http://hdl.handle.net/10985/6694 Sun, 19 Apr 2026 02:01:35 GMT 2026-04-19T02:01:35Z https://sam.ensam.eu:443/bitstream/id/065dd200-9205-4f10-903d-9fb60ce0be00/ http://hdl.handle.net/10985/6694 http://hdl.handle.net/10985/10566 Cavitation in a hydraulic system: The influence of the distributor geometry on cavitation inception and study of the interactions between bubbles ADAMA MAIGA, Mahamadou; BUISINE, Daniel; COUTIER-DELGOSHA, Olivier Hydraulic systems are often subjected to pressure drops, which may lead to cavitation. In systems such as power steering, hoist loads, or ventricular assist devices, distributors are generally used. Significant pressure losses can happen in a distributor due to gap and overlap, which may lead to cavitation development. However, this issue is almost never included in the conception of the distributors. In this study, the multibubble model of the modified Rayleigh–Plesset equation is applied to the rotary distributor of an oil hydraulic system. The influence of the overlap length, the gap, the rotation speed, and distributor inlet pressure on the cavitation and particularly the interactions between bubbles at cavitation inception are studied. The study highlights a critical length of the overlap; over this value, the overlap length influences significantly the cavitation duration and the void fraction. More generally, some geometrical details have a strong influence on cavitation. Optimization of these details in engine parts, taking account the occurrence of cavitation, would be an appropriate solution to reduce its effects. The study also demonstrates that the growth of small bubbles may be delayed by the interactions with the nearby bigger ones, even if the ambient pressure is lower than their theoretical critical pressure. They eventually collapse at the first moments of the cavitation development. However, if the ambient pressure drops further, that is, beyond a critical pressure, a small bubble gains enough inertial energy to overcome these interaction phenomena and thus to grow. The growth of small bubbles increases the interactions between bubbles and slows down the growth of nearby bigger ones. The results show that the interactions between bubbles are of primary importance in the first moments of the cavitation development, which suggests that they should be taken into account in the definition of the critical pressure. Thu, 01 Jan 2015 00:00:00 GMT http://hdl.handle.net/10985/10566 2015-01-01T00:00:00Z ADAMA MAIGA, Mahamadou BUISINE, Daniel COUTIER-DELGOSHA, Olivier Hydraulic systems are often subjected to pressure drops, which may lead to cavitation. In systems such as power steering, hoist loads, or ventricular assist devices, distributors are generally used. Significant pressure losses can happen in a distributor due to gap and overlap, which may lead to cavitation development. However, this issue is almost never included in the conception of the distributors. In this study, the multibubble model of the modified Rayleigh–Plesset equation is applied to the rotary distributor of an oil hydraulic system. The influence of the overlap length, the gap, the rotation speed, and distributor inlet pressure on the cavitation and particularly the interactions between bubbles at cavitation inception are studied. The study highlights a critical length of the overlap; over this value, the overlap length influences significantly the cavitation duration and the void fraction. More generally, some geometrical details have a strong influence on cavitation. Optimization of these details in engine parts, taking account the occurrence of cavitation, would be an appropriate solution to reduce its effects. The study also demonstrates that the growth of small bubbles may be delayed by the interactions with the nearby bigger ones, even if the ambient pressure is lower than their theoretical critical pressure. They eventually collapse at the first moments of the cavitation development. However, if the ambient pressure drops further, that is, beyond a critical pressure, a small bubble gains enough inertial energy to overcome these interaction phenomena and thus to grow. The growth of small bubbles increases the interactions between bubbles and slows down the growth of nearby bigger ones. The results show that the interactions between bubbles are of primary importance in the first moments of the cavitation development, which suggests that they should be taken into account in the definition of the critical pressure. http://hdl.handle.net/10985/15141 A new cavitation model based on bubble-bubble interactions ADAMA MAIGA, Mahamadou; BUISINE, Daniel; COUTIER-DELGOSHA, Olivier In this paper, a new model based on bubble-bubble interactions is proposed for cavitation. Unlike the well-known existing models (Rayleigh-Plesset, Gilmore), which are derived from the local balance equations in the vicinity of a