Fluid Forces

1
Fluid Forces

• Chapter 11

2
Fluid Drag Force

• Fluid Resistance Type of force that is known
  technically as DRAG FORCE.
• Two Types of DRAG FORCE
• SKIN FRICTION
• PROFILE DRAG

3
SKIN FRICTION-Surface Drag

• The drag caused by the fluid tending to rub along
  the surface of the body.
• The thin layer of fluid in contact with the solid
  surface of the body does not slide.

4
Skin Friction-continued

• Boundary Layer The region of relative motion
  between adjacent layers of fluid particles. This
  causes layers of air to rub near the surface of
  the body (friction). This friction varies from
  fluid to fluid. The more viscous (resistance to
  deformation) a fluid, the greater the layer
  rubbing.

5
Skin Friction-continued

• Skin Friction will increase when
• The velocity of air flow increases.
• The amount of surface area oriented parallel to
  the flow increases.
• The roughness of the surface increases.

6
PROFILE DRAG-pressure drag

• The most common drag force in sport.
• Created when the pressure difference on one side
  of the moving body is greater than that on the
  other side.

7
PROFILE DRAG-continued

• The velocity of the air flow past the object is
  too fast for the air to follow the contour of the
  trailing side of the object as it moves through
  space. This creates a BACK FLOW at the surface
  of the object. This causes the flow to separate
  from the surface contour. This separation in the
  boundary layer causes a large turbulent low
  pressure zone to be formed behind the object.

8
PROFILE DRAG-continued

• The cause of the turbulent motion behind a moving
  body is the force applied by the object to the
  fluid as it pushes its way through. It acts as a
  resistive force to the motion of the object.

9
PROFILE DRAG-continued

• Profile drag is given its name because of the
  PROFILE of the object (the area perpendicular to
  the flow of the fluid). Object with less profile
  smaller profile drag.

10
PROFILE DRAG-continued

• Profile drag is also called pressure drag because
  the differences between the pressures on the
  leading and trailing sides of the object create a
  SUCTION like effect acing against the motion of
  the object.

11
PROFILE DRAG-continued

• Profile drag is determined by
• The size of the low-pressure zone on the rear
  side of the object.
• The more abrupt the change of shape near the rear
  of the object.
• The orientation of a body relative to the fluid
  flow.

12
PROFILE DRAG-continued

• FORM DRAG is another name given to PROFILE OR
  PRESSURE DRAG because the form or shape of the
  object will determine how smoothly the object can
  cut through the fluid. Objects that are more
  streamlined in shape are those that tend to
  create a streamlined flow pattern so that
  turbulent flow is minimized.

13
Relative Influence of Factors Causing Drag

• Flow Velocity plays a major role in drag forces
  on objects.
• If velocity is doubled, drag is quadrupled

14
The Effects of Drag on Different Masses

• Increasing or decreasing the mass of a body has
  no effect on the magnitude of the drag force
  acting against it.
• The mass of the body DOES determine in part, how
  motion will be affected by that drag force.
• Golf Ball/Ping Pong Ball Example
• Basketball/Balloon Example

15
Fluid Drag as a Propulsive Force

• Drag Forces act in the same direction that the
  body is moving.
• Sailboat

16
Fluid Lift Forces

• AERODYNAMIC LIFT FORCE Always directed
  perpendicular to the oncoming flow and to drag
  force and is not always directed UPWARD.

17
Lift Forces-continued

• They influence the trajectory of projectiles with
  certain shapes.
• Discus
• Javelin
• Frisbee
• Ski jumpers
• Sky divers

18
Lift Force Acting on Shapes and Surfaces

• The object to be lifted must have a airfoil, or
  wind like shape.
• As air flows past the airfoil, it flows faster
  over the upper curved surface and slower
  underneath.

19
Lift Force Acting on Shapes and Surfaces-continued

• The flow-velocity difference existing between
  opposite sides of an object causes a pressure
  difference between the two sides.

20
Lift Force Acting on Shapes and Surfaces-continued

• BERNOULLIs PRINCIPLE Where the flow velocity
  is fast, the pressure is low where the flow
  velocity is slow, the pressure is high.

21
Lift Force Acting on Shapes and Surfaces-continued

• The existing pressure difference causes the
  airfoil to experience a force directed from the
  region of HIGH pressure to LOW pressure.
• The direction of this lift force is PERPENDICULAR
  to the flow direction past the object. (Drag
  force is directed parallel to the flow direction)

22
Lift Force Acting on Shapes and Surfaces-Continued

• Angle of Attack (If an object is not airfoil
  shaped, the difference in flow velocity and
  pressure may be achieved by tilting the object
  relative to the direction of flow past it.) THE
  ANGLE FORMED BTW THE MAIN PLANE OF THE OBJECT AND
  THE FLOW DIRECTION.

23
Lift Force Acting on Shapes and Surfaces-Continued

• As the ANGLE of ATTACK increases, the difference
  in flow velocity and pressure on opposite sides
  also increases, and LIFT FORCE IS GENERATED.
• Lift force will decrease and drag force will
  increase past a Critical Max Angle.
• Objects stall as a result of increasing drag
  force and the loss of lift force at a specific
  angle.

24
Lift Force Acting on Shapes and Surfaces-Continued

• The best angle of attack depends on the shape of
  the object and the relative speed and direction
  of fluid flow.

25
Hydrodynamics

• Skin Friction
• Profile Drag
• Wave Drag
• Propulsive Drag/Lift Force

26
Magnus Effect

• A lift force caused by a spin on an object is
  called a MAGNUS FORCE.
• Ball Example