The transition from laminar to turbulent flow can be caused by several factors, including the flow rate, viscosity, and surface roughness [1-4]. One of the most critical factors is the Reynolds number (which inherits both the viscous effect and the inertial forces - but here representing flow velocity for a given viscosity). When the Reynolds number is below a critical value, laminar flow dominates, and the fluid particles move in an ordered fashion. However, when the Reynolds number exceeds this critical value, the flow becomes unstable, and at that point, any small disturbance can trigger the transition to a turbulent flow. This critical Reynolds number (at which this transition takes place) depends on the geometry of the system, and it varies for different flow configurations.
Another factor that influences the transition to turbulence is the viscosity of the fluid. High viscosity fluids tend to resist deformation and flow in a more ordered manner, resulting in a laminar flow. In contrast, low viscosity fluids are more prone to deformation and exhibit turbulent behavior at lower Reynolds numbers.
Surface roughness can also cause laminar to turbulent transition by introducing small disturbances that can grow and trigger turbulent flow. Even small imperfections on the surface of a pipe or channel can generate eddies that eventually lead to turbulence.
In addition to these factors, external perturbations such as vibrations and thermal gradients can also trigger laminar to turbulent transition. For example, temperature differences in a fluid can induce density gradients that can generate flow instabilities and lead to turbulent flow.
One of the common ways to detect this transition is by measuring the skin friction coefficient, which is a measure of the shear stress acting on the surface. When the boundary layer becomes turbulent, the skin friction coefficient increases significantly. Other methods for detecting the transition include hot-wire anemometry, particle image velocimetry (PIV), and other pressure measurement techniques. [there's a big catch here!]
well, not quite!
The transition from laminar to turbulent can lead to changes in wall shear stress, which is the frictional force exerted by a fluid on a surface. Generally, in laminar flow, the fluid flows in smooth layers, with little mixing between adjacent layers, and the wall shear stress is directly proportional to the velocity gradient near the surface. In turbulent flow is erratic with rapid mixing and energy dissipation. The wall shear stress in turbulent flow is more complex and varies with the turbulent fluctuations.
During the transition from laminar to turbulent flow, the wall shear stress can either increase or decrease depending on the specific conditions of the flow
In some cases, as the flow transitions from laminar to turbulent, the wall shear stress may increase due to the onset of small-scale vortices that generate high velocity gradients near the surface. This can lead to enhanced mixing and energy dissipation, resulting in increased wall shear stress. In other cases, the wall shear stress may decrease due to the reduction of the velocity gradient near the surface as the flow becomes more turbulent. This fluctuation in wall shear can vary in time (due to the chaotic nature of turbulence) and space (due to the evolution of multi-sized coherent structures further downstream).
It's worth noting that it is very easy to mistake this transition for a flow separation. But how do we know separation then?!
In this context, flow separation refers to the phenomenon in which a fluid flow detaches or separates from a surface due to adverse pressure gradients or other flow conditions [5-7]. There are several types of flow separation that can occur in air flows over a wing or body, including:
As mentioned above, distinguishing between flow separation and laminar-to-turbulent transition in a boundary layer can be a tricky business - because both phenomena can result in similar changes to the flow field. However, there are certain characteristics that can help differentiate between the two.
As a matter of fact, flow separation occurs when the fluid flow over a surface detaches from the surface and creates a region of reverse flow. This results in the formation of vortices and eddies, which can be seen as large-scale fluctuations in the flow. Flow separation typically occurs at high angles of attack (not always!), where the pressure gradient is unfavorable for the flow to remain attached to the surface.
Laminar-to-turbulent transition occurs when the smooth and ordered flow in a laminar boundary layer becomes unstable and transitions (as in one of the cases) to a turbulent boundary layer. This can occur due to the presence of disturbances in the flow or changes in the Reynolds number.
Despite of the difficulty to differentiate between the two phenomena, there are certain characteristics that can help us, some of which are:
Laminar-to-turbulent flow transition and flow separation are two distinct phenomena in fluid dynamics, playing a crucial role in various engineering applications. Laminar-to-turbulent transition is the process where a smooth, orderly flow becomes irregular and chaotic, influenced by factors such as flow velocity, surface roughness, and external disturbances. Flow separation occurs when the boundary layer detaches from a solid surface due to an adverse pressure gradient, leading to performance issues like increased drag and reduced lift.
Distinguishing between these phenomena requires understanding their underlying mechanisms and analyzing fluid motion and pressure distribution. Laminar-to-turbulent transition is marked by increased fluid mixing and energy dissipation, while flow separation is characterized by recirculation zones and sudden pressure changes. Gaining a deeper understanding of these processes is essential for optimizing fluid systems, reducing significantly the operational cost and increasing efficiency.