single cavitation bubble, the present approach is based on the mutual interaction between two spherical bubbles of different sizes. The mass and momentum conservation equations, coupled with the local flowdivergence, lead to two equations for the evolution of the bubble radii and one equation for the local pressure. The bubble size variations predicted by the model are found in close agreement with the previous experimental data reported by Ohl [“Cavitation inception following shock wave passage,” Phys. Fluids 14(10), 3512–3521 (2002)]. The distinct radii of bubbles located close to each other, as well as the premature collapse of small bubbles during the initial stage of cavitation inception, are correctly reproduced by the model. The results generally show that bubble/bubble interactions play a primary role in the physics of cavitation inception, which is a preponderant phenomenon in cavitation-induced noise and erosion. The influence of the size of the nuclei on these interactions is discussed. During the expansion phases, the variations in the local flow divergence only slightly affect the growth of the big nuclei, which is mainly governed by their interaction with the neighboring bubbles, while it triggers the expansion of the small nuclei. Conversely, in the compression phase, the behavior of the bubbles is not influenced anymore by the initial size of the nuclei. It is also shown that large amplitude pressure variations resulting from the multiple collapses of small bubbles should be taken into account, in addition to the ambient pressure evolution, to calculate the instantaneous local pressure in the liquid and eventually evaluate the flow aggressiveness and the resulting erosion. Mon, 01 Jan 2018 00:00:00 GMT http://hdl.handle.net/10985/15141 2018-01-01T00:00:00Z ADAMA MAIGA, Mahamadou BUISINE, Daniel COUTIER-DELGOSHA, Olivier In this paper, a new model based on bubble-bubble interactions is proposed for cavitation. Unlike the well-known existing models (Rayleigh-Plesset, Gilmore), which are derived from the local balance equations in the vicinity of a single cavitation bubble, the present approach is based on the mutual interaction between two spherical bubbles of different sizes. The mass and momentum conservation equations, coupled with the local flowdivergence, lead to two equations for the evolution of the bubble radii and one equation for the local pressure. The bubble size variations predicted by the model are found in close agreement with the previous experimental data reported by Ohl [“Cavitation inception following shock wave passage,” Phys. Fluids 14(10), 3512–3521 (2002)]. The distinct radii of bubbles located close to each other, as well as the premature collapse of small bubbles during the initial stage of cavitation inception, are correctly reproduced by the model. The results generally show that bubble/bubble interactions play a primary role in the physics of cavitation inception, which is a preponderant phenomenon in cavitation-induced noise and erosion. The influence of the size of the nuclei on these interactions is discussed. During the expansion phases, the variations in the local flow divergence only slightly affect the growth of the big nuclei, which is mainly governed by their interaction with the neighboring bubbles, while it triggers the expansion of the small nuclei. Conversely, in the compression phase, the behavior of the bubbles is not influenced anymore by the initial size of the nuclei. It is also shown that large amplitude pressure variations resulting from the multiple collapses of small bubbles should be taken into account, in addition to the ambient pressure evolution, to calculate the instantaneous local pressure in the liquid and eventually evaluate the flow aggressiveness and the resulting erosion. http://hdl.handle.net/10985/10568 Interactions by exchange of volume between two spherical bubbles ADAMA MAIGA, Mahamadou; BUISINE, Daniel; COUTIER-DELGOSHA, Olivier In this paper, by using the system potential of two bubbles and with a special interest in the interaction by exchange of volume and without exchange of mass, a system of equations governing the evolution of two bubbles is proposed. This two-bubble model shows terms that do not appear in the models of interaction between bubbles. The twobubble model is compared with the modified Rayleigh–Plesset equation and a validation with the experimental study of Ohl [2002] is presented. The numerical results show that, on one hand, the development of small nearby bubbles can slow down the evolution of the biggest local one, while their disappearance can favor its growing. Furthermore, in the case of two bubbles in particular, the small bubble exchanges volume with the big one during their evolutions. On the other hand, contrary to the modified Rayleigh–Plesset model, the two-bubble model predicts appearance and disappearance