Parametric study of water column separation in Francis pump-turbine draft tube

The paper presents the modelling, simulation and analysis of the transient behaviour of a 340 MW pump-turbine in case of emergency shutdown in generation mode with particular attention to the possible draft tube water column separation. First, the model of a pumped storage power plant with simplified layout is setup with the EPFL software SIMSEN. This model includes a penstock feeding one 340 MW pump-turbine with the related rotating inertia and a tailrace tunnel. The model of the tailrace tunnel allows for water column separation simulation. Thus, the related SIMSEN model is introduced and validations are briefly presented. Then, the influence of the tailrace tunnel length and diameter on the minimum pressure in the draft tube is investigated through a parametric study first without water column separation model. Finally, the simulation results of the transient behaviour of the pump-turbine with downstream water column separation are presented for different degree of severity triggered by the submergence and the tailrace tunnel dimensions taken as parameters

Measurement of the Self-Oscillating Vortex Rope Dynamics for Hydroacoustic Stability Analysis

Flow instabilities in hydraulic machines often feature oscillating cavitation volumes, which locally introduce compliance and mass flow gain effects. These unsteady characteristics play a crucial role in one-dimensional stability models and can be determined through the definition of transfer functions for the state variables, where the cavitation volume is commonly estimated from the discharge difference between two points located upstream and downstream of the cavity. This approach is demonstrated on a test rig with a microturbine, featuring a self-oscillating vortex rope in its conical draft tube. The fluctuating discharges at the turbine inlet and the draft tube outlet are determined with the pressure-time method using differential pressure transducers. The cavitation volume is then calculated by integrating the corresponding discharge difference over time. In order to validate the results, an alternative volume approximation method is presented, based on the image processing of a high-speed flow visualization. In this procedure, the edges of the vortex rope are detected to calculate the local cross section areas of the cavity. It is shown that the cavitation volumes obtained by the two methods are in good agreement. Thus, the fluctuating part of the cavitation volume oscillation can be accurately estimated by integrating the difference between the volumetric upstream and downstream discharges. Finally, the volume and discharge fluctuations from the pressure-time method are averaged over one mean period of the pressure oscillation. This enables an analysis of the key physical flow parameters' behavior over one characteristic period of the instability and a discussion of its sustaining mechanisms

Implantation of the Femoral Component Relative to the Tibial Component in Medial Unicompartmental Knee Arthroplasty: A Clinical, Radiological, and Biomechanical Study

Knee-Society Meeting, ELECTR NETWORK, SEP, 2021Background: Unicompartmental knee arthroplasty (UKA) is a procedure with low morbidity and fast recovery. Anatomic implants or robotic-assisted UKA has been proposed to improve outcomes with precise positioning. Femoral component position (FCP) relative to the tibial insert could be a factor influencing the contact stresses. We aimed to evaluate the effect of the FCP relative to the tibial insert on clinical outcomes and stress distribution after medial UKA. Methods: Sixty-two medial fixed-bearing UKAs were evaluated at a minimum two-year follow-up using the Knee Society Score. Postoperative radiological evaluation performed on frontal X-rays classified the FCP relative to the tibial insert into the following: group M (medial), group C (central), and group L (lateral). A finite element model was developed to evaluate the biomechanical effects of the FCP relative to the tibial component. Results: The postoperative radiological evaluation showed 9 cases in group M, 46 cases in group C, and 7 cases in group L. The maximum knee flexion angle and the 2-year postoperative ``symptom'' and ``patient satisfaction'' scores of the Knee Society Score were significantly higher in group C. Compared with central positioning, a shift along the mediolateral axis leads to a displacement of the contact pressure center. Conclusion: The FCP relative to the tibial insert may increase patient outcomes at a minimum follow-up of two years after fixed-bearing medial UKA. Accordance between FCP and contact stresses on the polyethylene insert could be a contributing factor of long-term survival of UKA. (C) 2022 Elsevier Inc. All rights reserved

Implantation of the Femoral Component Relative to the Tibial Component in Medial Unicompartmental Knee Arthroplasty: A Clinical, Radiological, and Biomechanical Study

Knee-Society Meeting, ELECTR NETWORK, SEP, 2021Background: Unicompartmental knee arthroplasty (UKA) is a procedure with low morbidity and fast recovery. Anatomic implants or robotic-assisted UKA has been proposed to improve outcomes with precise positioning. Femoral component position (FCP) relative to the tibial insert could be a factor influencing the contact stresses. We aimed to evaluate the effect of the FCP relative to the tibial insert on clinical outcomes and stress distribution after medial UKA. Methods: Sixty-two medial fixed-bearing UKAs were evaluated at a minimum two-year follow-up using the Knee Society Score. Postoperative radiological evaluation performed on frontal X-rays classified the FCP relative to the tibial insert into the following: group M (medial), group C (central), and group L (lateral). A finite element model was developed to evaluate the biomechanical effects of the FCP relative to the tibial component. Results: The postoperative radiological evaluation showed 9 cases in group M, 46 cases in group C, and 7 cases in group L. The maximum knee flexion angle and the 2-year postoperative ``symptom'' and ``patient satisfaction'' scores of the Knee Society Score were significantly higher in group C. Compared with central positioning, a shift along the mediolateral axis leads to a displacement of the contact pressure center. Conclusion: The FCP relative to the tibial insert may increase patient outcomes at a minimum follow-up of two years after fixed-bearing medial UKA. Accordance between FCP and contact stresses on the polyethylene insert could be a contributing factor of long-term survival of UKA. (C) 2022 Elsevier Inc. All rights reserved