Laminar flows are smooth and streamlined, whereas turbulent flows are irregular and chaotic.
A low Reynolds number indicates laminar flow while a high Reynolds number indicates turbulent flow. The flow behavior drastically changes if it is laminar vs. turbulent.
In complex systems, the analysis of laminar and turbulent flow becomes crucial for efficient operational design.
The understanding of fluid flow behavior is critical when analyzing its effect in the design and simulation of fluid-dependent systems. For accurate fluid modeling in computational fluid dynamics (CFD), engineers and designers need to have a deeper understanding of flow paths and velocities within systems. In this article, we will discuss the differences between laminar vs. turbulent flow and learn how Reynolds number can sometimes be used to predict the onset of turbulence.
Laminar vs. Turbulent Flow
Fluid flow within a system can be either laminar or turbulent. The attributes of each flow type are different and the understanding of each is necessary for the design of the fluid system.
Consider running water through a hose. You may observe the water flowing in a straight line while the amount and rate of flow remain low. This smooth and streamlined nature of fluid motion is considered a laminar flow. Generally observed in viscous or low-velocity fluids, in laminar flow, the properties of the fluid at each point of the flow remain constant--including velocity and pressure. If the flow path is divided into several infinite layers, the fluid layers in laminar flow stay parallel to each other without mixing or disrupting the path of the adjacent layers. This very nature of the flow makes the use of laminar, low-velocity air circulation systems advantageous in medical or pharmaceutical labs and industrial warehouses, as this type of flow ensures smooth ventilation and prevents the accumulation of particulate matter or bacteria.
Turbulent flow is a common occurrence and can be observed in the flow of rivers, smoke coming out of exhaust pipes, or even experienced during turbulence in aircraft. This turbulent effect is due to the irregularity or obstruction on the path of the fluid flow. Unlike laminar flow, the fluid layers in turbulent flow can cross paths due to the continuous change in the magnitude and direction of the flow. Eddies or swirls can be observed in turbulent flow, and the apparent erratic flow behavior makes the analysis of turbulent flow a challenge. However, despite the challenges, turbulent flow analysis is important for industries, as most flows observed are turbulent. Turbulence analysis can help in the effective design of fluid distribution or mixing systems, support analysis of structures such as bridges or wind tunnels, and help automotive industries design fuel-efficient vehicles and aircraft. Turbulence prediction can be done to some extent by Reynolds number.
Using Reynolds Number to Determine Flow Pattern
Using numerical analysis to classify between laminar vs. turbulent flow can be tricky. However, it holds greater importance in industrial as well as academic settings. Reynolds number is an important dimensionless parameter that identifies the behavior of fluid based on attributes like viscosity or velocity of the fluid. Accordingly, the value of Reynolds number (Re) can be expressed as:
ρ is the density of the fluid
V is the fluid velocity
D is the hydraulic diameter (of pipe, tube, or duct).
μ is the fluid viscosity
Important takeaways from the Reynolds number calculation include:
- ☐ A flow is considered laminar if the Reynolds number is up to 2300. Viscous force is more prominent due to the slower flow rate.
- ☐ A flow is considered turbulent if the Reynolds number is greater than 3500. A faster and irregular flow path maximizes the inertial force in the system.
- ☐ Flow regime for the Reynolds number between 2300 to 3500 is considered transitional flow.
Identifying Different Flow Regimes for Effective CFD Analysis
Most industrial real-world systems are designed to involve either laminar or turbulent flows, so predicting the flow regime is paramount for effective CFD analysis and system design. However, analytical investigation of laminar vs. turbulent fluid flow regimes can be challenging, especially for systems with complex geometrical designs. This may require simulation across a range of Reynolds numbers and eddies in a fluid flow interface.
One solution is using a CFD interface that allows you to characterize and select the right flow regime. With the simulation capabilities of the Omnis platform, running CFD simulations in complex systems is easier. This flexible and robust tool facilitates the modeling of all flow regimes at the speed and fidelity levels required.