Instance-Independent View Serializability for Semistructured Databases

Semistructured databases require tailor-made concurrency control mechanisms since traditional solutions for the relational model have been shown to be inadequate. Such mechanisms need to take full advantage of the hierarchical structure of semistructured data, for instance allowing concurrent updates of subtrees of, or even individual elements in, XML documents. We present an approach for concurrency control which is document-independent in the sense that two schedules of semistructured transactions are considered equivalent if they are equivalent on all possible documents. We prove that it is decidable in polynomial time whether two given schedules in this framework are equivalent. This also solves the view serializability for semistructured schedules polynomially in the size of the schedule and exponentially in the number of transactions

Tyre/Road Noise due to Road Surface Texture Excitations - Experimental and Numerical Analyses

Tyre/road noise is the dominant noise source of vehicles travelling at speeds above 40 km/h. Hence, in urban environments, tyre/road noise is an important contributor to the overall environmental noise. Elevated environmental noise levels have several adverse effects on human well-being. Along with the increased social awareness of this issue the European Commission has put in place regulations aimed at restricting environmental noise levels. This PhD research focuses on tyre/road noise and more precisely the noise component caused by structural tyre vibrations due to road surface texture excitations. The main objective of this research is a deeper understanding of the structural tyre dynamic behaviour, tyre/road contact forces and tyre/road noise radiation of a rotating loaded slick tyre. To yield this understanding, industrially applicable experimental methods are developed and numerical methods, based on a highly detailed finite element model, are implemented and validated.status: publishe

Synopsis of the Experimental and Numerical Tire Dynamic Characterization

Both experimental as well as numerical analyses have been performed to establish the understanding of the influence of rolling on the tire dynamic behavior. The experimental analysis is based on accelerometer measurements on the inner liner of a tire. The numerical analysis is based on a highly detailed tire model, including complex material properties by means of frequency dependent viscoelastic material definitions. A comparative analysis is performed to evaluate the correspondence and differences between experimentally and numerically obtained modal parameters of a rolling tire. The analyses also show how the footprint contact, established due to loading of the tire, acts as a boundary condition for the structural waves and thus influences the dynamic behavior of the rolling tire. The results show that a rotating tire is subjected to Coriolis accelerations which make the wave speed of the positive- and negative-going wave to diverge from each other. This leads to complex or travelling wave mode shapes. The experimental analyses confirm the numerically predicted effects of rolling on the tire dynamic behavior.status: publishe

Analytical solution for free vibrations of rotating cylindrical shells having free boundary conditions

In this paper free vibrations of rotating cylindrical shells with both ends free are studied. The model used also allows for considering a flexible foundation supporting the shell in the sense of a radial and circumferential distributed stiffness. Furthermore, a circumferential tension (hoop stress) which may be due to pressurisation or centrifugal forces is taken into account. Natural frequencies and mode shapes are determined exactly for both stationary shells and for shells rotating with a constant angular speed around the cylinder axis. Trigonometric functions are assumed for the circumferential mode shape profiles, and a sum of eight weighted exponential functions is assumed for the axial mode shape profiles. The functional form of the axial profiles is shown to greatly vary with the roots of a characteristic bi-quartic polynomial that occurs in the process of satisfying the equations of motion. In the previously published work it has been very often assumed that the roots are two real, two imaginary, and two pairs of complex conjugates. In the present study, a total of eight types of roots are shown to determine the whole set of mode shapes, either for stationary or for rotating shells. The results using the developed analytical model are compared with results of experimental studies and very good agreement is obtained. Also, a parametric study is carried out where effects of the elastic foundation stiffnesses and the rotation speed are examined.publisher: Elsevier articletitle: Analytical solution for free vibrations of rotating cylindrical shells having free boundary conditions journaltitle: Engineering Structures articlelink: http://dx.doi.org/10.1016/j.engstruct.2016.11.008 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe

A simplified tire model based on a rotating shell

A number of simplified models of rotating tires have been suggested in the past. The belt had been modelled as a cylindrical shell, almost exclusively having simply supported boundary conditions. Such models are unable to represent certain groups of mode shapes due to the assumed zero radial displacements at the ends of the tire belt. In this paper, a model using free boundary conditions is reported. The developed model is able to approximate mode shapes that are similar to Rayleigh and Love symmetric and anti-symmetric mode shapes. Mode shapes, natural frequencies and their veering with rotation speed are calculated exactly.status: publishe