Analysis of the force characteristics of buried particles in a hydraulically collected flow field under density stratified flow

Hydraulic collection technology, owing to its low disturbance and high efficiency, holds broad prospects in applications such as seabed mineral extraction, biological resource harvesting, and waste recovery. However, existing studies are mostly based on “clear-water” conditions, ignoring the realistic scenario where target particles are buried by seabed sediments. In actual seabed environments, the sediment and the overlying seawater form a density-stratified system, which significantly alters the suction flow field structure and affects the forces acting on the particles. To address this, the present study proposes an idealized two-layer fluid model, simplifying the sediment layer as a Newtonian fluid with a higher density but the same viscosity as clear water, focusing on the core physical feature of density stratification while temporarily neglecting complex factors such as inter-particle friction and non-Newtonian rheology. Based on an experimentally validated volume-of-fluid two-phase flow numerical method, the effects of pipe Reynolds number (Rep = 100 000–200 000), density ratio (m = 1–2), and burial depth ratio (r = 0–1.25) on the vertical force coefficient of a buried particle are systematically investigated. The results show that large-scale vortical structures generated at the density-stratified interface enhance the energy transfer in the flow field, leading to a larger intensity and extent of the negative-pressure region above the particle than those under clear-water conditions. As the density ratio and burial depth ratio increase, the particle force coefficient exhibits an increasing trend. A higher-density medium increases the turbulent dissipation rate, whereas a greater burial depth compresses the flow field above the particle, further enhancing the negative-pressure effect. This study provides a reference for research on hydraulic collection technology.