What is Boundary Layer

Boundary Layer

The boundary layer is a very thin layer of air flowing over the surface of an aircraft wing, or like as well as other surfaces of the aircraft. The molecules directly touching the surface of the wing are virtually motionless. Each layer of molecules within the boundary layer moves faster than the layer that is closer to the surface of the wing. At the top of the boundary layer, the molecules move at the same speed as the molecules outside the boundary layer. This speed is called the free-stream velocity. The actual speed at which the molecules move depends upon the shape of the wing, the viscosity, or stickiness, of the air, and its compressibility (how much it can be compacted).

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Further, boundary layers may be either laminar (layered), or turbulent (disordered). As the boundary layer moves toward the center of the wing, it begins to lose speed due to skin friction drag. At its transition point, the boundary layer changes from laminar, where the velocity changes uniformly as one moves away from the object's surface, to turbulent, where the velocity is characterized by unsteady (changing with time) swirling flows inside the boundary layer.

Aerodynamic forces depend in a complex way on the viscosity of the fluid. As the fluid moves past the object, the molecules right next to the surface stick to the surface. The molecules just above the surface are slowed down in their collisions with the molecules sticking to the surface. These molecules in turn slow down the flow just above them. The farther one moves away from the surface, the fewer the collisions affected by the object surface. This creates a thin layer of fluid near the surface in which the velocity changes from zero at the surface to the free stream value away from the surface. Engineers call this layer the boundary layer because it occurs on the boundary of the fluid.

The Boundary Layer Method

In order to calculate the friction drag of an airfoil for a given flow condition (angle of attack, Reynolds number), an analysis of the viscous boundary layer is necessary. From the momentum loss in this small layer on the surface of the airfoil the drag can be derived. As the velocity distribution changes with angle of attack, the drag changes too. Also, the thickness of the boundary layer changes with Reynolds number.
The boundary layer module uses the velocity distribution derived by the panel method and performs its calculations based on the formulas presented in [14, 15, 16]. The method is a so called integral boundary layer method, which does not handle laminar separation bubbles or large scale separation (stall). The boundary layer module works best in the Reynolds number regime between 500'000 and 20'000'000.
The results of the boundary layer module are also used to correct lift, drag and moment coefficients empirically, if separation occurs. Additionally, a blending to separated, flat plate coefficients is performed for very high angles of attack. Having problem with Electric Flux Density keep reading my upcoming posts, i will try to help you.

The procedure starts at the stagnation point and marches along each surface, integrating simplified boundary layer equations. The integration follows a 2nd order Runge-Kutta scheme with stabilization by automatic step reduction. This can be a bit slow some times, but works more reliable than the simple Newton method used before. During the way towards the trailing edge, the method checks, whether transition from laminar to turbulent or separation occurs.