Massive Separation Research Articles

Enhancing BEM simulations of a stalled wind turbine using a 3D correction model

Nowadays wind turbine rotors are usually employed with pitch control mechanisms to avoid deep stall conditions. Despite that, wind turbines often operate under pitch fault situation causing massive flow separation to occur. Pure Blade Element Momentum (BEM) approaches are not designed for this situation and inaccurate load predictions are already expected. In the present studies, BEM predictions are improved through the inclusion of a stall delay model for a wind turbine rotor operating under pitch fault situation of -2.3° towards stall. The accuracy of the stall delay model is assessed by comparing the results with available Computational Fluid Dynamics (CFD) simulations data.

Exa PowerFLOW Simulations for the Sixth AIAA Drag Prediction Workshop

Unsteady simulations of the NASA Common Research Model using a new transonic lattice Boltzmann method are conducted for the Sixth AIAA CFD Drag Prediction Workshop. The context for the development of this unique new method is briefly described. The results of a grid convergence study are presented and discussed, and it is shown that the numerical method is capable of accurately predicting incremental drag changes. The occurrence of unsteady flow at higher angles of attack during an alpha sweep for the model including static aeroelastic wing deformations is discussed. Buffet is already shown to start locally on the wing for angles of attack considered for the workshop. Additional simulations for higher angles, deep into the buffet range and close to the high-speed stall of the model, are also shown and highlight the method’s capability to handle highly unsteady flows with massive turbulent separations.

Fluid–structure interaction of downwind sails: a new computational method

The spreading of high computational resources at very low costs led, over the years, to develop new numerical approaches to simulate the fluid surrounding a sail and to investigate the fluid–structure interaction. Most methods have concentrated on upwind sails, due to the difficulty of implementing downwind sailing configurations that present, usually, the problem of massive flow separation and large displacements of the sail under wind load. For these reasons, the problem of simulating the fluid–structure interaction (FSI) on downwind sails is still subject of intensive investigation. In this paper, a new weak coupled procedure between a RANS solver and a FEM one has been implemented to study the FSI problem in downwind sailing configurations. The proposed approach is based on the progressive increasing of the wind velocity until reaching the design speed. In this way, the structural load is also applied progressively, therefore, overcoming typical convergence difficulties due to the non-linearity of the problem. Simulations have been performed on an all-purpose fractional gennaker. The new proposed method has been also compared with a classic weak FSI approach. Comparable results have been obtained in terms of flying shape of the gennaker and fluid-dynamic loads. The most significant characteristic of the proposed procedure is the easiness to find a solution in a very robust way without convergence problem, and also the capability to reduce the simulation time with regard to the computational cost.

МАТЕМАТИЧНЕ МОДЕЛЮВАННЯ ДОЗВУКОВОГО ТУРБУЛЕНТНОГО ОБТІКАННЯ ПРОФІЛЮ NACA 0015, ЩО КОЛИВАЄТЬСЯ

Представлено результати розрахунків профілю NACA 0015, що коливається, для трьох режимів течії: слабкий відрив потоку, що відповідає середньому куту атаки 4°; розвинений відрив – 11° (докритичне обтікання профілю); масивний відрив – 15° (закритичне обтікання профілю). Для чисельного моделювання обтікання профілю, що коливається, застосовуються осереднені за Рейнольдсом нестаціонарні рівняння Нав'є-Стокса нестисливої рідини, замкнуті однопараметричною моделлю турбулентної в'язкості. Розроблений неявний скінчено-об'ємний чисельний алгоритм базується на методі штучної стисливості, що модифікований для розрахунку нестаціонарних задач. Аналізуються отримані поля завихренности, миттєві лінії течії, гістерезисні криві нестаціонарних аеродинамічних навантажень профілю для трьох характерних режимів обтікання. Розроблена методика дозволяє відтворювати структуру нестаціонарного відривного обтікання профілю, що коливається. Відмінності в аеродинамічних характеристиках стаціонарного профілю і профілю, що коливається, при одних і тих же кутах атаки добре узгоджуються з експериментальними даними. Порівняння результатів розрахунків обтікання профілю, що коливається з експериментальними даними і відомими розрахунками інших авторів показало перевагу моделі турбулентності SALSA в порівнянні з іншими моделями, що тестувались особливо при наявності масивного відриву потоку.

