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Experimental measurements often present corrupted data and outliers that can strongly affect the main coherent structures extracted with the classical modal analysis techniques. This effect is amplified at high frequencies, whose corresponding modes are more susceptible to contamination from measurement noise and uncertainties. Such limitations are overcome by a novel approach proposed here, the robust spectral proper orthogonal decomposition (robust SPOD), which implements the robust principal component analysis within the SPOD technique. The new technique is firstly presented with details on its algorithm, and its effectiveness is tested on two different fluid dynamics problems: the subsonic jet flow field numerically simulated, and the flow within an open cavity experimentally analyzed in [48]. The analysis of the turbulent jet data, corrupted both with salt and pepper and Gaussian noise, shows how the robust SPOD produces more converged and physically interpretable modes than the classical SPOD; moreover, the use of the robust SPOD as a tool for de-noising data, based on the signal reconstruction from de-noised modes, is also presented. Applying robust SPOD to the open cavity flow has revealed that it yields smoother spatial distributions of modes, particularly at high frequencies and when considering higher-order modes, compared to standard SPOD. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to investigate the feasibility of solving turbulent flows based on smoothed finite element method (S-FEM). Then, the differences between S-FEM and finite element method (FEM) in dealing with turbulent flows are compared. Design/methodology/approach: The stabilization scheme, the streamline-upwind/Petrov-Galerkin stabilization is coupled with stabilized pressure gradient projection in the fractional step framework. The Reynolds-averaged Navier-Stokes equations with standard k-epsilon model are selected to solve turbulent flows based on S-FEM and FEM. Standard wall functions are applied to predict boundary layer profiles. Findings: This paper explores a completely new application of S-FEM on turbulent flows. The adopted stabilization scheme presents a good performance on stabilizing the flows, especially for very high Reynolds numbers flows. An advantage of S-FEM is found in applying wall functions comparing with FEM. The differences between S-FEM and FEM have been investigated. Research limitations/implications: The research in this work is limited to the two-dimensional incompressible turbulent flow. Practical implications: The verification and validation of a new combination are conducted by several numerical examples. The new combination could be used to deal with more complicated turbulent flows. Social implications: The applications of the new combination to study basic and complex turbulent flow are also presented, which demonstrates its potential to solve more turbulent flows in nature and engineering. Originality/value: This work carries out a great extension of S-FEM in simulations of fluid dynamics. The new combination is verified to be very effective in handling turbulent flows. The performances of S-FEM and FEM on turbulent flows were analyzed by several numerical examples. Superior results were found compared with existing results and experiments. Meanwhile, S-FEM has an advantage of accuracy in predicting boundary layer profile. [ABSTRACT FROM AUTHOR]

Purpose: Impinging jets have been widely studied, and the addition of swirl has been found to be beneficial to heat transfer. As there is no literature on Reynolds-averaged Navier Stokes equations (RANS) nor experimental data of swirling jet flows generated by a rotating pipe, the purpose of this study is to fill such gap by providing results on the performance of this type of design. Design/methodology/approach: As the flow has a different behaviour at different parts of the design, the same turbulent model cannot be used for the full domain. To overcome this complexity, the simulation is split into two coupled stages. This is an alternative to use the costly Reynold stress model (RSM) for the rotating pipe simulation and the SST k-ω model for the impingement. Findings: The addition of swirl by means of a rotating pipe with a swirl intensity ranging from 0 up to 0.5 affects the velocity profiles, but has no remarkable effect on the spreading angle. The heat transfer is increased with respect to a non-swirling flow only at short nozzle-to-plate distances H/D < 6, where H is the distance and D is the diameter of the pipe. For the impinging zone, the highest average heat transfer is achieved at H/D = 5 with swirl intensity S = 0.5. This is the highest swirl studied in this work. Research limitations/implications: High-fidelity simulations or experimental analysis may provide reliable data for higher swirl intensities, which are not covered in this work. Practical implications: This two-step approach and the data provided is of interest to other related investigations (e.g. using arrays of jets or other surfaces than flat plates). Originality/value: This paper is the first of its kind RANS simulation of the heat transfer from a flat plate to a swirling impinging jet flow issuing from a rotating pipe. An extensive study of these computational fluid dynamics (CFD) simulations has been carried out with the emphasis of splitting the large domain into two parts to facilitate the use of different turbulent models and periodic boundary conditions for the flow confined in the pipe. [ABSTRACT FROM AUTHOR]

