| 1. |
- Thompson, David, et al.
(författare)
-
Assessment of measurement-based methods for separating wheel and track contributions to railway rolling noise
- 2018
-
Ingår i: Applied Acoustics. - : Elsevier. - 0003-682X .- 1872-910X. ; 140, s. 48-62
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Tidskriftsartikel (refereegranskat)abstract
- The noise produced during a train pass-by originates from several different sources such as propulsion noise, noise from auxiliary equipment, aerodynamic noise and rolling noise. The rolling noise is radiated by the wheels and the track and is excited by the wheel and rail unevenness, usually referred to as roughness. The current TSI Noise certification method, which must be satisfied by all new mainline trains in Europe, relies on the use of a reference track to quantify the noise from new vehicles. The reference track is defined by an upper limit of the rail roughness and a lower limit of the track decay rate (TDR). However, since neither the rail roughness nor the track radiation can be completely neglected, the result cannot be taken as representing only the vehicle noise and the measurement does not allow separate identification of the noise radiated by wheel and track. It is even likely that further reductions in the limit values for new rolling stock cannot be achieved on current tracks. There is therefore a need for a method to separate the noise into these two components reliably and cheaply. The purpose of the current study is to assess existing and new methods for rolling noise separation. Field tests have been carried out under controlled conditions, allowing the different methods to be compared. The TWINS model is used with measured vibration data to give reference estimates of the wheel and track noise components. Six different methods are then considered that can be used to estimate the track component. It is found that most of these methods can obtain the track component of noise with acceptable accuracy. However, apart from the TWINS model, the wheel noise component could only be estimated directly using three methods and un- fortunately these did not give satisfactory results in the current tests.
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| 2. |
- Afzal, Mohammad, 1987-, et al.
(författare)
-
A formulation of the Jacobian matrixfor 3D numerical friction contact model applied to turbine blade shroud contact
-
Ingår i: Journal of Sound and Vibration. - 0022-460X .- 1095-8568.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- An analytical expression is formulated to compute the Jacobian matrix for 3D friction contact modelling that eciently evaluates the matrix while computing the friction contact forces in the time domain by means of the alternate frequency time domain approach. The developed expression is successfully used for thecalculation of the friction damping on a turbine blade with shroud contact interface having an arbitrary 3Drelative displacement. The analytical expression drastically reduces the computation time of the Jacobian matrix with respect to the classical finite dierence method, with many points at the contact interface. Therefore,it also significantly reduces the overall computation time for the solution of the equations of motion,since the formulation of the Jacobian matrix is the most time consuming step in solving the large set of nonlinear algebraic equations when a finite dierence approach is employed. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of cyclic symmetry boundary condition for aperiodic structure. Implementation of the developed scheme in solving the equations of motion is proved to be effective and significant reduction in time is achieved without loss of accuracy.
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| 3. |
- Afzal, Mohammad, 1987-, et al.
(författare)
-
Adaptive control of normal load at the friction interface of bladed disks using giant magnetostrictive material
-
Ingår i: Journal of Vibration and Control. - 1077-5463 .- 1741-2986.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A novel application of magnetostrictive actuators in underplatform dampers of bladed disks is proposed for adaptive control of the normal load at the friction interface in order to achieve the desired friction damping in the structure. Friction damping in a bladed disk depends on many parameters such as rotational speed, engine excitation order, nodal diameter, contact stiffness, friction coefficient and normal contact load. However, all these parameters have a fixed value at an operating point. On the other hand, the ability to vary some of these parameters such as the normal contact load is desirable in order to obtain an optimum damping in the bladed disk at different operating conditions. Under the influence of an external magnetic field, magnetostrictive materials develop an internal strain that can be exploited to vary the normal contact load at the friction interface, which makes them a potentially good candidate for this application. A commercially available magnetostrictive alloy, Terfenol-D is considered in this analysis that is capable of providing magnetostrain up to 0.002 under prestress and a blocked force over 1500 N. A linearized model of the magnetostrictive material, which is accurate enough for a DC application, is employed to compute the output displacement and the blocked force of the actuator. A nonlinear finite element contact analysis is performed to compute the normal contact load between the blade platform and the underplatform damper as a result of magnetostrictive actuation. The contact analysis is performed for different mounting configurations of the actuator and the obtained results are discussed. The proposed solution is potentially applicable to adaptively control vibratory stresses in bladed disks and consequently to reduce failure due to high-cycle fatigue. Finally, the practical challenges in employing magnetostrictive actuators in underplatform dampers are discussed.
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| 4. |
- Afzal, Mohammad, 1987-, et al.
(författare)
-
Adaptive control of normal load at the friction interface of bladed disks using giant magnetostrictive material
- 2020
-
Ingår i: Journal of Intelligent Materials Systems and Structures. - : SAGE Publications. - 1045-389X .- 1530-8138. ; 31:8, s. 1111-1125
-
Tidskriftsartikel (refereegranskat)abstract
- A novel application of magnetostrictive actuators in underplatform dampers of bladed disks is proposed for adaptive control of the normal load at the friction interface to achieve the desired friction damping in the structure. Friction damping in a bladed disk depends on operating parameters, such as rotational speed, engine excitation order, nodal diameter normal contact load, and contact interface parameters, such as contact stiffness and friction coefficient. The operating parameters have a fixed value, whereas the contact interface parameters vary in an unpredictable way at an operating point. However, the ability to vary some of these parameters such as the normal contact load in a controlled manner is desirable to attain an optimum damping in the bladed disk at different operating conditions. Under the influence of an external magnetic field, magnetostrictive materials develop an internal strain that can be exploited to vary the normal contact load at the friction interface, which makes them a potentially good candidate for this application. A commercially available magnetostrictive alloy, Terfenol-D is considered in this analysis that is capable of providing magnetostrain up to 2 × 10-3 under prestress and a blocked force over 1500 N. A linearized model of the magnetostrictive material, which is accurate enough for a direct current application, is employed to compute the output force of the actuator. A nonlinear finite element contact analysis is performed to compute the normal contact load between the blade platform and the underplatform damper as a result of magnetostrictive actuation. The nonlinear contact analysis is performed for different actuator mounting configurations and the obtained results are discussed. The proposed solution is potentially applicable to adaptively control vibratory stresses in bladed disks and consequently to reduce failure due to high-cycle fatigue. Finally, the practical challenges in employing magnetostrictive actuators in underplatform dampers are discussed.
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| 5. |
- Afzal, Mohammad, et al.
(författare)
-
An analytical calculation of the Jacobian matrix for 3D friction contact model applied to turbine blade shroud contact
- 2016
-
Ingår i: Computers & structures. - : Elsevier. - 0045-7949 .- 1879-2243. ; 177, s. 204-217
-
Tidskriftsartikel (refereegranskat)abstract
- An analytical expression is formulated to compute the Jacobian matrix for 3D friction contact modeling that efficiently evaluates the matrix while computing the friction contact forces in the time domain by means of the alternate frequency time domain approach. The developed expression is successfully used for the calculation of the friction damping on a turbine blade with shroud contact interface having an arbitrary 3D relative displacement. The analytical expression drastically reduces the computation time of the Jacobian matrix with respect to the classical finite difference method, with many points at the contact interface. Therefore, it also significantly reduces the overall computation time for the solution of the equations of motion, since the formulation of the Jacobian matrix is the most time consuming step in solving the large set of nonlinear algebraic equations when a finite difference approach is employed. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of cyclic symmetry boundary condition for a periodic structure. Implementation of the developed scheme in solving the equations of motion is proved to be effective and significant reduction in time is achieved without loss of accuracy.
