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Purpose: The development of reusable space launchers requires a comprehensive knowledge of transonic flow effects on the launcher structure, such as buffet. Indeed, the mechanical integrity of the launcher can be compromised by shock wave/boundary layer interactions, that induce lateral forces responsible for plunging and pitching moments. Design/methodology/approach: This paper aims to report numerical and experimental investigations on the aerodynamic and aeroelastic behavior of a diamond airfoil, designed for microsatellite-dedicated launchers, with a particular interest for the fluid/structure interaction during buffeting. Experimental investigations based on Schlieren visualizations are conducted in a transonic wind tunnel and are then compared with numerical predictions based on unsteady Reynolds averaged Navier–Stokes and large eddy simulation (LES) approaches. The effect of buffeting on the structure is finally studied by solving the equation of the dynamics. Findings: Buffeting is both experimentally and numerically revealed. Experiments highlight 3D oscillations of the shock wave in the manner of a wind-flapping flag. LES computations identify a lambda-shaped shock wave foot width oscillations, which noticeably impact aerodynamic loads. At last, the experiments highlight the chaotic behavior of the shock wave as it shifts from an oscillatory periodic to an erratic 3D flapping state. Fluid structure computations show that the aerodynamic response of the airfoil tends to damp the structural vibrations and to mitigate the effect of buffeting. Originality/value: While buffeting has been extensively studied for classical supercritical profiles, this study focuses on diamond airfoils. Moreover, a fluid structure computation has been conducted to point out the effect of buffeting. [ABSTRACT FROM AUTHOR]

• Numerical simulations of cavitating internal flow in injectors. • Stochastic sensitivity analysis and calibration of cavitation model parameters. • The calibrated set-up gives accurate predictions of the main quantities of interest. • Improvements in predicting hydraulic flip in 3-phase flow conditions. A stochastic analysis of the cavitation model parameter sensitivity is carried out for internal flows relevant to injector configurations. Stochastic methodologies, namely generalized polynomial chaos and stochastic collocation, are used to obtain continuous response surfaces of the quantities of interest in the parameter space starting from a limited number of simulations. Cavitation is modeled through a transport equation for the void fraction closed by the Schnerr-Sauer relation, containing four free parameters. As for turbulence, the URANS equations are considered, together with two different closure models. The sensitivity to the cavitation model parameters is investigated, first, for a throttle geometry, for which experimental and LES data are available. First, two out of the four parameters are identified as the most important through a preliminary analysis based on 2D simulations, namely the vaporization and condensation factors. Then, the sensitivity of 3D simulation results to the previously identified most important parameters is investigated. The stochastic range of variability of the results contains the reference data. Thus, a parameter optimization is carried out in order to obtain the values giving the best agreement with the LES data. It is then shown that the cavitation parameter sensitivity is practically independent of the working fluid. Finally, it is shown that the calibrated cavitation model can be successfully applied to a different configuration, characterized by the hydraulic flip phenomenon, namely a 3-phase case in which liquid N-heptane flows from an inlet reservoir through a circular channel in an outlet reservoir where air is present. [ABSTRACT FROM AUTHOR]

The present paper addresses the issue of the strong dependency of eddy-resolving simulations for turbulent flows on the employed inflow conditions. Thus, the objective of this study is to analyze the influence of the inflow conditions on the external wall-bounded flow past the SD7003 airfoil and more precisely on the form and size of the laminar separation bubble. Motivated by the typically coarse resolution of the inlet region of the computational domain used for hybrid simulations, the synthetic turbulence is introduced within the flow field by a flexible source term treatment. The generated turbulent fluctuations are taken into account in the momentum equation by source terms and hence allow a shift from the inlet to a finer resolved region, where the damping of small structures due to the grid resolution is negligible. To provide a proper formulation of a synthetic turbulence inflow generator (STIG), the digital filter concept of Klein et al. (J. Comp. Phys. 186, 652–665, 2003) is merged with a large-eddy simulation (LES) as well as a hybrid LES-URANS method. The synthetically generated velocity fluctuations are distributed in an area of influence which is in accordance with the digital filter concept of the STIG. An automatic calculation of the dimension of the influence region is ensured by the employment of the integral scales which are used during the generation of the synthetic turbulence inflow generator inflow. The definition of the required input quantities for the STIG in case of the flow past a SD7003 airfoil at Re c = 60 , 000 and an angle of attack α = 4 ∘ are based on experimental data including a turbulence intensity of TI = 0.28%. Due to separation, transition and subsequent reattachment this is a demanding test case in which the shape and the size of the separation bubble strongly depends on the oncoming turbulence. The reference velocity profiles of the experimental measurements are compared with a wall-resolved LES and hybrid simulations performed on two grids with a coarser resolution. The evaluation of the results of the simulations applying the STIG and without turbulence intensity showed an improved level of agreement between the STIG based simulations and the experiment. Moreover, the turbulence intensity is varied to understand the behavior of the LSB in more detail. [ABSTRACT FROM AUTHOR]

