The flow around a four-bladed marine propeller in homogeneous inflow and in non-cavitating conditions is investigated using Large Eddy Simulation, LES. Explicit, using a k-equation eddy viscosity model, and implicit subgrid modeling are compared for both the standard LES formulation as well as a mixed formulation containing the, so called, scale similarity term. A wall-modeled approach is used on a relatively coarse grid, containing 5.5 million cells, for the full propeller in order to mimic a future applied computation including the ship hull. The implicit modeling is of particular interest in cavitation simulation, where the interaction between an explicit subgrid model and the liquid-vapor interface may cause numerical and modeling problems. All simulations yield fairly similar results, although the implicit LES gives better prediction of the global performance of the propeller. The agreement with experimental data is good close to the propeller, but the simulated flow structures diffuses quickly at the present grid resolution. 

This paper describes the M-Xplore extension of the Radioss software. The module contains facilities for the exploration of a parameterized finite element model design space. It supports facilities for interactive choice of variables and responses, definition of a sampling on a design space, automatic submission of the computations, and post-processing of the results. The computations are run automatically, either locally or in ASP-mode, i.e. as a client of a high-performance computing server. The software is described first in general, then we illustrate its exploration possibilities in terms of a model problem and a more typical application problem of crash simulation

Large eddy simulation (LES) has emerged as the next-generation simulation tool for handling complex engineering, geophysical, astrophysical, and chemically reactive flows. As LES moves from being an academic tool to being a practical simulation strategy, the robustness of the LES solvers becomes a key issue to be concerned with, in conjunction with the classical and well-known issue of accuracy. For LES to be attractive for complex flows, the computational codes must be readily capable of handling complex geometries. Today, most LES codes use hexahedral elements; the grid-generation process is therefore cumbersome and time consuming. In the future, the use of unstructured grids, as used in Reynolds-averaged NavierâStokes (RANS) approaches, will also be necessary for LES. This will particularly challenge the development of high-order unstructured LES solvers. Because it does not require explicit filtering, Implicit LES (ILES) has some advantages over conventional LES; however, numerical requirements and issues are otherwise virtually the same for LES and ILES. In this chapterwe discuss an unstructured finite-volume methodology for both conventional LES and ILES, that is particularly suited for ILES. We believe that the next generation of practical computational fluid dynamics (CFD) models will involve structured and unstructured LES, using high-order flux-reconstruction algorithms and taking advantage of their built-in subgrid-scale (SGS) models. ILES based on functional reconstruction of the convective fluxes by use of high-resolution hybrid methods is the subject of this chapter. We use modified equation analysis (MEA) to show that the leading-order truncation error terms introduced by such methods provide implicit SGS models similar in form to those of conventional mixed SGS models.

The predictive capability for Large Eddy Simulation (LES) for flows with unsteady separation at curved surfaces is investigated. LES with near wall modeling is applied and a range of subgrid models is evaluated for four different validation cases for which experimental flow measurement data is available. A novel model for the simulation of the effect of boundary layer tripping devices used in experiments is also proposed and evaluated. The validation focuses on the mean velocity distribution in the region of separation, as well as on the skin friction and the surface streamlines. The flow physics of separation for these four cases is illustrated and discussed in detail. 

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.

Almost all flows of practical interest are turbulent, and thus the simulation of turbulent flow and its diversity of flow characteristics remains one of the most challenging areas in the field of classical physics. In many situations the fluid can be considered incompressible; that is, its density is virtually constant in the frame of reference, moving locally with the fluid, but density gradients may be passively convected with the flow. Examples of such flows of engineering importance are as follows: external flows, such as those around cars, ships, buildings, chimneys, masts, and suspension bridges; and internal flows, such as those in intake manifolds, cooling and ventilation systems, combustion engines, and applications from the areas of biomedicine, the process industry, the food industry, and so on. In contrast to free flows (ideally considered as homogeneous and isotropic), wall-bounded flows are characterized by much less universal properties than free flows and are thus even more challenging to study. The main reason for this is that, as the Reynolds number increases, and the thickness of the viscous sublayer decreases, the number of grid points required to resolve the near-wall flow increases. The two basic ways of computing turbulent flows have traditionally been direct numerical simulation (DNS) and Reynolds-averaged NavierâStokes (RANS) modeling. In the former the time-dependent NavierâStokes equations (NSE) are solved numerically, essentially without approximations. In the latter, only time scales longer than those of the turbulent motion are computed, and the effect of the turbulent velocity fluctuations is modeled with a turbulence model. 

