Impulse and particle dislodgement under turbulent flow conditions

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-normal͒ probability density function ͑pdf͒ dependent on only the coefficient of variation of the impulse ͑impulse intensity͒. Relation of the impulse intensity to the particle Reynolds number, Re ‫ء‬ , 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 turbulent forces and impulse on a rough bed: Implications for initiation of bed material movement

“…For more details, see Celik et al [2010]. The sampling duration for each flow condition in all experiments was 15 min with an average data rate of 250 Hz.…”

“…By nondimensionalizing the random impulse variable, I, with its ensemble average value, I mean , asÎ ¼ I=I mean , Celik et al [2010] determined that the following log-normal expression represents the corresponding probability density function (pdf) well,…”

“…Hofland et al [2005] and Dwivedi et al [2010Dwivedi et al [ , 2011a report, in addition to pressure or force measurements, separate mobile grain experiments with simultaneously recorded particle image velocimetry (PIV) measurements. A number of other investigations report simple grain entrainment observations simultaneously with flow velocity (via either LDV or PIV) without providing separate force or pressure measurements, notably Balakrishnan [1997], Cameron [2006], and Celik et al [2010].…”

“…Using an indirect method for estimating the instantaneous force record (from the velocity measurements obtained upstream of a test grain via LDV) and computing the associated impulse, Celik et al [2010] proposed a log-normal distribution for the impulse parameter. The validity of this result is examined here by using direct measurements of the grain surface point pressures to determine impulse values.…”

Statistical Description on the Role of Turbulence and Grain Interference on Particle Entrainment from Gravel Beds

“…The pioneering studies by Einstein 31 (1950), Sutherland (1967) and Grass (1970) have contributed to the development of modeling 32 approaches that described particle entrainment using probabilistic concepts (e.g., Cheng Recently, two additional aspects were experimentally investigated that characterize physical 36 interactions at the sediment-flow interface. First, it was clarified that the duration of the 37 hydrodynamic forces, not only their magnitude, is important in determining whether a grain will be 38 entrained, thus arguing that a threshold criterion would be better described by an impulse rather 39 than a force balance (Diplas et al 2008;Celik et al 2010; Valyrakis et al 2010). Second, while most 40 past investigations have assumed turbulent flow and bed particle arrangement to be statistically 41 independent, evidence was found that they have dependence (Papanicolaou et al 2001; Dey et al 42 2011a; Tregnaghi et al 2012a).…”