134,750 research outputs found
Time Consistency of Fiscal and Monetary Policy: A Solution
No full textThis paper demonstrates how time consistency of the Ramsey policy–the optimal fiscal and monetary policy under commitment–can be achieved. Each government should leave its successor with a unique maturity structure for the nominal and indexed debt, such that the marginal benefit of a surprise inflation exactly balances the marginal cost. Unlike in earlier papers on the topic, the result holds for quite a general Ramsey policy, including time varying polices with positive inflation and positive nominal interest rates. We compare our results with those in Persson, Persson, and Svensson (1987), Calvo and Obstfeld (1990), and Alvarez, Kehoe, and Neumeyer (2004).time consistency; Ramsey policy; surprise inflation
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
General contact mechanics theory for randomly rough surfaces with application to rubber friction
No full textWe generalize the Persson contact mechanics and rubberfrictiontheory to the case where both surfaces have surface roughness. The solids can be rigid, elastic, or viscoelastic and can be homogeneous or layered. We calculate the contact area, the viscoelastic contribution to the friction force, and the average interface separation as a function of the sliding speed and the nominal contact pressure. We illustrate the theory with numerical results for the classical case of a rubber block sliding on a road surface. We find that with increasing sliding speed, the influence of the roughness on the rubber block decreases to the extent that only the roughness of the stiff counter face needs to be considered
Contact mechanics and rubber friction for randomly rough surfaces with anisotropic statistical properties
No full textIn this paper we extend the theory of contact mechanics and rubber friction developed by one of us (B.N.J. Persson, J. Chem. Phys. 115, 3840 (2001)) to the case of surfaces with anisotropic surface roughness. As an application we calculate the viscoelastic contribution to the rubber friction. We show that the friction coefficient may depend significantly on the sliding direction, while the area of contact depends weakly on the sliding direction. We have carried out experiments for rubber blocks sliding on unidirectionally polished steel surfaces. The experimental data are in a good qualitative agreement with the theory
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Theory of adhesion: Role of surface roughness
No full textWe discuss how surface roughness influences the adhesion between elastic solids. We introduce a Tabor number which depends on the length scale or magnification, and which gives information about the nature of the adhesion at different length scales. We consider two limiting cases relevant for (a) elastically hard solids with weak (or long ranged) adhesive interaction (DMT-limit) and (b) elastically soft solids with strong (or short ranged) adhesive interaction (JKR-limit). For the former cases we study the nature of the adhesion using different adhesive force laws (F ∼ u −n , n = 1.5–4, where u is the wall-wall separation). In general, adhesion may switch from DMT-like at short length scales to JKR-like at large (macroscopic) length scale. We compare the theory predictions to results of exact numerical simulations and find good agreement between theory and simulation results
Elastic contact between randomly rough surfaces: comparison of theory with (exact) numerical results
No full textThe effect of finite roughness size and bulk thickness on the prediction of rubber friction and contact mechanics
No full textWe present the numerical results for the viscoelastic and adhesive contribution to rubber friction for a tread rubber sliding on a hard solid with a randomly rough surface. In particular, the effect of the high- and low-frequency roughness power spectrum cut-off is investigated. The numerical results are then compared to the predictions of an analytical theory of rubber friction. We show that the friction coefficient for large load is given exactly by the theory while some difference between theory and simulations occur for small loads, due to a finite sample-size effects, whereas the contact area is almost unaffected by the low frequency cut-off. Finally, the role of a finite rubber thickness on viscoelastic friction and contact area is introduced and critically discussed. Interestingly, we show that classical rough contact mechanics scaling rules do not apply for this case
Theory of viscoelastic lubrication
No full textWe study the lubricated (wet) contact mechanics of a smooth hard cylinder sliding on a randomly rough nominally flat surface of a linear viscoelastic solid. We calculate the rolling and sliding friction, and study the transition from the boundary lubrication to the elasto-hydrodynamic lubrication regime. For the viscoelastic contact the minimum (average) separation does not monotonically increase with the sliding velocity, and the Stribeck curve exhibits new structures not shown for elastic solids
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