Visualization and laser measurements on the flow field and sand movement on sand dunes with porous fences

Abstract

The installation of windbreak sand fences around sand dunes is one of the most promising methods to suppress windblown sand movement. In the study reported in this paper, we investigated the influence and validity of a small fence mounted on a model sand dune, in order to understand the fence’s suppression mechanism on the sand movement. The flow field around the dune and the process of sand-dune erosion were measured using LDV, PIV, and laser-sheet visualization techniques. A non-porous fence was found to suppress sand movements in its upstream area, but to enhance erosion downstream of the fence. This intensive erosion was caused by separated shear flow from the leading edge of the fence. In this study, four levels of porosity rate of the fence were tested. The fence-porosity dependences of the turbulent flow field and the erosion were discussed. The shapes of eroded sand dunes were found to depend on the porosity rate. The relationship between the sand-dune erosion and the flow field around the dune was illustrated with schematic diagrams. We concluded that the most desirable fence porosity should be 30% in order to avoid dune erosion if installed at a middle height on the stoss surface of a dune. This porosity provides a mean velocity reduction with avoiding a separated flow, although the flow bleeding through the porous fence is accompanied by grid turbulence and induces serious erosion in a narrow space behind the fence. Furthermore, we confirmed that the empirical correlation of the critical friction velocity can be applied to sand movements influenced by a fence.