Changes In Natural Frequencies Research Articles

ДОСЛІДЖЕННЯ НЕРЕЗОНАНСНИХ КОЛИВАНЬ СИСТЕМИ «ТЕЛЕСКОПІЧНИЙ ГВИНТ – СИПКЕ СЕРЕДОВИЩЕ»

In the article it is substantiated the value of the angular speeds of rotation of the auger screw, which leads to the breakdown of its lateral vibrations. The dependences describing the law of change of amplitude or natural frequency at slowly variable length of the telescopic screw are deduced. Based on the Van der Paul’s method, in the developed system differential equations are obtained that determine the laws of change of amplitude and frequency of the wave process in the system of a telescopic propeller. It is established that for nonresonant oscillations for this system the main parameters of bending oscillations are a continuous flow of bulk medium - the screw does not depend on its small torsional oscillations and external periodic perturbation. The analysis of the given regression equations shows that to reduce the torque of the auger it is necessary to reduce the frequency of its rotation and the angle of the conveyor. The constructive diagram and the results of theoretical calculations for assessing the influence of constructive-kinematic parameters on the torque indicators of the telescopic screw conveyor are presented.

ОЦЕНКА ЭФФЕКТИВНОСТИ ИСПОЛЬЗОВАНИЯ ТРАДИЦИОННЫХ КОНСТРУКЦИЙ ПОДВЕСОК В СПЕЦИАЛИЗИРОВАННОМ АВТОМОБИЛЬНОМ ТРАНСПОРТЕ

In order to identify the reasons for the decrease in the efficiency of vehicles equipped with traditional suspension designs and to identify promis-ing areas for improving their efficiency, an analysis was made of the results of a review of studies obtained in the scientific field under consideration by foreign and Russian scientists. The article substantiates the relevance of increasing the efficiency of suspensions of specialized trucks and road trains operated in difficult changing climatic and road conditions. The main disadvantages that appear during the operation of vehicles equipped with traditional suspension designs are given. The consequences of the operation of such vehicles equipped with traditional suspension designs in conditions of a low level of road infrastructure are listed. The advantages of hydropneumatic suspensions, leaf springs made of composite materials in comparison with steel springs, independent suspension structures over dependent ones, semiactive air suspensions over passive and active ones, as well as regenerative suspensions are considered. The dependences of the change in the time between failures of the elements of the spring suspension, the estimated durability of the spring on the speed of movement, the acceleration of the sprung mass of the trailer of a road train when driving over bumps, the resistance forces of the hydraulic shock absorber on the ambient tem-perature are presented. The dependences of the change in natural frequencies and the damping coefficients of oscillations of the sprung and un-sprung masses of the car with an increase in the mass of its sprung part are given. The perspective directions for increasing the efficiency of suspensions of timber trucks and road trains identified as a result of the analysis are given.

Artificial neural network based delamination prediction in composite plates using vibration signals

Dynamic loading on composite components may induce damages such as cracks, delaminations, etc. and development of an early damage detection technique for delaminations is one of the most important aspects in ensuring the integrity and safety of composite components. The presence of damages such as delaminations on the composites reduces its stiffness and further changes the dynamic behaviour of the structures. As the loss in stiffness leads to changes in the natural frequencies, mode shapes, and other aspects of the structure, vibration analysis may be the ideal technique to employ in this case. In this research work, the supervised feed-forward multilayer back-propagation Artificial Neural Network (ANN) is used to determine the position and area of delaminations in GFRP plates using changes in natural frequencies as inputs. The natural frequencies were obtained by finite element analysis and results are validated by experimentation. The findings show that the suggested technique can satisfactorily estimate the location and extent of delaminations in composite plates.