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| 6. |
- Afzal, Mohammad, 1987-, et al.
(författare)
-
Numerical analysis of multiple friction contacts in bladed disks
-
Ingår i: International Journal of Mechanical Sciences. - 0020-7403 .- 1879-2162.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The damping potential of multiple friction contacts in a bladed disk, tip shroud and strip damper is investigated, showing that friction damping effectiveness can be potentially increased by using multiple friction contact interfaces. Friction damping depends on many parameters such as rotational speed, engine excitation order and mode family and therefore it is not possible to damp all the critical resonances using a single friction contact interface. For example, a strip damper is more effective for the low nodal diameters, where blade/disk coupling is strong. The equations of motion of the bladed disk with multiple friction contacts are derived in the frequency domain for a cyclic structure with rotating excitations and a highly accurate method is used to generate the frequency response function (FRF) matrix. Furthermore, a finite element contact analysis is performed to compute the normal contact load and the contact area of the shroud interface at operating rotational speed. The multiharmonic balance method is employed in combination with the alternate frequency time domain method to find the approximate steady state periodic solution. A low-pressure turbine bladed disk is considered and the effect of the engine excitation level, strip mass, thickness and the accuracy of FRF matrix on the nonlinear response curve are investigated in detail.
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| 7. |
- Afzal, Mohammad, et al.
(författare)
-
Numerical analysis of multiple friction contacts in bladed disks
- 2018
-
Ingår i: International Journal of Mechanical Sciences. - : Elsevier. - 0020-7403 .- 1879-2162. ; 137, s. 224-237
-
Tidskriftsartikel (refereegranskat)abstract
- The damping potential of multiple friction contacts in a bladed disk is investigated. Friction contacts at tip shrouds and strip dampers are considered. It is shown that friction damping effectiveness can be potentially increased by using multiple friction contact interfaces. Friction damping depends on many parameters such as rotational speed, engine excitation order and mode family and therefore it is not possible to damp all the critical resonances using a single kind of friction contact interface. For example, a strip damper is more effective for the low nodal diameters, where blade/disk coupling is strong. The equations of motion of the bladed disk with multiple friction contacts are derived in the frequency domain for a cyclic structure with rotating excitations. A highly accurate method is used to generate the frequency response function (FRF) matrix. Furthermore, a finite element contact analysis is performed to compute the normal contact load and the contact area of the shroud interface at operating rotational speed. The multiharmonic balance method is employed in combination with the alternate frequency time domain method to find the steady state periodic solution. A low-pressure turbine bladed disk is considered and the effect of the engine excitation level, strip mass, thickness and the accuracy of FRF matrix on the nonlinear response curve are investigated in detail.
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| 8. |
- Afzal, Mohammad, 1987-
(författare)
-
On efficient and adaptive modelling of friction damping in bladed disks
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work focuses on efficient modelling and adaptive control of friction damping in bladed disks. To efficiently simulate the friction contact, a full-3D time-discrete contact model is reformulated and an analytical expression for the Jacobian matrix is derived that reduces the computation time drastically with respect to the classical finite difference method. The developed numerical solver is applied on bladed disks with shroud contact and the advantage of full-3D contact model compared to a quasi-3D contact model is presented. The developed numerical solver is also applied on bladed disks with strip damper and multiple friction contacts and obtained results are discussed. Furthermore, presence of higher harmonics in the nonlinear contact forces is analyzed and their effect on the excitation of the different nodal diameters of the bladed disk are systematically presented. The main parameters that influence the effectiveness of friction damping in bladed disks are engine excitation order, contact stiffnesses, friction coefficient, relative motion at the friction interface and the normal contact load. Due to variation in these parameters during operation, the obtained friction damping in practice may differ from the optimum value. Therefore, to control the normal load adaptively that will lead to an optimum damping in the system despite these variations, use of magnetostrictive actuator is proposed. The magnetostrictive material that develops an internal strain under the influence of an external magnetic field is employed to increase and decrease the normal contact load. A linearized model of the magnetostrictive actuator is used to characterize the magnetoelastic behavior of the actuator. A nonlinear static contact analysis of the bladed disk reveals that a change of normal load more than 700 N can be achieved using a reasonable size of the actuator. This will give a very good control on friction damping once applied in practice.
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| 9. |
- Amiri, A., et al.
(författare)
-
A replacement model to simulate the nonlinear dynamics of electro-responsive liquid crystal coatings
- 2023
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226. ; 13:3
-
Tidskriftsartikel (refereegranskat)abstract
- An electric circuit replacement model is proposed to simulate the key nonlinear dynamics of electro-responsive liquid crystal polymer networks (LCNs). LCNs are known for having great potential to be integrated into smart functional surfaces due to their ability to generate various surface patterns. However, due to their complex molecular dynamics, low-order dynamic models that can accurately describe and predict their dynamic behavior are still lacking. In light of this research gap, we develop a lumped-parameter replacement model based on the observed dynamics in the experimental data and the physics of LCN dielectric properties. The unique assembly of lumped parameters in its simplest form describes the transformation of a high-frequency input voltage to a relatively slow increase in the local height of the LCN coating in between the electrodes, serving as an excitation mechanism to induce height change. The nonlinear dynamics of this height increase, as a function of both excitation frequency and voltage, is described by the proposed model. Furthermore, the comparison of the simulation results with the experimental data from LCN shows that key LCN response characteristics are captured well by the model. This model makes it possible to accurately predict and control the response of the electro-responsive LCN surfaces to obtain a predefined desired deformation pattern, which is a vital requirement for integrating them in haptic and smart surface devices.
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| 10. |
- Amlinger, Hanna, et al.
(författare)
-
Impact of PWM switching frequency on the radiated acoustic noise from a traction motor
- 2017
-
Ingår i: 2017 20th International Conference on Electrical Machines and Systems, ICEMS 2017. - : Institute of Electrical and Electronics Engineers Inc.. - 9781538632468
-
Konferensbidrag (refereegranskat)abstract
- The radiated acoustic noise from a traction motor at low speeds is dominated by the noise of electromagnetic origin. For a motor operated from pulse width modulated (PWM) converters, the switching frequency of the converter will have a large impact on the noise. The total harmonic distortion of the motor phase currents and thus also the exciting forces, will decrease with increasing switching frequency. Furthermore, changing the switching frequency will shift the frequencies of the exciting forces, hence have an influence on the coincidence with structural resonances of the motor. Tests have been performed on a traction motor and a decrease in sound pressure level with increasing switching frequency has been quantified and analyzed.
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| 11. |
- Amlinger, Hanna, et al.
