david valentin - Academia.edu (original) (raw)

Papers by david valentin

Journal of the Mechanical Behavior of Biomedical Materials

IOP Conference Series: Earth and Environmental Science

Erosive cavitation in hydraulic turbines affects severely the runner structure, increasing mainte... more Erosive cavitation in hydraulic turbines affects severely the runner structure, increasing maintenance costs and reducing the remaining useful life of the component. Actual vibro-acoustic techniques used to detect this kind of cavitation in hydraulic turbines are based on analysing high-frequency vibrations in different parts of the unit. Particularly, the demodulation of high frequency bands has proven to give relevant results regarding the erosive cavitation behaviour. However, demodulation is not absolutely conclusive since the excitation and the transfer function from the excitation to the measuring point, which depend on every particular prototype, are partially unknown. In this paper, the demodulation method to detect erosive cavitation in hydraulic turbines is reviewed and analysed in detail. To do so, first, an experimental study in a test rig in laboratory has been carried out. This test rig is based on a rotating disk instrumented with a piezoelectric patch and with different sensors, such as accelerometers and acoustic emission sensors, in the rotating and stationary parts. Different excitation patterns simulating erosive cavitation have been applied to the rotating disk. These patterns include pseudo-random excitations of different frequency bands modulated by one low carrier frequency, which model the erosive cavitation characteristics. In this way, it is possible to understand how mechanical vibrations similar to those produced by erosive cavitation are transmitted in such complex systems involving fluid and solid mediums. The knowledge obtained in the test rig helps to interpret the results obtained with demodulation analysis in prototypes. Particularly, in this paper these conclusions have been used to analyse erosive cavitation in a real Francis turbine.

Sensors (Basel, Switzerland), Jan 12, 2018

Submerged systems are found in many engineering, biological, and medicinal applications. For such... more Submerged systems are found in many engineering, biological, and medicinal applications. For such systems, due to the particular environmental conditions and working medium, the research on the mechanical and structural properties at every scale (from macroscopic to nanoscopic), and the control of the system dynamics and induced effects become very difficult tasks. For such purposes in submerged systems, piezoelectric patches (PZTp), which are light, small and economic, have been proved to be a very good solution. PZTp have been recently used as sensors/actuators for applications such as modal analysis, active sound and vibration control, energy harvesting and atomic force microscopes in submerged systems. As a consequence, in these applications, newly developed transducers based on PZTp have become the most used ones, which has improved the state of the art and methods used in these fields. This review paper carefully analyzes and summarizes these applications particularized to sub...

The Proceedings of the Space Engineering Conference

Journal of Fluids and Structures

Please check your proof carefully and mark all corrections at the appropriate place in the proof ... more Please check your proof carefully and mark all corrections at the appropriate place in the proof (e.g., by using on-screen annotation in the PDF file) or compile them in a separate list. Note: if you opt to annotate the file with software other than Adobe Reader then please also highlight the appropriate place in the PDF file. To ensure fast publication of your paper please return your corrections within 48 hours.

Journal of Physics: Conference Series

Francis turbines operate in many cases out of its best efficiency point, in order to regulate the... more Francis turbines operate in many cases out of its best efficiency point, in order to regulate their output power according to the instantaneous energy demand of the grid. Therefore, it is of paramount importance to analyse and determine the unstable operating points for these kind of units. In the framework of the HYPERBOLE project (FP7-ENERGY-2013-1; Project number 608532) a large Francis unit was investigated numerically, experimentally in a reduced scale model and also experimentally and numerically in the real prototype. This paper shows the unstable operating points identified during the experimental tests on the real Francis unit and the analysis of the main characteristics of these instabilities. Finally, it is shown that similar phenomena have been identified on previous research in the LMH (Laboratory for Hydraulic Machines, Lausanne) with the reduced scale model.

