Jens Nørkær Sørensen

Control of vortex breakdown in a closed cylinder with a small rotating rod

Publisher: Elsevier BV

Date: 11-2008

DOI: 10.1016/J.JFLUIDSTRUCTS.2008.06.015

Far-wake meandering induced by atmospheric eddies in flow past a wind turbine

Publisher: Cambridge University Press (CUP)

Date: 04-05-2018

DOI: 10.1017/JFM.2018.275

Abstract: A novel algorithm is developed to calculate the nonlinear optimal boundary perturbations in three-dimensional incompressible flow. An optimal step length in the optimization loop is calculated without any additional calls to the Navier–Stokes equations. The algorithm is applied to compute the optimal inflow eddies for the flow around a wind turbine to clarify the mechanisms behind wake meandering, a phenomenon usually observed in wind farms. The turbine is modelled as an actuator disc using an immersed boundary method with the loading prescribed as a body force. At Reynolds number (based on free-stream velocity and turbine radius) $Re=1000$ , the most energetic inflow perturbation has a frequency $\\unicode[STIX]{x1D714}=0.8$ –2, and is in the form of an azimuthal wave with wavenumber $m=1$ and the same radius as the actuator disc. The inflow perturbation is lified by the strong shear downstream of the edge of the disc and then tilts the rolling-up vortex rings to induce wake meandering. This mechanism is verified by studying randomly perturbed flow at $Re\\leqslant 8000$ . At five turbine diameters downstream of the disc, the axial velocity oscillates at a magnitude of more than 60 % of the free-stream velocity when the magnitude of the inflow perturbation is 6 % of the free-stream wind speed. The dominant Strouhal number of the wake oscillation is 0.16 at $Re=3000$ and keeps approximately constant at higher $Re$ . This Strouhal number agrees well with previous experimental findings. Overall the observations indicate that the well-observed stochastic wake meandering phenomenon appearing far downstream of wind turbines is induced by large-scale (the same order as the turbine rotor) and low-frequency free-stream eddies.

Control of confined vortex breakdown with partial rotating lids

Publisher: Cambridge University Press (CUP)

Date: 29-11-2013

DOI: 10.1017/JFM.2013.596

Abstract: Experiments were conducted to determine the effectiveness of controlling vortex breakdown in a confined cylindrical vessel using a small rotating disk, which was flush-mounted into the opposite endwall to the rotating endwall driving the primary recirculating flow. The results show that the control disk, with relatively little power input, can modify the azimuthal and axial flow significantly, changing the entire flow structure in the cylinder. Co-rotation was found to precipitate vortex breakdown onset whereas counter-rotation delays it. Furthermore, for the Reynolds-number range over which breakdown normally exists, co-rotation increases the bubble radial and axial dimensions, while shifting the bubble in the upstream direction. By contrast, counter-rotation tends to reduce the size of the bubble, or completely suppress it, while shifting the bubble in the downstream direction. These effects are lified substantially by the use of larger control disks and higher rotation ratios. A series of numerical simulations close to the onset Reynolds number reveals that the control disk acts to generate a rotation-rate-invariant local positive or negative azimuthal vorticity source away from the immediate vicinity of the control disk but upstream of breakdown. Advection of this source along streamlines modifies the strength of the azimuthal vorticity ring, which effectively controls whether the flow reverses on the axis, and thus, in turn, whether vortex breakdown occurs. The vorticity source generated by the control disk scales approximately linearly with rotation ratio and cubically with disk diameter this allows the observed variation of the critical Reynolds number to be approximately predicted.

Technical University Of Denmark

Location: Denmark

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ONERA

Location: France

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Linkage - International - Grant ID: LX0668992

Start Date: 04-2006

End Date: 12-2007

Amount: $40,000.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP1096444

Start Date: 01-2010

End Date: 12-2014

Amount: $350,000.00

Funder: Australian Research Council