Reducing Drag by streamlining http://edugen.wiley.com/edugen/courses/crs2436/crowe9771/crowe9771...
11.6 Reducing Drag by streamlining
An engineer can design a body shape to minimize the drag force. This process is called streamlining and is often
focused on reducing form drag. The reason for focusing on form drag is that drag on most bluff objects (e.g., a
cylindrical body at Re > 1000) is predominantly due to the pressure variation associated with flow separation. In
this case, streamlining involves modifying the body shape to reduce or eliminate separation. The impacts of
streamlining can be dramatic. For example, Fig. 11.4 shows that CD for the streamlined shape is about 1/6 of CD
for the circular cylinder when Re H" 5 × 105.
While streamlining reduces form drag, friction drag is typically increased. This is because there is more surface
area on a streamlined body as compared to a nonstreamlined body. Consequently, when a body is streamlined
the optimum condition results when the sum of form drag and friction drag is minimum.
Streamlining to produce minimum drag at high Reynolds numbers will probably not produce minimum drag at
very low Reynolds numbers. For example, at Re < 1, the majority of the drag of a cylinder is friction drag.
Hence, if the cylinder is streamlined, the friction drag will likely be magnified, and CD will increase.
Another advantage of streamlining at high Reynolds numbers is that vortex shedding is eliminated. Example 11.5
shows how to estimate the impact of streamlining by using a ratio of CD values.
EXAMPLE 11.5 COMPARI G DRAG O BLUFF A D
STREAMLI ED SHAPES
Compare the drag of the cylinder of Example 11.1 with the drag of the streamlined shape shown in
Fig. 11.4. Assume that both shapes have the same projected area.
Problem Definition
Situation: The cylinder from Example 11.1 is being compared to a streamlined shape.
Find: Ratio of drag force on the streamlined body to drag force on the cylinder.
Assumptions:
1. The cylinder and the streamlined body have the same projected area.
2. Both objects are two-dimensional bodies (neglect end effects).
Plan
1. Retrieve Re and CD from Example 11.1.
2. Find the coefficient of drag for the streamlined shape using Fig. 11.4.
3. Calculate the ratio of drag forces using Eq. (11.4).
Solution
1. From Example 11.1, Re = 7 × 105 and CD (cylinder) = 0.2.
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Reducing Drag by streamlining http://edugen.wiley.com/edugen/courses/crs2436/crowe9771/crowe9771...
2. Using this Re and Fig. 11.4 gives CD (streamlined shape) = 0.034.
3. Drag force ratio (derived from Eq. 11.4) is
Review
Notice that streamlining provided nearly a sixfold reduction in drag!
Copyright © 2009 John Wiley & Sons, Inc. All rights reserved.
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