Several flow control schemes were designed and tested to determine the most suitable method for reducing the momentum deficit in a rotor wake and thus attenuate rotor-stator interaction noise. A secondary concern of the project was to reduce the amount of blowing required air for wake filling and thus limit the efficiency penalty in an aircraft engine environment. Testing was performed in a linear blow down cascade wind tunnel, which produced an inlet Mach number of 0.345. The cascade consisted of five blades with the stagger angle, pitch, and airfoil cross-section representative of 90% span of the rotor geometry for NASA's Active Noise Control Fan (ANCF) test rig. The Reynolds number for the tests was based on inlet conditions and a chord length of 4 inches. Trailing edge jets, trailing edge slots, ejector pumps, and pressure/suction side jets were among the configurations tested for wake filling. A range of mass flow percentages were applied to each configuration and a pressure loss coefficient was determined for each. Considerable reduction in wake losses took place for discrete jet blowing techniques as well as pressure side and suction side jets. In the case of the pressure and suction side jets, near full wake filling occurred at 0.75% of the total mass flow. In terms of loss coefficients and calculated momentum coefficients, the suction/pressure surface jets were the most successful. Jets located upstream of the trailing edge helped to re-energize the momentum deficits in the wake region by using a flow pattern capable of mixing the region while also adding momentum to the wake. The slotted configuration was modeled after NASA's current blowing scheme and served as a baseline for comparison for all data. Digital particle image velocimetry was performed for flow visualizations as well as velocity analysis in the wake region. / Master of Science

An aeroacoustic investigation was performed to assess the effects of adding mass flow at the trailing edges of four stators upstream of an aircraft engine simulator. By using trailing edge blowing to minimize the shed wakes of the stators, the flow into the rotor was made more uniform. In these experiments a reduced number of stators (four) was used in a 1/14 scale model inlet which was coupled to a 4.1 in (10.4 cm) turbofan engine simulator with 18 rotors and 26 downstream stators. This study is a preliminary step toward a more in depth investigation of using trailing edge blowing to reduce unsteady rotor-stator interaction. Steady-state measurements of the aerodynamic flow field and acoustic far field were made in order to evaluate the aeroacoustic performance at three simulator speeds: 40%, 60%, and 88% of the design speed. The lowest test speed of 40% design speed showed the most dramatic reduction in radiated noise. Noise reductions as large as 8.9 dB in the blade passing tone were recorded at 40% design speed, while a tone reduction of 5.5 dB was recorded at 60% design speed. At 88% design speed a maximum tone reduction of 2.6 dB was recorded. In addition, trailing edge blowing reduced the overall sound pressure level in every case. For both the 40% design speed and the 60% design speed, the fan face distortion was significantly reduced due to the trailing edge blowing. The addition of trailing edge blowing from the four upstream stators did not change the total pressure ratio, and the mass flow added by the blowing was approximately 1%. The results of these experiments clearly demonstrate that blowing from the trailing edges of the stators is effective in reducing unsteady rotor-stator interaction and the subsequent forward radiated noise. / Master of Science

Boundary layer characteristics at the trailing edge of a tiltrotor blade model were measured using a flattened pitot probe and a single hot wire. The blade was mounted in Virginia Tech Stability Wind tunnel stationary on a turntable on the wind tunnel's upper wall with the tip pointing down. The measurement point was located at 1 mm behind the trailing edge to make it possible to measure the flow near the blade surface and measure the boundary layer on both sides of the trailing edge in a same run. Mean velocity profiles were measured for a variety of Reynolds numbers and angles of attack. Turbulence intensity and spectral measurements were performed using a single hot wire at the highest Reynolds number. Conclusion was reached that both of the flattened pitot probe and single hot wire are good for boundary layer thickness measurements. Displacement thickness, which is important in trailing edge noise prediction, was calculated from the profile data and fit using an algebra expression against the tip angle of attack. Once the relationship between tip angle of attack and local effective angle of attack is obtained by lifting line theory, the results can be used in the trailing edge noise prediction code. / Master of Science

This research performs an experimental study of a trailing edge blowing system that can adapt to variations in flow parameters and reduce the unsteady stator-rotor interaction at all engine operating conditions. The fan rotor of a 1/14 scale turbofan propulsion simulator is subjected to spatially periodic, circumferential inlet flow distortions. The distortions are generated by four struts that support a centerbody in the inlet mounted onto the simulator. To reduce the unsteady effects of the strut wakes on the rotor blades, the wake is re-energized by injecting mass from the trailing edge of the strut. Each strut is provided with discrete blowing holes that open out through the strut trailing edge. Each blowing hole is connected to a MEMS based microvalve, which controls the blowing rate of the hole. The microvalve is actuated by a signal voltage, generated by a PID controller that accepts free stream and wake axial flow velocities as inputs and minimizes their difference. To quantify the effectiveness of trailing edge blowing the far-field noise is measured in an anechoic chamber. The experiments are performed for two simulator test speeds, 29,500 rpm and 40,000 rpm, with and without trailing edge blowing. The maximum reduction recorded at 29,500 rpm is 8.2 dB, and at 40,000 rpm is 7.3 dB. Reductions of 2.9 dB and greater are observed at the first five harmonics of the blade passing frequency. The sound power level at the blade passing frequency, calculated from measured far-field directivity, is reduced by 4.4 dB at 29,500 rpm and by 2.9 dB at 40,000 rpm. The feasibility and advantage of active control is demonstrated by the ability of the system to respond to a step change in the inlet flow velocity, and achieve optimum wake filling in approximately 8 seconds. / Master of Science

The purpose of this study was to create a proof-of-concept piezoelectric actuator system capable of meeting the performance requirements necessary for actuation of a trailing edge flap for a helicopter main rotor blade. Due to extremely small displacements produced by piezoelectric actuators, their output is amplified several times in order to produce the required displacement for this device. The amplification is accomplished in two stages. The first stage, mechanical amplification, uses differential length lever arms to increase the piezoelectric actuator output. The second stage, hydraulic amplification, is coupled to the first stage and uses differential area pistons to further amplify the output of the mechanical amplifier. The actuation systems force and displacement output is characterized based on frequency.

