Wide Range Of Excitation Frequencies Research Articles

On vibration transmission in oscillating systems incorporating bilinear stiffness and damping elements

This paper investigates the dynamic characteristics and vibration transmission behaviour of a single oscillator and also coupled oscillators incorporating both bilinear stiffness and bilinear damping elements. Both harmonic balance analytical approximations and numerical integrations are used to obtain the steady-state dynamic response and vibration transmission behaviour. Vibration transmission within the oscillating systems is evaluated by time-averaged power flow variables and force transmissibility. For the single oscillator with bilinear elements, it is shown that a small bilinear stiffness ratio is beneficial in the suppression of force transmission in the high-frequency range. A large bilinear damping ratio can reduce the peak force transmissibility and also the peak time-averaged input power. For the coupled oscillators with interfacial bilinear elements, it is also found that a small bilinear stiffness ratio can reduce both the force transmissibility and the time-averaged transmitted power at high excitation frequencies. A combination of a small bilinear stiffness ratio and a large bilinear damping ratio can lead to effective mitigation of vibration transmission in a wide range of excitation frequencies. It is also shown that the inclusion of bilinear elements may cause large superharmonic or subharmonic response components, which result in high vibration transmission level. These results provide a better understanding of the effects of bilinear stiffness and bilinear damping on vibration transmission, and benefit future designs of vibration suppression systems with such elements.

Experimental study of stall control over an airfoil with dual excitation of separated shear layers

This paper investigates the stall control over an airfoil employing dual excitation of separated shear layers with dielectric barrier discharge plasma actuators. This novel approach is studied experimentally on the NACA 0015 airfoil at the angle of attack and Reynolds number of 14° and 0.3×106, respectively. To analyze the baseline flow, Time-averaged pressure distributions on the airfoil surfaces are evaluated with 40 pressure taps. Further, the dominant natural frequencies associated with flow structures in the airfoil wake are examined utilizing a hot-wire anemometer. Two plasma actuators, one on the suction side just upstream the separation point and the other on the pressure side at the trailing edge, are exploited to control the baseline flow. Based on the active actuators, three controlled cases are considered in the present study. In the first case, the frequency of natural vortex shedding and its different harmonics are employed to excite the suction side separated shear layer. In the second case, plasma actuation is applied to the shear layer at the trailing edge. Further, in the third case, both of the actuators are utilized to excite the separated shear layers on the suction side and the trailing edge, simultaneously. The results of the pressure coefficient distributions, as well as the aerodynamic coefficients, are compared for all cases. Moreover, the actuators energy consumption is evaluated for the controlled cases and a new efficiency parameter is presented. Accordingly, the first case prevents the stall phenomenon only at the wake mode frequency and its first super-harmonic. In addition, using the plasma actuator at the trailing edge solely is an ineffective flow separation control strategy. However, dual excitation is promising for a full stall control in a wide range of excitation frequencies. Analyses of the lift-to-drag ratio and the efficiency parameter reveal that the dual excitation case is the most efficient stall control technique.

Modeling dynamic behavior of MDOF systems with multiple bilinear springs

A computational scheme is presented in this paper to simulate dynamical behavior of multiple degrees of freedom (MDOF) systems with multiple bilinear springs. In the proposed scheme, a bilinear spring is modeled using by two parallel springs - a primary spring and a secondary spring. The primary spring is an ordinary linear spring having identical stiffness in tension and compression, and is active for tension and compression. The secondary spring, whose stiffness characterizes the bilinearity, is active only during compression. It is employed in connection with the Newmark integration method and the linear complementarity problem (LCP) formulation to obtain time-domain responses of dynamical systems with bilinear springs due to initial disturbances and harmonic excitations. The scheme described in this paper is effective in dealing with the sudden transition from tension to compression and vice versa simultaneously for all bilinear springs. Numerical results for bilinear oscillators with finite bilinearity ratios and impact oscillators with an infinite bilinearity ratio show that the proposed bilinear spring model is accurate, generic and valid for bilinearity ratios ranging from zero to infinity. Orderly and chaotic behavior of viscously damped 3-DOF system under harmonic excitation is studied for a wide range of excitation frequencies and bilinear ratios to demonstrate the effectiveness and applicability of the proposed model for MDOF bilinear systems.