of small bubbles in the neighborhood of the big bubble as it is observed in the experimental study of Ohl [2002]. The present findings show in particular that the interaction by exchange of volume can be very important in the cavitation born phase and it is necessary to take into account the interaction between bubbles as well as the disappearance of small ones on the evolution of the biggest local bubble. Also, this two-bubble model predicts an exchange of volume between both bubbles equal to zero when they are perfectly identical. Wed, 01 Jan 2014 00:00:00 GMT http://hdl.handle.net/10985/10568 2014-01-01T00:00:00Z ADAMA MAIGA, Mahamadou BUISINE, Daniel COUTIER-DELGOSHA, Olivier In this paper, by using the system potential of two bubbles and with a special interest in the interaction by exchange of volume and without exchange of mass, a system of equations governing the evolution of two bubbles is proposed. This two-bubble model shows terms that do not appear in the models of interaction between bubbles. The twobubble model is compared with the modified Rayleigh–Plesset equation and a validation with the experimental study of Ohl [2002] is presented. The numerical results show that, on one hand, the development of small nearby bubbles can slow down the evolution of the biggest local one, while their disappearance can favor its growing. Furthermore, in the case of two bubbles in particular, the small bubble exchanges volume with the big one during their evolutions. On the other hand, contrary to the modified Rayleigh–Plesset model, the two-bubble model predicts appearance and disappearance of small bubbles in the neighborhood of the big bubble as it is observed in the experimental study of Ohl [2002]. The present findings show in particular that the interaction by exchange of volume can be very important in the cavitation born phase and it is necessary to take into account the interaction between bubbles as well as the disappearance of small ones on the evolution of the biggest local bubble. Also, this two-bubble model predicts an exchange of volume between both bubbles equal to zero when they are perfectly identical. http://hdl.handle.net/10985/15495 Ventilation rates and thermal comfort assessment in a naturally ventilated classroom ALLAB, Yacine; KINDINIS, Andrea; CAUSONE, Francesco; TATTI, Anita; SIMONET, Sophie; BAYEUL-LAINÉ, Annie-Claude Ventilation systems are meant (i) to guarantee good indoor air quality (IAQ) by providing and distributing fresh air to the occupied/breathing zone and (ii) to dilute and remove pollutants emitted by indoor sources. On the other hand, inadequate ventilation rates can induce discomfort issues and excessive energy consumption. This study focuses on the performance assessment of natural ventilation strategies in university classrooms, which are characterized by a high occupancy level and the necessity to provide high levels of comfort to perform intellectual work. The high occupancy level creates challenging conditions both in terms of internal gains and CO2 concentration. This paper presents an experimental performance assessment of four natural ventilation strategies applied to a university classroom: single side ventilation, cross ventilation, stack ventilation with and without window supply. Each strategy is evaluated in terms of thermal comfort and air change rate measurements. Thermal comfort assessment were performed during occupancy period (physical parameter measurements and questionnaires) whereas air change rate measurements, based on tracer gas techniques, were performed during unoccupied periods. Fri, 01 Jan 2016 00:00:00 GMT http://hdl.handle.net/10985/15495 2016-01-01T00:00:00Z ALLAB, Yacine KINDINIS, Andrea CAUSONE, Francesco TATTI, Anita SIMONET, Sophie BAYEUL-LAINÉ, Annie-Claude Ventilation systems are meant (i) to guarantee good indoor air quality (IAQ) by providing and distributing fresh air to the occupied/breathing zone and (ii) to dilute and remove pollutants emitted by indoor sources. On the other hand, inadequate ventilation rates can induce discomfort issues and excessive energy consumption. This study focuses on the performance assessment of natural ventilation strategies in university classrooms, which are characterized by a high occupancy level and the necessity to provide high levels of comfort to perform intellectual work. The high occupancy level creates challenging conditions both in terms of internal gains and CO2 concentration. This paper presents an experimental performance assessment of four natural ventilation strategies applied to a university classroom: single side ventilation, cross ventilation, stack ventilation with and without window supply. Each strategy is evaluated in terms of thermal comfort and air change rate measurements. Thermal comfort assessment were performed during occupancy period (physical parameter measurements and