1,721,052 research outputs found
Separation delay through contoured transverse grooves on a 2D boat-tailed bluff body: Effects on drag reduction and wake flow features
No full textThe effectiveness of properly contoured transverse grooves in delaying the flow separation occurring on a two-dimensional boat-tailed bluff body is assessed through numerical simulations. The body has a cross-section with a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail. Three-dimensional Variational Multiscale Large Eddy Simulations are carried out at Re=Du∞∕ν=9.6×104, using a mixed finite-volume/finite-element method. The introduction of one contoured groove on each of the boat-tail lateral surfaces produces a significant delay of flow separation and a consequent increase of the base pressure, with a global drag reduction of the order of 9.7%. The wake dynamical structure remains qualitatively similar to the one typical of blunt-based two-dimensional bodies, with quantitative variations that are consistent with the reduction in wake width caused by boat tailing and by the grooves. The introduction of the grooves leads also to a regularization of the vortex shedding downstream of the body, which is more correlated in the spanwise direction. Finally, a few supplementary simulations show that the effect of the grooves is also robust to the variation of the geometrical parameters defining their location and shape
Flow around a 5:1 rectangular cylinder: Effects of upstream-edge rounding
Full text linkA sensitivity analysis of highly-resolved large-eddy simulations of the flow around a 5:1 rectangular cylinder to the introduction of a small rounding of the upstream edges is presented. Different values of the edge radius of curvature are considered, in a range such that they might reasonably be ascribed to manufacturing tolerances. A stochastic approach is adopted in order to build response curves of the quantities of interest as a function of the radius of curvature. The considered computational set-up, characterized by a fine numerical resolution and a low subgrid-scale (SGS) dissipation, predicts for the body having perfectly sharp edges a short mean recirculation length on the cylinder side, in disagreement with experimental data. On the other hand, even for the smallest considered radius of curvature, the length of the mean recirculation region increases significantly and, hence, the agreement with the experimental data is much improved. It is observed that the sharp edge introduces a higher level of turbulent fluctuations in the shear-layer at separation, which, if not artificially damped by numerical or SGS dissipation, grows faster and leads to a further upstream roll-up of the shear-layers and, hence, to a shorter mean recirculation region than in simulations with rounded edges
A Lagrangian probability-density-function model for collisional turbulent fluid-particle flows
No full textInertial particles in turbulent flows are characterised by preferential concentration and segregation and, at sufficient mass loading, dense particle clusters may spontaneously arise due to momentum coupling between the phases. These clusters, in turn, can generate and sustain turbulence in the fluid phase, which we refer to as cluster-induced turbulence (CIT). In the present work, we tackle the problem of developing a framework for the stochastic modelling of moderately dense particle-laden flows, based on a Lagrangian probability-density-function formalism. This framework includes the Eulerian approach, and hence can be useful also for the development of two-fluid models. A rigorous formalism and a general model have been put forward focusing, in particular, on the two ingredients that are key in moderately dense flows, namely, two-way coupling in the carrier phase, and the decomposition of the particle-phase velocity into its spatially correlated and uncorrelated components. Specifically, this last contribution allows us to identify in the stochastic model the contributions due to the correlated fluctuating energy and to the granular temperature of the particle phase, which determine the time scale for particle-particle collisions. The model is then validated and assessed against direct-numerical-simulation data for homogeneous configurations of increasing difficulty: (i) homogeneous isotropic turbulence, (ii) decaying and shear turbulence and (iii) CIT
Impact of spanwise extent of transverse grooves on drag reduction in boat-tailed bluff bodies: an experimental study
No full textThe paper describes the first experimental study on the application of small contoured grooves in boat-tailed bodies characterized by vortex shedding. In particular, we experimentally investigate the flow-separation delay and drag-reducing performance of spanwise-extruded and spanwise-discontinuous grooves. For this purpose, we consider groove geometries similar to those proposed and numerically investigated by Mariotti et al. (Eur J Mech B/Fluids 74:351–362, 2019) and Pasqualetto et al. (Fluids 7:121, 2022a). The Reynolds number, based on the freestream velocity and the model crossflow dimension, is Re=9.6·104. In addition to serving as an experimental confirmation of previous numerical studies, an important difference is that the present experiments were conducted with a freestream turbulence intensity of 0.9%, whereas the simulations were carried out with a freestream without turbulence. This extends the applicability of this flow control device to a situation closer to real-world or industrial applications. In the experiments, we measure pressure-drag variations for different configurations and flow correlations in the spanwise direction through pressure and hot-wire measurements. The results confirm the good performance of the grooves as passive flow-control devices and the capability of grooves to delay flow separation even in