TOWARD A MODEL FOR AIR-FLOW ON THE LEE SIDE OF AEOLIAN DUNES

The interaction between dunes and the primary wind results in a comple x pattern of secondary airflow on the lee side of dunes. From 15 dunes studied during transverse flow conditions at Padre Island in Texas, W hite Sands in New Mexico, and the Algodones in California, distinct no w regions can generally be recognized, with the overall flow structure comparing favourably to that proposed for subaqueous bedforms. Downwi nd of dunes with now separation is a back-now eddy that extends about four dune-brink heights downwind from the brink of the dune. Beyond th e separation cell, the velocity profiles can be divided into regions b ased upon segments separated by 'kinks' in the velocity pro files. The interior is an area above the dunes of relative high. wind speed but low velocity gradient. Beneath the interior is the wake, which consist s of two layers. The upper wake exhibits an uppermost portion where th e flow decelerates while the remainder exhibits accelerating flow, so that the overall velocity gradient decreases downwind. The lower wake exhibits low velocity gradients and wind speeds that accelerate downwi nd at all heights, but primarily near the top of the layer, thereby ca using the velocity gradient to increase downwind. At about eight dune heights downwind, the upper and lower wakes equilibrate to a single pr o file with the kink between them no longer apparent. The lowest recog nizable region is the internal boundary layer. It is recognized by a r elatively steep velocity gradient below the wake, and never exceeds a few tens of centimetres in height for our data set. Because of acceler ation and increasing shear stress within this layer, interdune flats a re at least potentially erosional. Overall, the wake and internal boun dary layer show a downward transfer of momentum from upper regions so that the flow recovers. Where flow separation does not occur, simple f low expansion down the lee-face causes flow deceleration.