Browsing by Subject "laminar flow"
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Item Computational Evaluation of a Transonic Laminar-Flow Wing Glove Design(2012-07-16) Roberts, Matthew WilliamThe aerodynamic benefits of laminar flow have long made it a sought-after attribute in aircraft design. By laminarizing portions of an aircraft, such as the wing or empennage, significant reductions in drag could be achieved, reducing fuel burn rate and increasing range. In addition to environmental benefits, the economic implications of improved fuel efficiency could be substantial due to the upward trend of fuel prices. This is especially true for the commercial aviation industry, where fuel usage is high and fuel expense as a percent of total operating cost is high. Transition from laminar to turbulent flow can be caused by several different transition mechanisms, but the crossflow instability present in swept-wing boundary layers remains the primary obstacle to overcome. One promising technique that could be used to control the crossflow instability is the use of spanwise-periodic discrete roughness elements (DREs). The Flight Research Laboratory (FRL) at Texas A&M University has already shown that an array of DREs can successfully delay transition beyond its natural location in flight at chord Reynolds numbers of 8.0x10^6. The next step is to apply DRE technology at Reynolds numbers between 20x10^6 and 30x10^6, characteristic of transport aircraft. NASA's Environmentally Responsible Aviation Project has sponsored a transonic laminar-flow wing glove experiment further exploring the capabilities of DRE technology. The experiment will be carried out jointly by FRL, the NASA Langley Research Center, and the NASA Dryden Flight Research Center. Upon completion of a wing glove design, a thorough computational evaluation was necessary to determine if the design can meet the experimental requirements. First, representative CAD models of the testbed aircraft and wing glove were created. Next, a computational grid was generated employing these CAD models. Following this step, full-aircraft CFD flowfield calculations were completed at a variety of flight conditions. Finally, these flowfield data were used to perform boundary-layer stability calculations for the wing glove. Based on the results generated by flowfield and stability calculations, conclusions and recommendations regarding design effectiveness were made, providing guidance for the experiment as it moved beyond the design phase.Item Design of an Instrumentation System for a Boundary Layer Transition Wing Glove Experiment(2012-08-23) Williams, Thomas 1987-Laminar flow control holds major promise for increasing aircraft efficiency and increasing laminar flow over aerodynamic surfaces could decrease drag by up to 30 percent. The Flight Research Lab at Texas A&M University has studied laminar flow over a wing with 30 degrees of leading edge sweep with Discrete Roughness Elements (DREs) installed and has indicated that DREs can be used to increase laminar flow at Reynolds numbers up to 7.5 million at Mach 0.3. A new project, termed SARGE, has been commissioned in conjunction with NASA for studying DREs on a swept wing glove at conditions relevant to jet transports. The SARGE project must have an instrumentation system capable of accurately measuring flow conditions and transition location on the suction side of the glove. Infrared (IR) thermography has been selected as the primary transition detection tool. A heat transfer analysis has shown that solar radiation will warm the surface of the glove above the adiabatic wall temperature and therefore the laminar region will appear to be warmer. The FLIR SC8000 IR camera has been selected for this application due to its ability to produce high-resolution images in the appropriate IR band. High quality air data is also required for the experiment. A five-hole probe will be used to measure flow angle and velocity near the glove. This instrument will provide meanflow conditions due to its limited frequency response. High quality pressure transducers coupled with careful probe calibration will allow for differential measurements to be made with an uncertainty of +/- 0.03 degrees. Static pressure ports and high frequency response Kulite transducers will also be employed. Hotfilm sensors will be used to verify the state of the boundary layer on the glove through spectral analysis. A unique hotfilm array has been proposed that will enable the measurement of traveling wave vectors through a spectral technique. An experiment on the Flight Research Lab's Cessna O-2 to investigate the veracity of this technique has also been suggested. Thermocouples will also be installed on the glove's surface to monitor temperatures and verify transition location. The layout of the hotfilms and thermocouples is also detailed.Item Experimental and numerical study of laminar forced convection heat transfer for a dimpled heat sink(2009-05-15) Park, Do SeoItem Study of Laminar Flow Forced Convection Heat Transfer Behavior of a Phase Change Material Fluid(2010-01-14) Ravi, GurunarayanaThe heat transfer behavior of phase change material fluid under laminar flow conditions in circular tubes and internally longitudinal finned tubes are presented in this study. Two types of boundary conditions, including uniform axial heat flux with constant peripheral temperature and uniform axial and peripheral temperature, were considered in the case of circular tubes. An effective specific heat technique was used to model the phase change process assuming a hydrodynamically fully-developed flow at the entrance of the tube. Results were also obtained for the phase change process under hydro dynamically and thermally fully developed conditions. In case of a smooth circular tube with phase change material (PCM) fluid, results of Nusselt number were obtained by varying the bulk Stefan number. The Nusselt number results were found to be strongly dependent on the Stefan number. In the case of a finned tube two types of boundary conditions were studied. The first boundary condition had a uniform axial heat flux along the axis of the tube with a variable temperature on the peripheral surface of the tube. The second boundary condition had a constant temperature on the outer surface of the tube. The effective specific heat technique was again implemented to analyze the phase change process under both the boundary conditions. The Nusselt number was determined for a tube with two fins with different fin height ratios and fin thermal conductivity values. It was determined that the Nusselt number was strongly dependent on the Stefan number, fin thermal conductivity value, and height of the fins. It was also observed that for a constant heat axial flux boundary condition with peripherally varying temperature, the phase change slurry with the internally finned tube performed better than the one without fins. A similar trend was observed during the phase change process with internal fins under the constant wall temperature boundary condition.Item The Effects of Step Excrescences on Swept-Wing Boundary-Layer Transition(2014-08-12) Duncan, Jr., Glen T.The immense fuel savings and environmental benefits of reducing aircraft skin-friction drag through laminar flow is well known. However, obtaining substantial laminar flow on an aircraft in an operational environment has proven to be a difficult challenge due to surface imperfections (e.g. 2-D steps and gaps, surface roughness, bug-strikes, paint chips, etc.). One controllable imperfection is a 2-D step through requiring certain manufacturing tolerances between adjacent wing panels. Current methods for designing a laminar-flow aircraft require designing and manufacturing with overly-restrictive tolerances obtained from unswept flat-plate experiments. This research aims at giving designers more realistic manufacturing tolerances for a typical, swept-wing transport aircraft. A 30? swept-wing model with a movable leading-edge extending to x/c = 0.15 is used in the flight environment and in a low-disturbance wind-tunnel to study the effect of 2-D step excrescences in a three-dimensional boundary layer. Forward- and aft-facing steps are modulated during the tests. The design of the test article and the internal actuation system is documented in detail. A structural and safety analysis is provided for the flight testing on a Cessna O-2A Skymaster. A flutter and handling-quality clearance flight proved the new test article is safe for the flight-testing experiments. Pressure measurements are compared with computational results, infrared thermography is used to globally detect boundary-layer transition, and hotwire measurements provide details of the boundary-layer profiles in the vicinity of the steps. An analysis of the results is provided including comparisons of both the wind tunnel and flight environment, and from experimental studies of an unswept model of similar 2-D pressure gradient. The crossflow instability is believed to dominate the transition process up to the critical step height, while the shear-layer instability dominates after the critical step height. The critical step height was found to be a function of unit Reynolds number. Also, the addition of leading-edge sweep with a similar 2-D pressure gradient substantially lowers the local Reynolds-based critical step height for forward-facing steps, while it is similar for the aft-facing steps. However, a substantial increase in the conventional laminar-flow tolerances can be made confidently if a favorable pressure gradient is implemented.