Engineering Failure Analysis

In this paper, an uncommon failure of a Pelton turbine has been analyzed. After the monitoring sy... more In this paper, an uncommon failure of a Pelton turbine has been analyzed. After the monitoring system detected a sudden increase in the vibration levels, the turbine was inspected. The inspection showed that a fragment of one bucket broke off during operation. Moreover there were several buckets with cracks, always located in the same side of the buckets. An analysis of the detached fragment revealed a fatigue problem. After the damage was found, the vibration signatures measured by the monitoring system before damage, with damage and after repair, were analyzed. Before damage occurred, an excessive axial vibration and the excitation of several natural frequencies of the turbine were detected in the measured vibration.

IOP Conference Series: Earth and Environmental Science, 2014

The dynamic response of submerged and confined disk-like structures is of interest in the flied o... more The dynamic response of submerged and confined disk-like structures is of interest in the flied of hydraulic machinery, especially in hydraulic turbine runners. This response is difficult to be estimated with accuracy due to the strong influence of the boundary conditions. Small radial gaps as well as short axial distances to rigid surfaces greatly modify the dynamic response because the fact of the added mass and damping effects. Moreover, the effect of the shaft coupling is also important for certain mode-shapes of the structure. In the present study, the influence of the added mass effect and boundary conditions on the dynamic behavior of a submerged disk attached to a shaft is evaluated through experimental tests and structuralacoustic coupling numerical simulations. For the experimentation, a test rig has been developed. It consists of a confined disk attached to a shaft inside a cylindrical container full of water. The disk can be fixed at different axial positions along the shaft. Piezoelectric patches are used to excite the disk and the response is measured with submersible accelerometers. For each configuration tested, the natural frequencies of the disk and the shaft are studied. Numerical results have been compared with experimental results. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

To avoid vibration problems, natural frequencies and mode-shapes of hydraulic impellers when inst... more To avoid vibration problems, natural frequencies and mode-shapes of hydraulic impellers when installed in the machine have to be known. To determine with good precision those natural frequencies is very important to well understand the influence of actual boundary conditions. Uncertainties in experimental and numerical results are frequently derived from parts of the runner that are very close to rigid walls (like hydraulic seals). The objective of this work is to determine the influence of small gaps in radial direction in the natural frequency of a submerged disk. This influence has been studied from an experimental and a numerically point of view. The experimental setup consisted in a disk inside a cylindrical container with water both made of stainless steel. Several experiments were carried out changing the gap values between disk and container in axial and in radial direction. In each case different tests were carried out changing the water height. Piezoelectric patches were u...

Mechanical Systems and Signal Processing, 2015

To avoid resonance problems in rotating turbomachinery components such as impellers, it is of par... more To avoid resonance problems in rotating turbomachinery components such as impellers, it is of paramount importance to determine the natural frequencies of these parts when they are under operation. Nevertheless, most of these rotating structures are inaccessible and in some cases submerged and confined. To measure the natural frequencies of submerged impellers from the rotating frame is complicated, because sensors have to be well fixed, withstand with large pressure and centrifugal forces. Furthermore, the signals have to be transmitted to the stationary frame. For this reason it may be advantageous to measure the natural frequencies with sensors placed on the casing. In this paper, the analysis of rotating disk-like structures submerged and confined has been performed from the stationary frame. Previously, an analytical model to determine the natural frequencies and mode shapes of the disk from the rotating frame is presented. Once natural frequencies and mode shapes are obtained in the rotating frame, the transmission to the stationary frame has been deduced. A rotating disk test rig has been used for the experimental study. It consist of a rotating disk that has been excited from the rotating frame with a piezoelectric patch and it response has been measured from both rotating and stationary frame. Results shows that for rotating submerged structures in heavy fluids such as water, not only the structural modes of the rotating part are different than for rotating structures in air, but also the transmission from the rotating to the stationary frame.

