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Fluid Properties and Units

Continuum. All materials, solid or fluid, are composed of molecules discretely spread and in continuous motion. However, in dealing with fluid-flow relations on a mathematical basis, it is necessary to replace the actual molecular structure by a hypothetical continuous medium, called the continuum..

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Fluid Properties and Units

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    1. Fluid Properties and Units CVEN 311

    2. Continuum All materials, solid or fluid, are composed of molecules discretely spread and in continuous motion. However, in dealing with fluid-flow relations on a mathematical basis, it is necessary to replace the actual molecular structure by a hypothetical continuous medium, called the continuum.

    3. Continuum In a continuum, the physical variable at a point in space is the averaged value of the variable in a small sphere. How good is the assumption?

    4. Dimensions and Units The dimensions have to be the same for each term in an equation Dimensions of mechanics are length time mass force temperature

    5. Dimensions and Units Quantity Symbol Dimensions Velocity V LT-1 Acceleration a LT-2 Area A L2 Volume ? L3 Discharge Q L3T-1 Pressure p ML-1T-2 Gravity g LT-2 Temperature T ? Mass concentration C ML-3

    6. Dimensions and Units Quantity Symbol Dimensions Density r ML-3 Specific Weight g ML-2T-2 Dynamic viscosity m ML-1T-1 Kinematic viscosity ? L2T-1 Surface tension ? MT-2 Bulk mod of elasticity E ML-1T-2

    7. Definition of a Fluid a fluid, such as water or air, deforms continuously when acted on by shearing stresses of any magnitude. - Munson, Young, Okiishi what is this?what is this?

    8. Fluid Deformation between Parallel Plates

    9. Shear Stress

    10. Fluid classification by response to shear stress Newtonian Ideal Fluid Ideal plastic

    11. Fluid Viscosity Examples of highly viscous fluids ______________________ Fundamental mechanisms Gases - transfer of molecular momentum Viscosity __________ as temperature increases. Viscosity __________ as pressure increases. Liquids - cohesion and momentum transfer Viscosity decreases as temperature increases. Relatively independent of pressure (incompressible)

    12. Example: Measure the viscosity of water The inner cylinder is 10 cm in diameter and rotates at 10 rpm. The fluid layer is 2 mm thick and 10 cm high. The power required to turn the inner cylinder is 50x10-6 watts. What is the dynamic viscosity of the fluid?

    13. Solution Scheme Restate the goal Identify the given parameters and represent the parameters using symbols Outline your solution including the equations describing the physical constraints and any simplifying assumptions Solve for the unknown symbolically Substitute numerical values with units and do the arithmetic Check your units! Check the reasonableness of your answer

    14. Role of Viscosity Statics Fluids at rest have no relative motion between layers of fluid and thus du/dy = 0 Therefore the shear stress is _____ and is independent of the fluid viscosity Flows Fluid viscosity is very important when the fluid is moving

    15. Dynamic and Kinematic Viscosity Kinematic viscosity (__) is a fluid property obtained by dividing the dynamic viscosity (__) by the fluid density

    16. Density and Specific Weight Density (mass/unit volume) r ___________ density of water: density of air at atmospheric pressure and 15 ?C: Specific Weight (weight per unit volume) g __________________

    17. Perfect Gas Law PV = nRT R is the universal gas constant T is in Kelvin

    18. Bulk Modulus of Elasticity Relates the change in volume to a change in pressure changes in density at high pressure pressure waves _________ ______ __________

    19. Vapor Pressure

    20. Cavitation

    21. Cavitation Damage

    22. Surface Tension Pressure increase in a spherical droplet

    23. Example: Surface Tension Estimate the difference in pressure (in Pa) between the inside and outside of a bubble of air in 20C water. The air bubble is 0.3 mm in diameter.

    24. Outline the solution Restate the goal Identify the given parameters and represent the parameters using symbols Outline your solution including the equations describing the physical constraints and any simplifying assumptions

    25. Viscosity Measurement: Solution

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