To identify the vegetation dynamics and relationship with the hydrothermal conditions in the Yellow River basin (YRB), the spatial–temporal variations of EVI, effective precipitation (Epr), accumulated temperature (At), and their relationships were obtained based on the MODIS EVI data and meteorological data from the YRB during 2001–2020. The results indicate that EVI trends increased during 2001 to 2020, especially in the farmland, forestland, and grassland ecosystems. Epr and At have also increased over the last 20 years. Epr mostly increased faster in the grassland, and water bodies and wetland ecosystems. At mostly increased faster in the water bodies and wetland, desert, and forest ecosystems. Affected by Epr and At, the correlation between the EVI and hydrothermal conditions varied under different hydrothermal conditions. Compared to the At, the Epr was the restrictive factor for the EVI variations in the terrestrial ecosystem in the YRB. In addition, the dynamical thresholds of the EVI, Epr, and At were confirmed. This study can improve the understanding of vegetation variations and their response to regional climate change, which is critical for ecological conservation and the high-quality development of the YRB.

The secondary flow is deflected under pressure and superimposed on the main flows. This research investigated its characteristics, including velocity gradient, vorticity, shear stress, and Reynolds stress in unpressurized circular pipes, through physical experiments and Computational Fluid Dynamics numerical simulations. Combining numerical simulations with the physical experiments under three flow rates (30 m /h, 35 m /h, and 40 m /h) and width–perimeter ratios ( W = 0.43, 0.4, and 0.35), the experimental data demonstrate the secondary-flow propagation in unpressurized circular pipes. The secondary flows manifest as deviations of velocities, substitutions of secondary vortices, and flips of shear stresses, which present decay tendencies and are negligible at 52 times the pipe diameter. The secondary flows are driven by the velocity gradient or Reynolds stress, the dominance of which shifts with the increase of diffusion distance. The secondary flow turbulence is reduced and smoothing when the width–perimeter ratio reaches a threshold (approximately W = 0.40) because of Dean vortices collisional depletion.