Natural Frequencies Of Structure Research Articles

Time dependent uncertain free vibration analysis of composite CFST structure with spatially dependent creep effects

• Uncertain free vibration analysis of composite concrete filled steel tubular structure is investigated. • Spatially dependent uncertain creep effect of the concrete core is considered. • Interval field model is adopted to model the spatially dependent uncertain parameters. • Robust bounds of natural frequencies of the structure can be determined. In this study, the time dependent free vibration analysis of composite concrete-filled steel tubular (CFST) arches with various uncertainties is thoroughly investigated within a non-stochastic framework. From the practical inspiration, both uncertain material properties and mercurial creep effect associated with such composite materials are simultaneously incorporated. Unlike traditional non-probabilistic schemes, both spatially independent (i.e., conventional interval models) and dependent (i.e., interval fields) interval system parameters can be comprised within a unified uncertain free vibration analysis framework for CFST arches. For the purpose of achieving a robust framework of the time-dependent uncertain free vibration analysis, a new computational approach, which has been developed within the scheme of the finite element method (FEM), has been proposed for determining the extreme bounds of the natural frequencies of practically motivated CFST arches. Consequently, by successfully solving two eigenvalue problems, the upper and lower bounds of the natural frequencies of such composite structures with various uncertainties can be rigorously secured. The unique advantage of the proposed approach is that it can be effectively integrated within commercial FEM software with preserved sharp bounds on natural frequencies for any interval field discretisation. The competence of the proposed computational analysis framework has been thoroughly demonstrated through investigations on both 2D and3D engineering structures.

Development of a Three-Degree-of-Freedom Ultrasonic Vibration Tool Holder for Milling and Drilling

This paper presents a three-degree-of-freedom (3-DOF) ultrasonic vibration tool holder for milling and drilling operations. The 3-DOF vibrations are generated by a novel actuator that contains three groups of piezoelectric rings actuating in X -, Y- , and Z -directions at the natural frequencies of the structure. The vibrations in XY plane are excited at the frequency but with slightly different amplitudes to produce an elliptical locus for intermittent tool-workpiece contact, which assists milling process, and the vibrations along Z -axis are used in drilling operations. The 3-DOF ultrasonic vibration actuator excites the structure's natural frequencies, which are above 16 kHz with amplitudes up to 20–25 $\mu$ m. A control system has been developed for tracking the resonance frequencies during cutting from the phase of actuator's impedance. Preliminary cutting tests show that the cutting forces are reduced by over 30% with the ultrasonic vibration assisted milling and drilling operations.

An adaptive‐passive retuning device for a pendulum tuned mass damper considering mass uncertainty and optimum frequency

A tuned mass damper (TMD) is one of the most used structural control devices. However, a traditional TMD has the disadvantage of high sensitivity to frequency deviation and difficulty adjusting the frequency. The optimal frequency for a TMD is dependent on the structural dominant frequency and the TMD mass ratio. Nevertheless, the actual structural modal mass is difficult to obtain, and the presence of a TMD may interfere with identification of the natural structural frequency. Aiming to control wind-induced vibration, an adaptive-passive retuning device is developed for a pendulum TMD called an adaptive-passive variable pendulum TMD (APVP-TMD). When it is time to adjust the pendulum, the mass will first be locked, and the structural frequency in this case is identified through wavelet transformation by an acceleration sensor and a microcontroller. It is found that this is actually the optimal frequency for the TMD. Then, a stepper motor will adjust the length of the pendulum under the guidance of the microcontroller. The effectiveness of APVP-TMD is verified through both discrete and continuous models. For the discrete model, a single-degree-of-freedom primary structure coupled with an APVP-TMD is presented as an experiment with analysis comparison, and a five-degree-of-freedom primary structure coupled with an APVP-TMD is proposed as a numerical simulation. For the continuous model, a wind-sensitive concrete chimney is controlled by an APVP-TMD as a case study. The results all show that the APVP-TMD can identify the optimal frequency and retune itself effectively, and the retuned TMD has better vibration control than the mistuned one.

