# Liquid Glass: The characteristics of Laminar Flow

Have you ever looked at flowing water, and it almost looks like glass because it is flowing so smoothly? That is called Laminar Flow. This flowing water is in contrast to turbulent flow. This type of flow can be measured in most liquids and gases (some are easier to calculate more accurately). Just as many factors (viscosity, velocity) affect the type of flow, the kind of flow affects the measurement/accuracy of many factors (flow rate, energy requirements). Knowing what type of flow you have/will have is essential for providing a variety of efficiencies and accuracies.

#### What is Turbulent Flow?

Laminar flow is a fluid flow type in which the fluid particles move in parallel layers, with each layer moving in the same direction. This type of flow occurs when a fluid flows in a smooth, continuous stream without mixing the layers.

Turbulent flow is an uneven flow of fluid characterized by changes in pressure and speed.

#### Determining Laminar vs. Turbulent

Determining whether a flow is laminar or turbulent is as easy as looking at it.

If the flow is:

• consistent and appears glass-like, it is laminar.
• changing direction or moving chaotically, it is turbulent
• somewhere in the middle, it is transient.

However, what if the flow is inside a closed container (cannot be observed) or is theoretical (in schematics for a plant)? In this case, you will need to know a few characteristics of the situation to determine the flow type.

#### Reynolds number

You can use the Reynolds number to determine if a flow will be laminar. The Reynolds number is the ratio of the inertia force to the viscous/friction force. The lower the number, the more laminar; the higher the number, the more turbulent. Different versions of the formula are used, depending upon available data for the calculation. If you know the kinematic viscosity, you can simplify the formula. If you don’t, you will need to know additional factors to solve the formula.

When determining the Reynolds number for a pipe or duct, you would use the hydraulic diameter instead of the length unit.

If the math seems daunting, check out The Engineer Toolbox’s free Reynolds Number calculator!

If the Reynolds number is:

• < 2300 = laminar
• >2300 AND <4000 = transient
• > 4000 = turbulent

Based on these formulas, you can conclude that a higher viscosity, lower velocity, and smaller flow lead to laminar flow. Furthermore, the opposite: lower viscosity, higher velocity, and larger flow lead to turbulent flow.

Obstacles can contribute to turbulent flow, but flow can remain parallel if the flow is already laminar. For example, rocks can break up the flow of a river, causing turbulence. However, sticking your finger in the stream of water in a fountain might only divert the flow instead of creating turbulence.

#### Industry Applications

Laminar flow and turbulent flow have different applications in the power generation industry.

Laminar flow is best for fluids/gases flowing through pipes/ducts. This flow requires less energy and is more consistent; thus, the flow rate is more accurately predicted.

Turbulent flow is best in tanks and situations where fluids need to mix. In laminar flow, the fluid remains parallel and wouldn’t be ideal as it does not mix.

Turbulent flow is more prevalent and easier to achieve, but how can you help to ensure laminar flow?

Here are just a few ways to limit turbulence in your flow:

• Prevent/limit restrictions such as valves, elbows, and couplings
• Do not mount the flow meter directly behind a restriction or add a turbulence filter between a valve and the flow meter
• Use the correct pipe lengths to limit speed

Below is a graph showing how the Reynolds number of water flow changes at different velocities depending on the hydraulic diameter in a steel pipe schedule 40. As you can see, different diameters will require different velocities to achieve a particular Reynolds number and vice versa.

Courtesy of The Engineer ToolBox

#### Additional Applications

Laminar and Turbulent flow isn’t just practical knowledge in an industrial setting. Laminar flow is the difference between life and death in a medical environment. Your blood must be laminar to avoid heart beating disruptions. In addition, intravenous fluids must be administered via laminar flow to provide consistent, uninterrupted medications to a patient.

#### Everyday Examples of Laminar Flow:

1. Blood flow
2. The air around a flying plane (turbulent air around the plane causes the infamous plane turbulence)
3. Dripping Honey (the high viscosity keeps it dripping smoothly)
4. Some Rivers and Canals (with little obstacles and waves)
5. Fountains
6. Faucets (when a faucet is turned on just slightly, the flow is usually at the correct velocity to present as laminar)
7. Smoke (short distances from disbursement)

#### Conclusion

Whether you know what laminar flow is or not, it has affected several aspects of your life, from the second you were born, from your consistent blood flow to your heart to the water flowing through your pipes, to the fluids producing power generation in your neighborhood. Laminar or Turbulent flow can determine life or death, consistent water pressure, or even uninterrupted/efficient power delivered to your home. Moreover, identifying and controlling the flow type is paramount in various applications including power generation.

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