Several Angles Of Attack Research Articles

Study of laminar separation bubble on low Reynolds number operating airfoils: RANS modelling by means of an high-accuracy solver and experimental verification

This work is devoted to the Computational Fluid-Dynamics (CFD) simulation of laminar separation bubble (LSB) on low Reynolds number operating airfoils. This phenomenon is of large interest in several fields, such as wind energy, and it is characterised by slow recirculating flow at an almost constant pressure.Presently Reynolds Averaged Navier-Stokes (RANS) methods, due to their limited computational requests, are the more efficient and feasible CFD simulation tool for complex engineering applications involving LSBs. However adopting RANS methods for LSB prediction is very challenging since widely used models assume a fully turbulent regime. For this reason several transitional models for RANS equations based on further Partial Differential Equations (PDE) have been recently introduced in literature. Nevertheless in some cases they show questionable results.In this work RANS equations and the standard Spalart-Allmaras (SA) turbulence model are used to deal with LSB problems obtaining promising results. This innovative result is related to: (i) a particular behaviour of the SA equation; (ii) a particular implementation of SA equation; (iii) the use of a high-order discontinuous Galerkin (DG) solver. The effectiveness of the proposed approach is tested on different airfoils at several angles of attack and Reynolds numbers. Numerical results were verified with both experimental measurements performed at the open circuit subsonic wind tunnel of Università Politecnica delle Marche (UNIVPM) and literature data.

Simulations of High Reynolds Number Air Flow Over the NACA-0012 Airfoil Using the Immersed Boundary Method

The immersed-boundary method is coupled to an incompressible-flow Reynolds-averaged Navier Stokes solver, based on a two-equation turbulence model, to perform unsteady numerical simulations of airflow past the NACA-0012 airfoil for several angles of attack and Reynolds numbers of 5.0×105 and 1.8×106. A preliminary study is performed to evaluate the sensitivity of the calculations to the computational mesh and to guide the creation of the computational cells for the unsteady calculations. Qualitative characterizations of the flow in the vicinity of the airfoil are obtained to assess the capability of locally refined grids to capture the thin boundary layers close to the airfoil leading edge as well as the wake flow emanating from the trailing edge. Quantitative analysis of aerodynamic force coefficients and wall pressure distributions are also reported and compared to experimental results and those from body-fitted grid simulations using the same solver to assess the accuracy and limitations of this approach. The immersed-boundary simulations compared well to the experimental and body-fitted results up to the occurrence of separation. After that point, neither computational approach provided satisfactory solutions.

Wind-tunnel tests on high-rise buildings: wind modes and structural response

The evaluation of pressure fields acting on slender structures under wind loads is currently performed in experimental aerodynamic tests. For wind-sensitive structures, in fact, the knowledge of global and local wind actions is crucial for design purpose. This paper considers a particular slender structure under wind excitation, representative of most common high-rise buildings, whose experimental wind field on in-scale model was measured in the CRIACIV boundary-layer wind tunnel (University of Florence) for several angles of attack of the wind. It is shown that an efficient reduced model to represent structural response can be obtained by coupling the classical structural modal projection with the so called blowing modes projection, obtained by decomposing the covariance or power spectral density (PSD) wind tensors. In particular, the elaboration of experimental data shows that the first few blowing modes can effectively represent the wind-field when eigenvectors of the PSD tensor are used, while a significantly larger number of blowing modes is required when the covariance wind tensor is used to decompose the wind field.

외부 오염물 증착에 의한 풍력 터빈 날개 단면의 공력 성능 저하 예측

In the present study, the effects of contaminant accumulation and surface roughness on the aerodynamic performance of wind turbine blade sections were numerically investigated by using a flow solver based on unstructured meshes. The turbulent flow over the rough surface was modeled by a modified <TEX>${\kappa}-{\omega}$</TEX> SST turbulence model. The calculations were made for the NREL S809 airfoil with varying contaminant sizes and positions at several angles of attack. It was found that as the contaminant size increases, the degradation of the airfoil performance becomes more significant, and this trend is further amplified near the stall condition. When the contaminant is located at the upper surface near the leading edge, the loss in the aerodynamic performance of the blade section becomes more critical. It was also found that the surface roughness leads to a significant reduction of lift, in addition to increased drag.

