Lecture 2 - marked.pdf - Introduction to...

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Course objective: “Learn to apply Newton’s Laws to Motion of Fluids” What is a fluid? 1. Put a solid in a larger, closed container and it does not change shape 2. A liquid will change its shape to conform to the container, up to the maximum depth of the liquid 3. A gas will completely fill the container, taking on the same boundaries as the container The word fluid is used to denote either a liquid or a gas . 27 Introduction to Fluids (Aerodynamics) – Sec 1.2 *from Çengel and Cimbala, Fluid Mechanics, 4 th edition
Definition of a fluid: “A substance that cannot be in static equilibrium under the action of tangential (shear) stresses” When a shear stress is applied: A solid experiences finite deformation & shear stress is usually proportional to the amount of deformation (strain). When a shear stress (oblique stress) is applied to a fluid, it will experience continuously increasing deformation; shear stress is usually proportional to the rate of deformation . ( Normal stress causes a volume change in a fluid – especially noticeable for a gas – but no further motion after reaching a new equilibrium) 28
29 Stress is defined as the force per unit area . Normal component: normal stress In a fluid at rest, the normal stress is called pressure Tangential component: shear stress A fluid at rest must have zero shear stress Normal stress: lim 𝑑𝑑𝐹𝐹 𝑛𝑛 𝑑𝑑𝑑𝑑 as 𝑑𝑑𝑑𝑑 → 0 𝜎𝜎 = 𝑑𝑑𝐹𝐹 𝑛𝑛 𝑑𝑑𝑑𝑑 Shear stress: lim 𝑑𝑑𝐹𝐹 𝑡𝑡 𝑑𝑑𝑑𝑑 as 𝑑𝑑𝑑𝑑 → 0 𝜏𝜏 = 𝑑𝑑𝐹𝐹 𝑡𝑡 𝑑𝑑𝑑𝑑 *from Çengel and Cimbala, Fluid Mechanics, 4 th edition
At the molecular level: a) Solid: molecules packed closely together in a rigid geometric structure.

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