Morphologic and Computational Fluid Dynamic Analysis of Sand Dune-Topographic Obstacle Interactions on Earth and Titan
Abstract:Earth and Titan have vastly different physical environments but similar landscapes. Sand dunes, like those found in Earth’s deserts, cover large areas of Titan’s equatorial region and are important records of climate. Titan’s linear dunes and their interaction with topographic obstacles within the dune fields suggest westerly wind flow, which is opposite the easterly flow predicted from several global climate models (GCMs). This interpretation of wind direction is largely based on the notion of the dunes as streamlines that flow around the obstacles. However, the mechanics of this behavior in granular, sandy material and bimodal flows are poorly understood. We examine the interactions between linear dunes and topographic obstacles by mapping the morphology of the obstacles and nearby dunes and using computational fluid dynamic (CFD) analysis of wind flow near obstacles.
We map crest line orientation, length, and spacing of the dunes using visible satellite imagery for Earth and radar imagery for Titan. Obstacles are mapped from both satellite imagery and digital terrain models (DTMs). Topographic information about the obstacles, taken from DTMs, and wind data for Earth and Titan, taken from either wind stations or GCMs, are input into a CFD model called WindNinja 2.2.0. We use the CFD model to estimate wind velocity and direction near the obstacles, calculate gross bedform normal transport, and determine dune orientations around the obstacles. Our results indicate greatest variations in wind velocity and direction for regions where wind initially strikes the topographic obstacle and uniform wind flow patterns upwind and downwind of the obstacle. The modeled dune orientations deviate greatest from the mean regional orientations around areas of elevation change. This behavior is consistent on both Earth and Titan. Identifying how dune orientations are affected by topography on Titan provides a new basis to evaluate wind directions on Titan.