IOP Conference Series: Earth and Environmental Science, 2014

To study the dynamic behavior of turbine runners (natural frequencies and mode shapes) not only t... more To study the dynamic behavior of turbine runners (natural frequencies and mode shapes) not only the added mass effect of still water has to be considered. Also the effect of rotation may not be neglected in the dynamic response. In the present study, the dynamic behavior of a rotating disk submerged in water is studied. For this purpose an experimental test rig has been developed. It consists of a rotating disk submerged in water that can be excited and its response can be measured from the rotating system by a slip ring system. For the excitation an impact device installed on the casing has been used. The response is measured with miniature accelerometers screwed on the disk. The influence of rotation on the dynamic response has been determined experimentally.

Journal of Fluids and Structures, 2014

Journal of Sound and Vibration, 2015

Sensors, 2014

In this paper, PZT actuators are used to study the dynamic behavior of a rotating disk structure ... more In this paper, PZT actuators are used to study the dynamic behavior of a rotating disk structure due to rotor-stator interaction excitation. The disk is studied with two different surrounding fluids-air and water. The study has been performed analytically and validated experimentally. For the theoretical analysis, the natural frequencies and the associated mode shapes of the rotating disk in air and water are obtained with the Kirchhoff-Love thin plate theory coupled with the interaction with the surrounding fluid. A model for the Rotor Stator Interaction that occurs in many rotating disk-like parts of turbomachinery such as compressors, hydraulic runners or alternators is presented. The dynamic behavior of the rotating disk due to this excitation is deduced. For the experimental analysis a test rig has been developed. It consists of a stainless steel disk (r = 198 mm and h = 8 mm) connected to a variable speed motor. Excitation and response are measured from the rotating system. For the rotating excitation four piezoelectric patches have been used. Calibrating the piezoelectric patches in amplitude and phase, different rotating excitation patterns are applied on the rotating disk in air and in water. Results show the feasibility of using PZT to control the response of the disk due to a rotor-stator interaction.

Energy and Buildings, 2013

ABSTRACT Fans are the most widespread system used to implement forced convection ventilation for ... more ABSTRACT Fans are the most widespread system used to implement forced convection ventilation for double glazed façades (DGF). However, the implementation of fans in a façade require an electrical supply for the motors, solid supports for the fans, and the installation has to meet fire safety regulations required by local laws. These facts, added to the need of a regular maintenance program for the fans and the possibility of noise and vibration generated by the moving parts of these equipment, might increase the final cost of the façade and reduce the comfort inside the building.In this paper the feasibility of using nozzles for DGF ventilation is evaluated. The nozzles selected for this study are based on the Coandă effect. For this investigation, a CFD model was used to simulate a Coandă nozzle. The modeled nozzle was simulated for different flow rate conditions and velocity and pressure fields obtained in the nozzle outlet were imposed as an inlet boundary condition in an upper crossed lateral ventilation model for a DGF. Results obtained for heat flux, and reductions in solar gain loads for different operating conditions were obtained and compared against previous results for vertical and horizontal DGF ventilation in a similar geometry.

Energies, 2013

Free convection is the most often used method in order to reduce solar load gains on a building w... more Free convection is the most often used method in order to reduce solar load gains on a building with double glazed façades (DGFs). However, depending on the climate factors, the thermal performance of a DGF may not be satisfactory and extra energy costs are required to obtain suitable comfort conditions inside the building. Forced ventilation systems are a feasible alternative to improve the thermal performance of a DGF in Mediterranean climates where large solar gains are a permanent condition throughout the year. In this paper the feasibility of using diverse forced ventilation methods in DGF is evaluated. In addition, an economical comparison between different mechanical ventilation systems was performed in order to demonstrate the viability of DGF forced ventilation. Moreover, an environmental study was carried out to prove the positive energetic balance on cooling loads between free and forced convection in DGF for Mediterranean climates. For this investigation, a CFD model was used to simulate the thermal conditions in a DGF for the different ventilation systems. Results obtained for heat flux, temperature and reductions in solar load gains were analyzed and applied for the economic and environmental research.

