The Nominal Thickness of the Boundary Layer

The boundary layer is a thin layer of fluid that forms on the surface of a solid object, such as an airfoil or a ship's hull, when it is exposed to a flowing fluid. This layer is characterized by a gradual transition from the no-slip condition at the surface to the velocity of the free stream.

Definition of Nominal Thickness

The nominal thickness of the boundary layer is defined as the distance from the solid surface at which the velocity of the fluid reaches a certain percentage of the velocity of the free stream. This percentage is typically used as a criterion to determine the thickness of the boundary layer.

Significance of the Nominal Thickness

The nominal thickness of the boundary layer is an important parameter in fluid dynamics and aerodynamics. It provides a measure of the scale of the flow near the surface and affects various flow characteristics, such as drag and heat transfer. Therefore, understanding and predicting the thickness of the boundary layer is crucial in many engineering applications.

Choice of Percentage

The choice of the percentage at which the velocity is considered to have reached the velocity of the free stream depends on the specific application and the desired level of accuracy. Different percentages can be used, but the most commonly used values are 99%, 97%, 95%, and 90%.

Correct Answer: Option B (99%)

The correct answer to the question is option B, which states that the nominal thickness of the boundary layer is defined when the velocity reaches 99% of the velocity of the free stream.

This choice of 99% is often used because it represents a sufficiently high percentage that captures the majority of the flow characteristics within the boundary layer. At this point, the velocity gradient across the boundary layer is relatively small, and the flow can be considered to be fully developed.

Using a higher percentage, such as 99%, ensures that the boundary layer thickness is accurately determined and provides a more conservative estimate for engineering calculations. It accounts for any potential variations in the flow field and allows for a more accurate representation of the flow near the surface.

In conclusion, the nominal thickness of the boundary layer is defined when the velocity reaches 99% of the velocity of the free stream. This choice of 99% ensures a more accurate estimation of the boundary layer thickness and is commonly used in engineering applications.