a turbulent freestream. The experiments elucidate the physical mechanism leading to the enhanced performance, specifically the reduction of friction losses due to the local recirculation embedded in the groove region. However, the experiments reveal a different behavior in terms of vortex shedding correlation in the spanwise direction with the introduction of grooves of different spanwise extents. Interestingly, the spanwise-extruded grooves exhibit a weaker increase in spanwise correlation of vortex shedding in experiments compared to simulations. This difference is likely due to the presence of freestream turbulence in the wind tunnel, which is absent in simulations. As expected, the introduction of the spanwise-discontinuous groove reduces vortex shedding correlation. Consequently, in experiments the adoption of spanwise-discontinuous grooves yields fewer benefits than those previously found numerically
Benchmark on the Aerodynamics of a Rectangular 5:1 Cylinder: an overview after the first four years of activity
No full textIn July 2008, a benchmark study on the aerodynamics of a stationary rectangular cylinder with chord-to-depth ratio equal to 5 (BARC) was launched. This paper gives an outline of the state of the art on the aerodynamics of 5:1 rectangular cylinders prior to the starting of BARC, and summarizes the results obtained by the contributors during the first four years of activity. The results of about 70 realizations of the BARC flow configuration obtained under a nominally common set-up in both wind tunnel experiments and numerical simulations are compared among themselves and with the data available in the literature prior to BARC, in terms of bulk parameters, flow and aerodynamic load statistics, pressure and force spanwise correlations. It is shown that the near wake flow,the base pressure and, hence, the drag coefficient obtained in the different flow realizations are in very good agreement. Conversely, the flow features along the cylinder lateral surfaces and, hence, the lift, are strongly sensitive to set-up and modelling, leading to a significant dispersion of both wind tunnel measurements and numerical predictions. Finally, a possible asymmetry of the time averaged flow has been recognized both in wind tunnel tests and in numerical simulations
Stochastic calibration of cavitation model parameters for simulations of 3-phase injector internal flows
No full textThe 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
Mixing sensitivity to the inclination of the lateral walls in a T-mixer
Full text linkOne of the simplest geometries for micro-mixers has a T-shape, i.e., the two inlets join perpendicularly the mixing channel. The cross-sections of the channels are usually square/rectangular, as straight walls facilitate experimental and modeling analysis. On the contrary, this work investigates through Computational Fluid Dynamics the effect of a cross-section with lateral walls inclined of an angle α as such an inclination may stem from different microfabrication techniques. Considering water as operating fluid, the same mixing performance as square/rectangular cross-sections is obtained for inclinations α≤3°; this indicates the maximum admissible error on the perpendicularity of the walls in the manufacturing process. Above this value, the presence of inclined walls delays the onset of the engulfment regime at higher Reynolds numbers, and for α≥23°the mixing is hampered dramatically, as the flow is unable to break the mirror symmetry and enter in the engulfment regime. At low Reynolds numbers, the mixing is moderately improved for α≥10°, because the vortex regime presents a lower degree of symmetry than that of T-mixers with straight walls
Stochastic sensitivity analysis of numerical simulations of injector internal flows to cavitation modeling parameters
No full textA 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
Effects of Spanwise-Discontinuous Contoured Transverse Grooves on Flow Separation and Vortex Shedding
No full textThe delay of boundary layer separation over curved solid surfaces is of great importance in many engineering applications. The proposed idea is to introduce small and suitably shaped grooves transverse to the flow (i.e., contoured transverse grooves) to passively generate local steady flow recirculations
A simple model for deep dynamic stall conditions
No full textThe present study is focused on modeling of dynamic stall behavior of a pitching airfoil. The deep stall regime is in particular considered. A model is proposed, which has a low implementation and computational complexity but yet is able to deal with different types of dynamic stall conditions, including those characterized by multiple vortex shedding at the airfoil leading edge. The proposed model is appraised against an extensive data set of experimental (α,CL) curves for NACA0012. The results of an existing widely used model, having comparable complexity, are also shown for comparison. The proposed model is able to well reproduce not only the classic curves of deep dynamic stall but also the curves characterized by lift oscillations at high angles of attack due to the shedding of multiple vortices. Furthermore, the model appears to be robust to variations of its parameters from the optimal values and of the airfoil geometry. Finally, the model is successfully implemented in a commercial CFD software and applied to the simulation of a vertical axis wind turbine within the actuator cylinder approach. The accuracy of the prediction of the turbine power coefficient in the whole rotation cycle is very good for the optimal working condition of the turbine, for which the model parameters were calibrated. Fairly good accuracy is also obtained in significantly different working conditions without any further calibration
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