Laminar vs. Turbulent Flow: Difference, Examples, and Why It Matters

Laminar flow occurs when the particles in a fluid move in one direction with little or no movement perpendicular to the flow direction. Turbulent flow occurs when fluid particles move perpendicular to the direction of flow, usually in swirls called eddies. Characteristics of the fluid, like flow rate, density, and viscosity, along with the geometry of objects the fluid flows in or around, determine when the flow transitions from laminar and how chaotic the turbulent flow regime is.

This critical fluid flow characteristic impacts everything from the noise a car makes to the fuel efficiency of an aircraft to the speed at which chemicals mix. Although fully laminar flow is theoretically possible, it is relatively rare in real-world applications, so engineers need to predict and manage laminar and turbulent flow in and around the objects they are designing.

Key Terms Used in Flow Characterization

An excellent place to start our look at the difference between laminar and turbulent flow is to lay out some of the critical terms engineers use to describe flow characterization.

Boundary Layer

A boundary layer is a thin layer of fluid next to a surface that the fluid flows past, in which the velocity varies from zero at the surface to the free-stream velocity of the fluid. The viscosity of the fluid creates a no-slip boundary condition on the surface. Free-stream velocity , running length, viscosity, and the amount of turbulence in the boundary layer determine the thickness of the boundary.

Bulk Velocity

The term bulk velocity refers to the overall average velocity of a fluid. It is calculated by measuring the volume flow rate divided by the cross-sectional area of the measurement plane.

Eddy

An eddy is the movement of fluid particles that deviates from the overall fluid flow direction. Eddies can be a swirl, a vortex, or simple fluctuations around the dominant flow direction.

Flow Separation or Boundary Layer Separation

Flow separation occurs when boundary layer flow moves away from a surface when the velocity next to the surface reverses due to an adverse pressure gradient.

Free Stream

The free stream is the area of flow outside of boundary layers.

Internal and External Flow

Internal flow describes situations when the fluid is bounded by a solid on all sides perpendicular to the flow direction. External flow describes fluid flowing around an object. Fluids behave differently if they flow inside something, like pipe flow, or around something, like an airplane wing.

Navier-Stokes Equations

The Navier-Stokes equations are a set of equations that describe the flow of viscous fluids. Computational fluid dynamics (CFD) programs combine the Navier-Stokes equations with additional equations to predict the behavior of most fluid flow situations.

Flow Regime

Flow regime, or flow pattern, is a description of a flow’s structure and behavior. Flow regime is determined by characteristics such as velocity, viscosity, phase, and laminar or turbulent flow.

Reynolds Number (Re)

The Reynolds number is a dimensionless value that characterizes the ratio between inertial and viscous forces in fluid flow. The value came from Osborne Reynolds’ experiments to understand how water flows in a pipe and when it transitions from laminar to turbulent. The ratio of internal and viscous forces strongly predicts when flow will transition from laminar to turbulent.

The Reynolds number equation is:

ρ = density of the fluid (kg/m3)

u = flow velocity (m/s)

L = characteristic dimension, such as pipe diameter, hydraulic diameter, equivalent diameter, chord length of an airfoil (m)

μ = dynamic viscosity of the fluid (Pa·s)

v = kinematic viscosity (m2/s)

Velocity Profile

A velocity profile is the velocity of fluid flow along an arbitrary straight line or flat plane. The line or plane is usually oriented perpendicular to the bulk flow direction or a surface. Velocity profiles show the velocity gradient in a boundary layer and are used to calculate mass flow rates.

Viscosity

The viscosity of a fluid is a measure of the resistance to deformation at a given rate. It characterizes the internal friction forces between parallel layers of fluid. 

What is Laminar Flow?

Laminar flow is a flow condition in which fluid particles follow smooth and steady streamlines with little movement of particles between adjacent layers. Laminar flow is characterized by relatively low Reynolds numbers because the viscous forces are much larger than the velocity. The type of fluid and fluid properties, along with the geometry and surface roughness of any solid objects that fluid flows around or through, contribute to how long the flow will remain laminar. The velocity profile of a laminar flow monotonically increases from zero to the free stream velocity through the boundary layer.

What is Turbulent Flow?

Turbulent flow is characterized by chaotic variations in the magnitude and direction of fluid particle velocity and amplitude of pressure. Turbulent flow is characterized by high Reynolds numbers, in which the velocity and characteristic dimension are much higher than the viscous damping of the fluid. How high depends on the fluid properties and the object the fluid is flowing in or around. Turbulent flow is highly irregular and almost impossible to predict or measure in detail. For this reason, engineers treat turbulence from a statistical perspective

Why Is It Important to Understand Both Laminar and Turbulent Flow?

Engineers care about laminar and turbulent flow because each flow regime impacts the physics of the fluid they are working with. Sometimes you may want to keep your flow laminar for as long as possible, and other times you may want turbulence. Here are a few situations engineers should be aware of and what role different flow patterns play.

