Search for: All records

The sedimentary bed morphology modulated by the wake flow of a wall-mounted flexible aquatic vegetation blade across various structural aspect ratios (AR=l/b, where l and b are the length and width of the blade, respectively) and incoming flow velocities was experimentally investigated in a water channel. A surface scanner was implemented to quantify bed topography, and a tomographic particle image velocimetry system was used to characterize the three-dimensional wake flows. The results showed that due to the deflection of incoming flow, the velocity magnitude increased at the lateral sides of the blade, thereby producing distinctive symmetric scour holes in these regions. The normalized morphology profiles of the sedimentary bed, which were extracted along the streamwise direction at the location of the maximum erosion depth, exhibited a self-similar pattern that closely followed a sinusoidal wave profile. The level of velocity magnitude enhancement was highly correlated to the postures of the flexible blade. At a given flow velocity, the blade with lower aspect ratios exhibited less significant deformation, causing more significant near-bed velocity enhancement in the wake deflection zone and therefore leading to higher erosion volumes. Further investigation indicated that when the blade underwent slight deformation, the larger velocity enhancement close to the bed can be attributed to more significant flow deflection effects at the lateral sides of the blade and stronger flow mixing with high momentum flows away from the bed. Supported with measurements, a basic formula was established to quantify the shear stress acting on the sedimentary bed as a function of incoming flow velocity and blade aspect ratio. 

Turbulence statistics and blade deformations of flexible emergent canopies impinged by water flows were experimentally investigated across a range of Reynolds numbers Reb=Ubb/ν (where Ub is the bulk incoming flow velocity, b is the blade width, and ν is the water kinematic viscosity) and blade aspect ratios AR=h/b (h is the blade length). Time-resolved particle image velocimetry was used to characterize both the deformation of flexible blades and the surrounding flow fields. Results showed that the blade deformation increased with the growth of both Reb and AR, with higher blade bending causing stronger variations in vertical profiles of streamwise velocities and Reynolds stresses. The drag produced by the presence of flexible canopies was identified as the dominant fluid loading balancing the pressure gradient. This term exhibited distinctive reduction near the water surface region with high blade deformation due to the large local blade inclination angle. Interestingly, in contrast to fully submerged flexible blades where the flow-induced drag increases monotonously with flow speed, a critical Reynolds number Reb,cri was observed, beyond which drag decreased with increasing flow speed until the blade became fully submerged. This phenomenon was explained with theoretical interpretations, which exhibited reasonable agreement with experimental results. Further analysis of unsteady flow dynamics revealed that Reynolds stress within the canopy was dominated by ejection events due to the absence of shear layer at the top of emergent canopy. Additionally, streamwise velocity spectra indicated that flow fluctuations inside the canopy were governed by periodic vortex shedding from blade. 

This work investigates the steady-state nonlinear dynamics of a large-deformation flexible beam model under oscillatory flow. A flexible beam dynamics model combined with hydrodynamic loading is employed using large deformation beam theory. The equations of motion discretised using the high-order finite element method (FEM) are solved in the time domain using the efficient Galerkin averaging-incremental harmonic balance (EGA-IHB) method. The arc-length continuation method and Hsu’s method trace stable and unstable solutions. The numerical results are in accordance with the physical experimental results and reveal multiple resonance phenomena. Low-order resonances exhibit hardening due to geometric nonlinearity, while higher-order resonances transition from softening to hardening influenced by inertia and geometric nonlinearity. A strong coupling between tensile and bending deformation is observed. The axial deformation is dominated by inertia, while bending resonance is influenced by an interplay between inertia, structure stiffness, and fluid drag. Finally, the effects of two dimensionless parameters, Keulegan and Carpenter number (KC) and Cauchy number (Ca), on the response of the flexible beam are discussed.