Assessment of RANS turbulence models and Zwart cavitation model empirical coefficients for the simulation of unsteady cloud cavitation

The numerical simulation of unsteady cavitation flows is sensitive to the selected models and associated parameters. Consequently, three Reynolds Average Navier-Stokes (RANS) turbulence models and the Zwart cavitation model were selected to assess their performance for the simulation of cloud cavitation on 2D hydrofoils. The experimental cavitation tests from a NACA65012 hydrofoil at different hydrodynamic conditions were used as a reference to tune the modeling parameters and the experimental tests from a NACA0015 were finally used to validate them. The effects of near wall grid refinement, time step, iterations and mesh elements were also investigated. The results indicate that the Shear Stress Transport (SST) model is sensitive to near wall grid resolution which should be fine enough. Moreover, the cavitation morphology and dynamic behavior are sensitive to the selection of the Zwart empirical vaporization, Fv, and condensation, Fc, coefficients. Therefore, a multiple linear regression approach with the single objective of predicting the shedding frequency was carried out that permitted to find the range of coefficient values giving the most accurate results. In addition, it was observed that they provided a better prediction of the vapor volume fraction and of the instantaneous pressure pulse generated by the main cloud cavity collapse.Postprint (published version

Unsteady numerical simulation of suction side leading edge cavitation in a Francis turbine runner

Cavitation appearance within the runner of the water turbines is a common problem that induces vibrations and provokes the erosion of the blades. Although many investigations have been carried out in simple configurations such as 2D and 3D hydrofoils in cavitation tunnels, few works have been done to simulate the cavitation dynamic behavior in actual turbine geometries. In this paper, the unsteady numerical simulation of suction side leading edge cavitation has been carried out for the GAMM Francis runner. For that, the two-phase model available in ANSYS¿ CFX and the Shear Stress Transport (SST) turbulence model have been used. A sensitivity analysis of various model parameters has been done and the results have been validated with experimental values obtained in a reduced scale model test utility at the best efficiency point. Special attention has been given to determine if a shedding process also exists and to understand the main hydrodynamic mechanisms involved. In conclusion, the instability of the leading edge cavitation seems to be mainly caused by field pressure perturbations originating from the guide vanes or the draft tube.Peer ReviewedPostprint (published version

Added mass effects on a Francis turbine runner with attached blade cavitation

To have a safe structural design, an analysis of the dynamic behavior of a Francis turbine runner with consideration of the added mass e ects of surrounding water is necessary during design phase. Both in design and at o -design operations, large-scale forms of attached cavitation may appear on runner blades and can change the added mass e ects of the surrounding fluid in relation to a single water domain. Consequently, a numerical investigation of the modal response of a Francis runner has been carried out by reproducing the presence of various sizes of leading edge cavitation (LEC) and trailing edge cavitation (TEC). The fluid–structure interaction problem has been solved by means of an acoustic-structural coupling method. The calculated added mass e ects with cavitation have been compared with those corresponding to the pure water condition without cavitation. Firstly, a single blade has been investigated to evaluate the level of significance for the proposed cavity shapes and dimensions. Afterwards, based on the results obtained, the complete runner structure has been considered, factoring in similar cavity shapes and locations. The results prove that significant added mass e ects are induced on the entire runner by the attached cavitation that increase the natural frequencies of the first modes. Moreover, the added mass e ects increase with cavity size and amplitude of blade deformation below the cavity.Postprint (published version

Full-scale wind turbine vibration signature analysis

A sample of healthy wind turbines from the same wind farm with identical sizes and designs was investigated to determine the average vibrational signatures of the drive train components during normal operation. The units were variable-speed machines with three blades. The rotor was supported by two bearings, and the drive train connected to an intermediate three-stage planetary/helical gearbox. The nominal 2 MW output power was regulated using blade pitch adjustment. Vibrations were measured in exactly the same positions using the same type of sensors over a six-month period covering the entire range of operating conditions. The data set was preliminary validated to remove outliers based on the theoretical power curves. The most relevant frequency peaks in the rotor, gearbox, and generator vibrations were detected and identified based on averaged power spectra. The amplitudes of the peaks induced by a common source of excitation were compared in different measurement positions. A wind speed dependency of broadband vibration amplitudes was also observed. Finally, a fault detection case is presented showing the change of vibration signature induced by a damage in the gearboxPeer ReviewedPostprint (published version

