Scale Of Free-stream Turbulence Research Articles

The effects of free-stream turbulence on the performance of a model wind turbine

Free-stream turbulence characteristics play an important role in the mechanisms of power harvesting for wind turbines. Acquisitions of power and thrust from a model wind turbine of diameter 0.18 m have been carried out in a wind tunnel for a wide range of turbulent base flows, with varying free-stream turbulence intensity in the range between 3% and 16% and integral timescale spanning from 0.1 to 10 times the turbine rotation period. The results demonstrate that power is significantly affected by both the inflow turbulence scales and its intensity, while thrust is scarcely affected by free-stream turbulence. Fluctuations in the generated torques are also measured, with their behavior dominated by the free-stream turbulence scale, and only moderately affected by turbulence intensity. The frequency response of thrust fluctuations has been measured for a selected subset of operating conditions, demonstrating that the turbine thrust is unaffected by high-frequency components in the inflow. Conclusions are drawn on the necessity to match both turbulence intensity and base flow frequency content in wind tunnel studies if realistic results are to be obtained from small-scale studies.

Length Scale of Free Stream Turbulence and Its Impact on Bypass Transition in a Boundary Layer

Length Scale of Free Stream Turbulence and Its Impact on Bypass Transition in a Boundary Layer

211 境界層遷移過程に対する主流撹乱の乱れ強さとスケールの影響について(OS2-3 乱流現象の実験とシミュレーション3,OS2 乱流現象の実験とシミュレーション)

211 境界層遷移過程に対する主流撹乱の乱れ強さとスケールの影響について(OS2-3 乱流現象の実験とシミュレーション3,OS2 乱流現象の実験とシミュレーション)

Turbine Aero-Heat Transfer Studies in Rotating Research Facilities

The present paper deals with the experimental aero-heat transfer studies performed in rotating turbine research facilities. Turbine heat transfer research had significant progress in the last few decades. Since the full-scale operational conditions of a modern gas turbine dictate high temperatures well in excess of 3600 o F and pressure ratios ranging from 20 to 50, experimental forced convection heat transfer research on the gas side of a rotating turbine environment is a technically challenging task. The current paper provides a limited review of turbine heat transfer research in various facilities including short-duration blow-down and large-scale/low-speed turbine systems. Since the final status of any forced convection heat transfer problem is closely related to the detailed structure of momentum transfer in highly 3D, unsteady, rotating, and turbulent viscous flow environment, emphasis will also be placed on pertinent turbine aerodynamic features existing in turbines. The most significant parameters to simulate in a rotating aero-heat transfer facility can be listed as Reynolds number based on the blade chord, Mach number for compressibility and shock wave effects, tip speed, intensity and scale of free-stream turbulence, Strouhal number for the unsteady wake passing effects, free-stream to wall temperature ratio, coolant to free-stream temperature ratio, specific heat ratio, molecular Prandtl number of the operating gas and a rotation number for turbine aero-heat transfer work performed under rotational conditions. A flow coefficient and a loading coefficient defined by the actual turbine hardware are typically maintained during laboratory testing.

Effect of the freestream turbulence scale and the leading edge shape on boundary layer laminar-turbulent transition

The effect of the leading edge shape and the turbulence scale on laminar-turbulent transition in the flat-plate boundary layer due to grid turbulence is investigated. In the experiments, the turbulence scale was changed by a factor of three and the bluntness radius of the edge by a factor of four, all other factors being fixed. It is shown that on the plate with a sharp edge the fluctuation growth rate and the laminar-turbulent transition point depend nonmonotonically on the turbulence scale. On the blunt plate transition occurs considerably earlier than on the sharp plate.

Simulation of the Effect of the Freestream Turbulence Parameters on the Boundary Layer of a Curvilinear Airfoil

Modified variants of differential turbulence models which make it possible continuously to calculate both the entire flow region with laminar, transition and turbulent regimes and local low Reynolds number zones are proposed for investigating the flow and heat transfer in the boundary layers developing in compressible gas flow past curvilinear airfoils. The effect of the intensity and scale of free-stream turbulence and their variability along the outer boundary layer edge, as well as the combined action of the turbulence intensity and the streamwise pressure gradient in flow past blade profiles, on the heat transfer and near-wall turbulence characteristics is analyzed. The numerical results are compared with experimental and theoretical data.

Effects of the length scale of free-stream turbulence and cylinder size on heat transfer in laminar separated flows

An experimental study is conducted to investigate the combined influence of length scale of free-stream turbulence, Λ, and cylinder diameter, D, on heat transfer from slightly heated circular cylinders in cross flow. By using a hot-wire anemometer, spectrum analysis of its signal with a Fourier analyzer is employed to investigate the separated shear layer formed behind the circular cylinder. Furthermore, a resistance thermometer is used to measure the time-averaged temperature behind the cylinder and the power-spectral density of the temperature fluctuation in the thermal boundary layer. The Nusselt number at the rear stagnation point varies with the relative scale Λ/D, although the Reynolds number is the same. Correspondingly, the laminar-to-turbulence transition region in the separated shear layer moves in the streamwise direction due to the change in the length scale ratio, and at the same time its variation gives a substantial increase in the power spectrum of the temperature fluctuation. It is disclosed that flow and heat transfer behind cylinders are affected by Λ/D, and are closely related with the streamwise movement of the transition region in the separated shear layer. A correlation equation including Λ/D as the parameter is derived to predict the heat transfer coefficient at the rear stagnation point.