Identification of faults through wavelet transform vis-à-vis fast Fourier transform of noisy vibration signals emanated from defective rolling element bearings

Fault diagnosis of rolling element bearings requires efficient signal processing techniques. For this purpose, the performances of envelope detection with fast Fourier transform (FFT) and continuous wavelet transform (CWT) of vibration signals produced from a bearing with defects on inner race and rolling element, have been examined at low signal to noise ratio. Both simulated and experimental signals from identical bearings have been considered for the purpose of analysis. The bearings have been modeled as spring-mass-dashpot systems and the simulated signals have been obtained considering transfer functions for the bearing systems subjected to impulsive loads due to the defects. Frequency B spline wavelets have been applied for CWT and a discussion on wavelet selection has been presented for better effectiveness. Results show that use of CWT with the proposed wavelets overcomes the short coming of FFT while processing a noisy vibration signals for defect detection of bearings.     

Seismic reliability of non-linear frames with PR connections using systematic RSM

An effective, efficient, and robust reliability analysis algorithm is proposed for non-linear structures, where seismic loading can be applied in the time domain. The method is developed specifically for steel frame structures considering all major sources of non-linearity, including geometry, material, and partially restrained (PR) connections. The non-linearity due to PR connections is modeled by moment-relative rotation curves using the four-parameter Richard model. For seismic excitation, the loading, unloading, and reloading behavior at PR connections is modeled using moment-relative rotation curves and the Masing rule. The proposed algorithm intelligently integrates the response surface method, the finite element method, the first-order reliability method, and an iterative linear interpolation scheme. The uncertainties in all the random variables including the four parameters of Richard model are considered. Two unique features of the proposed algorithm are that (1) actual earthquake time histories can be used to excite structures in the presence of major sources of non-linearity and uncertainty and (2) it is possible to estimate the risk corresponding to both the serviceability and strength limit states. The algorithm is verified using the Monte Carlo simulation technique. The verified algorithm is first used to study the reliability of a frame structure in the presence of PR connections with different degrees of flexibility. Then the algorithm is used to estimate the reliability of a frame structure excited by 13 actual recorded earthquake time histories, 12 of them recorded during the Northridge earthquake of 1994. As expected, the reliabilities of the frame are found to be quite different, when excited by several time histories of the Northridge earthquake.     

Time-domain seismic reliability of nonlinear structures

A novel reliability analysis technique is presented to estimate the reliability of real structural systems. Its unique feature is that the dynamic loadings can be applied in time domain. It is a nonlinear stochastic finite element logarithm combined with the response surface method (RSM). It generates the response surface around the most probable failure point and incorporates information of the distribution of the random variables in the RSM formulation. It is verified using the Monte Carlo simulation technique, and is found to be very efficient and accurate. Most sources of nonlinearlity and uncertainty can be explicitly incorporated in the formulation. The flexibility of connections, represented by moment-relative rotation(M-θ) curves, is addressed. After the Northridge earthquake of 1994, several improved steel connections were proposed. Structural Sesimic Design Associates (SSDA) tested several full-scale proprietory slotted web beam-column connections. The authors suggested(M-θ) curves for this connection using actual test data. Behaviours of steel frames, assuming the connections are fully restrained, partially restrained, consisting of pre and post-Northridge connections are evaluated and compared. Desirable features of the post-Northridge connections observed during testing are analytically confirmed. Laterally weak steel frame is then strengthened with concrete shear walls. Capabilities and the advanced nature of the method are demonstrated with the help of realistic examples. This paper is dedicated to Prof R N Iyengar of the Indian Institute of Science on the occasion of his formal retirement.     

Experimental investigation of spray characteristics of kerosene and ethanol-blended kerosene using a gas turbine hybrid atomizer

Gas turbines have wide application as prime movers in transportation and power generating sectors, most of which are driven by fossil fuels like kerosene. The conventional fuels are associated with problems of air pollution, and the fuel reserves are getting depleted gradually. Addition of ethanol in kerosene leads to better spraying characteristics. The present work deals with the spray characteristics of pure kerosene and 10%-ethanol-blended (by volume) kerosene using a novel gas-turbine hybrid atomizer. Here the inner air and outer air enter in the same and opposite directions, respectively, with respect to the fuel flow direction into the atomizer and a high swirling effect occurs outside the nozzle. The fuel stream is sandwiched between two annular air streams and the flow rate of inner and outer air is varied continuously. Various spray stages like distorted pencil, onion, tulip and fully developed spray regimes have been observed. The breakup length, cone angle and sheet width of the fuel stream are analysed directly from backlit imaging for different fuel and air flow rates. From the image processing, it is observed that breakup occurs at an early stage for 10%-ethanol-blended kerosene due to low viscosity of ethanol. It is also observed that at higher air flow rate, breakup occurs at an early stage due to turbulent nature of the fuel stream.     