Effects of attached blade cavitation on the natural frequencies of a Francis runner

Effects of attached blade cavitation on the natural frequencies of a Francis runnerPostprint (published version

Mass transfer rate effects on the cavitating vortex shedding flow around a circular cylinder at low Reynolds number

Cavitating vortex shedding is a flow phenomenon commonly encountered behind bluff bodies in hydraulic machinery and systems and the generated vapor bubbles significantly change the dynamic behavior of the vortices. In addition, the collapses of the cavities become a source of unwanted effects such as vibration, noise and material erosion. In this study, the numerical results of the cavitating vortex shedding behind a circular cylinder at low Reynolds number have been compared and analyzed using different mass transfer rate values between the vapor and the liquid water phases. For that, the transport equation for the vapor volume fraction coupled with the explicit source term based on the simplification of the Rayleigh-Plesset equation has been solved. Commonly, this explicit source term that accounts for the mass transfer rate is driven by a function of the pressure difference, the vapor volume fraction and several empirical factors. To understand the effects of this rate, its value has been gradually increased from its default value up to the infinite when the equilibrium assumption for the barotropic flow is satisfied. The obtained results show that as the rate is increased, higher gradients of the vapor volume fraction and of the pressure near the interface between the vapor and the liquid phases are predicted. However, the frequency of the vortex shedding is slightly affected by the increase of the mass transfer rate.Peer ReviewedPostprint (author's final draft

Desarrollo de conocimiento para la mejora del mantenimiento predictivo en el sector eólico

Postprint (published version

Experimental and numerical analysis of directional added mass effects in partially liquid-filled horizontal pipes

The change of pipe natural frequencies due to added mass effects has been investigated in two cylindrical horizontal pipes from empty to completely water filled cases with various intermediate partially-filled conditions. The added mass coefficients of the three first vertical and horizontal modes of vibration have been determined with both experimental modal analysis and finite element analysis (FEA) acoustic-structural numerical simulations, which showed good agreement. The vertical and horizontal added mass coefficients present different behaviors as a function of the water level. Moreover, the pipe cross sectional dimensions determine the magnitude of these effects. For generalization to any pipe size, dependency of the directional added mass coefficients with new vertical and horizontal added mass estimators has been found. These estimators can be used in practical situations with horizontally mounted cylindrical pipes as a reference to predict and quantify air content.Postprint (author's final draft

Large Eddy Simulation of the transient cavitating vortical flow in the wake of a hydrofoil

The Zwart mixture cavitation model and the Large Eddy Simulation (LES) turbulence model have been combined to simulate the transient cavitating flow in the wake of a hydrofoil. The hydrofoil is a 2D NACA0009 with a truncated trailing edge which has been extensively investigated experimentally in the EPFL high-speed cavitation tunnel. The simulated flow conditions correspond to a free stream velocity of 20 m/s (Re = 2 × 106) and an incidence angle of 0° for different levels of cavitation number, ¿¿, ranging from free cavitation to cavitation flow with different amounts of vapor. Under such hydrodynamic conditions and blunt trailing edge geometry, a Von Kármán vortex street takes place that is the object of the present study in order to ascertain how cavitation modifies the vortex structure and its dynamic behavior. The numerical results show a good agreement with the experimentally measured vortex shedding frequency without and with cavitation. As observed experimentally, the numerical results also predict an increase of the frequency induced by a significant change of the vortex properties. In order to identify the effects induced by the occurrence and development of cavitation, the obtained unsteady velocity field has been analyzed via Proper Orthogonal Decomposition (POD) technique to identify and understand changes in the flow structure. It has been found that the occurrence and development of cavitation can change the contribution of the POD modes to the total velocity filed. Moreover, the appearance of cavitation seems to stabilize the flow field unsteady behavior since high order POD modes are harmonically correlated with the first couple of basic POD modes.Peer ReviewedPostprint (published version

Added mass effects of attached cavitation on the blades of a Francis runner

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