Ch.8 Fluid Mechanics

What is fluid?

- anything that can flow and change shape quickly

What is fluid force?

- force exerted on the object by the fluid as the object is placed in the fluid or is moving relative to the fluid

What is buoyant force?

- upward force that acts on any objects that are placed in a fluid

What is dynamic fluid force?

- force that acts on objects when there is a relative motion between the object and the fluid
- collective force produced by fluid particles around the object

How do you find buoyant force?

- weight = buoyant force
- weight = volume x density

What will affect the weight of the fluid displaced?

- greater the volume and higher the density of the fluid displacement by the object, greater the weight of the fluid displaced

What is the boundary layer?

- a thin layer of fluid adjacent to the object’s surface when object is traveling through the fluid

What happens with the fluid in the boundary layer?

- fluid closest to object moves with the object
- fluid farthest from the object moves with the fluid around it

How is friction created within the boundary layer?

- it is created from the difference in velocity between the object and the fluid around it

What is the laminar flow?

- smooth and regular flow of fluid particles that will follow the contour of the object but eventually separates from the object

What affects the laminar flow following the object’s contour?

- smoother the shape of object, longer it follows object’s contour

What is turbulent flow and how does it develop?

- irregular movement of the fluid particles that occurs in areas of low pressure
- can develop on boundary layer or behind object

What is a wake?

- turbulent flow that develops behind the object

What effects the turbulent flow and size of wake?

- earlier the laminar flow separates from the object, greater turbulent flow and size of wake

How is dynamic fluid force produced?

- fluid particles produce force against the object

What are the components of dynamic fluid force?

- drag force
- lift force

What is drag force?

- acts along the direction of fluid that passes the object
- tends to resist object’s motion

What is lift force?

- acts perpendicularly to the flow of fluid that passes the object
- pushes object in perpendicular direction by turning or bending object’s path in motion

What 2 sources does drag force come from?

- surface drag
- pressure drag

What is surface drag and how is it created?

- friction between the object and the fluid
- created in boundary layer due to difference in velocity of object and the fluid

What is pressure drag?

- difference in pressure between the front and back sides of the object

What is the pressure drag component front of the object?

- collision between the object and the fluid collides results in an area of high pressure

What is the pressure drag component behind the object?

- wake creates an area of low pressure

What effects the pressure drag?

- greater pressure difference, greater pressure drag

What factors affect the drag force?

- relative velocity between fluid and object (greater velocity, greater force)
- characteristics of the object (smoothness, shape)
- characteristics of fluid the object is traveling through

What is relative velocity?

- object’s velocity relative to the fluid velocity

What is an example of relative velocity?

- person not moving (0 m/s) and wind (-5 m/s): 0 m/s - (-5 m/s) = 5 m/s
- person moving (5 m/s) and no wind (0 m/s): 5 m/s - 0 m/s = 5 m/s

What is projected frontal area and what its relationship to pressure drag?

- the cross-sectional area of the object perpendicular to fluid motion
- larger area, greater pressure drag

What is the relationship the object’s shape and pressure drag?

- smoother shape creates smaller turbulence behind it and smaller pressure drag
- rougher surface creates greater friction in boundary layer, and results in greater surface drap

What is a streamline?

- shape that presents very little resistance to a flow of air or water, increasing speed and ease of movement

How does density affect pressure drag and surface drag?

- higher density of fluid, greater pressure drag and surface drag

How does altitude and temperature affect density?

- higher altitude and temperature will result in lower fluid density

What is viscosity?

- measure of resistance to flow (thickness)

How does viscosity affect pressure drag and surface drag?

- higher viscosity, greater pressure drag and surface drag

How does altitude and temperature affect viscosity?

- high temperature, low fluid viscosity

What is drag coefficient?

- a value that quantifies the resistance of an object in fluid due to surface and pressure drag

What is drafting?

- trailing close to another rider to decrease the drag force acting on cyclists

How is lift force produced?

- when object turns the fluid flow (fluid is displaced in one direction, object is pushed in opposite direction

How can drag force and lift force be used to increase flight time?

- need to find optimal balance between drag and lift force to increase flight time

What is the angle of attack?

- angle of object relative to the direction of motion

What does angle of attack affect?

- greater angle of attack = greater lift force = longer flight time (good)
- greater angle of attack = greater drag force = greater resistance moving forward (bad)

What is the magnus effect?

- deflection in the path of spinning object caused by the lift force created by object’s spin
- causes object traveling through air to bend

What affects the magnus effect?

- increased linear and angular velocity = increase in magnus effect