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MENG382Lab7-Spring-2018ExternalFlow.docMENG 382
LABORATORY #7
External Flow
May 29th & 31st, 2018
Report submitted by:
____________________________________
MENG 382 Fluid Mechanics Lab #7, External Flow
Introduction: In this lab we examine the concept of external flow. This is a little different from pipe flow in that viscosity doesn’t necessarily dominate the flow. Now we need to consider inertia as a primary force to contend with. Forces created by external fluid flows are often given such names as, Lift and Drag. In this lab we will examine some of these forces in a wind tunnel. Recognize that Lift and Drag are calculated by the following equations:
A
V
C
Lift
L
2
2
1
r
=
Equation 1
A
V
C
Drag
D
2
2
1
r
=
Equation2Note that they are both functions of the square of the Velocity. Also, the term: 1/2ρV2 is often called the Dynamic Pressure and is given the symbol, q. That’s because if you were to stop the flow adiabatically, all of that flow velocity would convert into pressure. That’s what happens in a pitot tube that we often use to measure velocities in flows.
Figure 1, T-38 Talon Supersonic Trainer (note the pitot tube protruding from the nose of the jet).
Procedure: In external flows it is often difficult to accurately predict what is going to happen so we can use testing of scale models, such as in a Wind Tunnel. This lab is completed by finding out the Wind Tunnel-related answers to the following questions:
1. How are Wind Tunnels sized?
2. What is the size of the Wind Tunnel in our Fluid’s Lab?
3. Our Wind Tunnel uses a “Pitot Rake” to take its data. Derive the formula for a pitot tube from the General Energy equation.
4. What is the size of the largest Wind Tunnel in the world?
5. How “fast” can a Wind Tunnel go?
6. When taking data for the golf balls I usually run the wind tunnel at a velocity of 75 ft/sec. The diameter of the golf balls is 1.680 inches. Assuming standard temperature and pressure for the air calculate the Reynolds number for the golf ball.
7. Look in your textbook and find a Coefficient of Drag for a sphere for Laminar and Turbulent flow.
a. Smooth (Laminar flow) golf ball:______________________________________
b. Dimpled (Turbulent flow) golf ball:____________________________________
8. Using the air velocity from the Wind Tunnel and the Coefficient of Drag you just found, calculate the drag force on a Smooth Golf Ball (i.e. Laminar flow).
9. Using the air velocity from the Wind Tunnel and the Coefficient of Drag you just found, calculate the drag force on a Dimpled Golf Ball (i.e. Turbulent flow).
10. Why does the Dimpled Golf Ball have less drag?
11. Alan Shepard, the first American in space, is also famous for hitting a golf ball on the moon. How much drag force did this golf ball experience?
How to complete this lab report: Answer the questions, put your name on the front, and turn it in.