Highlights • A first Domain Decomposition (DD) method based on nodes weighting for the NIROM. • The DD uses a weighting constraint to achieve an equal accuracy in each subdomain. • The DD minimises the dynamic activity between subdomains. • The accuracy of the new DD based NIROM is improved compared to NIROM. • This method is validated using a realistic turbulent flow case at LSBU. Abstract In this paper, a new Domain Decomposition Non-Intrusive Reduced Order Model (DDNIROM) is developed for turbulent flows. The method works by partitioning the computational domain into a number of subdomains in such a way that the summation of weights associated with the finite element nodes within each subdomain is approximately equal, and the communication between subdomains is minimised. With suitably chosen weights, it is expected that there will be approximately equal accuracy associated with each subdomain. This accuracy is maximised by allowing the partitioning to occur through areas of the domain that have relatively little flow activity, which, in this case, is characterised by the pointwise maximum Reynolds stresses. A Gaussian Process Regression (GPR) machine learning method is used to construct a set of local approximation functions (hypersurfaces) for each subdomain. Each local hypersurface represents not only the fluid dynamics over the subdomain it belongs to, but also the interactions of the flow dynamics with the surrounding subdomains. Thus, in this way, the surrounding subdomains may be viewed as providing boundary conditions for the current subdomain. We consider a specific example of turbulent air flow within an urban neighbourhood at a test site in London and demonstrate the effectiveness of the proposed DDNIROM. [ABSTRACT FROM AUTHOR]

The growing availability of large scale computing power and facilities has caused a potential for increased accuracy in CFD simulations, allowing scientists and engineers to look beyond traditional Reynolds-Averaged Navier–Stokes (RANS approaches) in favor of high-fidelity simulations, characterized by high resolution in space and time, for industrially relevant flow configurations. In situ analysis and visualization is a promising solution in the exascale supercomputing era to reduce the size of data stored on disk and time spent for post-processing by using all available resources. This paper quantifies the impact of a tightly-coupled in situ approach based on ParaView Catalyst on three different codes, namely OpenFOAM, STREAmS, and MIGALE, which implement different numerical schemes and operate in different contexts (research field rather than industrial field). We show that the overhead is not negligible, it can be of the same order as the solution of the Navier–Stokes equations depending on the type of simulation, but in any case it does not prevent to solve the physical challenge under investigation. • The paper provides information about the actual implementation effort and computational cost of the approach to include in situ in a CFD code from scratch. • The paper quantifies the impact of a tightly-coupled in situ approach based on ParaView Catalyst on three different codes, namely OpenFOAM, STREAmS, and MIGALE, which they implement different numerical discretizations of the Navier–Stokes equations. • The overhead of in situ post-processing can be of the same order as the solution of the Navier–Stokes equations in high-fidelity simulations, characterized by high resolution in space and time. • The paper shows a limited impact of in situ visualization on the weak scalability of STREAmS in the range of 8-512 GPUs. [ABSTRACT FROM AUTHOR]