questionnaires) whereas air change rate measurements, based on tracer gas techniques, were performed during unoccupied periods. http://hdl.handle.net/10985/8951 INDOOR CLIMATE QUALITY ASSESSMENT IN HIGH SCHOOL CLASSROOM : VENTILATION STRATEGIES AND OCCUPANCY MANAGEMENT ALLAB, Yacine; KINDINIS, Andrea; BAYEUL-LAINÉ, Annie-Claude; SIMONET, Sophie; COUTIER-DELGOSHA, Olivier More than a well-being, providing a good Indoor Climate Quality (ICQ) in education buildings is necessary for students’ health and in order to maximize academic results. In the context of a research focusing on hybrid ventilation strategies, this paper presents the results of a first survey on thermal comfort and indoor air quality in high school classrooms; the results will be subsequently used to develop strategies on mixed mode ventilated classrooms. The objects of the study are the classrooms of ESTP, a civil engineering school located in Cachan, a suburban city in southern of Paris, France. The survey lasted one month during the mid-season. The classrooms are equipped with a mechanical ventilation system and with windows, so that various solutions can be tested based on natural, mechanical and hybrid ventilation. After a first period of monitoring and an analysis of initial conditions, ICQ was analyzed in different conditions according to the schedule and the occupancy of the investigated classrooms. Indoor climate quality is assessed using objective (measurement campaign) and subjective survey (questionnaire). The measuring campaign includes measurement of thermal comfort parameters (temperature, mean radiant temperature, air flow velocity, and humidity) and indoor air quality parameters (CO2 concentration, CO concentration and humidity). Thermal comfort is assessed using respectively analytic theory (ISO 7730) and extended PMV for non-air conditioned buildings (adaptive PMV). IAQ (indoor air quality) is evaluated according to the level of CO2 concentration. The questionnaire submitted to the students included 9 indexes for subjective assessment of thermal comfort and indoor air quality perception. It includes questions about sensation/preference of indoor climate quality and the satisfaction about the control of indoor climate. Additional issues are requested for PMV (Predicted Mean Vote) calculation (metabolism, clothing). According to the level of occupancy which characterizes various periods, it is found that the ICQ evolves during a school day. Moreover, the influence of the occupancy level was more significant on thermal comfort, especially in warm conditions. Wed, 01 Jan 2014 00:00:00 GMT http://hdl.handle.net/10985/8951 2014-01-01T00:00:00Z ALLAB, Yacine KINDINIS, Andrea BAYEUL-LAINÉ, Annie-Claude SIMONET, Sophie COUTIER-DELGOSHA, Olivier More than a well-being, providing a good Indoor Climate Quality (ICQ) in education buildings is necessary for students’ health and in order to maximize academic results. In the context of a research focusing on hybrid ventilation strategies, this paper presents the results of a first survey on thermal comfort and indoor air quality in high school classrooms; the results will be subsequently used to develop strategies on mixed mode ventilated classrooms. The objects of the study are the classrooms of ESTP, a civil engineering school located in Cachan, a suburban city in southern of Paris, France. The survey lasted one month during the mid-season. The classrooms are equipped with a mechanical ventilation system and with windows, so that various solutions can be tested based on natural, mechanical and hybrid ventilation. After a first period of monitoring and an analysis of initial conditions, ICQ was analyzed in different conditions according to the schedule and the occupancy of the investigated classrooms. Indoor climate quality is assessed using objective (measurement campaign) and subjective survey (questionnaire). The measuring campaign includes measurement of thermal comfort parameters (temperature, mean radiant temperature, air flow velocity, and humidity) and indoor air quality parameters (CO2 concentration, CO concentration and humidity). Thermal comfort is assessed using respectively analytic theory (ISO 7730) and extended PMV for non-air conditioned buildings (adaptive PMV). IAQ (indoor air quality) is evaluated according to the level of CO2 concentration. The questionnaire submitted to the students included 9 indexes for subjective assessment of thermal comfort and indoor air quality perception. It includes questions about sensation/preference of indoor climate quality and the satisfaction about the control of indoor climate. Additional issues are requested for PMV (Predicted Mean Vote) calculation (metabolism, clothing). According to the level of occupancy which characterizes various periods, it is found that the ICQ evolves during a school day. Moreover, the influence of the occupancy level