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Laminar and Turbulent Flow Fluid Mechanics Laminar and Turbulent Flow In fluid mechanics the difference between laminar and turbulent flow is crucial as these two flow regimes behave differently Laminar flow is characterised by smooth and even flow with minimal mixing between layers On the other hand turbulent flow is characterised by chaotic movement and contains swirling regions called eddies resulting in significant mixing of the fluid The Reynolds number is a parameter used to predict if flow will be laminar or turbulent This non dimensional parameter helps us understand the relative importance of inertial and viscous forces in determining the flow regime If viscous forces dominate flow is more likely to be laminar while if inertial forces dominate flow is more likely to be turbulent Most flows in the world around us are turbulent The flow of smoke out of a chimney and air behind a high speed car are usually turbulent However the flow of blood through vessels is mostly laminar which is fortunate as turbulent flow would require the heart to work much harder to pump blood around the body Pressure Drop in Pipe Flow In pipe flow one thing we are particularly interested in is pressure drop Across any length of pipe there will be a drop in pressure due to the frictional shear forces acting within the fluid The pressure drop in turbulent flow is much larger than in laminar flow which explains why the heart would have to work harder if blood flow was mostly turbulent The Darcy Weisbach equation is used to calculate pressure drop along the pipe For laminar flow the friction factor can be calculated easily as it is just a function of the Reynolds number However for turbulent flow calculating the friction factor is more complicated and defined by the Colebrook equation Engineers use the Moody diagram to look up friction factors for different flow conditions The Laminar Sublayer Even for turbulent flow viscous forces can be significant in the boundary layers that develop at solid walls The laminar or viscous sublayer is a very thin area close to the wall where viscous forces dominate and flow is essentially laminar Its thickness decreases as Reynolds number increases Above the laminar sublayer there is the buffer layer wher e both viscous and turbulent effects are significant and above the buffer layer turbulent effects are dominant For a given roughness the friction factors converge to a constant value meaning that flow in smooth pipes has a lower friction factor and smaller pressure drop than flow in rough pipes Surface roughness is important for turbulent flow because it introduces disturbances into the flow which can be amplified and result in additional turbulence This video is sponsored by Brilliant the best place to learn math and science online At high Reynolds number friction depends only on the relative roughness of the surface The thickness of the laminar sublayer is extremely thin making the effect of surface roughness governing Modelling turbulent flow through a pipe is simple but most scenarios are more complex and analysis of turbulent flow is complicated The complexity is due to the turbulent eddies that contain a lot of kinetic energy and the energy cascade makes the analysis of turbulent flow challenging Capturing the wide range of length scales associated with the turbulent eddies is one of the main challenges To analyse the flow we use experimentation or numerical methods including computational fluid dynamics CFD that involves solving the Navier Stokes equations CFD uses three main techniques to simulate flow including direct numerical simulation large eddy simulation and the Reynolds averaged Navier Stokes technique These techniques differ mainly in how they treat turbulence on different scales Experience and intuition are essential in determining the best techniques and models for a particular problem Engineering intuition is crucial when troubleshooting problems in the real world Brilliant is a math and science learning website and app that has courses covering a wide range of topics The scientific thinking course is great for engineers it intentionally ditches the math and puts emphasis on concepts using fun puzzles to develop engineering intuition

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UA CHEM 380 - Laminar and Turbulent Flow

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