The paper describes a multilevel MPI+OpenMP+OpenCL parallelization approach that provides complete portability across a wide range of hybrid supercomputer architectures. A parallel CFD algorithm for heterogeneous computing of turbulent flows is presented. It simulates the compressible Navier–Stokes equations using a cell-centered finite-volume method with polynomial reconstruction on unstructured hybrid meshes. A two-level partitioning is used for the workload distribution among computing devices of hybrid nodes. The overlap of communications and computations hides the data transfer expenses. The scalability is tested on various HPC systems including a fat node with 8 GPUs and supercomputers using up to 320 GPUs. Comparison of performance is presented for multicore CPUs, Intel Xeon Phi, various GPUs of AMD and NVIDIA. The heterogeneous execution using CPUs and GPUs is studied in detail. [ABSTRACT FROM AUTHOR]

The performance of parallel implicit Large Eddy Simulations (iLES) is investigated in conjunction with high-order weighted essentially non-oscillatory schemes up to 11th-order of accuracy. Simulations were performed for the Taylor Green Vortex and supersonic turbulent boundary layer flows on High Performance Computing (HPC) facilities. The present iLES are highly scalable achieving performance of approximately 93% and 68% on 1536 and 6144 cores, respectively, for simulations on a mesh of approximately 1.07 billion cells. The study also shows that high-order iLES attain accuracy similar to strict Direct Numerical Simulation (DNS) but at a reduced computational cost. [ABSTRACT FROM AUTHOR]

The burial and scour of objects at the bottom of the ocean is governed by a range of different factors and is thus a complex system to simulate with numerical methods. In this work, we present a sediment transport model based on the lattice Boltzmann method which is capable of predicting the influence of a unidirectional flow field on the development of the sand bed in the vicinity of arbitrarily shaped objects, even under turbulent conditions and relatively low resolution. The underlying lattice Boltzmann method for the fluid phase of the simulation is governed by the three-dimensional entropic multi-relaxation time collision model and further makes use of appropriate three-dimensional grid refinement and off-lattice boundaries. The corresponding framework was implemented in the open-source Palabos library and enables accurate turbulent flow simulations around arbitrarily shaped objects without the need to tune any specific model parameters. For the simulation of the sediment transport, the advection–diffusion equation is solved via the lattice Boltzmann method with an adapted version of the entropic multi-relaxation time collision model for a three-dimensional lattice with seven discrete lattice velocities (D3Q7). The presented sediment transport model thus inherits the unconditional numerical stability and the simplicity of the entropic multi-relaxation time collision model. Furthermore, erosion and sedimentation processes are included based on the critical Shields parameter, i.e. on the wall shear stress. An initial simulation of the flow and sediment transport around a horizontally bedded finite cylinder shows that the presented model is generally able to capture the major sediment processes which govern the development of the sand bed. [ABSTRACT FROM AUTHOR]

A conservative approach for MPI-based parallelization of tridiagonal compact schemes is developed in the context of multi-block finite-volume methods. For each block, an enlarged linear system is solved by overlapping a certain number of neighbour cells from adjacent sub-domains. The values at block-to-block boundary faces are evaluated by a high-order centered approximation formula. Unlike previous methods, conservation is retained by properly re-computing the common interface value between two neighbouring blocks. Numerical tests show that parallelization artifacts decrease significantly as the number of overlapping cells is increased, at some expense of parallel efficiency. A reasonable trade-off between accuracy and performances is discussed in the paper with reference to both the spectral properties of the method and the results of fully turbulent numerical simulations. [ABSTRACT FROM AUTHOR]

An adaptive dissipation scheme is developed that preserves scalar boundedness in a high-order finite difference framework satisfying the summation-by-parts (SBP) property. A sensor is introduced that switches from an SBP dissipation operator based on a high derivative order that is efficient at absorbing the highest energy modes to a second order derivative that targets energy across scales. Conditionally- and unconditionally-stable formulations are presented. Numerical experiments of a one-dimensional advection equation and a three-dimensional turbulent round jet demonstrate that scalar boundedness can be achieved within an acceptable threshold while retaining overall high-order accuracy. Although not demonstrated here, the same approach could be employed for shock capturing as well. • Present adaptive dissipation formulations to preserve scalar boundedness. • Implemented in high-order framework satisfying the summation-by-parts property. • Conditionally- and unconditionally stable formulations presented. • Boundedness errors significantly reduced in three-dimensional turbulent jet. [ABSTRACT FROM AUTHOR]

