Joseph Calantoni

Discrete particle model for sheet flow sediment transport in the nearshore

Fully three-dimensional discrete particle computer simulations of high-concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments. A simple impulse-momentum approach explains simulation results and forms the basis for adding an acceleration-related term to widely used energetics sediment transport formulae. Transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores. Simulations integrate F = ma and a corresponding set of equations for the torques for each sphere. Normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point, respectively; particle interactions are both inelastic and frictional. Pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag, added mass, and buoyancy forces on particles. Transport properties of the simulated granular-fluid assemblage are robust to large variations in material properties of the particles. Simulated transport rates agree with available experimental data for unsteady transport of coarse sands; the mode of bed load motion, dispersion of bed load particles, and particle segregation by size and density are qualitatively consistent with available particle-scale observations of bed load transport of natural particles.

Modelling Sheet-Flow Sediment Transport in Wave-Bottom Boundary Layers Using Discrete-Element Modelling

Sediment transport in oscillatory boundary layers is a process that drives coastal geomorphological change. Most formulae for bed–load transport in nearshore regions subsume the smallest–scale physics of the phenomena by parametrizing interactions amongst particles. In contrast, we directly simulate granular physics in the wave–bottom boundary layer using a discrete–element model comprised of a three–dimensional particle phase coupled to a one–dimensional fluid phase via Newtons third law through forces of buoyancy, drag and added mass. The particulate sediment phase is modelled using discrete particles formed to approximate natural grains by overlapping two spheres. Both the size of each sphere and the degree of overlap can be varied for these composite particles to generate a range of non–spherical grains. Simulations of particles having a range of shapes showed that the critical angle—the angle at which a grain pile will fail when tilted slowly from rest—increases from approximately 26° for spherical particles to nearly 39° for highly non–spherical composite particles having a dumbbell shape. Simulations of oscillatory sheet flow were conducted using composite particles with an angle of repose of approximately 33° and a Corey shape factor greater than about 0.8, similar to the properties of beach sand. The results from the sheet–flow simulations with composite particles agreed more closely with laboratory measurements than similar simulations conducted using spherical particles. The findings suggest that particle shape may be an important factor for determining bed–load flux, particularly for larger bed slopes.

Fast, large-scale, particle image velocimetry-based estimations of river surface velocity

A modified high-speed implementation of cross-correlation (CC) based, large-scale particle image velocimetry (LSPIV) was used to estimate the surface velocity of a river with video collected from a gray-scale camera. To improve the quality of results in the high-noise low-signal environment, we introduce a temporal correlation averaging (TCA) scheme that merges a small number of correlation surfaces in the time domain. The TCA scheme is combined with a multi-size macroblock (MMB) sampling method that provides correlation scores from four different macroblock sizes. The TCA scheme is also used in conjunction with a signal-level indicator computed on the macroblock. The signal-level indicator is used to reject correlation scores prior to computation and helps to keep noisy results out of the TCA. These modifications were tested by comparing LSPIV calculations to Acoustic Doppler Current Profiler measurements. The percent difference of measured velocity between LSPIV with TCA and MMB and without TCA and MMB when compared to the ADCP was reduced by as much as 30%. The low processing cost of our modifications along with an efficient multithread implementation of LSPIV facilitates high speed processing of up to a few thousand vector points at rates that exceed the capture speed of common hardware. HighlightsWe introduce a modified LSPIV method that uses temporal correlation averaging.Results from the modified LSPIV method were compared with ADCP results.Agreement between ADCP and LSPIV is improved when using our modifications.Processing speed remains consistent with unmodified LSPIV.

Lagrangian measurements of incipient motion in oscillatory flows

Incipient motion of coarse gravel-sized sediment was investigated under a range of oscillatory flows. This article examines the relative significance of shear stresses and pressure gradients in triggering motion, which was directly measured with electronic Smart Sediment Grains (SSGs). The data suggest that incipient motion was induced by the pressure gradients in flows with large accelerations, by the shear stresses in flows with low accelerations and greater shear, and by the combined effects in intermediate flows. A modified incipient motion criterion was evaluated accounting for the combined effects of the shear stresses and pressure gradients, which may be more widely applicable in the marine environment.

Development and Evaluation of an Autonomous Sensor for the Observation of Sediment Motion

AbstractMeasurements within the mobile bed layer have been limited by previous Eulerian-based technologies. A microelectromechanical system device, called a smart sediment grain (SSG), that can measure and record Lagrangian observations of coastal sediments at incipient motion has been developed. These sensors have the potential to resolve fundamental hypotheses regarding the incipient motion of coastal sediments. Angle of repose experiments verified that the sensor enclosure has mobility characteristics similar to coarse gravel. Experiments conducted in a small oscillating flow tunnel verified that the sensors detect incipient motion under various hydrodynamic conditions. Evidence suggests the influence of pressure-gradient-induced sediment motion, contrary to the more commonly assumed bed shear stress criterion. Lagrangian measurements of rotation measured with the newly developed SSG agreed to within 5% of the rotation estimates made simultaneously with high-speed video cameras.