was more significant on thermal comfort, especially in warm conditions. http://hdl.handle.net/10985/24481 Propagation and rupture of elastoviscoplastic liquid plugs in airway reopening model BAHRANI, S. Amir; HAMIDOUCHE, Souria; MOAZZEN, Masoud; SECK, Khady; DUC, Caroline; MURADOGLU, Metin; GROTBERG, James B.; ROMANO, Francesco The propagation and rupture of mucus plugs in human lungs is investigated experimentally by injecting synthetic mucus in a pre-wetted capillary tube. The rheology of our test liquid is thoroughly characterized, and four samples of synthetic mucus are considered in order to reproduce elastoviscoplastic regimes of physiological interest for airway reopening. Our experiments demonstrate the significant impact of the viscoplasticity and viscoelasticity of mucus. In support to our experiments, we propose a one-dimensional reduced-order model that takes into account capillarity, and elastoviscoplasticity. Our model manages to capture the cross-section averaged dynamics of the liquid plug and is used to elucidate and interpret the experimental evidence. Relying on it, we show that the liquid film thickening due to non-Newtonian effects favors plug rupture, whereas the increase of the effective viscosity due to higher yield stresses hinders plug rupture. As a result of such two effects, increasing the polymeric concentration in the mucus phase leads to a net increase of the rupture time and traveling length. Hence, non-Newtonian effects hinder airway reopening. Tue, 01 Feb 2022 00:00:00 GMT http://hdl.handle.net/10985/24481 2022-02-01T00:00:00Z BAHRANI, S. Amir HAMIDOUCHE, Souria MOAZZEN, Masoud SECK, Khady DUC, Caroline MURADOGLU, Metin GROTBERG, James B. ROMANO, Francesco The propagation and rupture of mucus plugs in human lungs is investigated experimentally by injecting synthetic mucus in a pre-wetted capillary tube. The rheology of our test liquid is thoroughly characterized, and four samples of synthetic mucus are considered in order to reproduce elastoviscoplastic regimes of physiological interest for airway reopening. Our experiments demonstrate the significant impact of the viscoplasticity and viscoelasticity of mucus. In support to our experiments, we propose a one-dimensional reduced-order model that takes into account capillarity, and elastoviscoplasticity. Our model manages to capture the cross-section averaged dynamics of the liquid plug and is used to elucidate and interpret the experimental evidence. Relying on it, we show that the liquid film thickening due to non-Newtonian effects favors plug rupture, whereas the increase of the effective viscosity due to higher yield stresses hinders plug rupture. As a result of such two effects, increasing the polymeric concentration in the mucus phase leads to a net increase of the rupture time and traveling length. Hence, non-Newtonian effects hinder airway reopening. http://hdl.handle.net/10985/22722 Experimental Analysis of an Axial Compressor Operating under Flow Distorsion BARETTER, Alberto; ROUSSETTE, Olivier; ROMANO, Francesco; DAZIN, Antoine; JOSEPH, Pierric In aircraft engines, compressor stages can encounter situations in which the flow is distorted at rotor inlet, for example during particular flight maneuvers, or due to the shape of the inlet of the airframe. The main objective of this paper is to investigate experimentally the effect of an inlet flow distortion on the internal flow dynamic and its consequences on the performance and operability of these machines. The distortion was generated by a porous plate grid installed upstream of the compressor. Eight total pressure rakes placed downstream of the grid were used to evaluate the distortion. Unsteady pressure measurements were performed on the casing at different axial and azimuthal positions to investigate the dynamic flow behavior at nominal conditions, at an operating point close to stall and during the transition to stall. 2D-2C PIV maps, synchronized with the runner position, were obtained on rotor blade-to-blade planes at three different spanwise positions (79%, 51%, 18% of the blade span). Three relative angular positions of the grid to the laser sheet were investigated at two different flow rates, namely the nominal flow rate and a flow rate close to stall. These three positions are corresponding to the laser sheet cutting through i/ the center of the grid’s wake and ii/ the two edges of the grid. The impact of the distortion on the performance of the compressor is analyzed and compared to existing models. The impact of the flow dynamic will also be considered, especially in operations close to the stall limit. Wed, 01 Jun 2022 00:00:00 GMT http://hdl.handle.net/10985/22722 2022-06-01T00:00:00Z BARETTER, Alberto ROUSSETTE, Olivier ROMANO, Francesco DAZIN, Antoine JOSEPH, Pierric In aircraft engines, compressor stages can encounter situations in which the flow is distorted