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

Scale-adaptive simulation of unsteady cavitation around a naca66 hydrofoil

Distances between consecutive aftershocks are analysed by means of mono- and multifractal theory with the aim of quantifying the complexity of the physical mechanism governing them, as well as their predictability and predictive instability. Hausdorff, Ha, and Hurst, H, exponents are determined by semivariograms and rescaled analysis, respectively. The exponent ß of the power law describing power spectral contents is also quantified. These three parameters permit a generation of fractional Gaussian noise, fGn, simulating distances. The complexity and predictive instability of physical mechanism generating the series of distances is quantified by means of the correlation dimension, µ*, the Kolmogorov entropy, ¿, and the Lyapunov exponents, ¿i, which are based on the reconstruction theorem formulation. Additionally, the multifractal detrended fluctuation analysis, MF-DFA, contributes with a different point of view to quantify the complexity of the series, in terms of fractal spectral width, W, spectral asymmetry, B, and the critical Hölder exponent, a0. By one hand, the MF-DFA is applied to the complete set of distances characterising the whole aftershock process. By the other hand, the MF-DFA is applied to segments of the series of distances with the aim of determining the evolution of the complexity since the mainshock up to the end of the stress relaxation process. Finally, an ARIMA multilinear regression process is applied to obtain some improvements, in comparison with fGn simulations, on the prediction of distances. The database for this analysis is obtained from the Southern California Seismic Network (SCSN) catalogue. Three series of aftershocks equalling to or exceeding magnitudes of 2.0, assuring seismic catalogue completeness, and associated with Landers (06/28/1992), Northridge (01/17/1994) and Hector Mine (10/16/1999) mainshocks are obtained. It is worth mentioning that common mono-multifractal behaviour for the three aftershocks series is not detected, whatever aftershock periods or segments of them are considered.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

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

Study of the cooling and vacuum systems of a miniature 12 MeV race-track microtron

With the aim of optimization, numerical simulations of the cooling and vacuum systems of a compact 12 MeV race-track microtron (RTM) which is under construction at the Technical University of Catalonia have been carried out. The hydraulic and thermal performance of the cool- ing system for various flow rates has been studied using the Computational Fluid Dynamics (CFD) software. A CFD model, previously validated with experimental pres- sure loss results, has permitted to simulate the cooling fluid temperature, inner wall temperatures and heat trans- fer coefficients at different sections of the RTM accelerat- ing structure. Conclusions concerning the current design and its possible optimization are discussed. Simulations of the RTM high vacuum conditions have been performed using the Monte-Carlo simulation package Molflow+. The pressure in the vacuum chamber, pumping tube con- ductance and maximum allowed throughput have been calculated. Also results of the vacuum chamber pumping out sessions are reported.Postprint (published version

Sensitivity analysis of zwart-gerber-belamri model parameters on the numerical simulation of francis runner cavitation

A series of numerical simulations of the 3D cavitation development in the GAMM Francis turbine runner were carried out with the Zwart-Gerber -Belamri (ZGB) two-phase model available in ANSYS ¿ CFX v16.2. The aim of the investigation w as to evaluate the influence of the ZGB parameters on the location, size and shape of the simulated cavitation. To begin, a sector of the entire fluid domain comprising a single blade was created taking profit from the rotational symmetry of the geometry a nd a stage simulation was set to simulate the flow field without cavitation. A grid independence analysis was carried out using the shear -stress transport (SST) turbulence model. Results such as the pressure drop and the torque at the best efficiency point (BEP) compared reasonably well with the experimental values, thus proving the model validity. The cavitation inception and development was simulated at BEP using the ZGB default parameters by progressively decreasing the sigma value. As expected, the onse t of cavitation took place at the blade suction side close to the junction with the band, and the length and area of the blade cavity increase d for decreasing sigma values. The cavity shape also showed a good agreement with the observations at the GAMM tur bine model. Finally, the values corresponding to the mean nucleation site diameter, the nucleation site volume fraction and the empirical coefficients of evaporation and condensation were modified individually while keeping the rest constant in order to ob serve the changes in cavitation behavior. As a result, it was concluded that all the parameters have a significant effect in the cavity length, the pressure distribution and the torque. Moreover, extreme values leading to unrealistic results were also found.Postprint (published version

Numerical simulation of the alba synchrotron light source cooling system response for failure prevention

The ALBA Synchrotron Light Source cooling system is designed with a common return pipe that interconnects the four consumption rings. Such configuration is believed to compromise its optimal operation. To understand its thermo-fluid dynamic behaviour, a detailed 1D model has been built comprising all the components such as the pipes, fittings, bends, valves, pumping stations, heat exchangers and so on, and the various regulation mechanisms. Preliminarily, the model results in steady state operating conditions have been compared with experimental measurements and the maximum deviations have been found below 13%. Then, a series of transient numerical simulations have been carried out to determine the system response. Specifically, effects of the blockage and leakage of a consumption line as well as the increase and decrease of heat duty for the tunnel rings have been investigated. As a result, the stability of the system has been evaluated and the operational limits have been estimated in front of hydraulic and thermal load variations. Moreover, particular behaviours have been identified which can be used to design monitoring and control strategies to prevent unexpected failures.Postprint (published version

Implicit large eddy simulation of unsteady cloud cavitation around a plane-convex hydrofoil

The present paper focuses on the erosive cavitation behavior around a plane convex hydrofoil. The Zwart-Gerber-Belamri cavitation model is implemented in a library form to be used with the OpenFOAM. The implicit large eddy simulation (ILES) is applied to analyze the three dimensional unsteady cavitating flow around a plane convex hydrofoil. The numerical results in the cases under the hydrodynamic-conditions, which were experimentally tested at the high speed cavitation tunnel of the École Polytechnique Fédérale de Lausanne (EPFL), clearly show the sheet cavitation development, the shedding and the collapse of vapor clouds. It is noted that the cavitation evolutions including the maximum vapor length, the detachment and the oscillation frequency, are captured fairly well. Furthermore, the pressure pulses due to the cavitation development as well as the complex vortex structures are reasonably well predicted. Consequently, it may be concluded that the present numerical method can be used to investigate the unsteady cavitation around hydrofoils with a satisfactory accuracy. © 2015 Publishing House for Journal of Hydrodynamics.Postprint (author's final draft

Numerical simulation of the alba synchrotron light source cooling system response to pump start-up and shut-down

The ALBA Synchrotron Light Source cooling system is submitted to regular pump start-ups and shut-downs. Moreover, pumps can trip due to motor power failures. As a result, the piping system can be subjected to surges and pressure oscillations. The 1D thermo-fluid simulation software Flowmaster¿ has been used to predict these transient conditions taking into account the fluid compressibility, the pipe elasticity, the characteristic time response of the check valves and the pump/motors moments of inertia. During pump start-ups, significant pressure rises are detected that can be reduced by readjusting the PID controller parameters. Unexpected pump shutdowns do not appear to provoke significant water hammer conditions. However, pressure fluctuations are generated mainly in the same pumping line but also in the rest of the system due to the particular common return configuration. In all the cases the pressure regulation mechanisms acting on the pump rotating speeds serve to attenuate the consequences of these transients. Finally, the feasibility of the model to simulate the effect on the system response of trapped air inside the pipes has also been evaluated.Postprint (published version