Numerical simulations of the aerodynamic response of circular segments with different corner angles by means of 2D URANS. Impact of turbulence modeling approaches

ABSTRACTThis paper presents the results of numerical and experimental investigations on the force coefficients and Strouhal numbers of circular segments considering different corner angles or chord to sagitta ratios. The research is motivated because these geometries are becoming increasingly popular in several engineering disciplines. The so-called D-section (semi-circular cylinder with a corner angle of ) has been experimentally studied in the past, since it is a galloping prone geometry. However, there is a lack of research for cases with different corner angles, and the numerical investigations related to this topic are particularly scarce. In this work, a 2D Unsteady Reynolds Averaged Navier–Stokes approach has been adopted aiming to study the circular segments at the sub-critical regime, considering corner angles from to , and the flow parallel to the rectilinear side. These sections were found to be particularly challenging since they present massive flow separation on the rectilinear side, alongside the inherent difficulties related to modeling the flow along curved surfaces at high Reynolds numbers. The impact of introducing low-Reynolds-number and curvature corrections in the SST turbulence model and the performance of the Transition SST model have been extensively studied.

An automatic hybrid numerical scheme for global RANS-LES approaches

A new automatic high-order hybrid central-difference/upwind scheme is developed for the finite-volume approximation of the inviscid fluxes within global hybrid RANS-LES approaches. Performance of the scheme is illustrated by examples of computations of three types of flow: a flow with massive separation with the use of Delayed Detached-Eddy Simulation (DDES), a flow with separation and reattachment with the use of DDES with shear-layer adapted subgrid length scale, and a fully attached flow with the use of Improved DDES (IDDES).

High-frequency counter-flow plasma synthetic jet actuator and its application in suppression of supersonic flow separation

We come up with a control strategy for suppression of supersonic flow separation based on high-frequency Counter-flow Plasma Synthetic Jet Actuator (CPSJA). The main purpose of this investigation is to verify if its control authority can be enhanced by the jet/shock interaction. We use a blunt nose to generate a bow shock, a step on a flat plate to introduce a massive separation in a Mach 2 wind tunnel, and the CPSJA to generate Plasma Synthetic Jet (PSJ). In this study, pulsed capacitive discharge is provided for an array of CPSJAs, which makes the actuation (discharge) frequency f1 = 1 kHz, f2 = 2 kHz and f3 = 3 kHz. We use the high-speed schlieren imaging and fast response pressure transducers as well as a numerical simulation to investigate the quiescent PSJ properties, the interaction between the jet and bow shock, and its disturbance effect on the downstream separated region. The schlieren images show that PSJ is characterized by a succession of vortex rings; the jet strength weakens with the increase of frequency. A 4.5 mN jet thrust is found for all the frequencies. The simulation results show that jet/shock interaction produces vorticity in the vortex ring of the jet, enhancing turbulent mixing in PSJ so that a great deal of momentum is produced into the flow. We found the downstream flow is significantly disturbed by the enhanced actuation. Actuation with frequency of f2, f3 which is close to the natural frequency fn of the separation bubble suppresses the separation with the upstream laminar boundary layer being periodically attenuated, which has a better control effect than f1. The control effect is sensitive to the position where PSJ interacts with the shear layer, but the amount of energy deposited in one pulse is not crucial in a separation reduction in the experiment.

Numerical Simulations of Low-Reynolds-Number Flow Past Finite Wings with Leading-Edge Protuberances

The use of various modeling approaches for the numerical simulation of low-Reynolds-number flow past wings of finite span with leading-edge protuberances is studied at pre- and poststall operating conditions. Variants of Reynolds-averaged Navier–Stokes simulations (including transitional modeling) and detached-eddy simulations (involving different options for shielding the boundary layer from the detached-eddy simulation limiter), as well as a low-Reynolds-number correction, are considered. An assessment of the capabilities exhibited by these modeling approaches is carried out, based on detailed comparisons against whole-flowfield and aerodynamic force measurements in a wind tunnel, at a Reynolds number of for wings with aspect ratios of 1 and 1.5. The results show that the use of detached-eddy simulation with an improved description of the boundary layer is required to reproduce the flow physics accurately at prestall conditions, where separation and subsequent reattachment are observed. At poststall regimes, massive flow separation occurs, and a choice of conservative shielding produces the best outcome with detached-eddy simulation. In addition, the present experimental dataset provides valuable information toward understanding the passive stall control mechanism generated by the use of leading-edge tubercles and its application to low-aspect-ratio wings.

