13 research outputs found
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Direct numerical simulation of the flow around an aerofoil in ramp-up motion
A detailed analysis of the flow around a NACA0020 aerofoil at Rec = 2 Ă 104 undergoing a ramp up motion has been carried out by means of direct numerical simulations. During the manoeuvre, the angle of attack is linearly varied in time between 0° and 20° with a constant rate of change of αrad = 0.12 Uâ/c. When the angle of incidence has reached the final value, the lift experiences a first overshoot and then suddenly decreases towards the static stall asymptotic value. The transient instantaneous flow is dominated by the generation and detachment of the dynamic stall vortex, a large scale structure formed by the merging of smaller scales vortices generated by an instability originating at the trailing edge. New insights on the vorticity dynamics leading to the lift overshoot, lift crisis, and the damped oscillatory cycle that gradually matches the steady condition are discussed using a number of post-processing techniques. These include a detailed analysis of the flow ensemble average statistics and coherent structures identification carried out using the Q-criterion and the finite-time Lyapunov exponent technique. The results are compared with the one obtained in a companion simulation considering a static stall condition at the final angle of incidence α = 20°
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Large-eddy and wall-modelled simulations of turbulent flow over two-dimensional river dunes
Turbulence models based on the Spalart-Allmaras Detached-Eddy Simulation (DES) approach are used to compute the turbulent flow over a two-dimensional dune geometry. DES was developed for massively separated flows, but has been applied as a wall model to attached flows as well. In attached shear layers, however, the lack of resolved eddies in the region where the model switches from a turbulence model to a Subfilter-Scale (SFS) one, results in an underprediction of the wall stress, and a shift in the logarithmic layer. The dune studied here is neither a fully attached flow nor a massively separated one, and allows us to investigate the accuracy of DES wall-models in intermediate cases of this type. Results are compared to a well-validated Large-Eddy Simulation (LES) database. DES based methods are found to be more accurate in this application, compared to attached boundary layers. All the methods required approximately 3% of the CPU time of the wall-resolved LES simulations. All methods gave similar results, but the Improved Delayed Detached Eddy Simulation seemed preferable because of the consistency of the trends it predicted
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Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics
We performed large-eddy simulations of flow over a series of three-dimensional dunes at laboratory scale (Reynolds number based on the average channel depth and streamwise velocity was 18 900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The bedform three-dimensionality was imposed by shifting a standard two-dimensional dune shape in the streamwise direction according to a sine wave. The statistics of the flow are discussed in 10 cases with in-phase and staggered crestlines, different deformation amplitudes and wavelengths. The results are validated qualitatively against experiments. The three-dimensional separation of flow at the crestline alters the distribution of wall pressure, which in turn may cause secondary flow across the stream, which directs low-momentum fluid, near the bed, toward the lobe (the most downstream point on the crestline) and high-momentum fluid, near the top surface, toward the saddle (the most upstream point on the crestline). The mean flow is characterized by a pair of counter-rotating streamwise vortices, with core radius of the order of the flow depth. However, for wavelengths smaller than the flow depth, the secondary flow exists only near the bed and the mean flow away from the bed resembles the two-dimensional case. Staggering the crestlines alters the secondary motion; the fastest flow occurs between the lobe and the saddle planes, and two pairs of streamwise vortices appear (a strong one, centred about the lobe, and a weaker one, coming from the previous dune, centred around the saddle). The distribution of the wall stress and the focal points of separation and attachment on the bed are discussed. The sensitivity of the average reattachment length, depends on the induced secondary flow, the streamwise and spanwise components of the channel resistance (the skin friction and the form drag), and the contribution of the form drag to the total resistance are also studied. Three-dimensionality of the bed increases the drag in the channel; the form drag contributes more than in the two-dimensional case to the resistance, except for the staggered-crest case. Turbulent-kinetic energy is increased in the separated shear layer by the introduction of three-dimensionality, but its value normalized by the plane-averaged wall stress is lower than in the corresponding two-dimensional dunes. The upward flow on the stoss side and higher deceleration of flow on the lee side over the lobe plane lift and broaden the separated shear layer, respectively, affecting the turbulent kinetic energy
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Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 2. Flow structures
