Search

In recent years, green shipping becomes one of the fundamental challenges for the marine industry: the limits imposed on ship emissions by IMO (International Maritime Organization) are increasingly stringent, especially in terms of SOx (sulfur oxides). The installation on board of scrubbers has proved to be a helpful solution to SOx abetment, in particular for the ships already in navigation: it allows to respect the limits imposed by the IMO even with the use of HFOs (Heavy Fuel Oils), so without the need to carry out a complete refitting of the propulsion system. However, such systems, usually installed in the funnels, have large dimensions. The integration between components is the best method to optimize the spaces, facilitating the installation of the scrubbers on board. The present work investigates a combined CFD-FEM (Computational Fluid Dynamics-Finite Element Method) methodology to evaluate the acoustic performances of a model-scale scrubber. Some papers in the literature consider the acoustic properties of SCRs (Selective Catalytic Reduction systems) for marine applications, while a thorough study on scrubbers’ performances is missing. Independent CFD or FEM calculations may evaluate the acoustic properties of the scrubber. However, the combined methodology reduces the computational burden by about 90% compared to the CFD modelling. Moreover, it gives the advantage of considering the influence of flow on acoustic properties, which is impossible for a fully FEM approach

In this paper the numerical simulations of the flow around an elastically-mounted circular cylinder with one degree of freedom (1-DOF) with small damping parameter and small mass ratio are presented. The simulations are performed for subcritical Reynolds numbers ( 2 . 0 ⋅ 1 0 3 Re 1 . 3 1 0 4 ), using the URANS (Unsteady Reynolds Averaged Navier Stokes) approach in combination with the SST (Shear Stress Transport) turbulence model. The simulations are carried out on 3D as well as 2D meshes. From the overall results it seems that the bi-dimensional approach could be effectively applied in the lower branch, while major discrepancies are present in the upper branch. The detailed analysis of the wake flow characteristics shows that the three dimensionality of the flow is stronger in the transition zones between the different branches. The extensive comparison between the computational simulations and the experimental results presented in this paper could be considered the “start point” in order to select the proper grid model depending on the requested accuracy.

The numerical predictions of a cavitating model scale propeller working in uniform and oblique flow conditions are presented. The cavitating phenomena are numerically reproduced using a homogeneous (mixture) model where three previously calibrated mass transfer models are alternatively used to model the mass transfer rate. The turbulence effect is modelled using the Reynolds Averaged Navier Stokes (RANS) approach. The simulations are performed using an open source solver. The numerical results are compared with available experimental data. For a quantitative comparison the propeller thrust is considered, while for a qualitative comparison, snapshots of cavitation patterns are shown. The thrust values obtained with the three different mass transfer models are very close to each other, however differences in the predicted cavitation patterns are observed. Moreover, some discrepancies between the numerical results and experimental data are reported.

The paper presents a numerical analysis of flow in a 6-jet Pelton turbine for 1-nozzle operation. Flow in the manifold with injectors was analysed in order to get inlet conditions for runner analysis, which was at first performed without cavitation. Numerically obtained efficiency value was compared to the measured value from the test rig. The simulation was repeated with cavitation included. Small vapour cavity at the inner side and a larger one at the back side of the buckets were observed. Detailed analysis of results showed that the conditions for cavitation pitting were not fulfilled. Additionally, with cavitation modelling the accuracy of efficiency prediction improved.

This paper presents results of the numerical analysis of two Pelton turbines: a 6-jet turbine for middle head and a 2- jet turbine for high head. For the 6-jet turbine losses in manifold, quality of the jets and turbine efficiency were predicted and validated with the experimental data. Additional improvement of accuracy of efficiency prediction was obtained with cavitation modelling. It was also checked that there was no interaction between the evacuating water sheets and the incoming jets. For a 2-jet turbine cavitation prediction was done. Small vapour cavity at the inner side and a larger one at the back side of the bucket were observed. Detailed analysis of cavitation and condensation processes showed that the conditions for cavitation pitting were not fulfilled.

