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Quantification of Enhanced Effects due to Dynamic Stall

Quantification of Enhanced Effects due to Dynamic Stall. P M V Subbarao Professor Mechanical Engineering Department. Development of Models to Account Acceleration Benefits. Pitching Dynamics of Blade In VAWT. Importance of Dynamics Stall in HAWT.

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Quantification of Enhanced Effects due to Dynamic Stall

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  1. Quantification of Enhanced Effects due to Dynamic Stall P M V Subbarao Professor Mechanical Engineering Department Development of Models to Account Acceleration Benefits........

  2. Pitching Dynamics of Blade In VAWT

  3. 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.

  4. 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, α0is the initial angle of attack.

  5. 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.

  6. A Section of A Turbine Blade Experiencing Gust • A rotational accelerations must act to stabilize the vortex, with three critical rotational accelerations affecting vortex attachment : • Angular acceleration (aang), • Centripetal acceleration (acen) • Coriolis acceleration (aCor) The rate of spanwise circulation redistribution generated as: For stable operation of rotor

  7. 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.

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

  9. Experimental Test Rigs

  10. Frequency of Dynamic Stall Occurrence

  11. Distribution of Frequency of Occurrence of Stall

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

  13. Quantification of The Enhanced lift in Linear Range • The step increase in lift at the onset of rotation is quanitified by fitting a straight line through the so-called linear region of the lift curve using experimental data. • Increasing the reduced frequency resulted in an increase in the lift at the onset of rotation. • The increase in lift is quantified using the Wagner’s function.

  14. Quantification of The Stall Effect The stall intensity factor is be defined using the lift slope

  15. Initial Remarks on DS • First two decades of 21st century deeply focused on the understanding of unsteady separation. • However, estimation of enhanced lift and drag due to dynamic stall still is still an unsolved problem in VWAT or HWAT. • A detail study is required for quantitative description of effects due to dynamic stall. • The estimation methodology is focused on understanding the shape of the lift vs. drag curve for an airfoil pitching at some constant and harmonic pitching rates.

  16. Factors for Development of Design Methodology • Factors influence Static Stall • Airfoil geometry • Surface roughness • Turbulence level • Reynolds number and Mach number. • Extra factors for Dynamic Stall: • Compressibility • Reduced frequency • Blade Geometry • Location of pitch axis

  17. Reduced Frequency • The reduced frequency defined as the ratio of convective time scale (c/U) and the oscillation time scale (1/α̊). • This parameter characterizes the unsteadiness of flow over the blade. • For an airfoil having a chord length ‘c’ and the airfoil motion is pitching at a constant rate of α̊, the reduced frequency is expressed as: The condition of  =1: The convective time scale is in the order of oscillation time scale. Means that the flow is highly unsteady. Lower the value of  : lower is the unsteadiness and hence the condition shifts gradually to steady state (static). For  < 0.05, the flow may be assumed to be static (steady).

  18. Effect of  on CL

  19. Effect of Reynolds Number on DS Re has minimal effect on the dynamic stall

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