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Influence of grid type and turbulence models on the numerical prediction of the flow around marine propellers working in uniform inflow
No full textIn 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
Influence of the mass transfer model on the numerical prediction of the cavitating flow around a marine propeller
No full textCavitating flows, which can occur in a variety of practical applications, can be modelled using a wide range of methods. One strategy consists of using the RANS (Reynolds Averaged Navier Stokes) approach along with 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 verify the influence of three widespread mass transfer models, mainly on the numerical predictions of the propeller performances. The models in question share the common feature of employing some empirical coefficients to tune the models of condensation and evaporation processes, which can influence the accuracy and stability of the numerical predictions. For this reason, and for a fair and congruent comparison, the empirical coefficients of the different mass transfer models are first equally well tuned using an optimization strategy.
The numerical predictions of the propeller performances based on the three different well-tuned mass transfer models are very close to each other. Unfortunately, the numerical cavitation patterns are slightly overestimated compared to the experimental ones, and the thrust breakdown is not properly predicted either.
Finally, we roughly verify that for the prediction of the model scale propulsive performances in the presence of the partial and tip vortex cavitation, the turbulence model, among those considered in this study, plays a minor role
Assessment and Tuning of CFD Cavitation Model Parameters using Advanced Optimization Techniques
No full textCavitating 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. Several mass transfer models
are available in literature, but at least, the most popular employ some empirical coefficients to guide the condensation and
evaporation processes. Moreover it seems that, to achieve accurate results, the empirical values of the models have to be properly tuned with respect to the solver and to the different cavitating regimes. For these reasons in this work we present a preliminary optimization strategy to properly tune and validate different mass transfer models. The optimization strategy is applied to the three popular mass transfer models, and the preliminary results for the cavitating flow around the Naca66(mod) hydrofoil are presented. The overall results suggest that the optimization strategy is stable, accurate and could be applied to additional mass transfer models and other cavitating flow phenomena
Comparison of Hexa-Structured and Hybrid-Unstructured Meshing Approaches for Numerical Prediction of the Flow Around Marine Propellers
No full textIn this paper we present a comparison between hexa-structured and hybrid-unstructured meshing approaches for the numerical prediction of the flow around marine propellers working in homogeneous flow (Open Water Conditions). The objective was to verify if the accuracy of the predictions based on structured meshes is significantly better than predictions based on hybrid meshes to justify the more difficult and time-consuming meshing strategy. The study was performed on two five-bladed propellers in model scale. Simulations were carried out with a commercial RANS solver, using a moving frame of reference approach and employing the SST (Shear Stress Transport) two equation turbulence model. Computational results from both meshing approaches were compared against experimental data. The thrust and torque coefficients were used as global quantities. Circumferentially averaged velocity components and root-mean square values of the turbulent velocity fluctuations, avaiable for one of the propellers, were used to indagate the local flow field. The computational results of global quantities for both meshing approaches were very close to each other and in line with experimental data. Also the local values of the flow were in line with the experimental data, exept for turbulent velocity fluctuations wich were underpredicted, especially in the case of the hybrid approach, where higher diffusivity was observed. The overall results suggest that for the prediction of the propulsive performances of marine propellers, at model scale, there are no significant differences, in term of accuracy, between structured and hybrid meshes but for a detailed study of the flow, the structured mesh seems to offer a better resolution
Numerical Predictions of the Cavitating and Non-Cavitating Flow around the Model Scale Propeller PPTC
No full textIn this work we simulate the non-cavitating and cavitating flow around the PPTC (Potsdam Propeller Test Case) model scale propeller. The calculations are carried out using the commercial CFD solver ANSYS CFX 12, and for the prediction of the cavitating behaviour three different mass transfer models, previously calibrated, are employed
Comparison of mass transfer models for the numerical prediction of sheet cavitation around a hydrofoil
No full textCavitating 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
Simulation of sheet and cloud cavitation with homogenous transport models
No full textThis 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
Numerical Predictions of Cavitating Flow around Model Scale Propellers by CFD and Advanced Model Calibration
Full text linkThe 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
Uncertainty Quantification of Turbulence Model Applied to a Cavitating Hydrofoil
Full text linkThis paper presents the Global Sensitivity Analysis of the coefficients of the standard k-ε turbulence model used in RANS (Reynolds Averaged Navier-Stokes) simulations aimed to predict the flow around a bi-dimensional hydrofoil operating at non-cavitating and cavitating flow regimes. The sensitivity analysis is treated by the Sobol Decomposition, where the Sobol Indices are computed through the Polynomial Chaos Expansion of the 2-nd order with a Point-Collocation Non-Intrusive approach. From the current results, it seems that the considered cavitating flow regime is less sensitive to the variability of the input parameters, at least for the prediction of lift and drag
Numerical investigation of the flow in axial water turbines and marine propellers with scale-resolving simulations
Full text linkThe 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
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