Browsing by Author "Dr. Ndaona Chokani, Committee Chair"

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Calibration Methods for a Constant Voltage Anemometer-Operated Hot-Wire in a Hypersonic Flow
(2003-09-17) Reimann, Craig Aaron; Dr. Hassan A. Hassan, Committee Member; Dr. Harvey J. Charlton, Committee Member; Dr. Ndaona Chokani, Committee Chair
The calibration of measurements made with a CVA-operated hot-wire, in a quiet hypersonic wind tunnel, are examined. Two calibration methods are evaluated. The first calibration method is based on the ratio of the power dissipated in the hot-wire and the difference between the hot- and cold-wire resistances. The second method is based on the traditional semi-empirical heat transfer relation for a heated wire in supersonic/hypersonic flow. The mean mass flux and total temperature derived from the calibration methods show overall good agreement with Navier-Stokes predictions. In general, the first method gives better results across the boundary layer. However, limitations in the calibration data meant no definitive assessment of the capability to yield fluctuation measurements could be made. The recently derived sensitivity coefficients for a CVA-operated hot-wire are used to obtain fluctuation measurements. Also, the fluctuating diagram method is used to evaluate the performance of the NASA Langley Mach 6 quiet tunnel and to detail the characteristics of the fluctuations in the laminar hypersonic boundary layer. Within the boundary layer, profiles of the measured rms fluctuations and the fluctuation diagrams show that the intensityof the boundary layer disturbance fluctuations are most intense in the vicinity of the critical layer. The static pressure fluctuations derived from the fluctuation diagrams verify that the fluctuations are less than 0.2%. This observation confirms previous qualitative observations of the performance of the Mach 6 quiet nozzle.
Distributed Actuation and Sensing on an Uninhabited Aerial Vehicle
(2003-08-12) Barnwell, William Garrard; Dr. Ndaona Chokani, Committee Chair; Dr. Charles E. Hall, Jr., Committee Member; Dr. Harvey J. Charlton, Committee Member
An array of effectors and sensors has been designed, tested and implemented on a Blended Wing Body Uninhabited Aerial Vehicle (UAV). This UAV is modified to serve as a flying, controls research, testbed. This effector/sensor array provides for the dynamic vehicle testing of controller designs and the study of decentralized control techniques. Each wing of the UAV is equipped with 12 distributed effectors that comprise a segmented array of independently actuated, contoured control surfaces. A single pressure sensor is installed near the base of each effector to provide a measure of deflections of the effectors. The UAV wings were tested in the North Carolina State University Subsonic Wind Tunnel and the pressure distribution that result from the deflections of the effectors are characterized. The results of the experiments are used to develop a simple, but accurate, prediction method, such that for any arrangement of the effector array the corresponding pressure distribution can be determined. Numerical analysis using the panel code CMARC verifies this prediction method.
High Order Spectral Analysis of Laminar Hypersonic Boundary Layers and CVA Application
(2002-08-07) Norris, Joseph David; Dr. Ndaona Chokani, Committee Chair
Three high order spectral analysis techniques - the short-time Fourier transform (STFT), the Fourier bispectrum, and the STFT bispectrum - are developed and used to characterize the nonlinear dynamics of a transitioning hypersonic boundary layer flow. The measurement of a hot-wire operated with a constant voltage anemometer (CVA) in the NASA Langley Mach 6 quiet tunnel are analyzed, and the effects of wall cooling and adverse pressure gradient on the nonlinear dynamics are examined. The STFT clearly identifies the modulation of the dominant second mode and its harmonic in the nonlinear region of the transition process. Phase modulation is observed to be the primary energy transfer mechanism, but in the latter nonlinear stages the role of amplitude modulation is increasingly important. The Fourier bispectrum quantifies the modulation as a low frequency phase coupled quadratic interaction. In addition, the Fourier bispectrum quantifies the dominant role of the second mode through its forcing of harmonic quadratic interactions. The STFT bispectrum identifies the transient stages of nonlinear interactions that are observed to be important in the nonlinear stages of transition. These nonharmonic, broadband interactions may offer new approaches for control of hypersonic transition. In a final element of this work, a new capability for the CVA is developed and demonstrated. A rapid and automated stepping of the wire voltage is developed, and used to obtain calibrated measurements in a short duration wind tunnel.