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Martín Avellanal,Mónica Martin-Corvillo,Laura Barrigon,Manuel Vazquez Espi,Clara M. Esteban Escolar 대한척추신경외과학회 2019 Neurospine Vol.16 No.2
Objective: To evaluate the direct costs of various spinal surgical procedures within 1 year of follow-up and to compare the profiles of neurosurgeons and orthopedic surgeons. Methods: All spinal procedures performed within a 10-month period in patients covered by a private insurance company were included. Costs related to the spinal interventions were systematically registered in the company database. Associated costs during the 1-year follow-up were recorded. Results: In total, 1,862 patients were included, with a total cost of €11,050,970, of whom 34.8% underwent noninstrumented lumbar decompression (€3,473), 27.1% dorsolumbar instrumented fusion (€6,619), 14.6% nucleoplasty (€1,323), 13.5% cervical surgery (€4,463), 3.4% kyphoplasty (€4,200), 2.9% scoliosis (€15,414), 1.2% oncologic surgery (€5,590), 0.5% traumatic compression (€7,844), and 4.7% (€1,343) other minor interventions (mainly rhizotomies). Approximately 42% of patients required reinterventions within the first year, with a global extra cost of €7,280,073; 11% were referred to the pain clinic, with a €114,663 cost; 55.5% were men; and the most common age range of patients who received an intervention was 65–75 years. Neurosurgeons performed 60% of all interventions. Noninstrumented lumbar operations were performed by neurosurgeons twice as often as instrumented operations, and they performed 76% of cervical operations. Orthopedic surgeons performed 2.5 times more instrumented than noninstrumented lumbar operations, and almost all scoliosis and rhizotomy procedures. Conclusion: The direct costs of spinal surgery in Spain were generally lower than those reported in other European Union countries and the United States. Neurosurgeons and orthopedic surgeons had different spine surgical profiles and costs.
Cavitation Surge in a Small Model Test Facility simulating a Hydraulic Power Plant
Yonezawa, Koichi,Konishi, Daisuke,Miyagawa, Kazuyoshi,Avellan, Francois,Doerfler, Peter,Tsujimoto, Yoshinobu Korean Society for Fluid machinery 2012 International journal of fluid machinery and syste Vol.5 No.4
Model tests and CFD were carried out to find out the cause of cavitation surge in hydraulic power plants. In experiments the cavitation surge was observed at flow rate, both with and without a surge tank placed just upstream of the inlet volute. The surge frequency at smaller flow rate was much smaller than the swirl mode frequency caused by the whirl of vortex rope. An unsteady CFD was carried out with two boundary conditions: (1) the flow rate is fixed to be constant at the volute inlet, (2) the total pressure is kept constant at the volute inlet, corresponding to the experiments without/with the surge tank. The surge was observed with both boundary conditions at both higher and lower flow rates. Discussions as to the cause of the surge are made based on additional tests with an orifice at the diffuser exit, and with the diffuser replaced with a straight pipe.
Susan-Resiga, Romeo,Muntean, Sebastian,Stein, Peter,Avellan, Francois Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.4
The flow in the draft tube cone of Francis turbines operated at partial discharge is a complex hydrodynamic phenomenon where an incoming steady axisymmetric swirling flow evolves into a three-dimensional unsteady flow field with precessing helical vortex (also called vortex rope) and associated pressure fluctuations. The paper addresses the following fundamental question: is it possible to compute the circumferentially averaged flow field induced by the precessing vortex rope by using an axisymmetric turbulent swirling flow model? In other words, instead of averaging the measured or computed 3D velocity and pressure fields we would like to solve directly the circumferentially averaged governing equations. As a result, one could use a 2D axi-symmetric model instead of the full 3D flow simulation, with huge savings in both computing time and resources. In order to answer this question we first compute the axisymmetric turbulent swirling flow using available solvers by introducing a stagnant region model (SRM), essentially enforcing a unidirectional circumferentially averaged meridian flow as suggested by the experimental data. Numerical results obtained with both models are compared against measured axial and circumferential velocity profiles, as well as for the vortex rope location. Although the circumferentially averaged flow field cannot capture the unsteadiness of the 3D flow, it can be reliably used for further stability analysis, as well as for assessing and optimizing various techniques to stabilize the swirling flow. In particular, the methodology presented and validated in this paper is particularly useful in optimizing the blade design in order to reduce the stagnant region extent, thus mitigating the vortex rope and expending the operating range for Francis turbines.
Nicolet, Christophe,Zobeiri, Amirreza,Maruzewski, Pierre,Avellan, Francois Korean Society for Fluid machinery 2011 International journal of fluid machinery and syste Vol.4 No.1
The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.