Control of Severe Shock-Wave/Boundary-Layer Interactions in Hypersonic Inlets

A theoretical analysis and numerical simulations are conducted to demonstrate the control capability of a splitter on the shock-wave/boundary-layer interactions in a generic hypersonic inlet with both a two-staged external compression and a strong cowl compression. The results show that, under the unstart condition, a well-designed splitter could suppress the massive cowl shock-induced separation, increase the fullness of the rampside boundary-layer profile at the isolator exit, and improve the inlet performance over the targeted operating Mach number range. With the splitter design, the severe shock-wave/boundary-layer interaction induced by the strong cowl shock is replaced by several weaker shock-wave/boundary-layer interactions with reduced shock intensity and boundary-layer thickness.

Forcing symmetry exchanges and flow reversals in turbulent wakes

Turbulent wakes past bluff bodies commonly present asymmetric flow states reminiscent of bifurcations in the laminar regime. Understanding the sensitivity of these states to flow forcing is crucial to the modelling and control of flow symmetry properties. In this study, the near wake of a rectangular bluff body in proximity to a wall is disturbed by the use of passive devices located between the model and the wall, upstream of the massive flow separation occurring at the blunt trailing edges. Due to the proximity to the boundary, the wake initially presents wall-normal asymmetry and a negative wall-normal pressure gradient along the base. The application of disturbances with variable size, however, sets flow symmetry along the wall-normal plane, leading to the intermittent spanwise wake reversals reported recently in the literature. A further increase in the size of perturbation suppresses wake switching, and wall-normal asymmetry is recovered, but with a positive wall-normal pressure gradient. The dynamical features of this bifurcation scenario can be retrieved using two coupled symmetry-breaking models for spanwise and wall-normal pressure gradients. This confirms the high sensitivity of the separated flow to external perturbations. More importantly, the results unify observations of the bluff-body wake topologies covered in previous investigations.

Passive flow control of a stalled airfoil using a microcylinder

The effect of passive flow control using a microcylinder near the leading edge of a stalled airfoil (NACA 0012) at a Reynolds number of 6 × 106 is investigated through a computational study. Detailed parametric analyses for the microcylinder are carried out based on Reynolds averaged Navier-Stokes (RANS) calculations in order to determine the optimal control parameters. Computations using delayed detached eddy simulation (DDES) for the controlled and uncontrolled cases are supplemented to capture the transient vortical structures in the massive separation region and further verify the numerical results from RANS. It is found that the total aerodynamic forces of the entire system are highly sensitive to the spacing between the surfaces of the airfoil and the microcylinder. When a superior set of control parameters is adopted, significant enhancement of lift coefficient and lift-to-drag ratio can be achieved after stall, accompanying a pronounced decrease of drag coefficient. Both RANS and DDES results indicate that the heavy stall can be effectively delayed and the extent of large separation region on the airfoil suction surface can also be diminished by such a little control device. A physical analysis of the flow fields is performed to illustrate the mechanisms of flow control for the improvement of airfoil aerodynamic performance.

Floquet stability analysis in the wake of a NACA0015 airfoil at post-stall angles of attack

