Study on Velocity Distribution Characteristics of Fully Developed Gap Flow with Composite Boundaries

Gap flows with composite moving–stationary boundaries are widely encountered in fluid machinery. The coupling of these two boundary effects directly alters flow characteristics including velocity distribution and pressure evolution. Most existing studies on gap flows focus on flows with fully stationary walls or a single moving wall, while systematic investigations into the coupling mechanism of dual boundaries remain insufficient. Therefore, taking the gap flow in a square pipe with composite boundaries as the research object, this paper adopted a combined method of theoretical analysis and physical experiments to investigate the influences of the velocity ratio ε (the ratio of the moving velocity of the flat plate to the cross-sectional average velocity of the gap flow), the Reynolds number (the gap flow Reynolds number based on the hydraulic diameter Dh = 2h, where h denotes the gap height) and the blockage ratio (the ratio of the height of the flat plate to the height of the square pipe) on the position of maximum flow velocity and velocity distribution within the parameter ranges: velocity ratio 0–1, Reynolds number 29,857–49,761, and blockage ratio 0.5–0.8. The dimensionless number η characterizing the position of maximum velocity was identified via theoretical derivation as a key parameter for solving velocity distribution. Experimental results indicated that an increase in the velocity ratio shifted the dimensionless position of maximum velocity toward the moving boundary. There existed a strong coupling effect between the Reynolds number and the velocity ratio: the Reynolds number exerted a weak influence on η under low velocity ratios, whereas the superposition of a high velocity ratio and a high Reynolds number drove the velocity peak markedly closer to the moving wall. A smaller blockage ratio made the maximum velocity position approach the stationary boundary, and the flow under the condition β = 0.5 possessed stronger anti-disturbance capacity. Velocity distribution curves demonstrated that a rising velocity ratio flattened the velocity profiles. At Re = 29,857, all curves under various conditions intersected at a characteristic point at y/h = 0.23, where the velocity varied oppositely with the velocity ratio on either side of the intersection. Increasing the Reynolds number raised the velocity across the entire flow field and improved velocity uniformity; meanwhile, the intersection point shifted toward the gap center, and the modulation effect of the velocity ratio on the flow field was enhanced. At a constant Reynolds number, a larger blockage ratio corresponded to higher velocity, and the intersection also moved toward the gap center, accompanied by greater dispersion of velocity distribution curves under different velocity ratios. The results will enrich the theoretical basis of gap flow and provide theoretical references for flow analysis and structural optimization under similar gap conditions.