In this paper, a methodology is presented for modelling underwater noise emissions from ships based on realistic vessel activity in the Baltic Sea region. This paper combines the Wittekind noise source model with the Ship Traffic Emission Assessment Model (STEAM) in order to produce regular updates for underwater noise from ships. This approach allows the construction of noise source maps, but requires parameters which are not commonly available from commercial ship technical databases. For this reason, alternative methods were necessary to fill in the required information. Most of the parameters needed contain information that is available during the STEAM model runs, but features describing propeller cavitation are not easily recovered for the world fleet. Baltic Sea ship activity data were used to generate noise source maps for commercial shipping. Container ships were recognized as the most significant source of underwater noise, and the significant potential for an increase in their contribution to future noise emissions was identified. 

Estimates of the noise source spectra of ships based on long term measurements in the Baltic sea are presented. The measurement data were obtained by a hydrophone deployed near a major shipping lane south of the island land. Data from over 2,000 close-by passages were recorded during a 3 month period from October to December 2014. For each passage, ship-to-hydrophone transmission loss (TL) spectra were computed by sound propagation modeling using 1. bathymetry data from the Baltic Sea Bathymetry Database (BSBD), 2. sound speed profiles from the HIROMB oceanographic model, 3. seabed parameters obtained by acoustic inversion of data from a calibrated source, and 4. AIS data providing information on each ship’s position. These TL spectra were then subtracted from the received noise spectra to estimate the free field source level (SL) spectra for each passage. The SL were compared to predictions by some existing models of noise emission from ships. Input parameters to the models, including e.g., ship length, width, speed, displacement, and engine mass, were obtained from AIS (Automatic Identification System) data and the STEAM database of the Finnish Metereological Institute (FMI). 

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.

Large Eddy Simulation is employed for the investigation of the wake instability ofa submarine propeller in open water conditions. The time-resolved velocity and pressure fields are computed in the near wake where the instability occurs. The study focuses in particular on the analysis of complete time series of the flow velocity at the probes in the wake. The results are compared with Laser Doppler Velocimetry measurements. © 2010 Institute of Ship Technology and Ocean Engineering.

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.

A review is presented of estimates for the grid resolution requirements for wall-resolved and wall-modeled large-eddy simulation, of flows with an important influence of turbulent boundary layers. The estimates are described within a classification scheme which is formulated based on the grid resolution relative to the length scales of the turbulent boundary layer. The grid resolution estimates are then applied to discuss the computational cost of ship hull hydrodynamics simulations, both in model and in full scale. The hulls of one submarine and one bulk carrier are included in this discussion. Two simulation cases are included in the paper to demonstrate a complete simulation methodology, to illustrate the implications of the grid resolution estimates, and to investigate the resulting predictive accuracy. The first simulation case consists of fully developed turbulent channel flow, for which a comparison is made with direct numerical simulation results. The second simulation case consists of the flow around an axisymmetric body, which is based on a bare-hull version of a generic submarine model, and for which wind tunnel measurement data are available for validation.

High Reynolds number wall bounded flow is here investigated using Large Eddy Simulation (LES), Detached Eddy Simulation (DES) and Reynolds Averaged Navier Stokes (RANS). The first case considered is the fully developed turbulent channel flow at Re, « 395,590,1800 and 10,000. This flow clearly indicates the development of the undisturbed boundary layer and related events, such as streaky structures, hairpin vortices and ejection events. The second case is the flow over an axisymmetric hill in a channel, here the flow contains complex structures such as a turbulent boundary layer with several unsteady separations and reattachments. It is three-dimensional due to both streamwise and spanwise pressure gradients on the lee-side of the hill. The shallowness of the separation region makes the flow a demanding test case for any computational fluid dynamics model. The third case is the flow past an axisymmetric submarine hull with an elliptic forebody and a smoothly tapered stern - the DARPA Suboff model AFF-1. This flow case is highly demanding due to the long midship section, on which the boundary layer is developed, in combination with the elliptic forebody and the tapered stern. Both LES and DES performs well in all cases considered, while RANS has slightly lower accuracy in the channel flow and the axisymmetric hull, and fails to predict some flow features for the axisymmetric hill. Also DES has some problems with the axisymmetric hill case, related to the inlet condition of the modified eddy viscosity.

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. 

