Search results

The purpose of this paper is to numerically investigate the mildly separated flow phenomena on a near-stall NACA0015 airfoil, by using Detached-Eddy Simulation (DES) type methods. It includes a comparison of different choices of underlying Reynolds-averaged Navier–Stokes model as well as subgrid-scale stress model in Large-Eddy simulation mode.

The unsteady flow phenomena are simulated by using delayed DES (DDES) and improved DDES (IDDES) methods, with an in-house computational fluid dynamics solver. Characteristic frequencies in different flow regions are extracted using fast Fourier transform. Dynamic mode decomposition (DMD) method is applied to uncover the critical dynamic modes.

Among all the DES type methods investigated in this paper, only the Spalart–Allmaras-based IDDES captures the separation point as measured in the experiments. The classical vortex-shedding and the shear-layer flapping modes for airfoil flows with shallow separation are also found from the IDDES results by using DMD.

The value of this paper lies in the assessment of five different DES-type models through the detailed investigation of the Reynolds stresses as well as the separation and reattachment.

This paper aims to investigate numerical solution of transonic flow around NACA0012 airfoil under sinusoidal pitch oscillation. Accordingly, effects of the amplitude and frequency of oscillations on aerodynamic coefficients are evaluated and the efficiency of the turbulent models, K-ω shear-stress transport (SST), scale adaptive simulation (SAS) and delayed detached eddy simulation (DDES), in simulation of the nonlinear phenomena – i.e. the interaction between shock and boundary layer and the shock oscillations – is studied.

K-ω SST, SAS and DDES models are used as turbulence approaches. The numerical results are compared with available experimental and numerical information.

According to the results inside the buffet boundaries, the DDES turbulent model expresses results that are more appropriate; however, SAS and SST models are not efficient enough in evaluating the characteristics of nonlinear flow.

In this research study, hybrid RANS-LES turbulence model is engaged to simulate transonic flow around pitching NACA0012 airfoil, and results are compared to the SAS and Reynolds Average Navier–Stocks simulations as well as available numerical and experimental data. In addition, effects of the amplitude and frequency of oscillations on aerodynamic coefficients are evaluated in buffet regions.

In organizations, individual user’s compliance with business processes is important from a regulatory and efficiency point of view. The restriction of users’ choices by implementing a restrictive information system is a typical approach in many organizations. However, restrictions and mandated compliance may affect employees’ performance negatively. Especially when users need a certain degree of flexibility in completing their work activity. The purpose of this paper is to introduce the concept of directive explanations (DEs). DEs provide context-dependent feedback to users, but do not force users to comply.

The experimental study used in this paper aims at investigating how DEs influence users’ process compliance. The authors used a laboratory experiment to test the proposed hypotheses. Every participant underwent four trials for which business process compliance was measured. Two trial blocks were used to cluster the four trials. Diagrammatic DEs were provided in one of the trial blocks, while textual DEs were provided in the other. Trial blocks were counterbalanced.

The results of the experiment show that DEs influence a user’s compliance, but the effect varies for different types of DEs. The authors believe this study is significant as it empirically examines design characteristics of explanations from knowledge-based systems in the context of business processes.

This study is certainly not without limitations. The sample used for this study was drawn from undergraduate information systems management students. The sample is thus not representative of the general population of organizations’ IT users. However, a student sample adequately represents novice IT users, who are not very familiar with a business process. They are particularly suitable to study how users react to first-time contact with a DE.

The findings of this study are important to designers and implementers of systems that guide users to follow business processes. As the authors have illustrated with a real-world scenario, an ERP system’s explanation can lack details on how a user can resolve a blocked activity. In situations in which users bypass restricted systems, DEs can guide them to comply with a business process. Particularly diagrammatic explanations, which depict actors, activities, and constraints for a business process, have been found to increase the probability that users’ behavior is business process compliant. Less time may be needed to resolve a situation, which can result in very efficient user-system cooperation.

This study makes several important contributions to research on explanations, which are provided by knowledge-based systems. First, the authors conceptualized, designed, and investigated a novel type of explanations, namely, DEs. The results of this study show how dramatic the difference in process compliance performance is when exposed to certain types of DEs (in one group from 57 percent on the initial trial to 82 percent on the fourth trial). This insight is important to derive design guidelines for DE, particularly when multimedia material is used.