This experimental study contains two points; part1 – turbine blade heat transfer under low Reynolds number flow conditions, and part 2 – trailing edge cooling and heat transfer. The effect of unsteady wake and free stream turbulence on heat transfer and pressure coefficients of a turbine blade was investigated in low Reynolds number flows. The experiments were performed on a five blade linear cascade in a low speed wind tunnel. A spoked wheel type wake generator and two different turbulence grids were employed to generate different levels of the Strouhal number and turbulence intensity, respectively. The cascade inlet Reynolds number based on blade chord length was varied from 15,700 to 105,000, and the Strouhal number was varied from 0 to 2.96 by changing the rotating wake passing frequency (rod speed) and cascade inlet velocity. A thin foil thermocouple instrumented blade was used to determine the surface heat transfer coefficient. A liquid crystal technique based on hue value detection was used to measure the heat transfer coefficient on a trailing edge film cooling model and internal model of a gas turbine blade. It was also used to determine the film effectiveness on the trailing edge. For the internal model, Reynolds numbers based on the hydraulic diameter of the exit slot and exit velocity were 5,000, 10,000, 20,000, and 30,000 and corresponding coolant – to – mainstream velocity ratios were 0.3, 0.6, 1.2, and 1.8 for the external models, respectively. The experiments were performed at two different designs and each design has several different models such as staggered / inline exit, straight / tapered entrance, and smooth / rib entrance. The compressed air was used in coolant air. A circular turbulence grid was employed to upstream in the wind tunnel and square ribs were employed in the inlet chamber to generate turbulence intensity externally and internally, respectively.

Mass transfer experiments were conducted to study the thermal performance characteristics of blockages with round and elongated holes, positioned in a 12:1 rectangular channel. Naphthalene sublimation technique was adopted to conduct experiments with four different blockage configurations, flow rates corresponding to Reynolds numbers (based on channel hydraulic diameter) of 7,000 and 17,000, and at three blockage locations. The hole area to channel area ratio for all four blockage configurations was the same at 0.196. The hole width was half the channel height, and the distance between consecutive blockages was twice the channel height. Average heat transfer, local heat (mass) transfer and overall pressure drop results were obtained. The thermal performance for a particular blockage configuration was measured in terms of the heat transfer enhancement and the friction factor ratio. Heat transfer enhancement was measured as a ratio of average Nusselt number on the blockage surface to the Nusselt number for a thermally fully developed turbulent flow in a smooth channel. Results indicate that this ratio ranged between 3.6 and 12.4, while the friction factor ratio varied between 500-1700. The blockage configuration with round holes was found to yield best thermal performance, while the configuration with largest hole elongation was nearly equal in thermal performance. In order to compare different blockage configurations, an average value of upstream and downstream side thermal performances was used. A general downward trend in Nusselt number ratio with elongation of holes was observed on the upstream side and a reverse trend was observed on the downstream side. An upward trend in the Nusselt number ratio with blockage hole elongation on the downstream side of a blockage was primarily due to jet reversal from the downstream blockage and its impingement on the downstream surface of the upstream blockage. Local experiments were performed to compare against the results from average experiments and also to gain insights into the flow behaviour. There was good agreement between the results from local and average mass transfer experiments. The average variation in Nusselt number ratio between local and average mass transfer experiments was about 5.06%.

Thesis (Ph.D)--Aerospace Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Hodges, Dewey; Committee Member: Bauchau, Olivier; Committee Member: Goldsman, David; Committee Member: Prasad, J.V.R.; Committee Member: Smith, Marilyn. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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The research effort presented in this dissertation consists of employing active trailing edge blowing control to reduce the unsteady stator-rotor interaction in a turbofan simulator. Two active flow control systems with different wake sensing approaches are successfully implemented on the engine simulator. The first flow control system utilizes Pitot probes as flow sensors. Use of Pitot probes as sensors is appropriate as a first step toward a more in depth investigation of active trailing edge blowing control. An upper performance limit in terms of wake-filling can be obtained and serves as the baseline in evaluating other control systems with indirect wake sensors. The ability of the system to achieve effective wake filling when subjected to a change in inlet flow conditions demonstrates the feasibility and advantage of active flow control. Significant tonal noise reductions in the far field are also obtained. The second control system involves using microphones as indirect wake sensors. The significance of these acoustic sensing approaches is to provide a practical TEB approach for realistic engines implementations. Microphones are flush mounted on the inlet case to sense the tonal noise at the blade passing frequency. The first sensing approach only uses the tone magnitude while the second novel sensing approach utilizes both the tone magnitude and phase as error information. The convergence rate of the second sensing approach is comparable with that of the Pitot-probe based experiments. The acoustic results obtained from both sensing approaches agree well with those obtained using Pitot probes as sensors. In addition to the experimental part of this research, analytical studies are also conducted on the trailing edge blowing modeling using an aeroacoustic code. An analytical model for trailing edge blowing is first proposed. This model is then introduced into the two-dimensional aeroacoustic code to investigate effect of various trailing edge blowing managements in the tonal sound generation. / Ph. D.