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  • International Journal of Emerging Researches in Engineering Science and Technology-

    Vol-2-Issue-4-April-2015-ISSN: 2393-9184

    27

    AbstractThis work involves usage of Reynolds Averaged

    Navier Stokes Equation (RANSE) based Computational Fluid

    Dynamics (CFD) approach for the determination of stall angle

    for Wortmann FX 79-W-151 aerofoil. Geometrical modelling of

    this aerofoil has been done using ANSYS ICEM CFD and

    simulations have been carried out using ANSYS CFX. Pitch

    motion which is a rotation about y-axis and occurring in x-z

    plane has been depicted here as yaw motion i.e. rotation about z

    axis in x-y plane so that the flow around the aerofoil section can

    be studied during rotation.Simulation of flow has been

    accomplished for various angles of attack for determining both

    static and dynamic stall. The limitation of RANSE solver with the

    angle of attack and the subsequent flow separation has also been

    studied.

    Index TermsAerodynamics,CFD,Dynamic stall,Static stall.

    I. INTRODUCTION

    OMPUTATIONAL FLUID DYNAMICS (CFD) involves

    solving of fluid flow problems by numerically solving the

    Navier-Stokes equations. Navier-stokes equations are of

    prime importance as they help solve majority of fluid flow

    problems as its simplification leads us to Eulers equation, Potential equations and Linearized potential equations. CFD

    helps in easier identification of flaws in the design and

    accurately predicts the fluid and thermal flows, thereby saving

    considerable time and cost in the production process. Thus

    CFD finds its application in various fields such as aerospace,

    marine, medical, automotive and chemical industries.

    In the present work, aerodynamic study of a flow around an

    aerofoil has been studied. Stall is an unfavorable condition

    that exists when the lift on the aerofoil decreases on increase

    in the angle of attack. This situation is highly undesirable as

    the aircraft will start spinning downwards losing its altitude.

    On reviewing various literatures, it can be understood that a

    quite a number of wind tunnel experiments have been done in

    estimating static and dynamic stall. Numerical studies using

    panel methods and Reynolds Averaged Navier Stokes

    Equation (RANSE) based CFD have been limited only to the

    Manuscript submitted for review on 20 January 2015.This work was

    supported by the Department of Mechanical engineering, SSET, Ernakulam,

    India.

    Praphul T, B.Tech, Mechanical engineering, SSET, Ernakulam, India. (e-

    mail: praphulmenon@gmail.com).

    Dr. Sheeja Janardhanan,Associate Professor, Department of Mechanical

    Engineering, SSET, Ernakulam, India. (e-

    mail:sheejajanardhanan@gmail.com).

    estimation of static stall. Thus the idea of this work is mooted

    based on [2]. This work deals with the measurements with

    sinusoidal pitching and constant angular velocities and

    dynamic stall characteristics. Under dynamic stall conditions,

    maximum value of lift coefficient was up to 80% higher than

    that for static lift. In the present work, pitch (rotation about y-

    axis) motions in x-z plane have been transformed to the x-y

    plane and consequent yaw (rotation about z-axis) motions

    have been targeted to find out stall angles in static and

    dynamic motions of the aerofoil.

    II. PROBLEM FORMULATION

    A. Geometrical modelling

    Geometrical modelling of the aerofoil has been carried out

    using ICEM CFD. Data points were imported to ICEM CFD

    using a .dat file obtained from UIUC airfoil coordinates

    database. The aerofoil was then modelled to the specifications

    as obtained from [2]. Fig. 1shows the aerofoil geometry.

    To simulate the wind tunnel experiment [2], a wind tunnel of

    dimensions 9m x 2.5mx 2m has been created around the

    aerofoil using ICEM CFD.

    Numerical Study on Aerofoil Stall

    Praphul Tand Sheeja Janardhanan

    C

    Fig. 1Aerofoil geometry

    Fig. 2.Wireframe model of the wind tunnel

  • International Journal of Emerging Researches in Engineering Science and Technology-

    Vol-2-Issue-4-April-2015-ISSN: 2393-9184

    28

    The inner domain and outer domain has been created using

    ICEM CFD to perform both the static and dynamic stall

    simulations on the aerofoil are as follows

    B. Meshing

    Structured H mesh has been chosen for the analysis. Grid

    independency study has been carried out using ANSYS CFX.