This paper aims to show results of numerical simulations of transonic flow around a supercritical airfoil at chord Reynolds number Re_c = 3 × 10⁶, with the aim of elucidating the mechanisms responsible for large-scale shock oscillations, namely, transonic buffet.

Unsteady Reynolds-averaged Navier–Stokes simulations and detached-eddy simulations provide a preliminary buffet map, while a high fidelity implicit large-eddy simulation with an upstream laminar boundary layer is used to ascertain the physical feasibility of the various buffet mechanisms. Numerical experiments with unsteady RANS highlight the role of waves travelling on pressure side in the buffet mechanism. Estimates of the propagation velocities of coherent disturbances and of acoustic waves are obtained, to check the validity of popular mechanisms based on acoustic feedback from the trailing edge.

Unsteady RANS numerical experiments demonstrate that the pressure side of the airfoil plays a marginal role in the buffet mechanism. Implicit LES data show that the only plausible self-sustaining mechanism involves waves scattered from the trailing edge and penetrating the sonic region from above the suction side shock. An interesting side result of this study is that buffet appears to be more intense in the case that the boundary layer state upstream of the shock is turbulent, rather than laminar.

The results of the study will be of interest to any researcher involved with transonic buffet.

This study aims to investigate the effects of electroactive morphing on the Airbus A320 Reduced Scale prototype of the H2020 N° 723402 European Research project smart morphing and sensing (SMS) for aeronautical configurations [1]^,[2].

The flow regimes correspond to low subsonic take-off conditions. The morphing is applied through the vibration and slight deformation of the near trailing edge region; respecting the way, this actuation has been applied on the experimental prototype using micro fibre composite actuators. Optimal frequency range has been used, associated with low amplitudes of deformation with the Arbitrary Lagrangian Eulerian methodology. This study used an adapted turbulence modelling with the organised eddy simulation (OES) as well as a hybrid approach delayed detached eddy simulation – with embedded OES (DDES–OES), able to sensitise and keep up the coherent structures development.

The morphing at an optimal frequency (300 Hz) and amplitude (0.7 mm), applied on a length (3.5 cm) near the trailing edge, has been studied at Reynolds number 1 million and incidence of 10°. The effects on the main flow instabilities and on the turbulent vortex structures are analysed using proper orthogonal decomposition. A modification of the wake structures and a formation of organised rows of vortices along the shear layer are obtained. This leads to a quasi-two-dimensional wake, benefits on the aerodynamic performance and a decrease of the frequency peaks in the spectrum, corresponding to an attenuation of the coherent structures.

This study provides a fundamental understanding of how the actuation modifies the coherent and turbulent vortex structures around the wing and in the wake.

The purpose of this article is to present numerical investigations of flow control with piezoelectric actuators on a backward facing step (BFS) and fluidic vortex generators on a NACA0015 aerofoil for the reattachment and separation control through the manipulation of the Reynolds stresses.

The unsteady flow phenomena associated with both devices are simulated using Spalart–Allmaras-based hybrid Reynolds averaged Navier-Stokes (RANS)/large eddy simulation (LES) models (detached eddy simulation (DES), delayed detached eddy simulation (DDES) and improved delayed detached eddy simulation (IDDES)), using an in-house computational fluid dynamics (CFD) solver. Results from these computations are compared with experimental observations, enabling their reliable assessment through the detailed investigation of the Reynolds stresses and also the separation and reattachment.

All the hybrid RANS/LES methods investigated in this article predict reasonable results for the BFS case, while only IDDES captures the separation point as measured in the experiments. The oscillating surface flow control method by piezoelectric actuators applied to the BFS case demonstrates that the Reynolds stresses in the controlled case decrease, and that a slightly nearer reattachment is achieved for the given actuation. The fluidic vortex generators on the surface of the NACA0015 case force the separated flow to fully reattach on the wing. Although skin friction is increased, there is a significant decrease in Reynolds stresses and an increase in lift to drag ratio.

The value of this article lies in the assessment of the hybrid RANS/LES models in terms of separation and reattachment for the cases of the backward-facing step and NACA0015 wing, and their further application in active flow control.

