Impulse and particle dislodgement under turbulent flow conditions

In this study, we investigated the role of turbulence fluctuations on the entrainment of a fully exposed grain near threshold flow conditions. Experiments were carried out to measure synchronously the near bed flow velocity and the particle movement for a range of flow conditions and resulting particle entrainment frequencies. We used a simplified bed geometry consisted of spherical particles to reduce the complexities associated with the variations in the bed and flow details in an effort to identify the underlying dominant physical mechanism. An analysis was performed based on common force approximations using near bed flow velocity. Turbulence fluctuations were treated as impulses, which are products of magnitude and duration of applied force. It is demonstrated that besides the magnitude of the instantaneous forces applied on a sediment grain, their duration is important as well in determining whether a particle will be entrained by a turbulent flow event. Frequency of particle entrainment varied remarkably with minute changes in gross flow parameters. Impulse imparted on the sediment grain by turbulent flow was found to be well represented by a log-normal distribution. We obtained a (log-normal) probability density function (pdf) dependent on only the coefficient of variation of the impulse (impulse intensity). Relation of the impulse intensity to the particle Reynolds number, Re∗, was established. The sensitivity of the computed impulse to the critical force level, as well as the influence of the critical impulse level on the dislodgement events, was explored. Particle entrainment probabilities were found using the derived pdf as well as experimental observations and a good agreement between the two is reported. Implications of the presented impulse concept and our experimental findings for sediment mobility at low bed shear stress conditions are also discussed

The effects of the 2007 global economic crisis on firm relocation factors: SME movements from Greece to Bulgaria

This chapter examines the ways in which the 2007 global economic crisis has influenced firm relocation factors. Firm mobility constitutes a dynamic process, with its aspects changing in response to significant broader processes, such as globalisation. Specifically, the recent crisis has modified the socio-economic conditions under which firms operate. In order to examine the crisis-driven changes in firm mobility, this chapter employs a comparative analysis of the pre- and post-crisis relocation of Greek small and medium-sized companies to Bulgaria, which has recently increased. Greece is at the epicentre of academic and political debate in Europe, being the European Union member state mostly affected by the crisis. In the context of the changing economic and institutional conditions, it is demonstrated that the significance of the firm relocation factors, such as labour cost and level of demand, records considerable differences between the pre- and the post-crisis period

Principles of bedload transport of non-cohesive sediment in open-channels

This text addresses the particular case of motion and causes of motion of granular material as bedload in the fluvial domain. The aim is to perform and overview of key concepts, main achievements and recent advances on the description of the processes involved in erosion, deposition and transport of sediment in open-channels. The theoretical functional relations describing both the initiation of motion and the sediment transport are introduced. The classical problem of the initiation of motion of particles is treated at grain and at reach scales, accounting for the stochastic nature of flow. Concepts of granular kinematics and methods for quantifying the sediment transport rate in rivers are presented. The latter results from the interactions between the flow and the particles on the bed surface. The sediment transport rate, which has been shown to have a stochastic behaviour, is converted to a lumped statistic distribution. Finally, some field and laboratory techniques for measuring sediment transport, accounting for its inherent fluctuations, are introduced.Peer ReviewedPostprint (published version