Fluid Survey 2

In the derivation of Bernoulli's formula, the presumption of the inviscid and incompressible flow is used. However in the real case, the viscosity can not be neglect as well as the density from the flow is usually not always frequent. Thus Bernoulli's equation can be not always right. For the lab, it is sensible to believe the flow is inviscid and incompressible. Firstly, the pitot was placed in the centre of the flow. The skin rubbing (effect of viscosity) is inversely proportionate to length. Therefore the effect of viscosity can be neglected inside the pitot. Second of all, the speed in the flow is much lower than the speed of audio under the sonic condition. Consequently , the Mach number is usually low enough to forget the transform of density of the handled volume as well as the controlled quantity is almost incompressible. That is why we could estimate the speed of the stream by Bernoulli's equation and continuity equation.

Because of the viscosity, the internal flow is restricted by the bounding walls and the effect develops during the whole flow. At the inflow region, the stream is nearly inviscid. After that, the boundary layers are developing along the duct which is called developing profile area. This is because the effect of viscosity is growing. On the centre in the duct, there may be an inviscid core stream. When the boundary layers will be merged, the flow is definitely fully developed and the velocity is certainly not affected by viscosity anymore. Meanwhile the stationary pressure diminishes due to the effect of viscosity (friction). The growing area of diffuser produces low velocity, which usually increases the pressure and unfavorable gradient. The fluid can be viscid as well as the boundary part is separated as a result of the spine flow and poor pressure recovery, in the event the angle is definitely large. The separation raises the stream losses. Besides, the larger angle leads to the sooner separation and heavier stream losses. If you have an abrupt enlargement, because of viscosity, huge vortex flow causes the flow losses and...


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