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I'm doing 'Heat and Mass transfer' at college and we're covering the topic on the hydrodynamic and thermal boundary layers.

I have a couple of questions, the answers to which are not given explicitly in any of my text books.

1. During open flow,why does laminar flow eventually have to become turbulentand why does that involve anincrease in thicknessof the layer?

2. Viscosity is due to 1. exchange of molecules from one layer to another in a fluid(momentum exchange) and 2. force of cohesion.

Therefore in laminar flow, where the flow is a lot more organized, and the adjacent layers stick to each-other, there would be larger force of cohesion. However, in turbulent flow, there is a lot of mixing of the molecules, so it would contribute to the create of large viscous forces.

Therefore,is the force of viscosity greater in laminar or in turbulent flow?

3.Heat exchange is greater in the turbulent layer, but so is the force of friction. So in practical life, would we prefer turbulent flow or laminar? ( So far I have formed an impression that the turbulent boundary layer is disadvantageous and unpredictable because its so 'messy' so I'll be really surprised if it turns out to be the better option').

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# The Hydrodynamic and Thermal boundary layers

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