An analytical method for vibro-impact between shrouded blades under a multiple-harmonic excitation

The impact vibration between shrouded blades under a multiple-harmonic excitation in an aircraft engine is analyzed in this paper to study the influence of parameters on dynamic behavior of the system. Aircraft engine blades undergo high vibrations caused by the centrifugal forces and gas forces during operation. The blade vibrations lead to high dynamic stress that causes high cyclic fatigue failure. In order to reduce the dynamic stress, a common technology is adding a shroud to each blade to generate a new damping from impact and friction between the shrouds. To investigate the impact vibration, a model composed of springs and a cantilever beam with a lumped mass is developed to simulate the cyclic symmetry structure of the shrouded blade in aircraft engines. The model is exerted a multiple-harmonic excitation to simulate the centrifugal force and the gas force during operation. Adopting multiple-harmonic excitation is closer to reality than harmonic excitation. The Euler-Bernoulli beam theory is utilized in deriving the equation of motion and the associated boundary conditions. Employing the Galerkin’s method, an approximate analytical solution is obtained by using the Fourier series method. Explicit expressions are obtained to calculate the responses of the system. Based on the expressions, parametric analysis is conducted to study the effect of the gap between shrouds, the mass ratio of the shroud to the blade and the stiffness ratio on the responses of the system. The numerical results indicate that there is an optimum mass ratio that makes the amplitude of the response lowest; the gap between shrouds and the stiffness ratio have effects on the responses and the resonance frequencies of the system. The analytical method used here for the approximate analytical solution can be extended to study other continuous systems

Energy-efficient query management scheme for a wireless sensor database system

Minimizing the communication overhead to reduce the energy consumption is an essential consideration in sensor network applications, and existing research has mostly concentrated on data aggregation and in-network processing. However, effective query management to optimize the query aggregation plan at the gateway side is also a significant approach to energy saving in practice. In this paper, we present a multiquery management framework to support historical and continuous queries, where the key idea is to reduce common tasks in a collection of queries through merging and aggregation, according to query region, attribute, time duration, and frequency, by executing the common subqueries only once. In this framework, we propose a query management scheme to support query partitioning, region aggregation and approximate processing, time partitioning and aggregation rules, multirate queries, and historical database. In order to validate the performance of our algorithm, a heuristic routing protocol is also described. The performance simulation results show that the overall energy consumption for forwarding and answering a collection of queries can be significantly reduced by applying our query management scheme. The advantages and disadvantages of the proposed scheme are discussed, together with open research issues

Dynamic characteristics of centrifugal pump induced by fluid and motor excitation forces

The combined dynamic characteristics of the centrifugal pump induced by the fluid and motor excitation forces are investigated in this paper. The coupling vibrations of a centrifugal pump during the operation are mainly caused by the fluid excitation and the motor excitation forces. The finite element model was constructed in this paper under the consideration of the fluid excitation which was obtained from the numerical simulation and the motor excitation force which came from the experiments; compared with the experimental results and well agreement, the components of the whole model were validated to be accurate enough for simulation. Applying the approach of the modal dynamics, the dynamic analysis was conducted to study the influence of the flow rate, the blade number, the exit installation angle and the outside diameter of impeller on the responses. The suggested optimal parameters were provided from the perspective of the vibration reduction. The results of the calculation are helpful to the designation and the safe operation of the centrifugal pumps

Numerical investigation on flow-induced structural vibration and noise in centrifugal pump

A full scale structural vibration and noise induced by flow was simulated by a hybrid numerical method. An interior flow field was solved by large eddy simulation firstly. The sliding mesh technique was applied to take into account the impeller-volute interaction. A sensitivity analysis on effects of near-wall grid size and sampling time on amplitude of pressure pulsations was performed to impose appropriate vibration exciting source. Computed modal of pump components was validated by experimental results, before the volute vibration and sound field were simulated using a coupled vibro-acoustic model. The numerical results indicated that the amplitude of pressure fluctuation, especially on those points located at near the volute tongue, strongly depended on near-wall grid size. The dominated frequency of the vibration velocity of volute was also blade-passing frequency (BPF), which was in according with frequency spectral characteristics of unsteady pressure fluctuation. Directivity distribution of radiation acoustic field excited by volute vibration was typical dipoles. This study shows that it is feasible to use the hybrid numerical method to evaluate the flow-induced vibration and noise generated in centrifugal pump

An Energy-Efficient and Fault-Tolerant Convergecast Protocol in Wireless Sensor Networks

The simple graph theory is commonly employed in wireless sensor networks topology control. An inherent problem of small-granularity algorithms is the high computing complexity and large solution space when managing large-scale WSNs. Computed transmission paths are of low fault tolerance because of unattended sensor nodes and frail wireless transmitting channels. This paper uses hyper-graph theory to solve these practical problems and proposes a spanning hyper-tree algorithm (SHTa) to compute the minimum transmitting power delivery paths set for WSNs convergecast. There are three main contributions of this paper: (1) we present a novel hyper-graph model to abstract large-scale and high connectivity WSNs into a robust hyper-tree infrastructure; (2) we present a precise mathematical derivation that solves the “hyper-tree existence” problem; (3) SHTa is proposed to compute the delivery paths set, which is the minimum power transmitting convergecast hyper-tree. Variable scale hyper-edges represented as computing units limit solution space and reduce computing complexity. Mutual backup delivery paths in one hyper-edge improve the capability of fault tolerance. With experiment results, SHTa computes short latency paths with low energy consumption, compared with previous algorithms. Furthermore, in dynamic experiments scenes, SHTa retains its robust transmitting quality and presents high fault tolerance