Some fluid modeling studies of flow and dispersion over two-dimensional low hills

The results of wind tunnel experiments on flow and dispersion over two-dimensional low and ‘gentle’ hills of different shapes and aspect ratios are discussed. The hill-induced influences on velocity and concentration fields are studied as functions of the hills‘ aspect ratio (the ratio of the half-length to the height of the hill).The speed-up of flow on the hilltop is shown to have an inverse relationship with the aspect ratio ‘a’, as predicted by several theories. However, this does not increase indefinitely as a → 0, but has a maximum value at some optimum aspect ratio greater than unity. The velocity variances and Reynolds stress increase rapidly in the near wake region in the lee of the hills. In the far wake region, beyond about five hill heights, the hill-induced perturbations in mean velocity, Reynolds stress and variances of velocity decay in inverse proportion to the distance behind the hill.Dispersion from elevated sources located on the top of ‘gentle’ hills is not significantly affected by the hills. For low-level sources on the top of steeper hills, where flow separation may occur, the ground level concentrations are reduced (by as much as a factor of 3). On the other hand, the ground level concentrations from sources on the lee side of the hills are considerably enhanced (by as much as a factor of 15) near the source, but slightly reduced far downwind. The influence of‘gentle’ hills bears an inverse relationship with the hills' aspect ratio. For steep hills, however, the hill influence is intimately related to the dimensions of the cavity region.