An experimental approach to sand particle cloud dispersion under stagnant and controlled turbulence: new insights for water engineering applications

Abstract

This thesis presents a comprehensive experimental investigation into the dispersion and evolution of sand particle clouds in water, emphasizing the development of a novel programmable facility capable of generating controlled turbulent background flow. A custom-built design including forty-pumps was designed to generate reproducible, near-zero- mean turbulence in a confined tank, where flow intensity could be adjusted through programmed activation sequences and the use of mesh screens with different openings. The facility enabled systematic variation of turbulence intensity from approximately 2.5% under weak forcing to 9.4% under strong forcing, allowing discrete and repeatable classification of weak, medium, and strong background turbulence regimes. This range overlaps with turbulence intensities reported in energetic natural environments, where field measurements in coastal and tidal flows indicate strong turbulence levels on the order of 9-12%. The designed system provides an unprecedented level of control and repeatability, enabling systematic exploration of particle-turbulence interactions under well-defined conditions. A MATLAB-based image analysis framework was established to extract quantitative parameters from high-contrast recordings. The workflow included background subtraction, binarization, inversion, air-bubble removal, boundary detection, and concentration tracking using the normalized red channel (R/255), where R, G, and B denote the red, green, and blue pixel intensities and R = G = B in this setup. In all experiments, sand particle clouds were generated by releasing dry sand through a vertical nozzle into a confined water-filled tank, forming negatively buoyant particle clouds without premixing with water. The tank geometry was fully confined, allowing controlled background turbulence to be imposed using the programmable pump array. Particle size was uniform within each test and characterized by the Stokes number, with multiple particle sizes examined systematically across the study. From the processed frames, penetration length, frontal velocity, lateral width, projected area, and an image based entrainment coefficient were determined. The effects of background turbulence intensity, particle size, and release geometry on particle cloud evolution were examined. Controlled turbulence was found to interrupt deep descent, enhance lateral spreading, accelerate breakup, and increase entrainment compared to stagnant ambient. Fine particles responded strongly to turbulent fluctuations, forming diffuse and laterally extended clouds, whereas coarse particles retained compact symmetry and settled quickly. Additional experiments were conducted in an immiscible oil–water system in which sand particles were released through the oil layer into the underlying water phase, allowing the effects of interfacial resistance and release configuration on sand particle cloud formation to be examined. Complementary tests in stagnant water revealed that higher release energy and aspect ratio promote early penetration and wider cloud development. Together, these studies established a unified framework for understanding the coupled effects of turbulence, geometry, and particle size on sand cloud evolution. The novel turbulence generation system and analysis methodology provide quantitative benchmarks for future investigations and offer insights applicable to sediment transport, dredging, wastewater discharge, and other particle-laden flow processes where controlled mixing and dispersion are essential. Keywords: Controlled background turbulence; Image-based analysis; Acoustic Doppler Velocimetry (ADV); Sand particle clouds