In this paper, we describe a higher-order finite volume solver for the RANS solution of turbulent compressible flows on unstructured meshes. Considering that higher-order least-squares reconstruction suffers from poor accuracy on highly anisotropic meshes with curvature, we propose a modification to this method which highly improves the accuracy of solution approximation and output quantities. In addition, robust implementation and coupling of a RANS turbulence model with a higher-order finite volume solver is described and an efficient solution strategy is adopted for the convergence to the steady-state solution. We present our results, ranging from second- to fourth-order, for the test case of turbulent flat plate, subsonic and transonic flows over two-dimensional airfoils and also turbulent flow over a multi-element configuration. These results are verified by comparison against well-established numerical and experimental values in the literature. Our results show the advantages of higher-order methods in obtaining a more accurate solution with fewer degrees of freedom and also fast and efficient convergence to the steady-state solutions. [ABSTRACT FROM AUTHOR]

In this work we apply a high-order discontinuous Galerkin (DG) finite element method to inviscid and turbulent internal flow problems. The equations here considered are the quasi-1D, 2D Euler equations and the RANS and k − ω equations in axisymmetric coordinates. The method here proposed is designed to ensure high-order accuracy in ducts and engine-like geometries using both explicit and implicit schemes for the temporal discretization of the governing equations. Absorbing Sponge Layer (ASL) boundary conditions are implemented to minimize the reflection of out-going waves at open boundaries. A shock-capturing technique is used to control the oscillations of high-order solutions around shocks. Accurate solutions of the hyperbolic equations are performed by means of the five-stage fourth-order accurate Strong Stability Preserving Runge-Kutta scheme, while the implicit Backward-Euler scheme is adopted for efficient steady state simulations of internal turbulent flows. Two types of test-problems have been considered, one focusing on the potential of DG method to solve ideal quasi-1D and 2D waves propagation and shock phenomena that may occur in ducts, and the other on its feasibility to provide high-order accurate solutions of multi-dimensional internal turbulent flows in geometries typical of internal combustion engine (ICE) applications. To clearly illustrate the performance of the high-order DG method, the results are compared with exact solutions, experimental data and second-order accurate solutions obtained with a finite volume commercial code. [ABSTRACT FROM AUTHOR]

A comprehensive study is conducted on a second-order low-dissipation low-dispersion (LD2) scheme in scale-resolving simulations of both incompressible and compressible flows, using a node-based unstructured CFD solver. The scheme deploys a higher order central reconstruction of the face values (up to fourth-order on structured meshes) and a matrix dissipation formulation to reduce the dispersive and dissipative numerical errors. The LD2 scheme is examined for compressible flow cases involving shock discontinuities, LD2-Compressible (LD2C), and is verified in a classical shock-tube problem. The scheme is then further verified in Large-Eddy Simulations (LES) of decaying isotropic turbulence (DIT) in comparison with available experimental data. It is shown that in scale-resolving simulations, the LD2C scheme is able to significantly improve the prediction as compared to a conventional second-order central scheme. The scheme is then further assessed and verified in hybrid Reynolds-Averaged Navier–Stokes (RANS)-LES computations for the subsonic and supersonic turbulent channel flow, where excellent agreement with reference DNS and correlations are observed. Moreover, a supersonic base flow is simulated using hybrid RANS-LES, where improved predictions are observed. The LD2C scheme exploits a shock sensor incorporating vorticity and is shown to improve the prediction of the resolved shear stress in the shear layer of compression. [Display omitted] • Low-dissipation and low-dispersion scheme for scale-resolving simulations. • Parametric study of numerical scheme for improved turbulence-resolving capabilities. • Vorticity-based shock capturing sensor for predicting high-speed flows. [ABSTRACT FROM AUTHOR]