An experimental investigation on the effectiveness of damping changes for damage detection in a Fibre Reinforced Plastic Plate

Fibre reinforced plastics (FRP) are extensively used because of their high specific strength and stiffness. However, throughout their service life, such materials are susceptible to damages, which when accumulated may result in catastrophic failure. Studies on effects of stiffness loss due to damage has been exercised by quite a large number of researchers but very less investigators have involved the variations in dissipative behaviour in combination with such stiffness loss. However, damage in these structures not only causes local reduction of stiffness but is also observed with increased dissipation behaviour. One of the most convenient ways to carry out health monitoring exercise to detect damages is to conduct systematic vibration testing. The present study investigates the likely changes in natural frequencies and mode shapes of a rectangular FRP composite plate along with damping factors due to the induced damage. The FRP plate considered has been modal tested and dynamic responses i.e., natural frequencies, mode shapes and modal damping factors are derived through curve-fitting technique from the frequency response function measurements. In this study, changes in dynamic responses due to a locally induced damage in the FRP plate is studied. For this, in order to maximize the effects of local reduction in the stiffness on the modal parameters, a partial-cut is provided in a localised region of the plate in the first damage case. In the second damage scenario, the partially-cut area has been filled with epoxy resin matrix to create a region with localised change in material properties and with limited stiffness loss. Damping changes has been observed to be significant and more sensitive than variations in natural frequencies and mode shapes. The study suggests that damping parameters along with stiffness changes together can be put into practice to develop an effective vibration-based health monitoring scheme

A hybrid model for modelling arbitrary cracks in isotropic plate structures

Damage modelling is essential in preliminary structural design and non-destructive damage detection. This paper investigates changes in natural frequencies of plates due to an arbitrarily aligned crack. A novel hybrid method couples an exact strip model for the intact part with a finite element model for the cracked part. The crack is modelled as a rotational spring with additional rotational freedoms being added to the finite element nodes. The method is validated against published results for through-the-depth cracks. Cracks with varying direction, location, depth and length are used to study the effects of changing the crack parameters.

Analysis and design optimization approach of vibration characteristics of an I shaped straight steel girder bridge

Bridges are subjected to environmental exposure like corrosion which may induce cracks. These cracks may cause change in natural frequency of bridge. The current study's goal is to explore the effect of crack on the vibration characteristics of bridge the design and modal analysis of bridge is conducted by ANSYS 21 simulation package. Two different designs of bridges are evaluated i.e. without crack and with crack.The effect of crack on natural frequency and mode shape is evaluated. The vibration characteristics of bridge structure changed with artificially induced transverse elliptical crack on girder.Techniques of optimization could help in determining the behaviour of the damage and extent to which it could be controlled via simulation technique without conducting tests on the actual model. Analysing the frequencies using ANSYS software, the validity of the presented numerical formulation is studied. Response spectrum methodology is used in studying the crack characteristic and optimization of crack variables. Sensitivity analysis showed the individual effect of each variable on the output parameters. Moreover, the response surface plots generated showed the peak and least deformation values of natural frequencies. Invaluable information which help practicing engineers better understand the vibration characteristics is provided.

Damage localization in beams based on the analysis of modal parameters

This paper presents a two-step method for damage localization in beams by combining natural frequencies and mode shapes. The general locations of the damage are first identified from an indicator developed using relative natural frequency change (RNFC) curves and the values of RNFCs. A curvature-mode-shape-based method is then utilized to determine the specific location of the damage in the second step. The proposed two-step method is verified by detecting damage in a simulated simply-supported beam. The identified damage location agrees well with the actual damage location. A strategy for fast and accurate damage localization based on general localization using natural frequencies and specific localization using mode shapes is the main novelty of the paper.

A New Ice Quality Prediction Method of Wind Turbine Impeller Based on the Deep Neural Network

More and more wind turbines are installed in cold regions because of better wind resources. In these regions, the high humidity and low temperatures in winter will lead to ice accumulation on the wind turbine impeller. A different icing location or mass will lead to different natural frequency variations of the impeller. In order to monitor the icing situation in time and in advance, a method based on depth neural network technology to predict the icing mass is explored and proposed. Natural-environment icing experiments and iced-impeller modal experiments are carried out, aiming at a 600 W wind turbine, respectively. The mapping relationship between the change rate of the natural frequency of the iced impeller at different icing positions and the icing mass is obtained, and the correlation coefficients are all above 0.93. A deep neural network (DNN) prediction model of ice-coating quality for the impeller was constructed with the change rate of the first six-order natural frequencies as the input factor. The results show that the MAE and MSE of the trained model are close to 0. The average prediction error of the DNN model is 4.79%, 9.35%, 3.62%, 1.63%, respectively, under different icing states of the impeller. It can be seen that the DNN shows the best prediction ability among other methods. The smaller the actual ice-covered mass of the impeller, the larger the relative error of the ice-covered mass predicted by the DNN model. In the same ice-covered state, the relative error will decrease gradually with the increase in ice-covered mass. In a word, using the natural frequency change rate to predict the icing quality is feasible and accurate. The research achievements shown here can provide a new idea for wind farms to realize efficient and intelligent icing monitoring and prediction, provide engineering guidance for the wind turbine blade anti-icing and deicing field, and further reduce the negative impact of icing on wind power generation.