Towards simulation of flapping wings using immersed boundary method

SUMMARYIn this work the immersed boundary method is applied to simulate incompressible turbulent flows around stationary and moving objects. The goal is to demonstrate that the immersed boundary technique along with a large eddy simulation approach is capable of simulating the effect of the so‐called leading edge vortex (LEV), which can be found in flapping wing aerodynamics. A Lagrangian method is used to approximate the solutions in the freshly cleared cells that lay within solid objects at one time step and emerge into fluid domain at the next time step. Flow around a stationary cylinder at ReD = 20, 40, and 3900 (based on cylinder diameter D) is first studied to validate the immersed boundary solver based on the finite volume scheme using a staggered grid. Then, a harmonically oscillating cylinder at ReD = 10 000 is considered to test the solver after the Lagrangian method is implemented to interpolate the solution in the freshly cleared cells. Finally, this approach is used to study flows around a stationary flat‐plate at several angles of attack and fast pitching flat‐plate. The rapidly pitching plate creates a dynamic LEV that can be used to improve the efficiency of flapping wings of micro air vehicle and to determine the optimum flapping frequency. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Integrated Experimental-Numerical Analysis of High-Agility Aircraft Wake Vortex Evolution

The presented investigation includes a combined experimental–numerical approach to quantify the wake vortex system of a high-agility aircraft from the near field up to the far field. Detailed near-field data are obtained by lowspeed wind-tunnel tests on a delta-canard configuration of 1:15 scale. The measurements are performed at several angles of attack applying advanced hot-wire anemometry. For a wake distance of up to 16 wing spans, mean and turbulentvelocity fieldsaremeasured.Theupstreamdataareusedtoinitializeimplicitlarge-eddysimulationsaimed tocomputethevelocity fieldsofthewakevortexsystemoverawakedistanceofupto50spans.Here,avalidationcase is shown comparing measured and calculated wake data over a distance from 4 to 16 spans, with the implicit largeeddy simulations initialized by the measured quantities at a position of two wing spans. Compared with the experimental data,the numericalresults showthe expectedlateral andvertical movementof thewakevortexsystem due to the interaction of the single vortices. The distributions of axial vorticity, crossflow velocities, and turbulence intensities match well with the experimental data. Inaddition, the dissipation process can be observed, resulting in a reduction of circulation. In the context of this study, the measured and computed velocity fields will be used to determine unsteady aerodynamic loads acting on a fighter aircraft encountering the wake. This is of great importance as wake induction may result in critical structural dynamic loads.

Incipient Cavitation Studied Under Strong Thermodynamic Effect

Incipient cavitation was studied under simulated cryogenic conditions on a NACA0015 hydrofoil in a tunnel filled with the perfluorinated ketone 2-trifluoromethyl-1, 1, 1, 2, 4, 4, 5, 5, 5-nonafluoro-3-pentanone. Through pressure measurements on the hydrofoil, laser-illuminated high-speed photography, and flash-illuminated photography, the extent of cavitation and the characteristic frequencies of its oscillation were studied under varying speeds in the range of 1.7-6.7 m/s and several angles of attack. The results presented in this paper are limited to a 5.1-degree angle of attack. It was found that the vapor formation was much stronger in fluoroketone than in cold-water tests at similar cavitation numbers. The formed bubbles were significantly smaller and there existed an extended speed range over which fluctuation amplitudes grew with no well-defined frequency peaks, as was observed in water.

