Laminar vs Turbulent Flow in flow measurement

When it comes to measuring flow, two crucial factors can greatly affect your flow measurement: laminar flow and turbulent flow. These phenomena play a significant role in the behaviour of flow meters and can impact the accuracy of flow measurements. Let's dive into the differences between laminar flow and turbulent flow, explore some tips to minimize the adverse effects of turbulence and learn why laminar flow is important for flow meters.

Understanding laminar flow

Laminar flow is an appearance where fluid particles move in smooth, parallel layers. This orderly movement allows for excellent fluid mixing and reduces energy loss, making it highly desirable in various flow applications. In laminar flow, the fluid's velocity remains relatively constant. This characteristic creates well-defined flow profiles, ensuring consistent and accurate flow measurements. Laminar flow is commonly observed at low flow rates and in situations where flow is unobstructed. Meaning there are not too many obstructions in the flow path like: valves, adapters or couplings.

3 Tips for you

Understanding turbulent flow

Turbulent flow occurs when the flow of a fluid is disrupted, often caused by restrictions like valves or adapters in combination with high fluid velocities. This phenomenon, known as the 'turbulence effect,' leads to chaotic and non-linear flow patterns. Turbulent flow is something you would like to prevent at the inlet of your flow measurement instrument, as it can affect the accuracy of your flow measurement. It is preferable to have a laminar flow just before your flow instrument. However, the instrument itself used as a flow controller, with a valve behind the meter, can cause a turbulent flow again. Not all kinds of flow meters experience this as disadvantageous. Mainly thermal flow meters using the bypass principle are sensitive to this effect. Flow meters based on the Coriolis working principle, ultrasonic flow meters or meters measuring the flow ‘in-line’ (also called Constant Temperature Anemometry) are not affected by turbulence.

Laminar flow versus turbulent flow

How do you know your flow is laminar or turbulent?

In general, it can be said that there are two types of flows: a laminar flow and a turbulent flow. You can see in the picture that laminar flow has been visualized by an experiment using ink in a cylindrical tube. The ink has been injected into the middle of a glass tube through which water flows. When the speed of the water is still low, the ink does not appear to mix with water, the streamlines are parallel; this is called laminar flow.

If the speed of the water increases, a sudden change will occur at a certain speed. The flow completely disrupts, and the water turns homogeneous through the ink. The streamlines are chaotic, not linear anymore, which is called turbulent flow.

Reynolds number: The key parameter

When do flow conditions lead to laminar flow or turbulent flow? This is determined by the Reynolds number (Re). The Reynolds number combines four variables:
•   Diameter of the tube
•   Speed of the fluid
•   Density of the fluid
•   Dynamic viscosity of the fluid

A low Reynolds number (≤ ca. 2300) indicates laminar flow, while a high Reynolds number (≥ ca. 3000) signifies turbulent flow. For values between 2300 and 3000, the flow can be transitional, exhibiting characteristics of both laminar and turbulent flow.

3 Tips to minimize the disadvantageous effects of turbulent flow

When you are using thermal mass flow meters based on the ‘bypass’ sensor principle, you should take these three tips into account:

1) Optimize process geometry;

Avoid unnecessary restrictions like valves, adapters, and elbow couplings in your flow process. These can induce turbulence. If they are necessary, consider using a turbulence filter between such components and the flow meter.

2) Limit flow speed;

Maintain the appropriate pipe length to limit flow speed. Aim for a minimal pipe length of at least 10 times the pipe diameter at the inlet of the instrument and 4 times the pipe diameter at the outlet (for flow meters). Using the correct pipe length helps in transitioning from turbulent to laminar flow.

3) Apply turbulence filters;

Turbulence filters help smooth out the flow and transform turbulent flow back into laminar flow before reaching the sensor. Some flow meters come equipped with integrated turbulence filters, providing an effective solution to counteract turbulence effects.

Why do you want laminar flow?

High accuracy; Laminar flow ensures that flow measurements remain consistent and reliable, leading to higher measurement accuracy. This is crucial in critical applications like chemical processing and pharmaceutical production.
Minimal pressure drop; With its smooth flow pattern, laminar flow minimizes pressure drop across flow meters, reducing energy consumption and optimizing system performance.
Ideal for low flow rates; Laminar flow is particularly desirable in low flow rate applications, where maintaining precision can be challenging in turbulent conditions.

It all depends on the process and application

It depends very much on the application what the consequences are of turbulent flow. In semicon processes for example, particularly in coating processes such as layer deposition, turbulent flow is a no-go! A stable process is essential here. However, in other coating processes, like flame spray techniques, the impact of turbulences will be less due to the high pressure in the flow. If you need advice on choosing the best flow meter for your application, please let us know or download our e-book.

Ask advice

Extended flow path inside a flow meter

Laminar flow vs Turbulent flow