A semi-active toroidal TLCD for multidirectional vibration reduction of structures

The toroidal tuned liquid column damper (TTLCD) is a recently developed device to mitigate multidirectional vibrations of structures. In this paper, a semi-active TTLCD (S-TTLCD), which can automatically identify the frequency change of structures and tune its natural frequency correctly by adjusting the liquid level, is proposed. The feasibility of the S-TTLCD is proved experimentally. The impacts of several important parameters, such as the multi-stage stiffness variation of structure, the pump flow rate, the mass ratio, and the damping ratio, on the performance of the S-TTLCD are evaluated numerically. The results confirm that the S-TTLCD is well adaptive and reduces the structural response more significantly when the structural natural frequency changes, compared to the passive TTLCD.

Experimental study on dynamic characteristics of a monopile foundation based on local scour in combined waves and current

Scour is one of the key factors causing the collapse of buildings (structures) in water. Pile foundations are often subject to the combined actions of waves and current, resulting in weakened soil overturning resistance and changes in natural vibration frequency, which may lead to vortex-induced vibration under the action of fluid flow and cause fatigue damage. A series of experiments under current-only, wave-only, and combined wave-current conditions were carried out in a wave-current flume to investigate the scour phenomenon around a monopile and the change in natural vibration frequency of the monopile foundation along the flow and vertical directions caused by local scour. The scour depth under combined wave-current conditions is greater than that under current-only conditions, and increases accordingly with the increase of the flow velocity and the decrease of the water depth. Furthermore, the scour phenomenon under different flow patterns was analyzed according to the variation of pulsation pressure. The empirical formula for the natural vibration frequency of the monopile foundation considering the local scour under current-only and combined wave-current conditions was established respectively. Compared with the experimental results, the maximum error is 10%, indicating that the empirical formula has a high degree of confidence. The above empirical formula obtained from the indoor scale model is applied to the field test, and the maximum relative error is 14.37%, which indicates that the empirical formula is also suitable for the prototype scale.

A study on the load characteristics of the anchorage according to the natural frequency change of the electric cabinet in a power plant

Due to the recent earthquakes in Korea, many cases of damage to anchorage of the electric cabinet in the power plant have been reported. Accordingly, for the purpose of reviewing the seismic safety, a study on the load characteristics transmitted to the anchorage during an earthquake is proceeding. It was investigated that the load transmitted to the anchorage also changes according to the change in the natural frequency of the cabinet. However, in the case of cabinets used in the actual field, it is necessary to prove the results of previous studies under more diverse conditions because they are composed of various stiffness. In this study, a structure replacing a cabinet was manufactured and the load transmitted to the anchorage under various conditions was analyzed through an experimental and analytical approach. As a result of this study, it was confirmed that the load transmitted to the anchorage increases as the natural frequency of the structure increases, and a correlation equation between the natural frequency of the structure and the transmission load of the anchorage was derived. Using the correlation equation proposed in this study is expected to help identify the load delivered to the cabinet anchorage under an earthquake.

Variable Natural Frequency Damper for Minimizing Response of Offshore Wind Turbine: Principle Verification through Analysis of Controllable Natural Frequencies