Element Level System Identification with Unknown Input with Rayleigh Damping

A novel system identification procedure is proposed for nondestructive damage evaluation of structures. It is a finite element-based time-domain linear system identification technique capable of identifying structures at the element level. The unique features of the algorithm are that it can identify a structure without using any input excitation information and it can consider both viscous and Rayleigh-type proportional damping in the dynamic models. The consideration of proportional damping introduces a source of nonlinearity in the otherwise linear dynamic algorithm. However, it will also reduce the total number of damping coefficients to be identified, reducing the size of the problem. The Taylor series approximation is used to transform a nonlinear set of equations to a linear set of equations. The proposed algorithm, denoted as the modified iterative least square with unknown input algorithm, is verified with several examples considering various types of structures including shear-type building, truss, and beams. The algorithm accurately identified the stiffness of structures at the element level for both viscous (linear) and proportional (nonlinear) damping cases. It is capable of identifying a structure even with noise-contaminated response information. An example shows how the algorithm could be used in detecting the exact location of a defect in a defective element. The algorithm is being developed further and is expected to provide an economical, simple, efficient, and robust system identification technique that can be used as a nondestructive defect detection procedure in the near future.     

Project Scheduling Using Fuzzy Set Concepts

Probabilistic methods are being used increasingly in construction engineering. However, when a parameter is expressed in linguistic rather than mathematical terms, classical probability theory fails to incorporate the information. The linguistic variables can be translated into mathematical measures using fuzzy set and system theory. A construction management problem, i.e., estimation of the duration of an activity, is solved using this theory. In order to implement the proposed technique, various membership functions need to be estimated using judgment or with the assistance of experts. The proposed technique is not sensitive to small variations in the membership values. This is a very desirable property. However, the method is sensitive to the choice of the fuzzy relations. The uncertainty in the fuzzy relations can be modeled along with other sources of uncertainty. The mean and variance of the parameters involved in the problem under consideration are estimated here using a new method. The method maximizes the product of the sum of the membership associations for a certain frequency of occurrence and the corresponding frequency of occurrence. One of the main advantages of the proposed technique is that it can be easily implemented in existing computer programs for project scheduling.     

Analysis of entropy generation due to natural convection in square enclosures with multiple discrete heat sources

Entropy generation due to natural convection in an enclosure heated locally from below with two isoflux sources was investigated. The flow and temperature fields were determined by numerical simulation of two-dimensional laminar conservation equations for mass, momentum and energy. For heaters of equal length and strength, the effects of Rayleigh number and heater position on flow and temperature fields and local entropy generation were examined. For heaters of unequal heater length and heater strength, the effects of heater length and strength ratios were investigated. Minimum entropy generation rate was achieved for the same condition at which the minimum peak heater temperature was obtained.     

Modeling of Evaporation and Combustion of Droplets in a Spray Using the Unit Cell Approach: A Review

A comprehensive review of modeling of evaporation and combustion of liquid fuel droplets using “unit cell” approach is presented. The review starts with a general introduction to the different regimes of droplet combustion and a brief overview of other techniques used for evaluating mutual interaction of droplets before presenting the cell model in detail. In this review, the major developments in this approach, encompassing effects of gas-phase convection, high pressure, and multicomponent composition of fuels, are reviewed for both dense and dilute sprays.     

The Effects of Magnetization Saturation on Thermomagnetic Convection in a Locally Heated Square Enclosure

Steady-state simulation study of thermomagnetic convection is presented for a square cavity with localized heat-sources. The external magnetic field conforms to Maxwell's equations. Effects of magnetization saturation of the ferrofluid medium on heat transfer enhancement are studied by invoking Langevin's law. Thermal interactions between the heaters and the fluid at convection-dominated regimes are visualized through streamline, heatline, and isotherm plots. The variation of Nu_avg,heater is depicted for increasing dipole strength of the magnetic field sources. Fluid-magnetization contours explain the layout of the Kelvin force-field. The positions of the magnets for maximum value of Nu_avg,heater are determined. For the same heat generation rate, effects of enclosure dimensions on heat transfer augmentation are studied.