INTRODUCTION

The angle between the relative wind and the chord line is the angle of attack of the airfoil. The aerodynamic properties are strongly influenced by the shape of the wing or blade section. The cross-sectional shape obtained by the intersection of the wing with the perpendicular plane is called an airfoil.

Boundary layers can be either laminar (stratified) or turbulent (turbulent), depending on the value of the Reynolds number. Aerodynamic forces are experienced when an airfoil is placed in the uniform flow of an air stream created due to the relative motion. Another assumption of the theory as presented here is that the flow is two-dimensional.

Next to any surface, the molecules of the air stick to the surface, as discussed in the properties of air sliding. The change in the velocity field also changes the pressure field around the ball [5].

Figure 1.1: Variation of Lift and Drag Forces with Angle of Attack for a Typical Aircraft of Symmetric Aerofoil [1]

Flows past a Two-dimensional Aerofoil

Aerofoil Terminology

The forward and rearmost points of the mean camber line are the leading and trailing edges, respectively. The maximum distance between the mean camber line and the chord line is Maximum camber and Maximum thickness is the distance between the upper and lower surfaces of an airfoil. It is the angle between the wing chord line and the direction of the flight path.

And the aspect ratio of a wing is the span (b) divided by the geometric mean chord (c). There were many similarities between these airfoils, but mainly two primary variables influence the shapes of an airfoil, namely the slope of the average camber line of the airfoil and the thickness distribution above and below this line. The first number specifies the maximum camber in percent of the chord [Abbott and Albert 1945 and 1959].

The second number indicates the position of maximum pitch in tenths of a chord [Abbott and Albert 1945 and 1959] and. The last two digits provide the maximum wing thickness as a percentage of the chord [Abbott and Albert 1945 and 1959].

Figure 2.10: The Drag coefficient vs. Angle of Attack graph of NACA 0015 symmetrical aerofoil [7]

CHAPTER-3 EXPERIMENTAL SETUP

Wind Tunnel AF 100

• Major Parts
• Effuser (Contraction Cone) - The Effuser accelerates the airflow linearly. A Large volume of low velocity air is converted into to a small
• Working Section
• Three Component Balance (AF A3)
• Balance Angle Feedback Unit
• Technical Data
• VERSATILE DATA ACQUISITION SYSTEM

A wind tunnel is a test device used in aerodynamic research to study the effects of air moving past solid objects. The air enters the tunnel through an aerodynamically designed spout that accelerates the air linearly to the work piece, passes through a grille, a diffuser and then to a variable speed axial fan. The test object is equipped with a sensitive balance to measure the forces generated by the air flow.

Wind tunnel AF 100 is an open-circuit subsonic wind tunnel for a wide range of investigations into aerodynamics with a working section of 300 mm by 300 mm and 600 mm long. The powers of three-component balance, electronic sensors on the optional wind tunnel instrumentation are connected to Versatile Data Acquisition System (VDAS). The working section of the wind tunnel is a square section with a clear roof, sides and floor.

A static pitot tube and a crossing pitot tube fit the working section upstream and downstream of all models. The working section has three-component balance (AF A3), which measures the angular position of models mounted on the balance in TecQuipment's Subsonic Wind Tunnel (AF100). To enable accurate data capture, monitoring and real-time display on a computer, a fully compatible TecQuipments Versatile Data Acquisition System (VDAS) has been used.

It also provides full adjustment of the angle of the model in the direction of the air flow. The balance angle feedback unit is an optional accessory for use with the three-component balance (AFA3) to measure the angular position of balance-mounted models in the subsonic wind tunnel (AF100). The angular feedback unit is mounted on the three-component balance attached to the wind tunnel.

The diffuser slows down the speed and ensures the uniformity of the air flow in the wind tunnel. The air then flows out to a variable speed axial fan and through a muffler unit to the atmosphere. VDAS is a highly accurate and efficient tool with hardware, software and accessories for digital automatic data acquisition.

Figure 3.2 : Effuse or Contraction Cone ( Side and Front View ).

Fabrication of the Profiles

The profiles have been retained in the working section of the wind tunnel AF100 with a hollow cylindrical steel rod. The cylindrical hollow rod has been attached to the profiles by drilling a hole with the drill and inserting the rod into it. A polysynthetic resin adhesive was placed in the hole so that the rod could not move or come out.

Figure 3.8(b): Sphere shaped profile.

