scholarly journals Transition Modeling Effects on Viscous/Inviscid Interaction Analysis of Low Reynolds Number Airfoil Flows Involving Laminar Separation Bubbles

1988 ◽  
Author(s):  
G. J. Walker ◽  
P. H. Subroto ◽  
M. F. Platzer
Keyword(s):  
Reynolds Number ◽  
Transition Zone ◽  
Interaction Analysis ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Positive Pressure ◽  
Laminar Separation ◽  
Transition Length ◽  
Low Reynolds ◽  
Transition Modeling

Estimating the low Reynolds number and off-design performance of axial turbomachine blades requires an accurate prediction of separation phenomena occurring on the blade surface. This paper discusses a viscous/inviscid interaction analysis of flow over a NACA 65-213 airfoil at a chord Reynolds number of 240,000 using a calculation method of Cebeci et al. The computed characteristics of a mid-chord laminar separation bubble are compared with experimental laser-doppler anemometer measurements of Hoheisel et al. Attention is focused on problems of modeling the laminar-turbulent transition zone within the viscous layer. A parametric study is undertaken to determine the location and extent of the transition zone which best models the observed separation bubble behavior. The required transition length is almost an order of magnitude smaller than that predicted from conventional transition length correlations. A physical model for this greatly reduced transition length in positive pressure gradient flows is proposed. The computational model correctly predicts most features of the separation bubble flow, but there are some significant discrepancies at reattachment which point to the need for improved turbulence modeling in this area. The inclusion of transverse pressure gradients associated with flow curvature in the viscous regions also appears very desirable for airfoils operating at Reynolds numbers around 105.

Experimental and Numerical Investigation of Transition Effects on a Low Reynolds Number Airfoil

2017 ◽  
Author(s):  
Michael J. Collison ◽  
Peter X. L. Harley ◽  
Domenico di Cugno
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Flow Visualisation ◽  
Transition Model ◽  
Laminar Separation ◽  
Oil Flow ◽  
Low Reynolds ◽  
Low Reynolds Number Airfoil

Low speed, small scale turbomachinery operates at low Reynolds number with transition phenomena occurring. In small consumer product applications, high efficiency and low noise are key performance metrics. Transition behaviour will partly determine the state of the boundary layer at the trailing edge; whether it is laminar, turbulent or separated impacts aerodynamic and acoustic performance. This study aimed to evaluate a commercially available CFD transition model on a low Reynolds number Eppler E387 airfoil and identify whether it was able to correctly model the boundary layer transition, and at what expense. CFD was carried out utilising the ANSYS Shear Stress Transport (SST) k-ω γ-Reθ transition model. The CFD progressed from 2D in Fluent v150, through to single cell thickness 3D (pseudo 2D) in CFX v172. An Eppler E387 low Reynolds number airfoil, for which experimental data was readily available from literature at Re = 200,000 was used as the validation case for the CFD, with results computed at numerous incidence angles and mesh densities. Additionally, experimental surface oil flow visualisation was undertaken in a wind tunnel using a scaled E387 airfoil for the zero incidence case at Re = 50,000. The flow visualisation exhibited the expected key features of transition in the breakdown of the boundary layer from laminar to turbulent, and was used as a validation case for the CFD transition model. The comparison between the results from the CFD transition model and the experimental data from literature suggested varying levels of agreement based on the mesh density and CFD solver in the starting location of the laminar separation bubble, with higher disparity for the position of the reattachment point. Whether 2D or 3D, the prediction accuracy was seen to worsen at high incidence angles. Finally, the location of the laminar separation bubble between CFD and oil flow visualisation had good agreement and a set of guidelines on the mesh parameters which can be applied to low Reynolds number turbomachinery simulations was determined.

