| 1. |
- Boij, Susann, et al.
(författare)
-
Reflection of sound at area expansions in a flow duct
- 2003
-
Ingår i: Journal of Sound and Vibration. - 0022-460X .- 1095-8568. ; 260:3, s. 477-498
-
Tidskriftsartikel (refereegranskat)abstract
- An analytical model for scattering at area discontinuities and sharp edges in flow ducts and pipes is presented. The application we have in mind is large industrial duct systems, where sound attenuation by reactive and absorptive baffle silencers is of great importance. Such devices commonly have a rectangular cross-section, so the model is chosen as two-dimensional. Earlier solutions to this problem are reviewed in the paper. The modelling of the flow conditions downstream of the area expansion, with and without extended edges, and its implications for the resulting acoustic modes are discussed. Here, the scattering problem is solved with the Wiener–Hopf technique, and a Kutta condition is applied at the edge. The solution of the wave equation downstream of the expansion includes hydrodynamic waves, of which one is a growing wave. Theoretical results are compared with experimental data for the reflection coefficient for the plane wave, at frequencies below the cut-on for higher order modes. Influence of the interaction between the sound field and the flow field is discussed. A region where the reflection coefficient is strongly Strouhal number dependent is found.
|
|
| 2. |
- Knudsen, Jacob, et al.
(författare)
-
Dynamic stability of weakly damped oscillators with elastic impacts and wear
- 2003
-
Ingår i: Journal of Sound and Vibration. - : Academic Press. - 0022-460X .- 1095-8568. ; 263:1, s. 175-204
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamics of non-linear oscillators comprising of a single-degree-of-freedom system and beams with elastic two-sided amplitude constraints subject to harmonic loads is analyzed. The beams are clamped at one end, and constrained against unilateral contact sites near the other end. The structures are modelled by a Bernoulli-type beam supported by springs using the finite element method. Rayleigh damping is assumed. Symmetric and elastic double-impact motions, both harmonic and sub-harmonic, are studied by way of a Poincaré mapping that relates the states at subsequent impacts. Stability and bifurcation analyses are performed for these motions, and domains of instability are delineated. Impact work rate, which is the rate of energy dissipation to the impacting surfaces, is evaluated and discussed. In addition, an experiment conducted by Moon and Shaw on the vibration of a cantilevered beam with one-sided amplitude constraining stop is modelled. Bifurcation observed in the experiment could be captured
|
|
| 3. |
- Al-Najjar, Basim, 1954-
(författare)
-
Accuracy, effectiveness and improvement of Vibration-based Maintenance in Paper Mills : Case Studies
- 2000
-
Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 229:2, s. 389-410
-
Tidskriftsartikel (refereegranskat)abstract
- Many current vibration-based maintenance (VBM) policies for rolling element bearings do not use as much as possible of their useful lives. Evidence and indications to prolong the bearings’ mean effective lives by using more accurate diagnosis and prognosis are confirmed when faulty bearing installation, faulty machinery design, harsh environmental condition and when a bearing is replaced as soon as its vibration level exceeds the normal. Analysis of data from roller bearings at two paper mills suggests that longer bearing lives can be safely achieved by increasing the accuracy of the vibration data. This paper relates bearing failure modes to the observed vibration spectra and their development patterns over the bearings’ lives. A systematic approach, which describes the objectives and performance of studies in two Swedish paper mills, is presented. Explanations of the mechanisms behind some frequent modes of early failure and ways to avoid them are suggested. It is shown theoretically, and partly confirmed by the analysis of (unfortunately incomplete) data from two paper mills over many years, that accurate prediction of remaining bearing life requires: (a) enough vibration measurements, (b) numerate records of operating conditions, (c) better discrimination between frequencies in the spectrum and (d) correlation of (b) and (c). This is because life prediction depends on precise knowledge of primary, harmonic and side band frequency amplitudes and their development over time. Further, the available data, which are collected from relevant plant activities, can be utilised to perform cyclic improvements in diagnosis, prognosis, experience and economy.
|
|
| 4. |
- Bengtsson, Johanna, 1974, et al.