The effect of the mass of a 2D elastically-mounted circular cylinder in cross-flow on its vortex-induced vibrations and on the related vortex shedding lift forces is analyzed via a single-degree-of-freedom multi-frequency model (sdof-mf). The mechanical system in question is characterized by low mass ratio, low structural damping and Reynolds number of order $$10^4$$. The proposed sdof-mf model relies on the decomposition of the total hydrodynamic force in a inertia/drag force, conventionally associated with the cylinder motion in still fluid, and an additional lift force associated to pure vortex shedding. The lift force is assumed to be composed by not-Fourier-dependent harmonics; this constitutes the key point of the proposed sdof-mf model. The parameters of this model are determined via a parameter identification method based, in this case, on VIV data obtained via CFD. The simulations are carried out changing systematically the values of the mass ratio, within the range of engineering practice, and covering a wide range of flow regimes including lock-in conditions. The results from the application of the sdof-mf model highlight the large influence of the mass ratio on the response of the cylinder and on the vortex shedding lift force. The effects are clearly visible on the maximum amplitude at lock-in, on the range of incident flow velocity over which synchronization occurs, on ultra/sub harmonic behavior and phase lag of the cylinder motion, and finally on the magnitude and harmonic content of the lift force induced by pure vortex shedding.

A novel single-degree-of-freedom multi-frequency model (sdof-mf) for the prediction of the Vortex Induced Vibrations (VIV) of an elastically mounted circular cylinder in two-dimensional cross flow is presented. The proposed model treats the total hydrodynamic force as sum of conventional Morison-like inertia and drag terms related to the cylinder motion in still fluid plus additional harmonics that account for the lift force induced by vortex shedding. Amplitudes, frequencies and phase lags of these harmonics are identified using a Parameter Identification (PI) procedure applied to time domain data of vortex induced forces, here obtained via CFD simulations. The proposed sharp identification via PI of the independent frequencies of the vortex shedding fluid force is the peculiarity of the proposed method. The model is assessed considering a wide range of flow regimes, including lock-in conditions. From the overall results, the proposed sdof-mf model exhibits promising but consistent capabilities in the reproduction of the vortex shedding forces and cylinder motion, in terms of both amplitudes and frequencies.

Pursuing the industrial goals of the project SOPHYA (Seakeeping Of Planing Hull YAchts, POR FESR 2014 2020 - 1.3.b Friuli Venezia Giulia Region), in particular those aimed at reducing the time-to-results of open source CFD simulations of planing hulls, a fully automated pre-processing procedure has been developed and tested. The procedure is based on open-source tools available within the OpenFOAM library. The pre-processing time is drastically reduced making the procedure fully-automated and user friendly, still making it robust and accurate. The novelty of the mesh generation procedure stands in the selection of the best mesh among a family of grids that share the same background octree-based mesh and differ from each other for the boundary-layer cells. This procedure allows to obtain high quality meshes with reduced number of cells that in turn allows a further reduction of the time-to-result through reduction of the CPU time. In addition to the mesh generation, the developed procedure provides a ready-to-run case folder for planing hull simulations. The proposed procedure allows a marked reduction of input parameters and file edits, along with better mesh quality and reduced risk of user error. The automated procedure is fully customizable and ultimately allows users to significantly reduce both pre-processing and computation time, a key aspect in CFD investigations at the design stage.

Cavitating phenomena, which may occur in many industrial systems, can be modelled using several approaches. In this study a homogeneous multiphase model, used in combination with three previously calibrated mass transfer models, is evaluated for the numerical prediction of cavitating flow around a marine propeller and a Kaplan turbine runner. The simulations are performed using a commercial computational fluid dynamics (CFD) solver and the turbulence effects are modelled using, alternatively, the Reynolds averaged Navier Stokes (RANS) and scale adaptive simulation (SAS) approaches. The numerical results are compared with available experimental data. In the case of the propeller the thrust coefficient and the sketches of cavitation patterns are considered. In the case of the turbine the efficiency and draft tube losses, along with the cavitation pattern sketches, are compared. From the overall results it seems that, for the considered systems, the three different mass transfer models can guarantee similar levels of accuracy for the performance prediction. For a very detailed investigation of the fluid field, slight differences in the predicted shapes of the cavitation patterns can be observed. In addition, in the case of the propeller, the SAS simulation seems to guarantee a more accurate resolution of the cavitating tip vortex flow, while for the turbine, SAS simulations can significantly improve the predictions of the draft tube turbulent flow.

