Quality and reliability of LES of convective scalar transfer at high Reynolds numbers

Numerical studies were performed to assess the quality and reliability of wall-modeled large eddy simulation (LES) for studying convective heat and mass transfer over bluff bodies at high Reynolds numbers (Re), with a focus on built structures in the atmospheric boundary layer. Detailed comparisons were made with both wind-tunnel experiments and field observations. The LES was shown to correctly capture the spatial patterns of the transfer coefficients around two-dimensional roughness ribs (with a discrepancy of about 20%) and the average Nusselt number (Nu) over a single wall mounted cube (with a discrepancy of about 25%) relative to wind tunnel measurements. However, the discrepancy in Re between the wind tunnel measurements and the real-world applications that the code aims to address influence the comparisons since Nu is a function of Re. Evaluations against field observations are therefore done to overcome this challenge; they reveal that, for applications in urban areas, the wind-tunnel studies result in a much lower range for the exponent m in the classic Nu∼Re m relations, compared to field measurements and LES (0.52–0.74 versus≈ 0.9). The results underline the importance of conducting experimentalor numerical studies for convective scalar transfer problems at a Re commensurate with the flow of interest, and support the use of wall-modeled LES as a technique for this problem that can already capture important aspects of the physics, although further development and testing are needed

Wake development and interactions within an array of large wind turbines

This paper presents first test results from wind tunnel studies of mean and turbulent wake characteristics within an array of large wind turbines. Up to now, a single rotating speed controlled 1:300 scale model of a 5MW-rated machine with a rotor diameter of 126m and a hub height of 90m is tested in a realistic model off-shore atmospheric boundary layer. The blade design is based on blade-element theory for low Reynolds number blade aerodynamics to comply with modelling requirements. Preliminary tests in a low-turbulence flow at a tip speed ratio of TSR=6 yielded a thrust coefficient of CT=0.52 which is within 5% of the predicted value of the theoretical design case with a lift coefficient of C= 0.6 (but a larger blade chord to mimic a higher C). Velocity measurements in the modelled off-shore boundary layer at several downstream positions suggest a transition from near to far wake at a downstream distance of approximately 4 rotor diameters D. At a downstream distance of 10D turbulence intensities in the wake of the single model turbine are still approximately twice as large as in the undisturbed boundary layer. Along with the high turbulence levels a velocity deficit of about 25% is found. Time averaged flow fields and lateral profiles of the vertical velocity clearly illustrate the characteristic swirl generated by the blade rotation, which persists until about a downstream distance of 7D

Wake development and interactions within an array of large wind turbines

This paper presents first test results from wind tunnel studies of mean and turbulent wake characteristics within an array of large wind turbines. Up to now, a single rotating speed controlled 1:300 scale model of a 5MW-rated machine with a rotor diameter of 126m and a hub height of 90m is tested in a realistic model off-shore atmospheric boundary layer. The blade design is based on blade-element theory for low Reynolds number blade aerodynamics to comply with modelling requirements. Preliminary tests in a low-turbulence flow at a tip speed ratio of TSR=6 yielded a thrust coefficient of CT=0.52 which is within 5% of the predicted value of the theoretical design case with a lift coefficient of C= 0.6 (but a larger blade chord to mimic a higher C). Velocity measurements in the modelled off-shore boundary layer at several downstream positions suggest a transition from near to far wake at a downstream distance of approximately 4 rotor diameters D. At a downstream distance of 10D turbulence intensities in the wake of the single model turbine are still approximately twice as large as in the undisturbed boundary layer. Along with the high turbulence levels a velocity deficit of about 25% is found. Time averaged flow fields and lateral profiles of the vertical velocity clearly illustrate the characteristic swirl generated by the blade rotation, which persists until about a downstream distance of 7D