Rapid flow separation for transient inflow conditions versus accelerating bodies: An investigation into their equivalency

Wong, Jaime G.; Mohebbian, Ali; Kriegseis, Jochen; Rival, David E.

doi:10.1016/j.jfluidstructs.2013.04.010

Cited by 20 publications

(15 citation statements)

References 21 publications

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“…Furthermore, vortex formation in trailing edge appears as a longer vortex sheet for accelerated flows, which increases the lift . It is also worthy to mention that differences in aerodynamic loads in pre stall or post stall conditions for different accelerations are also influenced by non‐circulatory (added mass) term . The non‐circulatory term is the same term in Theodorsen instability theorem, in which the lift is summation of circulatory and non‐circulatory forces.…”

Section: Resultsmentioning

confidence: 99%

Effect of acceleration on dynamic stall of airfoil in unsteady operating conditions

2014

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In this paper the effect of accelerated flow over a moving airfoil is considered and based on the flow field around the airfoil the dynamic stall is evaluated. In contrast to ordinary pitching motion, the dynamic stall evaluation in this study is performed with a different motion pattern, in which the airfoil has a heaving motion in one direction. This motion pattern is also similar to rotation of an element of blade in horizontal axis wind turbines (HAWTs). In present investigation, the Reynolds number is changed during simulation time and variations of this parameter from initial to final values are shown by acceleration parameter. The operating Reynolds number is more than 10 6 , and a S809 airfoil is selected to move with accelerations of 1, 4 and 6 m/s 2 in normal direction to free stream. To resolve accelerated flow filed in the two-dimensional computational domain and to achieve results within a reasonable computation time, the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations are employed. The governing equations are discretized based on the finite volume approach and semi-implicit method for pressure linked equations algorithm is used for pressure-velocity coupling. Furthermore, turbulence effect on flow field is accounted using shear stress transport (SST) k-ω model. It is shown that the accelerated flow can significantly influence on the aerodynamic loads and dynamic stall trend. This study may introduce a new concept regarding dynamic stall and aerodynamic loads when the rotational acceleration is involved in HAWTs. Copyright © 2014 John Wiley & Sons, Ltd. Initial time when the unsteady operating condition starts to occur (s) (2014) Published online in Wiley Online Library (wileyonlinelibrary.com).WIND ENERGY Wind Energ.

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Section: Resultsmentioning

confidence: 99%

Effect of acceleration on dynamic stall of airfoil in unsteady operating conditions

2014

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“…The approach that has been chosen to represent the time variation of the angle of attack is somehow controversial, since it does not reproduce standard wind tunnel experiments where the aerofoil is rotated around a revolution axis (usually the line through the centre of pressure of the aerofoil). However, the two approaches lead to similar results in terms of integral quantities, with some discrepancy in the shape and evolution of the wake (see Wong et al 32 ). Finally, the remaining boundary conditions are imposed by enforcing: impermeability and no slip conditions on the aerofoil wall, periodic conditions on the planes bounding the domain in the spanwise direction, and continuity of the flow variables through the top and bottom planes generated by the C-grid shape downstream of the trailing edge.…”

Section: Mathematical and Numerical Formulationmentioning

confidence: 90%

Direct numerical simulation of the flow around an aerofoil in ramp-up motion

2016

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A detailed analysis of the flow around a NACA0020 aerofoil at Rec = 2 × 104 undergoing a ramp up motion has been carried out by means of direct numerical simulations. During the manoeuvre, the angle of attack is linearly varied in time between 0° and 20° with a constant rate of change of α̇rad=0.12U∞/c. When the angle of incidence has reached the final value, the lift experiences a first overshoot and then suddenly decreases towards the static stall asymptotic value. The transient instantaneous flow is dominated by the generation and detachment of the dynamic stall vortex, a large scale structure formed by the merging of smaller scales vortices generated by an instability originating at the trailing edge. New insights on the vorticity dynamics leading to the lift overshoot, lift crisis, and the damped oscillatory cycle that gradually matches the steady condition are discussed using a number of post-processing techniques. These include a detailed analysis of the flow ensemble average statistics and coherent structures identification carried out using the Q-criterion and the finite-time Lyapunov exponent technique. The results are compared with the one obtained in a companion simulation considering a static stall condition at the final angle of incidence α = 20°.

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“…It should be noted that recent studies have indicated that oscillation of the body instead of the oscillation of the freestream are not equivalent even at low frequencies. For the steady incident velocity, a uniform freestream ( U = U ∞ ) is considered, and for the unsteady freestream velocity, U ∞ = U mean . Both SOS and COS have the same frequency of oscillation, f , with the same reduced frequency ( k ), where

k MathClass-rel= \frac{π italicf c}{U_{MathClass-rel\infty}} MathClass-punc.

…”

Section: Simulated Casesmentioning

confidence: 99%

Effects of nonuniform incident velocity on a dynamic wind turbine airfoil

Gharali

Johnson

2014

Wind Energy

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For further insight into the performance of a horizontal axis wind turbine blade section under yaw loads, a 2D numerical simulation of a pitching S 809 airfoil under dynamic stall with an unsteady incident velocity is presented. The streamwise incident velocity and pitch angle of incidence oscillated with the same frequency but with a range of phase differences, ÄˆÄ . Changingˆcaused variation of the results, which can be highlighted as significantly augmented and dramatically damped dynamic stall loads, both increasing and decreasing trends for vortex growth time duringˆincrease, a shifted location of the maximum loads and a change in the order of the vortex pair circulation in each cycle. The results showed strong dependency on the velocity and acceleration of the freestream during dynamic stall, which categorized the results in four individual subdomains with different behaviors.

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Rapid flow separation for transient inflow conditions versus accelerating bodies: An investigation into their equivalency

Cited by 20 publications

References 21 publications

Effect of acceleration on dynamic stall of airfoil in unsteady operating conditions

Effect of acceleration on dynamic stall of airfoil in unsteady operating conditions

Direct numerical simulation of the flow around an aerofoil in ramp-up motion

Effects of nonuniform incident velocity on a dynamic wind turbine airfoil

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