Latin American Theatre Review, 1979

Journal of the Mechanical Behavior of Biomedical Materials

IOP Conference Series: Earth and Environmental Science

Erosive cavitation in hydraulic turbines affects severely the runner structure, increasing mainte... more Erosive cavitation in hydraulic turbines affects severely the runner structure, increasing maintenance costs and reducing the remaining useful life of the component. Actual vibro-acoustic techniques used to detect this kind of cavitation in hydraulic turbines are based on analysing high-frequency vibrations in different parts of the unit. Particularly, the demodulation of high frequency bands has proven to give relevant results regarding the erosive cavitation behaviour. However, demodulation is not absolutely conclusive since the excitation and the transfer function from the excitation to the measuring point, which depend on every particular prototype, are partially unknown. In this paper, the demodulation method to detect erosive cavitation in hydraulic turbines is reviewed and analysed in detail. To do so, first, an experimental study in a test rig in laboratory has been carried out. This test rig is based on a rotating disk instrumented with a piezoelectric patch and with different sensors, such as accelerometers and acoustic emission sensors, in the rotating and stationary parts. Different excitation patterns simulating erosive cavitation have been applied to the rotating disk. These patterns include pseudo-random excitations of different frequency bands modulated by one low carrier frequency, which model the erosive cavitation characteristics. In this way, it is possible to understand how mechanical vibrations similar to those produced by erosive cavitation are transmitted in such complex systems involving fluid and solid mediums. The knowledge obtained in the test rig helps to interpret the results obtained with demodulation analysis in prototypes. Particularly, in this paper these conclusions have been used to analyse erosive cavitation in a real Francis turbine.

Sensors (Basel, Switzerland), Jan 12, 2018

Submerged systems are found in many engineering, biological, and medicinal applications. For such... more Submerged systems are found in many engineering, biological, and medicinal applications. For such systems, due to the particular environmental conditions and working medium, the research on the mechanical and structural properties at every scale (from macroscopic to nanoscopic), and the control of the system dynamics and induced effects become very difficult tasks. For such purposes in submerged systems, piezoelectric patches (PZTp), which are light, small and economic, have been proved to be a very good solution. PZTp have been recently used as sensors/actuators for applications such as modal analysis, active sound and vibration control, energy harvesting and atomic force microscopes in submerged systems. As a consequence, in these applications, newly developed transducers based on PZTp have become the most used ones, which has improved the state of the art and methods used in these fields. This review paper carefully analyzes and summarizes these applications particularized to sub...

The Proceedings of the Space Engineering Conference

Journal of Fluids and Structures

Please check your proof carefully and mark all corrections at the appropriate place in the proof ... more Please check your proof carefully and mark all corrections at the appropriate place in the proof (e.g., by using on-screen annotation in the PDF file) or compile them in a separate list. Note: if you opt to annotate the file with software other than Adobe Reader then please also highlight the appropriate place in the PDF file. To ensure fast publication of your paper please return your corrections within 48 hours.

Journal of Physics: Conference Series

Francis turbines operate in many cases out of its best efficiency point, in order to regulate the... more Francis turbines operate in many cases out of its best efficiency point, in order to regulate their output power according to the instantaneous energy demand of the grid. Therefore, it is of paramount importance to analyse and determine the unstable operating points for these kind of units. In the framework of the HYPERBOLE project (FP7-ENERGY-2013-1; Project number 608532) a large Francis unit was investigated numerically, experimentally in a reduced scale model and also experimentally and numerically in the real prototype. This paper shows the unstable operating points identified during the experimental tests on the real Francis unit and the analysis of the main characteristics of these instabilities. Finally, it is shown that similar phenomena have been identified on previous research in the LMH (Laboratory for Hydraulic Machines, Lausanne) with the reduced scale model.

