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Pressure distribution of cylinder in wind tunnel

Engineering and Science Unit Code: To obtain and compare theoretical and experimental surface pressure distributions on a circular cylinder. To obtain surface pressure distribution on an airfoil NACAand describes its significance in the wing design. In many engineering applications, it may be necessary to examine the phenomena occurring when an object is inserted into a flow of fluid. The wings of an airplane in flight, for example, may be analyzed by considering the wings stationary with air moving past them.

Certain forces are exerted on the wing by the flowing fluid that tend to lift the wing called the lift force and to push the wing in the direction of the flow drag force. Objects other than wings that are symmetrical with respect to the fluid approach direction, such as a circular cylinder, will experience no lift, only drag.

Drag and lift forces are caused by the pressure differences exerted on the stationary object by the flowing fluid. Skin friction between the fluid and the object contributes to the drag force but in many cases can be neglected. The measurement of the pressure distribution existing around a stationary cylinder in an air stream to find the drag force is the object of this experiment. Consider a circular cylinder immersed in a uniform flow.

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The streamlines about the cylinder are shown in Figure 1. Streamlines of flow about a circular cylinder. The fluid exerts pressure on the front half of the cylinder in an amount that is greater than that exerted on the rear half.

The difference in pressure multiplied by the projected frontal area 1 Fluid Mechanics II Lab Sheet of the cylinder gives the drag force due to pressure also known as form drag. Because this drag is due primarily to a pressure difference, measurement of the pressure distribution about the cylinder allows for finding the drag force experimentally. A typical pressure distribution is given in Figure 9.

  • Pressure distribution around a circular cylinder placed in a uniform flow;
  • This is housed beneath the test section portion;
  • Results and Discussion 4;
  • This is particularly useful for obtaining laminar flow;
  • The diffuser slows the speed of airflow in the wind tunnel;
  • Objects other than wings that are symmetrical with respect to the fluid approach direction, such as a circular cylinder, will experience no lift, only drag.

Pressure distribution around a circular cylinder placed in a uniform flow. Shown in Figure 2a is the cylinder with lines and arrowheads. The length of the line at any point on the cylinder surface is proportional to the pressure at that point. The direction of the arrowhead indicates that the pressure at the respective point is greater than the free stream pressure pointing toward the center of the cylinder or less than the free stream pressure pointing away. Note the existence of a separation point and a separation region or wake.

  • Set the fan motor frequency to 35 Hz;
  • Pressure distribution around a circular cylinder placed in a uniform flow.

The pressure in the back flow region is nearly the same as the pressure at the point of separation. To find the drag force, it is necessary to sum the components of pressure at each point in the flow direction.

Figure 2b is a graph of the same data as that in Figure 2a except that Figure 2b is on a linear grid. Figure 3 shows the effect of separated flow and the failure of the boundary layer theory. In Figure 3, surface pressure distributions Cp for inviscid flow and boundary layer flow on a circular cylinder.

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The actual laminar and turbulent boundary layer pressure distributions are surprisingly very different compared to theoretical predictions. Because of the boundary layer separation, the average surface pressure on the rear half of the cylinder is considerably less than that on the front half. Therefore, a large pressure drag is developed, even though due to small viscosity the viscous shear drag may be quite small.

No matter how small the viscosity, as long as it is not zero, there will be a boundary layer that separates from the surface, giving a drag that is, for the most part, independent of the viscosity.

Surface pressure distributions Cp for inviscid flow and boundary layer. Similar concepts hold for the airfoil as well. Over the front portion of the airfoil, the pressure decreases in the direction of flow favorable pressure gradientwhile the pressure increases in the direction of flow adverse pressure gradient. If the adverse pressure is not too great, the boundary layer fluid can flow into slightly increasing pressure region from C to the trailing edge in Figure 4a without separating from the surface.

However, if the adverse pressure gradient is too large, the boundary layer will separate from the surface as indicated in Figure 4b.

Flow visualization photographs of flow past an airfoil: A wind tunnel is really a fairly simple device. Most designs feature each of the five components described below.

