300 / 2024-02-29 10:31:54

Numerical investigation of flow and turbulent characteristics in submerged flexible canopies under fixed bending state

submerged flexible canopy flows; fixed bending deformation; shear layer; turbulent structure; canopy resistance

Submerged flexible canopies are widely distributed in rivers, coasts and lakes. When subjected to water flow, the canopies are usually bent and deformed to varying degrees, leading to changes in the turbulent structure and canopy resistance of the shear layer at the interface. To determine the fixed bending patterns of the canopy by fluid-solid coupling calculations, this study performs large eddy simulations of the canopy with different stiffnesses for the same bulk flow velocity (stem Reynolds number Re_d = 1200). These cases can reflect the influence of pure bending deformation on the canopy flows under the equilibrium state of vegetation. At the macro level, canopy bending causes a reduction of the overall water-blocking effect, shear strength and turbulent energy in the channel. The dense canopy flows develop towards sparse canopy flows or wall turbulent flows containing roughness height, and the velocity profile gradually approaches a logarithmic distribution, but the top of the canopy still possesses a similar large-scale coherent structure which penetrates into the bottom bed after a certain degree of deformation (canopy density C_dah_s = 0.26 - 0.37). Instead, the near-bed generates larger scaled eddies and stronger shear stresses (1.8 times) in magnitude by this time. Double-averaged profiles show that the maximum Reynolds stress and turbulent kinetic energy both decrease by about 76% as Cauchy number (Ca) rises from 1.5 to 750. Considered from the micro perspective, the bending deformation leads to a progressive dissipation of the clockwise recirculation zone at the rear tip of vegetation, and the flows bypass the top traveling downstream, thus causing lower momentum deficit. On the other hand, the enclosed counterclockwise circulation formed in the lower part relatively retards the process of canopy-scale vortices (K-H vortex) bottoming out.