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Performance recovery of a thick turbulent airfoil using a distributed closed-loop flow control system

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Abstract

This paper describes an experimental study aimed at controlling the performance of a thick airfoil, typical to the root section of a wind turbine blade. The main purpose is recovering decreased performance due to degraded surface quality, leading to decreased lift and increased drag. Since wind turbines are designed to operate for decades, the blades’ surface quality degradation due to environmental effects is unavoidable. This process promotes early transition to turbulent flow, leading to premature boundary layer separation in the post-transitional regime. In addition, non-uniform and unsteady wind speeds cause dynamic loads on the blade and on the overall turbine structure. Controlling unsteady and non-uniform loads by changing the blades’ (or its cross-section) performance will allow building larger, lighter and more durable to aging wind turbines. Active flow control (AFC) is a possible remedy to boundary layer separation, including rough surface effects. Currently, three arrays of synthetic jet actuators are controlled based on state estimation provided by feedback from hot-film and pressure sensors. The unsteady pressure sensors’ data are used to estimate the lift while the unsteady and un-calibrated hot-films data are used to determine the flow separation location and define the relative magnitude of actuation imparted by each of the three actuator rows. The aerodynamic results demonstrate that the “clean” turbine blade performance, with lift-based controller, is recovered by the closed-loop active flow control system at Reynolds numbers around half a million and excitation at Strouhal numbers larger than 10. The total closed-loop AFC system energy efficiency was measured and shown to increase by up to 60 % compared to the airfoil with degraded surface quality. The current results indicate the potential of a closed-loop AFC system to provide significant increase in the net energy harvesting capability of a wind turbine blade with degraded surface quality over a wide range of incidence angles and Reynolds numbers.

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Abbreviations

A airfoil :

Plan view area of the airfoil, b*c (m2)

A i :

A i or Amp, amplitude RMS of ith actuator slot (VAC)

A max :

Upper limit for excitation amplitude RMS in the system (VAC)

A (i)slot :

Cross-section area of the ith actuator’s exit slot (m2)

AFM3:

Third aerodynamic figure of merit (Stalnov et al. 2010) (Eq. 2), \( {\text{AFM3}} \equiv \frac{{[(L - D) \cdot U_{\infty } ]_{\text{controlled}} - 2 \cdot P_{\text{elect}} }}{{[(L - D) \cdot U_{\infty } ]_{\text{baseline}} }} \)

b :

Wing span (m)

c :

Wing chord (m)

\( C_{\text{d}} \) :

Drag coefficient, \( C_{\text{d}} \equiv \frac{D}{{\tfrac{1}{2}\rho U_{\infty }^{2} c \cdot b}} \)

C i :

Weighting coefficient for ith pressure sensor (Eq. 4), \( C_{i} = \frac{\text{SepLoc}}{{{\text{SensLoc}}_{i} }} \)

\( C_{\text{P}} \) :

Pressure coefficient, \( C_{\text{P}} \equiv \frac{{P - P_{\infty } }}{{\tfrac{1}{2}\rho U_{\infty }^{2} }} \)

\( \overline{{C_{\text{p}} }} \) :

Weighted average pressure coefficient (Eq. 3), \( \overline{{C_{\text{p}} }} = \frac{1}{q} \cdot \frac{{\sum\nolimits_{i = 1}^{5} {(C_{i} \cdot P_{i} )} }}{{\sum\nolimits_{i = 1}^{5} {C_{i} } }} \)

C L :

Lift coefficient, \( C_{\text{L}} \equiv \frac{L}{{\tfrac{1}{2}\rho U_{\infty }^{2} c \cdot b}} \)

\( C_{{{\upmu}}} \) :

Excitation momentum coefficient for N working actuator slots, \( C_{{{\upmu}}} \equiv \sum\limits_{i = 1}^{N} {\frac{{A_{\text{slot}}^{(i)} }}{{A_{\text{airfoil}} }}\left( {\frac{{U_{P}^{(i)} }}{{U_{\infty } }}} \right)^{2} } \)

D:

Drag force (N)

h :

Width of actuators exit slot (m)

\( K_{i} \) :

Weighting coefficient for defining ith actuator slot amplitude

L :

Lift force (N)

P Elect :

P Elect or Pow, power provided to the actuators (W)

P i :

The ith unsteady pressure sensor output voltage (V)

P ∞ :

Free-stream static pressure (N/m2)

q :

Free-stream dynamic pressure, \( \tfrac{1}{2}\rho U_{\infty }^{2} \) (Pa)

SensLoc i :

The ith sensor’s location (x/c)

SepLoc:

Boundary layer separation location (x/c)

Re :

Reynolds number, \( Re \equiv \frac{{\rho \cdot U_{\infty } \cdot c}}{\mu } \)

St :

St or F +, Strouhal number; \( \equiv \frac{f \cdot c}{{U_{\infty } }} \)

U ∞ :

Free-stream velocity, (m/s)

\( U_{\text{p}}^{(i)} \) :

Peak actuator slot exit velocity (m/s), superscript i is the number of the actuator slot

α :

Airfoil incidence angle (°)

μ :

Dynamic viscosity of the air (N s/m2)

ρ :

Density of the air (kg/m3)

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Acknowledgments

The assistance of Oksana Stalnov, Ilan Fono, Moshe Goldberg, Shlomo Pastuer, Avraham Blas, Eli Kronish, Mark Vassermann and Tomer Bachar is greatly appreciated. Partial financial support was provided by the Gordon family fund and the Meadow Fund.

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Authors and Affiliations

  1. School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, 69978, Tel Aviv, Israel

    Victor Troshin & Avraham Seifert

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  1. Victor Troshin
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  2. Avraham Seifert
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Correspondence to Avraham Seifert.

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This article is part of the collection Topics in Flow Control. Guest Editors J.P. Bonnet and L. Cattafesta.

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Troshin, V., Seifert, A. Performance recovery of a thick turbulent airfoil using a distributed closed-loop flow control system. Exp Fluids 54, 1443 (2013). https://doi.org/10.1007/s00348-012-1443-9

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