University of Texas at Arlington MAE 3183, Measurements II Laboratory Air Drag Force 1 Experiment #5 Air Drag ForceUniversity of Texas at Arlington MAE 3183, Measurements II Laboratory Air Drag Force 2 Introduction The effects of drag exist in everyone's life. Simply driving a car across town means that you have experienced drag. Your car engine must work a little harder to overcome the force present with drag and get you to your location on time. Just walking down the street as well will confront a person with drag. Although this drag is small and the speed at which you can walk is small, the effects still exist. Drag, or more specifically air drag, is a phenomenon that occurs as an object passes through a fluid. There are a few factors that determine the drag force that an object experiences. Some of the more obvious factors are shape, speed, fluid medium, and surface of the object. In some instances these factors are manipulated in order to either minimize or maximize drag. In other cases the drag forces must simply be known in order to design for other parameters possibly such as engine horsepower, structural strength, etc. Regardless of the need for finding the drag force, the need for an accurate calculation of this force persists. With this in mind, we experiment with shapes, speeds, and methods in order to draw insight on the ability to predict drag. In this experiment, we study the effect of shapes on drag at various Reynolds numbers and examine the validity to accurately predict drag using two different methods. Theory When a fluid flows around a stationary cylinder or when a cylinder moves through a stationary fluid, the fluid exerts a force on the cylinder called drag force. The sources of this drag are: (a) friction between the fluid and the surface of the cylinder, and (b) a non-uniform pressure distribution. The cylinder in the fluid stream presents a certain area perpendicular to the direction of fluid motion. This is called the planform area of the cylinder (length x width (diameter)) the fluid moves toward and is deflected around the cylinder, some of it’s momentum is transferred to the cylinder in the form of pressure on the projected area facing the flow. If the flow follows the contour of the cylinder, the pressure on the side facing the flow is balanced by the pressure on the reverse side in which case the pressure drag is very small or zero. (see Figure 1). This condition is described by potential theory where the fluid is ideal and is realized in real fluids at very low Reynolds numbers. At high Reynolds numbers, the flow does not follow the contour of the cylinder, i.e., the boundary layer grows more rapidly for an adverse pressure gradient and if the pressure gradient is large enough, separation may occur, and turbulent eddies form in the wake of the cylinder. In this case the pressure on the reverse side fails to recover (see Figure 2) leading to an unbalanced pressure distribution and pressure drag. Ordinarily, it is not practical to separate the viscous and pressure drag forces, and indeed, it is usually their sum in which we are interested. Therefore, the usual practice is to characterize their combined effects with two dimensionless parameters, the drag coefficient: Cd2FUAd2p (1) and the Reynolds number, ReDUD (2) where Fd is the drag force, U is the free stream velocity, Ap is the platform area, and D is the cord length of the shape (cylinder).University of Texas at Arlington MAE 3183, Measurements II Laboratory Air Drag Force 3 Figure 1, Ideal fluid flow around a cylinder Figure 2, Real fluid flow around a cylinder The drag coefficient may be determined experimentally in two ways. The most obvious method is to measure the drag force (Fd) and the velocity (U) directly and then calculate Cd from equation 1. The second method to determine drag force is to use the Moody chart with drag coefficient versus Reynolds number for known shapes. Topics to Review - Fluid Flow over an object - Reynolds Number - Drag Force and Drag Coefficient - Moody chart for known shapes Apparatus The Air Drag experiment consists of the following equipment: Wind Tunnel Traveling Pitot Tube Cylinder, airfoil, rectangular bar and sphere Data acquisition system The cylinder and airfoil have the following physical dimensions: Cylinder Chord 1.27 cm. Cylinder Length 26.67 cm. Airfoil Chord 1.27 cm. Airfoil Length 26.67 cm. Rectangular Bar Width 1.2 cm. Rectangular bar Length 26.67 cm. Sphere Diameter 6.23 cm. Air velocity in the tunnel is determined from u=2P (6) where P is the dynamic pressure measured with data acquisition system. When determining the drag coefficient by direct measurement of Fd and U, the value of u obtained from the pitot tube is close enough to U if the pitot tube is located well away from the region behind the cylinder.University of Texas at Arlington MAE 3183, Measurements II Laboratory Air Drag Force 4 Objective 1) To find Cd using the parameters obtained from the DAQ for four different shapes. 2) To investigate the effects of airflow around a cylinder and to determine the relationship between shape, Cd, and Reynolds number. Requirements 1) Determine the drag force for the cylinder and sphere using eq (1). (Cd can be found from a Cd versus Re plot which you should be able to find in any fluids textbook) 2) Create a Fd versus strain plot for the cylinder 3) Using the plot you created in step 2, determine Fd for the airfoil and rectangular bar and calculate Cd. 4) Plot (log-log) Cd versus Re for all the four shapes on one chart. 5) Compare your results with published data. Procedure 1) Make sure the fan is turned off. 2) Check the pitot tube. It should be located about midway between the cylinder and the top of the tunnel with the tip ahead of the leading edge of the shape. If it is not, turn the motor switch to the up or down position until the pitot tube is properly located. 3) Remove the screw stop from the front rail of the pitot tube slide, and move the pitot tube traversing mechanism toward the front of the wind tunnel (pitot tube body must not touch the cylinder, but measurement of the tunnel speed must be accomplished with the pitot tube and ahead of the cylinder). 4) Turn on the wind tunnel at appropriate speed levels (First reading should be when the fan is off and then 6 more readings by turning the knob from 1 to 6). 5) Open the