Heat Transfer

The movement of heat from an object to a fluid heavily depends on the flow velocity both against and perpendicular to the surface. High velocities and turbulence increase the heat flux from an object into the fluid around it. Engineers often design to increase turbulence in heating and cooling situations to maximize heat transfer between an object and a fluid.

Lift

Lift is a net force on one side of a solid object with fluid flowing around the object caused by a pressure rise on one side and a pressure drop on the other. Turbulence inside the boundary layer can increase the pressure differential, but high levels of turbulence in the free stream can decrease lift or cause unwanted oscillating forces on the object generating lift.

Drag

Drag is a force exerted by fluid in or past an object, applied in the direction of flow. In most cases, turbulence in a boundary layer increases the drag on an object. Designers spend a lot of time with simulation and wind tunnels, tweaking the aerodynamics of vehicles and aircraft to minimize drag.

Noise

When airflow around an object transitions to turbulent, the eddies can create sound waves in the audible range. Noise is not only wasted energy, but it can also be loud enough to become annoying or even unhealthy.

Mixing

One area in which turbulence can be a good thing is in mixing. In combustion, water treatment, and chemical manufacturing, engineers design systems in which the chaotic flow of turbulence mixes different fluids to improve the speed and efficiency of chemical reactions. 

Modeling Laminar and Turbulent Flow in Simulation

Laminar flow is well characterized by solving the Navier-Stokes equations in a general-purpose CFD tool like Ansys Fluent fluid simulation software or a tool focused on rotating machinery like Ansys CFX software. The same equations can predict turbulent flow, but the computational requirements for direct numerical simulation of turbulent flow are not practical. The number of equations needed to model an eddy accurately is on the order of the Reynolds number cubed. Because of this, users add additional equations to a model that approximates turbulent behavior with enough accuracy to answer engineering questions.

Ansys offers several resources, including free online courses on properly modeling laminar flow and turbulent flow. Here are some basic guidelines to create a strong foundation:

Modeling Laminar Flow in CFD

Modeling laminar flow is straightforward in a CFD tool. The most important task in modeling laminar flow is having sufficient accuracy to predict when the flow will transition to turbulent flow. Your mesh should include sufficient resolution in the boundary layers to capture the velocity profile accurately. It is also important to specify an accurate wall roughness and capture the surface geometry with sufficient resolution.

Predicting Laminar-turbulent Transition Flow in CFD

Although looking at the range of Reynolds numbers in a model can guide you in deciding where the transitional flow occurs, the suggested ranges refer to idealized cases that rarely arise in real applications. If you assume turbulent flow along the entire length of a model, you can over-predict the shear stress on the wall. That is why Ansys has pioneered the numerical prediction of transition flow based on the concept of local-correlation-based transition modeling (LCTM). To get this right, use a turbulence model that includes equations that accurately predict transitional flow.

Reynolds-averaged Navier-Stokes (RANS) Models for Turbulence

There are two classes of simplified equations for turbulent flow. The first class is RANS models. This approach decomposes flow quantities into their fluctuating and time-averaged components. RANS models are approximations based on empirical studies. There are many RANS models available. Here are some of the more commonly used RANS models:

• Spalart-Almaras (SA): A simple single equation model used in external aerodynamics.
• Two-equation Models: A family of RANS models based on the older k-ε and k-⍵ formulations. The shear stress transport (SST), baseline (BSL), and generalized k-⍵ (GEKO) models are used independently or in combination to predict turbulent flow in industrial applications accurately.

Some best practices for using RANS models are:

• Represent geometry as accurately as possible
• Keep inlets and outlets in areas of well-defined flow
• Make sure your fluid properties like density and viscosity are accurate
• Place fine resolution in boundary layers and transition gradually to coarser mesh regions
• Use iterative mesh refinement to converge on an accurate grid
• Use accurate boundary conditions
• Use second-order numerics if feasible
• Iterate on different RANS models or parameters within a model to match experimental data

Scale-resolving Simulation (SRS) Models for Turbulence

The second class of turbulence modeling, scale-resolving simulation, solves for turbulent fluid flow over time and space rather than averaging across time. Most applications of SRS use large eddy simulation (LES) models to solve for larger eddies while modeling the smaller eddies. LES models have been improved and validated over some time. They require more cells and longer runtimes compared to RANS models.

Increases in computing capability, especially the use of GPUs, enable the use of SRS models for industrial flows with a variety of SRS/RANS hybrid models, including:

• Scale-adaptive simulation (SAS)
• Detached eddy simulation (DES)
• Shielded detached eddy simulation (SDES)
• Stress-blended eddy simulation (SBES)
• Embedded LES (ELES)

Best practices for correctly using SRS models, especially LES models, are very different from those for RANS models. It is especially important to keep low-aspect ratio cells as turbulence eddies need to be resolved in all three space directions. In addition, strict time-step restrictions apply to ensure a proper time resolution of the turbulence field. Finally, LES quality strongly depends on the availability of specialized numerical treatments to minimize the impact of numerical dissipation.

Learn more about Fluent software’s wide range of turbulence models, including the industry-leading generalized k-ω (GEKO) model.