Investigation of the Propeller Slipstream with Particle Image Velocimetry

The flow around an 8-bladed propeller is analyzed using particle image velocimetry in the Airbus low-speed wind tunnel facility in Bremen, Germany. The propeller is mounted on a half-model equipped with two propellers with opposing rotation directions, where only the outboard propeller is investigated. For two cases with no thrust (C T = 0) and with thrust (C T = 0.1) the propeller slipstream is visualized at several angles of attack. In particular, the development of the slipstream over the wing is investigated with particle image velocimetry derived variables such as mean velocities, fluctuations, and turbulent kinetic energy. A detailed description of the slipstream returns valuable information on the vortical structures and periodic phenomena. An identification of such events is necessary for a future validation with numerical data. It allows for the analysis of parametric cases; first, a comparison between the thrust and no-thrust cases and, second, the effect of an increasing angle of attack. It is found that at the thrust cases, for higher angles of attack, vortices are formed due to interactions in the boundary layer.

G0501-1-3 平板翼の非定常キャビテーション流れに及ぼす翼端隙間の影響(キャビテーション,マイクロバブル)

Unsteady cavitating flow around a flat plate hydrofoil was studied in order to investigate the effect of tip leakage vortex cavitation on the unsteady character of the cavitating flow. A two-dimensional flat plate hydrofoil having rounded leading and trailing edges with/without a tip claearance was tested for several angles of attack. The cavitating flow was filmed from the top and the side simultaneously using two high speed video cameras, and the unsteady static pressures just upstream and dovmstream of the hydrofoil were also simultaneously measured with the high speed video camera recording. It was found that, with the tip clearance, the partial cavity at the same cavitation number became more or less shorter and thinner than that without the tip clearance. Even under the existence of the tip leakage vortex cavity, the Strouhal number rule of the unsteady cavitating flow based on the attached cavity length still stood in our experiments.

Hydrofoil Cavitation Under Strong Thermodynamic Effect

Cavitation was studied for a NACA0015 hydrofoil using a material that simulates cryogenic behavior. Several angles of attack and flow speeds up to 8.6m∕s were tested. The material used, 2-trifluoromethyl-1,1,1,2,4,4,5,5,5-nonafluoro-3-pentanone, hereafter referred to as fluoroketone, exhibits a strong thermodynamic effect even under ambient conditions. Static pressures were measured at seven chordwise locations along the centerline of the hydrofoil suction side and on the test section wall immediately upstream of the hydrofoil. Frequency analysis of the test section static pressure showed that the amplitude of the oscillations increased as the tunnel speed increased. A gradual transition corresponding to the Type II-I sheet cavitation transition observed in water was found to occur near σ∕2α=5 with Strouhal numbers based on chord dropping from 0.5 to 0.1 as the cavitation number was reduced. Flash-exposure high-speed imaging showed the cavity covering a larger portion of the chord for a given cavitation number than in cold water. The bubbles appeared significantly smaller in the current study and the pressure data showed increasing rather than constant static pressure in the downstream direction in the cavitating region, in line with observations made in literature for other geometries with fluids exhibiting strong thermodynamic effect.

A Lagrangian analysis of a two-dimensional airfoil with vortex shedding

Using invariant material manifolds and flow topology, the flow behavior and structure of flow around a two-dimensional Eppler 387 airfoil is examined with an emphasis on vortex shedding and the time-dependent reattachment profile. The examination focuses on low Reynolds number (Re = 60 000) flow at several angles of attack. Using specialized software, we identify invariant manifolds in the flow and use these structures to illuminate the process of vortex formation and the periodic behavior of the reattachment profile. Our analysis concludes with a topological view of the flow, including fixed points and a discussion of phase plots and the frequency spectrum of several key points in the flow. The behavior of invariant manifolds directly relates to the flow topology and illuminates some aspects seen in phase space during vortex shedding. Furthermore, it highlights the reattachment behavior in ways not seen before.

