Fluid Mechanics
Fluid Mechanics
2nd Year Mechanical and Building Services
Gerard Nagle Room 387 gerard.nagle@dit.ie
Phone Number: 01 402 2904 Office Hours: Wednesday’s, 2.00pm to 5.00pm
Fluids
In every day life, we recognise three states of matter, Solid, Liquids and Gas. Although different in many respects, liquids and gases have a common characteristic in which they differ from solids; they are fluids, lacking the ability of solids to offer permanent resistance to a deforming force. Fluids flow under the action of such forces, deforming continuously for as long as the force is applied. A fluid is unable to retain any unsupported shape. It flows under its own weight and takes the shape of any solid body with which it comes into contact. For example;
In the diagram above, deformation is caused by shearing forces, i.e. forces such as F, which act tangentially to the surfaces to which they are applied and cause the material originally occupying the space ABCD to deform to AB’C’D. This leads to the definition A fluid is a substance which deforms continuously under the action of shearing forces, however small they may be. Conversely, it follows If a fluid is at rest, there can be no shearing forces acting, and, therefore, all forces in the fluid must be perpendicular to the planes upon which they act.
Shear Stress in a Moving Fluid
There is no shear stress in a fluid at rest; shear stresses are developed when the fluid is in motion. If the particles of the fluid move relative to each other so that they have different velocities, they cause the original shape of the fluid to become distorted. Usually we are concerned with the flow past a solid boundary. The fluid in contact with the boundary adheres to it and will therefore have the same velocity as the boundary.
So, from the ABCD in the first diagram, which represents an element in a fluid with thickness s perpendicular to the diagram, the force F will act over an area A equal to . The force per unit area is the shear stress
2nd Year Mechanical and Building Services
Gerard Nagle Room 387 gerard.nagle@dit.ie
Phone Number: 01 402 2904 Office Hours: Wednesday’s, 2.00pm to 5.00pm
Fluids
In every day life, we recognise three states of matter, Solid, Liquids and Gas. Although different in many respects, liquids and gases have a common characteristic in which they differ from solids; they are fluids, lacking the ability of solids to offer permanent resistance to a deforming force. Fluids flow under the action of such forces, deforming continuously for as long as the force is applied. A fluid is unable to retain any unsupported shape. It flows under its own weight and takes the shape of any solid body with which it comes into contact. For example;
In the diagram above, deformation is caused by shearing forces, i.e. forces such as F, which act tangentially to the surfaces to which they are applied and cause the material originally occupying the space ABCD to deform to AB’C’D. This leads to the definition A fluid is a substance which deforms continuously under the action of shearing forces, however small they may be. Conversely, it follows If a fluid is at rest, there can be no shearing forces acting, and, therefore, all forces in the fluid must be perpendicular to the planes upon which they act.
Shear Stress in a Moving Fluid
There is no shear stress in a fluid at rest; shear stresses are developed when the fluid is in motion. If the particles of the fluid move relative to each other so that they have different velocities, they cause the original shape of the fluid to become distorted. Usually we are concerned with the flow past a solid boundary. The fluid in contact with the boundary adheres to it and will therefore have the same velocity as the boundary.
So, from the ABCD in the first diagram, which represents an element in a fluid with thickness s perpendicular to the diagram, the force F will act over an area A equal to . The force per unit area is the shear stress