(författare)
-
Operational deflection shapes of a PWM-fed traction motor
- 2016
-
Ingår i: Conference Proceedings of the Society for Experimental Mechanics Series. - Cham : Springer. - 9783319300832 ; , s. 209-217
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Konferensbidrag (refereegranskat)abstract
- Operational deflection shapes of an asynchronous traction motor for railway applications are investigated. The radiated noise from the tractionmotor on a train is, especially at low speeds, dominated by noise generated by electromagnetic forces. The tested motor is fed by a pulse-width-modulated (PWM) frequency converter for which the voltage is modulated as a series of pulses that are switched with a certain frequency. In this case, PWM force lines can be expected to influence the radiated noise. Therefore, detailed knowledge about the frequencies and deflection shapes of vibrations generated by PWM forces is of great importance for understanding and controlling the radiated noise and its spectral content. Vibration levels are measured on the stator shield and the operational deflection shapes are studied for several PWM switching frequencies and motor speeds. The deflection shapes with the largest vibration levels are determined. These are then compared to the expected excitation resulting from the pure PWM force lines. Changing the switching frequency, will shift the frequencies of the exciting forces. An appropriate selection of the PWM switching frequency is therefore important for the resulting acoustic radiation from the motor.
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| 12. |
- Amlinger, Hanna
(författare)
-
Reduction of Audible Noise of a Traction Motor at PWM Operation
- 2018
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A dominating source for the radiated acoustic noise from a train at low speeds is the traction motor. This noise originates from electromagnetic forces acting on the structure resulting in vibrations on the surface and thus radiated noise. It is often perceived as annoying due to its tonal nature. To achieve a desirable acoustic behavior, and also to meet legal requirements, it is of great importance to thoroughly understand the generation of noise of electromagnetic origin in the motor and also to be able to control it to a low level.In this work, experimental tests have been performed on a traction motor operated from pulse width modulated (PWM) converter. A PWM converter outputs a quasi-sinusoidal voltage created from switched voltage pulses of different widths. The resulting main vibrations at PWM operation and their causes have been analyzed. It is concluded that an appropriate selection of the PWM switching frequency, that is the rate at which the voltage is switched, is a powerful tool to influence the noise of electromagnetic origin. Changing the switching frequency shifts the frequencies of the exciting electromagnetic forces. Further experimental investigations show that the trend is that the resulting sound power level decreases with increasing switching frequency and eventually the sound power level reaches an almost constant level. The underlying physical phenomena for the reduced sound power level is different for different frequency ranges. It is proposed that the traction motor, similar to a thin walled cylindrical structure, shows a constant vibration over force response above a certain frequency. This is investigated using numerical simulations of simplified models. Above this certain frequency, where the area of high modal density is dominating, the noise reducing effect of further increasing the switching frequency is limited.
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| 13. |
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| 14. |
- Aulitto, Alessia, et al.
(författare)
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Experimental study of a slit in the presence of a bias flow under medium- and high-level acoustic excitations
- 2023
-
Ingår i: International Journal of Spray and Combustion Dynamics. - : SAGE Publications. - 1756-8277 .- 1756-8285. ; 15:2, s. 117-126
-
Tidskriftsartikel (refereegranskat)abstract
- This work presents an experimental investigation of the acoustic properties of a slit in the presence of a bias flow under moderate- and high-acoustic excitations. Impedance tube experiments are discussed for a geometry inspired by deep punching resulting in a cut in the plate. The acoustic transfer impedance of the plate is discussed for several bias flow velocities, acoustic excitation, and different frequencies. In the range considered for this study, a bias flow appears to have two main effects, globally enhancing the sound absorption of the plate and creating a protective layer downstream of the plate due to the interaction between the slits. A maximum of the enhancement factor is found at a specific ratio between the acoustic velocity and the bias flow velocity. Two simple asymptotic behaviors are found, dominated by the flow or by the acoustic excitation, respectively. The behavior of the inertance is complex. Globally the inertance decreases with decreasing flow Strouhal number while its dependency on the amplitude of the acoustic velocity is less obvious.
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| 15. |
- Boere, Stijn, et al.
(författare)
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Tyre/road interaction model for the prediction of road texture influence on rolling resistance
- 2014
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Ingår i: International Journal of Vehicle Design. - 0143-3369 .- 1741-5314. ; 65:2-3, s. 202-221
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Tidskriftsartikel (refereegranskat)abstract
- A novel modelling approach to predict the influence of road texture on the rolling resistance of car tyres is presented where the large static tyre deformations and the small texture induced tyre vibrations are treated separately. The energy dissipation due to the large continuous cyclic deformation of the tyre cross section for a treadless tyre subject to nominal load on a smooth road is determined in a non-linear steady-state rolling analysis on an FEM tyre model. The additional energy dissipation resulting from the con tact forces and tyre vibrations due to the combined effect of the tread profile and the road texture, are determined based on a modal representation of the deformed tyre. The predicted rolling resistance coefficients are compared to experimental data. Although an offset in the absolute rolling resistance levels can be observed, the model predicts the correct trend regarding the increase of rolling resistance with increasing texture depth.
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| 16. |
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| 17. |
- Botling, Fredrik, et al.
(författare)
-
Combined Experimental and Analytical Vibro-Acoustic Model of an Electrical Motor
- 2016
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A vibro-acoustic modal model of a traction motor forrailway applications is presented based on an experimentalmodal analysis of the system. Noise requirements forrailway traction motors are getting more and moredemanding as part of the overall levels of new rolling stock.It is therefore of great interest to understand and predict thevibro-acoustic behaviour of electromagnetic noisegenerated by traction motors. The modal parameters arederived from an experimental modal analysis. The primarysource for the radiated sound of the tested traction motor isthe radial deflections of the stator shield. The modalparameters for the radial deflections are implemented in areduced order modal model in a state space format usingMatlab/Simulink. Only the structural modes that have amatch in both the frequency and the spatial domain withthe electromagnetic excitation force will cause animportant vibro-acoustic response. This makes it possibleto create an accurate and efficient reduced order modalmodel with only a fraction of the total number of structuralmodes. The simulation results from the modal model arecompared to measured levels of vibration and acousticmeasurements of the motor.
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| 18. |
- Botling, Fredrik
(författare)
-
Modelling and simulation of electromagnetic audible noise generated by traction motors
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An annoying tonal noise is produced by modern electrical trains duringacceleration and deceleration. This noise is caused by electromagneticforces generating structural vibrations, especially from the traction motors.The electromagnetic noise is dominant at low train speeds and affectsboth the passengers on the train and on platforms, as well as peopleliving near the track. The focus on this issue has increased the last years,both regarding legislation, contractual requirements and also because ofexpectations from citizens and travelers. To be able to design low noiseelectric drive systems, a thorough understanding of the cause and thepossibility to predict the electromagnetic noise is needed. This thesisdescribes the modelling and simulation of an complete multi-physicsreal-time environment for prediction and analysis of the electromagneticnoise. The simulation results are validated against measurements of thestructural vibration and acoustic response of a real traction motor fed bya power converter running in the entire operational range.
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| 19. |
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| 20. |
- Botling, Fredrik, et al.
(författare)
-
Modelling framework for electromagnetic noise generation from traction motors
- 2016
-
Ingår i: Journal of Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Cham : Springer Nature. - 1612-2909.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Electromagnetic noise from traction motors is often the single most dominating noise source from trains at low trainspeeds. This electromagnetic noise is tonal and annoying for both passengers on the train and people near the track andon the platform. A thorough understanding of the cause and the prediction of the electromagnetic noise is needed to beable to design low noise components. This paper describes a real time multi-physics modelling framework forprediction of the audible electromagnetic noise generated by traction motors fed by power converters. The cause of theelectromagnetic noise is influenced by several different domains. To be able to fully simulate the final acoustic sound,all these domains needs to be modelled and simulated together. Simulations of some relevant operational conditionsfor acoustic noise generation has been performed and discussed. Some of these results have been compared to realmeasurements from a converter fed traction motor.