Resonance causes extreme stress, acceleration of fatigue, and reduction in lifespan of offshore wind structures. The main factors that cause resonance are environmental loads such as wind and waves, and dynamic loads caused by rotor movement. Estimation of the natural frequency at the design stage is highly uncertain, and natural frequency changes occur due to various factors during long-term operation. Therefore, it is important to ensure structural safety from resonance through a vibration-monitoring system or an additional damper. In this study, the effect of seawater existing inside the substructure on the natural frequency of the structure was dealt with. The natural frequency estimation equation for a fixed offshore wind structure was derived with the “inner fluid simplification assumption”. The finite element modal analysis was performed to verify the principle of Variable Natural Frequency Damper (VNFD), a system that controls the natural frequency of offshore wind structures through a pump, and to find the range of natural frequency control. As a result, interior fluid affects the natural frequency of the wind turbine support structure. Specifically, the variable natural frequency range was very low, at about 0.027% for the monopile model at a depth of 10 m, but increased rapidly to about 3.66% at a depth of 70 m. Furthermore, when estimating the natural frequency of a fixed offshore wind turbine in deep water without consideration of interior fluid, the estimates can be higher than with consideration of it.

Monitoring abnormal vibration and structural health conditions of an in-service structure from its SHM data

Vortex-induced vibration occurred recently in several important engineering structures in China. Very large mono-frequency periodic vibration was noted. This study reports on the analysis of recorded data from the structural health monitoring (SHM) system of a suspension bridge with the aim to develop some indicators that may be useful to evaluate if there is any abnormal vibration or change of health conditions of a structure in future. The power spectrum of measured acceleration responses is plotted in three-dimensions of time, frequency and power. Acceleration responses from 16 sensors on the target bridge deck in 233 days are analyzed. The evolution of different dynamic parameters in the period encompassing the occurrences of large vortex-induced vibration are reported. The changes in these parameters are identified to be closely associated with some temporary installations on the deck and temporary measures for vibration suppression. The changes in vibration power at different natural frequencies of the structure can be used to demonstrate the evolution of the unexpected event. The trend and abrupt changes of natural frequencies and modal components show the flow of vibration energy between different vibration modes. In addition to the intuitive three-dimension power spectral plot, two state vectors are also developed from the power spectrum. The identification of abnormal vibration and structural state can then be carried out automatically with the novelty detection method and a threshold. The analysis of the SHM acceleration responses of the bridge deck demonstrates the implementation procedure and the effectiveness of the proposed method.

A LABVIEW AND INTERNET OF THINGS BASED AUTOMATION MEASUREMENT SYSTEM FOR MONITORING AND ANALYZING STRUCTURE

The civil infrastructure systems play a critical role in the world; however, their average age is becoming higher and higher. Most of these structures need inspection, maintenance or replacement in order to ensure safety requirement of structures as well as users. Structural health monitoring systems has drawn attention from many organizations to manage crucial structures. Applied programming and Internet of Things (IoT) has received a great attention in the last decades due to its potential and capacity to be integrated into any complex system. This paper presents the use LabVIEW, IoT platform and hardwares to build an automated measurement system. The proposed solutions are demonstrated in this paper will help building robust and reliable measurement systems for crucial applications. The measurement system is used to evaluate the health of structures through the changes of natural frequencies.

Seismic Response Evaluation of High-Steep Slopes Supported by Anti-Slide Piles with Different Initial Damage Based on Shaking Table Test.

In order to study the instability development process of the slope reinforced by anti-slide piles under earthquake conditions, the dynamic response characteristics of the slope are usually taken as the main characteristics, and the model test and numerical simulation are the main research methods. In this paper, a shaking table model test is designed and completed to investigate the influence of anti-slide piles with different initial damage on the failure mode of high and steep slope under earthquake conditions. The changes in velocity, strain and natural frequency during slope vibration are tested in combination with cloud maps when sinusoidal waves of different accelerations with a peak value of 5 Hz are applied. Thus, the differences of slope failure development process and dynamic response characteristics are obtained. The experimental results show that the anti-slide pile with different initial damage has obvious influence on the slope instability process. Under the condition of good anti-slide pile quality, the failure development of the slope behind the pile is limited to soil sliding on top of the slope, slope sliding and overburden sliding; the front slope foot of pile mainly forms shear belt and local sliding. With the decrease in the initial mass of the anti-slide pile, the slope failure develops into topsoil sliding, slope sliding and deep integral sliding; analogously, the failure of the slope in front of the pile develops into a whole slip along the slip belt. The natural frequency cloud map can directly reflect the damage location of the slope, and the frequency change rate is positively correlated with the cumulative shear strain. It shows that the macro-failure characteristics of the model slope change well when the natural frequency is used as the sensitive index to measure the influence of vibration on the model slope. The threshold value of the natural frequency change rate can distinguish different development stages of the slope; 1% is the threshold value of stage II, and 1.5% is the threshold value of stage III.

