An Approach to Measuring Step Excrescence Effects in the Presence of a Pressure Gradient

Abstract

An experimental study was undertaken to determine the effects of step excrescences on boundary layer transition using a unique ground test facility in which the test model was propelled though still air. The models used were designed to have a nominally constant pressure gradient so that the results would be relevant to laminar flow aircraft whose wings often have long runs of mildly favorable pressure gradient. The models had an integrated continuously adjustable twodimensional step, which could be adjusted to be forward-facing or aft-facing. The large model was used to increase the Reynolds numbers examined so that the results are applicable to laminar flow flight vehicles. Multiple measurement methods, including Preston tubes, hot wires, accelerometers, a boundary layer traverse, and static pressure taps were used to provide comparison data, and to add to the physical understanding of the results. The propelled-model test approach required that the instrumentation be self-contained and ride along with the model as the carrier vehicle moved down the test track. Due to the relatively short times available for data-taking (approximately 15-30 seconds per run), the initialization and data analysis techniques had to be tailored for this application. Nomenclature C p= pressure coefficient

Hot-Wire Measurements of the Influence of Surface Steps on Transition in Favorable Pressure Gradient Boundary Layers

“…Building on the experimental and computational work performed as part of the Manufacturing Tolerances for Laminar Flow (MEATLOAF) project 1,2 , this study (present paper and two companion papers of Bender et al 17 and Drake et al 18 ) develops general criteria for the influence of surface steps on transition using a unique experimental facility. The experimental project was conducted in a Towing Wind Tunnel (for more details see the companion papers 17,18 ) where a test model can be towed (moved) in still air allowing low background disturbance flow over the model. Models were designed to cover wide range of pressure gradients and the maximum transition Reynolds number reached on the models was more than 4x10 6 .…”

“…Models were designed to cover wide range of pressure gradients and the maximum transition Reynolds number reached on the models was more than 4x10 6 . The focus of the present paper is the experimental hot-wire anemometry investigations of the boundary layer flow on two models and a comparison with skin friction measurements obtained using arrays of Preston probes 17 . The goals of these measurements were: to document transition onset for a moving model in open air with natural disturbance environment ("flight" conditions) rather than for a static model in a conventional wind tunnel with an artificial disturbance environment; to find the mechanism of the transition in varied flow conditions such as stream-wise pressure gradient and size of excrescences; to compare the results on transition location obtained from velocity fluctuations and spectra measurements with those obtained from skin friction measurements.…”

“…The models were mounted vertically on the HART carrier vehicle between two splitter plates (figure 2). For more details about the test facility and model design refer to Bender et al 17 .…”

Step Excrescence Effects for Manufacturing Tolerances on Laminar Flow Wings

“…Measurements were made of skin friction distributions to determine transition locations. 10 Hot wire measurements were also performed.…”

“…From the measured skin friction distributions, transition locations were determined for each step tested. 10 Both forward-facing and aft-facing steps were examined. The sizes of the steps were selected based on their observed influence on the transition location: small steps were initially tested, their heights increased until the transition location was moved forward from the no-step location, step height continued to be increased until the transition location was observed to be immediately downstream of the step.…”