An unstructured grid generation approach for wall-modelled LES is proposed. The applicability of the approach is demonstrated for the simulation of the flow around an axisymmetric body, at Re-number 5.48·106, which is representable of model scale ship hydrodynamics. A numerical trip wire must be employed to induce resolved fluctuations in the simulated boundary layer. The predictive accuracy of the simulation technique is evaluated for the flow around the axisymmetric body, and for the computation of a turbulent boundary layer on a flat plate. For the flat plate, comparison is made with results from direct numerical simulation. For the axisymmetric body, results from wall-modelled LES, RANS and experimental measurements are compared. 

We present new bounds for the solution of the resolvent equation for plane Couette flow. Both analytic methods and computation, using the Chebyshev tau spectral method, are used. The emphasis is on determining the Reynolds number-dependence of the estimates. The main result is the introduction of a weighted norm, which leads to optimal asymptotic behavior of the resolvent for large Reynolds numbers.

We prove nonlinear stability for finite amplitude perturbations of plane Couette flow. A bound of the solution of the resolvent equation in the unstable complex half-plane is used to estimate the solution of the full nonlinear problem. The result is a lower bound, including Reynolds number dependence, of the threshold amplitude below which all perturbations are stable. Our result is an improvement of the corresponding bound derived in [3].

Sufficient conditions for nonlinear stability of viscous shock wave solutions of systems of conservation laws are given. The analysis applies to strong shocks of Lax type but is restricted to perturbations with zero mass. We use the Laplace transform and reduce the question of stability to a spectral condition for the resolvent equation of the linearized problem.

The flow around a stream-wise oscillating circular cylinder in a steady uniform flow is computed using large eddy simulations (LES). A finite volume method with capability to handle moving meshes is employed. The results are used for comparison with experiments for validation of the algorithms. Another goal of the investigation is to obtain information, complimentary to that of the experiments, about the process of vortex shedding, the structure of the flow in the wake and the relation between this and the forces acting on the cylinder. 

Lately there has been a regenerated interest in approximative methods for calculating low frequency electromagnetic scattering in lossy media. In this paper we focus on scattering from smooth bodies submerged in a dissipative half-space. The proposed method utilizes a surface integral formulation which is discretized using the boundary element method (BEM) and point matching. Results are given for various electromagnetic dipole sources. The incoming field is obtained from an extension of the classical ideas found in Banos [1] utilizing the Foster-Lien integral. The dyadic Green’s function needed in the surface integral equation is calculated using this quasi-static approach. The approximation is validated by a comparison with results from direct numerical integration of the Sommerfeld integrals. The speed and accuracy of the two methods is discussed.

A crucial component in the statistical simulation of a computationally expensive model is a good design of experiments. In this paper we compare the efficiency of the columnwise-pairwise (CP) and genetic algorithms for the optimization of Latin hypercubes (LH) for the purpose of sampling in statistical investigations. The performed experiments indicate, among other results, that CP methods are most efficient for small and medium size LH, while an adopted genetic algorithm performs better for large LH. Two optimality criteria suggested in the literature are evaluated with respect to statistical properties and efficiency. The obtained results lead us to favor a criterion based on the physical analogy of minimization of forces between charged particles suggested in Audze and Eglais (1977. Problems Dyn. Strength 35, 104-107) over a ’maximin distance’ criterion from Johnson et al. (1990. J. Statist. Plann. Inference 26, 131-148).'

A general method for computational fluid dynamics with boundaries moving in any prescribed fashion, is presented. The method adapts the boundary fitted mesh to the changing spatial domain by deforming it and, when necessary due to grid quality requirements, regenerating it. Finite volumes are used for the discretisation of Navier-Stokes equations, the mesh is regenerated by the advancing front method and an elliptic differential equation, or an analytical expression, is used to compute the mesh motion. Results are presented for two model problems.

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.

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. 

The use of a precursor simulation of fully developed turbulent channel flow for the generation of turbulent boundary layer (TBL) inflow data is investigated. Based on the desired properties of the TBL, a complete procedure is described for how to specify the precursor simulation. The key feature of the specification is to match the momentum thickness of the precursor to that of the inflow TBL. The inflow data is then constructed from time- and space-dependent flow data in a cross-plane of the precursor. The proposed procedure removes the need to rescale the flow data and thus violate the governing equations, as is common practice in other state-of-the-art inflow generation methods for TBLs. The adaption length of the generated TBL is investigated using wall-resolved large-eddy simulation (WRLES) for a zero-pressure gradient (ZPG-) TBL, with a momentum thickness Reynolds number in the interval 830–2 400. The results are compared with a solution obtained using a standard rescaling procedure for the inflow data. The adaption length is shown to be similar for the two methods. Practical differences and advantages of the proposed of method, as compared to other inflow generation techniques, are assessed. These involve the role of the auxiliary simulation, channel flow in the present case, in the overall computational procedure, as well as data handling, initial transients and adaption lengths.