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| 21. |
- Botling, Fredrik, et al.
(författare)
-
Vibro-acoustic modal model of a traction motor for railway applications
- 2016
-
Ingår i: Conference Proceedings of the Society for Experimental Mechanics Series. - Cham : Springer. - 9783319300832 ; , s. 197-208
-
Konferensbidrag (refereegranskat)abstract
- A vibro-acoustic modal model of a traction motor for railway applications is presented based on an experimental modal analysis of the system. Noise requirements for railway traction motors are getting more and more demanding as part of the overall levels of new rolling stock. It is therefore of great interest to understand and predict the vibro-acoustic behaviour of electromagnetic noise generated by traction motors. The modal parameters are derived from an experimental modal analysis. The primary source for the radiated sound of the tested traction motor is the radial deflections of the stator shield. The modal parameters for the radial deflections are implemented in a reduced order modal model in a state space format using Matlab/Simulink. Only the structural modes that have a match in both the frequency and the spatial domain with the excited electromagnetic force will cause important vibro-acoustic response. This makes it possible to create an accurate and efficient reduced order modal model with only a fraction of the total number of structural modes. The simulation results from the modal model are compared to measurements of operational deflection shapes and acoustic measurements of the motor.
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| 22. |
- Chen, Lin Strobin, et al.
(författare)
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ACOUSTIC SCATTERING BEHAVIOR OF A 2D FLAME WITH HEAT EXCHANGER IN CROSS-FLOW
- 2016
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Ingår i: ICSV 2016 - 23rd International Congress on Sound and Vibration. - : International Institute of Acoustics and Vibrations. - 9789609922623
-
Konferensbidrag (refereegranskat)abstract
- In practical heat production systems, premixed flames with cold heat exchanger in cross-flow is a widely used configuration. Self-excited thermoacoustic instabilities often occur in such systems. A practical way to predict the presence of the instabilities is the network model approach. In the present study, the configuration flame - heat exchanger is analyzed numerically. We first analyze the system as a network of segregated elements. Based on the resulting acoustic scattering matrix, the role of the heat exchanger as an amplifier of the flame resonant frequencies will be discussed. Then, results from the 1D network modeling are compared to results of compressible numerical simulations, performed for several distances between flame and heat exchanger. Finally, the limits to the validity of the segregated network model approach are discussed.
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| 23. |
- Ganji, Hamed F., et al.
(författare)
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A framework for suppression of thermoacousitc instability using the nyquist criterion
- 2023
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Ingår i: Proceedings of the 29th International Congress on Sound and Vibration, ICSV 2023. - : Society of Acoustics.
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Konferensbidrag (refereegranskat)abstract
- To simulate complex multiphysics and multiscale phenomena, such as thermoacoustic combustion instabilities, a comprehensive model is often created by combining subsystems. This approach allows the contribution of acoustic terminations to be separated from the acoustic response of the flame, potentially providing a systematic approach to suppressing thermoacoustic instabilities. In this paper, we present a framework for developing terminations and silencers using the Nyquist criterion and winding number. Firstly, we determine the dispersion relation to establish the system's eigen-frequencies. Then, by distinguishing between the Nyquist diagram and the Nyquist criterion and using the argument (arg) of the dispersion relation, we propose two thermoacoustic stability criteria. This approach is similar to the theory used in microwave technology. Using this methodology, we can develop a systematic method for characterizing the quality of specific combustion devices and provide guidelines for designing stable acoustic embeddings in thermoacoustic systems. To validate this approach, we will perform an eigenvalue analysis on a classical duct-flame-duct test case, and the results will demonstrate the effectiveness of this analogy in solving thermoacoustic problems.
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| 24. |
- Ganji, Hamed F., et al.
(författare)
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Assessment of the acoustic scattering matrix of a heat exchanger using ssCFD-LNSE simulation
- 2022
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Ingår i: Internoise 2022. - : The Institute of Noise Control Engineering of the USA, Inc..
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Konferensbidrag (refereegranskat)abstract
- The risk of thermoacoustic instability is present in any combustion appliance. The instability results from a closed loop feedback between unsteady combustion, heat-transfer and acoustic modes of the system. To predict the system acoustics, all constituting elements of the appliance need to be modelled. A heat-exchanger is a element where the gas faces complex fluid dynamics and heat transfer processes. Therefore, modelling of (thermo)-acoustic properties of a heat exchanger is challenging. In this paper, a computational approach is proposed to characterize the acoustic properties of a generic heat exchanger in both laminar and turbulent flow regimes. A hybrid Computational Fluid Dynamics - Computational Aero-Acoustics (CFD-CAA) method is used based on the linearized Navier-Stokes equation, called ssCFD-LNSE. The core idea in this approach is to efficiently model acoustic wave propagation with inclusion of mean flow and temperature fields. ssCFD-LNSE is performed by splitting the quantities of the total field into a mean part (obtained from CFD) and a (acoustic) perturbation part modelled within the LNSE solver. The goal of this research is to assess the two-port acoustic scattering matrix of an array of tubes, as a generic model of a heat-exchanger, with a hot cross flow.
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| 25. |
- Ganji, Hamed F., et al.
(författare)
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Characterization and identification of thermoacoustic behavior of flames anchored on burner decks with multiple perforations; Transfer Function (de)composition approach
- 2022
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Ingår i: Internoise 2022 - 51st International Congress and Exposition on Noise Control Engineering. - : The Institute of Noise Control Engineering of the USA, Inc..
-
Konferensbidrag (refereegranskat)abstract
- The appearance of thermoacoustic instability in combustion systems depends on thermoacoustic property (e.g., Transfer Function (TF)) of used burner/flame. Therefore, an attractive approach to cope with the instability is the purposeful design of the burner thermoacoustics. One of the ideas of how the flame TF can be altered/designed is based on the heuristic idea that the acoustic response of one flame can be counteracted by the appropriately phased response of another flame. For the particular case of premixed, burner deck anchored conical flames, the TF depends on the diameter of perforation. It suggests the concept of combining different size and shape of perforations in one burner deck. In the present work, the acoustic response of sintered ceramic fibre burners with mixed perforation is investigated using the TF (de)composition principle. By this approach, the cumulative flame TF can be represented as a weighted sum of elemental TF's of the groups of flames on the basis of the additive nature of the individual flame heat release rate. The capability of this principle to offer a designing framework for optimization of burner deck patterns aiming desirable acoustic characteristics will be tested by a course of measurements. Possible simplifications and extensions of the TF (de)composition principle will be discussed.