We present a numerical scheme geared for high performance computation of wall-bounded turbulent flows. The number of all-to-all communications is decreased to only six instances by using a two-dimensional (pencil) domain decomposition and utilizing the favourable scaling of the CFL time-step constraint as compared to the diffusive time-step constraint. As the CFL condition is more restrictive at high driving, implicit time integration of the viscous terms in the wall-parallel directions is no longer required. This avoids the communication of non-local information to a process for the computation of implicit derivatives in these directions. We explain in detail the numerical scheme used for the integration of the equations, and the underlying parallelization. The code is shown to have very good strong and weak scaling to at least 64 K cores. [ABSTRACT FROM AUTHOR]

A numerical model was built to study the mechanism of sedimentary bedform development in hydraulically smooth turbulent flows. The model consisted of a module for flows, a module for sediment transport, and a module for bed surface evolution. The flow was unsteady, three-dimensional and modeled by a Large-Eddy-Simulation (LES) method coupled with an Immersed-Boundary-Method (IBM). Governing equations were discretized on a fixed Cartesian grid by Finite Difference Method. Sediment (bedload) transport was estimated by Van Rijn formula corresponding to bed shear stress distribution obtained from the flow solution. The bed surface evolution was adapted to the sediment flux and described by the Exner–Polya equation. Updated bed surface was then used as the boundary for solving the flow field in the next time step. Time-advancement was discretized by the Adams–Bashforth method. The model was first validated by two test cases of bed shear stress and turbulence statistics over fixed sinusoidal bed surfaces. It was then employed to study initiation and development of bedforms from an initially flat bed to fully developed forms. Formation of newly and successively formed bedforms, growing and downstream propagating process of existing bedforms were very close to experimental observations. Instantaneous bed shear stress and corresponding sediment flux around evolving bedforms, which were difficult to observe in experiments, were also well produced by this model. [ABSTRACT FROM AUTHOR]

In this paper, high-order finite-element discretizations consisting of discontinuous Galerkin (DG) and streamline upwind/Petrov Galerkin (SUPG) methods are investigated and developed for solutions of two- and three-dimensional compressible viscous flows. Both approaches treat the discretized system fully implicitly to obtain steady state solutions or to drive unsteady problems at each time step. A modified Spalart and Allmaras (SA) turbulence model is implemented and is discretized to an order of accuracy consistent with the main Reynolds Averaged Navier-Stokes (RANS) equations using the present high-order finite-element methods. To accurately represent the real geometry configurations for viscous flows, high-order curved boundary meshes are generated via a Computational Analysis PRogramming Interface (CAPRI), while the interior meshes are deformed subsequently through a linear elasticity solver. The mesh movement procedure effectively avoids the generation of invalid elements that can occur due to the projection of curved physical boundaries and thus allows high-aspect-ratio curved elements in viscous boundary layers. Several numerical examples, including large-eddy simulations of viscous flow over a three-dimensional circular cylinder and turbulent flows over a NACA 4412 airfoil and a high-lift multi-element airfoil at high angles of attack, are considered to show the capability of the present high-order finite-element schemes in capturing typical viscous effects such as flow separation and to compare the accuracy between the high-order DG and SUPG discretization methods. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a combination of numerical methods applied to thermohydrodynamic lubrication problems with cavitation is presented. It should be emphasized the difficulty of the nonlinear mathematical coupled model involving a free boundary problem, but also the simplicity of the algorithms employed to solve it. So, finite element discretizations for the hydrodynamic and thermal equations combined with upwind techniques for the convection terms and duality methods for nonlinear features are proposed. Additionally, a model describing the movement of the shaft is provided. Considering the shaft as a rigid body this model will consist of an ODE system relating acceleration of the center of gravity and external and pressure loads. The numerical experiments of mechanical stability try to clarify the position of the neutral stability curve. Finally, a rotating machine for ship propulsion involving both axial and radial bearings operating with nonconventional lubricants (seawater to avoid environmental pollution) is analyzed by using laminar and turbulent inertial flows. [Copyright &y& Elsevier]

