1,721,044 research outputs found
Generalized Liquid-Based Damping Device for Passive Vibration Control
Get PDFThis paper presents a liquid-based device with fluid-induced damping, stiffness and inertia effects. The proposed concept is modelled and experimentally studied. A lumped parameter fluid dynamics approach is used to model the flow-induced energy dissipation, inertia and volumetric compressibility. It is shown that the developed 5-state nonlinear dynamic model can be modified to represent a range of previously established models. A reference set of model parameters is determined from the calibration data obtained from a novel device demonstrator. The model’s functional and parametric changes are shown to enable the device specializations which can approximate simpler as well as ideal devices such as dampers, springs and inerters. This work also demonstrates that the interaction between all three principal fluid effects opens routes to dynamic device tuning and frequency-selective damping
Vibration Control in a Helicopter with Semi-Active Hydraulic Lag Dampers
No full textThis paper describes the research on the use of semi-active hydraulic lag dampers for vibration control in helicopters. A semi-active vibration control of the hydraulic lag damper is investigated in the context of a midsized production helicopter in steady level flight. The semi-active operation of the damper is achieved via periodic flow restrictor modulation. The influence of the modulations and constraints on the nonrotating hub loads of a five-bladed main rotor is evaluated. The selected harmonic modulation frequencies are determined by the combination of the hydromechanical coupling in the damper and the mechanical filtering effect of the symmetric rotor. The modulation frequencies of 3, 5, and 7 per revolution are used to modify the 5-per-revolution lateral and longitudinal hub loads, whereas the frequencies of 4 and 6 per revolution are efficient at modifying the 5-per-revolution hub yaw moment. The harmonic modulations larger than seven per revolution are not used due to limited damper responses at these frequencies. The out-of-plane hub loads and moments cannot be significantly influenced by the rotor-damper configuration considered. A maximum load reduction of 72% on the lateral hub load component decreases to 58% when considering a limited damper response authority due to the relief valve activation
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Vibration control of a rotating Timoshenko beam-tendon system via internal guiding inerter-dampers
Get PDFA guided active tendon concept is proposed to modify the dynamics of the main rotor blades to enable the rotorcraft to operate in the wider range of working conditions. Because some blade modes are not sensitive to the applied axial force, in this paper, a new vibration suppression method for a rotating Timoshenko beam axially loaded by a tendon with the inerter-damper guiding elements is proposed and studied. The equations of motion are derived by means of the Lagrange’s equation with the use of the energy expressions, and the modal and harmonic analyses are carried out. The analytical model is verified and compared with an independently formulated finite element model. It is found that inerter-dampers introduce non-local modes. To tune the chosen beam-dominated modes using inerter-dampers without introducing the additional resonances, the new optimal tuning strategy is proposed. When one inerter-damper is used, the inertance and damping coefficient for the optimal tuning of the higher modes are lower than those for the lower modes. When the inerter-damper is used to tune a single mode optimally, its function is analogous to a viscous damper for the lower modes, but it is similar to a rigid attachment for the higher modes. The maximum attainable damping ratio tends to decrease with the reduction of the applied axial force and increase of the rotational speed. The optimal performance of the single inerter-damper at different locations varies. For example, it is found that the damping ratio of the second beam-dominated mode reaches the maximum value with the inerter-damper located between 0.5-0.6L from the fixed end, where the required inertance and damping coefficient for the optimal tuning reach the minimum. More inerter-dampers can be used to tune a larger number of the beam-dominated modes optimally and simultaneously. The inerter-dampers are shown to effectively suppress the beam-dominated modes in wide axial force and speed ranges. In addition, inerter-dampers can be used to support the same function as the rigid tendon-guiding attachments of increasing the frequencies of the tendon-dominated modes. However, they cannot alter the critical compressive force. By integrating inerter-dampers into the tendon-loaded blades internally, the modes insensitive to the applied axial force can be suppressed to mitigate resonant vibrations
Theoretical and experimental modal analysis of a beam-tendon system
