| technique has been developed to directly measure skin friction in the turbulent boundary layer of a three-dimensional supersonic combustion flow. A floating element cantilever beam configuration has been designed to detect two components of the small tangential shear forces created by the flow passing over the non-intrusive floating element. Although skin friction has been measured successfully in cold flow before, this gauge is uniquely designed for testing in high temperature, high heat flux environments. The application which specifically prompted the development of this technique was the propulsion system for the National Aero-Space Plane. An appreciable need exists to determine the skin friction inside the combustor of this scramjet engine, and the three-dimensional, supersonic, reacting flowfield is difficult to predict numerically. Gauges were tested in a variety of supersonic combustion facilities, with each combustor requiring customization of the gauge design to the test conditions and geometry of the model wall. Maintaining constant temperature of the strain gages inside the sensor throughout the test cycle is of critical importance, due to their sensitivity to temperature changes. Over the course of the project, many parameters which may affect the measurement were identified and examined individually. Changes were adapted into the design when necessary, including tailoring the design to insure that the floating element temperature remains close to that of the surrounding wall. For this reason, a numerical heat transfer study was undertaken to check that the gauge could be adapted for new test conditions. The output from the gauge during supersonic combustion was found to be repeatable for the same nominal input conditions, and this repeatablity was consistent from facility to facility. Results in two-dimensional, supersonic cold flow compare well with predicted flat plate values. Results indicate that... |
