Numerical simulation of flow over EPPLER 387 at low Reynolds number and comparison with experiment (original) (raw)
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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.
Accurate laminar-turbulent prediction is very much important to understand the complete performance characteristics of any airfoil which operates at low and medium Reynolds number. In this article, a numerical study has been performed over two different thick airfoils operating at low Reynolds number using k-ω SST, k-kl-ω and Spalart-Allmaras (SA) RANS models. The unsteady two dimensional (2D) simulations are performed over NACA 0021 and NACA 65-021 at Re 120,000 for a range of angle of attacks. The performances of these models are assessed through aerodynamic lift, drag and pressure coefficients. To obtain better comparison, the simulated results are compared with the experimental measurements and XFOIL results as well. In this present study, it is found that the k-kl-ω transition model is capable of predicting correct lift, drag coefficient and separation bubble as reported in experiments. At high angles of attack, this model fails to predict performance variables accurately. The SA and SST models are fail to predict laminar separation bubble. However, At high angle of attack, SA model shows better predictions compared to k-kl-ω and k-ω SST models.
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.
Numerical and Experimental Flow Analysis of Moving Airfoils with Laminar Separation Bubbles
AIAA Journal, 2007
Experimental measurements and unsteady RANS simulations of the low-Reynoldsnumber flow past an SD7003 airfoil with and without plunge motion at Re = 60k are presented, where transition takes place across laminar separation bubbles. The experimental data consist of high-resolution phase-locked PIV measurements in a wind tunnel and a water tunnel. The numerical simulation approach includes transition prediction which is based on linear stability analysis applied to unsteady mean flow data. The numerical results obtained for steady onflow are validated against PIV-data and published force measurements. Good agreement is obtained for specific turbulence models. Flows with plunge motion reveal strong effects of flow unsteadiness on transition and the resulting laminar separation bubbles which are well captured in the simulations.
Validation of the RANS-simulation of laminar separation bubbles on airfoils
Aerospace Science and Technology, 2006
This paper presents RANS simulations of the low-Reynolds-number flow past an SD7003 airfoil at Re = 6 × 10 4 , where transition takes place across a laminar separation bubble. The transition prediction procedure using an approximate envelope method as well as a linear stability solver is discussed. The numerical results are validated against PIV-and force measurements obtained in several wind-and watertunnels and are also compared to XFOIL results. Good agreement is found within the operational range of the airfoil.
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...
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.
A study of long separation bubble on thick airfoils and its consequent effects
International Journal of Heat and Fluid Flow, 2015
A parametric study has been performed to analyse the flow around the thick-symmetric NACA 0021 airfoil in order to better understand the characteristics and effects of long separation bubbles (LoSBs) that exist on such airfoils at low Reynolds numbers and turbulence intensities. In the article, the prediction capabilities of two recently-developed transition models, the correlation-based c-Re h model and the laminar-kinetic-energy-based j-j L -x model are assessed. Two-dimensional steady-state simulations indicated that the j-j L -x model predicted the separation and reattachment process accurately when compared with published experimental work. The model was then used to study the attributes and the effects of LoSBs as a function of the angle of attack, freestream turbulence intensity and Reynolds number. It was observed that LoSBs considerably degrade the aerodynamic performance of airfoils and lead to abrupt stall behaviour. It is, furthermore, illustrated that the presence of the LoSB leads to an induced camber effect on the airfoil that increases as the airfoil angle of attack increases due to the upstream migration of the bubble. An increase in the Reynolds number or turbulence levels leads to a reduction in the bubble extent, considerably improving the airfoil performance and leading to a progressive trailing-edge stall.
Aerodynamic Analysis of Low Reynolds Number Flows
2009
An aerodynamic analysis of low-Reynolds number flows is presented. The focus is placed on the laminar separation bubbles, a peculiar phenomenon of these kind of flows. The only simulations techniques feasible to be applied to complex configurations appear to be the methods based on the Reynolds Averaged Navier Stokes equations. A critical point is the turbulence modelling. In fact, the turbulence models are calibrated for flows at high Reynolds number with separation in the turbulent regime. The flow over a flat plate with an imposed pressure gradient, and around the Selig-Donavan 7003 airfoil is considered. Large eddy simulations have also been performed and used as reference for the RANS results. Laminar separation bubbles have been found by the Spalart-Allmaras and the k-w SST turbulence models. The models have been used without prescribing the transition location and assuming low values of the free-stream turbulence. The main results have been achieved for the k-w SST turbulence...