Mangrove trees form dense networks of prop roots in coastal intertidal zones. The interaction of mangroves with tidal and river flows is fundamental to the preservation of estuaries and shorelines by providing water filtration, protection against erosion, and habitat for aquatic animals. The impact of mangrove roots on unidirectional flows is mainly characterized by hydrodynamics including drag, flow structures, transport, and fluid-structure interaction. In this work, we focus on the drag coefficient and flow structure downstream of simplified mangrove root models. The mangrove roots were modeled as a cluster of uniformly distributed rigid circular cylinders (patch), with a frontal area per unit volume $\left(a\right)$. Direct force measurements were made in a recirculating water flume. In addition, the unsteady wake was measured using particle image velocimetry (PIV). The models were tested for a Reynolds number range of 600 to 12 000 based on the patch diameter $\left(\mathrm{R}{\mathrm{e}}_D=\frac{\rho UD}{\mu }\right)$, in order to resemble natural conditions. A new length scale, the “effective diameter,” is proposed by comparing the Strouhal number of the patches with the analytical Strouhal number of a canonical cylinder in the flow field that produces the same vortex shedding. We compared different length scales to characterize the hydrodynamics, including the patch diameter $\left(D\right)$, equivalent length proposed by Mazda $\left(L_E\right)$, and an effective diameter $D_{\mathrm{eff}}$. A universal empirical curve describing the drag coefficient based on $\left(L_E\right)$ and $D_{\mathrm{eff}}$ is also presented. It was found that the effective diameter was able to capture competing parameters including patch diameter, porosity, and cylinder diameters into a single parameter to obtain the drag coefficient of the physical models. The results revealed that the time-average drag coefficient decreased with an increased Reynolds number and porosity. It was found that the ratio of $\frac{C_D}{\mathrm{St}}$ to the blockage parameter $(C_{D}aD)$ exhibits a linear relationship, indicating that the parameter $\mathrm{St}aD$ is constant for all patches considered. This finding was also valid using equivalent length and effective diameter as the characteristic lengths. In addition, based on time-resolved PIV results downstream of the physical model, we found that the vorticity magnitude decays and the vortex structure is more streamlined with an increase in porosity. This analysis of the hydrodynamics of mangrove rootlike models can also be extended to predict values of drag coefficient in other canopy flows, including submerged arrays, flexible elements, and bio-inspired coastal infrastructures.

• Received 23 August 2017

DOI:https://doi.org/10.1103/PhysRevFluids.3.073801