at rotor inlet, for example during particular flight maneuvers, or due to the shape of the inlet of the airframe. The main objective of this paper is to investigate experimentally the effect of an inlet flow distortion on the internal flow dynamic and its consequences on the performance and operability of these machines. The distortion was generated by a porous plate grid installed upstream of the compressor. Eight total pressure rakes placed downstream of the grid were used to evaluate the distortion. Unsteady pressure measurements were performed on the casing at different axial and azimuthal positions to investigate the dynamic flow behavior at nominal conditions, at an operating point close to stall and during the transition to stall. 2D-2C PIV maps, synchronized with the runner position, were obtained on rotor blade-to-blade planes at three different spanwise positions (79%, 51%, 18% of the blade span). Three relative angular positions of the grid to the laser sheet were investigated at two different flow rates, namely the nominal flow rate and a flow rate close to stall. These three positions are corresponding to the laser sheet cutting through i/ the center of the grid’s wake and ii/ the two edges of the grid. The impact of the distortion on the performance of the compressor is analyzed and compared to existing models. The impact of the flow dynamic will also be considered, especially in operations close to the stall limit. http://hdl.handle.net/10985/21615 Experimental and Numerical Analysis of a Compressor Stage under Flow Distortion BARETTER, Alberto; GODARD, Benjamin; ROUSSETTE, Olivier; ROMANÒ, Francesco; BARRIER, Raphael; DAZIN, Antoine; JOSEPH, Pierric On many occasions, fan or compressor stages have to face azimuthal flow distortion at inlet, which affects their performance and stability. These flow distortions can be caused by external events or by some particular geometrical features. The aim of this work is to propose a joined numerical and experimental analysis of the flow behavior in a single axial compressor stage under flow distortion. The distortions are generated by different grids that are placed upstream to the rotor. Experimentally, the flow analysis is based on the measurements obtained by a series of unsteady pressure sensors flush-mounted at the casing of the machine rotor. URANS computations are conducted using the elsA software. The flow distortion is simulated by a drop of stagnation pressure ratio at the inlet boundary condition. The study is focusing first on the ability of a pressure drop, imposed as an inlet boundary condition in CFD, to reproduce accurately the effect of a flow distortion. The analysis is conducted using singular value decomposition (SVD) and dynamic mode decomposition (DMD). A special attention is then paid, on the experimental level, to the arising of rotating stall, from the onset of the instability up to completely developed stall cells. Fri, 01 Jan 2021 00:00:00 GMT http://hdl.handle.net/10985/21615 2021-01-01T00:00:00Z BARETTER, Alberto GODARD, Benjamin ROUSSETTE, Olivier ROMANÒ, Francesco BARRIER, Raphael DAZIN, Antoine JOSEPH, Pierric On many occasions, fan or compressor stages have to face azimuthal flow distortion at inlet, which affects their performance and stability. These flow distortions can be caused by external events or by some particular geometrical features. The aim of this work is to propose a joined numerical and experimental analysis of the flow behavior in a single axial compressor stage under flow distortion. The distortions are generated by different grids that are placed upstream to the rotor. Experimentally, the flow analysis is based on the measurements obtained by a series of unsteady pressure sensors flush-mounted at the casing of the machine rotor. URANS computations are conducted using the elsA software. The flow distortion is simulated by a drop of stagnation pressure ratio at the inlet boundary condition. The study is focusing first on the ability of a pressure drop, imposed as an inlet boundary condition in CFD, to reproduce accurately the effect of a flow distortion. The analysis is conducted using singular value decomposition (SVD) and dynamic mode decomposition (DMD). A special attention is then paid, on the experimental level, to the arising of rotating stall, from the onset of the instability up to completely developed stall cells. http://hdl.handle.net/10985/24441 Experimental and Numerical Analysis of a Compressor Stage under Flow Distortion BARETTER, Alberto; GODARD, Benjamin; JOSEPH, Pierric; ROUSSETTE, Olivier; ROMANO, Francesco; BARRIER, Raphael; DAZIN, Antoine On many occasions, fan or compressor stages have to face azimuthal flow distortion at inlet, which affects their performance and stability. These flow distortions can be caused by external events or by some particular geometrical features. The aim of this