[1] Large-eddy simulation (LES) of turbulence in plant canopies has traditionally been validated using bulk statistical quantities such as mean velocity and variance profiles. However, turbulent exchanges between a plant canopy and the atmosphere are dominated by large-scale coherent structures, and therefore LES must also be validated using statistical tools that are sensitive to details of coherent structures. In this study, LES and measurements using particle image velocimetry (PIV) are compared near the top of the canopy by means of a quadrant-hole analysis of turbulent kinetic energy, vorticity, and dissipation rate. The LES resolves coarse features of individual corn plants and uses the Lagrangian scale-dependent dynamic subgrid model. At the measurement location, there is good agreement between the LES predictions and the field data in terms of most conditionally sampled quantities, confirming the applicability of LES for fundamental studies of vegetation-air interactions and coherent structures. The simulation results confirm that sweeps (the fourth-quadrant events) contribute the largest fraction of turbulent kinetic energy, vorticity, and dissipation rate inside the plant canopy. The magnitudes of the vorticity and dissipation rate at the top of the canopy are highest in the first quadrant (rare events of outward interactions).

SUMMARYThis paper presents a numerical method that couples the incompressible Navier-Stokes equations with the level set method in a curvilinear co-ordinate system for study of free surface ows. The ÿnite volume method is used to discretize the governing equations on a non-staggered grid with a four-step fractional step method. The free surface ow problem is converted into a two-phase ow system on a ÿxed grid in which the free surface is implicitly captured by the zero level set. We compare di erent numerical schemes for advection of the level set function in a generalized curvilinear format, including the third order quadratic upwind interpolation for convective kinematics (QUICK) scheme, and the second and third order essentially non-oscillatory (ENO) schemes. The level set equations of evolution and reinitialization are validated with benchmark cases, e.g. a stationary circle, a rotating slotted disk and stretching of a circular uid element. The coupled system is then applied to a travelling solitary wave, and two-and three-dimensional dam breaking problems. Some interesting free surface phenomena are revealed by the computational results, such as, the large free surface vortices, air entrapment and splashing of the water surge front. The computational results are in excellent agreement with theoretical predictions and experimental data, where they are available.

Turbulent flow in a corn canopy is simulated using large-eddy simulation (LES) with a Lagrangian dynamic Smagorinsky model. A new numerical representation of plant canopies is presented that resolves approximately the local structure of plants and takes into account their spatial arrangement. As a validation, computational results are compared with experimental data from recent field particle image velocimetry (PIV) measurements and two previous experimental campaigns. Numerical simulation using the traditional modelling method to represent the canopy (field-scale approach) is also conducted as a comparison to the plant-scale approach. The combination of temporal PIV data, LES and spatial PIV data allows us to couple a wide range of relevant turbulence scales. There is good agreement between experimental data and numerical predictions using the plant-scale approach in terms of various turbulence statistics. Within the canopy, the plant-scale approach also allows the capture of more details than the field-scale approach, including instantaneous gusts that penetrate deep inside the canopy.

Turbulent open-channel flow over a two-dimensional laboratory-scale dune is studied using large eddy simulation. Free-surface motion is simulated using a level set method. Two subgrid scale models, namely, dynamic Smagorinsky and dynamic two-parameter models, are employed to assess model effects on the free surface. It is found that the two models have very similar performance in predicting the free-surface turbulence. Two flow depths are simulated to investigate the effects of water depth on flow coherent structures and turbulence statistics. In the deep-water flow, experimental data are used to assess for the numerical predictions of the mean flow field and turbulence statistics. They are found to be in good agreement. In the shallow-water flow, there is strong interaction between the free-surface and large-scale vortical structures emanating from the bed, increasing turbulence intensity and free-surface disturbance. The simulations predict streaky structures in the wall layer after flow reattachment in the deep-water flow, but not in the shallow-water case, suggesting that interaction between the free surface and the flow structures is significantly affected by the flow depth.

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