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Wind Tunnel- Force

This experiment is done on a wind tunnel. For general information of this windtunnel and the data that can be obtained from it, refer the wind tunnel fundamentals tab.

Lab Objectives:

• Measure the Lift and Drag forces experienced by an Unsymmetric NACA airfoil mounted on Load cells, at varying velocities and fixed andgle of attack.

  • Plot the Lift and Drag forces versus the freesttream velocities, as shown below, and verify the second-order relation between the aerodynamic forces and the velocities.

  • Calculate the Lift and Drag force coefficients and compare to the on-screen values.



            In this lab the characteristics of airfoil lift and drag with Varying Velocities will be investigated. The airfoils are not only useful for aircraft. The investigation described herein applies to many fluid dynamic scenarios like wind turbine blades, wings on F1 cars, helicopter blades, propeller blades and hydrofoils. An Airfoil is a body designed to produce lift from the movement of the fluid around it. Specifically lift is a result of circulation in the flow produced by the airfoil.



Figure 1: Pressure distribution of an airfoil and generation of lift and drag



The  tunnel  balance is  three component  type  (  three forces )  designed  using  the electrical  strain  gauges  to indicate  separately  on the  digital  indicator. The balance is intended for indicating the lift , drag & side force in case of airfoils, and drag force only in case of bluff bodies,Viz., spherical, Hemi - spherical, Flat disc. These models are mounted on the string (Vertical square rod) situated exactly beneath the test section. The output from the lift, drag & side forces (strain gauges) are connected to the respective multi - pin sockets provided at control panel. (Picture available in Wind Tunnel Overall Theory document).

The lift coefficient 

The lift coefficient is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and some flow conditions on lift. The lift coefficient expresses the ratio of the lift force to the force produced by the dynamic pressure (q∞) times the area. By knowing the lift coefficient, we can predict the lift that will be produced under a different set of velocity, density (altitude), and area conditions using the lift equation. For given air conditions, shape, and inclination of the object, we have to determine a value for CL to determine the lift.

Where L is the lift force, ρ is air density, Ʋ is the true air speed, A is the planform area (projected area of the wing) and CL is the lift coefficient.

The Drag coefficient

            The drag coefficient is a number used to model all of the complex dependencies of shape, inclination, and flow conditions on aircraft drag. The drag coefficient expresses the ratio of the drag force to the force produced by the dynamic pressure times the area.  In a controlled environment (wind tunnel) we can set the velocity, density, and area and measure the drag produced. Through division we arrive at a value for the drag coefficient. As pointed out on the drag equation slide, the choice of reference area (wing area) will affect the actual numerical value of the drag coefficient that is calculated.  We can predict the drag that will be produced under a different set of velocity, density (altitude), and area conditions using the drag equation.

For given air conditions, shape, and inclination of the object, we must determine a value for CD to determine drag. Determining the value of the drag coefficient is more difficult than determining the lift coefficient because of the multiple sources of drag. The drag coefficient given above includes form drag, skin friction drag, wave drag, and induced drag components.

Where FD is the drag force, [ ho]  is the mass density of the fluid [v]  is the velocity of the object relative to the fluid, A is the reference area, and CD is the drag coefficient.


Variation of Lift and Drag forces with Freestream Wind Speed:

The experiment is to demonstrate the induced aerodynamic loads on the turbine blades due to the freestream wind, and their variation with the Wind Speed. As the formulae above suggest, the student may expect a quadratic variation of the forces with respect to velocities due to the v^2 term. But NOTE that the Force Coefficients are constant at a given angle of attack (0 degrees in this experiment). Only the actual Forces vary due to v^2 with the freestream Wind Speed.



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