Vortex Trapping by Different Cavities on an Airfoil

Abstract: This paper presents results of an experimental investigation on the flow in a trapped vortex cell, embedded into a flat plate, and interacting with a zero-pressure-gradient boundary layer. The objective of the work is to describe the flow features and elucidate some of the governing physical mechanisms, in the light of recent investigations on flow separation control using vortex cells. Hot-wire velocity measurements of the shear layer bounding the cell and of the boundary layers upstream and downstream are reported, together with spectral and correlation analyses of wall-pressure fluctuation measurements. Smoke flow visualisations provide qualitative insight into some relevant features of the internal flow, namely a large-scale flow unsteadiness and possible mechanisms driving the rotation of the vortex core. Results are presented for two very different regimes: a low-Reynolds-number case where the incoming boundary layer is laminar and its momentum thickness is small compared to the cell opening, and a moderately high-Reynolds-number case, where the incoming boundary layer is turbulent and the ratio between the momentum thickness and the opening length is significantly larger than in the first case. Implications of the present findings to flow control applications of trapped vortex cells are also discussed.

Abstract: In the present research, a Computational Fluid Dynamics (CFD) investigation is carried out for analyzing the simultaneous effect of suction and cavity for controlling flow separation on NACA 0012 airfoil. Hence, a perpendicular suction jet (jet = -90°) is employed with Rjet equal to 0.15 at Ljet = 0.1c. Simultaneously, a cavity is used at 90% of chord length (0.9c) with 20 mm width and 10 mm depth. The fluid flow is assumed to be 2D turbulent, and incompressible. The results demonstrate that lift coefficient has raised by 30% and drag coefficient has decreased by 40% at α = 14° by using simultaneous suction and cavity. The flow control method improves lift to drag ratio and stall angle has increased from 14° to 22°. Consequently, the flow separation has been delayed, the recirculation zone has gone downstream and completely eliminated by utilizing simultaneous suction and cavity as an effective flow control method.

Abstract: Engineered surface textures can manipulate boundary layers affecting fluid drag We study periodic, infinitely long spanwise grooves on a laminar boundary layer over a plate for 1000 $R\phantom{\rule{0}{0ex}}{e}_{L}$ 25000 Below a certain width-to-depth aspect ratio (AR), a primary vortex inside each groove causes the freestream to ``slip over'', reducing skin friction Increasing AR poses a tradeoff in drag reduction due to pressure drag from groove vertical walls Overall, transverse grooves for laminar flow can reduce total drag up to 10% compared to a flat plate, despite increasing the wetted surface area

Abstract: This paper presents a review of the control of flow separation from solid surfaces by periodic excitation. The emphasis is placed on experimentation relating to hydrodynamic excitation, although acoustic methods as well as traditional boundary layer control, such as steady blowing and suction, are discussed in order to provide an appropriate historical context for recent developments. The review examines some aspects of the excited plane mixing-layer and shows how its development lays the foundation for a basic understanding of the problem. Flow attachment to, and separation from, a deflected flap is then shown to be a paradigm for isolating controlling parameters as well as understanding the basic mechanisms involved. Particular attention is paid to separation control on airfoils by considering controlling parameters such as optimum reduced frequencies and excitation levels, performance enhancement, efficiency, reduction of post-stall unsteadiness, compressibility and other important features. Additional topics covered include excitation of separation bubbles, control and exploitation of diffuser flows, three-dimensional effects, the influence of longitudinal curvature and possible applications to unmanned air vehicles. The review closes with some recent developments in the control and understanding of incompressible dynamic stall, specifically illustrating the control of dynamic stall on oscillating airfoils and identifying the crucial time-scale disparity between dynamic stall and periodic excitation.

Abstract: When work began on the Darrieus vertical axis wind turbine (VAWT) program at Sandia National Laboratories, it was recognized that there was a paucity of symmetrical airfoil data needed to describe the aerodynamics of turbine blades. Curved-bladed Darrieus turbines operate at local Reynolds numbers (Re) and angles of attack (..cap alpha..) seldom encountered in aeronautical applications. This report describes (1) a wind tunnel test series conducted at moderate values of Re in which 0 less than or equal to ..cap alpha.. less than or equal to 180/sup 0/ force and moment data were obtained for four symmetrical blade-candidate airfoil sections (NACA-0009, -0012, -0012H, and -0015), and (2) how an airfoil property synthesizer code can be used to extend the measured properties to arbitrary values of Re (10/sup 4/ less than or equal to Re less than or equal to 10/sup 7/) and to certain other section profiles (NACA-0018, -0021, -0025).

Abstract: Frictionless flows with finite voticity are usually made determinate by the imposition of boundary conditions specifying the distribution of vorticity ‘at infinity’. No such boundary conditions are available in the case of flows with closed streamlines, and the velocity distributions in regions where viscous forces are small (the Reynolds number of the flow being assumed large) cannot be made determinate by considerations of the fluid as inviscid. It is shown that if the motion is to be exactly steady there is an integral condition, arising from the existence of viscous forces, which must be satisfied by the vorticity distribution no matter how small the viscosity may be. This condition states that the contribution from viscous forces to the rate of change of circulation round any streamline must be identically zero. (In cases in which the vortex lines are also closed, there is a similar condition concerning the circulation round vortex lines.)The inviscid flow equations are then combined with this integral condition in cases for which typical streamlines lie entirely in the region of small viscous forces. In two-dimensional closed flows, the vorticity is found to be uniform in a connected region of small viscous forces, with a value which remains to be determined—as is done explicitly in one simple case—by the condition that the viscous boundary layer surrounding this region must also be in steady motion. Analogous results are obtained for rotationally symmetric flows without azimuthal swirl, and for a certain class of flows with swirl having no interior boundary to the streamlines in an axial plane, the latter case requiring use of the fact that the vortex lines are also closed. In all these cases, the results are such that the Bernoulli constant, or ‘total head’, varies linearly with the appropriate stream function, and the effect of viscosity on the rate of change of vorticity at any point vanishes identically.

Abstract: We review recent advances in understanding, modeling, and controlling oscillations in the flow past a cavity. The fundamental mechanisms underlying cavity flow oscillations have been known for at least 40 years, but suppressing these oscillations in a reliable and robust way is still a challenge today. Interest in controlling the flow past a cavity is motivated by aerospace applications, but in addition, cavity flows provide an attractive canonical problem for exploring general flow control techniques. The focus is on recent advances in modeling these flows, and in controlling them, using both open-loop and closed-loop techniques. A relatively new perspective is that cavity oscillations may not always be self-sustained, but under some flow conditions may be lightly damped resonances, sustained by external disturbances such as boundary layer turbulence. Areas in which our understanding is incomplete, and which deserve further study, are discussed, in particular the effects of high-frequency open-loop forcing, fundamental limitations of feedback control for a given configuration of sensors and actuators, and the development of a feedback design methodology that respects the limited range of validity of the available dynamical models.

Abstract: In a numerical simulation the choice of a turbulence model must be a compromise between physical modelling and computational cost. The CIRA RANS flow solver has been applied, by employing a large set of turbulence models, to typical aerodynamic applications for which certified experimental data are available in literature. The transonic flows over an airfoil and a wing placed in a wind tunnel, both characterized by a strong shock-boundary layer interaction with an induced separation, and the high lift flow around a multi component airfoil are taken into consideration. An evaluation, in terms of accuracy and numerical behaviour, of some common turbulence models ranging from one-equation to high order, using the same code and numerics, is presented. Satisfactory and consistent results have been achieved; the more sophisticated the turbulence model the more accurate the simulation is. The SST Menter κ – ω turbulence model has shown, for the applications investigated, the best compromise between the physical capabilities and the numerical stiffness.