Shipping is a critical component of global trade but also accounts for a substantial portion of global greenhouse gas emissions. Recognising this issue, the International Maritime Organisation (IMO) has implemented new measures aimed at determining the energy efficiency of all ships and promoting continuous improvements, such as the Energy Efficiency Existing Ship Index (EEXI). As Computational Fluid Dynamics (CFD) can be used to calculate the EEXI value, RISE-SSPA1 and Flowtech have developed a CFD-based method for predicting full-scale ship performance with SHIPFLOW v7.0, which meets the new requirements of IMO. The method is validated through an extensive comparison study that examines the delivered power and propeller rotation rate between full-scale CFD predictions and high-quality sea trials using 14 common cargo ships of varying sizes and types. The comparison between the CFD predictions and 59 sea trials shows that both delivered power and RPM can be predicted with satisfactory accuracy, with an average comparison error of about 4% and 2%, respectively. The numerical methods used in this study differ significantly from the majority of the state-of-the-art CFD codes, highlighting their potential for future applications in ship performance prediction. Thorough validation with a large number of sea trials is essential to establish confidence in CFD-based ship performance prediction methods, which is crucial for the credibility of the EEXI framework and its potential to contribute to shipping decarbonisation.

This report demonstrates the qualifications of RISE to carry out CFD for ship self-propulsion, thus predicting the delivered power. The procedures were fully developed at SSPA which became fully integrated into the Maritime Department of RISE by 2023-01-01. An outline is given of the best-practice guidelines used at SSPA/RISE and how they comply with the relevant ITTC recommendations for verification and analysis. In addition, an overview is given of previous validation studies performed for a wide range of ships, including comparison with both model-scale and full-scale data. Complete references are provided to reports and publications in which these SSPA studies and methods are described in detail.

Noise generated by turbulent flow over high-aspect ratio bluff bodies is of interest in many engineering applications including the design wind turbines, aircraft and marine vessels. This study investigates the noise produced by a large span circular cylinder in cross-flow at a Reynolds number based on diameter (ReD) of 2.2×104. Large eddy simulations and the Ffowcs Williams and Hawkings acoustic analogy were used to simulate the aerodynamic and aeroacoustic fields around both full- and reduced-span cylinders, with aspect ratios of 18.75 and 4.0 respectively. At ReD=2.2×104, there is well-documented evidence of a low-frequency modulation of the fluctuating lift force, which is evident in the present results. The modulation means that very long runtimes are required to reach statistical convergence for the full-span cylinder. The modulation is not observed in the reduced-span simulation results, which significantly reduces the time taken to reach statistical convergence. The sound pressure levels (SPL) predicted from the full-span simulation are consistently 3-6 dB below experimental values. The SPLs predicted by scaling the reduced span simulation were in better agreement with the measured values, particularly around the vortex shedding frequency. These results show that more accurate far-field acoustic predictions can be obtained by scaling the results from the reduced-span simulation, when compared to the full-span predictions. 

The predictive accuracy of wall-modelled LES is influenced by a combination of the subgrid model, the wall model, the numerical dissipation induced primarily by the convective numerical scheme, and also by the density and topology of the computational grid. The latter factor is of particular importance for industrial flow problems, where unstructured grids are typically employed due to the necessity to handle complex geometries. Here, a systematic simulation-based study is presented, investigating the effect of grid-cell type on the predictive accuracy of wall-modelled LES in the framework of a general-purpose finite-volume solver. Following standard practice for meshing near-wall regions, it is proposed to use prismatic cells. Three candidate shapes for the base of the prisms are considered: a triangle, a quadrilateral, and an arbitrary polygon. The cell-centre distance is proposed as a metric to determine the spatial resolution of grids with different cell types. The simulation campaign covers two test cases with attached boundary layers: fully-developed turbulent channel flow, and a zero-pressure-gradient flat-plate turbulent boundary layer. A grid construction strategy is employed, which adapts the grid metric to the outer length scale of the boundary layer. The results are compared with DNS data concerning mean wall shear stress and profiles of flow statistics. The principle outcome is that unstructured simulations may provide the same accuracy as simulations on structured orthogonal hexahedral grids. The choice of base shape of the near-wall cells has a significant impact on the computational cost, but in terms of accuracy appears to be a factor of secondary importance.

