Flow is considered laminar when the air exhibits a smooth, parallel pattern when it comes in contact with aircraft surfaces.
A laminar boundary layer produces less drag.
CFD numerical simulation of laminarity facilitates the aerodynamic design of airplanes.
Model of laminar flow in an airplane
Laminar flow has been a topic of interest in industrial as well as academic settings for a long time. Engineers and systems designers working on aviation projects, fluid distribution networks, and even heating and cooling applications have explored the laminarity factor for its potential advantages in turbulence management and contamination reduction. In aerospace industries, laminar flow has been sought after for the benefit it provides in minimizing drag and helping aircraft reduce emissions by using less power and burning less fuel.
The study of laminar airflow for the critical design of aerodynamic components can be simplified with the help of CFD (computational fluid dynamic) numerical simulation.
In this article, we will learn about the laminar flow in airplanes, the laminar boundary layer, other influencing components, and discuss the role of CFD tools in analyzing these aerodynamic concepts.
Laminar Flow in Airplanes
Laminar flow is categorized from the smooth and parallel flow paths that a fluid exhibits without intermixing. In aviation, laminar flow essentially represents the smoothness that air exhibits when it comes in contact with aircraft surfaces. When the air passes through the contours of the aircraft fuselage or wings, the streamlined feature of the airflow may change. The turbulence thus induced can have a negative influence on aircraft control. This includes the effect on drag and lift. The airfoil should therefore be designed well enough to preserve the laminar regime of the airflow to minimize drag and increase the lift of wings.
When this air moves at low velocity, a streamlined flow of air is observed, which is the laminar flow of air. To maintain this laminarity, the airfoil is usually made thinner with a pointed edge and made progressively thicker while maintaining near-symmetry of the wing cross-section. With the increase in velocity, the turbulence increases, producing drag and affecting the quality of the flight.
Laminar Flow Boundary Layer
The boundary layer for different types of flows
Air possesses some level of viscosity. This viscosity results in the formation of a thin boundary layer of air on the aircraft surface. The figure above shows the boundary layer for different types of flows. The laminar flow zone can typically be determined with Reynolds number, which in the case of external flow occurs at less than 500,000. Depending on the surface roughness, the transition can start earlier or after this point.
As the laminar boundary layer facilitates a quick and smooth flow, less friction drag is produced. A well-designed laminar flow airfoil can have almost half the level of drag as turbulent flow. The total drag force acting on an airplane surface can be analyzed using the following formula:
Note that Cf is the drag coefficient, ρ is the fluid density, and v is the flow velocity for the surface area A’.
For a wide range of Reynolds numbers (Re), the laminar drag coefficient can be calculated as:
In an effort to reduce the friction drag, designers tend to make the aircraft surface as smooth as possible. Any irregularities, even as small as dirt, are enough to disrupt the laminar boundary layer, which causes an increase in drag.
The Role of CFD Tools in Facilitating Aerodynamic Design
In the analysis of the aerodynamic properties of a proposed airplane design, the evaluation of the laminar boundary layer, drag, and lift can be done easily through modeling and simulation. CFD tools such as Omnis can facilitate the easy analysis of the governing boundary layer equations in aerodynamics. And, with solutions such as Pointwise, comprehensive mesh generation can be done to analyze the simulation accuracy of the airfoil. With high-fidelity CFD simulations, the stability of laminar flow in airplanes can be investigated to improve the aerodynamic properties of the design.