Boundary Layer Theory for Wind Turbine Blades P

Boundary Layer Theory for Wind Turbine Blades P M V Subbarao Professor Mechanical Engineering Department Creation and Control of Laminar Boundary Layers……

Major Steps in Selection of Correct Airfoil for WT Blade • First Consideration : Identification of important design variables and selection of suitable values for the same. • Second consideration: Control the performance over multiple operating points.

Multipoint Description of the Performance Curve • Typically, airfoil design requirements include; • information regarding Clmax (point C) • as well as the operating range over which low drag is achieved (points A to B). • These requirements can be translated into specific characteristics to be embodied in the velocity or pressure distribution on the surface of airfoil.

Multi Point Design Method • Low drag at points A and B requires extended runs of laminar flow on the lower and upper surfaces. • The high lift requirement is achieved by limiting the leading edge suction peak behavior. • Each of these must be achieved at the corresponding Wind Velocities. • Collectively this approach is referred to as Multipoint Design. • This is clearly a desired feature of Design for Performance.

WT Blade Design for performance

Deeper Studies on Structure of Airfoil Boundary Layer

Industrial Use of Boundary Layer Information At any streamwise location :

Structure of Boundary Layer Suction Surface of an Airfoil Low Reynolds number airfoil flows are principally distinguished by their associated laminar separation bubbles. • Serious attention has to be focused on laminar separation bubbles, because they are the leading culprit to the degradation in performance relative to airfoils at higher Reynolds numbers.

Structure of Boundary Layer Suction Surface of an Airfoil @ Low Reynolds Numbers

Validity of Prandtl’s 2 D Boundary Layer Equations • The system of equations for Prandtl boundary-layer equations in stretched Cartesian co-ordinates.

Remarks on Prandtl’s Idea • The essence of Prandtl’s idea was discussed and agreed without any indication of possible problems in implementing it for an arbitrary body. • The function p(x), which is determined by the inviscid flow, predicts the boundary layer. • The main problem which will arise is that of boundary layer separation. • It turns out that this BL cannot be continued indefinitely along the surface of the body.

Remarks on Prandtl’s Idea • What can occur is: – the ejection of vorticity into the free stream, – a breaking away of the boundary layer from the surface, and – the creation of free separation streamline. • Separation is part of the stalling of an airfoil at high angles of attack.

BL Equations at The Surface of Airfoil If the external pressure gradient, , then, at the wall Must decrease faster as y increases

Zero Pressure Gradient Location on Suction Surface At zero pressure gradient location at the wall Is also zero, as u is zero near surface

Adverse Pressure Gradient If the external pressure gradient, , then, at the wall Must increase initially and then decrease as y increases Increasing distance in downstream