CFL3D/OVERFLOW Results for DLR-F6 Wing/Body and Drag Prediction Workshop Wing

A series of overset grids was generated in response to the Third AIAA CFD Drag Prediction Workshop (DPW-III) which preceded the 25th Applied Aerodynamics Conference in June 2006. DPW-III focused on accurate drag prediction for wing/body and wing-alone configurations. The grid series built for each configuration consists of a coarse, medium, fine, and extra-fine mesh. The medium mesh is first constructed using the current state of best practices for overset grid generation. The medium mesh is then coarsened and enhanced by applying a factor of 1.5 to each (I, J, K) dimension. The resulting set of parametrically equivalent grids increase in size by a factor of roughly 3.5 from one level to the next denser level. Computational fluid dynamics simulations were performed on the overset grids using two different Reynolds-averaged Navier-Stokes flow solvers: CFL3D and OVERFLOW. The results were postprocessed using Richardson extrapolation to approximate grid-converged values of lift, drag, pitching moment, and angle of attack at the design condition. This technique appears to work well if the solution does not contain large regions of separated flow (similar to that seen in the DLR-F6 results) and appropriate grid densities are selected. The extra-fine grid data helped to establish asymptotic grid convergence for both the OVERFLOW FX2B wing/body results and the OVERFLOW DPW-W1/W2 wing-alone results. More CFL3D data are needed to establish grid convergence trends. The medium grid was used beyond the grid convergence study by running each configuration at several angles of attack so drag polars and lift/pitching moment curves could be evaluated. The alpha sweep results are used to compare data across configurations as well as across flow solvers. With the exception of the wing/body drag polar, the two codes compare well qualitatively showing consistent incremental trends and similar wing pressure comparisons.

Effect of Computational Method on Discrete Roughness Correlations for Shuttle Orbiter

A reanalysis of discrete roughness boundary-layer transition data using a consistent computational method for comparison to other published results has been completed. The primary objective of this effort was to investigate the influence of the computational approach on the resulting transition correlation. The experimental results were previously obtained on Space Shuttle Orbiter models in the NASA Langley Research Center 20-Inch Mach 6 Air Tunnel. The phosphor thermography system was used to monitor the status of the boundary layer via global heat- transfer images of the orbiter windward surface. The existing roughness transition database included a variation in the size and location of discrete roughness trips along the centerline of 0.0075-scale models at an angle of attack of 40 deg. Various correlative approaches were attempted, with the roughness transition correlations based on edge properties providing the most reliable results. When a consistent computational method is used to compute edge conditions, transition data sets for different moderately blunt configurations at several angles of attack are shown to collapse to a well-behaved correlation. The shuttle experimental dataset presented herein, therefore, can be used to calibrate the preferred computational method of the end user for use in the future designs of the next-generation space access vehicles.

Unsteady Computations of Abrupt Wing Stall Using Detached-Eddy Simulation

Unsteady computational fluid dynamics calculations are presented of the abrupt wing stall phenomenon on the preproduction F/A-18E using detached-eddy simulation. Detached-eddy simulation combines the efficiency of a Reynolds-averaged turbulence model near the wall with the fidelity of large-eddy simulation in separated regions. Because it uses large-eddy simulation in the separated regions, it is capable of predicting the unsteady motions associated with separated flows. Detached-Eddy Simulation has been applied to predict the unsteady shock motion present on the F/A-18E at transonic speeds over several angles of attack. Solution-based grid adaption is used on unstructured grids to improve the resolution in the separated region

Supercavitation Hydrofoil Performance and Torsional Flutter Margin.

Supercavitation hydrofoil applied to high-speed marine propeller or hydro-machinery blade runs into unsteady behaviors such as cavitation breakdown and hydraulic flutter in some operation range. The hydrofoil performance was experimentally estimated and compared with the wedge performance using the cavitation tunnel and the torsional vibration apparatus with three component load cells. This experiment was carried out at several angles of attack in the region from subcavitation to supercavitation. At a general steady state but including some cavitation breakdowns, the hydrofoil has the most superior time mean lift/drag ratio about 6<Cl/Cd<8 at in all cavitation regions. But, the ratio drastically decreases as the angle of attack increases. Fluctuating lift coefficient C'l due to the cavitation breakdown reaches up to about 10% of time mean lift coefficient Cl. At the hydrofoil pitching motion, the torsional flutter margin of the hydrofoil extends to higher reduced frequency side as the angle of attack increases and has the nearly same margin of the wedge in all cavitation regions.