Highlights: [•] Thermal turbulence is studied from a strictly numerical point of view. [•] The formulation is based on the variational multiscale method. [•] The orthogonal subgrid scale method is extended to variable density flows. [•] The dissipative structure of the formulation is analyzed. [•] The turbulent channel flow has been solved obtaining solutions convergent to DNS. [Copyright &y& Elsevier]

Highlights: [•] WENO-JS, -M, -Z, and -MZ are evaluated for implicit large eddy simulation. [•] Non-reacting and reacting high-speed air/ jets are simulated. [•] Numerical experiments do not show any obvious advantages of improved WENO schemes. [ABSTRACT FROM AUTHOR]

• A novel progressive locally power-law preconditioning method (LPLPM) is employed to analyze turbulent flows. • The turbulence model coupled with the governing equations is the BSL k − ω turbulent model. • The preconditioning factors are automatically adapted using velocity-field sensors from the computational domain. • A sensitivity evaluation, including the numerical and turbulent parameters, is conducted to study the performance of the LPLPM. • LPLPM is also compared with the other preconditioning methods, including SPM, MPM, and ACM. • LPLPM enhances the rate of convergence considerably compared to other preconditioning approaches. In this study, an extended progressive preconditioning technique called the locally power-law preconditioning method (LPLPM) is employed for analyzing unsteady and steady turbulent flows for the first time. This technique's effectiveness and convergence rate are compared to the standard and Malan's preconditioning methods, along with the artificial compressibility approach. In all methods, a finite volume and the four-stage local time Runge-Kutta schema are applied to discretize the space and the time derivative terms of governing equations, respectively. In the LPLPM, the preconditioning matrix is automatically adapted using flow-field velocity sensors via a power-law relation. The turbulence model coupled with the governing equations is the BSL- k − ω model. To investigate the efficiency and accuracy of the LPLPM, different test cases are performed for flows over S809, NACA0012, and ONERA airfoils at various Reynolds numbers (1.0 × 106 to 5.25 × 106) and angles of attack (0 to 16 (deg)). A sensitivity evaluation is performed to investigate the impacts of numerical variables on the solution. The results reveal that the LPLPM is accurate, robust, and computationally efficient in simulating turbulent flows and significantly improves the convergence rate up to 71% and 43% reductions in the iteration number for simulating steady and unsteady turbulent flows, respectively. [ABSTRACT FROM AUTHOR]

This study discusses the effect of combined first-order, bulk and wall reactions on the overall intensity of mass withdrawal in smooth pipes. A one-dimensional model for the transport of high Schmidt number compounds (where the Schmidt number is the ratio of the fluid viscosity to the solute diffusivity) is extended with the effect of a first-order bulk reaction. Profiles of velocity and eddy diffusivity are obtained with a standard high Reynolds number k-ε closure in combination with a modified Van Driest wall function. By comparing the results of the 1D model to an analytical asymptotic high Sc approximation, it is shown that the interaction between bulk and wall reactions is very weak. In terms of the decay coefficient, a maximum deviation of 4% is observed for very high bulk demand due to the attenuation of the streamwise solute mass flux. The parameter range for which the concentration profiles can be safely assumed to be uniform, both in the viscous sublayer and in the bulk, is established. [ABSTRACT FROM AUTHOR]

