Browsing by Author "Celik, Ahmet Ozan"
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- Experimental Investigation of the Role of Turbulence Fluctuations on Incipient Motion of SedimentCelik, Ahmet Ozan (Virginia Tech, 2011-05-04)The movement of granular material along a streambed has been a challenging subject for researchers for more than a century. Predicting the limiting case of nearly zero bedload transport, usually referred to as threshold of motion or critical condition, is even more challenging due to the highly fluctuating nature of turbulent flow. Numerous works have advocated that the peak turbulent forces, randomly occurring in time and space with magnitudes higher than the average, initiate the bed material motion. More recent findings have shown that not only the magnitude of the peak turbulent forces acting on individual grains but their duration as well have to be considered for determining the incipient conditions. Their product, or impulse, is better suited for specifying such conditions. The goal of this study was to investigate the mechanism responsible for initiation of sediment motion under turbulent flow conditions. The impulse concept was investigated by utilizing appropriate measurement methods in the laboratory for determining the condition of incipient motion. The experimental program included measurements of particle entrainment rates of a mobile grain and turbulence induced forces acting upon a fixed grain for a range of flow conditions. In addition, near bed flow velocities were measured synchronously with both the entrainment and pressure measurements at turbulent resolving frequencies. Results of this work covered the limitations and uncertainties associated with the experimental methods employed, and the description of the inadequacies of existing incipient motion models via the impulse framework. The extreme sensitivity of bed material activity to minute adjustments in flow conditions was explained by the associated change in the frequency of impulse events. The probability density function proposed for impulse was used together with the critical impulse to estimate the particle entrainment rate for a range of flow conditions. It was shown that the impulse events with potential to dislodge the grain were occurring mostly during sweep type of flow structures. The impulse events were also typically accompanied by positive lift forces. The force patterns showed that the positive peaks in the lift consistently occurred before and after the impulse events in the drag force. The magnitude of these lift forces were significantly higher in the wake of a cylinder compared to that of uniform flow conditions. The time average lift force in the wake of a cylinder was also observed to be positive with magnitudes reaching more than 30% of the submerged weight of the particle. The cylinder caused the downstream turbulence intensity to increase slightly but the particle entrainment rate to increase significantly. This finding provided a physically based explanation for the modification of turbulent force fluctuations and resulting changes in the particle movement rates by such unsteady flow conditions.
- Impulse and particle dislodgement under turbulent flow conditionsCelik, Ahmet Ozan; Diplas, Panayiotis; Dancey, Clinton L.; Valyrakis, Manousos (American Institute of Physics, 2010-04-01)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.
- Instantaneous pressure measurements on a spherical grain under threshold flow conditionsCelik, Ahmet Ozan; Diplas, Panayiotis; Dancey, Clinton L. (Cambridge University Press, 2014-02-01)The aim of this investigation was to experimentally examine the surface pressures and resulting forces on an individual sediment grain whose size is comparable to the scales of the turbulent channel flow in an effort to discern details of the flow/grain interaction. This was accomplished by measuring the pressure fluctuations on the surface of a coarse, fully exposed, spherical grain resting upon a bed of identical grains in open channel turbulent flow. This spherical particle was instrumented with low-range, high-frequency-response pressure transducers to measure the individual surface pressures simultaneously on its front, back, top and bottom. The local flow velocity was measured synchronously with a laser Doppler velocimeter. The flow and sediment are near threshold conditions for entrainment with the channel and particle Reynolds numbers varying between 31000-39000 and 330-440 respectively. The emphasis was on determining the characteristics of the flow field with the potential to dislodge a spherical grain under uniform flow conditions as well as in the wake of a circular cylinder placed spanwise across the flow in otherwise fully developed open channel flow. It is concluded that the streamwise velocity near the bed is most directly related to those force events (and associated individual surface pressure distributions) crucial for particle entrainment. The lift force was observed to momentarily reach values which can be consequential for particle stability, although it is poorly correlated with the fluctuating normal velocity component. Turbulence intensity near the bed, rather than being the causative factor for increased force fluctuations, was shown to be an indicator of changes in the average lift force experienced by the grain during the application of extreme drag forces, at least for this particular flow condition (the upstream, spanwise-mounted circular cylinder). This effect is known to alter the sediment transport rates significantly. The characteristics of the temporal durations of flow events about the local maxima in the stagnation pressure, drag and lift forces, using a conditional sampling method, revealed the prevalence of sweep-type near-bed flow events in generating favourable conditions for particle dislodgement, although the dominant feature is the positive streamwise velocity fluctuation, not the normal velocity component. The duration of such events was the highest in the fourth and first quadrants in the u, w plane, inducing high impulses on the grain.
- Instantaneous turbulent forces and impulse on a rough bed: Implications for initiation of bed material movementCelik, Ahmet Ozan; Diplas, Panayiotis; Dancey, Clinton L. (American Geophysical Union, 2013-04-01)The overall objective of this study is to identify 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. This is pursued here by examining particle dislodgment under uniform and cylinder wake-flow experiments. The critical impulse concept is investigated more rigorously by measuring directly the pressures at four points on the surface of a fixed test grain. The number of impulse events determined from these experiments increases by more than an order of magnitude, over a modest change of roughness Reynolds number. Furthermore, they are well described by a log-normal probability density function. Both results are consistent with those obtained from similar experiments via indirect (velocity-based) impulse calculations and reported in a prior contribution. This comparison supports the use of the velocity record for determining instantaneous hydrodynamic forces and impulses instead of the more difficult approach of measuring the pressure fluctuations directly. The present results demonstrate the dominant role the local, streamwise velocity component plays on particle dislodgment. This is attributed to the large impulse content and occasionally strong positive lift force associated with flow events, exhibiting pronounced positive streamwise velocity fluctuations. The majority (approximate to 70%) of these events occur in the fourth quadrant, while a significant number (approximate to 22%) appear as first-quadrant episodes. It was also determined that wake flows can increase substantially particle entrainment via enhanced lift and increased turbulence intensity.