Project

Flow over an airfoil

SShashikanth B G29 Sept 2019

OBJECTIVE

To illustrate the variation of lift and drag force with the angle of attack of an airfoil.

INTRODUCTION

Wings, horizontal tail surfaces, vertical tails surfaces, and propellers are all examples of airfoils. An airfoil is a device which gets a useful reaction from air moving over its surface. The component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called drag. This force on an airfoil is vital in many applications and is primarily the result of its angle of attack and shape. Therefore, the study of the variation of drag and lift force with the angle of attack is important in the design of many engineering systems.

THEORY

A stationary fluid exerts only normal pressure forces on the surface of a body immersed in it. A moving fluid exerts tangential shear forces on the surface because of the no-slip condition caused by viscous effects. Both of these forces, in general, have components in the direction of flow, and thus the drag force is due to the combined effects of pressure and wall shear forces in the flow direction. The components of the pressure and wall shear forces in the direction normal to the flow tend to move the body in that direction, and their sum is called lift. For two-dimensional flows, the resultant of the pressure and shear forces can be split into two components: one in the direction of flow, which is the drag force, and another in the direction normal to flow, which is the lift, as shown in the figure below. For three-dimensional flows, there is also a side force component in the direction normal to the page that tends to move the body in that direction.

Airfoils are highly-efficient lifting shapes, able to generate more lift than similarly sized flat plates of the same area, and able to generate lift with significantly less drag.

The magnitude of the lift generated by an object depends on the shape of the object and how it moves through the air. For thin airfoils, the lift is directly proportional to the angle of attack, which is the angle between the chord line of an airfoil and the vector representing the relative motion between the body and the fluid through which it is moving. As an object moves through the air, air molecules stick to the surface. This creates a layer of air near the surface called a boundary layer that, in effect, changes the shape of the object. The flow turning reacts to the edge of the boundary layer just as it would to the physical surface of the object. At high angles, the boundary layer may lift off or \"separate\" from the body and create an effective shape much different from the physical shape. This condition is called a stalling.

METHODOLOGY

Generation of Airfoil Model:

We import the data file containing airfoil cross-section x, y, z co-ordinates using \'curve through XYZ points\'. The thickness is considered as 0.15m.

Setup:

Since we are interested in two-dimensional analysis, a two-dimensional computational domain in XY plane whose length from trailing edge and the leading edge is 11m and 5 m respectively.

Thermodynamic Parameters:

To calculate the value of properties at different time interval, we enable the time-dependent option. We analyse the flow for 5 seconds with 1 second time step.

We use air the fluid to study the flow simulation.

Then we specify the initial condition, velocity in x-direction=600 m/s.

Mesh:

We use the default global mesh setting with a refined mesh(level=3) locally near the solid-liquid interface.

Goals:

Since the calculation domain is large, the solution takes time. Thus to get the point of convergence we define global and surface goals. We stop the solution when the force on the surface in the x and y-direction reach a stable value.

RESULTS

Case i: Angle of Attack= 0 degree

Figure showing solver.

Lift and drag force convergence.