Critical speed

About: Critical speed is a research topic. Over the lifetime, 2764 publications have been published within this topic receiving 31365 citations.

Design and Implementation of a Critical Speed-Based DVFS Mechanism for the Android Operating System

Abstract: DVFS is an efficient energy saving technique for processors during program execution time. A critical speed-based DVFS mechanism that we have implemented on the Android operating system is introduced. Our studies indicate that due to memory accesses, decreasing the frequency may not always reduce the energy consumption. A critical speed is thus defined as the CPU frequency with which the energy consumption can be minimized. In our mechanism, a prediction equation based on the correlation of the memory access rate and the critical speed was constructed and used to choose a suitable frequency and voltage dynamically at run time. Our initial experiment results show that for real applications running on Android, the energy consumption can be effectively reduced.

Investigation of the dynamic characteristics of a dual rotor system and its start-up simulation based on finite element method

Abstract: Recently, the finite element method (FEM) has been commonly applied in the engineering analysis of rotor dynamics. Gyroscopic moments, rotary inertia, transverse shear deformation and gravity can be included in computational models of rotor-bearing systems. In this paper, a finite element model and its solution method are presented for the calculation of the dynamics of dual rotor systems. A typical structure with two rotor shafts is discussed and the procedure for obtaining the coupling motion equations of the subsystems is illustrated. A computer program is developed to solve critical speeds and to simulate the transient motion. The influence of gyroscopic moments on co-rotation and counter-rotation is analyzed, and the effect of the speed ratio on critical speed is studied. The dynamic characteristics under different conditions of increasing speed during start-up are demonstrated by comparison with transient nodal displacements. The presented model provides a complete foundation for further investigation of the dynamics of dual rotor systems.

Dynamics of Plate Generated by Moving Harmonic Loads

Abstract: A thin plate resting on a Winkler elastic foundation subject to a moving harmonic load can be used as the model for highway and airport pavement under moving vehicle load and many other applications. The study of dynamic response of the plate thus becomes very important. In this paper we study the dynamic displacement of a plate caused by a moving harmonic line and point load. The solution is represented by the convolution of dynamic Green's function of plate. An approximate relationship between critical load velocity and critical frequency is established analytically. It is found that the maximum displacement response occurs at the center of the moving load and travels at the same speed with the load.

A parametric dynamic study on hunting stability of full dual-bogie railway vehicle

Abstract: Analysis is performed on the hunting stability of a full railway vehicle system composed of a vehicle body, two bogie frames, and two wheelsets for each bogie frame. Incorporated into this analysis are the nonlinear heuristic creep and flange-rail contact models. The results show that the hunting speed is most sensitive to the primary longitudinal and lateral stiffnesses, and the nonlinear heuristic creep model plays a key role in confining the hunting speed within a physically reasonable range. Eigenanalysis is performed to investigate the dynamic behavior of the vehicle in the vicinity of the hunting speed. The results reveal that there exists not only the most dominant pair of complex conjugate roots, but also its shadowed roots. The roles of these two principal pairs of eigensolutions in the hunting motion are thoroughly explored via numerical studies using bifurcation analysis and an orbital representation. It is shown that the nonlinear hunting motions before the modal transition speed mainly refer to the principal mode, and those after the critical speed refer not only to the principal mode, but also to its shadowed mode, which supports the necessity of the dual-bogie railway vehicle model in the hunting analysis.

Elimination of Unstable Ranges of Rotors Utilizing Discontinuous Spring Characteristics: An Asymmetrical Shaft System, an Asymmetrical Rotor System, and a Rotor System With Liquid

Abstract: Unstable vibration occurs in the vicinities of the major critical speeds of asymmetrical shaft and rotor systems. It occurs also in a wide rotational speed range higher than the major critical speed of a shaft with a hollow disk partially filled with liquid. The occurrence of the unstable vibrations is a serious problem because the amplitude increases exponentially, and finally, the system is destroyed. The active vibration control can suppress unstable vibrations but the method is generally complicated and costly. No simple effective method to suppress unstable vibrations has been developed yet. In the previous paper, the authors proposed a simple method by utilizing discontinuous spring characteristics, which can suppress steady-state resonances. This paper shows that this method is also effective to suppress unstable vibrations. By using this method, the unstable vibrations can be changed into almost periodic motions, and the amplitudes are suppressed to the desired small level even in an unstable range. The validity of the proposed method is also verified by experiments.