Surface Tension in Froude Models

A criterion for avoiding surface tension effects in Froude models is obtained analytically for uniform flows. Experiments show that surface tension does not, in practice, limit the scale in curvilinear flow Froude models. In uniform flows the least scale is determined by instrumentation accuracy and Froude number or by the need to exceed a limiting Reynolds number. The least scale is found by including a surface energy term in the energy equation or by considering the celerity of small free surface disturbances. The scale is chosen so that experimental reading error scatter will mask any dependence on surface tension. Experiments on three weir geometries, for which flow at one head is interpreted as an imperfect Froude model of flow at a different head, are used to examine surface tension effects in Froude models of curvilinear flows. The viscosity was kept essentially constant and the surface tension varied using an additive. For all practical purposes variations due to surface tension are masked by the inherent scatter of the data.