A complete approach for wall-modeled large-eddy simulation (WMLES) is demonstrated for the simulation of the flow around a bulk carrier in the model scale. Essential components of the method are an a-priori estimate of the thickness of the turbulent boundary layer (TBL) over the hull and to use an unstructured grid with the appropriate resolution relative to this thickness. Expressions from the literature for the scaling of the computational cost, in terms of the grid size, with Reynolds number, are adapted in this application. It is shown that WMLES is possible for model scale ship hydrodynamics, with âŒ108 grid cells, which is a gain of at least one order of magnitude as compared with wall-resolving LES. For the canonical case of a flat-plate TBL, the effects of wall model parameters and grid cell topology on the predictive accuracy of the method are investigated. For the flat-plate case, WMLES results are compared with results from direct numerical simulation, RANS (Reynolds-averaged Navier-Stokes), and semi-empirical formulas. For the bulk carrier flow, WMLES and RANS are compared, but further validation is needed to assess the predictive accuracy of the approach. 

The velocity signal of a high quality wall-resolving large eddy simulation (WRLES) of fully-developed turbulent channel flow at ReÏ = 1000 is spatially averaged over cubic boxes of size corresponding to possible choices for grid-cell size in a wall-modeled (WM)LES of the same flow. Two box sizes are considered, as well as multiple wall-normal locations of the center of the box. After applying filtering in time, the generated velocity signals are used to study algebraic wall models with respect to their ability to accurately predict the wall shear stress, ϯw. In particular, models based on the Spalding and Reichardt laws are examined. The sensitivity of ϯw with respect to the wall-normal distance of the velocity sampling point, h, the wall model and its parameters, and also to the resolution of the WMLES grid is addressed. It is shown that by using wall models with the parameters calibrated to fit the WRLES mean velocity profiles, the mean of the wall shear stress can be accurately predicted, however, no improvement for the fluctuations of this quantity is achieved. To avoid dependence of the mean predicted ϯw on h, an integrated formulation of algebraic wall models is proposed and applied to Reichardt law, leading to improved results. Finally, an idea is described and examined to increase the correlation between the predicted ϯw and reference wall shear stress through dynamically adjusting the wall model parameters. To facilitate similar studies, the generated datasets for a-priori study of WMLES are made publicly available. Copyright © Crown copyright (2018).All right reserved.

Results are reported from wall-modelled large eddy simulations (WMLES) of a zero pressure gradient flat-plate turbulent boundary layer (TBL) flow performed using unstructured computational meshes. In particular, two meshes are considered: a hex-dominant and a polyhedral. The resolution of the meshes is kept constant with respect to the local thickness of the TBL. The WMLES predictions are evaluated by comparison with reference data from direct numerical simulation (DNS) and semi-empirical expressions for the development of integral quantities along the TBL. Good agreement is observed for the skin friction coefficient, mean streamwise velocity and the Reynolds stresses. Also, the influence of the location of the sampling (matching) point of the employed algebraic wall-stress model is investigated. It is found that moving the sampling point to the third consecutive off-the-wall cell centre leads to a significant improvement in the prediction of the mean wall shear stress, as opposed to sampling for the wall-adjacent cell. 