Air exhalation in the laryngeal area produces a complex transitional flow. A detailed investigation of the resulting flow features is very interesting from a fundamental point of view and useful to support medical researchers regarding treatment options. In the current study, the highly complex, unsteady, transitional flow in a patient-specific geometry has been described by three different numerical approaches, (1) using the unsteady Reynolds-averaged Navier–Stokes (URANS) equations with the k − ω shear stress transport (SST) turbulence model, (2) by large-eddy simulation (LES) with the WALE subgrid-scale model, and finally (3) using direct numerical simulation (DNS) – without any model needed. These three approaches deliver different pictures of the resulting flow, obviously with noticeable differences in terms of accuracy, but also regarding the associated computational efforts. The present article is devoted to the detailed comparison of these three approaches, using the DNS results as a reference. It has been found that even though URANS is able to predict with a fair accuracy the large-scale features of velocity and pressure in an average sense, it fails to predict correctly most relevant turbulent features; large differences are found concerning fluctuations, power spectral density, or spectral entropy. On the other hand, LES is able to reproduce both small-scale and large-scale flow features in a good agreement compared to DNS; additionally, LES delivers an appropriate description of the evolution of spectral entropy and can thus be used to predict flow transition. Obviously, DNS delivers an even more detailed and accurate view of the flow properties, but this comes with a tremendous increase in computing effort. For this configuration, a high-quality LES appears as best compromise between accuracy and computational requirements and, therefore, best suitable for medical studies of transitional airflows. All data-sets are publicly available under Abdelsamie et al. [1]. • Laryngeal exhalation flow computed in a patient-specific geometry. • Providing DNS reference data for a patient-specific geometry, all reference data-sets are publicly available under https://data.mendeley.com/datasets/2c6vy2px9y. • Three numerical approaches are compared: DNS, LES, URANS. • URANS can predict neither relevant turbulent properties nor flow transition. • Good agreement is found between LES and DNS for all important quantities. [ABSTRACT FROM AUTHOR]

In this work we investigate the use of adaptive linearly implicit Rosenbrock-type Runge–Kutta and Explicit Singly Diagonally Implicit Runge–Kutta schemes to integrate in time high-order Discontinuous Galerkin space discretizations of the incompressible Navier–Stokes (INS) and Reynolds Averaged Navier–Stokes (URANS) equations. The objective of this activity is to assess the efficiency and accuracy of the considered schemes coupled with a time-step adaptation technique for incompressible URANS simulations. The schemes have been first investigated for the computation of the laminar travelling waves and of the turbulent flow around a circular cylinder at a Reynolds number R e = 5 × 10 4 , verifying the convergence order, a simple relation to set the system tolerance starting from the tolerance of the adaptation strategy, and their computational efficiency. Finally, the best scheme resulting from our analysis has been applied to the URANS simulation of the flow through a vertical axis wind turbine, comparing the results with CFD and experimental data available in literature. [ABSTRACT FROM AUTHOR]

Unsteady Reynolds Average Navier–Stokes simulation and Zonal Detached Eddy Simulation of an OAT15A airfoil in transonic buffet conditions are used to investigate the abilities of several stochastic estimation methods to reconstruct and predict turbulent flow. In particular the comparison between the two simulations estimations allows us to identify limitations of these methods in relation to some characteristics of turbulent flow. A sensor location optimization algorithm is proposed and tested, as well as its extension to the choice of delays for the multi-time-delay stochastic estimation. The impact of Gaussian noise is also assessed. [ABSTRACT FROM AUTHOR]