We perform a Floquet stability analysis in the wake of a NACA0015 airfoil, with four angles of attack, α=20°, 17.5°, 15°, and 12.5°, considered. The central aim is to predict the secondary instabilities at the fixed moderate angles of attack, which are sufficiently large for massive separation from the airfoil, while at the same time allowing the formation and travelling of surface vortex on the airfoil, differentiating from the cases with very large angles of attack. We divide the angles of attack into two groups. We report that mode C is the first unstable mode for both groups, and which is also the only unstable mode for the small angles of attack, i.e., α=15° and α=12.5°, while for the high angles of attack, i.e., α=20° and α=17.5°, four unstable modes are observed. They are mode A, mode quasi-periodic, mode SL, and mode SS. The modes SL and SS are both subharmonic but with different wave numbers. We conjecture that these two subharmonic modes are resulted from the splitting of mode C. A comparison in the base flow topologies between the two groups shows that the different instability behaviors are probably due to their different flow patterns of the base flows. Three-dimensional direct numerical simulations (DNSs) have also been employed to study the physical realizability of the dominant unstable modes. A good consistency between the Floquet analysis and the 3D DNS results is achieved, indicating that the dominance of the linear instability is responsible for such a three-dimensional flow as the Reynolds number is not far from the critical value. Moreover, we find that the critical Reynolds numbers for the onset of three-dimensional instability fall into the range of 159.7–234.2 by defining this new Reynolds number according to the width of the flow wake. These values are very close to that of a bluff body. Furthermore, we note that the corresponding Strouhal numbers are around 0.17 for all the angles of attack, implying the relevance of three-dimensional instabilities to the wake dynamics or more specifically the fluctuation in the wake.

Behavior of Shock Train in Curved Isolators with Complex Background Waves

To study the duct curvature effects on a shock train in rectangular hypersonic inlet/isolator models, four curved isolators are designed and tested at a freestream Mach number of 4.92. Strong interactions between the shock train and background waves are encountered. In small curvature isolators, the shock train intersects with the left-running or right-running shock stably, with the massive separation situated near the top or bottom wall, respectively; whereas the large separation always locates near the bottom wall in large curvature isolators. When the shock train switches from intersecting with one background shock to another, rapid forward movement occurs in large curvature isolators; whereas violent flow oscillations may appear in small curvature isolators. The radial pressure gradient direction in the shock train region alternatively changes from being centripetal to being centrifugal in small curvature isolators, whereas the direction remains centripetal in large curvature isolators due to the enhanced left-running expansion waves and right-running compression waves. In addition, the amplitude of the radial pressure gradient increases with the duct curvature, which induces significant secondary flow and accumulates a large amount of low-energy fluid near the bottom surface. On the other hand, the streamwise adverse pressure gradient in the shock train decreases with the duct curvature, resulting in a longer and weaker shock train, and the maximum sustainable backpressure ratio decreases with a percentage as high as 17% of that of straight isolator. Besides, the root-mean-square values of static pressure on the exit plane can be reduced to some extent by increasing the duct curvature. In general, large duct curvature has detrimental effects on the shock train characteristics, but adequate duct curvature can help to avoid violent flow oscillation and decrease pressure fluctuating level.

Hybrid Reynolds-Averaged/Large-Eddy Simulation Methodology from Symbolic Regression: Formulation and Application

A unified hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation closure is presented that is built from data-driven methods. This is a novel way to construct such models that does not impose constraints. Direct numerical simulation data is filtered, and the ratio of resolved to unresolved energy is used to fit an ideal length scale damping function for the unified framework. This study shows the viability of using high-fidelity data, not just for a priori testing but for the complete creation of lower-fidelity methods. Alongside the physical model, a convection scheme is proposed that marries well to it. This numerical scheme ensures that the damped turbulence model is provided the appropriate ratio of stability to accuracy. Furthermore, an additional function guarantees that the large-eddy simulation mode is only active in vortical flow. This hybrid closure is then applied to two industrially relevant yet very different geometries for which reliable reference data exist. The periodic hills test case is characterized by a confined separation and reattachment, whereas the tandem cylinders exhibits massive separation with wake interference patterns. For the latter, aeroacoustic predictions are of paramount importance. On both geometries, two mesh levels are tested. This a posteriori validation program provides a stern test for the new closure. For both resolutions on both geometries, the new formulation performs extremely well. Each case is also predicted using an established closure, and comparison to this further illustrates the potential of the current framework.

Separation of Flame and Nonflame-retardant Plastics Utilizing Magneto-Archimedes Method

In physical recycling process, the quality of recycled plastics becomes usually poor in case various kinds of plastic materials are mixed. In order to solve the problem, we tried to separate flame and nonflame-retardant plastics used for toner cartridges as one example of mixed plastics by using magneto-Archimedes method. By using this method, we can control levitation and settlement of the particles in the medium by controlling the density and magnetic susceptibility of the medium and the magnetic field. In this study, we introduced the separation system of plastics by the combination of wet type specific gravity separation and magneto-Archimedes separation. In addition, we examined continuous and massive separation by introducing the system which can separate the plastics continuously in the flowing fluid.