work is to propose a joined numerical and experimental analysis of the flow behavior in a single axial compressor stage under flow distortion. The distortions are generated by different grids that are placed upstream to the rotor. Experimentally, the flow analysis is based on the measurements obtained by a series of unsteady pressure sensors flush-mounted at the casing of the machine rotor. URANS computations are conducted using the elsA software. The flow distortion is simulated by a drop of stagnation pressure ratio at the inlet boundary condition. The study is focusing first on the ability of a pressure drop, imposed as an inlet boundary condition in CFD, to reproduce accurately the effect of a flow distortion. The analysis is conducted using singular value decomposition (SVD) and dynamic mode decomposition (DMD). A special attention is then paid, on the experimental level, to the arising of rotating stall, from the onset of the instability up to completely developed stall cells. Mon, 01 Nov 2021 00:00:00 GMT http://hdl.handle.net/10985/24441 2021-11-01T00:00:00Z BARETTER, Alberto GODARD, Benjamin JOSEPH, Pierric ROUSSETTE, Olivier ROMANO, Francesco BARRIER, Raphael DAZIN, Antoine On many occasions, fan or compressor stages have to face azimuthal flow distortion at inlet, which affects their performance and stability. These flow distortions can be caused by external events or by some particular geometrical features. The aim of this work is to propose a joined numerical and experimental analysis of the flow behavior in a single axial compressor stage under flow distortion. The distortions are generated by different grids that are placed upstream to the rotor. Experimentally, the flow analysis is based on the measurements obtained by a series of unsteady pressure sensors flush-mounted at the casing of the machine rotor. URANS computations are conducted using the elsA software. The flow distortion is simulated by a drop of stagnation pressure ratio at the inlet boundary condition. The study is focusing first on the ability of a pressure drop, imposed as an inlet boundary condition in CFD, to reproduce accurately the effect of a flow distortion. The analysis is conducted using singular value decomposition (SVD) and dynamic mode decomposition (DMD). A special attention is then paid, on the experimental level, to the arising of rotating stall, from the onset of the instability up to completely developed stall cells. http://hdl.handle.net/10985/24485 Coherent Particle Structures in High-Prandtl-Number Liquid Bridges BARMAK, Ilya; ROMANO, Francesco; KANNAN, Parvathy Kunchi; KUHLMANN, Hendrik C. Clustering of small rigid spherical particles into particle accumulation structures (PAS) is studied numerically for a high-Prandtl-number (Pr = 68) thermocapillary liquid bridge. The one-way-coupling approach is used for calculation of the particle motion, modeling PAS as an attractor for a single particle. The attractor is created by dissipative forces acting on the particle near the boundary due to the finite size of the particle. These forces can dramatically deflect the particle trajectory from a fluid pathline and transfer it to certain tubular flow structures, called Kolmogorov–Arnold–Moser (KAM) tori, in which the particle is focused and from which it might not escape anymore. The transfer of particles can take place if a KAM torus, which is a property of the flow without particles, enters the narrow boundary layer on the flow boundaries in which the particle experiences extra forces. Since the PAS obtained in this system depends mainly on the finite particle size, it can be classified as a finite-size coherent structure (FSCS). Mon, 01 Feb 2021 00:00:00 GMT http://hdl.handle.net/10985/24485 2021-02-01T00:00:00Z BARMAK, Ilya ROMANO, Francesco KANNAN, Parvathy Kunchi KUHLMANN, Hendrik C. Clustering of small rigid spherical particles into particle accumulation structures (PAS) is studied numerically for a high-Prandtl-number (Pr = 68) thermocapillary liquid bridge. The one-way-coupling approach is used for calculation of the particle motion, modeling PAS as an attractor for a single particle. The attractor is created by dissipative forces acting on the particle near the boundary due to the finite size of the particle. These forces can dramatically deflect the particle trajectory from a fluid pathline and transfer it to certain tubular flow structures, called Kolmogorov–Arnold–Moser (KAM) tori, in which the particle is focused and from which it might not escape anymore. The transfer of particles can take place if a KAM torus, which is a property of the flow without particles, enters the narrow boundary layer on the flow boundaries in which the particle experiences extra forces. Since the PAS obtained in this system depends mainly on the finite particle size, it can be classified as a finite-size coherent structure (FSCS).