Browsing by Subject "Operational modal analysis"

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Estimation of helicopter rotor loads from blade structural response
(2020-03-27) Uehara, Daiju; Sirohi, Jayant; Goldstein, David B; Ravi-Chandar, Krishnaswa; Bennighof, Jeffrey K; Bhagwat, Mahendra J
Measuring the load distribution along a helicopter rotor blade has been one of the most challenging tasks in experimental aeromechanics. Conventional loads measurements with on-blade instrumentation, such as pressure transducers for airloads and strain gages for structural loads, require the experimentalist to overcome a large number of technical barriers; for example, sensor integration to the rotor blade structure, sensor failure due to strong centrifugal forces, and influence of sensor installation on rotor blade dynamics. The goal of this dissertation is to develop a new, combined experimental and theoretical methodology to estimate helicopter rotor loads without using these conventional on-blade sensors. The rotor loads estimation methodology begins with the measurement of blade structural deformation measurements using non-contact, optical, time-resolved Digital Image Correlation (DIC). The time-resolved DIC technique successfully showed its capability of measuring the three-dimensional deformation time history of a rotating blade for both a small- and a large-scale rotor in hover. The modal properties (natural frequencies, mode shapes, damping ratios, and modal coordinates) of the blade in the rotating-frame were then extracted from the deformation time history using Natural Excitation Technique - Eigensystem Realization Algorithm (NExT-ERA) and Complexity Pursuit (CP), which are operational modal analysis (OMA) algorithms. The first three modes were identified by the OMA algorithms and well correlated with a numerical model. Rotor loads were then finally estimated based on the measured deformations and blade modal characteristics. Having validated the present approach incrementally with measurements performed on rotors at different scales, configurations, and operating conditions, the current study estimated the spanwise lift distribution and integrated thrust at the hub for a 2 m-diameter, two-bladed, isolated single rotor in hover. Due to a lack of participating modes (only the first and second flap modes), the estimated sectional lift distribution did not capture the lift loss typically observed at regions of the blade tip and induced by trailing tip vortices. Nevertheless, the mean value of the estimated thrust at the rotor hub was within 5% of the measured value for all the operating conditions
Operational modal analysis of a rotating cantilever beam using high-speed digital image correlation
(2015-12) Rizo-Patron, Sergio Sebastian; Sirohi, Jayant; Ravi-Chandar, Krishnaswamy
A novel procedure to perform an operational modal analysis on a rotating cantilever beam is described. This procedure uses Digital Image Correlation (DIC) to measure the deformation of a beam from images captured with a pair of high-speed digital cameras. Modal parameters including natural frequencies and mode shapes are determined from the deformation data through application of the Ibrahim Time Domain method. The procedure was validated on a 2 m diameter, Mach-scale helicopter rotor, excited by a jet of compressed air. Images of the rotor blade were captured at a sampling rate of 1000 Hz at rotational speeds up to 900 RPM. The out-of-plane deformation of the rotor was measured with a spatial resolution of 7.2 mm and an accuracy of 60 μm, or 0.006% of the rotor radius. The first three flap bending modes were identified at each rotational speed and compared to an analytical model of the system. It was found that the analytical model over-predicted the natural frequencies due to differing boundary conditions between the model and the experiment, and so the analytical frequencies were scaled to the results of a rap test using traditional frequency domain analysis. The scaled analytical and experimental natural frequencies agreed to within 0.2% in the best case and 10.0% in the worst case. The experimental mode shapes were also found to closely match the analytical model. The results of this test demonstrate the ability of this procedure to determine the modal parameters of rotating cantilever beams.