Transient simulations of flow in a Francis turbine were performed with a goal to predict pressure pulsation frequencies and amplitudes caused by rotating vortex rope at part load operating regime. Simulations were done with the SAS SST turbulence model with curvature correction on basic and refined computational meshes. Without cavitation modelling too small values of frequency and amplitudes were obtained. With mesh refinement the calculated amplitudes were a bit closer to the measured values, while the accuracy of predicted frequency did not improve at all. Agreement between measured and numerical values was significantly improved when cavitation was included in simulations. In addition, the predicted value of the dominant frequency was slightly more accurate when, in the Zwart et al. cavitation model, the default condensation and evaporation model constants were replaced with previously calibrated ones.

In this study, the objective of the optimization of a double-suction pump is the maximization of its hydraulic efficiency. The optimization is performed, by means of the modeFRONTIER optimization platform, in steps. At first, by means of a DOE (Design of Experiments) strategy, the design space is explored, using a parameterized CAD representation of the pump. Suitable metamodels (surrogates or Response Surfaces), which represent an economical alternative to the more expensive 3D CFD model, are built and tested. Among different metamodels, the evolutionary design, radial basis function and the stepwise regression models seem to be the most promising ones. Finally, the stepwise regression model, trained on a set of 200 designs and constructed with only five the most influential input design parameters, was chosen as a potentially applicable metamodel.

In this work we analyze the influence of grid type and turbulence model on the numerical prediction of the flow around marine propellers, working in uniform inflow. The study is carried out comparing hexa-structured meshes with hybrid-unstructured meshes using the SST (Shear Stress Transport) turbulence model and the BSL-RSM (Baseline-Reynolds Stress Model) turbulence model. The simulations are carried out with a commercial CFD solver. The numerical results are compared with the available experimental data of two propellers in model scale. The comparison is carried out evaluating the global field values, represented by the thrust and torque coefficients, and also considering some local field values measured in the propeller wake. The computational results suggest that, for the numerical predictions of the propeller open water propulsion characteristics, the hexa-structured and hybrid-unstructured meshes can guarantee similar levels of accuracy. Nevertheless hybrid-unstructured meshes seem to exhibit a more diffusive character than hexa-structured meshes, and thus they are less suited for detailed investigations of the flow field. Finally, the two different turbulence models behave similarly on both types of meshes, with the BSL-RSM turbulence model providing only slightly better predictions than the computationally more economical SST turbulence model.

The paper presents numerical simulations of flow in a model of a high head Francis turbine and comparison of results to the measurements. Numerical simulations were done by two CFD (Computational Fluid Dynamics) codes, Ansys CFX and OpenFOAM. Steady-state simulations were performed by k-epsilon and SST model, while for transient simulations the SAS SST ZLES model was used. With proper grid refinement in distributor and runner and with taking into account losses in labyrinth seals very accurate prediction of torque on the shaft, head and efficiency was obtained. Calculated axial and circumferential velocity components on two planes in the draft tube matched well with experimental results.

In this study the impeller geometry of a double-suction pump ensuring the best performances in terms of hydraulic efficiency and reluctance of cavitation is determined using an optimization strategy , which was driven by means of the modeFRONTIER optimization platform. The different impeller shapes (designs) are modified according to the optimization parameters and tested with a computational fluid dynamics (CFD) software, namely ANSYS CFX. The simulations are performed using a decoupled approach, where only the impeller domain region is numerically investigated for computational convenience. The flow losses in the volute are estimated on the base of the velocity distribution at the impeller outlet. The best designs are then validated considering the computationally more expensive full geometry CFD model. The overall results show that the proposed approach is suitable for quick impeller shape optimization.

This paper introduces the results of correlated numerical models study carried out to analyse cavitating flows. The flow field of steady attached sheet cavitation and the case of unsteady cavitation behaviour with quasi-periodic fluctuations is analysed with different homogenous cavitation transport models. Three models in form of additional transport equations for water volume fraction are combined with the RANS (Reynolds Averaged Navier–Stokes) equations and calibrated for the cavitating flow around the NACA66 (MOD) hydrofoil by means of an optimisation strategy. In the second stage, the optimised models are applied to the case of internal unsteady cavitating flow in Venturi type section. The results obtained using calibrated models are very close to each other, and agree well with the available experimental data, indicating that the optimisation process is recommended as a general – first step tool for mathematical models validation.

