Lecture 5 - Sedimentary notes PDF

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Lecture 5: Fundamentals of Particle Settling Boundary Layers: - One of the factors that controls if the flow is turbulent or laminar flow- The nature of fluid molecular motions is to concentrate the drag force and the region of velocitydecrease into a thin layer close to the plate surface known as t...

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Lecture 5: Fundamentals of Particle Settling 1) Boundary Layers: - One of the factors that controls if the flow is turbulent or laminar flow - The nature of fluid molecular motions is to concentrate the drag force and the region of velocity decrease into a thin layer close to the plate surface known as the boundary layer - Boundary layer is very thin - Turbulent flow has significant mixing - The thickness of a boundary layer depends on whether the flow close to the surface is laminar or turbulent - The criterion for determining this is the value of the Reynolds number of the flow close to the surface - At the start the Reynolds number is small due to the fact that x is small (distance from the start of the plate) so more like a laminar flow - As you move away from a plate, x is getting larger, Reynolds number then increases so is more like a turbulent flow - Flow is always laminar where you are very close to the plate — as velocity is slow from the plate, so get a laminar flow in a turbulent flow - Above the boundary layer = the flow is not affected by that particle - Below the boundary layer = the flow is affected by that particle 2) Particle Settling: - There are two regimes of flow, so we expect two equations to describe particle settling – one for each regime - For Low Reynolds number (laminar) = Stokes law — used for Reynolds number < 0.1 - For High Reynolds number (turbulent) - cant actually find the proper settling = Need experiments - Particle Reynolds number normally tells you its about the flow going around the particle - Only use Stokes Law for very small particles, settling at very slow speeds - The motion of the particle causes liquid to be displaced either side of the particle - This particle need to use kinetic energy to move the fluid out of the way — energy loss associated with the particle moving through a fluid — this energy loss is called a drag force - Buoyancy force acts upwards — density is less than the particle and the volume is the same as the particle - Drag force acts upwards - Mass x gravity acts downwards - As speed of particle increases the drag increases — therefore must have a constant velocity as forces are the same — terminal velocity - Drag force will be different in laminar and turbulent flow - For very slow flow around a sphere (very low Reynolds numbers (...