In Computational Fluid Dynamics, accurately modeling fluid flow is crucial for obtaining reliable results. One key distinction in flow analysis is between internal and external flow. Understanding these types of flow, their physical characteristics, and their implications for CFD modeling is essential for effective simulations. This blog post explores internal and external flow, highlighting their differences and how these distinctions impact CFD modeling.

What Are Internal and External Flows?

Internal Flow

Internal flow refers to fluid movement within a confined or enclosed boundary, such as inside pipes, ducts, or channels. The fluid is bounded by solid surfaces that significantly influence its behavior.

External Flow

External flow involves fluid moving around or over an object that is not confined, such as airflow around an aircraft, flow over a building, or the movement of water around a ship. The fluid interacts with the object and the surrounding environment.

Physical Differences Between Internal and External Flow

Boundary Conditions

Internal Flow: The flow is constrained by solid boundaries, leading to a well-defined flow path. The boundary conditions are often set on the walls of the pipe or duct.

External Flow: The flow is influenced by the interaction with both the object and the free stream or ambient environment. Boundary conditions are set at the surface of the object and often extend to the far field.

Flow Development

Internal Flow: The flow typically develops from a fully developed state to a less developed state depending on the length of the flow path. The flow profile becomes fully developed after a certain distance from the inlet.

External Flow: The flow is usually unbounded and does not reach a fully developed state. It evolves around the object and can be affected by the shape and orientation of the object.

Velocity Profile

Internal Flow: Velocity profiles are influenced by the pipe or duct geometry, leading to a parabolic or fully developed profile in laminar flow cases.

External Flow: Velocity profiles are influenced by the shape of the object and the free stream conditions, leading to complex flow patterns such as boundary layers, wakes, and vortices.

Pressure Distribution

Internal Flow: Pressure distribution is primarily influenced by frictional effects and flow rate. It typically decreases along the length of the flow due to friction losses.

External Flow: Pressure distribution is influenced by the interaction with the object, leading to pressure changes around the object due to flow separation, wakes, and other aerodynamic effects.

Differences in CFD Modeling

Domain Definition

Internal Flow: The computational domain is defined by the geometry of the pipe, duct, or channel. The domain is typically closed and requires careful attention to boundary conditions on the internal surfaces.

External Flow: The computational domain extends beyond the object to capture the interaction with the free stream or ambient flow. The domain must be large enough to account for the full development of the flow and minimize boundary effects.

Mesh Generation

Internal Flow: Meshing focuses on the geometry of the pipe or duct, often requiring finer mesh near the walls to capture boundary layer effects. Structured meshes are commonly used.

External Flow: Meshing needs to capture complex flow patterns around the object, including boundary layers, wakes, and vortices. Unstructured or hybrid meshes are often used to handle the complexity of the flow.

Boundary Conditions

Internal Flow: Inlet boundary conditions typically specify velocity or mass flow rate, and outlet conditions often involve pressure or mass flow. Wall conditions include no-slip or slip boundaries.

External Flow: Inlet boundary conditions include free-stream velocity or pressure, and outlet conditions are often defined as far-field or pressure outlets. Surface boundary conditions include wall interactions, slip conditions, and specific heat transfer conditions.

Solver Settings

Internal Flow: Solvers are tuned to handle fully developed or developing flow profiles within a confined geometry. Incompressible or compressible flow solvers may be used depending on the application.

External Flow: Solvers need to handle complex interactions with the surrounding environment, including turbulence modeling, flow separation, and pressure recovery. Advanced turbulence models and larger domains are often required.

Simulation Objectives

Internal Flow: Objectives often include analyzing pressure drop, flow distribution, and heat transfer within pipes or ducts.

External Flow: Objectives focus on understanding aerodynamic performance, flow patterns around objects, and drag or lift forces.

Conclusion

Internal and external flows present distinct challenges and opportunities in CFD modeling. Internal flows are confined within geometries such as pipes and ducts, while external flows involve interactions with unbounded environments around objects. Understanding the physical differences between these flow types and their implications for CFD modeling helps in setting up accurate simulations and achieving reliable results.

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