---
title: "Laminar sub-layer; laminar vs turbulent BL"
subject: "Fluid Mechanics"
chapter: 8
key: "semester_3/fluid_mechanics:laminar-sublayer-laminar-vs-turbulent-bl"
source: https://www.bctengineer.com/q/semester_3/fluid_mechanics/laminar-sublayer-laminar-vs-turbulent-bl
site: BCT Engineering
---

# Laminar sub-layer; laminar vs turbulent BL

Fluid Mechanics · Chapter 8: Boundary Layer theory

- Asked in: 2078 Bhadra, 2+2 marks
- Video: [Understanding Laminar and Turbulent Flow](https://www.youtube.com/watch?v=9A-uUG0WR0w) — The Efficient Engineer
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/fluid_mechanics/laminar-sublayer-laminar-vs-turbulent-bl
- Whole chapter: https://www.bctengineer.com/md/semester_3/fluid_mechanics/chapter-8

## Question

What is laminar Sub-layer? Differentiate between the characteristics of laminar and turbulent boundary layer.

## Short answer

The laminar sublayer is a very thin viscous layer next to the wall beneath a turbulent boundary layer, where velocity varies linearly and shear is purely viscous. Laminar boundary layers have orderly flow, lower thickness (δ ∝ √x), and separate easily; turbulent layers have random eddies, greater thickness (δ ∝ x^0.8), higher shear, and resist separation better.

## Answer

#### Laminar sub-layer

In a turbulent boundary layer, a very thin layer next to the solid surface still has laminar flow, because the wall damps out the eddies there. In this layer the velocity changes almost linearly with $y$ and the shear is purely viscous, $\tau_0 = \mu\,(du/dy)$. Its thickness is

$$
\delta' = \frac{11.6\,\nu}{u_*},\qquad u_* = \sqrt{\tau_0/\rho}
$$

It is only a small fraction of the total boundary layer thickness, but it controls wall shear and heat transfer, and decides whether a surface is hydraulically smooth or rough.

#### Laminar vs turbulent boundary layer

| Point | Laminar BL | Turbulent BL |
|---|---|---|
| Flow | Smooth, orderly layers | Random eddies and mixing |
| Where | Near leading edge, $Re_x < 5\times10^5$ | Downstream, $Re_x > 5\times10^5$ |
| Shear stress source | Viscosity only | Viscosity plus turbulent (Reynolds) stress |
| Velocity profile | Parabolic type, gradual | Fuller, steeper at wall ($\approx 1/7$ power law) |
| Thickness | $\delta = 5x/\sqrt{Re_x}$, grows slowly | $\delta = 0.37x/Re_x^{0.2}$, grows faster |
| Wall shear | Lower | Higher |
| Separation | Separates easily under adverse pressure gradient | Resists separation better |

## Check yourself

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

1. Where is the laminar sub-layer and why does it exist?
   A. At the edge of the laminar boundary layer, due to free stream
   B. At the leading edge only
   C. Next to the wall in a turbulent boundary layer, because the wall damps the eddies
   D. Far from the wall, due to low velocity
   Answer: C. Close to the wall eddies are suppressed so the flow remains viscous and laminar. (Key point: Laminar sub-layer)

2. What governs shear stress in the laminar sub-layer?
   A. Turbulent Reynolds stress
   B. Viscosity only: tau = mu du/dy, with a nearly linear profile
   C. Pressure gradient only
   D. Gravity
   Answer: B. The sub-layer is thin and its velocity varies almost linearly with y. (Key point: Shear in the sub-layer)

3. Which is correct for a turbulent boundary layer compared with a laminar one?
   A. Fuller velocity profile, higher wall shear and better resistance to separation
   B. Parabolic profile and lower wall shear
   C. Lower wall shear and separates more easily
   D. Thinner layer that grows more slowly
   Answer: A. Turbulent mixing gives a fuller profile (1/7 power), more wall shear and delays separation. (Key point: Laminar vs turbulent BL)

4. How does thickness grow in the two boundary layers?
   A. Both grow as sqrt(x)
   B. Laminar: 0.37x/Re^0.2; turbulent: 5x/sqrt(Re)
   C. Both stay constant along the plate
   D. Laminar: delta = 5x/sqrt(Re_x); turbulent: delta = 0.37x/Re_x^0.2, growing faster
   Answer: D. The turbulent layer grows roughly as x^0.8 versus x^0.5 for laminar. (Key point: Thickness growth)

## Related in Fluid Mechanics

- [Boundary layer concept and related terms](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Abl-concept-terms-separation) — 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
- [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
- [BL growth along thin plate, examples](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Abl-growth-thin-plate-examples) — Fluid Mechanics, chapter 8
- [BL, displacement thickness and ratio, 1/7 profile](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Abl-thickness-displacement-ratio-1-7) — Fluid Mechanics, chapter 8
- [BL development along thin flat plate](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Abl-development-flat-plate-features) — Fluid Mechanics, chapter 8

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