Engineering Failure Analysis

In this paper, an uncommon failure of a Pelton turbine has been analyzed. After the monitoring sy... more In this paper, an uncommon failure of a Pelton turbine has been analyzed. After the monitoring system detected a sudden increase in the vibration levels, the turbine was inspected. The inspection showed that a fragment of one bucket broke off during operation. Moreover there were several buckets with cracks, always located in the same side of the buckets. An analysis of the detached fragment revealed a fatigue problem. After the damage was found, the vibration signatures measured by the monitoring system before damage, with damage and after repair, were analyzed. Before damage occurred, an excessive axial vibration and the excitation of several natural frequencies of the turbine were detected in the measured vibration.

IOP Conference Series: Earth and Environmental Science, 2014

The dynamic response of submerged and confined disk-like structures is of interest in the flied o... more The dynamic response of submerged and confined disk-like structures is of interest in the flied of hydraulic machinery, especially in hydraulic turbine runners. This response is difficult to be estimated with accuracy due to the strong influence of the boundary conditions. Small radial gaps as well as short axial distances to rigid surfaces greatly modify the dynamic response because the fact of the added mass and damping effects. Moreover, the effect of the shaft coupling is also important for certain mode-shapes of the structure. In the present study, the influence of the added mass effect and boundary conditions on the dynamic behavior of a submerged disk attached to a shaft is evaluated through experimental tests and structuralacoustic coupling numerical simulations. For the experimentation, a test rig has been developed. It consists of a confined disk attached to a shaft inside a cylindrical container full of water. The disk can be fixed at different axial positions along the shaft. Piezoelectric patches are used to excite the disk and the response is measured with submersible accelerometers. For each configuration tested, the natural frequencies of the disk and the shaft are studied. Numerical results have been compared with experimental results. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

To avoid vibration problems, natural frequencies and mode-shapes of hydraulic impellers when inst... more To avoid vibration problems, natural frequencies and mode-shapes of hydraulic impellers when installed in the machine have to be known. To determine with good precision those natural frequencies is very important to well understand the influence of actual boundary conditions. Uncertainties in experimental and numerical results are frequently derived from parts of the runner that are very close to rigid walls (like hydraulic seals). The objective of this work is to determine the influence of small gaps in radial direction in the natural frequency of a submerged disk. This influence has been studied from an experimental and a numerically point of view. The experimental setup consisted in a disk inside a cylindrical container with water both made of stainless steel. Several experiments were carried out changing the gap values between disk and container in axial and in radial direction. In each case different tests were carried out changing the water height. Piezoelectric patches were u...

Mechanical Systems and Signal Processing, 2015

To avoid resonance problems in rotating turbomachinery components such as impellers, it is of par... more To avoid resonance problems in rotating turbomachinery components such as impellers, it is of paramount importance to determine the natural frequencies of these parts when they are under operation. Nevertheless, most of these rotating structures are inaccessible and in some cases submerged and confined. To measure the natural frequencies of submerged impellers from the rotating frame is complicated, because sensors have to be well fixed, withstand with large pressure and centrifugal forces. Furthermore, the signals have to be transmitted to the stationary frame. For this reason it may be advantageous to measure the natural frequencies with sensors placed on the casing. In this paper, the analysis of rotating disk-like structures submerged and confined has been performed from the stationary frame. Previously, an analytical model to determine the natural frequencies and mode shapes of the disk from the rotating frame is presented. Once natural frequencies and mode shapes are obtained in the rotating frame, the transmission to the stationary frame has been deduced. A rotating disk test rig has been used for the experimental study. It consist of a rotating disk that has been excited from the rotating frame with a piezoelectric patch and it response has been measured from both rotating and stationary frame. Results shows that for rotating submerged structures in heavy fluids such as water, not only the structural modes of the rotating part are different than for rotating structures in air, but also the transmission from the rotating to the stationary frame.