The design of a space launcher requires some considerations about the unsteady loads and heat transfer occurring at the base of the structure. In particular, these phenomena are predominant during the early stage of the flight. This paper aims to evaluate the ability of the unstructured, high order finite-volume CFD solver FLUSEPA, developed by Airbus Safran Launchers, to accurately describe these phenomena.

This paper first performs a steady simulation on a base flow around a four-clustered rocket configuration. Results are compared with NASA experiments and Loci-CHEM simulations. Then, unsteady simulations of supersonic H2/air reacting mixing layer based on the experiment of Miller, Bowman and Mungal are performed. Three meshes with different cells number are used to study the impact of spatial resolution. Instantaneous and time-averaged concentrations are compared with the combined OH/acetone planar laser-induced fluorescence imaging from the experiment.

FLUSEPA satisfactorily predicts the base heat flux at the base of a four-clustered rocket configuration. NASA Loci-CHEM reactive simulations indicate that afterburning plays an important role and should not be neglected. The unsteady reactive computation of a supersonic mixing layer shows that FLUSEPA is also able to accurately predict flow structures and interactions. However, the complexity of the experiment and the lack of details concerning the facility prevents from obtaining satisfactory converged results.

This study is the first step on the development of a cost-effective method aiming at predicting unsteady loads and heat transfer on space launchers using an unsteady and reactive model for the CDF calculations. It uses original techniques such as conservative CHIMERA-like overset grids, local re-centering of fluxes and local adaptive time-stepping to reduce computational cost while being robust and accurate.

The quality of aeroelastic predictions strongly depends on the quality of aerodynamic predictions. At the boundary of a typical flight envelope, special flow conditions may arise, which challenge the conventional Reynolds-averaged Navier–Stokes (RANS) approach beyond reasonable limits.

Test Case 3 of the Second AIAA Aeroelastic Prediction Workshop is a representative test case, where the flow over a supercritical wing separates downstream of the shock waves and generates large turbulent lengthscales.

In this study, RANS predictions are compared to those obtained in this particular test case with the more sophisticated hybrid RANS–large eddy simulation (LES) approach, in particular with the Spalart–Allmaras–delayed detached eddy simulation model. Results are indeed closer to experimental data.

However, the costs associated with this approach are much higher. It is argued that adopting hybrid RANS–LES modelling is not a simple model switch.

This paper aims to investigate the impact of aerofoil camber on the performance of micro-air-vehicle-scale cycloidal propellers.

First, experiments were conducted to validate the numerical methodology. After that, three turbulent models were compared to select the most accurate one. Then, 2D numerical simulation was carried out on 11 aerofoils with different cambers, including five cambered aerofoils, one symmetrical aerofoil and five inverse cambered aerofoils. The inverse cambered aerofoils are symmetrical about the chord line to the corresponding cambered ones.

The cycloidal propeller with large cambered aerofoil gives the lowest hovering efficiency, but with symmetrical aerofoil or small inverse cambered aerofoil shows the highest. Also, blades with large cambered aerofoil display high performance at the upper part of its trajectory, while with symmetrical aerofoil or the inverse cambered aerofoil have their best at the lower part. In addition, intensified downwash can be observed in the rotor cage for all cases. When a blade runs through the top-left part of its circle path, all cases display the feature of deep dynamic stall. When the blade travels through the nadir of its path, the actual angle of attack is close to zero due to the strong downwash. Furthermore, there exits intensified blade-vortex interaction induced by the preceding blade for large cambered aerofoils at the lower-right part of its trajectory.

This paper develops a new cycloidal propeller which is more efficient than the one already present.

This paper discovers that the aerofoil camber is a vital design parameter in the performance of cycloidal propeller, and the authors expect that the rotor with deformable aerofoil on camber would achieve much higher efficiency.

If we are to deliver services that encourage recovery from mental illness, it is essential that there is a vision of a future where recovery from mental illness is both possible and understood by communities, services and staff. Piers Allott provides a brief introduction to the concept of recovery from ‘mental illness’, proposes a vision for recovery, and flags up ways in which recovery practice can be taken forward.