A numerical study of tuned liquid damper based on incompressible SPH method combined with TMD analogy

Tuned liquid dampers (TLDs) are economical dynamic vibration absorbers which are increasingly being used for structural response control. Nevertheless, a thorough understanding of the sloshing behavior inside the TLD is essential for efficient design. In this paper, a truly incompressible smoothed particle hydrodynamics (ISPH) method is employed to circumvent the difficulties regarding the simulation of fully nonlinear sloshing motion induced by large amplitude excitations. The behavior of the TLD with damping screens is studied numerically over a wide range of excitation frequencies and amplitudes. It is found through comparison with the existing experimental data that the ISPH model is a powerful tool for accurate predictions of the TLD behavior. The amplitude-dependent properties of an equivalent tuned mass damper (TMD) model, which is capable of reproducing the energy dissipation characteristics of the TLD, are derived from the simulations results. Good agreement between the numerical and experimental results is indicative of the equivalent model’s ability to predict the response of structure–TLD systems, suggesting that the ISPH model can properly simulate the TLD behavior. The effect of screen placement on the performance of structure–TLD systems is also investigated. Findings reveal that improved performance of TLD can be realized under high amplitude excitations by placing the damping screens in the locations corresponding to the higher sloshing modes.

Noise analysis and improvement of a micro-electro-mechanical-systems fluxgate sensor

An integrated micro solenoid fluxgate sensor with a Co-based amorphous core is presented in this work based on the Micro-Electro-Mechanical System (MEMS) technologies. As the advantage of miniaturization, the micro fluxgate sensor is found to be operated under wide excitation frequency range of 100 kHz–1 MHz. Ultra-low frequency noise (<0.1 Hz) always arises in zero field time-domain signal acquisition under full excitation frequency range. An additional broadband transformer is placed before the excitation end of the sensor to eliminate the ultra-low frequency noise. The noise power density of the sensor working at 600 kHz is analyzed. The result shows the noise is 1.15 nT/Hz1/2@1 Hz without the transformer and 0.11 nT/Hz1/2@1 Hz with the transformer, while the noise rms level is 5.38 nT and 0.23 nT in the frequency range of 50 mHz–10 Hz respectively. For the MEMS micro fluxgate sensor, more stringent electromagnetic compatibility design for the sensor signal system is needed to eliminate additional noise derived from the electromagnetic radiated interference and the voltage disturbance.

Experimental Evaluation and Modeling of the Damping Properties of Multi-Layer Coated Composites

In this work, the dissipative properties of different coating solutions are compared and a beam mechanical model, taking into account of dissipative actions at the interface between different layers is proposed. The aim is to find optimal coatings to be employed in the production of composites with high damping properties. The investigated coating layers are obtained from different materials and production processes, and are applied on different metallic substrates. The composite specimens, in the form of slender beams, are tested by means of forced excitation dynamic measurements. Force and displacement experimental data, in a wide range of excitation frequencies, are used to estimate the system damping properties. Homogeneous, uncoated specimens are also tested for comparison. A specific identification procedure is used to identify the specimens stress-strain relationship in the frequency domain. The ratio of the imaginary part and the modulus of the specimen estimated complex frequency response function is considered as a measurement of the damping behaviour. A modified third order multi layered beam model, based on the zig-zag beam theory, is proposed. The model takes into account the contribution to the damping behaviour of the frictional actions and slipping at the interface between layers. Frictional actions are modelled by means of a complex, elasto-hysteretic contribution.

Analytical Expressions of Hydro-Seismic Forces on Dams

Analytical expressions for the determination of hydro-seismic forces acting on a rigid dam with irregular upstream face geometry in presence of a compressible viscous fluid are derived through a linear combination of the natural modes of water in the reservoir based on a boundary method making use of complete sets of complex T-functions.Analytical expressions for the determination of hydro-seismic forces acting on a rigid dam with irregular upstream face geometry in presence of a compressible viscous fluid are derived through a linear combination of the natural modes of water in the reservoir based on a boundary method making use of complete sets of complex T-functions. The formulas obtained for distributions of both shear forces and overturning moments are simple, computationally effective and useful for the preliminary design of dams. They show clearly the separate and combined effects of compressibility and viscosity of water. They also have the advantage of being able to cover a wide range of excitation frequencies even beyond the cut-off frequencies of the natural modes of the reservoir. Key results obtained using the proposed analytical expressions of the hydrodynamic forces are validated using numerical and experimental solutions published for some particular cases available in the specialized literature.