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

The flow around an axisymmetric hill, mounted in a channel with a fully developed approach flow, is investigated. The flow contains complex structures such as a turbulent boundary layer with several unsteady separations and reattachments. It is highly three-dimensional due to both streamwise and spanwise pressure gradients on the leeside of the hill. The shallowness of the separation region makes the flow a very demanding test case for any computational fluid dynamics model. Three different strategies are used in this study: Reynolds-averaged Navier-Stokes (RANS), large eddy simulation (LES), and detached eddy simulation (DES). The computed flow, in terms of velocity and pressure profiles, compared with measurement data and the results show that LES and DES are indeed capable of handling this complicated flow in a correct way whereas RANS clearly fails to predict several important flow features. Furthermore, the influence of the size of the computational domain, the grid resolution and the inflow boundary conditions is also studied. It is found that the pressure field is sensitive to the location of the inlet and the DES model is very sensitive to the inlet boundary condition on the eddy viscosity. To significantly improve the predictions, it is believed that the near-wall resolution must be increased substantially, in particular in the spanwise direction, or a better wall handling has to be incorporated. 

The effect of grid resolution on a large eddy simulation (LES) of a wall-bounded turbulent flow is investigated. A channel flow simulation campaign involving a systematic variation of the streamwise (Îx) and spanwise (Îz) grid resolution is used for this purpose. The main friction-velocity-based Reynolds number investigated is 300. Near the walls, the grid cell size is determined by the frictional scaling, Îx+ and Îz+, and strongly anisotropic cells, with first Îy+ ⌠1, thus aiming for the wall-resolving LES. Results are compared to direct numerical simulations, and several quality measures are investigated, including the error in the predicted mean friction velocity and the error in cross-channel profiles of flow statistics. To reduce the total number of channel flow simulations, techniques from the framework of uncertainty quantification are employed. In particular, a generalized polynomial chaos expansion (gPCE) is used to create metamodels for the errors over the allowed parameter ranges. The differing behavior of the different quality measures is demonstrated and analyzed. It is shown that friction velocity and profiles of the velocity and Reynolds stress tensor are most sensitive to Îz+, while the error in the turbulent kinetic energy is mostly influenced by Îx+. Recommendations for grid resolution requirements are given, together with the quantification of the resulting predictive accuracy. The sensitivity of the results to the subgrid-scale (SGS) model and varying Reynolds number is also investigated. All simulations are carried out with second-order accurate finite-volume-based solver OpenFOAM. It is shown that the choice of numerical scheme for the convective term significantly influences the error portraits. It is emphasized that the proposed methodology, involving the gPCE, can be applied to other modeling approaches, i.e., other numerical methods and the choice of SGS model. 

Grid requirements for LES of wall-bounded flows are considered. The setting is a zero pressure gradient turbulent boundary layer on a flat plate, but the results are intended to be of use generally for the simulation of flows with an important influence of turbulent boundary layers. The basis for the grid estimates are expressions for the thickness and the viscous length scale of a turbulent boundary layer. The literature is reviewed, and a new power law is proposed, the coefficients of which have been determined using recent high-Re experimental data. An estimation for the number of grid points required for NWM-LES is derived, which is more general than previously published such estimates. A complete simulation methodology, including a numerical tripping device for transition to turbulence in the boundary layer, is demonstrated for NWM-LES of a flat plate turbulent boundary layer. The predictive accuracy is assessed by comparison with DNS data.

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.

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.

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 flow around the DTMB5415 surface combatant hull with and without bilge keels is studied numerically by the use of a Large Eddy Simulation (LES) method. The main purpose for bilge keels is to reduce roll motions when operating in ocean waves. The flow resistance should at the same time be affected as little as possible. The computations presented here are for straight course in calm water conditions and show only a small change in the wake of the hull and there by a minimal effect on the resistance. The computations are made as a preparatory step before investigating other conditions, such as yaw angle.