(författare)
-
Evaluations of effects due to low-frequency noise in a low demanding work situation
- 2004
-
Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 278:1-2, s. 83-99
-
Tidskriftsartikel (refereegranskat)abstract
- Noise sources with a dominating content of low frequencies (20-200 Hz) are found in many occupational environments. This study aimed to evaluate effects of moderate levels of low-frequency noise on attention, tiredness and motivation in a low demanding work situation. Two ventilation noises at the same A-weighted sound pressure level of 45 dB were used: one of a low-frequency character and one of a flat frequency character (reference noise). Thirty-eight female subjects worked with six performance tasks for 4 h in the noises in a between-subject design. Most of the tasks were monotonous and routine in character. Subjective reports were collected using questionnaires and cortisol levels were measured in saliva. The major finding in this study was that low-frequency noise negatively influenced performance on two tasks sensitive to reduced attention and on a proof-reading task. Performances of tasks aimed at evaluating motivation were not significantly affected. The difference in work performance was not reflected by the subjective reports. No effect of noise was found on subjective stress or cortisol levels.
|
|
| 5. |
- Birgersson, F., et al.
(författare)
-
Application of the spectral finite element method to turbulent boundary layer induced vibration of plates
- 2003
-
Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 259:4, s. 873-891
-
Tidskriftsartikel (refereegranskat)abstract
- The spectral finite element method and equally the dynamic stiffness method use exponential functions as basis functions. Thus it is possible to find exact solutions to the homogeneous equations of motion for simple rod, beam, plate and shell structures. Normally, this restricts the analysis to elements where the excitation is at the element ends. This study removes the restriction for distributed excitation, that in particular has an exponential spatial dependence, by the inclusion of the particular solution in the set of basis functions. These elementary solutions, in turn, build up the solution for an arbitrary homogeneous random excitation. A numerical implementation for the vibration of a plate, excited by a turbulent boundary layer flow, is presented. The results compare favourably with results from conventional modal analysis.
|
|
| 6. |
|
|
| 7. |
- Birgersson, F., et al.
(författare)
-
Modelling turbulence-induced vibration of pipes with a spectral finite element method
- 2004
-
Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 278:05-apr, s. 749-772
-
Tidskriftsartikel (refereegranskat)abstract
- The vibration of pipes is studied here using the Arnold-Warburton theory for thin shells and a simplified theory valid in a lower frequency regime. The vibrational response is described numerically with the spectral finite element method (SFEM), which uses the exact solutions of the equations of motion as basis functions. For turbulence excitation, the set of basis functions was extended to include particular solutions, which model a spatially distributed excitation. An efficient numerical solution to homogeneous random excitation is presented and the results compare favourably with wind tunnel measurements.
|
|
| 8. |
- Boden, Hans, et al.
(författare)
-
Aeroacoustic research in Europe : the CEAS-ASC report on 1999 highlights
- 2000
-
Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 237:3, s. 477-482
-
Tidskriftsartikel (refereegranskat)abstract
- This paper is a report on the highlights of aeroacoustics research and development in Europe in 1999, compiled from the information provided to the Aeroacoustics Specialists Committee (ASC) of the Confederation of European Aerospace Societies (CEAS). CEAS presently comprises the national Aerospace Societies of France (AAAF), Germany (DGLR), Italy (AIDAA), The Netherlands (NVvL), Spain (AIAE), Sweden (FTEF), Switzerland (SVFW) and the United Kingdom (RaeS).
|
|
| 9. |
- Boden, Hans, et al.
(författare)
-
Linearity tests for in-duct acoustic one-port sources
- 2000
-
Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 237:1, s. 45-65
-
Tidskriftsartikel (refereegranskat)abstract
- Acoustic one-port source data are commonly used to predict the plane wave sound generation in duct and pipe systems connected to fluid machines. The source data are usually determined experimentally, which assumes that linear time-invariant system theory can be used. Since some machines such as IC-engines and compressors generate very high sound levels in the connecting ducts or pipes it is of interest to investigate whether the assumption of linearity is justified. Linearity tests for linear system identification when both input and output signals can be measured are common in the literature. In the case when only the output signal can be measured linearity tests are not so readily found. This paper presents two different linearity coefficients for determining whether an acoustic one-port source under test is linear. Their sensitivity to random noise and their ability to detect non-linearities are investigated by simulations and measurements on several types of machines.
|
|
| 10. |
|
|