Aeroelastic modal dynamics of floating wind turbines in anisotropic conditions based on Floquet analysis

A reduced order model of a floating wind turbine with 22 degrees of freedom has been implemented and validated in comparison to the multibody, hydro-servo-aero-elastic code hGAST. The multi-blade Coleman transformation is applied and Floquet stability analysis is performed. This allows on one hand, to verify the actual Floquet implementation against the standard Coleman-based eigenvalue analysis in cases the transformed dynamic system has constant coefficients and on the other hand to analyze the effects of different anisotropies on stability. Mass imbalance of the rotor and yaw misalignment have been specifically addressed. In the mass imbalance scenario, the homogeneous results are not substantially altered while in the wind yaw scenario, there is slight change of the flapwise natural frequencies and of the tower lateral frequency as well as of the corresponding damping characteristics.

Changes of Natural Frequencies of Shafts Due to Pitting Corrosion

Changes of Natural Frequencies of Shafts Due to Pitting Corrosion

Contemporary Approaches to the Vibration Diagnostics of Rotating Shafts

Rotating shafts are the most vulnerable part of steam turbines. They are subjected to a wide range of static and dynamic loading combined with high temperature. Different types of damage of rotating shafts in most cases are the result of this long-term combined loading. To avoid catastrophic failure of structural elements during operation of turbines, it is urgent to use reliable damage detection methods. A lot of investigations demonstrated that most appropriate as applied to the turbine shafts in operation are the vibration-based methods, in particular, the change of natural frequencies, the appearance of non-linear resonances, the change of vibrational spectrum and others. The paper contains the comparative analysis of most effective approaches to the vibration diagnostics of damage in rotating shafts of steam turbines.

Deterministic coherence resonance analysis of coupled chaotic oscillators: fractional approach

Recently, the stabilization of chaos by chaos has attracted the attention of researchers. A small deviation between the natural frequencies of unidirectionally coupled chaotic oscillators can cause the emerge of a coherence resonance in the slave oscillator for a given coupling strength. In this work, we investigate the phenomenon of coherence resonance for a coupled Rössler system under the influence of fractional operators in the frequency response of the slave oscillator. Based on the analysis of the fractional Rössler system, we determine a range of fractional orders in which the system retains its instability, and an analysis of the frequency response of the system to changes in natural frequency is developed. Finally, the frequency response of the slave system to changes in the fractional derivative order, the natural frequency of the master system, and the coupling strength is analyzed, and how these changes promote the occurrence of coherence resonance is examined.

Method for Adjusting Torsional Natural Frequencies of Powertrains with Novel Coupling Design

Torsional vibrations are inherently present in every rotating powertrain. In resonant conditions, torsional vibrations can be significantly amplified. A typical method to reduce the torsional vibration particularly at resonance is to modify the torsional natural frequencies with the component design. Commonly, a straightforward method for the modification is the adjustment of the torsional stiffness of a coupling. This study presents a method to modify the torsional natural frequencies using a coupling design with continuously adjustable torsional stiffness. The presented coupling design is investigated with torsional analysis and experimental measurements. Torsional analysis was utilized to predict the effects of varying the coupling stiffness to the torsional natural frequencies of a powertrain. The experimental measurements were conducted by attaching the adjustable stiffness coupling to the powertrain and measuring the change in the torsional natural frequencies while the torsional stiffness of the coupling was adjusted. The torsional natural frequencies were determined from the measurements by identifying the resonance induced torsional vibrations from the vibration response of the powertrain. The torsional vibrations were excited to the system by a load motor. The measurements showed that the first torsional natural frequency of the powertrain changed from 15.6 Hz to 43.0 Hz as the torsional stiffness of the coupling was adjusted. The results of the torsional analysis and the experimental measurements were compared to determine the performance of the realized coupling. The results indicated that the torsional natural frequencies determined by torsional analysis agree well with the experimentally measured results. The prediction errors were generally below ±5%, which is typically considered as a margin for accurate torsional analysis.