This work presents a feature-rich open-source library for wall-modelled large-eddy simulation (WMLES), which is a turbulence modelling approach that reduces the computational cost of standard (wall-resolved) LES by introducing special treatment of the inner region of turbulent boundary layers (TBLs). The library is based on OpenFOAM and enhances the general-purpose LES solvers provided by this software with state-of-the-art wall modelling capability. The included wall models belong to the class of wall-stress models that account for the under-resolved turbulent structures by predicting and enforcing the correct local value of the wall shear stress. A review of this approach is given, followed by a detailed description of the library, discussing its functionality and extensible design. The included wall-stress models are presented, based on both algebraic and ordinary differential equations. To demonstrate the capabilities of the library, it was used for WMLES of turbulent channel flow and the flow over a backward-facing step (BFS). For each flow, a systematic simulation campaign was performed, in order to find a combination of numerical schemes, grid resolution and wall model type that would yield a good predictive accuracy for both the mean velocity field in the outer layer of the TBLs and the mean wall shear stress. The best result, â1% error in the above quantities, was achieved for channel flow using a mildly dissipative second-order accurate scheme for the convective fluxes applied on an isotropic grid with 27000 cells per ÎŽ 3 -cube, where ÎŽ is the channel half-height. In the case of flow over a BFS, this combination led to the best agreement with experimental data. An algebraic model based on Spalding’s law of the wall was found to perform well for both flows. On the other hand, the tested more complicated models, which incorporate the pressure gradient in the wall shear stress prediction, led to less accurate results. Program Summary: Program Title: libWallModelledLES Program Files doi: http://dx.doi.org/10.17632/m8dnsnp4nd.1 Licensing provisions: GPLv3 Programming language: C++ Nature of problem: Large-eddy simulation (LES) is a scale-resolving turbulence modelling approach providing a high level of predictive accuracy. However, LES of high Reynolds number wall-bounded flows is prohibitively computationally expensive due to the need for resolving the inner region of turbulent boundary layers (TBLs) [1]. This inhibits the application of LES to many industrially relevant flows [2] and prompts for the development of novel modelling techniques that would modify the LES approach in a way that allows it to retain its accuracy (at least away from walls) yet significantly lower its computational cost. Solution method: Wall-modelled LES (WMLES) is an approach that is based on complementing LES with special near-wall modelling that allows to leave the inner layer of TBLs unresolved by the computational grid. Many types of wall models have been proposed [1,3], commonly tested within the framework of in-house research codes. Here, an open-source library implementing several wall models is presented. The library is based on OpenFOAM, which is currently the most widely used general-purpose open-source software for computational fluid dynamics

Noise generated by the interaction of inflow turbulence with a lifting surface may be of interest for a number of hydrodynamic applications, including a propeller operating in non-cavitating condition. In the present work, simulation-based methods are applied for the prediction of the radiated noise from a wing in strong inflow turbulence, at a chord-based Reynolds number, Rec = 256000, and a Mach number of 0.09. The case has also been investigated experimentally, and a comparison is made between measurement and simulation results. Large-eddy simulation is employed to predict the flow, with synthetic turbulence fluctuations applied at the inflow boundary. The LES-results are then used to compute the acoustic source terms of an acoustic analogy. The sensitivity of the results to inflow turbulence level and turbulent length scale is investigated. The analysis is focused on the unsteady flow features associated with flow noise generation.

In this study, the sources of uncertainty of hot-wire anemometry (HWA) and oil-film interferometry (OFI) measurements are assessed. Both statistical and classical methods are used for the forward and inverse problems, so that the contributions to the overall uncertainty of the measured quantities can be evaluated. The correlations between the parameters are taken into account through the Bayesian inference with error-in-variable (EiV) model. In the forward problem, very small differences were found when using Monte Carlo (MC), Polynomial Chaos Expansion (PCE) and linear perturbation methods. In flow velocity measurements with HWA, the results indicate that the estimated uncertainty is lower when the correlations among parameters are considered, than when they are not taken into account. Moreover, global sensitivity analyses with Sobol indices showed that the HWA measurements are most sensitive to the wire voltage, and in the case of OFI the most sensitive factor is the calculation of fringe velocity. The relative errors in wall-shear stress, friction velocity and viscous length are 0.44%, 0.23% and0.22%, respectively. Note that these values are lower than the ones reported in other wall-bounded turbulence studies. Note that in most studies of wall-bounded turbulence the correlations among parameters are not considered, and the uncertainties from the various parameters are directly added when determining the overall uncertainty of the measured quantity. In the present analysis we account for these correlations, which may lead to a lower overall uncertainty estimate due to error cancellation Furthermore, our results also indicate that the crucial aspect when obtaining accurate inner-scaled velocity measurements is the wind-tunnel flow quality, which is more critical than the accuracy in wall-shear stress measurements.