---
title: "Distorted model and its importance"
subject: "Fluid Mechanics"
chapter: 10
key: "semester_3/fluid_mechanics:distorted-model-importance"
source: https://www.bctengineer.com/q/semester_3/fluid_mechanics/distorted-model-importance
site: BCT Engineering
---

# Distorted model and its importance

Fluid Mechanics · Chapter 10: Similitude and physical modeling

- Asked in: 2072 Chaitra, 1+2 marks
- Video: [Fluid Mechanics: Topic 13.6.3 - Distorted models](https://www.youtube.com/watch?v=5ZxWCMthyU0) — CPPMechEngTutorials
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/fluid_mechanics/distorted-model-importance
- Whole chapter: https://www.bctengineer.com/md/semester_3/fluid_mechanics/chapter-10

## Question

Define distorted model and its importance in model analysis.

## Short answer

A distorted model uses different horizontal and vertical scale ratios (distortion >1), breaking geometric similarity. It is vital for large flat prototypes (rivers, estuaries) to save space and cost, keep flow turbulent, allow measurable depths and velocities, and study sediment transport, though results need corrections.

## Answer

A **distorted model** is a scale model in which the scale ratios are not the same in all directions, i.e. the horizontal scale $L_r$ differs from the vertical scale $h_r$. Geometric similarity is therefore not complete. The distortion is $h_r/L_r$ (greater than 1).

#### Importance (why it is used)

1. **Space and cost**: for large and flat prototypes such as rivers, harbours, estuaries and tidal basins, an undistorted model would be either too big or so shallow that the depth would be a few millimetres.
2. **Avoids scale effects**: a larger vertical scale gives enough depth to keep the flow turbulent, avoiding surface tension and viscosity effects, and keeps the Reynolds number high.
3. **Measurable quantities**: depths, velocities and slopes become large enough to measure accurately.
4. **Movable beds**: it gives enough slope and tractive force to study scour, silting and sediment transport.
5. **Better visualisation** of flow patterns, waves and currents.

The disadvantages are that the results need correction factors and that velocity distributions, bed forces and wave patterns are not exactly reproduced.

## Check yourself

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

1. How is a distorted model defined?
   A. All scales same
   B. Horizontal and vertical scales differ
   C. Different fluid
   D. Larger than prototype
   Answer: B. Geometric similarity is not complete. (Key point: Definition)

2. Why is distortion used for rivers?
   A. Rivers are too deep
   B. Rivers are viscous only
   C. Undistorted model would be too large or too shallow
   D. To reduce cost of fluid
   Answer: C. Depth would be a few mm. (Key point: Space)

3. How does distortion help?
   A. It removes gravity
   B. It makes slope zero
   C. It eliminates correction factors
   D. Greater depth keeps flow turbulent and avoids surface tension and viscosity effects
   Answer: D. Larger vertical scale gives enough depth. (Key point: Scale effects)

4. What is a disadvantage?
   A. Results need correction factors
   B. More space
   C. No measurement
   D. Needs larger depth
   Answer: A. Velocity and wave patterns not exactly reproduced. (Key point: Disadvantage)

## Related in Fluid Mechanics

- [Distorted vs undistorted model; Pi theorem](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Adistorted-vs-undistorted-buckingham-principle) — Fluid Mechanics, chapter 10

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