![]() ![]() ![]() As we can see, the critical Reynolds number is not always clustered from 2,000 to 2,500, as is sometimes quoted in guides on CFD simulations. The numbers above are order of magnitude approximations. This produces a wide variety of maximum Reynolds numbers for laminar flow, as shown in the table below.Ĭritical Reynolds number for laminar flowĢ00-2,000, depending on orientation angle The shape of the system will impose some boundary conditions, which will limit the flow rate and thus the magnitude of inertial forces acting on the fluid. Some flows over simple shapes have lower limits on the Reynolds number for laminar flow than others. The transition to turbulent flow for flows over simple shapes is a helpful example to understand how the same transition might arise in a more complex geometry. It is therefore important to understand the limit on Reynolds number for laminar flow in real systems so that flow behavior can be controlled. Most importantly, due to the complex geometry of real systems, a portion of the flow in one region of a system might be laminar, while the flow might transition to turbulence in some other region. For compressible flows, the density also plays an important role, as very dense, low viscosity fluids will be more likely to enter the turbulent flow regime. In general, for a given fluid, the viscous forces are fixed for an incompressible fluid, so the transition to laminar flow will depend entirely on length scale (L) or flow velocity (u). The next question concerns exactly how low the inertial forces need to be. In particular, when the inertial forces are sufficiently low, the Reynolds number might be below the critical threshold for growth of turbulence and the flow will be laminar. In the above definition, we can see what is required to ensure laminar flow. ![]() Reynolds number definition in terms of forces acting on a fluid We can see why this is the case when we examine the definition of the Reynolds number: At high flow rates beyond some critical level, the flow will become progressively more turbulent as the Reynolds number for the flow increases. In a broad sense, there is a range of Reynolds numbers where flow is guaranteed to be laminar. In this article, we’ll examine some upper limits on Reynolds number for laminar flow in simple systems, followed by the high-level process for extracting Reynolds number in more complex systems with CFD simulations. Complex systems might not have such well-defined limits on laminar flow, so these need to be determined as part of systems design. There is an eventual transition to turbulence as the Reynolds number describing the flow increases, which might be undesirable behavior in some systems. When working with a simple closed system, such as a circular pipe, the Reynolds number in laminar flow is well-defined. This summative, dimensionless quantity has significant explanatory and predictive power in fluid mechanics and is often used as a starting point to understand fluid behavior in complex systems. The flow behavior in a system, including the range of possible parameters that define laminar flow, is summarized in terms of the Reynolds number. Laminar flow is the most basic type of flow discussed in fluid mechanics problems, both in classrooms and in industry. Laminar flow tends to occur in Reynolds number values below approximately 2300 for enclosed systems, but the critical Reynolds number could be very different in other systems.ĬFD simulations can be used as a starting point to determine the critical Reynolds number that defines a transition to turbulent flow.Īt high Reynolds numbers, the flow across the bottom surface of this airplane will become turbulent The Reynolds number in laminar flow can vary over a wide range, depending on system geometry. ![]()
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