Laminar Flow

Laminar flow is a concept in physics that refers to the smooth flow of fluid. The fluid can be in liquid or gaseous form. Laminar flow is the opposite of turbulent flow, which is a rough, choppy movement. The flow of fluids, whether liquid or gas, is dependent on the speed, or velocity, and the thickness, or viscosity, of the substance. Laminar flow can occur naturally or can be created by increasing the resistance through which the substance is flowing. This concept is an important factor in the movement of vehicles, especially aircrafts. The force can also be harnessed to help control contaminants in laboratories and maintain “clean rooms” for scientific work.

Background

The concept of laminar flow has its origins in the study of fluid dynamics. For centuries, scientists have noticed that there were ways to affect the flow of water, which, in turn, would affect how boats and other objects floated. Ancient Greeks such as Archimedes studied flotation and devised rules about the flow of water, but it would be many years before any significant study was done into how and why water is sometimes smooth and sometimes choppy and rough. Leonardo da Vinci conducted extensive experiments involving the flow of water and of air—he drew models of flying devices that mimicked the action of birds’ wings—and accurately recorded descriptions of the effect of friction and drag on experimental vehicles.

In the seventeenth century, Isaac Newton developed several laws that related to this concept, including his laws of motion and of viscosity. Several mathematicians built on his theories over the next century, but they mostly created formulas using a nonexistent perfect fluid. Their theories were founded on the idea of a fluid that did not experience any friction. Since no such fluid exists in the real world, their theories were of limited use. Later in the eighteenth century, scientists began experimenting with real-world fluids and developed the science of hydraulics.

During the nineteenth century, scientists made several key advancements in the study of fluid dynamics. William Froude and his son, Robert, created models to test water flow. John William Strutt and Osborne Reynolds made key discoveries as well. Reynolds conducted a famous pipe experiment that revealed a quantity later known as the Reynolds number, which can be used to predict how fluids will flow in different situations.

In the early days of the twentieth century, German engineer Ludwig Prandtl determined one of the key theories of fluid dynamics. His paper, published in 1904, described how fluids (whether liquid or gaseous) have several layers. These include a boundary layer and another outer layer. His boundary-layer theory became an important part of determining how fluids flow. It also gave laminar flow its name; laminar comes from laminae, which is derived from a Latin word meaning “thin layer.”

Overview

According to the principles of Prandtl’s theory, laminar flow—sometimes called streamline flow—occurs when a liquid or gaseous fluid moves in parallel layers without any disruption between them. This state is most likely to occur when the fluid is flowing at a slower speed. Consequently, anything that interferes with this slow flow and increases the speed of the fluid, such as moving from a wider river to a narrower stream, will increase the speed and most likely disrupt the fluid’s smooth flow.

This is also where the Reynolds number comes into effect. The Reynolds number has no dimension and refers to the ratio between how fast the liquid moves and how thick or viscous it is. A fluid that is both very thick and very slow-moving—such as some oils—will have a low Reynolds number and a smooth flow. A very thin liquid that is moving quickly will likely be the opposite of laminar or turbulent.

The concept of laminar flow is also important to aeronautics. Where flight is concerned, laminar flow is considered the smooth flow of air over the parts of a plane, rocket, or other aircraft. The front and wings of an aircraft are designed to allow air to flow smoothly over these areas and help provide the lift the craft needs to fly. When the airflow is broken as it moves over the upper surface of the craft, it creates turbulence that interferes with the smooth flight of the vehicle.

The first aircraft intentionally designed with a laminar airfoil to take advantage of this property of fluid dynamics was the P-51 Mustang, which was used by the US military during World War II. The P-51 was so successful because of the properties of laminar flow. The area of fluid (in this case, the gaseous fluid air) forms a very thin boundary layer directly against the wing or any other surface. This area essentially stays with the plane even as it moves. If the layer directly next to that—the outer layer—encounters no interference, it moves smoothly over the boundary layer, and the result is laminar flow. If something interferes with the ability of the outer layer to flow over the boundary layer—as when a suitcase-loaded roof rack is placed on the top of an otherwise aerodynamic car roof—drag is created and turbulence results.

Laminar flow can also be put to work in a laboratory to help control contaminants. Since the 1960s, the Centers for Disease Control and Prevention (CDC) has used the principles of laminar flow to move air in certain laboratories at one speed and in one direction. This helps prevent cross-contamination and keeps conditions sterile. When combined with filtration systems, these laminar flow setups can stop particulates and microorganisms from becoming airborne and spreading throughout the room.

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