The Enchanting Business of Bernoulli

Introduction

"My business today has most uncommon enchantment. And of all the things I like to do, of which there are hundreds in this business of demonstrations, this, indeed, enchants my soul the most. It has to do with the principle of Bernoulli." - Julius Sumner Miller

Julius Sumner Miller, 1960s TV personality, got really excited about showing Bernoulli's Principle in action. So excited in fact, that we decided to perform a simple experiment showing how our B1100-1 USB Barometers could detect the pressure changes caused by increasing air velocity. For our own demonstration of Bernoulli's Principle, we attached several time-synched barometers to a 2008 Honda Civic and collected data while driving down the highway.

Objective

Demonstrate B1100-1's capabilities to measure pressure by observing how air speed affects pressure at different points on a vehicle.

Demonstrate the B1100-1's new time synchronization capabilities.

Setup

We used six time-synched barometers and set them to take data twice a second. To make sure that air turbulence did not enter the plastic case and cause an unnatural buildup of pressure inside the USB Barometer, we secured the case caps with masking tape. Using 3M Command Strips, we attached a barometer on the bumper, hood, roof, rear window, trunk, and fender of a 2008 Honda Civic.


Config.txt Settings

Samplerate = 500
Interleave = 10
Samplesperfile = 7200
statusIndicators = normal

Barometers on Bumper and Hood
Barometers on Bumper and Hood

Barometer on Roof
Barometer on Roof

Barometers on Rear Window, Trunk, and Fender
Barometers on Rear Window, Trunk, and "Fender"

Test Procedure

Once the USB Barometers were stuck onto the Civic and turned on, we took off down the highway. From about 12:37 to 12:45 we went a constant 45 mph at a near constant altitude. At 12:45, we sped up to 50 mph. Soon we crossed over a bridge, reaching the highest altitude (107 ft) at 12:49:43. From 12:51 to 1:00, we went at 45 mph but were stopped by several traffic lights. From 1:00 to 1:05 we went 65 mph, stopped, turned around, and went 70 mph until 1:08. We then drove to GCDC Headquarters at 40 mph.

Results

Graph of All Car Flow Data

After collecting the data, the results from all six barometers were copied into Excel. The above graph shows the different pressures read over the entire trip. Areas where the Civic went a specific, constant speed are highlighted and labeled at the bottom of the graph. One can see that as the car sped up, the pressure increased on the bumper of the car and decreased on the roof of the car. Pressure did not change very much in other areas, although the pressure on the hood of the car appears to increase and decrease at the same times the roof changed, but at a much lower magnitude.

The highlighted point on the graph in which all of the lines of data have a pressure decrease (near 51:35.7) is not due to a change in the car's velocity, but in the altitude of the car. At this point the car was going over a high bridge. The altitude change caused a drop in atmospheric pressure.

Average Pressures over Different Positions

The above graph shows the differences in average pressure between different positions on the car at 50 and 70 MPH.

Discussion

This experiment shows us how velocity affects the pressures on different areas of the car. The pressures in these different areas behave differently because of the way the air particles interact with a certain position of the car.

The bumper shows an increase of pressure when the velocity of the car increases. This is because the bumper feels the ram pressure of the car. Ram pressure affects the area of an object in which air particles are directly colliding with the object moving through the fluid. Because of these direct collisions, ram pressure increases when velocity increases. It is much like holding one's hand out of a car window with the palm facing forward. The person's hand feels an increased pressure as it deflects the air around it.

The hood and roof show a decrease in pressure when the velocity of the car increases. The air running over these areas show us a clear application of Bernoulli's Principle. Bernoulli's Principle states that as fluid velocity increases, static pressure decreases. Static pressure is the pressure felt by an object or person suspended in the fluid and moving with it. It is the pressure felt when air molecules run over the top of your hand with your palm faced down. For the car, static pressure is seen when air particles flow over the car instead of colliding with it head on. We can deduce from the pressure readings that air starts picking up speed as it goes up the hood of the Civic and goes the fastest over the roof.

The rear areas of the Civic do not have much of a change in pressure. As air runs off the roof of the car, the air velocity has slowed and the pressure returned almost to the static air pressure. This is due to the turbulent air pocket located behind the vehicle. If the sensors were moved out of the turbulence, the velocity would increase to the driving speed of the car, and the pressure would again decrease.

The other purpose of this experiment was to demonstrate the Real Time Clock synching capabilities of the B1100-1 USB Barometer. The preciseness of the Real Time Clocks can be seen as the car drives over the bridge. All barometers show the car reaching the highest point on the bridge at the exact same time. This assures that the pressure readings for the rest of the experiment occur with the velocity of the car being exactly the same at all points on the car at a certain time.

Conclusion

The B1100-1 has a very precise Real Time Clock that allows for the collection of time-synched data. It also allows for the measuring of pressure due to fluid velocity, a truly "enchanting" bit of business.