---
title: "Boundary layer concept and related terms"
subject: "Fluid Mechanics"
chapter: 8
key: "semester_3/fluid_mechanics:bl-concept-terms-separation"
source: https://www.bctengineer.com/q/semester_3/fluid_mechanics/bl-concept-terms-separation
site: BCT Engineering
---

# Boundary layer concept and related terms

Fluid Mechanics · Chapter 8: Boundary Layer theory

- Asked in: 2074 Asoj, 5 marks
- Video: [Boundary layer concept](https://www.youtube.com/watch?v=qOrhmKzwcfA) — Pavika fluid says Tawi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/fluid_mechanics/bl-concept-terms-separation
- Whole chapter: https://www.bctengineer.com/md/semester_3/fluid_mechanics/chapter-8

## Question

Define boundary layer concept. Explain the terms boundary layer thickness, laminar sub-layer and point of separation of boundary layer with sketch.

## Short answer

The boundary layer is a thin viscous layer near a solid surface where velocity rises from zero to free-stream U, outside which flow is inviscid. Boundary layer thickness δ is where u = 0.99U. A laminar sublayer under a turbulent layer has purely viscous shear. The separation point is where the adverse pressure gradient makes (du/dy)wall = 0 and flow detaches.

## Answer

#### Boundary layer concept

When a real fluid flows past a solid body, fluid at the surface sticks to it (no-slip). Velocity increases from zero at the wall to the free-stream velocity $U$ over a thin layer. Viscous shear is large only within this layer, called the **boundary layer**. Outside it the flow is treated as ideal (potential) flow. This idea (Prandtl, 1904) lets the flow be split into a thin viscous layer and an outer inviscid region.

#### Boundary layer thickness ($\delta$)

It is the distance from the wall at which the velocity equals 99% of the free-stream velocity: $u = 0.99U$ at $y = \delta$.

#### Laminar sub-layer

Under a turbulent boundary layer there is a very thin layer next to the wall in which flow stays laminar and the shear is purely viscous: $\tau_0 = \mu\,du/dy$. Its thickness is $\delta' = 11.6\nu/u_*$.

#### Point of separation

It is the point on the surface where flow moving against an adverse pressure gradient ($dp/dx > 0$) is brought to rest at the wall, so $(du/dy)_{y=0} = 0$. Downstream the flow near the wall reverses and the boundary layer leaves the surface, forming a wake.

```text
 U -> ===========================
            turbulent BL         separation
                      ~~~~~~~~~~~~  S
 solid ========================\===\====
      laminar sub-layer        <-- back flow
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What did Prandtl's boundary layer concept allow?
   A. Ignoring viscosity everywhere
   B. Treating all flow as turbulent
   C. Treating the fluid as compressible
   D. Splitting the flow into a thin viscous layer and an outer inviscid region
   Answer: D. Viscous shear is large only in the thin layer; outside, ideal flow applies. (Key point: Boundary layer concept)

2. Which defines boundary layer thickness?
   A. Distance where u = 0.5U
   B. Distance where du/dy = 0
   C. Distance where u = 0.99U
   D. Distance where tau = mu U
   Answer: C. Edge of the layer is conventionally at 99% of free-stream velocity. (Key point: Thickness definition)

3. What is the shear in the laminar sub-layer?
   A. Purely turbulent
   B. Purely viscous, tau_0 = mu du/dy
   C. Zero
   D. Equal to dynamic pressure
   Answer: B. The sub-layer has thickness 11.6 nu/u* and viscous shear only. (Key point: Laminar sub-layer)

4. What marks the point of separation?
   A. (du/dy) at the wall becomes zero under an adverse pressure gradient
   B. du/dy at the wall is maximum
   C. The pressure gradient is favourable
   D. u reaches U at the wall
   Answer: A. After this point there is back flow and a wake. (Key point: Point of separation)

## Related in Fluid Mechanics

- [Boundary layer separation and stagnation point](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Abl-separation-stagnation-point) — Fluid Mechanics, chapter 8
- [Laminar sub-layer; laminar vs turbulent BL](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Alaminar-sublayer-laminar-vs-turbulent-bl) — Fluid Mechanics, chapter 8
- [BL zones sketch and displacement thickness](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Abl-characteristics-sketch-displacement) — Fluid Mechanics, chapter 8
- [BL, displacement and momentum thickness](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Abl-thickness-displacement-momentum-applications) — Fluid Mechanics, chapter 8

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