Video observations of bed form morphodynamics in a meander bend

A new optical remote sensing technique for estimating water depth from an oblique camera view is described. The water surface and the bed were imaged simultaneously to create time-dependent maps of the water surface velocities and the bed elevations that can be used to validate numerical models at high spatial and temporal resolution. The technique was applied in a sandy meander bend at the University of Minnesota Saint Anthony Falls Laboratory Outdoor StreamLab. The root mean square differences between optical estimates of the bed and in situ observations ranged between 0.01 and 0.03 m. Mean bed form wavelength was 0.73 m and mean crest height was 0.07 m, but both varied with distance around the meander bend. Bed form classification varied with distance downstream, and sinuosity of bed forms varied with local radius of curvature. Bed form roughness scaled similarly to other natural riverine environments although wavelength and height magnitude and variability were larger than predicted by empirical formulations for straight reaches. Bed form translation rate varied between 1 and 5 mm s−1. Estimates of velocity from particle image velocimetry (PIV) on the water surface were ∼10% higher than in situ observations collected ∼0.05 m below the water surface. Using the PIV observations to drive simple equations for bed load sediment flux, we explained up to 72% of the observed variance in downstream sediment flux. The new methodology described here provides nonintrusive, high spatial and temporal resolution measurements of both the bed and the flow.

Incipient motion of surf zone sediments

Incipient motion experiments were conducted with natural gravel, acetate beads, and coarse-gravel-sized electronic grains called Smart Sediment Grains in a Small-Oscillatory Flow Tunnel. Measurements of fluid velocity were made using Particle Image Velocimetry. The strength of the fluid shear stresses and the pressure gradients were examined for a range of oscillatory flow conditions at the onset of motion of the sediment particles to determine which mechanism had induced particle motion. The three sediment types utilized in these experiments facilitated an assessment of the effects of sediment grain size diameter, shape, and density on incipient motion. Results suggested that the onset of sediment motion was dominated by the pressure gradients for flows with small orbital excursion amplitudes, by the shear stresses for flows with large orbital excursion amplitudes and by the combined effects for intermediate flows. The denser, angular gravel required greater free-stream accelerations to trigger sediment motion than the spherical, less dense acetate beads, and Smart Sediment Grains. A combined parameter for incipient motion that accounts for the simultaneous effects of both shear stresses and pressure gradients while depending on the static coefficient of friction and the packing concentration of the mobile bed layer was evaluated for accuracy using a range of sediment types. The results suggested that the combined parameter may be a better indicator of sediment mobilization under oscillatory flows than the typically assumed shear stress criterion.

A pressure boundary integral method for direct fluid-particle simulations on Cartesian grids

We consider a new Cartesian grid method for direct numerical simulations of fully coupled interaction of incompressible flow and spherical particles, based on a discontinuous extension of the pressure Poisson equation (PPE) across particle boundaries. We give a complete mathematical description of the boundary-integral treatment of the discontinuous PPE that includes the derivation of a new pressure boundary condition for accelerating boundaries and the solution of the system of boundary integral equations using spherical harmonics expansions. The model was validated with the standard test for finite Reynolds number flow around a sphere and with a novel test using the analytical solution for the Stokes flow past two adjacent spheres moving with the same velocity. The model capability and numerical efficiency was demonstrated with simulations for the collective settling of groups of 64-512 particles.

Summertime conditions of a muddy estuarine environment: the EsCoSed project contribution.

As part of the Estuarine Cohesive Sediments (EsCoSed) project, a field experiment was performed in a highly engineered environment, acting as a natural laboratory, to study the physico-chemical properties of estuarine sediments and the associated hydro-morphodynamics during different seasons. The present contribution focuses on the results obtained from the summertime monitoring of the most downstream part of the Misa River (Senigallia, Italy). The measured hydrodynamics suggested a strong interaction between river current, wave forcing and tidal motion; flow velocities, affected by wind waves traveling upstream, changed significantly along the water column in both direction and magnitude. Surficial salinities in the estuary were low in the upper reaches of the estuary and exceeded 10 psu before the river mouth. Montmorillonite dominated the clay mineral assemblage, suggesting that large, low density flocs with high settling velocities (>1 mm s(-1)) may dominate the suspended aggregate materials.

Laboratory observations of sand ripple evolution using bimodal grain size distributions under asymmetric oscillatory flows

ABSTRACT Calantoni, J., Landry, B.J. and Penko, A.M., 2013. Laboratory observations of sand ripple evolution using bimodal grain size distributions under asymmetric oscillatory flows The heterogeneity of sand beds has been suggested to significantly impact the resulting sand ripple morphodynamics. However, the majority of previous experiments for sand ripple morphodynamics were conducted using only unimodal grain size distributions. Here we performed a series of ripple growth and transition experiments in a small oscillatory flow tunnel in the Sediment Dynamics Laboratory at the U.S. Naval Research Laboratory. Sand beds were constructed from mixtures of two unimodal sands median grain sizes of 0.30 mm (blue) and 0.70 mm (white), respectively. Experiments were performed with compositions of bimodal mixtures with percent by mass of 10/90, 25/75, 50/50, 75/25, 90/10. Additionally, similar experiments were performed for each of the unimodal cases (i.e., 100/0, 0/100). For each experiment, starting from a planar bed, three different flow forcing conditions were applied in sequential blocks (with minimum of one-hour duration) until the ripples appeared to be uniform and in equilibrium. We analyzed ripple characteristics such as migration rate, wavelength, height, and steepness as a function of the mobility number. Over a range of nearly identical mobility numbers, we observed opposing trends with migration rates increasing in one block forcing and decreasing in another, where the two blocks were comprised of different combinations of the semiexcursion amplitude and oscillatory frequency. The results suggested that the commonly used mobility number might not be appropriate to characterize ripple migration rates, especially for sediment beds composed of bimodal size mixtures. Overall, wavelength, height, and steepness are consistent with empirical ripple predictors. However, observed subtleties existed among the different forcing blocks across the same range of grain size distributions.