Purpose – The purpose of this paper is to provide the current state of the art in the development of a computer code combining an immersed boundary method with a conjugate heat transfer (CHT) approach, including some new findings. In particular, various treatments of the fluid-solid-interface conditions are compared in order to determine the most accurate one. Most importantly, the method is capable of computing a challenging three dimensional compressible turbulent flow past an air cooled turbine vane. Design/methodology/approach – The unsteady Reynolds-averaged Navier–Stokes (URANS) equations are solved within the fluid domain, whereas the heat conduction equation is solved within the solid one, using the same spatial discretization and time-marching scheme. At the interface boundary, the temperatures and heat fluxes within the fluid and the solid are set to be equal using three different approximations. Findings – This work provides an accurate and efficient code for solving three dimensional CHT problems, such as the flow through an air cooled gas turbine cascade, using a coupled immersed boundary (IB) CHT methodology. A one-to-one comparison of three different interface-condition approximations has shown that the two multidimensional ones are slightly superior to the early treatment based on a single direction and that the one based on a least square reconstruction of the solution near the IB minimizes the oscillations caused by the Cartesian grid. This last reconstruction is then used to compute a compressible turbulent flow of industrial interest, namely, that through an air cooled gas turbine cascade. Another interesting finding is that the very promising approach based on wall functions does not combine favourably with the interface conditions for the temperature and the heat flux. Therefore, current and future work aims at developing and testing appropriate temperature wall functions, in order to further improve the accuracy – for a given grid – or the efficiency – for a given accuracy – of the proposed methodology. Originality/value – An accurate and efficient IB CHT method, using a state of the art URANS parallel solver, has been developed and tested. In particular, a detailed study has elucidated the influence of different interface treatments of the fluid-solid boundary upon the accuracy of the computations. Last but not least, the method has been applied with success to solve the well-known CHT problem of compressible turbulent flow past the C3X turbine guide vane. [ABSTRACT FROM AUTHOR]

In this paper unsteady RANS computations are used to study the inception of rotating stall in a transonic centrifugal compressor taking into account realistic installation effects on performance, as commonly found nowadays due to space limitations. To this end, the effect of an ideal uniform inlet conditions (normally found for long straight inlet) is compared with inlet distortions generated by a bent pipe installed just in front of the impeller. The numerical techniques that have to be applied to correctly represent the rotating stall are explained in detail: all simulations are done modeling the whole annulus of the radial machine, using high performance computing to represent the non-periodic phenomena leading to the stall inception. Moreover, stable boundary conditions are employed, with the aim to avoid the inception of large unphysical surge cycles. When an uniform inlet flow to the compressor is considered, the formation of 8 blockage cells rotating in the same direction of the compressor is pointed out. On the other hand, when the elbow is installed in front of the impeller, the distorted flow suppresses the formation of a rotating stall pattern. [ABSTRACT FROM AUTHOR]

In near-wall turbulence modeling it is required to resolve a thin layer nearby the solid boundary, which is characterized by high gradients of the solution. An accurate enough resolution of such a layer can take most computational time. The situation even becomes worse for unsteady problems. To avoid time-consuming computations, a new approach is developed, which is based on a non-overlapping domain decomposition. The boundary condition of Robin type at the interface boundary is achieved via transfer of the boundary condition from the wall. For the first time interface boundary conditions of Robin type are derived for a model nonstationary equation which simulates the key terms of the unsteady boundary layer equations. In the case of stationary solutions the approach is automatically reduced to the technique earlier developed for the steady problems. The considered test cases demonstrate that unsteady effects can be significant for near-wall domain decomposition. In particular, they can be important in the case of the wall-function-based approach. [ABSTRACT FROM AUTHOR]

An implicit predictor–corrector method is presented for the simultaneous integration of the six degrees-of-freedom (DOF) equations of motion for a manoeuvring submarine and the unsteady Reynolds-Averaged Navier Stokes (URANS) equations describing the vehicle hydrodynamics. The novel method uses coefficient-based hydrodynamic models for estimating the Jacobian matrix for Newton iteration. The method is applied to emergency rising and horizontal plane zig-zag manoeuvres. It is shown to converge faster at each timestep than under-relaxed fixed-point iteration with an optimum relaxation parameter. A simple model containing only primary linear hydrodynamic coefficients that are relatively easy to estimate or measure was found to be adequate for modelling the Jacobian matrix in these simulations. [ABSTRACT FROM AUTHOR]