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| 26. |
- Gomez, Erik
(författare)
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Centrifugal pendulum vibration absorbers in heavy-duty truck powertrains : Modelling, simulation and experimental investigations
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The heavy-duty vehicle industry is facing big challenges to reduce CO2 emissions. Although electrification is on the rise, the combustion engine will used for some time to come. Unfortunately, CO2 reduction methods, such as downspeeding, down-sizing and increased cylinder gas-pressure, result in increased torsional vibrations and noise which must be addressed. Conventional torsional vibration reduction methods alone, such as the clutch-damper, will not suffice. The centrifugal pendulum vibration absorber (CPVA) is a torsional vibration reduction device that has not conventionally been used in heavy-duty vehicles and is herein investigated.This work presents analytical, simulation and experimental investigations of the CPVA. First, a model of a centrifugal pendulum vibration absorber with a general suspension architecture is derived with Kane's method. The model allows for different pendulum paths and relative rotation of the pendulum body with respect to the rotor. A normal-force friction loss of the pendulum is developed together with a measurement method to determine the friction coefficient. The measurement method may also be used to validate the pendulum model parameters without any special test-apparatus other than standard accelerometers and a data acquisition system. The developed CPVA model is then included in a torsional model of the complete powertrain including gas-pressure based excitation from the engine model. The powertrain model is developed to simulate transient and quasi-steady state conditions in the time-domain. Also, a continuous model of the end-stops of the pendulum are implemented which also facilitates the simulation.It is concluded that it is important to include the system dynamics of the powertrain in the final design of the CPVA. Also, by exploiting the frequency veering property of the CPVA, powertrain resonances can be completely eliminated.
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| 27. |
- Gomez, Erik, et al.
(författare)
-
Low rotational-speed aspects of centrifugal pendulum vibration absorbers
- 2020
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Ingår i: Proceedings of ISMA 2020 - International Conference on Noise and Vibration Engineering and USD 2020 - International Conference on Uncertainty in Structural Dynamics. - : Katholieke Universiteit Leuven, Dept Werktuigkunde. ; , s. 3053-3063
-
Konferensbidrag (refereegranskat)abstract
- The performance of the centrifugal pendulum vibration absorber (CPVA) is investigated in the low rotational-speed regime in the context of a heavy-duty combustion engine. With the given dimensions and properties of heavy-vehicle engines, special considerations must be taken into account in the analysis of the CPVAs in the low speed regime. In this study, bifilar-suspended pendula absorbers attached to the engine flywheel, tuned to absorb the main engine firing order, is modeled analytically. Friction damping is included in the pendulum model and the effects of the performance by varying the pendulum path curve is studied in the low rotational-speed regime. The low rotational-speed is due to engine down-speeding, resulting in excessive torsional vibration in the driveline. In order to maintain or improve NVH and fulfill legal requirements, the excess torsional vibration must be addressed to gain the emission benefits of down-speeding. Simulation and analysis of the CPVA behaviour is therefore paramount for heavy-vehicle manufacturers to understand and thus make well balanced decisions to fulfill environmental goals.
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| 28. |
- Gomez, Erik, et al.
(författare)
-
Multiple-order excitation and response of centrifugal pendulum vibration absorbers
- 2018
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Ingår i: Proceedings of ISMA 2018 - International Conference on Noise and Vibration Engineering and USD 2018 - International Conference on Uncertainty in Structural Dynamics. - : KU Leuven - Departement Werktuigkunde. - 9789073802995 ; , s. 4305-4319
-
Konferensbidrag (refereegranskat)abstract
- The multiple-order response of a rotor equipped with a centrifugal pendulum vibration absorber (CPVA) is investigated in this study. CPVAs have been used in the abatement of torsional vibrations since the 1930s and have been extensively researched but seldom with a focus on the full multiple-harmonic response. Although the firing order in reciprocating engines dominates in amplitude, the higher-order harmonics have impact on driveline-related noise issues. This investigation is conducted in order to understand if an higher-order amplification can stem from the design of the CPVA or if this phenomenon is due an interaction between other powertrain components. Therefore, an isolated rotor-CPVA system is studied. The investigated rotor is subjected to an oscillating torque consisting of single- and multiple-order harmonics. The equations of motion are derived by Kane s method and the multiple-harmonic response of the system is analyzed. The linear steady-state response is compared with numerical time integration of the equations of motion.
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| 29. |
- Gomez, Erik, et al.
(författare)
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Normal-force dependant friction in centrifugal pendulum vibration absorbers : Simulation and experimental investigations
- 2021
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Ingår i: Journal of Sound and Vibration. - : Academic Press. - 0022-460X .- 1095-8568. ; 492
-
Tidskriftsartikel (refereegranskat)abstract
- A model of the normal-force dependant friction loss between the rotor and pendula is developed for bifilar centrifugal pendulum vibration absorbers (CPVAs). The normal-force is dominantly dependant on the rotor rotational velocity but also dependant on the pendulum path. Simulation results of the pendulum with the proposed normal-force friction model agree well with novel experimental results. Order-response simulations of the rotor-CPVA model with the proposed friction model reveal accuracy improvements on the prediction of the rotor oscillation amplitude at different rotational velocities in comparison to existing friction formulations. Derivation of the equations of motion of the rotor-CPVA is performed by Kane's method and solved numerically. The developed experimental setup is used to validate the model parameters such as friction coefficients, path parameters and relative pendulum rotation without necessitating a special test-apparatus other than standard vibration measurement equipment. CPVAs are commonly used to reduce torsional vibration created by reciprocating engines. To reduce emissions, heavy-duty vehicles manufactures are downsizing and downspeeding the combustion engine while maintaining the power output. Unfortunately, this gives rise to increased torsional vibration in the powertrain. The CPVA is a device that can reduce the torsional vibration and thus help fulfill environmental goals.
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| 30. |
- Gomez, Erik R., et al.
(författare)
-
Powertrain shuffle-mode resonance suppression by means of flywheel mounted torsichrone centrifugal pendulum vibration absorbers
- 2022
-
Ingår i: Journal of Sound and Vibration. - : Elsevier BV. - 0022-460X .- 1095-8568. ; 534
-
Tidskriftsartikel (refereegranskat)abstract
- A flywheel mounted centrifugal pendulum absorber (CPVA) is designed to completely suppress the low frequency shuffle-mode resonance of a heavy-duty truck powertrain by exploiting the eigenfrequency veering property of the CPVA. The shuffle-mode is excited by the half-order torque of a five-cylinder engine in a downspeeded powertrain during cruising conditions. It is shown that the design of the CPVA may be performed by linear analysis of the CPVA and powertrain system. The linear design is then validated by full nonlinear simulations of the powertrain. A complete suppression of the shuffle-mode resonance is achieved with relatively small total mass of the pendula which makes it highly possible to implement in practice. Downspeeding and downsizing of combustion engines, without sacrificing the power output, are methods to reduce emissions, although, resulting in increased noise and vibration using conventional vibration reduction technology. The CPVA is an order tuned device that can help to reduce the increased vibrations and thus help to reduce the environmental impact of heavy-duty vehicles.
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| 31. |
- Gomez, Erik, et al.
(författare)
-
Torsional vibrations in heavy-truck powertrains with flywheel attached centrifugal pendulum vibration absorbers
- 2022
-
Ingår i: Mechanism and machine theory. - : Elsevier BV. - 0094-114X .- 1873-3999. ; 167
-
Tidskriftsartikel (refereegranskat)abstract
- A nonlinear centrifugal pendulum vibration absorber (CPVA) with normal-force dependant friction loss is investigated in a torsional model of a downspeeded powertrain of a heavy-truck. The engine model includes gas-pressure excitation and the existing pendulum model is extended to include a continuous formulation of end-stops at the end of the pendulum-path. Furthermore, the friction loss of the pendulum is experimentally determined. A pendulum-path parameter-study in the complete powertrain model is conducted to consider the effects of the system dynamics on the CPVA. It is shown that the performance of the CPVA is affected by the powertrain system-dynamics and thus important to consider in the design of the CPVA. Downspeeding of the engine by appropriate gearing of the driveline is a measure to decrease the CO2 emissions. However, downspeeding increases the torsional vibration and noise of the powertrain with conventional torsional vibration reduction methods. The CPVA can be used to reduce the torsional vibration and thus facilitate to reach environmental goals.