A new approach to the particle-in-cell method for the computation of three-dimensional incompressible unsteady flows

A hybrid three-dimensional particle-in-cell (PIC) method is proposed. This 3D PIC technique combined with a classical three-dimensional vortex particle method is applied to the calculation of unsteady inviscid and incompressible flow. More precisely the wake roll-up behind an inclined flat plate is studied. A spreading procedure of the vorticity, a direct interaction between nodes of the grid and a mesh generated from the plate discretization present a few undesirable grid effects, but ensure a continuous application of the slip condition to the plate. In this way the total circulation is nearly conserved and, in the case of a separated flow at high angle of attack, the particles are prevented from crossing the wing. Numerical results are compared with experiment for several angles of attack and aspect ratios.

Unsteady Flow Around an Elliptic Cylinder in the Critical Reynolds Number Regime.

Experimental investigation of the flow around an elliptic cylinder of axis ratio 3 : 1 was carried out for several angles of attack and a wide range of Reynolds numbers. The flow characteristics were studied by measuring the fluid force and the surface pressure fluctuation. In the critical Reynolds number regime, a discontinuous change of flow state was observed. This change was accompanied by the hysteresis effect. The fluid force and the pressure fluctuation decreased markedly in the critical flow state.

Unsteady Cavitating Flow Around an Elliptic Cylinder.

Experimental investigations on the unsteady cavitating flow around an elliptic cylinder of axis ratio 3 : 1 were carried out for several angles of attack and a wide range of cavitation numbers. The flow characteristics were studied by measuring the fluid force and the pressure fluctuation. Under non cavitating conditions, a dramatic change of flow state was observed in the critical Reynolds number regime. This change of flow state has a great influence on the unsteady characteristics of cavitating flow.

Calculation of three-dimensional low Reynolds number flows

An interactive boundary-layer stability-transition approach is used to calculate the performance characteristics of an infinite swept wing at low Reynolds numbers for several angles of attack. The in viscid flow solutions are obtained from an inviscid method based on conformal mapping and the viscous flow solutions from an inverse boundary-layer scheme which uses the Hilbert integral formulation to couple the inviscid and viscous flow. The onset of transition is calculated by the e method, based on twoand three-dimensional versions of linear stability theory. Calculated results for an infinite swept wing with an Eppler airfoil cross section are presented for sweep angles corresponding to A = 30, 40, and 45 deg, and for Reynolds numbers of 3 x 10 and 4.6 x 10. The effect of sweep angle on lift and drag coefficients is investigated together with the accuracy of predicting the onset of transition with two versions of the e method.

Effect of wind-tunnel walls on airfoil droplet impingement

A two-dimensiona l computer code, capable of modeling wind-tunnel walls, was written to calculate the impingement of water droplets on an airfoil in an incompressible, inviscid flow. The calculated flowfield was validated by comparison to standard wind-tunnel wall correction methods. When run without the walls present, the calculated droplet impingement compared well to a free-air code. Once validated, the code was run for an NAG A 4412 at several angles of attack, tunnel heights, and modified inertia parameters. The effect that the tunnel walls had on the local impingement efficiency and the total collection efficiency was examined. Additionally, changes in the limits of impingement and changes in the origin of tangent trajectories are discussed with applications to icing tunnel testing. The effects of the walls are found to be small for reasonable tunnel height-to-chord ratios due to canceling near- and far-field aerodynamic effects on the droplet. The effect on total collection efficiency changes sign when droplet diameter (modified inertia parameter) is increased due to a change in the relative importance of droplet inertia and drag. CD = E = Fr = g = H = h — K = K(} = Rtl = S =