Cavitation Surge in a Small Model Test Facility Simulating a Hydraulic Power Plant
Koichi Yonezawa,Daisuke Konishi,Kazuyoshi Miyagawa,François Avellan,Peter Doerfler,Yoshinobu Tsujimoto 한국유체기계학회 2012 International journal of fluid machinery and syste Vol.5 No.4
Model tests and CFD were carried out to find out the cause of cavitation surge in hydraulic power plants. In experiments the cavitation surge was observed at flow rates higher and lower than the swirl free flow rate, both with and without a surge tank placed just upstream of the inlet volute. The surge frequency at smaller flow rate was much smaller than the swirl mode frequency caused by the whirl of vortex rope. An unsteady CFD was carried out with two boundary conditions: (1) the flow rate is fixed to be constant at the volute inlet, (2) the total pressure is kept constant at the volute inlet, corresponding to the experiments without/with the surge tank. The surge was observed with both boundary conditions at both higher and lower flow rates. Discussions as to the cause of the surge are made based on additional tests with an orifice at the diffuser exit, and with the diffuser replaced with a straight pipe.
Chen, Changkun,Nicolet, Christophe,Yonezawa, Koichi,Farhat, Mohamed,Avellan, Francois,Tsujimoto, Yoshinobu Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.3
The effects of acoustic modes in the penstock on the self-excited oscillation in hydraulic power system were studied by assuming a finite sound velocity in the penstock. The flow in the draft tube is considered to be incompressible assuming that the length of the draft tube is smaller than the wavelength of the oscillation. It was found that various acoustic modes in the penstock can become unstable (amplified) by the diffuser effect of the draft tube or the effect of swirl flow from the runner. Their effects on each mode are discussed.
Experimental Study and Numerical Simulation of Cavity Oscillation in a Conical Diffuser
Chen, Chang-Kun,Nicolet, Christophe,Yonezawa, Koichi,Farhat, Mohamed,Avellan, Francois,Miyazawa, Kazuyoshi,Tsujimoto, Yoshinobu Korean Society for Fluid machinery 2010 International journal of fluid machinery and syste Vol.3 No.1
Based on the one-dimensional stability analysis, the self-excited oscillation in hydraulic power generating systems was studied by a simple experiment and numerical simulation. It was shown that a cavity in a conical diffuser can cause surge. With the diffuser, a high amplitude and low frequency oscillation occurs at low cavitation number. This oscillation was not observed with the straight pipe. It was confirmed that the diffuser effect of the draft tube can be the cause of the full load surge in hydraulic power system. Numerical results were also analyzed to check the validity of the one-dimensional stability analysis.
Nicolet, Christophe,Alligne, Sebastien,Kawkabani, Basile,Simond, Jean-Jacques,Avellan, Francois Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.4
This paper presents a numerical simulation study of the transient behavior of a $2{\times}340MW$ pump-turbine power plant, where the results show an unstable behavior at runaway. First, the modeling of hydraulic components based on equivalent schemes is presented. Then, the 2 pump-turbine test case is presented. The transient behavior of the power plant is simulated for a case of emergency shutdown with servomotor failure on Unit 1. Unstable operation at runaway with a period of 15 seconds is properly simulated using a 1-dimensional approach. The simulation results points out a switch after 200 seconds of the unstable behavior between a period of oscillations initially of 15 seconds to a period of oscillation of 2.16 seconds corresponding to the hydraulic circuit first natural period. The pressure fluctuations related to both the rigid and elastic water column mode are presented for oscillation mode characterization. This phenomenon is described as a switch between a rigid and an elastic water column oscillation mode. The influence of the rotating inertia on the switch phenomenon is investigated through a parametric study.
Alligne, S.,Nicolet, C.,Allenbach, P.,Kawkabani, B.,Simond, J.J.,Avellan, F. Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.4
Hydroelectric power plants are known for their ability to cover variations of the consumption in electrical power networks. In order to follow this changing demand, hydraulic machines are subject to off-design operation. In that case, the swirling flow leaving the runner of a Francis turbine may act under given conditions as an excitation source for the whole hydraulic system. In high load operating conditions, vortex rope behaves as an internal energy source which leads to the self excitation of the system. The aim of this paper is to identify the influence of the full load excitation source location with respect to the eigenmodes shapes on the system stability. For this, a new eigenanalysis tool, based on eigenvalues and eigenvectors computation of the nonlinear set of differential equations in SIMSEN, has been developed. First the modal analysis method and linearization of the set of the nonlinear differential equations are fully described. Then, nonlinear hydro-acoustic models of hydraulic components based on electrical equivalent schemes are presented and linearized. Finally, a hydro-acoustic SIMSEN model of a simple hydraulic power plant, is used to apply the modal analysis and to show the influence of the turbine location on system stability. Through this case study, it brings out that modeling of the pipe viscoelastic damping is decisive to find out stability limits and unstable eigenfrequencies.