The numerical predictions of the cavitating flow around two model scale propellers in uniform inflow are presented and discussed. The simulations are carried out using a commercial CFD solver. The homogeneous model is used and the influence of three widespread mass transfer models, on the accuracy of the numerical predictions, is evaluated. The mass transfer models in question share the common feature of employing empirical coefficients to adjust mass transfer rate from water to vapour and back, which can affect the stability and accuracy of the predictions. Thus, for a fair and congruent comparison, the empirical coefficients of the different mass transfer models are first properly calibrated using an optimization strategy. The numerical results obtained, with the three different calibrated mass transfer models, are very similar to each other for two selected model scale propellers. Nevertheless, a tendency to overestimate the cavity extension is observed, and consequently the thrust, in the most severe operational conditions, is not properly predicted.

Cavitating flows, which can occur in a variety of practical cases, can be modelled with a wide range of methods. One strategy consists of using the RANS (Reynolds Averaged Navier Stokes) equations and an additional transport equation for the liquid volume fraction, where mass transfer rate due to cavitation is modelled by a mass transfer model. In this study, we compare three widespread mass transfer models available in literature for the prediction of sheet cavitation around a hydrofoil. These models share the common feature of employing empirical coefficients, to tune the models of condensation and evaporation processes, that can influence the accuracy and stability of the numerical predictions. In order to compare the different mass transfer models fairly and congruently, the empirical coefficients of the different models are first well tuned using an optimization strategy. The resulting well tuned mass transfer models are then compared considering the flow around the NACA66(MOD) and NACA009 hydrofoils. The numerical predictions based on the three different tuned mass transfer models are very close to each other and in agreement with the experimental data. Moreover, the optimization strategy seems to be stable and accurate, and could be extended to additional mass transfer models and further flow problems.

The numerical predictions of cavitating flow around a marine propeller working in non-uniform inflow and an axial turbine are presented. The cavitating flow is modelled using the homogeneous (mixture) model. Time-dependent simulations are performed for the marine propeller case using OpenFOAM. Three calibrated mass transfer models are alternatively used to model the mass transfer rate due to cavitation and the two-equation SST (Shear Stress Transport) turbulence model is employed to close the system of the governing equations. The predictions of the cavitating flow in an axial turbine are carried out with ANSYS-CFX, where only the native mass transfer model with tuned parameters is used. Steady-state simulations are performed in combination with the SST turbulence model, while time-dependent results are obtained with the more advanced SAS (Scale Adaptive Simulation) SST model. The numerical results agree well with the available experimental measurements, and the simulations performed with the three different calibrated mass transfer models are close to each other for the propeller flow. Regarding the axial turbine the effect of the cavitation on the machine efficiency is well reproduced only by the time dependent simulations.

The accurate prediction of the performances of axial water turbines and naval propellers is a challenging task, of great practical relevance. In this paper a numerical prediction strategy, based on the combination of a trusted CFD solver and a calibrated mass transfer model, is applied to the turbulent flow in axial turbines and around a model scale naval propeller, under non-cavitating and cavitating conditions. Some selected results for axial water turbines and a marine propeller, and in particular the advantages, in terms of accuracy and fidelity, of ScaleResolving Simulations (SRS), like SAS (Scale Adaptive Simulation) and Zonal-LES (ZLES) compared to standard RANS approaches, are presented. Efficiency prediction for a Kaplan and a bulb turbine was significantly improved by use of the SAS SST model in combination with the ZLES in the draft tube. Size of cavitation cavity and sigma break curve for Kaplan turbine were successfully predicted with SAS model in combination with robust high resolution scheme, while for mass transfer the Zwart model with calibrated constants were used. The results obtained for a marine propeller in non-uniform inflow, under cavitating conditions, compare well with available experimental measurements, and proved that a mass transfer model, previously calibrated for RANS (Reynolds Averaged Navier Stokes), can be successfully applied also within the SRS approaches.