---
title: "Air flow through openings numerical (0.3 m, 0.4 m)"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:airflow-079a"
source: https://www.bctengineer.com/q/semester_6/building_technology/airflow-079a
site: BCT Engineering
---

# Air flow through openings numerical (0.3 m, 0.4 m)

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2079 Asoj, 5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/airflow-079a
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Air is flowing through a two square opening of size 0.3 m and 0.4 m in a 5m × 5m × 5m room, with a wind of 4 km/hr blowing at 90° to the opening. Take coefficient of effectiveness = 0.68. Calculate the rate of air flow.

## Short answer

For wind-driven ventilation through two openings in series, effective area A = (A1×A2)/√(A1²+A2²) = 0.07844 m²; airflow Q = E×A×V = 0.68 × 0.07844 × 1.111 = 0.0593 m³/s = 213.4 m³/h.

## Answer

#### Given
- Inlet and outlet openings: 0.3 m × 0.3 m and 0.4 m × 0.4 m, so $A_1 = 0.0900\ \text{m}^2$ and $A_2 = 0.1600\ \text{m}^2$
- Wind speed $v = 4$ km/h $= 4/3.6 = 1.1111$ m/s, at 90° to the opening
- Coefficient of effectiveness $E = 0.68$ (already accounts for the wind direction; the room size does not enter the formula)

#### Formula
For wind-driven ventilation through two openings in series, the effective area is

$$
A = \frac{A_1 A_2}{\sqrt{A_1^2 + A_2^2}}, \qquad Q = E\, A\, V
$$

#### Calculation
$$
\begin{aligned} A &= \frac{0.0900 \times 0.1600}{\sqrt{0.0900^2 + 0.1600^2}} = 0.07844\ \text{m}^2 \\ Q &= 0.68 \times 0.07844 \times 1.1111 = 0.05927\ \text{m}^3/\text{s} \end{aligned}
$$

**Answer: $Q = 0.0593$ m³/s $= 213.4$ m³/h.**

## Check yourself

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

1. For two openings in series the effective area is
   A. A1 A2 / sqrt(A1^2 + A2^2)
   B. A1 + A2
   C. A1 x A2
   D. (A1 + A2)/2
   Answer: A. Series openings combine like this; A = 0.0784 m2. (Key point: Effective area)

2. Does the room size enter the flow Q = E A V?
   A. Yes, as volume
   B. No, it does not
   C. Yes, as height
   D. Yes, as floor area only
   Answer: B. Only E, A and V are used. (Key point: Room size)

3. 4 km/h in m/s is
   A. 14.4
   B. 0.24
   C. 1.111
   D. 4
   Answer: C. 4/3.6 = 1.111 m/s. (Key point: Speed)

4. The flow is about
   A. 0.593 m3/s
   B. 0.0059 m3/s
   C. 1.2 m3/s
   D. 0.0593 m3/s
   Answer: D. 0.68 x 0.07844 x 1.1111 = 0.0593 m3/s = 213 m3/h. (Key point: Result)

## Close variants set in other exams

- [Air flow through openings numerical (0.36 m, 0.4 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aairflow-078p) — Building Technology, chapter 1, asked 2×

## Related in Building Technology

- [Building types, mechanical ventilation, air flow](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aairflow-078c) — Building Technology, chapter 1