Entropy Analysis of the Interaction between the Corner Separation and Wakes in a Compressor Cascade

The corner separation in the high-loaded compressors deteriorates the aerodynamics and reduces the stable operating range. The flow pattern is further complicated with the interaction between the aperiodic corner separation and the periodically wake-shedding vortices. Accurate prediction of the corner separation is a challenge for the Reynolds-Averaged Navier–Stokes (RANS) method, which is based on the linear eddy-viscosity formulation. In the current work, the corner separation is investigated with the Delayed Detached Eddy Simulation (DDES) approach. DDES results agree well with the experiment and are systematically better than the RANS results, especially in the corner region where massive separation occurs. The accurate results from DDES provide a solid foundation for mechanism study. The flow structures and the distribution of Reynolds stress help reveal the process of corner separation and its interaction with the wake vortices. Before massive corner separation occurs, the hairpin-like vortex develops. The appearance of the hairpin-like vortex could be a signal of large-scale corner separation. The strong interaction between corner separation and wake vortices significantly enhances the turbulence intensity. Based on these analyses, entropy analysis is conducted from two aspects to study the losses. One aspect is the time-averaged entropy analysis, and the other is the instantaneous entropy analysis. It is found that the interaction between the passage vortex and wake vortex yields remarkable viscous losses over the 0–12% span when the corner separation has not yet been triggered; however, when the corner separation occurs, an enlarged region covering the 0–30% span is affected, and it is due to the interaction between the corner separation and wake vortices. The detailed coherent structures, local losses information and turbulence characteristics presented can provide guidance for the corner separation control and better design.

Comparative studies of numerical methods for evaluating aerodynamic characteristics of two-dimensional airfoil at low Reynolds numbers

ABSTRACTThis study investigates the predictability of the aerodynamic performance of some numerical methods at low Reynolds numbers and their dependency on the geometric shape of airfoil. We conducted three-dimensional large-eddy simulations (3-D LES), two-dimensional laminar simulations (2-D Lam), and Reynolds-averaged Navier–Stokes simulations with Baldwin–Lomax (2-D RANS(BL)) and Spalart–Allmaras (2-D RANS(SA)) turbulence models. Although there is little discrepancy between the 3-D LES, 2-D Lam, and 2-D RANS(SA) results in terms of the lift and drag characteristics, significant differences are observed in the predictability of the separation and reattachment points. The predicted lift, separation, and reattachment points of the 2-D Lam are qualitatively similar to those of the 3-D LES, except for high angles of attack at which a massive separation occurs. The 2-D RANS(SA) shows good predictability of the lift and separation points, but it does not estimate reattachment points accurately. The 2-D RANS(BL) fails to predict the precise separation points, which results in a poor lift predictability. These characteristics appear regardless of the airfoil geometry shapes. The results suggest that a 2-D Lam without any turbulence models can be used to estimate qualitative airfoil aerodynamic characteristics at the low Reynolds numbers.

Aeroacoustic’s Investigation on High-Lift Device by using a Modern Hybrid RANS/LES-Model