Dynamic interactions of multiple wall-mounted flexible flaps

Coherent waving interactions between vegetation and fluid flows are known to emerge under conditions associated with the mixing layer instability. A similar waving motion has also been observed in flow control applications, where passive slender structures are used to augment bluff body wakes. While their existence is well reported, the mechanisms which govern this behaviour, and their dependence on structural properties, are not yet fully understood. This work investigates the coupled interactions of a large array of slender structures in an open-channel flow, via numerical simulation. A direct modelling approach, whereby the individual structures are fully resolved, is realised via a lattice Boltzmann-immersed boundary-finite element model. For steady flow conditions at low–moderate Reynolds number, the response of the array is measured over a range of mass ratio and bending rigidity, spanning two orders of magnitude, and the ensuing response is characterised. The results show a range of behaviours which are classified into distinct states: static, regular waving, irregular waving and flapping. The regular waving regime is found to occur when the natural frequency of the array approaches the estimated frequency of the mixing layer instability. Furthermore, after normalising with respect to the natural frequency of the array, the frequency response across the examined parameter space collapses onto a single curve. These findings indicate that the coherent waving mode is in fact a coupled instability, as opposed to a purely fluid-driven response, and that this specific regime is triggered by a lock-in between the fluid and structural natural frequencies.

Diet and Psychosis: A Scoping Review

This paper presents a new robust method namely, unified interval Chebyshev-based random perturbation method, to tackle hybrid random interval structural natural frequency problem. In the proposed approach, random perturbation method is implemented to furnish the statistical features (i.e., mean and standard deviation) and Chebyshev surrogate model strategy is incorporated to formulate the statistical information of natural frequency with regards to the interval inputs. The comprehensive analysis framework combines the superiority of both methods in a way that computational cost is dramatically reduced. This presented method is thus capable of investigating the day-to-day based time-variant natural frequency of structures accurately and efficiently under concrete intrinsic creep effect with probabilistic and interval uncertain variables. The extreme bounds of the mean and standard deviation of natural frequency are captured through the embedded optimization strategy within the analysis procedure. Three particularly motivated numerical examples with progressive relationship in perspective of both structure type and uncertainty variables are demonstrated to justify the computational applicability, accuracy and efficiency of the proposed method.

Fluid–Structure Interactions on a Tunable Store in Complex Cavity Flow

Fluid–structure interactions were studied on a store with tunable structural natural frequencies in complex cavity flow. Different leading-edge geometries, doors, and internal inserts generated cavity pressure fields more representative of an actual aircraft bay. The store loading and response was characterized using point pressure and accelerometer measurements, while high-frequency pressure-sensitive paint captured the distribution of surface pressure. In the complex cavity, the store responded to the cavity resonant tones not only in the streamwise and wall-normal directions but also in the spanwise direction. That spanwise response to cavity tones was not observed in a simple rectangular cavity because the flow in the spanwise direction was uniform and in phase. This different behavior highlights the importance of using a representative bay geometry and internal store for the prediction of the structural response in a flight environment.

Stochastic interval analysis for structural natural frequencies based on stochastic hybrid perturbation edge-based smoothing finite element method

Realistically calculating of natural frequencies is crucial for studying dynamical characteristics of structures in engineering. In this paper, the gradient smoothing technique is first applied into the conventional finite element method (named ES-FEM), to improve the accuracy of the deterministic calculation of natural frequencies. Then, the hybrid random and interval uncertain parameters are introduced into the proposed ES-FEM based on hybrid stochastic and interval perturbation method. Expressions for the mean value and variance of natural frequencies are derived by combining the random interval perturbation method and the random interval moment method. The upper and lower bounds of mean value and variance of natural frequencies are calculated based on hybrid perturbation vertex method. The proposed method (denoted ESHPVM) is compared with the different methods. The high accuracy and efficiency of the proposed methods are verified by two numerical examples.

