On the Separation of Turbulent Boundary Layers

S T U D E N T S of fluid mechanics agree now rather generally that the separation of the boundary layer from a wing surface is the cause of burbling and is responsible, therefore, for the complicated phenomena near the stall. There is consequently a great practical as well as scientific interest in obtaining an analytical method of predicting such separation. One method of analyzing flow separation in the case of a turbulent boundary layer was proposed by Gruschwitz and seemed to offer a practical solution. The limited experimental evidence in favor of this method, however, was not sufficient to command confidence in its general application. In the present research an attempt was made to verify the Gruschwitz method by an extended experimental examination of the turbulent boundary layer over a typical wing profile at high Reynolds Numbers. The data here presented lead to the conclusion that the Gruschwitz method does not describe the actual phenomena and is, therefore, not the general solution hoped for. The evidence now seems to point to a dependence of flow separation upon several other variables besides the pressure gradient and the thickness of the boundary layer which need further experimental examination. Such investigation is now under way at the Massachusetts Institute of Technology and a further report will be made in a subsequent paper. The mechanics of flow separation depends upon the motion of the air within the boundary layer. The particles within the boundary layer are retarded by the surface friction and are usually carried against a rising pressure by the momentum transferred to them from outside of the boundary. If the pressure rise is sufficient the particles lose energy, come to rest, and separation occurs. If the momentum is transferred from the exterior stream by molecular exchange purely, the motion is defined as laminar, while the motion is defined as turbulent if the exchange is molar. All boundary layers begin at a stagnation point and remain laminar for a certain distance downstream whatever may be the character of the exterior flow or of the surface. If the prevailing velocity is sufficiently high and the surface sufficiently large, however, the layer will become turbulent and continue so throughout the remainder of its length. The chief characteristic which distinguishes the turbulent flow from the laminar is its much greater ability to transport momentum laterally. This gives it a correspondingly greater ability to proceed against a rising pressure. The separation of laminar boundary layers has recently been studied by von Karman and Millikan who explained the variation of the maximum lift coefficient with low Reynolds numbers in the case of certain wing profiles. The separation of the turbulent boundary layer, however, is of greater importance at the higher Reynolds numbers used in engineering since it is responsible for most of the burbling phenomena met in practice. The studies presented here are an outgrowth of a recent investigation by Gruschwitz on the behavior of turbulent boundary layers for various pressure gradients. Since this method is used for comparison with the experimental data here reported, it will be well to review it briefly.