Ahmet Çelik

TL;DR: It is demonstrated that in addition to the magnitude of the instantaneous turbulent forces applied on a sediment grain, the duration of these turbulent forces is also important in determining the sediment grain's threshold of motion, and that their product is better suited for specifying such conditions.

TL;DR: In this article, a new criterion for the onset of entrainment of coarse sediment grains is presented, based on the product of force and its duration, or impulse, which is a more appropriate and universal criterion for identifying conditions for particle dislodgement.

Abstract: 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.

TL;DR: In this paper, the authors investigated the physical mechanisms responsible for the entrainment of an exposed particle subject to rapidly fluctuating hydrodynamic forces in the case of channel flow with a fully rough boundary.

TL;DR: A fuzzy model for ACC in multistage processes is developed and it is solved by Genetic Algorithms and the proposed approach is applied in an engine valve manufacturing firm and the model is solve by GAs.