Highlights: [•] Hybrid LES–URANS method for turbulent wall-bounded internal and external flows. [•] Unique modeling approach based on an explicit algebraic Reynolds stress model. [•] Dynamic LES–URANS interface criterion including transition onset. [•] Evaluation based on internal flow through two asymmetric diffusers. [•] External flow past an airfoil including laminar separation bubble and transition. [Copyright &y& Elsevier]

Highlights: [•] Analysis of discharge effects in the base region of supersonic airfoils. [•] Low purge rate reduce the base drag. [•] There is an optimum purge rate minimizing shock intensity. [•] A second shock system is created at excessive discharge rate. [•] Shock intensity linearly reduces with increasing base pressure. [Copyright &y& Elsevier]

A comprehensive evaluation of the predictive capability and computational costs of Reynolds Averaged Navier–Stokes (RANS), Unsteady RANS (URANS), Delayed Detached-Eddy Simulation (DDES), and Lattice-Boltzmann Method (LBM) methodologies was carried out for flow around the SAE notchback and the DrivAer fastback models. From the comparison, all methods predicted drag within roughly 10% of experimental values. However, this accuracy was misleading for RANS as the predicted flowfield deviated from both the unsteady methods and experiments in regions where flow separation was developing. This was especially true around the rotating wheel flows of the DrivAer model, where RANS predicted extensive separation around the rear wheels to the point that it changed the structure of the vehicle wake. In these regions of highly separated flow, the unsteady methods proved to be much more accurate and robust, although comparisons of the flow around the rotating wheels proved inconclusive as to which methods provided suitable predictive capabilities. Despite improved flow predictions, URANS and DDES proved to be significantly more expensive than RANS, while LBM provided comparable accuracy to URANS/DDES with substantially reduced costs. • CFD methods are compared for predicting flow around benchmark SAE and DrivAer models. • RANS performs poorly in regions of flow separation despite good prediction in drag. • URANS, DDES, and LBM give better predictions behind the wheels and at the base. • LBM provides comparable accuracy to URANS and DDES with substantially reduced costs. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates the numerical simulation of the aerodynamics of biomass burners operating in small-scale, fixed-grate technologies. The efficiency of these boilers is largely determined by the fluid patterns originated in the combustion chamber, as a consequence of the interaction of primary and secondary inlets. A set of CFD computations have been carried out for a case-study burner, seeking the comparison for the isothermal-flow solutions given by Reynolds Averaged Navier–Stokes equations (RANS) and by Unsteady RANS equations (URANS). The influence of both spatial and temporal discretization is discussed, using the Grid Convergence Index (GCI) based on Richardson extrapolation. The results indicate that RANS solutions are slightly more sensitive to grid parameters, while URANS solutions show a better convergence behavior. Validation has been reasonably achieved by comparing the URANS velocity profiles against experimental measurements. As a consequence, a mathematical tool is now available to support design modifications of the biomass burner, combining simplicity, reliability and economy. [Copyright &y& Elsevier]

Abstract: Nowadays, unsteady RANS methods are the only techniques usable in the industry to design a control device based on a periodic excitation of the flow. This paper aims at evaluating the potential of such methods by comparing their performances with respect to LES computations taken as a reference. The chosen test case is a rounded backward facing step proposed by Dandois et al. . The frequency response of this flow to the periodic excitation of a synthetic jet is computed both with LES and with four different URANS models. It is demonstrated that the optimal frequency identified by best URANS models is close to the one of LES. Nevertheless, the amplitude of separated bubble surface reduction is underestimated by URANS models. [Copyright &y& Elsevier]

