Why Does Airflow Decrease When Blowing Directly Into a Pipe?

[swerider]

if you use your mouth to blow air through a pipe (an A4 page rolled into a pipe for simplicity) with your mouth against the pipe, the outlet flow is much less compared to when you blow through the pipe with your mouth away from it. In terms of fluid mechanics, why does this happen?

 

[minger]

Because if you have you lips against the paper, then the only air that can come out of the paper. Basically, in order for air to go out, air must come in. If your lips are against the paper, you're stopping air from coming in.

 

[Mene]

This phenomenon can be explained by the concept of boundary layer in fluid mechanics. When you blow air through the pipe with your mouth against it, the air has to flow past your mouth before entering the pipe. This creates a thin layer of air, known as the boundary layer, which sticks to the surface of your mouth and slows down the airflow. As a result, the outlet flow is reduced.

On the other hand, when you blow through the pipe with your mouth away from it, the air can enter the pipe more directly without encountering the boundary layer. This allows for a smoother and faster flow, resulting in a higher outlet flow.

Additionally, when your mouth is against the pipe, the air has to make a sharp turn to enter the pipe, causing an increase in air pressure and further slowing down the flow. This is known as the Coanda effect, where a fluid tends to follow the curvature of a surface.

In summary, the difference in outlet flow when blowing through the pipe with your mouth against it or away from it can be attributed to the presence of a boundary layer and the Coanda effect. These factors affect the fluid dynamics and result in variations in airflow.