Get PDFTheoretical and experimental modal analysis of the system consisting of the Euler-Bernoulli beam axially loaded by a tendon is studied in the paper. The beam-tendon system is modelled using a set of partial differential equations derived by Hamilton’s principle and the coupling between the beam and the tendon is ensured by the boundary conditions. Theoretical modal analysis is conducted using a boundary value problem solver and the results are thoroughly experimentally validated using a bench-top experiment. In particular, the effect of the tendon tension on the modal properties of the system is studied. It is found that by increasing the tension, the natural frequencies of the beam decrease while the natural frequencies of the tendon increase. It is also shown that these two sets of modes interact with each other through frequency loci veering. The effect of the tendon mass is also experimentally and numerically studied and it is shown that lighter tendon produces fewer vibration modes in the studied frequency region. Two further numerical studies are conducted to demonstrate the effect of the tendon on the torsional modes of the beam, and to study the structural stability. Overall, an excellent agreement between the numerical and experimental results is obtained, giving the confidence in the derived theoretical model
Free vibration analysis of a rotating pre-twisted beam subjected to tendon-induced axial loading
Get PDFThe main objective of this paper is to investigate free vibration of a rotating pre-twisted beam with bending-bending-torsion coupling that is axially loaded by a tendon. The tendon is connected to the tip of the beam, passes through its body, and is fixed and loaded at the axis of rotation. Due to their connection, the motion of the beam influences the motion of the tendon and vice versa. The equations of motion of the beam-tendon system are introduced and solved numerically by a combination of a boundary value problem solver and differential quadrature method to obtain the natural frequencies and mode shapes of the system. The numerical implementation is firstly validated against literature for a non-rotating beam-tendon system and a rotating pre-twisted blade. Then, the effect of the tendon on the free vibration of the system is studied for a wide range of loading cases and rotation speeds. From the computed modal characteristics, it is found that the presence of a tendon leads to the frequency shift of beam's natural frequencies as well as frequency loci veering between the beam-dominated and tendon-dominated modes. Both of these effects strongly depend on the beam-tendon rotation speed.</p
Free vibration and stability analysis of a cantilever beam axially loaded by an intermittently attached tendon
Get PDFFree vibration and stability analysis of the system consisting of the Euler–Bernoulli beam axially loaded by a tendon is studied in the paper. The tendon is attached to the cantilever beam at the tip as well as in several spanwise locations using mechanical attachment points. This novel beam-tendon system is modelled using a set of partial differential equations and the coupling between the beam and the tendon is ensured by the boundary and continuity conditions. Computational free vibration analysis is conducted using a boundary value problem solver and the results are thoroughly experimentally validated using a bench-top experiment. In particular, the effect of the number of the attachment points and their location on the frequency-loading diagram of the beam-tendon system is investigated for the first time. It is found that the applied axial load causes a frequency shift of beam-dominated natural frequencies, and frequency loci veering between beam-dominated and tendon-dominated modes. Both of these features are shown to be dependent on a number and location of the attachment points. In addition, the effect of the attachment points on the structural stability of the system is numerically studied and it is observed that the critical force of the system with the intermittently attached tendon is always higher than for the system with no attachment points
Theoretical and experimental free vibration analysis of a beam-tendon system with an eccentrically placed tendon
Get PDFTheoretical and experimental free vibration analysis of a beam-tendon system is presented in this paper. The system consists of a thin-walled cantilever beam with an open cross-section and a tip mass which is loaded by an eccentrically placed tendon. The novel beam-tendon system is modelled using a set of partial differential equations and numerical free vibration analysis is conducted using a boundary problem solver. The results are thoroughly validated using a bench-top experiment and a high-fidelity finite element model. The effect of the tendon location on the frequency-loading diagrams is systematically investigated for the first time. The results demonstrate that the location of the tendon significantly influences natural frequency shifts caused by the applied axial load and it is observed that some natural frequencies can even increase with the increasing compressive axial loading for certain locations of the tendon. In addition, veering between the beam-dominated and tendon-dominated modes is studied and the structural stability of the system is discussed. The paper concludes with suggestions of practical applications of a beam-tendon system with an eccentrically placed tendo
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