Flow separation control using unsteady pulsed suction through endwall bleeding holes in a highly loaded compressor cascade

Unsteady pulsed suction (UPS) as a novel unsteady flow control (UFC) technique is applied in a certain highly loaded compressor cascade for the control of flow separations. Some related aerodynamic parameters such as excitation frequency and time-averaged suction flow rate are studied in detail. UPS and steady constant suction (SCS) are investigated to analysis comparatively the control effects of flow separations with the same time-averaged suction flow rate. The results show that UPS can provide appreciable improvement of cascade performance in a wide range of excitation frequencies. There are different optimum frequencies under different time-averaged suction flow rates, but the optimum frequencies are both an integer multiple of the natural frequency of vortex shedding. Based on the optimum frequency, the total pressure loss coefficients under the time-averaged suction flow rates ms=0.4% and ms=0.53% are reduced by 9.4% and 14.2%, respectively. The time-averaged suction flow rate plays a more crucial role than the excitation frequency. Simultaneously, UPS can provide a better flow control effect than SCS with the same time-averaged suction flow rate in the control of flow separation. The time-averaged suction flow rate range that can obtain positive effects for UPS is wider than that for SCS. Additionally, the potentials of UPS at off-design points are also validated.

Dynamics of liquid sheet breakup in the presence of acoustic excitation

We present the results of experimental investigations on the effect of acoustic excitation on the breakup of the liquid sheet formed by oblique impingement of two liquid jets for varying Weber number over a wide range of excitation frequencies (100–2000 Hz) and sound pressure levels (50–120 dB). Visualization studies reveal violent sheet flapping, reduction in the sheet breakup length and width, impact wave amplification and rise in droplet density shed from the sheet. Overall, increase in excitation frequency increases the threshold sound pressure level at which the sheet responds. Reduction in the sheet breakup length and width with frequency is observed at some selected frequencies, which rules out the possibility of resonance. For smooth sheet regime, visual observations show that the dominant sheet breakup mechanism shifts from Rayleigh–Plateau (R–P) to Kelvin–Helmholtz (K–H) instability. It is shown using temporal analysis that the reduction in the sheet breakup length may be attributed to periodic sheet collapse and reformation. Drop shedding frequencies in flapping liquid sheet are seen to synchronize with those excitation frequencies which are lower than or nearly equal to the fundamental frequency without excitation. Acoustic excitation reduces the mean droplet size significantly without altering the drop size distribution pattern that follows a universal Gamma distribution.

Real-time hybrid simulation of structures equipped with viscoelastic-plastic dampers using a user-programmable computational platform

A user-programmable computational/control platform was developed at the University of Toronto that offers real-time hybrid simulation (RTHS) capabilities. The platform was verified previously using several linear physical substructures. The study presented in this paper is focused on further validating the RTHS platform using a nonlinear viscoelastic-plastic damper that has displacement, frequency and temperature-dependent properties. The validation study includes damper component characterization tests, as well as RTHS of a series of single-degree-of-freedom (SDOF) systems equipped with viscoelastic-plastic dampers that represent different structural designs. From the component characterization tests, it was found that for a wide range of excitation frequencies and friction slip loads, the tracking errors are comparable to the errors in RTHS of linear spring systems. The hybrid SDOF results are compared to an independently validated thermalmechanical viscoelastic model to further validate the ability for the platform to test nonlinear systems. After the validation, as an application study, nonlinear SDOF hybrid tests were used to develop performance spectra to predict the response of structures equipped with damping systems that are more challenging to model analytically. The use of the experimental performance spectra is illustrated by comparing the predicted response to the hybrid test response of 2DOF systems equipped with viscoelastic-plastic dampers.

Thermoplastic Dielectric Elastomer of Triblock Copolymer with High Electromechanical Performance.

Dielectric elastomer (DE) actuators have been shown to have promising applications as soft electromechanical transducers in many emerging technologies. The DE actuators, which are capable of large actuation strain over a wide range of excitation frequencies, are highly desirable. Here, the first single-component DE of a triblock copolymer with attractive electromechanical performance is reported. Symmetric poly(styrene-b-butyl acrylate-b-styrene) (SBAS) is designed and synthesized. The SBAS actuator exhibits about 100% static actuation area strain and excellent dynamic performance, as evidenced by a wide half bandwidth of 300 Hz and a very high specific power of 1.2 W g-1 within the excitation frequency range of 300-800 Hz.