This study focuses on the development, validation and application of the interdisciplinary computational fluid dynamics/computational aeroacoustics (CFD/CAA) method with the name Flight-Physics Simulator AEOLus (FPSAEOLus). FPS-AEOLus is based on enhanced conservative, anisotropic, hybrid Reynolds-averaged Navier-Stokes/ Large-Eddy Simulation (RANS/LES) techniques to solve an aerodynamic flow field by applying the unsteady, compressible, hyperbolic Navier–Stokes equations of second order. The two-layer SSG/LRR- ω differential Reynolds stress turbulence model presented, combining the Launder- Reece-Rodi (LRR) model near walls with the Speziale-Sarkar-Gatski (SSG) model further apart by applying Menter's blending function F1. Herein, Menter's baseline ω-equation is exploited for supplying the length scale. Another emphasis is put on the anisotropic description of dissipation at close distance to the solid wall or in wake area for describing the friction-induced surface-roughness behaviour in viscous fluid physics and swirling wake effects. For that purpose, the SSG/LRR-ω seven-equations Reynolds stress turbulence model with anisotropic extension was realized, therefor the theory is described in general. Beyond that, a modified delayed detached-eddy simulation (MDDES) and a scale adaptive simulation (SAS) correction to capture the stochastic character of a large-eddy-type unsteady flow with massive flow separations in the broad band is implemented. To demonstrate the time-dependent noise propagation having wave interference a linearized Euler equation (LEE) model using a combined Momentum- and Lamb-vector source have been applied into the CFD/CAA - method. The DLR 15 wing, a High-Lift device in landing configuration having a deployed slat and landing flap is studied experimentally and numerically. The first part of the application deals with the steady flow investigation; however, the same turbulence model is used for the unsteady flow case without the enclosed time derivatives. The second part concentrates on unsteady modelling for the Navier–Stokes and Linearized Euler field. With this new combined CFD/CAA - method, steady and unsteady numerical studies for the high-lift wing configuration for discovering the aerodynamic and –acoustic propagation effects are shown, discussed and when experimental data were available validated. The High-Lift wing has a constant sweep angle of Λ=30° to investigate possible cross-flow; to realize this, periodic boundary conditions were set in spanwise direction

Development of flow structures in the near-field wake region of the Common Research Model

The flow in the near-field wake region of a generic transport aircraft configuration has been studied using both unsteady RANS and delayed detached-eddy simulations. The NASA Common Research Model has been used for this purpose, as experimental data obtained with this model in the cryogenic European Transonic Wind Tunnel during the ESWIRP test campaign were available for comparison and validation. Focusing on high angles of attack of \(\alpha = 16^\circ\) and \(\alpha = 18^\circ\), the results form the basis for the study of flow phenomena occurring at stall conditions. Such flight conditions are characterized by massive flow separation at the wing and highly unsteady flow in the wake. In a first step, the numerical results are compared with available test data using global force coefficients and pressure distributions on the wing. Both show a good agreement of numerical and experimental results, indicating slight deviations of the pressure variable at the inboard wing sections. In a second step, the turbulent structures of the near-field wake are examined using the proper orthogonal decomposition method based on snapshots (two-dimensional instantaneous flow fields). Frequencies of the first mode-pair match the dominant frequencies of the lift and velocity wake spectra very well.

Surface pressure determination: a comparison between PIV-based methods and PSP measurements

The present work investigates the challenges in measuring the surface pressure distribution using particle image velocimetry (PIV) in comparison with pressure sensitive paint (PSP) measurements. The aim of the investigation is to assess if PIV may be regarded as a complementary method to PSP, or even as a possible replacement at least for 2D flow analysis or at low speed, where PSP fails. The investigation is based on two different test cases: (1) the attached flow around a NACA-0012 airfoil and (2) the separated flow around a backward-facing step (BFS). Both cases are investigated at transonic flow conditions. For the airfoil, both PIV and PSP measurements were conducted for a direct comparison. At such high Reynolds number, the boundary layer thickness is very small, and therefore, the pressure in the flow field outside the boundary layer can be easily determined from the velocity data with an isentropic flow assumption. The results of the PIV-based surface pressure reconstruction and the one of PSP are compared with the static pressure taps installed on the model surface. For the BFS test case, synthetic PIV images are produced from a Large Eddy Simulation (LES). In this case, flow separation as well as strong viscous effects appear which inhibit the use of the isentropic flow assumption. Therefore, the Reynold averaged momentum equation is directly used and the pressure is determined solving the Poisson equation which is obtained by applying the divergence operator to the momentum equation. The results are compared with the pressure distribution on the surface given by the LES simulation. This case aims to analyze the accuracy of PIV-based surface pressure determination in case of massive flow separation. It will be shown that the deviations to the expected results are caused by the spatially discretization as well as by spatially averaging, which occurs in a standard PIV measurement. In addition, the effect of the velocity uncertainty on the estimated surface pressure distribution biases the result significantly. This implies that PSP will be hardly replaced by PIV.