The overall design creates high-speed, low-turbulence airflow through the test section and allows researchers to measure the resulting forces on the model being tested. The purpose of the straighteners is to straighten the airflow. The nozzle takes a large volume of low-velocity air and reduces it to a small volume of high-velocity air without creating turbulence. The test section is where the test article and sensors are placed.

The diffuser slows the speed of airflow in the wind tunnel. The air flow is generated by using a centrifugal fan.

Air is drawn by the centrifugal fan into the settling chamber through a faired inlet and passes through a rectangular screen flow conditioning section before being accelerated through the contraction section into the test section. The flow then passes through the diffuser section into the centrifugal fan and is discharged into the room. The test-section air velocity control is accomplished by a variable frequency inverter. Figure 5 is a schematic of the side view of the circular cylinder.

The cylinder is placed in the test section of the wind tunnel which is operated at a pre-selected velocity. The pressure taps are attached to a multi-tube inclined U-tube manometer board.

Only the first 10 taps are connected. For two different approach velocities, measure and record the pressure distribution about the circular cylinder. An airfoil NACA is also mounted in the test section.

  1. Most designs feature each of the five components described below. A typical pressure distribution is given in Figure 9.
  2. This is particularly useful for obtaining laminar flow. Schematic of the experimental apparatus used in this experiment.
  3. When we look at absolute aerodynamic quantities like the lift force or the pitching moment, we find that those quantities depend on a large number of fluid parameters like the density, viscosity and temperature and additionally on the flow speed and model size. There are several types of corrections that need to be applied here.
  4. To obtain and compare theoretical and experimental surface pressure distributions on a circular cylinder. We measure aerodynamic quantities in the middle of the airfoil section and assume that the flow is approximately two-dimensional.

However, only the first 7 taps are connected. Schematic of the experimental apparatus used in this experiment. Record the room temperature and barometric pressure. Ensure the connection is done properly.

  1. Tighten the set screw using screw driver.
  2. Objects other than wings that are symmetrical with respect to the fluid approach direction, such as a circular cylinder, will experience no lift, only drag. The reading should be negative reading.
  3. It was found that the magnitude and area of the sound source intensity distribution in the near wake of the semi-circular cylinder were reduced compared with those of the circular cylinder. A typical pressure distribution is given in Figure 9.
  4. You need to obtain the data of maximum air speed for various fan motor frequencies from another group doing the Experiment 1 on the same day.

Allow the flow in tunnel stabilize for about minutes. Ensure the measuring point of the tube is in-line with the air flow. Tighten back the screw. The reading of the inclined manometer is fluctuating, kindly take the average reading.

Place the counter weight on the rear part of the test model holder stand. Tighten the set screw using screw driver. Ensure the first pressure port is in parallel direction of the air flow before tightening it.

  • The purpose of the straighteners is to straighten the airflow;
  • This manometer is used for studying the pressure distribution across the various models;
  • Place the counter weight on the rear part of the test model holder stand;
  • Set the fan motor frequency to 35 Hz;
  • A much better way is to non-dimensionalize the aerodynamic quantities with flow speed and model size and to look at similarity parameters rather than fluid parameters;
  • Because this drag is due primarily to a pressure difference, measurement of the pressure distribution about the cylinder allows for finding the drag force experimentally.

Close all the windows of the test section with respective cover. Ensure the connection is correct whereby the first port should be connected to the first U-tube manometer and so on.

Set the fan motor frequency to 35 Hz. Allow the flow in the tunnel stabilize for minutes. Record the readings in Table 2. The reading should be negative reading. Ensure the airfoil is in parallel direction of the air flow before tightening it. Record the readings in Table 3. For SI units, 1. Discuss the pressure distributions on the circular cylinder.

Also, compare both Cp profiles with those shown in Figure 3. You need to obtain the data of maximum air speed for various fan motor frequencies from another group doing the Experiment 1 on the same day.

Discuss and interpret the finding from the graph. However, you will share the data with your group members. You need to describe them properly. Provide a sample calculation. Coordinate with your group members to avoid presenting the same sample calculation.

Your report should include: