Annie Ross

Lightweight damping of composite sandwich beams: Experimental analysis

The purpose of this article is to experimentally study the damping of composite sandwich beams with lightweight honeycomb core. The top and bottom facesheets are made of carbon/epoxy layers with partial interleaved viscoelastic layers. A new damping approach consisting of selectively targeting the inflection points of the bending mode shapes is proposed. At the nodes, the shearing deformation in the beam is maximal, and so is the strain in the viscoelastic layers. The experimental investigation of damping is made by means of standard impact tests using an instrumented hammer performed on beam specimens. The nodes are determined experimentally by moving a small accelerometer along the beam axis and by measuring the amplitude of the acceleration at each point. This novel damping approach keeps the damping ratio as high as the ratio obtained with standard (full coverage) surface damping treatment while reducing the added mass by almost 50%. A comparison of the results obtained in this study with experimental and numerical results found in the literature leads to the conclusion that the most efficient way of damping this type of sandwich structure is to modify and/or improve the viscoelastic properties of the core.

Forward projection of transient sound pressure fields radiated by impacted plates using numerical Laplace transform.

Forward propagation of the transient sound pressure radiated by an impacted plate is presented. It is shown that direct and inverse time domain discrete Fourier transforms, involved in Fourier transform based near-field acoustical holography (NAH), lead to aliasing errors in the reconstructed time signals. Adding trailing zeros to the initial time signals is an inefficient way to reduce time aliasing errors. Hence, the numerical Laplace transform is introduced and a Fourier transform based transient NAH (TNAH) approach is formulated. An error measure is introduced to compare both NAH and TNAH with respect to the propagation distance and the location of the observation point in the projection plane. The percentage of error with TNAH is reduced by more than a factor of 10 without adding trailing zeros to the initial signals. Simulation results are validated experimentally using a free Plexiglas plate impacted at its center.

Application of CHMMs to two-phase flow pattern identification

In this paper, the application of continuous hidden Markov models (CHMMs) in identifying two-phase flow patterns is investigated. Air-water two-phase flows were realized in a transparent vertical tube with a 2m length and a 19mm inside diameter. Local void fraction signals were collected using a single-step index multimode optical fiber probe located at the center and mid-length of the tube. Walsh-Hadamard transform, an autoregressive model and an innovative method based on the passage length of the phases were used to extract signal features required in the CHMM implementation. CHMMs were trained for nine reference flow conditions, and were used to identify the flow patterns of 60 different flow conditions. Two different approaches were compared to treat log-likelihood results: maximum total likelihood and maximum likelihood. The results from the passage length-based method, in combination with the maximum total likelihood approach, were in relatively good agreement with a theoretical flow pattern map and photographs of the flow captured during the experiments. In sum, the results showed that a CHMM has a good potential in identifying two-phase flow patterns.

Dynamic Interaction Between a Straight Tube and an Anti-Vibration Bar

Fretting-wear is a known problem in steam generator U-tubes. These tubes are supported by flat bars called anti-vibration bars (AVB) in the plane of the U-bend. Clearances between the tubes and the bars are designed to be minimal, but cumulative tolerances and manufacturing variations may lead to clearances larger than expected. Large clearances may result in ineffective support leading to in-plane and out-of-plane motion causing fretting-wear and impact abrasion. In the present work, the problem is investigated with a single two span tube, an anti-vibration bar at mid-span and a local excitation force. The dynamic behavior of a tube with simple supports at both ends and an anti-vibration bar at mid-span is characterized. The influence of clearance, preload and tilt of the support on the dynamics of the tube are investigated experimentally. The results indicate that the fretting-wear work-rate is very low with preloads, reaches a maximum around a zero clearance and diminish again for larger clearances. The tilt of the anti-vibration bar in our experiments seems to change the dynamic behavior of the tube.Copyright

Two-Phase Damping in Vertical Pipe Flows: Effect of Void Fraction, Flow Rate and External Excitation

Predicting vibration effects in steam generators requires good knowledge of two-phase damping ζ2φ. The purpose of this work is to correlate two-phase damping in axial flow with tube transversal excitation frequency and magnitude. The test section consists of a stiff square tube subjected to internal axial flow of air-water mixture. The hydraulic diameter is 3 inches. The tube is supported with linear bearings and fitted with flexible tubing on both ends to allow motion in the transverse direction. Compression springs allow setting the natural frequency of the tube oscillation. A motor provides transverse sinusoidal excitation to the tube assembly. ζ2φ is determined from the frequency response function. As a result of this study, ζ2φ is represented as a function of excitation frequency and amplitude, void fraction and flow rate. Specific information is gained through high frame rate videos of the oscillating tube, including bubble transverse velocity and size for low void fraction, and flow pattern transitions. Indeed, it is suspected that two-phase damping is partly caused by the work rate of virtual mass forces of the gas phase. Better knowledge of the physical process involved in two-phase damping will allow better modeling and prediction of tube behavior.© 2014 ASME

