This paper discusses the impact of scaling effects in the performance of a standalone wing sail, comparing experiments and numerical simulations. The experimental data of a standalone wing sail with a NACA0015 section, tested in the R.J. Mitchell wind tunnel at the University of Southampton, are compared with wind tunnel tests run at KTH Royal Institute of Technology, where the L2000 wind tunnel and wing sail had a smaller scale, and with Computational Fluid Dynamics simulations of three different cases. For the numerical simulations, first the model scale Southampton wind tunnel was simulated. Then, keeping the same scale, the wind tunnel domain was substituted with a larger domain to simulate an open-field condition, and analyse the presence of blockage effects. Finally, full-scale simulations were achieved keeping the same scale of the model scale open-field simulations, and reaching the full-scale Reynolds number by varying the viscosity of the fluid. The flow in the numerical simulations is modelled with the RANS equations and the k − ω SST turbulence model, knowing about its limitations in simulating stall conditions, but judged to be satisfactory for a preliminary study about scale effects. The range of model scale Reynolds numbers covered by both experimental campaigns spans from 2.2x105 to 6.7x105, while the full-scale Reynolds number is equal to 7.9x106, covering a range representative of most wind propulsion technologies. The main conclusions are that the simulations capture well the shape of the lift curve up to an angle before stall, after which the simulations diverge from the experiments. In full-scale, higher lift coefficients are reached, and the lift curve shows a different behaviour than in model scale, with a longer linear region and a more abrupt stall.

A new formulation for scale-resolved simulations of turbulence with variable resolution (VR) is proposed. A delayed detached-eddy simulation (DDES) model based on the k-ω framework is extended with VR-terms representing the commutation terms arising from variable resolution defined in terms of the DDES length scale. The VR-terms are responsible for the exchange of turbulence kinetic energy between the resolved and unresolved partitioning of the computational representation of turbulent flow. The new formulation is implemented in a general-purpose CFD code and applied on two cases, namely, a mixing shear layer and a wall-mounted hump and have been compared with and combined with the baseline model and two additional grey-area mitigation (GAM) formulations. The proposed method is shown to provide the mechanism for the exchange of energy between unresolved and resolved representation of the flow and to enhance the transition from modelled to resolved turbulence and thus improve the prediction of the resolved Reynolds stresses, development of the vorticity thickness for the shear layer flow and the skin friction recovery length for the hump flow.

A new Grey-Area Mitigation (GAM) method for hybrid RANS-LES is presented.

Reynolds-averaged Navier-Stokes (RANS) simulations of a single wind turbine wake in the neutral atmospheric pressure-driven boundary layer (PDBL) are conducted and compared to RANS simulations with inflow based on the more traditional log-law. The latter is valid in the neutral atmospheric surface layer (ASL), while the PDBL is a better representation of the whole atmospheric boundary layer (ABL). It is found that the wake results of the two types of simulations become more similar for increasing ABL height to rotor diameter ratio. In fact, the ASL is shown to be a special asymptotic case of the PDBL. The RANS simulations are also compared to a large-eddy simulation (LES) PDBL case, where it is found that both the ASL and PDBL RANS simulations compare well with the reference LES data in the wake region, while the RANS PDBL compares better with the data in the upper region of the domain.

We seek to develop a closure model to enable scale-resolving simulation (SRS) of a turbulent flow to optimally switchover from a Reynolds-averaged Navier-Stokes (RANS) calculation at the wall to a specified degree of resolution in the wake or free-stream region. The closure model is derived by (i) using the physical principle that the total energy of resolved and unresolved scales should be conserved in the switchover region and (ii) establishing consistency with equilibrium boundary layer scaling of the partially resolved field. The model development is performed in the context of a partially averaged Navier-Stokes (PANS) scale-resolving method by quantifying and modeling the commutation terms resulting from varying resolutions in the wall-normal direction. The resulting wall-modeled PANS (WM-PANS) is used to compute the turbulent channel flow in the Re,. range 180 - 8000. The influence of the RANS-SRS switchover location on the computed flow field is examined. It is then demonstrated that the mean flow is reproduced with reasonable accuracy at modest computational effort without discernible log-layer mismatch even at the highest Reynolds number considered. While the Reynolds stresses are also recovered accurately over most of the flow domain, a noticeable computational transition from RANS to unsteady SRS flow behavior is observed and the underlying physics is examined. Irrespective of the location of computational transition, the unsteady features of the flow away from the wall are well captured. It is demonstrated that the proposed closure is able to inject the appropriate amount of resolved turbulence without the need for artificially generated synthetic turbulence. Overall, WM-PANS presents an accurate and computationally viable option for scale-resolving computations of near-wall high-Reynolds number flows.

