Flow over an airfoil
AIM:
Model the flow over a NACA0017 Aerofoil, For the angle of attacks 0,2,4,6,8,10 compare the lift and drag forces
INTRODUCTION:
Airflow over the aerofoil is the prime factor in aerodynamics. Flow pattern has tremendous effect on aerodynamic reaction on aerofoil.
There are patterns of airflow, airflow terminology, effect of patterns of flow and importance of streamlining objects.
TERMINOLOGY
Streamlines:
Lines which show the direction of the fluid at any particular moment are called streamlines. A stream of moving air (or any fluid) consists of many particles. If particles moves in the same direction, then their path may be traced by smokes emitted in the airflow or colors in the fluid stream (e.g. in moving water). The streaks of colors (called streaklines) or smokes show the stream lines. The path taken by any one particle in such stream is a streamline.
Streamline flow:
Streamline flow, is the flow in which the fluid particles move in an orderly manner and retain the same relative positions in successive cross-sections of the flow tube. In a streamline flow, all flow particles follow streamlines and produce stream tube.
Stream tube:
A stream tube or tube of flow is considered to be an imaginary boundary defined by streamlines drawn so as to enclose a tubular region of fluid. In other words, a collection of streamlines forming a closed curve or tube, as in given Figure, it is referred to as a stream tube. Since the walls of the stream tube are streamlines, there can be no flow through the wall or no fluid crosses the boundary of such a tube. That means, there is no cross flow of particles in a streamline flow.
Laminar flow: Stream line flow is some times called laminar flow. In other words, a flow which maintains the shape of the body over which it is flowing (Figure) is the laminar flow.
Turbulent flow: This is a flow in which the fluid particles may move perpendicular as well as parallel to the surface of the body and undergo eddying or unsteady motions. This may result in considerable thickening of the airflow and lead to break-up.
Incompressible flow: Incompressible flow is a flow in which the density does not change from point-to-point.
The compressibility effect starts to play when speed of air equals the speed of sound through it. The range of speed below the level of sound speed is called the subsonic speed range. The speed equal to the speed of sound is called the sonic speed and the range above it is called supersonic speed range.
Subsonic and supersonic flow: Flow of air with the speed below sonic speed is called subsonic flow and that with the speed in the supersonic range is called the supersonic flow.
Airflow at subsonic range is treated as incompressible flow while at supersonic range it is treated as compressible flow.
EFFECT OF FLOWPATTERN OVER OBJECTS
Flow pattern over an object especially the aerodynamically designed objects (e.g. wings of an aeroplane) has very important effect. Aerodynamic reaction for producing desired lift forces with minimum unwanted drag forces is the prime objective of aerodynamically shaping the wings and other aerodynamic surfaces. Properly shaped objects that ensures flow over it is laminar/streamlined is called streamlined object and the stream line flow over such objects (e.g. wings) has the contribution towards producing lift, while the cross flow creating turbulence destroys lift and produces more drag which is an evil for most part of the flight.
If air could have been seen as shown Figure, these air patterns of wakes and turbulences created by edges and corners of flying objects in the air. These patterns are creating huge obstructions in the form of drag forces to be imposed as loads for engine thrusting the aircraft forward with so much penalty of extra fuel burn.
Besides, these drag produced on the aircraft because of its drag by the wakes and turbulences, stalling of wings may happen due to turbulence over wings resulting in the loss of lift forces.
BOUNDARY LAYER AND ITS CONTROL:
Control of boundary layer is very important for a flight.
The layer of air over a surface that is slower moving in relation to the rest of the fast moving slipstream is called the boundary layer.
Friction between the surface and the air particles does not permit the flow over the surface to attain the local free stream velocity immediately. The particle just over the surface has about the zero velocity, the next upward layer has little more than zero velocity and the next upward one has further more velocity. This variation in velocity continues until attaining the full velocity of the free air stream. Thus, there is a layer, adjacent to the surface, in the air stream where air velocity varies from ZERO to free stream velocity. This layer is the boundary layer.
Within the boundary layer, there exists relative velocity exist i.e. one upper layer slides over the adjacent lower layer; thus all the fluid friction (due to air viscosity) comes into play in the boundary layer. More the viscosity, more is the friction. More the boundary layer thickness more is the amount of frictional force that the body is to overcome to move forward.
The layers of air near the surface retard the layers farther away – owing to the friction between them, i.e. the viscosity – and so there is a gradual increase in velocity as the distance from the surface increases. The maximum velocity – that of the free airflow – is reached sooner over a smooth surface than over a rough one, and since it is the shearing action between the layers which causes the resistance it follows that a smooth surface will have less resistance than a rough surface. Over a smooth surface like that of a wing most of the increase in velocity may occur within a millimetre or so of the surface as compared with the few metres which we mentioned earlier in connection with an aerodrome surface.
- Laminar Boundary Layer:The boundary layer can be termed as laminar when there is no cross flow of air particles between the layers.
- Turbulent Boundary Layer:The boundary layer can be termed as turbulent when there is cross flow of air particles between the layers. Due to lose of energy, flow first becomes wavy in transition and then grows into turbulence.
The usual tendency is for the boundary layer to start by being laminar near the leading edge of a body, but there comes a point, called the transition point, when the layer tends to break away from the surface and become turbulent and thicker (Figure). As the speed increases the transition point tends to move further forward, so more of the boundary layer is turbulent and the skin friction greater.
Formation and development of boundary layer: When a body such as an aircraft wing is immersed in a fluid, which is flowing past it, the fluid molecules in contact with the wings surface tend to be brought to rest by friction and stick to it. The next molecular layer of the fluid tends to bind to the first layer by molecular attraction but tends to shear slightly creating movement with respect to the first stationary layer. This process continues as successive layers shear slightly relative to the layer underneath them. This produces a gradual increase in velocity of each successive layer of the fluid (say air) until the free stream relative velocity is reached some distance away from the body immersed in the fluid.
