Polytechnic University of Catalonia

About: Polytechnic University of Catalonia is a education organization based out in Barcelona, Spain. It is known for research contribution in the topics: Finite element method & Population. The organization has 16006 authors who have published 45325 publications receiving 949306 citations. The organization is also known as: UPC - BarcelonaTECH & Technical University of Catalonia.

Temperature effects on the hydraulic behaviour of an unsaturated clay

Abstract: The influence of temperature on the hydraulic properties of unsaturated clays is of major concern in the design of engineered barriers in underground repositories for high-level radioactive waste disposal. This paper presents an experimental study centred on the investigation of the influence of temperature on soil hydraulic properties related to water retention and permeability. Laboratory tests were conducted on artificially prepared unsaturated fabrics obtained from a natural kaolinitic-illitic clay. Special attention is given to the testing procedures involving controlled suction and temperature oedometer cells and the application of the vapour equilibrium technique at high temperatures. Retention curves at different temperatures show that total suction tends to reduce with increasing temperatures at constant water content. Temperature influence on water permeability is more relevant at low matric suctions corresponding to bulk water preponderance (inter-aggregate zone). Below a degree of saturation of 75% no clear effect is detected. Experimental data show that temperature dependence on permeability at constant degree of saturation and constant void ratio is smaller than what could be expected from the thermal change in water viscosity This behaviour suggests that phenomena such as porosity redistribution and thermo-chemical interactions, which alter clay fabric and pore fluid, can be relevant.

An Algebraic Framework for Diffie---Hellman Assumptions

Abstract: We put forward a new algebraic framework to generalize and analyze Diffie---Hellman like decisional assumptions which allows us to argue about security and applications by considering only algebraic properties. Our $$\mathcal {D}_{\ell ,k}\text{- }\textsf {MDDH}$$Dl,k-MDDH Assumption states that it is hard to decide whether a vector in $$\mathbb {G}^\ell $$Gl is linearly dependent of the columns of some matrix in $$\mathbb {G}^{\ell \times k}$$Gl×k sampled according to distribution $$\mathcal {D}_{\ell ,k}$$Dl,k. It covers known assumptions such as $$\textsf {DDH},\, 2\text{- }\textsf {Lin}$$DDH,2-Lin (Linear Assumption) and $$k\text{- }\textsf {Lin}$$k-Lin (the k-Linear Assumption). Using our algebraic viewpoint, we can relate the generic hardness of our assumptions in m-linear groups to the irreducibility of certain polynomials which describe the output of $$\mathcal {D}_{\ell ,k}$$Dl,k. We use the hardness results to find new distributions for which the $$\mathcal {D}_{\ell ,k}\text{- }\textsf {MDDH}$$Dl,k-MDDH Assumption holds generically in m-linear groups. In particular, our new assumptions $$2\text{- }\textsf {SCasc}$$2-SCasc and $$2\text{- }\textsf {ILin}$$2-ILin are generically hard in bilinear groups and, compared to $$2\text{- }\textsf {Lin}$$2-Lin, have shorter description size, which is a relevant parameter for efficiency in many applications. These results support using our new assumptions as natural replacements for the $$2\text{- }\textsf {Lin}$$2-Lin assumption which was already used in a large number of applications. To illustrate the conceptual advantages of our algebraic framework, we construct several fundamental primitives based on any $$\textsf {MDDH}$$MDDH Assumption. In particular, we can give many instantiations of a primitive in a compact way, including public-key encryption, hash proof systems, pseudo-random functions, and Groth---Sahai NIZK and NIWI proofs. As an independent contribution, we give more efficient NIZK and NIWI proofs for membership in a subgroup of $$\mathbb {G}^\ell $$Gl. The results imply very significant efficiency improvements for a large number of schemes.

The Influence of Fractional Diffusion in Fisher-KPP Equations

Abstract: We study the Fisher-KPP equation where the Laplacian is replaced by the generator of a Feller semigroup with power decaying kernel, an important example being the fractional Laplacian. In contrast with the case of the standard Laplacian where the stable state invades the unstable one at constant speed, we prove that with fractional diffusion, generated for instance by a stable Levy process, the front position is exponential in time. Our results provide a mathematically rigorous justification of numerous heuristics about this model.