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| 32. |
- Hoeijmakers, Maarten, et al.
(författare)
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Accuracy assessment of thermoacoustic instability models using binary classification
- 2013
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Ingår i: International Journal of Spray and Combustion Dynamics. - : SAGE Publications. - 1756-8277 .- 1756-8285. ; 5:3, s. 201-223
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Tidskriftsartikel (refereegranskat)abstract
- We apply binary classification theory to assess the (in)stability prediction accuracy of thermoacoustic models. It is shown that by applying such methods to compare a large set of stability predictions and experiments it is possible to gain valuable qualitative insight in different aspects of prediction quality. The approach is illustrated with a 2-port model and a large experimental data set. The presented framework provides an unified and practical tool to answer questions such as (i) What is the chance that a stable prediction will be correct? and (ii) How conservative is the model? It is shown that the most suitable quality indicator is strongly dependent on the actual purpose of the model. The method provides a solid starting point for model comparison and optimization.
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| 33. |
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| 34. |
- Hoeijmakers, Maarten, et al.
(författare)
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Flame dominated thermoacoustic instabilities in a system with high acoustic losses
- 2016
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Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 169, s. 209-215
-
Tidskriftsartikel (refereegranskat)abstract
- The thermoacoustic stability behaviour of a flame is experimentally investigated in the presence of large acoustic losses. Recently it has become clear that under such conditions an instability can occur due to an intrinsic local feedbackloop at the heat source. The experimental results confirm that despite significant acoustic losses, thermoacoustic instabilities can still be present. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 35. |
- Hoeijmakers, Maarten, et al.
(författare)
-
Flames in context of thermo-acoustic stability bounds
- 2015
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 35, s. 1073-1078
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Tidskriftsartikel (refereegranskat)abstract
- Bounds are derived for the acoustic losses such that a thermoacoustic system with a given flame can be guaranteed to be stable. The analysis is based on the flame's acoustic input-to-output properties represented by its scattering matrix. The developed analytical and numerical techniques allow estimating the maximum reflection coefficients (equivalently - acoustic losses) which are sufficient to ensure stable operation of a given burner. It is shown that the calculated numerical upper-bound is less conservative than the analytical one. The frequency dependence of the required acoustic losses provides (i) a thermo-acoustic signature of the flame and (ii) guidelines for the proper design of the up-and downstream acoustics from the flame. The method is illustrated on two burners/flames of premixed multiple Bunsen type. The frequency dependence of the upper bounds allows to identify those frequency ranges where the flame is more likely to cause instability of the complete system.
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| 36. |
- Hoeijmakers, Maarten, et al.
(författare)
-
Intrinsic instability of flame-acoustic coupling
- 2014
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 161:11, s. 2860-2867
-
Tidskriftsartikel (refereegranskat)abstract
- This paper shows that a flame can be an intrinsically unstable acoustic element. The finding is clarified in the framework of an acoustic network model, where the flame is described by an acoustic scattering matrix. The instability of the flame acoustic coupling is shown to become dominating in the limit of no acoustic reflections. This is in contrast to classical standing-wave thermoacoustic modes, which originate from the positive feedback loop between system acoustics and the flame. These findings imply that the effectiveness of passive thermoacoustic damping devices is limited by the intrinsic stability properties of the flame.
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| 37. |
- Hosseini, N., et al.
(författare)
-
Development of a numerical model for obtaining flame transfer function in a simplified slit burner with heat exchanger
- 2014
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Ingår i: 21st International Congress on Sound and Vibration 2014, ICSV 2014. ; , s. 1581-1588
-
Konferensbidrag (refereegranskat)abstract
- The goal of the present work is to develop a model to investigate the interactions between a burner and a heat exchanger, looking from a hydrodynamic and thermo-acoustic point of view. Thermo-acoustics of laminar premixed flames in open air have been studied by several researchers in the past decades. However, a burner in open air may behave differently compared to if situated in a combustion chamber enclosure. This could be related to the effect of temperature, the effect of flame shape (e.g. flames impinging on the heat exchanger walls in compact boilers), distribution of the flame on the burner deck, etc. The behavior of a linear array of a multiple Bunsen-type flames is studied on a 2D geometry in a CFD code to simulate the effects of heat exchanger addition on the thermo-acoustics of the flames. The location of the heat exchanger surface is varied for different conditions of power. A step profile velocity perturbation is used for obtaining the response of the flame represented within the flame transfer function approach. The calculated gain and phase of the flame transfer function are compared for different cases as the indicator of the thermo-acoustic behavior. Results show that the main parameter having considerable influence is the burner load and flame shape. The setup to experimentally investigate these effects will be constructed in near future.
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| 38. |
- Hosseini, N., et al.
(författare)
-
Evaluating thermoacoustic properties of heating appliances considering the burner and heat exchanger as acoustically active elements
- 2018
-
Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 191, s. 486-495
-
Tidskriftsartikel (refereegranskat)abstract
- Heat exchangers are an essential constituent part of many combustion systems. The thermoacoustic instability in such systems is a common problem and it has been studied extensively. However, the heat exchanger has not gained much attention in the field of combustion thermoacoustics, leading to a lack of knowledge about the thermoacoustic interactions between the burner and the heat exchanger. In this paper, a modeling approach is introduced to study these interactions in an academic representation of a heating appliance, comprised of a perforated slit burner and a tube heat exchanger. Both elements are considered thermally and acoustically active. A CFD model is used in a two-dimensional domain to simulate the response of the system to small amplitude broadband velocity perturbations. The thermochemical and acoustic coupling between the burner and the heat exchanger is investigated and a method is introduced to decouple their effects and study them separately. The extents to which this method is valid are addressed by varying the distance between the elements. Results show that as long as the flames do not impinge on the heat exchanger surface, a linear network modeling approach can be applied to construct the acoustic response of the composed configuration from the responses of its constituting elements. This approach requires registering the average velocity on a properly chosen intermediate plane between the burner and heat exchanger. Choosing this plane may be to some point difficult, i.e. when the burner and heat exchanger are close and cannot be considered independent. Moreover, when flame impingement occurs, the interactions between the flame and heat exchanger affect their individual thermoacoustic behaviors and the burner plus heat exchanger assembly needs to be considered as one coupled acoustic element. Particularly, flame impingement changes the phase of the heat absorption response of the heat exchanger and it may significantly alter the acoustic properties of the coupled assembly. The physics lying behind the effects of such interactions on the thermoacoustics of the system is discussed. The obtained results signify that a correct stability prediction of an appliance with burner and heat exchangers requires considering active thermoacoustic behavior of both elements as well as their interactions.
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| 39. |
- Hosseini, N., et al.