We performed large-eddy simulations of the flow over a series of three-dimensional (3D) dunes at laboratory scale. The bedform three-dimensionality was imposed by shifting a standard two-dimensional (2D) dune shape in the streamwise direction according to a sine wave. The turbulence statistics were discussed in Part 1 of this article (Omidyeganeh & Piomelli, J. Fluid Mech., vol. 721, 2013, pp. 454â483). Coherent flow structures and their statistics are discussed concentrating on two cases with the same crestline amplitudes and wavelengths but different crestline alignments: in-phase and staggered. The present paper shows that the induced large-scale mean streamwise vortices are the primary factor that alters the features of the instantaneous flow structures. Wall turbulence is insensitive to the crestline alignment; alternating high- and low-speed streaks appear in the internal boundary layer developing on the stoss side, whereas over the node plane (the plane normal to the spanwise direction at the node of the crestline), they are inclined towards the lobe plane (the plane normal to the spanwise direction at the most downstream point of the crestline) due to the mean spanwise pressure gradient. Spanwise vortices (rollers) generated by KelvinâHelmholtz instability in the separated shear layer appear regularly over the lobe with much larger length scale than those over the saddle (the plane normal to the spanwise direction at the most upstream point of the crestline). Rollers over the lobe may extend to the saddle plane and affect the reattachment features; their shedding is more frequent than in 2D geometries. Vortices shed from the separated shear layer in the lobe plane undergo a three-dimensional instability while being advected downstream, and rise toward the free surface. They develop into a horseshoe shape (similar to the 2D case) and affect the whole channel depth, whereas those generated near the saddle are advected downstream and toward the bed. When the tip of such a horseshoe reaches the free surface, the ejection of flow at the surface causes âboilsâ (upwelling events on the surface). Strong boil events are observed on the surface of the lobe planes of 3D dunes more frequently than in the saddle planes. They also appear more frequently than in the corresponding 2D geometry. The crestline alignment of the dune alters the dynamics of the flow structures, in that they appear in the lobe plane and are advected towards the saddle plane of the next dune, where they are dissipated. Boil events occur at a higher frequency in the staggered alignment, but with less intensity than in the in-phase alignment
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Large-eddy simulation of two-dimensional dunes in a steady, unidirectional flow
We performed large-eddy simulations of the flow over a typical two-dimensional dune geometry at laboratory scale (the Reynolds number based on the average channel height andmean velocity is 18,900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The results are validated by comparison with simulations and experiments in the literature. The flowseparates at the dune crest, generating a shear layer that plays a crucial role in the transport of momentum and energy, and the generation of coherent structures. The turbulent kinetic energy budgets show the importance of the turbulent transport and mean-flow advection in the bulk flow above the shear layer. In the recirculation zone and in the attached boundary layers production and dissipation are the most important terms. Large, coherent structures of various types can be observed. Spanwise vortices are generated in the separated shear layer due to the Kelvin-Helmholtz instability; as they are advected, they undergo lateral instabilities and develop into horseshoe-like structures, are tilted downward, and finally reach the surface. The ejection that occurs between the legs of the vortex creates the upwelling and downdrafting events on the free surface known as "boils." Near-wall turbulence, after the reattachment point, is affected by large streamwise Taylor-Görtler vortices generated on the concave part of the stoss side, which affect the distribution of the near-wall streaks
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Numerical Simulation of a Passive Control of the Flow Around an Aerofoil Using a Flexible, Self Adaptive Flaplet
© 2018 The Author(s) Self-activated feathers are used by almost all birds to adapt their wing characteristics to delay stall or to moderate its adverse effects (e.g., during landing or sudden increase in angle of attack due to gusts). Some of the feathers are believed to pop up as a consequence of flow separation and to interact with the flow and produce beneficial modifications of the unsteady vorticity field. The use of self adaptive flaplets in aircrafts, inspired by birds feathers, requires the understanding of the physical mechanisms leading to the mentioned aerodynamic benefits and the determination of the characteristics of optimal flaps including their size, positioning and ideal fabrication material. In this framework, this numerical study is divided in two parts. Firstly, in a simplified scenario, we determine the main characteristics that render a flap mounted on an aerofoil at high angle of attack able to deliver increased lift and improved aerodynamic efficiency, by varying its length, position and its natural