Influence of Viscosity, Density and Surface Tension on Two-Phase Damping

Internal two-phase flow is common in piping systems. Such flow may induce vibration that can lead to premature fatigue or wear of pipes. In the nuclear industry in particular, failure of piping components is critical and must be avoided. Two-phase damping is considered part of the solution, since it constitutes a dominant component of the total damping in piping with internal flow. However, the energy dissipation mechanisms in two-phase flow are yet to be fully understood. The purpose of this paper is to explore the relationships between two-phase damping and fluid properties. Simple experiments were carried out in a clear vertical clamped-clamped tube to verify the effects of fluid properties on two-phase damping. Various fluids, such as air, alcohol, pure water, sugared water, glycerol, and perfluorocarbon, were combined to obtain different controlled mixtures and to determine the effect of surface tension, density and viscosity on two-phase damping. Two-phase damping ratios were obtained from free transverse vibration measurements on the tube. Two sets of experiments with stagnant and moving continuous phase were conducted. Based on dimensional analysis, we obtained a semi-empirical model for two-phase damping in bubbly and slug flow. The Void fraction and Bond number are shown to be major parameters of two-phase damping, which is described as a kinetic energy transfer from the tube to the continuous phase through added mass of the dispersed phase.Copyright

Method for finding optimal exponential decay coefficient in numerical Laplace transform for application to linear convolution

In this paper, a method based on the numerical Laplace transform is used for calculating the full linear convolution of real or complex signals. An algorithm for obtaining the last N values of the convolution is presented, along with a method for finding an optimal value for the decay coefficient of the transform. It is shown that the use of the numerical Laplace transform formulation allows the calculation of each half of the linear convolution independently, which has computational benefits. The numerical Laplace transform is expressed as the fast Fourier transform of signals that have been premultiplied by a decreasing exponential window characterized by decay coefficient c. The error of the resulting linear convolution depends on the value of the decay coefficient; undervalue results in the generation of wrap-around error whereas overvalue causes amplification of Gibbs phenomenon. In this paper, a formula that optimizes the value of the decay coefficient is developed. A trade-off value for c is obtained and error analysis shows that it outperforms other coefficients proposed in the literature when applied to the calculation of linear convolution. The relative errors obtained are of the order of 10-6% and 10-9% for single and double precisions. The NLT is used for the calculation of the full linear convolution.The computational advantages are investigated for such calculation.A formula for optimizing the decay coefficient of the NLT is proposed.The coefficient is a trade-off value between wrap-around and Gibbs errors.Relative errors with the proposed coefficients are in the order of 10e-6 to 10e-9.

Dynamic characterization of viscoelastic materials used in composite structures

The main focus of this paper is the experimental comparative analysis of the viscoelastic properties of acrylic- and silicon-based viscoelastic materials. These materials are widely used in the aeronautic industry for structural and/or damping applications. It is therefore required to determine their viscoelastic properties such as shear modulus and loss factor following their integration to aircraft composite structures. The influence of material thickness, bonding quality, curing and cocuring with composite material was evaluated using dynamic mechanical analysis machine in plan shear configuration. This comparative analysis in the 0–600 Hz frequency range provides useful information that should be taken into consideration when designing bonded joints for structural and/or damping applications. The small variation of the acrylic-based material loss factor over a wide range of frequencies suggests that this material is a good candidate for damping applications at room temperature in metallic structures where no curing is required. However, cocuring of acrylic-based material with composite laminae increases its shear modulus up to six times with respect to the uncured material whereas its loss factor is reduced by about 20%. On the other hand, the silicon-based material remains stable after thermal treatment (curing and cocuring) suggesting that it is well suited for in situ damping treatment involving laminate composite structure. However, at room temperature its shear modulus after cocuring is lowered by almost 25% due to poor bonding quality, which depends on the nature of the substrate.

Repeatability of force signals in aerial circus straps

The aim of this study is to develop a method for assessing movement variability of circus acrobats. An analysis of the repeatability of force signals is used to quantify variability. Six students from the National Circus School of Montréal performed 5–10 trials of an acrobatic movement called dislock in aerial circus straps while tension force was measured at the hanging point of the aerial apparatus. The repeatability of force signals was calculated with three statistical methods: time-averaged standard deviation, intraclass correlation and root mean square error. These methods were compared with the ratings of a circus coach who ranked each acrobat’s trial with regard to the movement variability. The standard deviation and the intraclass correlation methods are commonly used to quantify the agreement between measurements in biomechanics, while the root mean square error method is regularly employed to quantify the agreement between measurements and a model. All participants performed the movement with little variability (intraclass correlation ⩾ 0.8). The results of the three methods were in good agreement with the coach’s assessment. The root mean square error method, in particular, showed perfect agreement and is therefore considered the best measure of repeatability. In the future, the proposed method could be used by coaches or artists training alone, allowing a new form of feedback.

Sparse regularization for reconstructing transient sources with time domain nearfield acoustical holography

In this paper, the ℓ1-norm sparse regularization method is applied to the time domain reconstruction of transient acoustic fields such as impulse noise. This method properly reconstructs the back-propagated sound field where its amplitude should be null: for transient sources, this occurs mostly for positions and times that precede the arrival of the first wave front. Therefore, it significantly reduces causal errors typically found in time domain reconstruction when standard Tikhonov regularizations is applied. The reconstructions obtained from both Tikhonov and sparse regularization methods are compared using a transient baffled piston model, and show that the global root-mean-square (RMS) error is significantly reduced when using sparse regularization. The improvement provided depends on the level of sparsity of the reconstructed signal. For the studied cases, it can represent a reduction of the global RMS error by up to a factor of 3. The performance of Pareto frontier curve for predicting the optimal sparse regularization parameter is examined; it leads to accurate predictions especially for lower noise levels. Finally, sparse regularization is applied to experimental data over time and spatial domains in order to obtain an accurate reconstruction of the transient sound field produced by an impacted plate.