School of Mechanical Engineering Seminar
Wednesday, October 24, 2012 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Re-suspension of particles in an oscillating grid turbulent flow

Hadar Traugott

MSc student under supervision of Dr. Alex Liberzon

The incipient motion condition for particulate material exposed to a moving fluid constitutes the central problem for sediment transport in river, coastal areas and atmospheric flows. Furthermore, it is an important mechanism in a variety of engineering applications, such as: detection of explosives and pneumatic conveying. Despite a significant progress in the field of sediment transport during the past decades, description of the mechanisms responsible for the initiation of particle motion from a surface and re-entrainment into suspension remains a challenge. This is partially due to the technical difficulties to quantify the forces applied on the particles and the collection of high-resolution data of particle displacement simultaneously.  In this study we investigated the necessary conditions for initial entrainment of spherical particles from smooth bed into zero-mean-shear turbulent flow in an oscillating grid chamber. Particle image velocimetry (PIV) was used to determine the properties of turbulent flow. Moreover, three-dimensional particle tracking velocimetry (3D-PTV) was used to synchronously measure local flow conditions and track the entrainment of individual test particles through the various phases of the re-suspension. The results provide further insight into the re-suspension process of spherical particles in the transitional range of particle size Reynolds numbers 2< Re_p <500 through identification of the dominant mechanisms that cause re-suspension and better description of the role hydrodynamic forces play in this process.

School of Mechanical Engineering Seminar
Wednesday, october 24, 2012 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Laser Slope Gauge for measuring 2D wave slopes

Edan Sanker

MSc Student of Prof. Lev Shemer

A 2D wave's surface slope gauge was designed and constructed in a small scale wind-wave tank. The instrument is based on Snell's law of refraction. A narrow vertical laser beam pierces the water surface from below and deviates at an angle dependent on the instantaneous surface slope. The refracted beam passes a Fresnel lens and then hits a diffusive screen placed at the focal distance from the lens. The deflection of the beam for this configuration does not depend on the instantaneous surface elevation and can be observed using a high frequency position sensing detector. This is an electro-optic device which converts an incident light spot into continuous 2D position data. The measurement results are thus independent of neither wave height nor light intensity.

The instrument has numerous advantages. It is capable of measuring the surface slope without physical contact with the water surface. Another great advantage is that it measures slopes in two directions simultaneously thus providing directional information, specifically it allows investigation of the angular distribution of wind generated waves. This is in contrast to a single-point measurements using a wave gauge that are incapable of providing data of wave propagation direction. Another advantage is that the instrument can be used for the investigation of small-amplitude capillary waves. These waves have high frequency and often ride on large amplitude gravity waves. It's difficult to measure very high frequency waves (>50 Hz) using the wave gauge because of their extremely small amplitudes. However, these small scale waves exhibit increasing surface slope at higher frequencies thus, making it easier to investigate these waves using the Laser slope gauge. Based on the current configuration the instrument can measure slopes up to about . In addition, the instrument can be moved along the test section so that it is capable of measuring wave slopes at different fetches.

To test the instrument, monochromatic mechanical waves were generated in the flume by a wedge-type wave maker. Preliminary tests provided comprehensive data which verified the instrument's capability of measuring the instantaneous 2D surface slope correctly and with high precision. The verification process involved comparison of the surface slope with respect to the surface elevation measurements by a conventional wave gauge which were simultaneously sampled at the same fetch. Figures 1 and 2 show a typical time series of the 2D surface slope components. Figure 3 exhibits a typical comparison between the slope gauge and the wave gauge measurements evaluating the down wind surface slope component.

Figure 1: Typical Time Series of the Downwind Slope Component.

Figure 2: Typical Time Series of the Crosswind Slope Component.

Figure 3: Typical Comparison of the Downwind Slope Component Measured by the Laser Slope Gauge and the Wave Gauge.