Mean airflow patterns upwind of topographic obstacles and their implications for the formation of echo dunes: A wind tunnel simulation of the effects of windward slope

Secondary airflow plays an important role in dune formation and development. In the case of the echo dunes that form upwind of an obstacle, it introduces considerable complexity because both the initiation and development processes are influenced by airflow patterns. In this study, we measured the variations of airflow in front of obstacles with different windward (stoss) slope angles by means of particle-image velocimetry in a series of wind tunnel tests. The windward slope angle was the key factor in determining the secondary airflow patterns. The horizontal velocities decreased and the vertical velocities increased as the airflow approached perpendicular to the obstacle, and a vortex of reversed flow formed in front of obstacles with a stoss slope of 60 degrees or steeper. The positions of airflow separation and of the core of the reversed vortex were a function of the windward slope angle. Depending on the position of the reverse vortex and its significance for the formation of echo dunes, the windward slope could be divided into three groups (65 degrees or less, 70 degrees-75 degrees, and 80 degrees or more). The horizontal velocity profiles deviated from a log linear distribution, resulting in five airflow regions with different rates of change of horizontal velocity. We discuss the significance of these velocity variations for sand accumulation and the effects of each airflow region on echo dune formation.