For an aircraft, subject to laminar flow over its wing section, the thickness of the boundary layer is seldom more than 1 mm. The thinner the boundary layer, the less the drag and the greater the efficiency of the lift producing surface. Since friction reduces the energy of the air flowing over an aircraft wing, it is important to keep wing surfaces and other lift producing devices as clean and as smooth as possible. This will ensure that energy losses in the air close to the boundary are minimized and efficient laminar flow is maintained for as long as possible.
Mach number: The Mach Number (M) refers to the speed at which an aircraft is traveling in relation to the speed of sound. Thus, a Mach Number of 0.5 means that the aircraft is traveling at half the speed of sound. Both the speed of the aircraft and the speed of sound are true speeds.
Realms of Flight:
The aerodynamic characteristics can be divided into three basic realms, or regimes, all based on Mach numbers.
(i) Subsonic flight: Flight below Mach 0.75 is called subsonic flight, and in this speed range all of the airflow is subsonic.
(ii) Transonic flight: Perhaps the most difficult realm of flight is that between Mach 0.75 and 1.20, because at this speed some of the airflow is subsonic, while other flow is supersonic. At these speeds, shock waves form and move around. The aerodynamic centre of the aerofoil shifts from 25% of the chord at subsonic speeds to 50% in supersonic flight, and this shift causes large changes in aerodynamic trim and stability.
(iii) Supersonic flight: Flight above Mach 1.20 is smooth and efficient, since all of the airflow is supersonic and the shock and expansion waves are attached and are stationary.
Flight at extremely high speeds-that is, flight above Mach 5.00-is called hypersonic flight, and thermal limitations are encountered as well as aerodynamic problems.
SHOCK WAVE FORMATION AND DEVELOPMENT
All the points on the surface of a wing produce pressure waves. If the airflow is subsonic, these pressure waves can all move away- ahead of the wing, but when the critical Mach Number is reached and the speed becomes sonic speed at some point on the wing, pressure waves move forward only as far as this point and then “pick-up” to form a Shock Wave.
If the speed is increased further the shock wave starts to move towards the trailing edge as the supersonic area above the wing grows larger. Another shock wave will form below the wing and will also move towards the trailing edge.
As the speed of the wing through the air is increased the wing shook waves move back towards the trailing edge and when the wing reaches sonic speed a Bow Wave forms ahead of the leading edge.
BODY OF THE CONTENT:
As we have to perform flow over naca aerofoil0017, the coordinates of the specified aerofoil is being added into the curve through xyz points through which the profile is being generated, then after closing the curve, extruding it to a certain amount to perform flow simulation over it.
In flow simulation adding wizard creating the new project, parameters are external flow, time depending and initial Velocity in x-direction as 600m/s.
Now editing the computational bar to 2-D simulation as increasing the area of computational field generally 10-15 times at the exit and 5 times at the entry w.r.t the length of the aerofoil.
Now in setting up the mesh adding the local mesh at the surface of the aerofoil for getting better results and enabling the advanced channel refinement, the mesh would look something like shown below
Now adding some goals as we required the force along x-direction which will basically gives us the drag force and the force along the y-direction which will give us the lift of the aerofoil.
For creating the study for different angle attack just create the clone of generated study and elevate the curve to the given angles and perform the same flow simulation to get the results.
OBSERVATIONS:
For 0o angle of attack the following results are obtained.
Velocity cut-plot:
Drag-force:
Lift:
The lift is varying drastically up and down as the speed is supersonic and due to shape of the aerofoil too.
For 2o angle of attack the following results are obtained.
As we are performing a supersonic flow so showing the mach number plot will make much more sense as it will relate to the speed of sound as well the which type od flow is happening around the surface of aerofoil.
Mach number cut-plot:
As from the above plot it can be seen that at the edge point of the aerofoil only the mach number is less than 1 which is a sub-sonic speed else the mach is above the value of 1.
Drag-force:
Lift:
For 4o angle of attack the following results are obtained.
Mach number cut-plot:
Drag-force:
Lift:
For 6o angle of attack the following results are obtained.
Mach number cut-plot:
Drag-force:
Lift:
For 8o angle of attack the following results are obtained.
Mach number cut-plot:
Drag-force:
Lift:
For 10o angle of attack the following results are obtained.
Mach number cut-plot:
Drag-force:
Lift:
CONCLUSION:
The Angle of Attack is the angle at which relative wind meets an Aerofoil. It is the angle formed by the Chord of the aerofoil and the direction of the relative wind or the vector representing the relative motion between the aircraft and the atmosphere.
From the above graphs obtained it is clear that the drag force increases with the increase in the AOA (Angle of Attack).
The magnitude of the drag generated by an object depends on the shape of the object and how it moves through the air. For thin airfoils, the drag is nearly constant at small angles (+/- 5 degrees). As the angle increases above 5 degrees, the drag quickly rises because of increased frontal area and increased boundary layer thickness. 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 which, in effect, changes the shape of the object. The flow reacts to the edge of the boundary layer just as it would to the physical surface of the object. To make things more confusing, the boundary layer may lift off or "separate" from the body and create an effective shape much different from the physical shape.
In case of lift except the 1st one as the angle of attack was zero the lift was fluctuating in between +ve and -ve the other graph shows a relation which confines that the lift also increases with the increase in AOA.
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 for small angles (within +/- 10 degrees). For higher angles, however, the dependence is quite complex. 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.