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Paul Ziadé ◽  
Mark A. Feero ◽  
Philippe Lavoie ◽  
Pierre E. Sullivan
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Base Flow ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Layer Development

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

Comparative Investigation of Laminar Separation Bubble on a Wing at Low Reynolds Number

2020 ◽  
Vol 12 (3) ◽  
Author(s):  
M.P. Uthra ◽  
A. Daniel Antony
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Flow Characteristics ◽  
Suction Side ◽  
Aerodynamic Performance ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Air Vehicles

Most admirable and least known features of low Reynolds number flyers are their aerodynamics. Due to the advancements in low Reynolds number applications such as Micro Air vehicles (MAV), Unmanned Air Vehicles (UAV) and wind turbines, researchers’ concentrates on Low Reynolds number aerodynamics and its effect on aerodynamic performance. The Laminar Separation Bubble (LSB) plays a deteriorating role in affecting the aerodynamic performance of the wings. The parametric study has been performed to analyse the flow around cambered, uncambered wings with different chord and Reynolds number in order to understand the better flow characteristics, LSB and three dimensional flow structures. The computational results are compared with experimental results to show the exact location of LSB. The presence of LSB in all cases is evident and it also affects the aerodynamic characteristics of the wing. There is a strong formation of vortex in the suction side of the wing which impacts the LSB and transition. The vortex structures impact on the LSB is more and it also increases the strength of the LSB throughout the span wise direction.

Suction control of laminar separation bubble over an airfoil at low Reynolds number

2017 ◽  
Vol 233 (1) ◽  
pp. 81-90 ◽  
Author(s):  
Juanmian Lei ◽  
Qingyang Liu ◽  
Tao Li
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Aerodynamic Characteristics ◽  
Economic Effects ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Suction Control ◽  
Low Reynolds ◽  
Suction Hole

A laminar separation bubble appears generally on the NACA2415 airfoil at low Reynolds number. In this paper, suction control of the laminar separation bubble over the NACA2415 airfoil at low Reynolds number are simulated. The effects of suction control on the flow field and the aerodynamic characteristics of the airfoil are focused on at different angles of attack. Numerical simulations show that employing the γ-Reθt transition model coupling the k–ω shear stress transport turbulence model can predict the laminar separation bubbles accurately. The results indicate that suction control can delay the transition, decrease the velocity gradient of the boundary layer and inhibit the production of the separation bubble. The effect of the suction control becomes better with the suction location getting closer to the separation bubble and the suction speed (the suction gas speed of suction hole) getting faster. The figure of merit is introduced to evaluate energy consumption of the suction control. In consideration of the economic effects, the suction control is suitable for the larger angle of attack situation at low Reynolds number.

scholarly journals Laminar Separation Bubble and Flow Topology of NACA 0015 at Low Reynolds Number

CFD Letters ◽  
2021 ◽  
Vol 13 (10) ◽  
pp. 36-51
Author(s):  
Mohamed Ibren ◽  
Amelda Dianne Andan ◽  
Waqar Asrar ◽  
Erwin Sulaeman
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Flow Field ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Angle Of Attack ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Naca 0015

The development of sophisticated unmanned aerial vehicles and wind turbines for daily activities has triggered the interest of researchers. However, understanding the flow phenomena is a strenuous task due to the complexity of the flow field. The engaging topic calls for more research at low Reynolds numbers. The computational investigations on a two-dimensional (2D) airfoil are presented in this paper. Numerical simulation of unsteady, laminar-turbulent flow around NACA 0015 airfoil was performed by using shear-stress transport (SST) model at relatively low Reynolds number (8.4 × 104 to 1.7 × 105) and moderate angles of attack (0 ≤ α ≤ 6). In general, on the suction side, with increasing Reynolds number and angles of attack, separation, and reattachment point shifts upstream and concurrently shrinking the size of the laminar bubble. However, On the pressure side, the laminar bubble is seen to move toward the trailing edge at the relatively same size as the angle of attack increases. Moreover, the variations in the angle of attack have more influence on the laminar separation bubble characteristics as compared to the Reynolds number. The reattachment points were barely observed for the range of the angles of attack studied. At very high angles of attack, it is recommended to simulate the flow field using large eddy simulation or direct numerical simulation since the flow is considered three-dimensional and detached from the surface thus forming a complex phenomenon.

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