This experiment is done on a wind tunnel. For general information of this windtunnel and the data that can be obtained from it, please refer introductory theory given in wind tunnel fundamentals tab.
•Measure the velocities downstream of a symmetric airfoil inclined at 10 deg to freestream using a Pitot probe (shown in fig below behind the blade model).
• Obtain the velocity profile, as shown in Fig. below, by plotting the velocities versus the downstream vertical location. It is useful to visualize by plotting in reverse, that is, Vertical probe position on y axis and velocites on x axis. But remember, the independent varying parameter is the vertical probe location.
- The effect of the wake behind the blade model is seen by the low velocity at -20 or -30 mm vertical position.
DRAG OF AN AIRFOIL
In this lab the characteristic of airfoil drag due to flow separation will be investigated by the wake survey method. 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. However, the pressure distribution resulting in Lift also produces unwanted Drag. The drag, though, is useful in aerodynamic braking and spoilers.
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.
VELOCITY MEASUREMENT (PITOT PROBE)
One very important use of wind tunnels is to visualize flow patterns and measure the pressure at a selected point in the flow field and compute the corresponding speed of air. The equation relates the speed of the fluid at a point to both the mass density of the fluid and the pressures at the same point in the flow field. For steady flow of an incompressible fluid for which viscosity can be neglected, the fundamental equation has the form
Where V is the speed of the fluid, P0 is the total, also called the stagnation, pressure at that point of measurement, and p is the static pressure at the same point. This equation comes from the application of Bernoulli’s equation for the steady flow of an incompressible and inviscid fluid along a streamline. Bernoulli’s equation is typically obtained by integrating Euler’s equations along a streamline. It will be recalled that Euler’s equations are a special case of the Navier -Stokes equations, when the viscosity of the fluid has been neglected. The Navier-Stokes equations, in turn, are obtained from Newton’s second law when it is applied to a fluid for which the shear deformation follows Newton’s law of viscosity
Flow over streamlined bodies create separation region in the aft end due to high pressure gradients. This region, called wake, is because the flow is unable to negotiate the steep pressure gradient, creating a large wake of dead air behind the airfoil. Inside this separated region, flow is recirculating and part of the flow is actually moving in the opposite direction; so-called reversed flow. This separated flow is due to the viscous forces not being able to overcome the inertial forces. The energy in the wake is directly proportional to the momentum loss across the body, that results in Drag. By the method of wake survey using Pitot probes we can capture the velocity profile downstream of an airfoil (as shown in figure). The velocities may then be used in the momentum integral equation to determine the drag (Ref. Houghton and Carpenter, Aerodynamics for Engineering Students). This is very useful in cases where strain gage balancing can't be used.