MECH103

Professor Roberts

Fall 2008

Lab #9– Drag (Windtunnel)

When an object is fully immersed in a moving fluid, forces are exerted on the object. These forces are known (from the aeronautical background) as lift and drag. The classic and most useful example is the airfoil. But engineering problems involving the forces exerted on a solid body when a fluid flows by it no longer belongs exclusively to the area of aerodynamics. It is also important to many other fields including design, electronics and the environment.

In the most recent years, the behavior of air (fluid)flowing around vehicles has been studied thoroughly in many automotive companies. These tests have produced significant changes in the design of these vehicles, making them smoother and more streamlined. In the electronics industry, the cooling of electronic components has become critical. Designing flow paths and measuring the amount of drag force experienced by the component is very important to the mounting of the component in various units and chassis.

Drag

This particular experiment is concerned with the total of all horizontal components of the exerted forces applied to an object (moving or stationary) by a fluid (stationary or moving). This total force is defined as drag. This force is a resistance force and is the combined effect of two causes:

Friction Dragfriction between the object and the fluid

Pressure Drag a pressure difference between the leading and trailing sides

of the object.

The friction drag is primarily a function of the surface area in contact with the moving fluid, commonly referred to as the wetted surface. Reducing the size of the area will reduce the friction drag.

The primary cause of pressure drag is the "appearance" of the object relative to the flow stream and how the fluid will flow around the object. If the object does not change the flow drastically, the overall pressure difference will be minimal. The smoother the object is, the less changes in the direction of the fluid around the object, therefore producing a lower pressure drag. This makes streamlining desirable. But, streamlining can often increase the "wetted" surface area causing an increase in friction drag. It is important to remember that for most cases, drag is unwanted, and normally the objective is to minimize it. Figure 1-1, shows the effect of streamlining

Figure 1-1 Streamlining

FRONTAL AREA, Afront, and SURFACE AREA, Asurf

As noted above, drag is highly influenced by area. The frontal area, Afront, of a shape or object is also called the projected area. It is the area that an observer would see if the observer were riding on the fluid approaching the object. Figure 1-2 shows a truck model with the fluid velocity approaching it. The frontal area of the truck is the projection of the truck body and the streamlining of the cab or other additives not seen.

Figure 1-2. Frontal Area, Afront

Streamlining of the cab or adding skirts and flairs would affect the surface area, Asurf. and would help to decrease the surface drag. But care must be given in the design when working with the different areas. Figure 1-3, shows a streamlined object viewed from the flow stream in different directions. As can be seen, reversing the leading and trailing ends of the object will not change the frontal area nor will it change any of the first six variables listed above but will greatly effect the drag force.

Figure 1-3. Two objects of the same size, different shapes.

The objects in Figure 1-4 are very different but have the same frontal area and same surface area but like Figure 1-3, the drag on each will be quite different.

Figure 1-4. Identical objects with different orientation.

DRAG EQUATION

Total drag for most models and shapes is a function of the following variables;

  1. the frontal area (Afront) of the object,
  2. the surface area (Asurf) of the object,
  3. the density of the fluid,
  4. the viscosity of the fluid,
  5. the velocity of the fluid,
  6. the shape of the object,
  7. the orientation of the object relative to the flow.

A mathematical expression for the drag can be developed as:

D= Cd[(V2/2) Afront][eq 1]

where:D = the drag force,N or lbs

Cd= the drag coefficient (dimensionless),

 = the density of the fluid,kg/m3 or slugs/ft3

V = the velocity of the fluid or model, m/s or ft/s

Afront= the frontal or projected area,m2 or ft2

It is conventional and far more beneficial to determine values of the drag coefficient. By measuring the drag force experimentally and calculating the Cd, investigations can be made relative to the behavior of the flow of fluids about different objects. This can be done efficiently and economically using models in wind tunnels.

Therefore,

Cd = D/ [(V2/2) Afront] [eq 2]

Experiment DRAG

OBJECTIVES: a. To compare the drag forces on objects of different shapes at various air speeds.

b. To become familiar with the use of dimensionless numbers.

APPARATUS:Various models

Subsonic wind tunnel

Wind tunnel balance

PROCEDURE:

Models1. For each model, measure the maximum height, maximum width and length using the various measuring scales supplied.

  1. Sketch the front view of each model as well as possible and label the measurements at each point of design change.

3. Determine the frontal area, Afront

  1. Sketch the side view of each model and label the measurements at each point of design change.

5. Determine the surface area, Asurf for the sides.

  1. Sketch the top view of each model as well as possible and label the measurements at each point of design change.

7. Determine the surface area, Asurffor the top.

8. Determine the total surface area, Asurf by adding the sides and top surface areas.

Wind tunnel

1. Study the subsonic wind tunnel unit, the various control panels and computer software. Determine the controls foradjusting the wind tunnel speed, the velocity meter for speed measurement and the drag measuring scale.

  1. Lock the drag model in place with the mounting knurled nut making sure the model is pointing upward towards the air stream.
  1. Close the wind tunnel opening and start the fan, being careful that all items have been removed from the wind tunnel opening.
  1. Adjust the speed on the computer to obtain the desired speed and measure the drag value on the model. Repeat for all test speeds.
  1. Repeat steps 2, 3 and 4 for all models assigned.
  1. Close the program by clicking on to the red dot on the menu bar.
  1. Shut the power to the fan and lift/drag control panel.

PRESENTATION OF RESULTS:

  1. Tabulate the measurements made on the models.
  1. List the drag force for each model at each air velocity.
  1. Calculate the drag coefficient for each model using equation 2 and the measured velocity. Also use the approximate frontal area. Calculate the air density using the atmospheric pressure and air temperature of the day.
  1. Compare the results obtained for each given model at the specified velocities.

Data Sheet

Fan Speed Setting (%)

/

ReferenceTunnelVelocitym/s

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Model 1Drag

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Model 2Drag

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Model 3Drag

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Model 4Drag

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Model 5Drag

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PITOT TUBE (DP)

Barometric Pressure:______

Air Temperature:______

Results

ReferenceTunnelVelocitym/s

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Model 1DragCoeff.

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Model 2DragCoeff.

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Model 3DragCoeff.

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Model 4DragCoeff.

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Model 5DragCoeff.

mech103-drag1