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1 1.0INTRODUCTION In fluid dynamics, drag is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid. Drag force depend on velocity and it is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of drag is viscous friction, the turbulent drag is independent of viscosity. Simple wind tunnels use elementary principles of fluid mechanics to study lift and drag forces on a solid body. A beam balance typically measures changes in elongation at the base of the body. The flow-related force can be divided into three named components, a drag (drag force), which acts in the flow direction, a lift (lift force) and a side force, all perpendicular to each other. The lift usually is in the direction so that it does a useful job, for instance upwards for an airplane in horizontal flight or downwards for inverted wings in race car.
2 1.1 Objectives To measure the drag coefficient CD, over a range of velocities in the test section for prism (body base surface facing upstream and body base surface facing downstream) 1.2 Theory Drag is the component of force on a body acting parallel to the direction of relative motion. The drag force, FD, was written in functional form FD = f1(d, V, μ, ρ). Application of Bucking Pi Theorem resulted in two dimensionless П parameters that written in function forms as -----------------(1) Note that d2 is proportional to the cross-sectional area (A = length x height) and therefore we could write -------(2) Although Eq 2 was obtained for sphere, the form of equation is valid for incompressible flow over any body, the characteristic length used in the Reynolds Number depends on body shape. The drag coefficient, CD, any body defined as -------------(1.2) ÷÷øöççèæ=μrrVdfdVFD22221(Re)21332fVdfAVFD=÷÷øöççèæ=μrrAVFCDD221r=
4 3.0PROCEDURES 1)Dimension of the prism was measured. 2)Inlet section (Figure2) was pulled to open the test section (Figure 3) 3)The balance arm (Figure 5) installed into the test section 4)Fitted body surface facing flow (Figure 6 ) to the balance arm. 5)Closed the inlet section and cardboard (Figure 4) was placed to close the space at the test section. 6)Balanced the arm and reading from the drag scale is taken. 7)The blower fan was switch on and flow the velocity to 8 m/s.