Drag Force Measurement

Topics: Fluid dynamics, Aerodynamics, Viscosity Pages: 5 (1200 words) Published: May 17, 2013



1. Objectives

* Calculation of velocity profile in the wake of a two-dimensional cylinder. * Estimate the coefficient of drag and compare with values in the literature and from the force balance. 2. Introduction

Drag is an unavoidable consequence of an object moving through a fluid. Drag is the force generated parallel and in opposition to the direction of travel for an object moving through a fluid. Drag can be broken down into the following two components: * Form drag (or pressure drag) - dependent on the shape of an object moving through a fluid * Skin friction - dependent on the viscous friction between a moving surface and a fluid, derived from the wall shear stress. [3]

The drag coefficient is a function of several parameters like shape of the body, Reynolds Number for the flow, Froude number, Mach number and Roughness of the Surface. [5] Objects drag coefficients are mostly results of experiments.

3. Apparatus and Techniques

3.1 Apparatus:

Figure 1: wind tunnel

Figure 2: manometer inclined 45 degree in H2O mm

Figure 3: speed readout in m/s.

Figure 4: barometer in hg cm
Figure 4: barometer in hg cm
Figure 5: thermometer in Degree Celsius
Figure 5: thermometer in Degree Celsius

Figure 6: regular circular cylinder 3cm in height 2.5 cm in diameter.

3.2 Procedures and Techniques:

At the start, we measured the room temperature and pressure to calculate the density of the air in the laboratory room using thermometer* and barometer** shown in figures 4&5. Using the wind tunnel as shown in Figure1 we measured pressure and controlled speed. At first, we measure the initial conditions to use them as a reference for future calculations. Then we set the wind tunnel at a several velocities*** and wrote down the readings of the manometer shown in fig2. 4.3 Uncertainties:

*Thermometer = ± 1 degree
**Barometer = ± 5 hg cm
***Speed readout = ± 2.5 m/s

4. Results and Discussions

Equation 1 can be used to calculate the drag force

FD=ρwu2dy-ρU∞A∞ (1)
ρ is air density (kg/m^3), w is width of the cylinder (m), u velocity of the flow near the cylinder (m/s), U∞ velocity of the flow in free stream (m/s), A∞ area of cylinder with rake (m^2).

Calculate the velocity (u) by using numeric integration by Simpson rule as shown in equation 2

(2). [1]

To calculate the velocity (u) from the dynamic pressure we used equation 3

u=2*Po-Pρ (3)
Podynamic pressure (pa), P static pressure (pa).

By Substituting the Valuables of the drag force we calculated from equation 1 in Equation 4 we calculated the drag Coefficient to compare with figure 10.

CD=FD0.5ρU∞2A (4)
CD drag coefficient, A face area of the cylinder (m^2).

Shown in figures 9, 8&7 are the results of the velocities across the wind tunnel in different motor speeds, which can be seen; on the speed readout shown in figure 3 by integrating the values numerically (equation 2) the drag force can be calculated.

Figure 7

Figure 8

Figure 9

4.1 Uncertainties:
SD=inx2i-x2mn-1 Equation 5 where (SD) is the stander deviation, xi measured value, xm mean value, n number of measure's. Up=t∝/2,v*SDn
Equation 6: Up uncertainty.

Pressure total= ±12.2 H2o mm.
Pressure static= ± 2.5 H2o mm.
Air density= ± 0.004 kg/m^3.
u= ± 6 m/s
FD= ± 0.99 N

5. Conclusions

5.1 Summary:
Setup the test and take all parameters, calculate velocity from dynamic pressure equation, take deferent runs and calculate uncertainty, set it in a graph and compare with previous tests.

5.2 Conclusion:
* The faster the flow goes the higher drag force becomes. * The opposite goes for the drag coefficient; the faster the flow goes the lower the drag coefficient becomes. 6.
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