New hybrid RANS/LES methods are proposed where explicit algebraic Reynolds stress modelling is introduced both in the RANS and LES regions. Both in the form of an IDDES method allowing for wall-modelled LES simulations and in the form of an DDES methods where attached boundary layers are shielded within RANS. The methods are extensions of the SST-(I)DDES metods by Gritskevich, et al., Flow, turbulence and combustion 88 (3), 431?449, 2012. In channel flow the log region is well predicted for different Reynolds numbers similar as with the baseline SST-IDDES, but the EARSM-IDDES give a more rapid transition to turbulence closer to the wall. The model gives reasonable results on very coarse meshes, basically similar with the baseline SST-IDDES, for periodic hill flow at Re = 10.000 and 37.000 Also for the 3D Stanford diffuser, the EARSM-IDDES gives good results, here superior to the SST-IDDES, which has problems in fully resolving the turbulence in the inlet channel. The model in DDES mode is tested on the developing shear layer. The so-called grey area problem is substantially mitigated compared with the SST-DDES resulting in a more physically correct and fully developed resolved turbulence.

Reynolds-averaged Navier-Stokes (RANS) simulations of wind turbine wakes are usually conducted with two-equation turbulence models based on the Boussinesq hypothesis; these are simple and robust but lack the capability of predicting various turbulence phenomena. Using the explicit algebraic Reynolds stress model (EARSM) of Wallin and Johans son (2000) can alleviate some of these deficiencies while still being numerically robust and only slightly more computationally expensive than the traditional two-equation models. The model implementation is verified with the homogeneous shear flow, half-channel flow, and square duct flow cases, and subsequently full three-dimensional wake simulations are run and analyzed. The results are compared with reference large-eddy simulation (LES) data, which show that the EARSM especially improves the prediction of turbulence anisotropy and turbulence intensity but that it also predicts less Gaussian wake profile shapes.

In a recent study (Želi et al. in Bound Layer Meteorol 176:229–249, 2020), we have shown that the explicit algebraic Reynolds-stress (EARS) model, implemented in a single-column context, is able to capture the main features of a stable atmospheric boundary layer (ABL) for a range of stratification levels. We here extend the previous study and show that the same formulation and calibration of the EARS model also can be applied to a dry convective ABL. Five different simulations with moderate convective intensities are studied by prescribing surface heat flux and geostrophic forcing. The results of the EARS model are comparedto large-eddy simulations of Salesky and Anderson (J Fluid Mech 856:135–168, 2018). It is shown that the EARS model performs well and is able to capture the counter-gradient heat flux in the upper part of the ABL due to the presence of the non-gradient term in the relation for vertical turbulent heat flux. The model predicts the full Reynolds-stress tensor and heat-flux vector and allows us to compare other important aspects of a convective ABLsuch as the profiles of vertical momentum variance. Together with the previous studies, we show that the EARS model is able to predict the essential features of the ABL. It also shows that the EARS model with the same model formulation and coefficients is applicable over awide range of stable and moderately unstable stratifications.

This paper will present the main results of the aerodynamic design and analysis for flow control applied to trailing edge of wings and profiles. This work has been conducted in the framework of the European project AFLoNext aiming at developing technologies allowing for an improvement of the performance and loads situation in the operational domain. The technologies are expected to provide an increase in aerodynamic efficiency and a structural weight reduction for the design flight conditions with a potential for 1–2% fuel savings and corresponding emission reduction. Numerical simulations are performed on 2D and 3D test cases. Where available, a comparison with experimental data is performed. High-speed flow is considered, to investigate a transonic configuration representative of cruise conditions. Trailing edge devices (TED) such as fluidic Gurney flaps or micro-jets for circulation control are used for assessing the possibility of delaying the buffet onset or increasing the maximum achievable lift, thus extending the flight envelope of an aircraft. The purpose of the present paper is to present the result of the work performed by the different partners involved in the project.

For turbulent flows that are influenced by an active scalar, the Reynolds stresses and scalar flux are coupled in a complicated way, which makes it difficult to model these flows. A framework has been derived for obtaining explicit algebraic Reynolds-stress and scalar-flux models for two-dimensional mean flows with stratification. For the specific case of stably stratified parallel shear flows, the derived model was shown to give good results. As an extension of these results, two more cases are considered: unstable stratification in a horizontal channel and natural convection in a vertical channel.