The investigation of vibration characteristics on the bolted disk–drum joints structure

In a bolted disk–drum joints structure, assembly parameters, such as the number and pretightening force of the tightening bolts, will affect the vibration characteristics of the structure. In this article, the influences are discussed. First, the influences of contact and structural stiffness on the natural characteristics of the joint structure are analyzed theoretically. Considering the micro-contact between the contacting surfaces of the bolts’ connected structure, the stress diffusion between the bolt head and the disk is investigated. Then, the relationship between the tightening torque and contact stiffness, as well as the relationship between the number of tightening bolts and structural stiffness, is established. Furthermore, the relationship between assembly parameters and the natural frequencies is obtained. Hence, the regularity of the influence on the structural natural frequencies can be summarized as a consequence. The results of experiment showed that the minimum error between theoretical calculation and experimental data was 1.5%, which provides a reliable analysis tool for the investigation of vibration characteristics on the bolted disk–drum joints structure.

Investigation of liner cavitation considering coupling effect between engine vibration and acoustic field of water coolant passage

The engine vibration gives rise to pressure fluctuation of water coolant; therefore, the coupled system between engine structure and acoustic field of water coolant passage needs to be established to calculate the response of engine vibration and water coolant pressure fluctuation. A rectangular tank is used to evaluate water pressure induced by structural impact vibration. The dynamic characteristics of structural-acoustic coupled system are discussed by changing water depth. Considering the resonance effect, the relationship between natural frequencies of structure and acoustic field is examined by variation of sound speed. The equation of motion of engine structure coupled with acoustic field of water coolant passage is established to calculate coolant pressure fluctuation during engine working condition. The effect of piston slap forces on pressure fluctuation is evaluated. The influence of acoustic characteristics of water coolant passage on pressure fluctuation is discussed by varying sound speed.

Analysis of structures subjected to crowd loads

The movement of people on any structure induces dynamic crowd load. The enthusiastic potential behavior of people from overwhelming happiness in concert, shows etc. on structures such as stadiums, auditorium, grandstands, bridges, malls and convention centers causes synchronized rhythmic movements. Resonance between forcing frequency and one or more natural frequencies of the structure resulting from human-structure synchronization leads to strong vibrations. Such vibrations not only affect structural stability, but also create discomfort among people. Pedestrian induced lateral vibration of Millennium Bridge, London is a classic example for the same. Crowd loads impose vertical as well as horizontal loads on the structure. Both these loads induce vibrations in corresponding directions. Beyond an acceptable limit of vibration level, people may feel uncomfortable. If there is a mode of low natural frequency, even a significantly low forcing frequency can lead to strong vibrations, which in turn may cause panic. The present research focuses on analysis of a simply supported slab strip subjected to crowd load. The crowd-structure interaction is analytically modelled and the modal parameters and the response of the occupied structure are determined. Furthermore, the effect of various parameters such as crowd size, crowd location and crowd activity on the behavior of occupied structure is studied. The comfort level of the people on the structure is also estimated. It is observed that crowd parameters affect the behavior of the system as well as the comfort level of the occupants. It is further seen that the trend of variation of natural frequency of the occupied structure with variation in crowd properties changes with change in the empty structure properties.

Dynamic Modeling and Analysis of a Horizontal Operating 3-Axis Machine for Friction Stir Welding

A mathematical lumped model for horizontal operating three axes machine for Friction Stir Welding (FSW) of heat exchangers' tube to tubesheet, is formulated using Finite Element Analysis (FEM) in conjunction with Lagrange's equation formulations. The equations of motion are obtained in matrix form using Lagrange's equation. The system consists of 14 degrees of freedom. The structure's natural frequencies, together with the output motion at two main points, are obtained by solving the generalized eigenvalue problem resulting from the system matrices using both Solidworks (SW) and Matlab. Dynamic behavior is simulated by providing dynamic motion of the same points on the structure under cyclic forces/torques at different frequencies. These studies were performed with the spindle located at the center of the machine frame and enduring traverse loadings. Frequency predictions and displacement responses obtained from lumped model are compared with those provided by FEM and found in excellent agreement.

