Form drag, also known as pressure drag, arises because of the shape and size of the object. The pressure drag is proportional to the difference between the pressures acting on the front and back of the immersed body and the frontal area.
This type of drag force is also an interesting consequence the Bernoulli’s effect. According to Bernoulli’s principle, faster-moving air exerts less pressure.
This causes that there can be a pressure difference between surfaces of the object. The general size and shape of the body are the most important factors in form drag. Generally, bodies with a larger presented geometric cross-section will have higher drag than thinner bodies.
As can be seen from the figure, the drag force arises from the difference between the pressures acting on the front and back of the immersed body. For this force can be calculated (for this case) simply using the definition of pressure as:
Since the head loss is roughly proportional to the square of the flow rate in most engineering flows, the total drag coefficient can be determined by simply adding the friction and pressure drag coefficients:
Reactor Physics and Thermal Hydraulics:
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- White Frank M., Fluid Mechanics, McGraw-Hill Education, 7th edition, February, 2010, ISBN: 978-0077422417