(författare)
-
Intrinsic thermoacoustic modes and their interplay with acoustic modes in a Rijke burner
- 2018
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Ingår i: International Journal of Spray and Combustion Dynamics. - : Sage Publications. - 1756-8277 .- 1756-8285. ; 10:4, s. 315-325
-
Tidskriftsartikel (refereegranskat)abstract
- The interplays between acoustic and intrinsic modes in a model of a Rijke burner are revealed and their influence on the prediction of thermoacoustic instabilities is demonstrated. To this end, the system is examined for a range of time delays, temperature ratios and reflection coefficients as adjustable parameters. A linear acoustic network model is used and all modes with frequency below the cut-on frequency for non-planar acoustic waves are considered. The results show that when reflection coefficients are reduced, the presence of a pure ITA mode limits the reduction in the growth rate that usually results from a reduction of the reflection coefficients. In certain conditions, the growth rates can even increase by decreasing reflections. As the time delay of the flame and thus the ITA frequency decreases, the acoustic modes couple to and subsequently decouple from the pure ITA modes. These effects cause the maximum growth rate to alternate between the modes. This investigation draws a broad picture of acoustic and intrinsic modes, which is crucial to accurate prediction and interpretation of thermoacoustic instabilities.
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| 40. |
- Hosseini, N., et al.
(författare)
-
Transfer function calculations of segregated elements in a simplified slit burner with heat exchanger
- 2015
-
Ingår i: 22nd International Congress on Sound and Vibration, ICSV 2015. - : International Institute of Acoustics and Vibrations. - 9788888942483
-
Konferensbidrag (refereegranskat)abstract
- A simplified burner-heat exchanger system is numerically modelled in order to investigate the effects of different elements on the response of the whole system to velocity excitation. We model the system in a 2D CFD code, considering a linear array of multiple Bunsen-type flames with heat exchanger tubes downstream the flames. Thermoacoustic instability is one of the main issues in lean premixed combustion systems, especially domestic boilers. In compact condensing boilers the close distance between the burner surface and the heat exchanger has increased the importance of studying the interactions between the flames and the heat exchanger. The elements corresponding to the heat balance in the system are the flame as heat source and burner deck and heat exchanger as heat sinks. We use both transfer function and transfer matrix approaches to identify the response of these elements to a step function excitation of velocity at the inlet of the domain. Steady-state simulations show that the contribution of the burner deck to the heat balance of the whole system is negligible, leaving the flame and heat exchanger as main contributors to the response of the system. We separately investigate the behavior of these two elements by modeling cases with flame only and heat exchanger only. Then we calculate the behavior of the combined system and compare it to the results of modeling a case with flame and heat exchanger together. These results show that, assuming linear behavior of the elements, it is possible to predict the system behavior via its constructing elements. Further investigations of the effects of other parameters and the limits, within which the assumptions are valid, are currently in progress.
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| 41. |
- Jerrelind, Jenny, et al.
(författare)
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Effects of non-linear wheel suspension bushing on vehicle response
- 2012
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Ingår i: Proceedings of the ASME Design Engineering Technical Conferences And Computers And Information In Engineering Conference, Vol 6. - : ASME Press. - 9780791845059 ; , s. 615-622
-
Konferensbidrag (refereegranskat)abstract
- This work presents an analysis of the effects of non-linear characteristics of a top mount bushing in the wheel suspension of a vehicle when evaluating vehicle characteristics such as comfort and handling. The investigation is performed by comparing simulation results from a quarter car model when using a non-linear bushing model and an approximated linear bushing model. It is revealed when analysing the results that there are differences in the response when comparing measures such as sprung mass acceleration, rattle space ratio and tyre-ground contact force. The conclusion is that the more detailed bushing model mainly affects the acceleration levels especially at high frequencies where the linear model underestimates the acceleration. The rattle space ratio and tyre-ground contact force are also affected but not to the same extent.
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| 42. |
- Kojourimanesh, Mohammad, et al.
(författare)
-
Evaluation of the thermo-acoustic instability frequency and growth rate via input reflection coefficient measurement for central heating equipment
- 2024
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 263
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental measurement methods and theoretical evaluations based on low-order modeling approaches for both the growth rate and frequency at the onset of thermo-acoustic combustion instability are proposed, and their performance is evaluated. The developed techniques are demonstrated through a systematic measurement of the linear growth rate and frequency of evolving oscillations in the laboratory setup and also applied for the thermo-acoustic qualification of an industrial domestic boiler and a heat cell unit (combination of a burner with a heat-exchanger). Generic measurements have been done for a burner deck with premixed surface-stabilized Bunsen-type flames. The industrial domestic boiler and the heat cell unit are equipped with burners of a similar type but differ by their perforation pattern. They have been tested at different conditions and the experimental and theoretical results are compared. Two modeling strategies are tested: 1- in Laplace domain, with estimating a rational function in the complex domain to fit the measured frequency response, 2- exclusively in frequency domain, without estimating a rational function. Both methods include measurement of the frequency response of two reflection coefficients from i) the upstream part of the system, Rup, and ii) burner with flame completed by the downstream part of the appliance, Rin. Within the first approach, a procedure for an analytic continuation of the measured frequency response to the complex domain is applied and complex eigenfrequencies are calculated by solving the corresponding dispersion equation. An alternative approach was proposed by Kopitz and Polifke and allows estimating both the frequency of oscillation and the growth rate from the analysis of the polar plot of the system's characteristic equation in the frequency domain. The comparison shows that the unstable frequencies can be predicted accurately by both tested modeling strategies. This conclusion holds also for the tested industrial applications. The prediction of the instability growth rates is closer to the measured one when the modeling method in the complex domain is used. However, the frequency domain analysis provides less accurate, but still reasonable estimates of the growth rates and frequencies. Moreover, a good overview of thermo-acoustic performance of each industrial boiler/burner at different conditions is obtained via Rin measurements.
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| 43. |
- Kojourimanesh, Mohammad, et al.
(författare)
-
Intrinsic thermo-acoustic instability criteria based on frequency response of flame transfer function
- 2022
-
Ingår i: Internoise 2022. - : The Institute of Noise Control Engineering of the USA, Inc..
-
Konferensbidrag (refereegranskat)abstract
- A study of Intrinsic Thermo-Acoustic (ITA) instability behavior of flames anchored to a burner deck is performed by introducing a mapping between the Flame Transfer Function, FTF(s), defined in the complex (Laplace) domain and the experimentally measured Flame Frequency Response, FFR(iω). The conventional approach requires a system identification procedure to obtain the FTF(s) from the measured FFR(iω). Next, root-finding techniques are applied to define the complex eigenfrequencies. The common practice is to fit the FTF(s) by a rational function that may lead to artifacts like spurious poles and zeros. The purpose of the present work is to establish instability criteria which are directly applicable in the frequency domain. The particular case is considered where the acoustic boundary conditions at both sides of the flame are anechoic. Therefore, the pure ITA mode is treated. First, the causality of the measured FFR(iω) is checked. Then, the criteria of the ITA mode instability applicable to the FFR(iω) phase and magnitude, are derived. Causality properties are used to find the unstable frequency, growth rate, and even the maximum possible value of the linear growth rate. In addition, a procedure is explained to reconstruct the flame transfer function in the complex plane s from the measured flame frequency response which could be an alternative method to study the FTF behavior in the complex domain instead of its estimation with a rational function.
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| 44. |
- Kojourimanesh, M., et al.
(författare)
-
Mobius transformation between reflection coefficients at upstream and downstream sides of flame in thermoacoustics systems
- 2021
-
Ingår i: "Advances in Acoustics, Noise and Vibration - 2021" Proceedings of the 27th International Congress on Sound and Vibration, ICSV 2021. - : Silesian University Press.