Abstract: This paper may be considered as a sequel to one of our earlier works pertaining to the development of an upwind algorithm for meshless solvers. While the earlier work dealt with the development of an inviscid solution procedure, the present work focuses on its extension to viscous flows. A robust viscous discretization strategy is chosen based on positivity of a discrete Laplacian. This work projects meshless solver as a viable cartesian grid methodology. The point distribution required for the meshless solver is obtained from a hybrid cartesian gridding strategy. Particularly considering the importance of an hybrid cartesian mesh for RANS computations, the difficulties encountered in a conventional least squares based discretization strategy are highlighted. In this context, importance of discretization strategies which exploit the local structure in the grid is presented, along with a suitable point sorting strategy. Of particular interest is the proposed discretization strategies (both inviscid and viscous) within the structured grid block; a rotated update for the inviscid part and a Green-Gauss procedure based positive update for the viscous part. Both these procedures conveniently avoid the ill-conditioning associated with a conventional least squares procedure in the critical region of structured grid block. The robustness and accuracy of such a strategy is demonstrated on a number of standard test cases including a case of a multi-element airfoil. The computational efficiency of the proposed meshless solver is also demonstrated. [ABSTRACT FROM AUTHOR]

In this paper we investigate the impact of the floating-point precision and interpolation scheme on the results of direct numerical simulations (DNS) of turbulence by pseudo-spectral codes. Three different types of floating-point precision configurations show no differences in the statistical results. This implies that single precision computations allow for increased Reynolds numbers due to the reduced amount of memory needed. The interpolation scheme for obtaining velocity values at particle positions has a noticeable impact on the Lagrangian acceleration statistics. A tri-cubic scheme results in a slightly broader acceleration probability density function than a tri-linear scheme. Furthermore the scaling behavior obtained by the cubic interpolation scheme exhibits a tendency towards a slightly increased degree of intermittency compared to the linear one. [Copyright &y& Elsevier]

Abstract: An innovative computational model is presented for the large eddy simulation (LES) modeling of multi-dimensional unsteady turbulent flow problems in external flow field. Based on the LES principles, the model uses a pressure projection method to solve the Navier–Stokes equations in transient condition. The turbulent motion is simulated by Smagorinsky sub-grid scale (SGS) eddy viscosity model. The momentum equation of the flow motion is solved using a three-step finite element method (FEM). The external flow field is simulated using a boundary element method (BEM) by solving a pressure Poisson equation that assumes the pressure as zero at the infinity. Through extracting the boundary effects on a specified finite computational domain, the model is able to solve the infinite boundary value problems. The present model is used to simulate the flows past a two-dimensional square rib and a three-dimensional cube at high Reynolds number. The simulation results are found to be reasonable and comparable with other models available in the literature even for coarse meshes. [Copyright &y& Elsevier]

Scale resolving numerical methods are necessary to accurately predict the afterbody flows of space launchers. Numerical workflows built for such computations have to be designed in order to obtain a minimal numerical dissipation to resolve fine turbulent structures, a sufficient robustness to capture eventual shock waves and both a reasonable computational cost and an acceptable user workload enabling comparative design studies. This article presents the development of a hybrid numerical framework based on Ducros's sensor and designed to switch from a low-dissipation formulation in presence of vortical structures to a robust formulation around high gradients. This hybrid workflow is used with ZDES, including its latest automatic mode (ZDES mode 2 (2020)), to simulate a transonic space launcher afterbody experiment with and without a cold air propulsive jet [1,2]. The salient physical properties of the base flow are investigated and the evaluation of computed results follows the extended nomenclature for validation of simulation techniques [3] from level 0 to 5. The framework is thus validated for instantaneous and mean flow visualizations, base and extension mean pressure coefficient distribution, pressure fluctuation levels, one-point and two-point spectral analyses. Such an efficient automatic RANS/LES strategy could be well suited to study realistic launcher afterbody geometries with propulsive jet(s). [ABSTRACT FROM AUTHOR]