    Mesh size with density of 1.5 million cellshas been thus

    chosen as the adequate mesh size.For performing the analysis

    of static and dynamic stall, the same mesh density as obtained

    through grid independency was retained. Meshing strategy

    involved for carrying out the static and dynamic stall

    simulations has been based on [6]where an interface mesh

    with circular cylindrical inner domain encompassing a ship

    and cuboidal fluid domain were developed and used for

    hydrodynamic analysis. This type of mesh reduces the

    complexity of rotating the aerofoil and remeshing each time

    for an orientation, facilitating the rotation of cells in the inner

    domain alone, thus saving considerable time and effort.

    C. Boundary conditions

    Simulations carried out for wind tunnel experiment and stall

    (both static and dynamic) involved the following boundary

    conditions.

    The boundary conditions are as follows:

    Aerofoil wall no slip wall

    Bottom wall no slip wall

    Inlet inlet

    Outlet outlet

    Side 1 wall no slip wall

    Side 2 wall no slip wall

    Top Opening

    Turbulent intensity was kept below 1%, air flow speed was

    taken to be 50 m/s as in [2].

    The turbulence model has been taken ask- SST as found

    from [1].

    Fig. 3. Wireframe model of the inner domain.

    Fig. 4. Wireframe model of the outer domain.

    Fig. 5. Mesh model selected after performing the grid independency

    study.

    Fig. 6. Mesh model of the inner domain.

    Fig. 7. Mesh model of the outer domain.

    Fig. 8. Boundary condition as applied to the aerofoil

  • International Journal of Emerging Researches in Engineering Science and Technology-

    Vol-2-Issue-4-April-2015-ISSN: 2393-9184

    29

    D. CFD analysis

    1) Static stall

    Static stall is the stall condition observed for the flow over the

    aerofoil at a fixed angle of attack. For static stall estimations,

    the inner domain was rotated about the z axis in counter

    clockwise as well as clockwise directions for various angles of attack. The various angles of attack under consideration here

    are from 0 degree to 15 degree in steps of 2.5 degree

    increments for each angle.

    2) Dynamic stall

    Dynamic stall occurs when there is sudden change in the angle

    of attack by the aerofoil. A sinusoidal rotation about z-axis

    was given to the inner domain in CFX pre in order to obtain

    yaw movement. The sinusoidal function is as follows:

    ta sin (1)

    = Amplitude of oscillation.

    a=Maximum amplitude of oscillation = 0.2618 rad

    = Angular frequency of oscillation = 0.5 rad/sec

    t = instantaneous time in s

    .txt, .xlsx files were written to give inputs to the CFX solver

    for interpolating the values. Animation on the dynamic stall

    analysis was also performed.

    III. RESULTS

    A. CFD Validation

    Validation of CFD was done by comparing the values of

    coefficient of lift obtained for the aerofoil at zero degree of attack between the CFX simulation and the experimental value

    obtained from [2]as shown

    The values thus are close to each other thus validating the mesh and CFD analysis.

    B. Static stall

    1) Anticlockwise direction

    It is observed that the stall occurs between 12.5 and 15

    degrees. This is because of the flow separation that occurs

    between 12.5 and 15 degrees that causes the lift force to drop

    causing stall.

    Fig. 9. Boundary condition as applied to the wind tunnel

    Fig. 10. Graph plot comparing values of coefficient of lift vs. angle of

    attack for static stall simulations performed in anticlockwise direction.

    0, 0.216

    2.5, 0.414

    5, 0.571

    7.5, 0.728

    10, 0.92

    12.5, 0.95

    13.75, 0.896

    15, 0.937

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    1

    0 2.5 5 7.5 10 12.5 13.75 15

    Co

    effi

    cien

    t o

    f li

    ft (

    Cl)

    Angle of attack (degrees)

    Static Stall in Yaw (Anticlockwise Direction)

    TABLE I

    COMPARISON OF COEFFICIENT OF LIFT

    Experiment (Mohlmann, 2007)

    Present CFD

    lC 0.2 0.2144

    Fig. 11. Velocity and pressure plots on the aerofoil at 0 degree angle of

    attack.

  • International Journal of Emerging Researches in Engineering Science and Technology-

    Vol-2-Issue-4-April-2015-ISSN: 2393-9184

    30

    Fig. 13. Velocity and pressure plots on the aerofoil at 5 degree angle of

    attack in the anti-clockwise direction.

    Fig. 14. Velocity and pressure plots on the aerofoil at 7.5 degree angle

    of attack in the anti-clockwise direction.

    Fig. 12. Velocity and pressure plots on the aerofoil at 2.5 degree angle

    of attack in the anticlockwise direction