The boundary layer flow differs significantly between internal and external flows due to the presence or absence of confining boundaries.
Understanding Boundary Layer Flow
A boundary layer is a thin layer of fluid adjacent to a solid surface where the flow velocity ranges from zero at the surface (due to the no-slip condition) to the free stream velocity away from the surface. This layer is characterized by significant velocity gradients and viscous effects.
Internal Flow
Internal flow refers to fluid motion that is completely bounded by solid surfaces. A classic example is flow through a pipe or duct.
- Confined Growth: The boundary layer in internal flow develops along the inner walls of the conduit. However, its growth is restricted by the presence of other walls.
- Boundary Layer Interaction: As the flow progresses, the boundary layers growing from opposite walls may eventually merge. This merging results in a fully developed flow where the viscous effects dominate the entire cross-section.
- Examples:
- Air flowing through ventilation ducts
- Water flowing in a pipe
- Oil flowing in engine passages
External Flow
External flow involves fluid motion around a body immersed in a large, unconfined fluid region. Examples include flow over an airplane wing or around a car.
- Unrestricted Growth: The boundary layer in external flow can grow relatively freely in one direction (away from the surface) because the fluid domain is unbounded.
- Pressure Gradients: External flows are often characterized by significant pressure gradients that influence the boundary layer's behavior. These gradients can lead to boundary layer separation, which can drastically alter the flow field and increase drag.
- Examples:
- Air flowing over an airplane wing
- Wind flowing around a building
- Water flowing around a submarine
Key Differences Summarized
The table below highlights the key differences between boundary layer flow in internal and external flows:
| Feature | Internal Flow | External Flow |
|---|---|---|
| Boundaries | Fully bounded by solid surfaces | Partially bounded; immersed in a fluid |
| Boundary Layer Growth | Restricted by surrounding walls | Relatively unrestricted in one direction |
| Flow Development | Leads to fully developed flow | Can lead to boundary layer separation |
| Dominant Effect | Viscous effects eventually dominate entire flow | Pressure gradients significantly influence flow |
In summary, internal flows have boundary layers that are restricted by walls, leading to fully developed flow, whereas external flows have boundary layers that can grow freely, potentially leading to separation influenced by pressure gradients.