• Optimization techniques are applied to turbulence model calibration of a planar mixing layer. • The Nelder–Mead simplex method was faster and more optimal than a genetic algorithm for the current work. • Sensitivity to the optimization function is observed. • Adjusting three pressure-strain coefficients is most effective. • Adjusting additional coefficients provides diminishing returns. This article discusses the use of numerical optimization procedures to aid in the calibration of turbulence model coefficients. Such methods would increase the rigor and repeatability of the calibration procedure by requiring clearly defined and objective optimization metrics, and could be used to identify unique combinations of coefficient values for specific flow problems. The approach is applied to the re-calibration of an explicit algebraic Reynolds stress model for the incompressible planar mixing layer using the Nelder–Mead simplex algorithm and a micro-genetic algorithm with minimally imposed constraints. Three composite fitness functions, each based upon the error in the mixing layer growth rate and the normal and shear components of the Reynolds stresses, are investigated. The results demonstrate a significant improvement in the target objectives through the adjustment of three pressure-strain coefficients. Adjustments of additional coefficients provide little further benefit. Issues regarding the effectiveness of the fitness functions and the efficiency of the optimization algorithms are also discussed. [ABSTRACT FROM AUTHOR]

• No-slip boundary treatments are directly compared in two turbulent flows. • Local turbulent statistics around solid interfaces are provided. • IBM cannot predict dissipation rate correctly near solid interfaces. • IBM has better robustness in suppressing acoustic noise in LBM. In the first part of this study [1], we compared the performances of two categories of no-slip boundary treatments, i.e. , the interpolated bounce-back schemes and the immersed boundary methods in a series of laminar flow simulations within the lattice Boltzmann method. In this second part, these boundary treatments are further compared in the simulations of turbulent flows with complex geometry to provide a next-level assessment of these schemes. Two non-trivial turbulent flow problems, a fully developed turbulent pipe flow at a low Reynolds number, and a decaying homogeneous isotropic turbulent flow laden with a large number of resolved spherical particles are considered. The major problem of the immersed boundary method revealed by the present study is its incapability in computing the local velocity gradients inside the diffused interface, which can result in significantly underestimated dissipation rate and viscous diffusion locally near the particle surfaces. Otherwise, both categories of the no-slip boundary treatments are able to provide accurate results for most of turbulent statistics in both the carrier and dispersed phases, provided that sufficient grid resolutions are used. The criteria of sufficient grid resolutions for each examined flow are also addressed in this document. [ABSTRACT FROM AUTHOR]

• New measures of the propagation error in a DNS database for the turbulent flow in a square duct are provided. • Data-driven models that have the Reynolds stress tensor as target are subjected to this error. • We propose to have as target, the Reynolds force vector, the divergence of the Reynolds stress tensor. • Results show that the proposed approach is promissing. Data-driven turbulence modeling is receiving considerable attention specially when Direct Numerical Simulations (DNS) are the physics-informed learning environment and Reynolds average Navier–Stokes (RANS) simulations are the injected environment. A caveat of such approach, is that some studies indicate the existence of an intrinsic error in the Reynolds stress tensor provided by reputable DNS databases that, although small, lead to a reconstructed mean velocity field with a flagrant inaccuracy. This fact imposes a huge challenge in data-driven and traditional RANS models and is becoming a concern in the very recent literature. In the present work, we propose to replace the Reynolds stress tensor by its divergence, the Reynolds force vector , as a target for the machine learning technique. Since the Reynolds force vector can be computed from first order statistics, this estimate is not contaminated by the errors associated with applying the divergence onto the Reynolds stress tensor available in the DNS databases, circumventing the problem exposed above. The turbulent flow through a square duct was chosen as the case to be analyzed. Employing a κ –ϵ RANS model as injection environment, the non-persistence-of-straining tensor to compose the set of inputs, and a neural network architecture as the ML technique to bridge the injected and learning environments, we compared the proposed strategy with the approach commonly used in the literature, i.e. to correct the Reynolds stress tensor. The results demonstrate that the Reynolds force vector correction is able to reconstruct the mean velocity field with a higher fidelity with respect to the DNS data. [ABSTRACT FROM AUTHOR]