As fluid flows around an object, such as a cylinder, the pressures and velocities close to the object constantly change. According to inviscid potential flow theory, the pressure distribution around a cylinder is symmetric, not only horizontally but also vertically, upstream and downstream of the cylinder. This results in a zero net drag force.
However, experimental results give different flow patterns, pressure distributions and drag coefficients because the inviscid potential theory does not take into account fluid viscosity, which differs greatly from reality.Taking viscosity of the fluid into account, we can further understand real flow patterns around a cylinder.
First, a boundary layer is developed along the cylinder as a result of viscous forces. These viscous forces cause skin friction drag, which is a drag force caused by the friction of the fluid moving across the surface of the object.
Since the cylinder is a bluff body, meaning that it is not streamlined, flow separation occurs and a low pressure wake forms behind the object. This leads to an even bigger form of drag due to a pressure differential.
The characteristics of this flow pattern rely on the Reynolds number. The Reynolds number is a dimensionless number used to describe fluid, and it is a ratio of the inertial forces to the viscous forces. Rho infinity is the density of the fluid, V infinity is the free stream velocity, D is the diameter of the cylinder, and mu is the dynamic viscosity of the fluid.
Below a Reynolds number of about 4, the flow pattern shows very little flow separation behind the cylinder. As the Reynolds number increases, flow separation increases. Below a Reynolds number of about 40, we see a fixed pair of vortices in the wake.
At higher Reynolds number, the vortices shift to a vortex street with a pattern of alternating vortices caused by a process called vortex shedding. At even higher Reynolds number, after the laminar boundary layer has undergone the transition to turbulent, the wake becomes disorganized.
Finally, at very high Reynolds number and turbulent flow, we see the wake become narrower and fully turbulent.
In this lab, we will subject a cylinder with 24 pressure ports to fluid flow in a wind tunnel. We will then use the pressure measurements at each pressure tap to examine the pressure distribution and determine the drag forces on the cylinder.
For this experiment, use an aerodynamics wind tunnel with a test section of 1 ft by 1 ft. Also, obtain an aluminum cylinder with 24 built-in ports for pressure tubes. A manometer panel with 24 columns will also be needed.
To begin, first remove the top cover of the test section. Insert the tubes that connect to the cylinder ports through the slit in the bottom of the test section. Then mount the cylinder on top of the turntable orienting it so that port zero is facing upstream.
Replace the top cover of the test section, and connect the 24 pressure tubes labeled zero through 23 to the corresponding ports of the manometer panel.
Once all of the tubes are properly connected, start the wind tunnel. Increase the wind speed to 60 miles per hour and record all of the 24 pressure measurements by reading the manometer. Now, set the wind speed back to zero and turn off the wind tunnel. Open the test section.
Now, modify the cylinder by securing a 1-mm diameter string vertically between ports 3 and 4, which is equivalent to theta equal to 52.5°. Keep the string as straight as possible while taping it in place. Tape another string between ports 20 and 21, which is theta equal to 307.5°. These strings will disturb the air flow. Use a pin to puncture holes through the blue tape so that the ports can sense the flow pressures.
Then, close the test section. Turn the wind tunnel back on, and increase the wind speed back to 60 miles per hour. Record the 24 pressure measurements using the manometer.
When finished, set the wind speed back to zero and turn off the wind tunnel. Disconnect the tubes from the manometer. Then open the test section and remove the cylinder.
Now, let's interpret the results. First, we can determine the Reynolds number using the free stream velocity, which was 60 miles per hour. The diameter of the cylinder, viscosity and density of the free stream are known. Thus, the Reynolds number is equal to 1.78 x 105.
At this Reynolds number, we can expect a flow pattern as shown, where flow separation occurs and results in a turbulent low pressure wake behind the cylinder. This pressure differential leads to drag.
Now, let's look at our experimental data, in this case for the clean cylinder. Due to symmetry, we will look at only ports 1 through 12. Theta is the angular position of the port, and P-gage is the manometer reading.
First, calculate the non-dimensional pressure coefficient for each port where rho infinity and V infinity are the free stream density and velocity, respectively. Do the same calculation for the disturbed cylinder.
If we plot the experimental results for each cylinder as compared to the ideal, we can see that the stagnation point, or theta equal to zero, the pressure coefficient is at its maximum for both the clean and disturbed cylinders. Before theta equal to 60°, the clean and disturbed cylinders agree well with the ideal data.
After 60°, they deviate from the ideal as they form a low pressure region at the back of the cylinder. If we recall the expected flow pattern, we can see that in the wake region of the flow pattern, we should see turbulent vortices and eddies. This phenomenon corresponds well with the low pressure regions measured for both cylinders.
However, differences between the two arise where the strings were added to the cylinder, where the clean cylinder experiences a lower pressure region in the wake than the disturbed cylinder. This is because the disturbed flow tends to wrap around the cylinder more before the flow separation occurs. The boundary layer, which starts as laminar, transitions to turbulent immediately after the disturbance.
You can see that it wraps around the disturbed cylinder more than the clean cylinder, which is always laminar before the flow separation. Because the disturbed flow has a higher back pressure in the wake, it should have a lower drag force. Let's confirm this hypothesis.
First, calculate drag, FD, as shown using the angular position of each pressure port, the angular distance with adjacent ports, the gage pressure at each port, and the radius of the cylinder. Once we've calculated drag for each cylinder, we can calculate the non-dimensional drag coefficient, CD, for each cylinder.
As expected, the drag coefficient is lower for the disturbed cylinder than the clean cylinder. These results also explain why golf balls are dimpled. The dimples cause turbulent boundary layer flow and therefore lower the drag.
In summary, we learned about the characteristic flow patterns observed at different Reynolds numbers and the transition to turbulent flow. We then subjected cylinders to cross flow in a wind tunnel and measured the pressure distribution along their surfaces to determine the drag forces on each.