Secondary separation from a slender wing (original) (raw)
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An Experimental Study of the Laminar Flow Separation on a Low-Reynolds-Number Airfoil
Journal of Fluids Engineering, 2008
An experimental study was conducted to characterize the transient behavior of laminar flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds number of 70,000. In addition to measuring the surface pressure distribution around the airfoil, a high-resolution particle image velocimetry (PIV) system was used to make detailed flow field measurements to quantify the evolution of unsteady flow structures around the airfoil at various angles of attack (AOAs). The surface pressure and PIV measurements clearly revealed that the laminar boundary layer would separate from the airfoil surface, as the adverse pressure gradient over the airfoil upper surface became severe at AOA≥8.0deg. The separated laminar boundary layer was found to rapidly transit to turbulence by generating unsteady Kelvin–Helmholtz vortex structures. After turbulence transition, the separated boundary layer was found to reattach to the airfoil surface as a turbulent boundary layer when the adverse pressure...
Cornell University - arXiv, 2022
We investigate the spatial distributions and production mechanisms of vorticity and turbulent kinetic energy around a finite NACA 0018 wing with square wingtip profile at Re c = 10 4 and 10 • angle of attack with the aid of Direct Numerical Simulation (DNS). The analysis focuses on the highly inhomogeneous region around the tip and the near wake; this region is highly convoluted, strongly three-dimensional, and far from being self-similar. The flow separates close to the leading edge creating a large, open recirculation zone around the central part of the wing. In the proximity of the tip, the flow remains attached but another smaller recirculation zone forms closer to the trailing edge; this zone strongly affects the development of main wing tip vortex. The early formation mechanisms of three vortices close to the leading edge are elucidated and discussed. More specifically, we analyse the role of vortex stretching/compression and tilting, and how it affects the strength of each vortex as it approaches the trailing edge. We find that the three-dimensional flow separation at the sharp tip close to the leading edge plays an important role on the subsequent vortical flow development on the suction side. The production of turbulent kinetic energy and Reynolds stresses is also investigated and discussed in conjunction with the identified vortex patterns. The detailed analysis of the mechanisms that sustain vorticity and turbulent kinetic energy improves our understanding of these highly three dimensional, non-equilibrium flows and can lead to better actuation methods to manipulate these flows.
An Experimental Investigation on the Flow Separation on a Low-Reynolds-Number Airfoil
45th AIAA Aerospace Sciences Meeting and Exhibit, 2007
An experimental investigation was conducted to study the transient behavior of the flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds numbers of 68,000. A high-resolution PIV system was used to make detailed flow field measurements in addition to the surface static pressure distribution mapping around the airfoil. The measurement results visualized clearly that a separation bubble would be generated on the airfoil upper surface if the adverse pressure gradient is adequate. The length of the separation bubble could be up to 20% of airfoil chord length and its height only about 1% of the cord length. The transient behavior of the flow separation on the airfoil, which includes the "taking-off" of the laminar boundary layer from the airfoil surface at the separation point, the generation of unsteady Kelvin-Helmholtz vortex in the separated boundary layer, the rapid transition of the separated laminar boundary layer to turbulent flow, the reattachment of the turbulent flow to the airfoil surface to form separation bubble, and the burst of the separation bubble to cause airfoil stall, were elucidated clearly and quantitatively from the detailed flow field measurements.
Physical Review Fluids
We investigate the spatial distributions and production mechanisms of vorticity and turbulent kinetic energy around a finite NACA 0018 wing with square wingtip profile at Re c = 10 4 and 10 • angle of attack with the aid of Direct Numerical Simulation (DNS). The analysis focuses on the highly inhomogeneous region around the tip and the near wake; this region is highly convoluted, strongly three-dimensional, and far from being self-similar. The flow separates close to the leading edge creating a large, open recirculation zone around the central part of the wing. In the proximity of the tip, the flow remains attached but another smaller recirculation zone forms closer to the trailing edge; this zone strongly affects the development of main wing tip vortex. The early formation mechanisms of three vortices close to the leading edge are elucidated and discussed. More specifically, we analyse the role of vortex stretching/compression and tilting, and how it affects the strength of each vortex as it approaches the trailing edge. We find that the three-dimensional flow separation at the sharp tip close to the leading edge plays an important role on the subsequent vortical flow development on the suction side. The production of turbulent kinetic energy and Reynolds stresses is also investigated and discussed in conjunction with the identified vortex patterns. The detailed analysis of the mechanisms that sustain vorticity and turbulent kinetic energy improves our understanding of these highly three dimensional, non-equilibrium flows and can lead to better actuation methods to manipulate these flows.
Three-Dimensional Solution of Flows over Wings with Leading-Edge Vortex Separation
AIAA Journal, 1976
A computer program has been developed for the solution of the subsonic, three-dimensional flow over wings with leading-edge vortex separation. The documentation is divided into two volumes, Part I and Part II. This volume is Part II of the documentation containing the description of the computer program. It consists of three sections presenting the Program Logic, the Description of Subroutines, and the Program Listing.
Numerical Study of Laminar Separation Flow Over a Low Reynolds Number Airfoil
Current article is a numerical study on an airfoil to predict laminar separation flow at low Reynolds numbers. 0 0 Two angles of attack 4 & 6 were chosen for current investigation at a Reynolds number of 70,000. Various RANS turbulence models were utilized to ascertain their prediction capabilities of laminar separation flows over airfoils at low Reynolds numbers. Low Reynolds number flows are predominantly laminar, which even with slightest disturbance tend to separate from the surface of the object in flow. However due to transition flow tends to reattach thus forming a laminar separation bubble between separation and reattachment points. This is a comparative study of RANS based turbulence models and Mach contours with streamline plots around airfoil are provided for various turbulence models. Of the various models utilized during the current investigation it was observed that transition models were able to clearly predict the laminar separation flow and the presence of laminar separation bubble. Other turbulence models however could not predict flow separation.
1993
The detailed unsteady separation structure of a low-Reynolds-number Eppler 387 airfoil was numerically studied using a time accurate artificial compressibility code. Periodic vortex shedding from the separation was observed in all cases studied. The boundary layer developed into a structure which was reminiscent of a free shear flow, where vortex roll-up and pairing were the dominant features. Although the small-scale turbulence was not modelled in this study, the time-averaged results of global parameter (C, , Cd) and local parameter (C> compared well with the experimental data of McGhee, Walker & Millard [1988]. The time-averaged results exhibited many features of a transitional bubble. namely the nearly stagnant recirculating fluid downstream of the separation point and an abrupt increase of the surface pressure in the reattachment region. This suggests that the unsteady large-scale structure controls the separation on low-Reynolds-number airfoils and small-scale turbulence only plays a secondary role.
Laminar Separation Bubble And Tip Vortex Interaction Over Rigid Wing Withlow Aspect Ratio
Laminar Separation Bubble And Tip Vortex Interaction Over Rigid Wing Withlow Aspect Ratio, 2016
In this study, laminar sepa ra tion bub ble and tip vortices interaction over NACA4412 ri gid wing with low aspect ratio were investigat ed both exp eri mentally and numerically. The experi ment s were performed in an ope n circuit wind tunnel which had low free str eam turbulence intensity at low Reynolds numbers. In the experi me ntal study, the smoke-wire and oil flow visuali zati on technique were used for the flow visuali zation over the NACA 4412 aerofoil surface. In the numerical study, ANSYS FLUENT software was utili ze d and SST Transitio n mod el was used. It was seen that lami nar separation bubble and tip vortices which were occurred ove r th e NACA441 2aerofoil coal esced into each other , and observed that these tip vortices caused to reattach ment of the flow over the aerofoil tip. In the numerical results, three-dimensiona l beh avior of flow over the ae rofoil was observed and it was deduced that there was a good agre e ment betwee n obtai ned experi mental and numerical results.
Influence of Turbulence Modeling in Capturing Separated Flow over Delta Wing at Subsonic Speed
The visualization of three-dimensional coherent structures present in fluid flow over a delta wing configuration is a challenging task for current CFD research. These turbulent structures have to be resolved by using proper turbulence modeling with combination of computational speed and accuracy for high angles of attack aerodynamic design. With this aim, fully structured grids on a half span delta wing were generated with the grids refined near the wall to capture the physics of vortex flow in both the span- and stream-wise directions. The onset of flow separation and the amount of flow separation have been under-predicted by SA, Standard k-epsilon and RNG k-epsilon turbulence models. The primary vortex suction peak is situated conically on the wing and diminishes in magnitude as the trailing edge is approached. The cutting plane technique was utilized for the visualization of vortex breakdown phenomenon at high angles of attack. On the basis of comparison of computed surface pressure plots with that of experimental data and visualization of different contours, Shear Stress Transport model (SST) is found to be a good choice for prediction of separated flow over delta wing configurations. Moreover, surface flow visualizations have shown profound sensitivity of the vortex structures to the angle of attack. Keywords: turbulence modeling; vortical flow; primary and secondary flow separations; vortex breakdown
International Journal of Heat and Fluid Flow, 2006
The present study analyses the successive transition steps in the flow around a high-lift wing configuration, as the Reynolds number increases in the low and moderate range (800-10,000), by the Navier-Stokes approach. The flow system is mainly governed by two kinds of organised modes appearing successively as the Reynolds number increases: the von Kàrmàn and the shear layer mode. A period-doubling scenario characterises the first 2D stages of the von Kàrmàn mode up to Reynolds number 2000, where the shear-layer mode becomes predominant. The successive stages of the 3D transition are also analysed in detail. In a second step, the effect of wall suction has been studied both in 2D and 3D flows around the NACA0012 airfoil at 20°of incidence and a Reynolds number of 800. This study has the objective to optimise the aerodynamic coefficients and to attenuate the mentioned 3D transition effects in the near wake. The receptivity of the flow to the suction is clearly shown and the suction position on the wall has been optimised according to the improvement of the aerodynamics coefficients.