Opto-Mechanical Structure Design and Natural Frequency Analysis of Payload for Remote Sensing Satellite

This paper presents natural frequency and stress of opto-mechanical structure. In this study, an opto-mechanical structure was investigated.The opto-mechanical structure includes lens assembly and detector head assembly (DHA). The opto-mechanical structure requires high stability and position accuracy. In this design and analysis study, wide ranges of the opto-mechanical structural analysis have been performed including: Natural frequency analysis and dynamic analysis. Opto-mechanical design includes material selection and mechanical design. Payload structure design will be based on stiffness. Hence, Natural frequency is major design criteria for opto-mechanical design of payload structure. At higher vibration during launching time, resonance will be created and parts will be misaligned or may be damaged. Objective of this work is to reach maximum natural frequency with minimum stress on opto-mechanical structure.

考虑轴向力作用结构自振频率的简化计算

考虑轴向力作用结构自振频率的简化计算

Finite element modeling of masonry wall with mortar 1pc : 4 lime : 10 sand under lateral force

This paper gives a detailed presentation of three-dimensional Finite Element Model that has been constructed for Masonry Wall under lateral force by using Abaqus software. This research aimed to investigate the behavior of Masonry Walls under lateral force and developed load-displacement curve. From the result that The numerical model using the Abaqus Software can represent the load-displacement curve of Masonry Walls due to lateral forces, Numerical Results with the Abaqus Software obtained that the magnitude of the load on condition of Plastic is 82.13 KN and experimental results obtained of 81 KN. There is a difference of 1.4%, Based on Abaqus Software with Numerical results obtained a compressive strength of masonry wall fm = 1.8 MPa with a modulus of elasticity = 150 MPa, Calculation of the natural frequency of structures with Abaqus Software is obtained as the difference of 2.13-2.92% with the test results Hakas (2017)

Simplified theoretical treatment of lateral structural properties under crowd excitation

Walking crowd-induced lateral vibrations on slender structures have attracted considerable attention. The improper vibration of a structure adversely affects human comfort. To explore the effects of crowds on lateral structural properties, a pedestrian from a crowd is simulated by a walking bipedal robot. A simplified theory for structural vibration is proposed based on the assumptions of uniform distribution and synchronized walking of pedestrians. This theory can be used to describe the effect of the change in crowd size on lateral structural damping and the frequency of the structure. The method can estimate the variations in structural properties in the case of a certain crowd for engineering design. The results show that the increase in crowd size results in decrease in structural natural frequency in the lateral direction, but increases structural damping. The influence of the crowd on structural properties agrees well with the non-simplified model. Change in walking frequency has little influence on the structural properties. However, the continuous increase in crowd size on structure top causes a non-convergent amplification of dynamic response under a resonant walking excitation. This research provides a quantitative assessment on the effect of crowd size on the change in structural properties for structural design or serviceability evaluation.

A mass lumping scheme for the second-order numerical manifold method

The numerical manifold method (NMM) has been employed to solve many types of engineering problems. To improve accuracy of the traditional NMM, a second-order NMM with six-node triangular mesh was recently proposed. This second-order NMM is further applied for structural dynamic problems in this study. In order to reduce time consumption in solving large scale simultaneous algebraic equations, a mass lumping scheme which is suitable for the second-order NMM is proposed. A series of numerical examples show that natural frequencies of structures assessed from the proposed lumped mass matrix (LMM) are more accurate than those from the consistent mass matrix (CMM). In addition, even in the implicit time integration scheme to predict dynamic responses of structures, the CMM can be replaced by the proposed LMM without losing accuracy.