• The POD data compression method properly conserves the flow spectral content in LES. • The leading edge vortex can induce the transition of the CROR blades boundary layer. • The majority of losses in CROR are induced by the front and rear rotor boundary layer. • The maximum of losses is generated on the rear region of the blades. This paper presents the study of the losses generated in a Counter Rotating Open Rotor (CROR) configuration at three different operating conditions (approach, cutback and sideline). Unsteady Reynolds Averaged Navier-Stokes (URANS) and Large-Eddy Simulation (LES) approaches are used and compared to describe the flow field and the mechanisms of loss. Since no common circumferential periodicity occurs in the two blade rows of the configuration (11 blades for the front rotor and 9 for the rear rotor), a full 360 ∘ simulation would be required. In order to reduce the related computational cost, a phase-lagged assumption approach is used. This method enables to perform unsteady simulations on multi-stage propulsive configurations including multiple frequency flows with a computational domain reduced to one single blade passage for each row. The phase-lagged approach requires a large data storage reduced in the study by a data compression method. The data compression method is based on a Proper Orthogonal Decomposition (POD) replacing the traditional Fourier Series Decomposition (FSD). The inherent limitation of the phase-shifted periodicity assumption remains with the POD data storage but this compression method alleviates some issues associated with the FSD, especially spectrum content issues. The analysis of the losses generated in the configuration is based on an entropy formulation. In particular, the losses are split between boundary layer contributions and the remaining domain where wakes and secondary flows occur. The study shows the influence of the leading edge vortex on the suction side boundary layer transition of the front and rear rotor blades at high rotational speed (cutback and sideline). The main source of losses is associated with the suction side boundary layer over the front and rear rotor blades with a main peak of loss production at around 75% of the blade chord. [ABSTRACT FROM AUTHOR]

• Numerical simulation of 3 phase flows in injectors: liquid in air with cavitation • Calibration of cavitation model parameters using gPC • The calibrated set-up robust to change in the injector geometry and conditions • The calibrated set-up improves the predictions compared to default set-up The work focuses on the calibration of cavitation model parameters for the numerical simulation of three-phase injector flows. Cavitation is modeled through a transport equation for the void fraction closed by the Schnerr-Sauer relation. The vaporization and condensation factors contained in this model are considered for calibration against experimental data available for a test-case characterized by fuel injection in a reservoir filled of air through an axisymmetric channel. In spite of the simplified geometry, this flow configuration is representative of a real injector and contains most of the complex physical phenomena which may be encountered in injector flows, as turbulence, cavitation and hydraulic flip, i.e. a back flow of air from outside the injector along the whole length of the channel replacing the cavitating regions. Since a direct calibration would imply huge computational costs, not affordable in practical applications, response surfaces of the quantities of interest are built through generalized Polynomial Chaos. These response surfaces, which can be obtained starting from only a few deterministic simulations, have been then used to carry out the parameter calibration. The quantities of interest taken into consideration are the critical cavitation point (CCP), i.e. the value of the outlet pressure at which the flow inside the injector can be considered choked (for a fixed inlet pressure), and the mass-flow-rate (MFR) at the CCP. To further reduce the computational costs, calibration is carried out by using axisymmetric simulations. It has been then checked that the obtained cavitation model gives accurate results also in three-dimensional simulations of the actual geometry. Moreover, this set-up has been applied to two different complex one-hole injector geometries, i.e. a sector of real injector geometries, and the results have been compared against available experimental data. The calibrated cavitation model set-up appears to be robust, giving good predictions also in conditions significantly different from those in which it has been obtained. [ABSTRACT FROM AUTHOR]

• We apply near-wall domain decomposition for essentially unsteady turbulent flows. • We apply nonlocal in time interface boundary conditions (IBC). • We realize them in two ways via solving an auxiliary problem and Fourier method. • We validate the criteria for essential unsteadiness in the IBC. • We demonstrate the unsteady correction in the IBC can significantly affect accuracy. In near-wall turbulence modeling it is necessary to resolve a thin boundary layer containing high gradients of the solution. An accurate enough resolution of such a layer can take most of the computational time. The situation becomes even worse for unsteady problems. To avoid time-consuming computations, in the present paper a new approach is developed, which is based on a non-overlapping domain decomposition. The boundary condition of Robin type at the interface boundary between domains is constructed by transferring the original boundary condition from the wall. For the first time, recently developed unsteady interface boundary conditions of Robin type are used for the unsteady Reynolds Averaged Navier-Stokes equations. The interface boundary conditions contain a memory term which takes into account the nonlocal effect in time to be taken into account for essential unsteady problems. In the case of stationary solutions the new approach automatically reduces to the technique earlier developed for the steady problems. The considered test cases demonstrate that the effect of the memory term can be significant for the accuracy of the near-wall domain decomposition. The criteria for importance of the memory term in the interface boundary condition are formulated and confirmed for turbulent flows. [ABSTRACT FROM AUTHOR]