Papers by Keyword: Critical Velocity

Abstract: The cold spraying process is numerically modeled using Lagrangian and Arbitrary Lagrangian Eulerian (ALE) techniques. The simulations were performed to predict the critical velocity of spherical aluminum particles deposited on the aluminum substrate. ALE technique was found to be more suitable than the Lagrangian technique. Using Lagrangian and ALE techniques, the critical velocity for aluminum was predicted as 605 m/s and 770 m/s. Critical velocity was in between 770-775 m/s, as reported in the literature. The Lagrangian technique's capability is limited in capturing large deformations associated with cold spraying. However, this technique requires less computational ability and is quicker than the ALE technique. The jet formation was prominent in the case of the Lagrangian technique, and hence the difference between the numerically estimated value of critical velocity and experimentally measured velocity is more. The Compression ratio was found to increase with an increase in impingement velocity.

Abstract: Flow-induced vibrations occur in some of the internal components of a nuclear reactor. When specific conditions are present, these vibrations may result in excessive deformations or fatigue that can generate mechanical damage. Several boiling water reactor (BWR) of nuclear power plants (NPP) have experienced failures in the jet pump assembly due to flow-induced vibration (FIV) which could be caused by acoustic pulsations derived from recirculation pumps, vibration induced by turbulence and vibration induced by leakage at the slip joint. The purpose of this paper is to establish a viable numerical methodology to evaluate the fluid-structural interaction at the slip joint of a jet pump. In this analysis, the fluid-structural interaction was evaluated with the finite element method and finite volume method with ANSYS^® code in the case of two steel plates with a divergent gap. Results show that a critical velocity could cause fluidelastic instability, if only one flow in a two-way fluid-structural interaction was considered. This is one of the phenomena that could take place at the slip joint of a jet pump assembly.

Abstract: Aeroelasticity on airplane wing has a significant impact on the efficiency and the safety of a flight. Therefore, the study of aeroelasticity problems is a great attention in wing design process. To analyze this, three primary subjects of aeroelastic phenomena that the examination has to focus on, which are wingtip oscillation amplitude, flutter frequency and critical flutter velocity of the wing. [1] As these subjects are highly dependent on structures and materials of the wings, therefore, the aim of this paper is to introduce a structural calculation which is the combination of three experimental models and the infinite element method. Supercritical is chosen as the sample airfoils of the simulation. The model wings are made from different materials and size in order to create varied wing structures, thereby a comprehensive analysis is accomplished and the flutter velocity is also restricted to appropriate values within the working range of experiment devices. Simultaneously, Infinite element method using ANSYS software to simulate the phenomena on the same model wings is also conducted as a verification for the precision of the experimental models. In conclusion, obtained results from the structural calculation have a high applicability in the preliminary design stage of airplane wings, by making comparisons between two or more chosen airfoils to conclude which is the better one in term of wing sustainability and aeroelasticity resistance.

Abstract: The main goal of this paper is, that if we want to correctly identify dynamic model of drive shaft made of carbon-epoxy composite, we must take into account the influence of deformation velocity on elasticity characteristic. In the paper, Authors showed the effect of passing shaft resonance (1st critical speed). It was shown in the conditions of acceleration and braking with different increases of rotational speed. When examining the results obtained with empirical and model research, it has been shown that in the structural design of carbon fiber drive shafts, the main factor affecting the right development of the structure should be dynamic criterion, since the only use of static criteria leads to big errors.

Abstract: This paper concerns the smoke control modes and the critical ventilation velocity when the subway tunnel on fires. The standard for the smoke control mode is making sure the smoke exhausting in the shortest way. The critical ventilation velocity means it is just sufficient to prevent the smoke spreading upstream. The critical velocity in different heat release rates obtained though theoretical analysis and computer simulation. In the end, a simple formula to calculate the critical velocity can be fitting out.

Abstract: The generalized model experiments were designed and conducted in the pressure pipeline to investigate the characteristic of sediment incipient motion. There were some differences between the measured sediment critical velocity and the predictive one which was calculated by traditional formulas. In this paper, the analyses of velocity distribution and its corresponding effect on the sediment incipient motion in the pressure pipeline are done. Then, a simple but available formula which is suitable for the prediction of sediment critical velocity in the pressure pipeline is proposed. Finally, this formula is verified by experimental data and good agreements are observed between the recorded data and predictions.

Abstract: Dynamic behavior of a semi-infinite elastic beam to a moving single sinusoidal pulse was theoretical investigated. An analytical model was developed based on the Bernoulli-Euler beam theory. The solutions of the deflection and stress of beam were obtained by using the superposition principle and applying the techniques of Fourier transform. It is found that when the moving pulse reaches a critical velocity for a given moving pulse duration, the maximal absolute value of stress in beam attains its maximum value.

Abstract: Critical velocity is a very important parameter in smoke control of tunnel fires and the variation of critical velocity against fire heat release rate is also one of the most important issues in tunnel fire researches. In this paper, a simplified physical model of a tunnel was established and the predictions of critical velocity for fire sizes in the 5-100MW range were carried out by FDS simulations. The FDS-predicted dimensionless critical velocities were compared with the values calculated by Wu and Bakar’s model. The result indicated that when the heat release rate was relatively small, Q≤30MW, the critical velocity increased with the increasing of heat release rate and varied as the one-third power of the heat release rate; when Q≥40MW, the growth rate of critical velocity became very small; after Q reach to 60MW, the critical velocity was almost unchanged with the increasing of heat release rate. In addition, the values of critical velocity calculated by Wu and Bakar’model which was derived from small-scale gas fire tests were underestimated. Therefore, the model suggested by Wu and Bakar is not suitable for critical velocity prediction in tunnel fires.

Abstract: The critical velocity for an infinite cylindrical shell subjected a moving load with a constant velocity is analyzed in this paper. It is found that the critical velocity problem can be translated into a distribution of the real roots of a quadruplicate equation, which can be solved by using Descartes sign method and complete discrimination system for polynomials. Our research shows that the number of the critical velocities for an infinite cylindrical shell always is even number. Furthermore the longitudinal wave velocity is not one critical velocity for the shell. Our results are different from the conclusion drawn by other authors that there are three critical velocities in an infinite shell, and the longitudinal wave velocity is the maximum critical velocity. Then further studies are needed to clarify these questions.

Abstract: Known from the gas dynamic model of cold spraying, the properties of process gas including gas temperature, gas pressure and gas source have great influence on the critical velocity of powder. The influences of the three kinds of gas properties on the quality of the NiCoCrAlY coating on the magnesium substrate by the experiments of the cold gas dynamic spraying were discussed in this study. The results of the experiments show that, when the pressure value of process gas is 3.5MPa, the density of the coating is perfect, and there are obvious layered structures. The porosity is only 0.45%. The influence of gas temperature on the coating quality is the influence of that on the oxygen content. When the temperature of the process gas is confined to 600°C~700°C, the oxygen content is low and the coating comprehensive quality is best. On the premise that other parameters are kept constant, when nitrogen and helium gas are used together, the coating quality is better.