engine efficiency

High stiffness seals for rotor critical speed control

An annular seal is analyzed in which the inlet clearance is larger than the outlet clearance; the flow path may be either stepped or tapered. This design produces radial stiffness 1.7 to 14 times that of a constant clearance seal having the same minimum clearance. When sealing high pressure fluids, such a seal improves rotor stability and can be used to shift troublesome critical speeds to a more suitable location

Not Peace, but a Sword: Navy v. Egan and the Case Against Judicial Abdication in Foreign Affairs

In the United States\u27 system of separation of powers, the judiciary must safeguard the rights of individuals from abuses by the political branches of government. Yet, when it comes to matters touching foreign affairs, scholars such as John Yoo and jurists such as Antonin Scalia argue that the executive branch is entitled to virtually unreviewable discretion. They point to Navy v. Egan for support. There, the Court held that an administrative body that hears appeals from adverse actions against government employees was precluded from reviewing the merits of security clearance determinations because the executive branch deserves super-strong deference in foreign affairs. An examination of the disastrous consequences of Egan crystallizes the constitutional and functional arguments against super-strong deference to the executive-both in foreign affairs generally and in the security clearance process specifically. The case has prompted lower courts to deny plaintiffs an independent forum in which to bring constitutional claims related to security clearance denials and revocations. Egan\u27s progeny flouts the longstanding principle that an individual who suffers a constitutional injury is entitled to an appropriate remedy. Furthermore, by abdicating its duty to check executive power in the security clearance context, the judiciary has fortified deficiencies inherent to executive agency decisionmaking, namely tunnel vision, path dependency, and imperialist tendencies. Abdication has also enabled a systematic denial of clearances to candidates with foreign connections. Without a diverse counterterrorism workforce, the United States lacks the operational proficiency and the legitimacy to wage a successful war on terrorism. This Note is the first to call on the judiciary to reclaim the right to exercise judicial review of the merits of security clearance determinations. Furthermore, it charts a path for lower courts to reopen judicial review of the merits of security clearance determinations, provide injured plaintiffs with a remedy, deter future racial discrimination, and avert a chilling effect on agency decisionmakers

Rotordynamic Forces from Discharge-to-Suction Leakage Flows in Centrifugal Pumps: Effects of Geometry

The rotordynamic forces generated by the fluid flow through the impeller leakage path of a centrifugal pump are now well established. The present paper examines the effects of modifying the leakage path geometry by changing the front shroud, from a conical shape to a more typical curved design, and the effects of low pressure seal design on these forces. It is found that only the cross-coupled stiffness is affected by the change of path geometry. Changing the low pressure seal from an axial to a radial clearance does, however, significantly affect the rotordynamic forces. A bulk flow numerical model is found to predict the same general result for the low pressure seal tests. The model agrees with the general trends with increasing leakage flow coefficient exhibited by the data, but appears to underpredict the magnitude of the normal force

Moving Mugabe's Mountain: Zimbabwe's Path to Arrears Clearance and Debt Relief

Zimbabwe is embarking on a long, complex, and difficult journey to rebuild its economy, which has been shattered by years of neglect and destructive government policies. If the final political hurdles are overcome? hopefully sooner rather than later—the new coalition government will be faced with making politically difficult decisions and addressing conclusively the economic ghosts of the past. One of those ghosts is more than $5 billion owed to external creditors, of which nearly 80 percent is arrears. Clearing Zimbabwe’s external debt arrears and securing comprehensive debt relief will be a critical step in eventual recovery. By doing so, the government will remove a crippling burden on its budget, investment climate, and overall macroeconomic environment. More important to the success of the coalition government, it will unlock hundreds of millions of dollars in new external assistance for critical reconstruction programs that will improve the Zimbabwean people’s quality of life. This paper provides a detailed overview of the arrears clearance and debt relief processes. The purpose is to spark a debate in creditor capitals and hopefully to buttress the government’s analytical foundation. With a strong dedicated team of experts—and support from relevant donor agencies— the Zimbabwean government will one day conclusively address its crushing debt burden and proceed with the rebuilding of a once vibrant and proud nation.zimbabwe; reconstruction; external debt; economic development; debt relief