-
Konferensbidrag (refereegranskat)abstract
- An alternative approach to assess the conditions for the onset of thermoacoustics combustion instability is proposed. The method is based on the analysis of the relation between the reflection coefficient of combustor upstream section (subsystems) with respect to the burner/flame, Rup, and the input reflection coefficient, Rin, as observed from the inlet of the burner with flame and all downstream subsystems of a combustion appliance. The instability of the combustor can be determined by evaluating the system dispersion relation expressed in the terms of these two reflection coefficients. The properties of the relation between Rin, the reflection coefficient of the burner downstream subsystems, Rdn, and elements of the flame transfer matrix, TM, of the burner are investigated. This relation has the form of the well-known Mobius transformation. Using the well-developed theory of the transformation, we derive the necessary conditions of Rdn to ensure that the magnitude of Rin becomes less than 1 in a frequency range. This condition results in a passive thermoacoustics stability of the system's operation. Furthermore, an optimum value of Rdn is derived which provides a minimum value of Rin at given entries of the burner TM. The practical application of the developed theory provides suitable criteria and guidelines for designing passive acoustic properties at the downstream side of the combustion appliance.
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| 45. |
- Kojourimanesh, Mohammad, et al.
(författare)
-
Stability criteria of two-port networks, application to thermo-acoustic systems
- 2022
-
Ingår i: INTERNATIONAL JOURNAL OF SPRAY AND COMBUSTION DYNAMICS. - : SAGE Publications. - 1756-8277 .- 1756-8285. ; 14:1-2, s. 82-97
-
Tidskriftsartikel (refereegranskat)abstract
- System theory methods are developed and applied to introduce a new analysis methodology based on the stability criteria of active two-ports, to the problem of thermo-acoustic instability in a combustion appliance. The analogy between thermo-acoustics of combustion and small-signal operation of microwave amplifiers is utilized. Notions of unconditional and conditional stabilities of an (active) two-port, representing a burner with flame, are introduced and analyzed. Unconditional stability of two-port means the absence of autonomous oscillation at any embedding of the given two-port by any passive network at the system's upstream (source) and downstream (load) sides. It has been shown that for velocity-sensitive compact burners in the limit of zero Mach number, the criteria of unconditional stability cannot be fulfilled. The analysis is performed in the spirit of a known criterion in microwave network theory, the so-called Edwards-Sinsky's criterion. Therefore, two methods have been applied to elucidate the necessary and sufficient conditions of a linear active two-port system to be conditionally stable. The first method is a new algebraic technique to prove and derive the conditional and unconditional stability criteria, and the second method is based on certain properties of Mobius (bilinear) transformations for combinations of reflection coefficients and scattering matrix of (active) two-ports. The developed framework allows formulating design requirements for the stabilization of operation of a combustion appliance via purposeful modifications of either the burner properties or/and of its acoustic embeddings. The analytical derivations have been examined in a case study to show the power of the methodology in the thermo-acoustics system application.
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| 46. |
- Kojourimanesh, M., et al.
(författare)
-
Thermo-acoustic flame instability criteria based on upstream reflection coefficients
- 2021
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 225, s. 435-443
-
Tidskriftsartikel (refereegranskat)abstract
- A prospective method to assess thermo-acoustic instabilities based on two reflection coefficients measured from the upstream side of the burner is presented and experimentally validated. In order to compose a model which allows predicting the onset of thermo-acoustic instability of combustion in a practical appliance, one has to characterize the thermo-acoustic properties of the burner including the flame as an acoustically active element and acoustic properties of all other (usually passive) components of the combustion appliance both upstream as well as downstream of the burner. This kind of modeling strategy usually faces serious practical problems related to the need of measurements/modeling at the hot downstream part of the system. In the present work, we propose a measurement and a system modeling approach which relies on two acoustic measurements, namely reflection coefficients, only at the cold (burner upstream) part of the combustion appliance. Both reflection coefficients, termed upstream and input, can be readily measured using standard acoustic techniques. The need to measure the input reflection coefficient of an acoustically active subsystem may impose difficulties related to the acoustic instability of the measurement setup itself. The approach and technical solution to handle this problem via a special modification of the excitation source (loudspeaker box) is proposed. The dispersion relation to search for system eigen frequencies is represented in a form that couples the reflection coefficients of the upstream part of the appliance and input reflection coefficient from the downstream part as observed through the burner with flame. This form of the dispersion relation is commonly used in the theory of radio-frequency circuits and recently introduced for thermo-acoustic problems. The proposed method is applied to burners with premixed burner-stabilized Bunsen-type flames. The observed instability conditions and oscillation frequencies are compared with predictions of the proposed modeling approach and reveal good correspondence.
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| 47. |
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| 48. |
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| 49. |
- Liu, Hao, 1986-
(författare)
-
Wave Modelling Techniques for Medium and High Frequency Vibroacoustic Analysis Including Porous Materials
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical methods based on wave modelling are explored for the vibroacoustic analysis of wave propagation, sound transmission and interior noise in vehicles and buildings at medium and high frequencies. The presence of sound absorbing porous materials in practical engineering structures is also considered. The wave modelling techniques provide computational efficiency and physical insight, and two such methods having these advantages are developed in this thesis namely: the semi-analytical finite element method and the wave expansion method.The semi-analytical finite element method is applicable to structures which have constant properties in one direction, and it uses a finite element discretization of the cross-section and analytical functions in the third direction. Equations of motion are derived from this method to study wave propagation characteristics, which help understand the vibroacoustic behavior of structures. These characteristics may also be used by high frequency techniques, such as statistical energy analysis. The wave propagation in sandwich panels with a poroelastic core, which is modeled with Biot's theory, is investigated thoroughly.The semi-analytical finite element method retains the flexibility of the finite element method on geometry and also dramatically increases the computational speed thanks to the orthogonality of the analytical functions when used to calculate forced response. The calculated response of partitions is integrated into diffuse field sound transmission loss calculations of, for example, built-up train floor partitions and multilayer panels lined with porous materials. The calculations are computationally efficient and show good agreement with measurements, thus it is interesting for industrial optimizations.The wave expansion method uses a priori defined plane wave solutions to the Helmholtz equation for approximation of the sound field in geometrically complex enclosures. It reduces the requirements regarding the number of degrees of freedom compared to the finite element method, which, furthermore, is polluted by dispersion errors. Therefore, the wave expansion method is particularly appealing for high frequency (or large wavenumber) calculations. Its application in interior sound field predictions is assessed within the automobile context.
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| 50. |
- Liu, Hao, et al.
(författare)
-
Wave propagation in sandwich panels with a poroelastic core
- 2014
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Ingår i: Journal of the Acoustical Society of America. - : Acoustical Society of America (ASA). - 0001-4966 .- 1520-8524. ; 135:5, s. 2683-2693
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Tidskriftsartikel (refereegranskat)abstract
- Wave propagation in sandwich panels with a poroelastic core, which is modeled by Biot's theory, is investigated using the waveguide finite element method. A waveguide poroelastic element is developed based on a displacement-pressure weak form. The dispersion curves of the sandwich panel are first identified as propagating or evanescent waves by varying the damping in the panel, and wave characteristics are analyzed by examining their motions. The energy distributions are calculated to identify the dominant motions. Simplified analytical models are also devised to show the main physics of the corresponding waves. This wave propagation analysis provides insight into the vibro-acoustic behavior of sandwich panels lined with elastic porous materials.
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