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Discussion

Discussion: “Unsteady RANS Simulations of Wells Turbine Under Transient Flow Conditions” (Hu and Li, ASME J. Offshore Mech. Arct. Eng., 140(1), p. 011901)

[+] Author and Article Information
Tiziano Ghisu

Department of Mechanical,
Chemical and Materials Engineering,
University of Cagliari,
Cagliari 09123, Italy
e-mail: t.ghisu@unica.it

Francesco Cambuli

Department of Mechanical,
Chemical and Materials Engineering,
University of Cagliari,
Cagliari 09123, Italy
e-mail: cambuli.f@unica.it

Pierpaolo Puddu

Department of Mechanical,
Chemical and Materials Engineering,
University of Cagliari,
Cagliari 09123, Italy
e-mail: puddu@unica.it

Irene Virdis

Department of Mechanical,
Chemical and Materials Engineering,
University of Cagliari,
Cagliari 09123, Italy
e-mail: ire.virdis@studenti.unica.it

Mario Carta

Department of Mechanical,
Chemical and Materials Engineering,
University of Cagliari,
Cagliari 09123, Italy
e-mail: m.carta8@studenti.unica.it

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received June 13, 2018; final manuscript received January 15, 2019; published online March 14, 2019. Assoc. Editor: Ould el Moctar.

J. Offshore Mech. Arct. Eng 141(4), 045501 (Mar 14, 2019) (5 pages) Paper No: OMAE-18-1072; doi: 10.1115/1.4042875 History: Received June 13, 2018; Revised January 15, 2019

The work by Hu and Li (2018, “Unsteady RANS Simulations of Wells Turbine Under Transient Flow Conditions,” ASME J. Offshore Mech. Arct. Eng., 140(1), p. 011901) presents the numerical simulation of a high-solidity Wells turbine by means of a computational fluid dynamics (CFD) (Reynolds-averaged Navier–Stokes (RANS)) approach. A key aspect highlighted by the authors is the presence of a hysteretic loop in the machine's performance curves, due (according to their explanation) to the interaction of vortices shed by the blade with the blade circulation, which is responsible for the aerodynamic forces. It is our opinion that this work contains some serious errors that invalidate the results. In this brief discussion, we aim to demonstrate how the hysteresis found and discussed by the authors should not be present in the turbine analyzed in Hu and Li (2018, “Unsteady RANS Simulations of Wells Turbine Under Transient Flow Conditions,” ASME J. Offshore Mech. Arct. Eng., 140(1), p. 011901), and it is unlikely to be present in any Wells turbine. The fact that Hu and Li find hysteresis in their simulations is most likely caused by numerical errors due to an insufficient temporal discretization. This and other inaccuracies could have been avoided with a more careful consideration of the available literature.

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Figures

Grahic Jump Location
Fig. 1

Comparison of steady simulations for three grids with approximately 1 × 106, 2 × 106, and 3 × 106 cells over the operating range of the turbine

Grahic Jump Location
Fig. 2

Temporal evolution of torque coefficient for a problem studied in Ref. [3], with different time-step sizes: (a) complete simulated history and (b) fourth period

Grahic Jump Location
Fig. 3

False hysteresis caused by insufficient temporal discretization and its disappearance when using an appropriate time-step size

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