Design and Analysis of Wind Turbines using Dynamic Stall Effects P M V Subbarao Professor Mechanical Engineering Department Diagnostics of Most Probable Off-design Conditions ........

Random Distribution of Wind

Extraneous Wind Conditions Wind Shear Wind Shifts Wind Gusts

OCCURRENCE OF DYNAMIC STALL in HAWT A dynamic stall event occurs on a blade section when the local angle of attack rapidly increases through the static stall point. Turbulence, shifts in wind direction or magnitude. Wind shear, or upstream flow disturbances can alter the local velocity. These can create blade angle of attack changes sufficient to drive dynamic stall. Dynamic stall can be characterized by an extremely large suction peak value at or near the leading edge. Dynamic stall is expected to occur if the peak pressure coefficient (Cp) was less than -10.

Importance of Dynamics Stall in HAWT The lifetime of major wind turbine components is typically far less than their 20-30 year design lifetime. Components, such as generators and blades, are frequently subjected to dynamic loading far in excess of their design loads. A primary source of the excessive fatigue and failure is speculated to be derived from the unsteady aerodynamics. Unsteady aerodynamics is due to dynamic inflow, turbulence, and dynamic stall. Unsteady detached flows are responsible for the fatigue cycles with the highest peak-to-peak loading for both blade and rotor shaft bending, reducing turbine lifetime. Therefore, an understanding of the flow physics which dictate these forces would be beneficial in designing more reliable wind turbines.

The Effect of Gusts on Angle of Attack Under steady operating conditions, the turbine blade is designed to maintain a constant circulation profile over the span of the blade. However, when a gust impinges on a blade the change in angle of attack across the blade Δα is highly non-uniform.     Δα(r) is the change in angle of attack as a function of radius r, ΔV is the change in the free stream velocity, α0 is the initial angle of attack.

Effect of Gust on Spanwise Variation of AOA Both the change in magnitude and the spanwise gradient of angle of attack are largest in the near-hub region. Creates a scope for Dynamic stall behavior.

Three dimensional Nature of Flow In transient flow conditions wind turbines develop a gradient in angle of attack along the blade span. This generates a spanwise vorticity gradient. It is postulated that, in combination with the spanwise flow induced by rotational accelerations, this spanwise vorticity gradient acts to redistribute circulation along the span of the blade towards the root. For locations near hub this redistribution would result in a greater magnitude of local circulation. This increases the lift experienced near hub.

The Distribution of Inflow Conditions Over 20,577 Blade Rotational Cycles

Experimental Test Rigs

Frequency of Dynamic Stall Occurrence

Distribution of Frequency of Occurrence of Stall

The Frequency of Dynamic Stall Occurrence at Two or More Span Locations During A Cycle

Dynamic Controls Systems Typical HAWT blades have a twist distribution such that the entire blade is at the same angle of attack for a given wind speed. Therefore, when dynamic stall occurs, it will affect extremely large portions of the blade at the same time. By adjusting blade pitch, the entire blade can be put into a operational regime in which dynamic stall is not likely to occur. Doing so should yield a significant reduction in rotor loads.

True Model for A VAWT???

The Vision of A Genious

Dynamic vortex shedding for a straight-bladed vertical axis turbine

Imagine a 3D world & Reality of 3D Imagination Twisty vortex strings that produce infinite V -type velocities as distance from the filament goes to zero. These twisty vortices are similar to tornados. How to quantify?

The bounded vortex for an ith blade element Each of the blade element is a bound vortex lament called a Lifting line. According to the airfoil theory , the lift can also be formulated in terms of the two-dimensional sectional lift coefficient (Cl), the chord length (c) and the relative velocity (Vrel) as The strength of the bound vortex can be expressed as

The Shed Vortex The relationship between the strength of the bound vortex of blade element i at time step j and the strength of the shed vortex from this element is given by the Kelvin's law. This Law requires that the circulation around any closed curve remains constant over time. It satisfies the following equation Where, si,j is the strength of the shed vortex at element i and time step j.

The journey of Shed Vortices In reality, shed vortices are released continuously at the trailing edge. The newest shed vortices are placed at x % from the current location of the trailing edge to the location of the trailing edge at the previous time step. The other shed vortices are transported by the calculated velocity.

Occurrence of Trailing Vortices In order to satisfy Helmholtz's theorems, a trailing vortex must emit from the blade at each blade element. This emission occurs at a location, where the strength of each discrete bound vortex changes to generate the varying circulation a (lift) along the span.

Occurrence of Trailing Vortices The strength of a trailing vortex is the difference between the strengths of the bound vortices from where it emits. It can be seen in The Field Generated by A VAWT is A System of Vortex Filaments........ This system of vortices generate a complex velocity field. Velocity field generates Pressure Field…..

Biot–Savart Law The contribution from the infinitesimal length vortex The contribution from the finite length vortex The Gaussian kernel Here, r is the position  is the circulation of vortex r denotes the complex conjugate of r

Topics for Course Project DSV based Design of Vertical Axis Wind Turbines. Effect of DSV on performance and control of HAWT. Vortex Lattice Method of Design for VAWT. Collect atleast 5 international journal papers published after 2000. No two groups can have more than two same papers. A group which collect all the research papers different from other groups will get 10% bonus marks. Use original statements and uniform symbols used in lectures. Graphs can directly take from papers.