CHAPTER-4 EXPERIMENTAL RESULTS

• Collection of Experimental Data
• Aerodynamic Characteristics of NACA 0015
• Aerodynamic Characteristics of NACA 4415
• Aerodynamic Characteristics of the Cylinder
• Aerodynamic Characteristics of the Sphere

For investigation of the aerodynamic characteristics of four airfoils, such as cylinder, sphere, symmetric airfoil NACA 0015 and swept airfoil (NACA 4415) objects. The experimental data were obtained at different Reynolds number and angles of attack. The two airfoils (symmetrical NACA 0015 and curved NACA 4415) were tested from -3˚ to 21˚ angles of attack by 3˚. The variation of drag coefficient with Reynolds number for symmetrical airfoil NACA 0015 is shown in the following Figure 4.1 to Figure 4.9.

The drag coefficient simultaneously decreases and curves downward with increasing air Reynolds number for each angle of attack. But as the Reynolds number increases, the lift coefficient increases and the drag coefficient decreases. The variation of drag coefficient with Reynolds number for a curved airfoil (NACA 4415) is shown in Figures 4.12 to 4.20.

The drag coefficient decreases at the same time slightly and gives descending lines as the Reynolds number of the air increases for each angle of attack. The Reynolds number curves shown in Figure 4.23 show a significant increase in CD with an increase in Reynolds number for the cylinder. Theoretically at the subsonic airspeed level the curve is a straight line in the Reynolds number range.

But experimentally increasing the Reynolds number means that the drag coefficient gradually increases at low Reynolds number and becomes a straight line at high Reynolds number [4]. Theoretically, in the subsonic level of airspeed, the curve goes down in the Reynolds number range. Reynolds number for the sphere, the curves presented in Figure 4.24 show a significant decrease in CD with an increase in Reynolds number for all geometries, although the value of the drag coefficient is much higher than the ideal values.

Figure 4.1: Drag Coefficient vs. Reynolds Number at -3˚ angle of attack for NACA 0015

CHAPTER-5 DISCUSSIONS

Comparison of Coefficient of Drag Force of Theoretical Result

The variation of coefficient of drag with Reynolds number for different angles of attack of NACA 0015 airfoil is shown in Figure 5.1. It is seen that the drag coefficient tends to decrease with the increase of Reynolds number. It is also found that with increasing angle of attack the values ​​of drag coefficient increase respectively.

Similarly, the variation of the drag coefficient with Reynolds number for different angles of attack of the NACA 0015 airfoil is shown in Figure 5.2. Here the drag coefficient will decrease linearly with the increase in Reynolds number and at higher angle of attack. For the cylinder, figure 5.3 represents the evolution of the drag coefficient according to the Reynolds number at different angular positions.

Theoretically, in subsonic level of airspeed, the ideal curve is a straight line in this range of Reynolds number. However, practically at low to high Reynolds number, the Drag coefficient increases gradually and forms an approximate constant line up to a certain range of Reynolds number in the following figure [4]. Again, theoretically, the sphere at subsonic level of airspeed shows a downward parabolic curve in the range of Reynolds number which has been shown in Figure 2.6.

But the drag coefficient values ​​are much higher than the ideal values ​​for the geometry. From the investigation, it was found that the rounded airfoil NACA 4415 has a lower drag coefficient than the symmetrical airfoil NACA 0015. Thus, first NACA 4415 and then NACA 0015 have a better aerodynamic shape than a cylinder or sphere.

So Drag forces are easy to predict and control for aerofoil than cylinder or sphere. From the above comparisons it is clear that the cam airfoil is clearly the most efficient aerodynamic shape and the sphere is the least desirable aerodynamic shape which greatly disturbs the airflow.

Figure 5.2: Drag Coefficient vs. Reynolds Number at different angle of attack for NACA 4415

CHAPTER-6 CONCLUSIONS AND

Conclusion

The main objective of this thesis is to study the drag forces of profiles with different shapes at different Reynolds numbers. The obtained results and the graph model seem to agree quite well with the experimental and theoretical one, especially in the curve of drag coefficient vs. Reynolds number.

Symmetric airfoil (NACA 0015) and rounded airfoil (NACA 4415) give low drag coefficient than cylinder and sphere, so airfoils are desirable aerodynamic shapes where high lift forces are required, such as airplanes, turbine blades, etc. The fluid flow essentially follows the contours of the airfoils, so we say it is streamlined, and a body that is not streamlined is a bluff. The flow of fluid over a pale body follows the contours of that body only partially or not at all, so the shape of the cylinder and sphere are bluffs.

The traction force over a cylinder or sphere is required for the design of the vehicles, building forms and also the airframe. The importance of the experimental study is to clarify the physical phenomena that the theory describes. And it has been shown that the practical physical phenomena and theories vary widely.

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