Particle motion was observed using high-speed imaging technology to investigate the fluid forces acting on particles in flowing water.The drag coefficient and lift coefficient were obtained by solving the mechanical equations for saltating spheres and natural sediments.Experimental results manifest that:1)The average drag coefficient for particles with nonuniform velocity in flowing water,particles with nonuniform velocity in stilling water,and particles with uniform velocity in stilling water all decrease with the increase of Reynolds number.The difference between these coefficients diminishes when the particle-fluid relative velocity approaches the settling velocity.When the rela-tive velocity equals the settling velocity,the coefficients are approximately equal.2)Shape has a greater influence on lift coefficient than on drag coefficient;natural sediments exhibit larger average drag coefficient compared to spheres,whereas spheres demonstrate higher average lift coefficient than natural sediments.The equations for drag coefficient and lift coefficient of spheres and natural sediments are established,and the calculated values agree well with measured data.
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