Large-Eddy Simulation Study of Turbulent Flow over Dune Field
The interactive feedback between turbulent flow and dune morphology is significant to understand the formation of dune field and turbulent flow physical attributes. Different kinds of deductions and formulas are proposed via sorts of field measurements and experimental observations, some of which indeed could provide highly precise predictions to sediment transportation and dune morphodynamics. However, it becomes harder to forecast them when the sediment particles suspended by critically high Reynolds number wind regime, given the unsteady turbulent mixing process and wide range of turbulent length scales. In terms of that, the aero/hydro-dynamical effect on dune morphodynamics has been widely studied by analyzing aeolian process, where turbulent coherent structures induced local wind shear enhancement gets revealed by experimental observations and numerical simulations. Several significant characteristics of turbulent flow over dune field such as the dominance of large length scale of turbulent eddies and inertial effect make Large-Eddy Simulation (LES) become a very promising study methodology. In this work, the solid-fluid mutual effects in dune field has been comprehensively demonstrated via LES method associated with Particle Image Velocimetry (PIV) data validation. To approach the universality and veracity, idealized and realistic dune field have been both studied in this work. The idealized barchan dune field consists of four different stages with decreasing streamwise offset but same spanwise offset, aimed to reveal aeolian effects in dune field morphological complexity – offset merger interaction. White Sands National Monument (WSNM) is located at the Tularosa Basin in southern New Mexico, which has been adopted as the ideally realistic case. The simulations in different mesh density are displayed at the end to show the grid insensitivity of this work. The high credibility of LES results has been verified by the PIV results in Appendix A. Through LES, the interactive motion induced downwind dune asymmetric erosion gets well elaborated associated with a coherent structure – interdune roller, where differential helicity calculation reveals its positive streamwise rotation, scouring the sediment on the interdune surface of downwind dune. The decreasing interdune spacing effectively enhances the local momentum flux – flow channeling, which impinges on the stoss face of downwind dune and elevates the surface shear magnitude. The wake centerline misalignment – wake veering – has been observed in LES and PIV results. Isosurfaces of conditionally-averaged and instantaneous Q criterion complementarily reveal the hairpin vortex shedding from the dune brinkline and the streamwise distance of vortex core, which is revealed to be proportional to dune crest height, associated with the constant shedding frequency St = 0.25 after the practice of wavelet analysis. The Reynolds-averaged streamwise vorticity transport equation explain the interdune roller is a consequence of vortex stretching and flow channeling. Meanwhile, through the Reynolds-averaged data initiated simulation, the genesis of turbulent coherent structure has been fully understood, that is generated from streamwise vortex roller (interdune roller) in interdune region and spanwise vortex roller surrounding the brinkline. With wide range of turbulent spectrum involved, the spanwise vortex roller breaks down into aggregations of small rollers which get stretched and tilted into successive hairpin vortices shedding off from dune brinklines along wake centerlines. While, the streamwise vortex roller also support the hairpin evolving processes in small dune leeward within interdune region. The WSNM results favorably testify this turbulence structural model in DFSL as well. Finally, the integral length scale is calculated on different elevations in all cases. The integral length profiles of ideal and realistic case indicate self-similar heterogeneity attribute of turbulent eddies in dune-field boundary layer. The scaling practice of spatial integral length concludes the mixing-layer analogy of the dune-field obstructed shear flow in roughness sublayer and the effectivity of attached-eddy hypothesis at higher located turbulent structures overlaying roughness sublayer.