Hypersonic Fluid–Structure Interactions Due to Intermittent Turbulent Spots on a Slender Cone

Fluid–structure interactions due to intermittent turbulent spots were studied on a 7 deg half-angle cone in the Sandia Hypersonic Wind Tunnel at Mach 5 and 8 and in the Purdue Boeing/U.S. Air Force Office of Scientific Research Mach 6 Quiet Tunnel. A thin composite panel was integrated into the cone and the response to boundary-layer disturbances was characterized by accelerometers on the backside of the panel. Under quiet flow conditions at Mach 6, the cone boundary layer remained laminar. Artificially generated turbulent spots excited a directionally dependent panel response. When the spot generation frequency matched a structural natural frequency of the panel, resonance would occur, and responses over 200 times greater than under a laminar boundary layer were obtained. At Mach 5 and 8 under noisy flow conditions, natural transition driven by the wind-tunnel acoustic noise dominated the panel response. A single turbulent spot could not change this forced response. However, at higher spot burst rates, an elevated vibrational response was observed during transition at frequencies corresponding to the distribution of turbulent spots in the transitional flow. Once turbulent flow developed, the structural response dropped because the intermittent forcing from the spots no longer drove panel vibration.

Comparison Between Performance of a Wireless MEMS Sensor and an ICP Sensor in Shaking Table Tests

Wireless sensors are more favorable in measuring structural response compared to conventional sensors. This is because they are easier to use with no issues with cables and are considerably cheaper. There are several applications that can be used in recording and analyzing data from MEMS sensor installed on an iPhone. The Vibration App is one of the applications used and there has not been adequate research conducted in analyzing the performance of this App. This paper analyzed the performance of the Vibration App by comparing it with the performance of an ICP sensor. Results show that natural frequency results are more accurate (error less than 5%) in comparison to the amplitude results. This means that built- in MEMS sensor in smartphones are good at estimating natural frequency of structures. In addition, it was seen that the results became more accurate at higher frequencies (5.0Hz and 10.0Hz).

Study on vertical vibration control of long-span steel footbridge with tuned mass dampers under pedestrian excitation

This paper aims to study crowd-induced vibration control of long-span steel footbridges with different dynamic characteristics by combining methods of site measurement and numerical simulation. Four kinds of footbridge models with sensitive vertical natural frequencies of easily generating human-bridge resonance under pedestrian loading are designed based on an actual steel footbridge. The numerical models are firstly validated by comparative investigation between the site measurement and the simulation. Detailed study on dynamic responses of the four footbridges with and without controlling systems of tuned mass damper (TMD) and multiple tuned mass damper (MTMD) is then conducted under 13 cases of crowd random excitations, rhythmic running and jumping excitations. Results show that the numerical simulation agrees well with the site measurement data. TMD system is found to be highly efficient in reducing vibration responses only when the excitation frequency is basically consistent with the structural natural frequency, which obviously limits the application of TMD system in footbridges as wider excitation frequency bandwidth is caused by human activities. MTMD system are demonstrated to be with higher vibration absorption robustness appearing predominant and stable capacity of reducing structural vibrations under all the crowd random and rhythmic excitations for the four footbridges.

Experimental Comparison Study on Stress Relief for Welded Low Carbon Steel (St 37) Bar by Vibration Mechanism and Heat Treatment Process

In this paper An Experimental process of vibration stress relief (VSR) and post-weld heat treatment (PWHT) after welding the resonant vibration is implemented. Depending on the vibration frequency as the indications for stress relief for a Welded Low Carbon Steel (St. 37) Bar. The vibration frequency should be applied to the structure and the structure natural frequency should be close each other in vibration mode, and with small vibration amplitude which will be amplified during vibration under a 7 N force of frequency which it close to the natural frequency of the welded bar (at resonance) for 20 mints. When the amplitude of vibration in the stress relief became large, then the treatment will be more effective. The (PWHT) is employed for comparison, and indication that the stress been relief are take place by mean of metallurgical and mechanical properties development and change in the natural frequency before and after treatment lead to change the amplitude for small to larger also indicate that the stress been relived.