Exploring Fluid Dynamics; Laminar, Turbulent, and Transitional Flow

The study of fluid dynamics describes the laminar, turbulent, and transitional flow of liquids and gasses. If water flowing through a plumbing system maintains a constant velocity at any point, laminar flow exists. At any point in a pipe, the motion, speed, and direction of the water remain consistent.

The term “laminar flow” also describes how flowing water breaks down into a series of concentric layers that move at different speeds. Although the layers may move at different speeds, no disruptions to flow exist. Water flowing within a pipe has the greatest velocity in its innermost layer. As the layers move farther from the center layer, each has a progressively slower velocity. The outer layer contacts the pipe surface and has no velocity. Friction increases between each layer throughout the stream of water and a portion of the kinetic energy found within the flowing water dissipates into heat energy.

Water Properties Contribute to the Transition from Laminar Flow to Turbulent Flow

In contrast to the properties of water in laminar flow, the properties of water in turbulent flow constantly change. For example, the motion, speed, and direction of water caught in turbulent flow constantly change.

Transitional flow mixes the characteristics of laminar and turbulent flow. Water caught in a transitional flow has constant changes in motion, speed, and direction at the center of the flow while the motion, speed, and direction remains constant at the edges.

The transition from laminar to turbulent flow water occurs because of the properties of water. Of those properties, viscosity, density, and velocity have the greatest impact on flow patterns. 

• Viscosity measures the internal friction of a liquid or its resistance to flow. Thicker liquids have a molecular structure that increases the internal friction. The molecular structure of thinner liquids--such as water--decreases the resistance to flow. Along with varying the thickness of the liquid, viscosity also varies with temperature. Increases in temperature causes the average speed of molecules in water to increase and attraction between molecules to decrease. As a result, viscosity decreases with increasing temperature.

• While density represents the weight of water relative to a specific volume, the density of water can change with temperature or if the water contains dissolved solids.

• Velocity is the speed of the water flow. Because a higher velocity causes a greater flow rate, several relationships exist. The flow rate equals the volume of water passing by the same location through an area during a specific period of time. If the flow remains constant but the area around the flow decreases, the velocity increases in proportion to the decrease in area. As a result, the flow rate remains directly proportional to the magnitude of speed and the size of a pipe.

Forces Cause the Transition to Turbulent Flow

As water flows in pipes, through pumps, around shapes, and within fluids, a positive frontal force pushes against the object, the drag occurs along the sides of an object, and negative pressure exists in the downstream side of fluid movement. Each instance illustrates the types of forces that act on liquids or the forces exerted by liquids.

Each of those forces become apparent as water flows in pipes, through pumps, around shapes, and within fluids. 

• Cohesion describes the attraction between water molecules. Because no water molecules occur above the surface of water, a stronger bond exists in two different ways. Adjacent molecules at the surface bond to one another. Strong bonds also exist between molecules at the surface and just below the surface.

• Surface tension occurs because of cohesion. Working as an inward net force that holds the surface layer of water together, cohesion causes the surface of water at rest to contract to the smallest possible surface area. Increases in surface tension increase the contraction and the resistance of the surface. As a result, the surface layer becomes a barrier between the atmosphere and water.

• Along with cohesion, water also has adhesive properties. Because water molecules have a positive end and a negative end, the molecules attract to the opposite charge of molecules that make up other substances. As an example, water adhesion allows raindrops to stick to a variety of surfaces.

• Shearing stress refers to the impact of gravity, pressure differences, and other forces on laminar water flow. Applying shear forces--no matter the size--to resting water causes the liquid to continuously deform or change. In contrast, a moving liquid resists shear forces.

While the shear of a fluid has a directly proportional relationship to the applied force, it is inversely proportional to viscosity. For laminar flow, the amount of shear stress depends on viscosity. Density determines the amount of shear stress for turbulent flow.

Performance Depends on Flow

As water flows through a pump, the pumping action increases fluid velocity and mixes the laminar flow layers into vortexes and eddies that define turbulent flow. Water moving at higher speeds causes instability within laminar flow. Increasing the speed causes the transition from laminar flow to transitional flow and finally to turbulent flow. Rather than velocity occurring in the direction of flow, velocity develops in different directions.

Within those vortexes and eddies, pressure and velocity vary in space and time. The changes in pressure and velocity combine with random movements and allow greater amounts of fluid to contact the inner surfaces of the pump and result in increased drag. Changes in pressure and temperature also change the viscosity of water. At moderate to high temperatures, the cohesion of water begins to deform with changes in pressure. Lower water temperatures cause increased viscosity, more resistance to flow, and more friction.

Hard water adds a greater negative impact on performance. Changes in pressure allow the buildup of calcium carbonate, calcium sulfate, strontium sulfate, barium sulfate, silica, silicates, and concentrations of minerals that exceed the solubility of water. Because the boiling point for hard water changes because of the mineral content, the viscosity also changes. As the accumulation of calcium and magnesium minerals increases, the water has a higher density than pure water, sinks to the bottom, and becomes more stationary. In addition, a relationship exists between density and viscosity. Water that has higher mineral content has higher viscosity and decreased flow.

The decrease in solubility allows alkaline, non-alkaline, and silica-based scale to form on surfaces that contact water. Scale deposits in residential, commercial, and industrial water systems restrict the flow of water, plug nozzles, and negatively impact the efficiency of pumps and heat exchangers. The accumulation of scale pushes equipment to work harder, increases energy use, and reduces performance.

 Magnation Water Technologies Use Turbulent Flow

The products produced by Magnation take advantage of water properties, physical forces, and turbulent flow to condition water. Applying passive magnetic and electromagnetic water conditioning causes water to gain an electrical charge and the agitation needed to break apart weakly bonded mineral clusters. Improved water flow carries calcium away and prevents minerals from adding to scale deposits. In addition, the application of strong magnetic fields reduces surface tension and the losses caused by friction. Molded helical grooves and tapering inside Magnation’s products increase pressure and create the turbulent flow needed to separate water from dissolved solids.

To learn more about the advantages offered through the Magnation Water Technologies product line, contact a professional at 888 820 0363 or submit a contact form.