A novel approach for the design and analysis of nonlinear dampers for automotive suspensions

This paper proposes an analytical technique for frequency analysis and the design of nonlinear dampers to further improve ride dynamics performance of vehicle suspensions over a wide range of excitation frequencies. Using the energy balance method (EBM), the proposed methodology estimates the equivalent linear damping coefficient of any nonlinear passive damper whose force is a general function of the damper’s relative displacement and relative velocity. Knowing the equivalent linear damping coefficient makes it possible to perform a frequency analysis of the suspension ride performance with any nonlinear damper. Some specific criteria are defined to design the desired form of equivalent linear damping coefficient which provides a high/small damping ratio at low-/high-frequency excitations, so the corresponding nonlinear damping force required to obtain improved ride performance of the suspension using a 1-degree-of-freedom quarter car model is also defined. A sensitivity analysis is then performed to provide a design guideline. The results show that the dependency of the equivalent damping coefficient either relative to the velocity of the suspension (velocity-dependent damper) or the relative displacement of the suspension (position-dependent damper) could provide a variable damping ratio leading to better vibration isolation over the excitation frequency. A noticeable ride dynamic performance can be reached over the entire range of the excitation frequency by designing a nonlinear damper such that its equivalent linear damping ratio becomes a desired function of both its relative displacement and relative velocity (position-velocity-dependent damper).

Aerodynamic response of internal passages to pulsating inlet supersonic conditions

Abstract Supersonic flow passages benefit from high specific mass-flow rate, which can potentially be used in fluid machinery with the highest power density. Currently, the maximization of the propulsive efficiency, on forthcoming power plant systems, requires reduced engine cores, while for distributed local-power plant units, small gas turbine engines are ideal. Hence, ultra-compact fluid machinery is an enabling technology for future propulsion and power generation applications. In this context, the use of supersonic flow within internal passages allows the downsizing of the turbomachinery. However, the efficient operation of supersonic internal passages is a challenge due to shock losses and a constrained operability. Therefore, fully supersonic internal passages are typically discarded, and their unsteady operation scarcely documented. This paper presents a detailed characterization of the unsteadiness experienced across internal flow passages exposed to pulsating supersonic conditions. The analysis was performed in a wide range of excitation frequencies. To generalize the results the fluctuations are expressed in terms of a non-dimensional reduced frequency, i.e. ratio of excitation period to the propagation wave time. This aerodynamic characterization was performed with three-dimensional unsteady Reynolds-Averaged Navier Stokes simulations, revealing a hysteresis like behavior and amplitude modulation between the inlet and the outlet distinct from the steady prediction. The paper provides a numerical approach, based on a one-dimensional Euler solver that can predict the unsteady response of internal passages to the inlet varying conditions.

Spin-wave propagation in ultra-thin YIG based waveguides

Spin-wave propagation in microfabricated 20 nm thick, 2.5 μm wide Yttrium Iron Garnet (YIG) waveguides is studied using propagating spin-wave spectroscopy (PSWS) and phase resolved micro-focused Brillouin Light Scattering (μ-BLS) spectroscopy. We demonstrate that spin-wave propagation in 50 parallel waveguides is robust against microfabrication induced imperfections and extract spin-wave propagation parameters for the Damon-Eshbach configuration in a wide range of excitation frequencies. As expected from its low damping, YIG allows for the propagation of spin waves over long distances; the attenuation lengths is 25 μm at μ0H = 45 mT. Moreover, direct mapping of spin waves by μ-BLS allows us to reconstruct the spin-wave dispersion relation and to confirm the multi-mode propagation in the waveguides, glimpsed by propagating spin-wave spectroscopy.

Influence of the excitation frequency on the density of helium metastable atoms in an atmospheric pressure dielectric barrier discharge

Diffuse dielectric barrier discharges in atmospheric-pressure helium can be sustained over a wide range of excitation frequencies (from, but not restricted, 25 kHz to 15 MHz). The aim of the present paper is to identify the specific characteristics of the discharge modes that can be sustained in this frequency range, namely, the atmospheric-pressure Townsend-like discharge (APTD-L) mode, the atmospheric-pressure glow discharge (APGD) mode, the Ω mode, the hybrid mode, and the RF-α mode. This is achieved experimentally, by measuring the density of helium metastable atoms, which are known to play a driving role on the discharge kinetics. This density is measured by means of two absorption spectroscopy methods, one using a spectral lamp and the other one using a diode laser as a light source. The first one provides the time-averaged atom densities in the singlet He(21S) and triplet He(23S) metastable states, while with the second one we access the time-resolved density of He(23S) atoms. Time-averaged measurements indicate that the He(23S) density is relatively low in the APTD-L, the Ω and the RF-α modes (&amp;lt;4×1016 m−3) slightly higher in the APGD mode (2–7×1016 m−3), and still higher (&amp;gt;1×1017 m−3) in the hybrid mode. The hybrid mode is exclusively observed for frequencies from 0.2 to 3 MHz. However, time-resolved density measurement shows that at 1 MHz and below, the hybrid mode is not continuously sustained. Instead, the discharge oscillates between the Ω and the hybrid mode with a switching frequency about the kilohertz. This explains the significantly lower power required to sustain the plasma as compared to above 1 MHz.

