Ambuj Dwivedi

Hydrodynamic Forces Generated on a Spherical Sediment Particle during Entrainment

The objective of this research is to study the relationship between the coherent flow structures and the hydrodynamic forces leading to entrainment of a spherical bed sediment particle for a rough bed uniform turbulent flow. Two types of experiments, namely, movable and fixed balls, were conducted using spherical roughness-element beds with particle image velocimetry to measure the instantaneous flow-velocity field. Miniature piezoelectric pressure sensors were used to capture the instantaneous pressure on the surface of the sphere. Movable ball experiments reveal the predominance of large sweep structures at the instant of entrainment. Fixed ball experiments carried out at entrainment conditions show the importance of both vertical and horizontal pressure gradients on the ball leading to entrainment. Probability distribution function plots of pressures based on quadrant analysis of velocities also reveal the higher probability of occurrence of high magnitude force induced by sweep ( Q4 ) events.

Testing and calibration of smart pebble for river bed sediment transport monitoring

The smart pebble (smart particle), SP, has been developed for the past two years to monitor sediment transport in riverbeds. The implementation is based on use of small size and low cost acceleration and angular motion sensors. In this stage, the project is focused on calibrating and testing the final version of the SP as well as its packaging in a 4-cm diameter spherical package. The calibration was done in two stages; individual sensor calibration and complete system calibration. The complete SP unit was tested under linear motions generated by a shake table, and 2D rotational motions using two manually controlled servomotors. Offline digital signal conditioning was done in MATLAB. The preliminary results show that the system has relatively large amplitude error due to low sampling frequency. Experiments conducted by sampling a 1-Hz sinusoidal signal at different rates show that to keep the amplitude error of the system under 5% the sampling rate has to be at least 10 times the maximum bandwidth of the signals acquired from sensors.

Role of Turbulence and Particle Exposure on Entrainment of Large Spherical Particles in Flows with Low Relative Submergence

AbstractLaboratory experiments of sediment entrainment were carried out in a flume for different flow configurations with particle image velocimetry (PIV) used to measure the flow velocity. The experiments were done in a low submergence condition. Measurements revealed that the vertical distribution of streamwise flow velocity at the instant of entrainment is significantly different from the temporal mean flow profile. The role of turbulence for a given flow depth is found to decrease with an increase in exposure of the particle. An expression for threshold velocity is proposed that accurately represents the enhanced near-bed turbulence during entrainment. A Shields-type parameter, expressed in terms of critical near-bed flow velocity rather than shear velocity, is used to define the sediment transport threshold.

Low-Cost Autonomous 3-D Monitoring Systems for Hydraulic Engineering Environments and Applications With Limited Accuracy Requirements

The details of developing autonomous 3-D motion monitoring systems based on commercial off-the-shelf (COTS) motion sensors for hydraulic environments are discussed. Possible areas of application, are river bed sediment transport monitoring and monitoring the agitation and other physical parameters inside milk vats with a mechanized agitator. Simplified calculations of inertial navigation systems (INSs) such as Euler angle method, MATLAB programs for further processing, power management systems for autonomous operation including the possibility of inductive power transfer (IPT) and use of microelectromechanical systems (MEMS) technology are discussed. Experimental results for proof of concept systems are highlighted.

Design Of ‘Smart’ Pebble For Sediment Entrainment Study

The ‘Smart’ pebble is a small self-equipped datalogger having a 40 mm outer diameter with a built-in sensor for acceleration and rotation measurements. This pebble has the capability to detect instantaneous acceleration due to turbulence in fluid, which is indicative of hydrodynamic forces acting on the pebble. The pebble stores the data continuously in its internal memory. These data can subsequently be transferred by serial port to a computer for detailed analysis. Test runs using the smart pebble have been carried out to validate accurate functioning of the acceleration and gyro sensor. The acceleration of the smart pebble was compared to the acceleration of the shake table. The results show the excellent capability of the pebble to record the instantaneous forces acting on the pebble during its entrainment.