IOP Conference Series: Earth and Environmental Science, 2014

To study the dynamic behavior of turbine runners (natural frequencies and mode shapes) not only t... more To study the dynamic behavior of turbine runners (natural frequencies and mode shapes) not only the added mass effect of still water has to be considered. Also the effect of rotation may not be neglected in the dynamic response. In the present study, the dynamic behavior of a rotating disk submerged in water is studied. For this purpose an experimental test rig has been developed. It consists of a rotating disk submerged in water that can be excited and its response can be measured from the rotating system by a slip ring system. For the excitation an impact device installed on the casing has been used. The response is measured with miniature accelerometers screwed on the disk. The influence of rotation on the dynamic response has been determined experimentally.

Journal of Fluids and Structures, 2014

Journal of Sound and Vibration, 2015

Sensors, 2014

In this paper, PZT actuators are used to study the dynamic behavior of a rotating disk structure ... more In this paper, PZT actuators are used to study the dynamic behavior of a rotating disk structure due to rotor-stator interaction excitation. The disk is studied with two different surrounding fluids-air and water. The study has been performed analytically and validated experimentally. For the theoretical analysis, the natural frequencies and the associated mode shapes of the rotating disk in air and water are obtained with the Kirchhoff-Love thin plate theory coupled with the interaction with the surrounding fluid. A model for the Rotor Stator Interaction that occurs in many rotating disk-like parts of turbomachinery such as compressors, hydraulic runners or alternators is presented. The dynamic behavior of the rotating disk due to this excitation is deduced. For the experimental analysis a test rig has been developed. It consists of a stainless steel disk (r = 198 mm and h = 8 mm) connected to a variable speed motor. Excitation and response are measured from the rotating system. For the rotating excitation four piezoelectric patches have been used. Calibrating the piezoelectric patches in amplitude and phase, different rotating excitation patterns are applied on the rotating disk in air and in water. Results show the feasibility of using PZT to control the response of the disk due to a rotor-stator interaction.

Energy and Buildings, 2013

ABSTRACT Fans are the most widespread system used to implement forced convection ventilation for ... more ABSTRACT Fans are the most widespread system used to implement forced convection ventilation for double glazed façades (DGF). However, the implementation of fans in a façade require an electrical supply for the motors, solid supports for the fans, and the installation has to meet fire safety regulations required by local laws. These facts, added to the need of a regular maintenance program for the fans and the possibility of noise and vibration generated by the moving parts of these equipment, might increase the final cost of the façade and reduce the comfort inside the building.In this paper the feasibility of using nozzles for DGF ventilation is evaluated. The nozzles selected for this study are based on the Coandă effect. For this investigation, a CFD model was used to simulate a Coandă nozzle. The modeled nozzle was simulated for different flow rate conditions and velocity and pressure fields obtained in the nozzle outlet were imposed as an inlet boundary condition in an upper crossed lateral ventilation model for a DGF. Results obtained for heat flux, and reductions in solar gain loads for different operating conditions were obtained and compared against previous results for vertical and horizontal DGF ventilation in a similar geometry.

Energies, 2013

Free convection is the most often used method in order to reduce solar load gains on a building w... more Free convection is the most often used method in order to reduce solar load gains on a building with double glazed façades (DGFs). However, depending on the climate factors, the thermal performance of a DGF may not be satisfactory and extra energy costs are required to obtain suitable comfort conditions inside the building. Forced ventilation systems are a feasible alternative to improve the thermal performance of a DGF in Mediterranean climates where large solar gains are a permanent condition throughout the year. In this paper the feasibility of using diverse forced ventilation methods in DGF is evaluated. In addition, an economical comparison between different mechanical ventilation systems was performed in order to demonstrate the viability of DGF forced ventilation. Moreover, an environmental study was carried out to prove the positive energetic balance on cooling loads between free and forced convection in DGF for Mediterranean climates. For this investigation, a CFD model was used to simulate the thermal conditions in a DGF for the different ventilation systems. Results obtained for heat flux, temperature and reductions in solar load gains were analyzed and applied for the economic and environmental research.

Latin American Theatre Review, 1979