frequency. Later on, a detailed direct numerical simulation analysis is used to understand the origin of the aerodynamic benefits introduced by the flaplet movement induced by the interaction with the flow field. The parametric study that has been carried out, reveals that an optimal flap can deliver a mean lift increase of about 20% on a NACA0020 aerofoil at an incidence of 20 o degrees. The results obtained from the direct numerical simulation of the flow field around the aerofoil equipped with the optimal flap at a chord Reynolds number of 2 à 10 4 shows that the flaplet movement is mainly induced by a cyclic passage of a large recirculation bubble on the aerofoil suction side. In turns, when the flap is pushed downward, the induced plane jet displaces the trailing edge vortices further downstream, away from the wing, moderating the downforce generated by those vortices and regularising the shedding cycle that appears to be much more organised when the optimal flaplet configuration is selected
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The PELskin project: part IVâcontrol of bluff body wakes using hairy filaments
The passive control of bluff body wakes using a sparse layer of elastic hairy filaments has been investigated via a series of numerical simulations and compared to selected experiments under well-controlled boundary conditions. It has been found that a distribution of filaments spaced half of the dominant three dimensional instability and resonating with the main shedding frequency can drastically delay the three dimensional transition of the wake behind a circular cylinder. It will also be shown that when using a pair of rows of filaments symmetrically spaced by an azimuthal angle, the wake topology can be deeply affected as well as the value of the integral force coefficients of the cylinder. In the most favourable case, a coupled three dimensional transition delay and strongly reduced values of the drag and of the lift fluctuation can be simultaneously achieved. These results hold also for higher Reynolds-number flows as shown in experiments on a cylinder with hairy flaps attached to the aft part. The lock-in effect of structural vibration of the flaps with the vortex shedding is assumed to be the reason for a sudden change in the shedding cycle as soon as the motion amplitude is high enough to modify the wake. In line with this hypothesis, it has been demonstrated that a long elastic filament pinned on the centerline of a forced spatially developing mixing layer can interact with the vortex dynamics delaying the pairing process-leading to a reduced thickness of the layer. These findings show that a properly designed fluid structure interaction can indeed lead to technological benefits in terms of wake control: drag reduction, vibration control and possibly palliation of aeroacoustic emissions
Large eddy simulation of air flow and pollution dispersion around buildings
Bibliography: p. 193-207some pages are in colourThe study of pollution dispersion in urban environment require more accurate prediction of air flow. The high Reynolds number air flow in the atmospheric boundary layer results in fully turbulent flow. Turbulent flow over three-dimensional obstacles is still challenging in engineering; however, a greater understanding of it is necessary to environment management. This work is an effort to provide a new code to simulate air flow around a few simple obstacles and predict the pollution dispersion in the atmospheric boundary layer for a single case. Simple but reliable methods and models were adopted in developing the code in C++ to numerically solve the fluid flow equations using large eddy simulation (LES). In addition, the mass transfer equation was numerically solved to predict the pollution dispersion around a cube mounted on the ground in the atmospheric boundary layer. Several cases were examined by the code for fluid flow simulation. The first and simplest simulation was conducted for a cube obstacle in the atmospheric boundary layer and was selected for the pollution dispersion case as well. A few parameters were changed for this problem to investigate the effects of the Reynolds number, grid spacing, and sub-grid scale constant on the flow patterns. The fluid flow around a cube mounted on a plane in a channel was the second problem. Finally, the fluid flow around two parallel rectangles in the atmospheric boundary layer with a little passage between them was investigated. All the results obtained from simulations were assessed with experimental data from the literature. However, the major differences between simulation setups and experimental models have been considered in the discussion. The most important difference between experiments and simulation was the approaching flow into the obstacles. Simulations were carried out by laminar inflow condition; however, in the experiments, fully turbulent flow had been used. As a result, the flow pattern was changed significantly; so that in the simulation with the laminar approaching flow, the wake behind the obstacle increased and the vortex on the roof disappeared. There is a good agreement between the simulation result and experimental data in most of the case. In fact, the flow in the channel resulted in more reliable outcome than the flow in atmospheric boundary layer. The developed code showed the capability of accurately predicting the pollution dispersion accurately with some modifications