NASA/MSFC FY88 Global Scale Atmospheric Processes Research Program Review

Interest in environmental issues and the magnitude of the environmental changes continues. One way to gain more understanding of the atmosphere is to make measurements on a global scale from space. The Earth Observation System is a series of new sensors to measure globally atmospheric parameters. Analysis of satellite data by developing algorithms to interpret the radiance information improves the understanding and also defines requirements for these sensors. One measure of knowledge of the atmosphere lies in the ability to predict its behavior. Use of numerical and experimental models provides a better understanding of these processes. These efforts are described in the context of satellite data analysis and fundamental studies of atmospheric dynamics which examine selected processes important to the global circulation

Prevention of extreme roll motion through measurements of ship's motion responses

PhD ThesisExploring the operational links between a sea state and a ship’s heading and speed provides the opportunity to continuously monitor dynamic stability behaviour; and hence to avoid significant changes of stability in adverse weather. Significant changes of stability at sea can lead to dangerous transient situations and eventually stability failure. Despite its importance, the current intact stability (IS) criteria do not evaluate or consider the dynamics of the motion responses of a vessel in a wave environment. In this thesis, the full six degrees of freedom motion responses of two models have been tested in irregular waves under intact vessel conditions. The general modelling approach for a mathematical model was based on numerical simulations at different speeds, sea conditions and angle of heading relative to the waves. In the second model, a physical model was tested in a towing tank under similar simulated environmental conditions to that employed for the first model. The investigation was limited to the effects of encountered frequency components and the associated magnitude of energy of the ship’s motion responses. An analysis of heave, pitch and roll motion confirmed the vulnerability of the model to certain wave-excited frequency ranges. This particular range of frequency results in an adverse effect on the amplitude of the responses, and these were closely related to the natural mode frequencies and related coupling effects. It was confirmed that the roll motion maintains its highest oscillation amplitude at around the natural frequency in all sea conditions regardless of ship heading angles. It was also observed that spectral analysis of the heave and pitch responses revealed the wave peak frequency. Roll is magnified when the peak frequency of the waves approaches the natural roll frequency, therefore keeping them sufficiently apart avoids potentially large motion responses. It was concluded that peak frequency and associated magnitude are the two important inherent characteristics of motion responses. Detection of the most influential parameters of encountered waves through measurements of heave and pitch responses could be utilised to provide a method to limit the large motion of a ship at sea. The measurement of waves whilst a ship is underway is a major challenge, whereas ship motion, which is relatively easily measured, is a good indirect reflection of the encountered wave characteristics and which can be measured, stored and analysed using Prevention of extreme roll motion through measurements of ship’s motion responses iv on-board equipment. Motion responses are considered as continuous signals with a time-dependent spectral content, and signal processing is a suitable technique for detection, estimation and analysis of recorded time-varying signals. The method is fast enough to be considered as an on-board real-time monitoring of dynamic stability. Signal processing techniques are used in the detection and estimation of the influential parameters of a wave environment through the analysis of motion responses. The variables of the system were detected by spectral analysis of the heave and pitch motions. These variables are the peak wave frequencies and associated magnitudes which can cause a large roll motion when reasonably close to the ship’s natural roll frequency. The instantaneous frequency (IF) present in the signal is revealed through spectral analysis of short-time Fourier transforms (STFT) in less than a minute. The IF is a parameter of practical importance which can be used in real-time on-board decision making processes to enable the vessel to take actions in order to avoid large roll motions

Self-excited vibrations controlled by damping at blade root joints of turbine disks

