A temperature rise decreases the viscosity and density of liquids. The more viscous, or less slippery, a fluid the harder it is to get shearing between layers. The high viscosity prevents rapid velocity changes occurring between layers. The sub layer in viscous fluids is thicker than in low viscosity fluids.
At low speeds the whole flow across a pipe is laminar and the fluid slides over itself. As the speed becomes faster eddies start to form and cross the fluid layers. A transition from laminar to turbulent flow develops.
At still higher velocities the flow in the core of the pipe becomes turbulent with swirling eddies throughout. Figure 2 shows where the various flow regions occur at a tank nozzle.
The laminar sub layer is always present against the pipe wall. But as the velocity rises the energetic swirling eddies begin to impact more deeply and the sub layer begins to thin. At still higher velocities the sub layer thins further and the taller roughness peaks stick into the turbulent region. This minimises the losses along the pipe. There is a very much greater loss of pressure in turbulent flow.
The pipe system designer has to strike a practical balance between increasing the pipe diameter to reduce energy loss and keeping the diameter small to lower installation costs. Elbows, bends, reducers, branch tees and flanges all cause individual minor pressure losses.
When a fluid is forced to change direction, or go around a disruption, eddies are produced. These new twisting eddies interfere with the flow pattern and produce additional pressure losses. The greatest pressure losses occur at sudden diameter and direction changes. Most of the loss occurs in the downstream eddy wake. When designing a pipe run gradually blend-in changes to the flow pattern. Unlike a liquid a gas is compressible and can be squashed. When a gas is compressed the density increases — as the pressure is released the density decreases.
Gas flowing into a pipe starts at a pressure, temperature and associated density. The frictional losses along the pipe cause a pressure loss. The "magic" is that air passing over the surface of the paper causes reduced pressure from above on the paper. Normal atmospheric pressure below the paper pushes it upward. This simple demonstration also illustrates the principle on which airplanes fly.
Air flying over the wings of the airplane produces a lifting effect from below on the wings. Boundary layer effects. Bernoulli's principle works very well in many cases. But assuming that fluids have no viscosity, as Bernoulli did, does introduce some errors in real life. The reason for these errors is that even in fluids with very low viscosity, the fluid right next to the solid boundary sticks to the surface.
This effect is known as the no-slip condition. Thus, however fast or easily the fluid away from the boundary may be moving, the fluid near the boundary has to slow down gradually and come to a complete stop exactly at the boundary.
This effect is what causes drag on automobiles and airplanes in spite of the low viscosity of air. The treatment of such flows was considerably simplified by the boundary layer concept introduced by German physicist Ludwig Prandtl — in According to Prandtl, a fluid slows down only in a thin layer next to the surface.
This boundary layer starts forming at the beginning of the flow and slowly increases in thickness. It is laminar in the beginning but becomes turbulent after some period of time.
Since the effect of viscosity is confined to the boundary layer, the fluid away from the boundary may be treated as ideal. Shape and drag. Moving automobiles and airplanes experience a resistance or drag due to the viscous force of air sticking to their surface.
Another source of resistance is pressure drag, which is due to a phenomenon known as flow separation. This happens when there is an abrupt change in the shape of the moving object, and the fluid is unable to make a sudden change in flow direction and stay with the boundary.
In this case, the boundary layer gets detached from the body, and a region of low pressure turbulence or wake is formed below it, creating a drag on the vehicle due to the higher pressure in the front. That is why aerodynamically designed cars are shaped so that the boundary layer remains attached to the body longer, creating a smaller wake and, therefore, less drag. There are many examples in nature of shape modification for drag control.
The sea anemone, for instance, with its many tentacles, continuously adjusts its form to the ocean currents in order to avoid being swept away while gathering food. Fluorescent Light ». Fluid Dynamics forum.
Words to Know Boundary layer: The layer of fluid that sticks to a solid surface and through which the speed of the fluid decreases. Compressibility: The property that allows a fluid to be compressed into a smaller volume. The gas most commonly encountered in everyday life is air; therefore, scientists have paid much attention to its flow conditions. Wind causes air to move around buildings and other structures, and it can also be made to move by pumps and fans.
One area of particular interest is the movement of objects through the atmosphere. This branch of fluid dynamics is called aerodynamics, which is "the dynamics of bodies moving relative to gases, especially the interaction of moving objects with the atmosphere," according to the American Heritage Dictionary.
Problems in this field involve reducing drag on automobile bodies, designing more efficient aircraft and wind turbines, and studying how birds and insects fly. Generally, fluid moving at a higher speed has lower pressure than fluid moving at a lower speed. This phenomenon was first described by Daniel Bernoulli in in his book " Hydrodynamica ," and is commonly known as Bernoulli's principle.
It can be applied to measure the speed of a liquid or gas moving in a pipe or channel or over a surface. This principle is also responsible for lift in an aircraft wing, which is why airplanes can fly. Because the wing is flat on the bottom and curved on the top, the air has to travel a greater distance along the top surface than along the bottom.
To do this, it must go faster over the top, causing its pressure to decrease. This makes the higher-pressure air on the bottom lift up on the wing. Scientists often try to visualize flow using figures called streamlines, streaklines and pathlines.
McDonough defines a streamline as "a continuous line within a fluid such that the tangent at each point is the direction of the velocity vector at that point. A streakline, according to McDonough, is "the locus [location] of all fluid elements that have previously passed through a given point. However, in the case of turbulent or unsteady flow, these lines can be quite different. Most problems in fluid dynamics are too complex to be solved by direct calculation.
In these cases, problems must be solved by numeric methods using computer simulations. This area of study is called numerical or computational fluid dynamics CFD , which Southard defines as "a branch of computer-based science that provides numerical predictions of fluid flows. Small changes at the beginning can result in large differences in the results. The accuracy of simulations can be improved by dividing the volume into smaller regions and using smaller time steps, but this increases computing time.
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