Introduction to Wind Tunnel

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  • Topic: Aerodynamics, Airfoil, Drag equation
  • Pages : 10 (3533 words )
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  • Published : November 14, 2012
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ABSTRACT
The basic concept and operation of subsonic wind tunnel was demonstrated in this experiment by conducting airfoil drag analysis on a NACA 0015 airfoil. The small subsonic wind tunnel along with apparatus such as, the manometer rake, the inclined manometer and the pitot - static tube were used with different baffle settings to record varying pressure readings. To achieve this objective, some assumptions were made for the lower range of subsonic flow to simplify the overall analysis. From the obtained aerodynamic measurements using a pitot-static tube mounted ahead of the airfoil at the test section, the actual velocity was determined and by relating it to the theoretical velocity, the velocity coefficient was calculated. The velocity coefficient varies for each baffle setting by a factor of decimals, thus the velocity coefficient can be used as a correction factor. Further, the coefficients of drag were calculated for the following angles of attack, 10o, 15o, and 20o and were compared with the published values. INTRODUCTION

The wind tunnel is an absolute necessity to the development of modern aircrafts, as today, no manufacturer delivers the final product, which in this case can be civilian aircrafts, military aircrafts, missiles, spacecraft, and automobiles without measuring its lift and drag properties and its stability and controllability in a wind tunnel. Benjamin Robins (1707-1751), an English mathematician, who first employed a whirling arm to his machine, which had 4 feet long arms and it, spun by falling weight acting on a pulley however, the arm tip reached velocities of only few feet per second. [4]

Figure 1: Forces exerted on the airfoil by the flow of air and opposing reaction on the control volume, by Newton’s third law. [1] This experiment will determine drag forces experienced by a NACA 0015 airfoil, subjected to a constant inlet velocity at various baffle settings with varying angles of attack. DATA ANALYSIS, THEORATICAL BACKGROUND AND PROCEDURE

Apparatus in this experiment as shown in the figure 2, consisted of a small subsonic wind tunnel. The wind tunnel had an inlet cross-section of 2304 in2 and an outlet crosses section of 324 in2. A large compressor forced air (from room) into the inlet through the outlet tunnel and back into the room. This creates a steady flow of air and a relative high velocity can be achieved in the test section. Instrumentation on the wind tunnel consisted of an inclined manometer and a pitot-static tube in the test section also a manometer rake behind the tested objet (airfoil NACA 0015). The manometer rake consisted of 36 inclined manometers; number 36 is used as a reference for the static pressure. All other manometer measures the pressure behind the object in the airflow.

Figure 2: Wind tunnel set up with instrumentation [5]
Before the experiment was performed the laboratory conditions were recorded, the room temperature was measured to be 22.5 C (295.65) and the atmospheric pressure 29.49 inHg (99853.14Pa). Theory
The setup of this experiment includes a NACA 0015 airfoil placed in the wind tunnel. Considering the cross-sectional area A1, velocity V1, and the density of air p1 at the inlet and similarly the cross-sectional area A2, velocity V2, and the density of air p2 at the outlet and by assuming that no mass is lost between the inlet-outlet section, we get the mass conservation equation, p1 V1 A1 = p2 V2 A2 (1). Further, the airflow can be assumed to be incompressible for this experiment due to low velocity, the equation (1) can be reduced to V1 A1 = V2 A2 (2), moreover, the air is assumed to be inviscid, the Bernoulli’s equation, p1+12ρV12=p2+12ρV22 (3) and the equation (2) can be reduced to Vth=2(p1-p2)/ρ1-A2A12 (4) in order to find the theoretical velocity. The pitot - static tube is used to calculate the actual velocity of the flow by using, Vact= 2(p2-p1)ρ (5). Furthermore, the velocity coefficient can be calculated using, Cv=VactVth (6). The...
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