The purpose of this thesis is the development of a methodology for the calculation of the non-linear aero-elastic behavior of a bladed disk to be used in an industrial process. The non-linear aero-elastic phenomena of a bladed disk for aeronautical applications are studied in the presence of friction contacts using a one-way coupled method. The calculation is performed using a method based on the Harmonic Balance Method (HBM) and the balance between the energy introduced by the unsteady aerodynamics on the blade airfoil and the dissipative energy. The HBM method is preferred with respect to the Direct Time Integration (DTI) for the strong reduction of the computation time that HBM technique allows in spite of an acceptable level of approximation when nonlinearities are introduced and the response is periodic. The nonlinearity is introduced by purposely developed contact elements, placed at the blade root-joints, that produce additional stiffening and damping in the system due the introduction of contact stiffnesses and friction forces based on Coulomb’s law. The aero-elastic equilibrium will be investigated through a Parameter variation of the Limit Cycle Oscillations (LCO) of the system using two different approaches: the physical approach and the modal approach. The effect of such variations will be highlighted in order to demonstrate what are the parameters that influence most the blade amplitude, both for the CFD and the mechanical simulation. In particular, the uncertainty in the definition of the contact parameters at the blade root will be taken into account by varying the friction coefficient and the normal force distribution on the blade root joint. Finally, the results of the analysis will be compared with the experimental data produced with a cold-flow test rig to verify if the sensitivity study associated to the simplifications introduced in the method are compatible with the measured response

6th International congress of the Serbian society of mechanics: Review

Ovaj rad prikazuje najvažnije informacije o 6. kongresu Srpskog društva za mehaniku, koji je održan na Tari od 19. do 21. juna 2017. Kongres je organizovan od strane Srpskog društva za mehaniku. Dat je kratak prikaz najznačajnijih radova predstavljenih na ovom kongresu, a koji se bave teorijskom i primenjenom mehanikom.This paper presents the most important information and describes the activities of the 6th Congress of the Serbian Society of Mechanics which was held on mountain Tara, on 19- 21 June, 2017. The Congress was organized by the Serbian Society of Mechanics. Brief summaries of the plenary lectures and some of 99 accepted papers, which admittedly attracted the most interest were shown as well

Tribo-dynamic analysis of hypoid gears in automotive differentials

Torsional vibrations in differentials of Rear Wheel Drive vehicles are of major importance for the automotive industry. Hypoid transmissions, forming the motion transfer mechanism from the driveshaft to the wheels, suffer from severe vibration issues. The latter are attributed to improper mesh between the mating gear flanks due to misalignments, variation of contact load and shifting of the effective mesh position. For certain operating conditions, the gear pair exhibits high amplitude motions accompanied with separation of the mating surfaces. Ultimately, single or even double-sided vibro-impact phenomena evolve, which have been related to noise generation. This thesis attempts to address these issues by effectively analysing the dynamic behaviour of a hypoid gear pair under torsional motion. The case study considered is focused on a commercial light truck. The major difference of the employed mathematical model to prior formulations is the usage of an alternative expression for the dynamic transmission error so that the variation of contact radii and transmission error can be accounted for. This approach combined to a correlation of the resistive torque in terms of the angular velocity of the differential enables the achievement of steady state, stable periodic solutions. The dynamic complexity of systems with gears necessitates the identification of the various response regimes. A solution continuation method (software AUTO) is employed to determine the stable/unstable branches over the operating range of the differential. The ensuing parametric studies convey the importance of the main system parameters on the dynamic behaviour of the transmission yielding crucial design guidelines. A tribo-dynamic investigation aims at expanding the dynamic model from pure dry conditions to a more integrated elastohydrodynamic (EHL) approach. Analytical and extrapolated solutions are applied for the derivation of the film thickness magnitude based on the kinematic and loading characteristics of the dynamic model. The temperature rise is governed mainly by conduction due to the thin lubricant films. The generated friction is also computed as a function of the viscous shear and asperity interactions. The effective lubricant viscosity is greatly affected by the pressure increase due to the resonant behaviour of the contact load. The final part of this work is involved with a feasibility study concerning the application of Nonlinear Energy Sinks (NES) as vibration absorbers, exploiting their ability for broadband frequency interaction. Response regimes associated with effective energy absorption are identified and encouraging results are obtained, showing the potential of the method