Understanding Laminar Sub Layer | Civil Engineering Optional Notes for UPSC PDF Download

The Laminar Sublayer

It is a region in fluid dynamics that exists adjacent to a solid boundary, such as the surface of a wall or a boundary between two fluid layers. It is characterized by smooth, orderly flow of fluid particles, which move in parallel layers, or laminas, along the surface. This region typically exists in the immediate vicinity of the solid boundary and is influenced by the no-slip condition, where fluid particles adhere to the solid surface and have zero velocity relative to it. 

The laminar sublayer is important in boundary layer theory, which studies the behavior of fluid flow near solid surfaces. It plays a role in determining the overall characteristics of boundary layers, such as boundary layer thickness and velocity profiles. In many practical engineering applications, the laminar sublayer is of interest because it can affect heat transfer, mass transfer, and drag forces experienced by objects moving through fluids. 

Applications of Laminar Sub Layer

• Heat Transfer: In applications involving heat exchange, such as in cooling systems, heat exchangers, or electronic cooling, understanding the behavior of the laminar sublayer is crucial. It influences the rate of heat transfer between the solid surface and the fluid, affecting the efficiency of heat exchange processes. 
• Drag Reduction: Control of the laminar sublayer can be important for reducing drag in various engineering applications. By manipulating the characteristics of the laminar sublayer, such as through the use of riblets or other surface modifications, engineers can reduce the overall drag experienced by objects moving through fluids, leading to increased efficiency and performance, particularly in aerospace and automotive industries. 
• Boundary Layer Control: Understanding and controlling the laminar sublayer is essential for managing the overall boundary layer behavior. This is important in aerodynamics, where engineers seek to optimize the flow around aircraft wings, vehicle bodies, or other structures to minimize drag and improve efficiency. Turbine Design: In the design of turbines and other rotating machinery, the laminar sublayer plays a role in determining the boundary layer characteristics on blades or other surfaces. By optimizing the flow within the laminar sublayer, engineers can enhance the performance and efficiency of turbines.

Solved Example: The thickness of laminar sub layer 'd' is given by (with usual notations)

(a) 11.6u_*/v

(b) u_*/(11.6v) 

(c) 11.6v/u_*

(d) v/u_{*
Ans: (c)}

On the basis of NIKURADSE’s Experiment, the boundary is classified as
Hydrodynamically smooth if $\frac{𝑘}{𝛿}<0.25$
The boundary in transition if $0.25<\frac{𝑘}{𝛿}<6.0$
Hydrodynamically rough if $\frac{𝑘}{𝛿}>6$
Where,
k is the average height of roughness, δ is the thickness of the laminar sublayer
Where,
For the flow over a flat plate, the laminar sublayer thickness is given by

$𝛿\ast =\frac{11.6}{\times }$Where, $𝑈\ast =\frac{𝜏}{0}$
U_* is shear velocity, ν is the kinematic viscosity