Boundary Layer

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What is a Boundary Layer?

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In fluid mechanics, the term boundary layer refers to a thin layer of flowing liquid or gas in contact with a surface like a boundary layer in pipe flow or the surface of an aeroplane wing. The fluid present in the boundary is subjected to shearing forces, and there exists a range of velocities across the boundary from maximum to zero. This occurs only when the fluid comes in contact with the surface of an object.


Boundary layer concepts in an aircraft wing are thicker toward the trailing edge while thinner at the leading edge. The boundary layer flow is generally turbulent in the downstream or trailing portion and laminar at the top or upstream.

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What is Boundary Layer Theory?

Boundary-Layer theory states that when a real fluid flows over a solid body, the boundary's velocity remains zero only if the boundary is stationary. However, if the object moves away from the boundary in a perpendicular direction, the rate increases to the free stream velocity; this means a velocity gradient- (du/day). Velocity gradient  (du/day) does not exist outside the boundary as the outside boundary layer velocity is equal and constant to the free stream velocity. The development of the boundary layer is of three regions- laminar, turbulent, and transition. 

Boundary Layer Theory in Fluid Mechanics

The boundary layer theory in fluid mechanics states that when a fluid has a relative motion to the surface, the liquid particles next to it adheres. This adhering mechanism is known as the 'no-slip condition. Through viscosity, this layer then creates a barrier or resists the next layer, thus slowing it down, which, in turn, affects the layer above it, and this mechanism goes on. 

Therefore, when an object moves away from the surface, it experiences fluid layers of increasing velocity till the object reaches the layer where the fluid holds no velocity reduction or moves toward free stream velocity. Theoretically, this occurs at infinity. For brevity's sake, boundary layer definition is the thickness starting from the surface to the point that comprises 99 per cent fluid velocity of the free stream velocity. 

Therefore, the real fluid phenomena are restricted within the boundary layer, and this is why an object experiences friction drag or vice-versa. The layer will also keep growing along the surface's length. 

For instance, the ripples around the boat or a canoe in calm water are limited to only a certain distance from the canoe or boat's body. This is a simple visualisation of the boundary layer around the boat or canoe.

Boundary Layer in Aerodynamics

The boundary layer in aerodynamics is significant because the shape of the aerofoil changes effectively in its presence. The boundary layer flow can be attached to the aerofoil's surface at lower angles of attack, resulting in Laminar flow, or it can be separated from the aerofoil surface at high angles of attack, resulting in a separated flow. 

The nature of the boundary's characterisation regards Reynold's number, which determines the ratio between inertial and viscous forces. If the viscous forces dominate the inertia forces, the boundary remains attached to the aerofoil resulting in a laminar flow. If the inertia forces dominate the viscous forces, then the boundary is no longer attached to the aerofoil resulting in a separated flow. The viscous flow acts parallel to the surface of the aerofoil resulting in shear forces. 

In aerodynamics, the boundary layer definition will be in terms of viscosity. Thus, the boundary layer is a region in the vicinity of the viscous forces' aerofoil surface.

FAQ (Frequently Asked Questions)

1. What is the Boundary Layer in Connection to the Physical Attributes?

A boundary layer flow is defined as an area present next to the surface and holds significant effects of wall frictional forces. Since the area of interest remains parallel to the surface, the surface region is assumed to be impervious to the flow. The velocity is nearly parallel to the surface. The boundary flow is from left to right, leading to an upstream phase known as 'leading edge' where the surface begins. At the leading edge or coordinate system origin, the boundary flow immediately next to the surface starts to experience frictional forces due to the 'no slip' boundary condition.

An example of the boundary layer concept is the boundary layer in pipe flow, where a thin layer of flowing liquid or gas comes in contact with the surface.

2. State the Boundary Layer Thickness and the Displacement Thickness.

The boundary layer thickness is the distance from the boundary to the point where the fluid's velocity is approximately equal to 99 per cent of the free stream velocity or the surface velocity of an inviscid flow. The boundary layer thickness is denoted by δ (a delta).

The displacement thickness is observed inside the fluid's boundary layer velocity is less than the free stream velocity. Therefore, the discharge is less in the area. To compensate for the reduced velocity discharge in that area, the boundary is displaced outward in the perpendicular direction by a certain distance. This distance is known as the displacement thickness and is dented by δ*.

3. Define Turbulent Flow and Laminar Flow and State the Methods that Prevent Separation.

In Turbulent flow, the adjacent layers of the fluid cross each other as the particles of fluid move randomly instead of moving in a streamlined path for the flow inside a pipe. If Rs is more significant than 4000, the flow is considered to be turbulent. 

Laminar flow is where the fluid flows in the layer, and there is no intermixing with each other. 

The methods for preventing the boundary layer separation include providing a bypass in the slotted area, present guide blades in the end terminal, and supply of additional energy from a pavilion. Proving a tripwire ring in the laminar area helps the flow go over the sphere, and providing slight divergence in a diffuser also prevents separation. Another standard method is rotating the boundary in the direction of flow.