The assessment of boundary layer two-dimensionality

Abstract

SummaryThe results of an experimental evaluation of three assessment criteria for boundary layer two-dimensionality are presented. The criteria include the so-called ‘two pin test', the momentum balance principle and the cross-stream variation in local skin friction coefficient. Extensive spanwise measurements in the flows investigated highlight the relative merits and limitations of the criteria imposed.The momentum balance method is found to be the most informative technique, with the proviso that the test data must be interpreted with some care if measurements are restricted along a single streamwise axis.The two pin test is found to provide a worthwhile complimentary addition to momentum balance data, whereas the spanwise variation in local skin friction coefficient is less conclusive as regards flow two-dimensionality.Excepting the case of adverse pressure gradient flows, the momentum balance equation is seen as a viable experimental means of estimating the local skin friction coefficient in boundary layers where measurement is problematical, as in the case of transitional boundary layers. In such cases, the two pin test could be used as the sole means of confirming the flow two-dimensionality.