Vibration transmission through an impacting mass-in-mass unit, an analytical investigation

Impacting events are discontinuous and non-smooth in nature; thus, resulting in various forms of complex nonlinear dynamics. A numerical algorithm has been developed based on the analytical solution to distinguish different classes of impacting responses. The main advantages of the solver are that it can either stop the integration process after automatically identifying the steady state solution or continue until the maximum time is reached in case of the chaotic type response. To identify the frequency of higher periodic response, a periodicity coefficient has been defined as the frequency ratio of excitation and the system's response. The effect of coefficient of restitution on the different dynamic responses is also discussed within the scope of the paper. The amplitude of the vibration of the main mass is reduced due to the presence of the multi-periodic and chaotic impacting responses for a wide range of excitation frequencies. These characteristics make impact dampers ideal for applications in wideband vibration insulation and a unit of wideband nonlinear metamaterial.

Important role of magnetization precession angle measurement in inverse spin Hall effect induced by spin pumping

Here, we investigate the spin Hall angle of Pt in a Ni80Fe20/Pt bilayer system by using a broadband spin pumping and inverse spin Hall effect measurement. An out-of-plane excitation geometry with an application of external magnetic field perpendicular to the charge current direction is utilized in order to suppress unwanted galvanomagnetic effects. Magnetization precession angle (θC) on ferromagnetic resonance for wide excitation frequency range (4–14 GHz) is estimated from the rectification voltage of anisotropic magnetoresistance (AMR) and a conventional method of using the microwave power in a coplanar waveguide. A marked difference in θC profiles for the different methods is observed, resulting in the large variation in estimated values of spin current density at Ni80Fe20/Pt interface. The frequency dependence of the spin current density estimated using the AMR effect is found to be similar to that of the inverse spin Hall voltage. We obtain the frequency-invariant spin Hall angle of 0.067 ± 0.002.

Robust identification of the mechanical properties of viscoelastic non standard materials by means of frequency domain experimental measurements

This work presents an identification procedure for the constitutive model of viscoelastic non-standard materials, such as Functionally Graded Materials (FGM). A generalized Kelvin model of arbitrary order is selected, making it possible to define the material constitutive relationship by means of the ratio of polynomials in the frequency domain. Since ill-conditioning may occur at the numerical identification stage of high order model parameters, a novel approach to identify the model optimal order and parameters, mainly employing a orthogonal polynomial basis, is proposed in this paper. Least square error fitting techniques employing a classic monomial and a Forsythe orthogonal polynomial basis are compared by starting from numerically estimated measurements with noise. The selected approach is used to fit dynamical measurement data obtained from real test specimens in a wide excitation frequency range. The optimal model results obtained by fitting real experimental data are presented and discussed.

Ground-borne vibration control in a building-like structure using multi-positive position feedback combined with sliding mode control

This work deals with the structural and dynamic analysis of a building-like structure consisting of a three-story building with one type of actuators, that is, a passive/active electromechanical actuator. The base of the structure is perturbed using an electromagnetic shaker, which provides forces with a wide range of excitation frequencies, including noisy excitations emulating traffic, underground railways and earthquakes, common in Mexico City. The overall mechanical structure is modeled using Euler-Lagrange method and validated using experimental modal analysis techniques. The active controls laws are synthesized to actively attenuate the noise and vibration system response caused by noise excitation forces acting on the base of the structure. The control scheme is obtained using multi-positive position feedback (MPPF) combined with sliding mode (SM) to improve the closed-loop system response, which is able to simultaneously attenuate the three vibration modes of the primary system. Some simulation and experimental results are included to illustrate the overall system performance.

An energy flow study of a double-deck tunnel under quasi-static and harmonic excitations

This paper presents a comparison between the vibration energy flow radiated by a double-deck tunnel and the one radiated by a simple tunnel when both are excited by constant or by harmonic moving loads. For both cases, the radiated energy is computed using a three-dimensional semi-analytical model of the system. The total energy radiated upwards is presented for a wide range of load speeds, when a constant moving load is considered, and for a wide range of excitation frequencies, when the excitation is a harmonic moving load. Significant differences have been obtained, first, for constant loads moving at very high speeds and, second, for harmonic loads moving at typical speeds for underground trains.