Fluid Mechanics Notes eBook
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1.0 INTRODUCTION In general matter can be distinguished by the physical forms known as solid, liquid, and gas. The liquid and gaseous phases are usually combined and given a common name of fluid. Solids differ from fluids on account of their molecular structure spacing of molecules and ease with which they can move . The intermolecular forces are large in a solid, smaller in a liquid and extremely small in gas. Fluid mechanics is the study of fluids at rest or in motion. It has traditionally been applied in such area as the design of pumps, compressor, design of dam and canal, design of piping and ducting in chemical plants, the aerodynamics of airplanes and automobiles. In recent years fluid mechanics is truly a high-tech discipline and many exciting areas have been developed like the aerodynamics of multistory buildings, fluid mechanics of atmosphere, sports, and micro fluids. 1.1 DEFINITION OF FLUID A fluid is a substance which deforms continuously under the action of shearing forces, however small they may be. Conversely, it follows that 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.
Dr. Nagaraj Sitaram, Professor, Civil Department, SBMJCE, Bangalore
Shear force, F y
x Fluid deforms continuously under the action of a shear force yx
dFx f Deformation Rate dA y
Shear stress in a moving fluid Although there can be 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, causing the original shape of the fluid to become distorted. If, on the other hand, the velocity of the fluid is same at every point, no shear stresses will be produced, since the fluid particles are at rest relative to each other. Differences between solids and fluids The differences between the behaviour of solids and fluids under an applied force are as follows i.
For a solid, the strain is a function of the applied stress, providing that the elastic limit is not exceeded. For a fluid, the rate of strain is proportional to the applied stress.
The strain in a solid is independent of the time over which the force is applied and,