The emergence of supersonic flow on wind turbines

Abstract

The future generation of wind turbines will be characterised by longer and more flexible blades. These large wind turbines are facing higher Reynolds numbers, as a consequence of longer chord lengths and increased relative wind speeds. Higher tip speeds, however, also result in an increased Mach number. Although the maximum tip speed in steady design conditions may remain (well) below the critical value, the presence of turbulence, wind gusts, blade deflections, etc. in combination with the flow acceleration over the airfoil surface, may cause a significant increase in the velocity perceived over the blade surface. We have evaluated the operational conditions of the IEA 15MW reference turbine using OpenFAST in normal design and off-design conditions to demonstrate that, if unabated, near-future wind turbines will be at risk of suffering from local supersonic flow. The driving factor is identified to be inflow turbulence, however, the tip airfoil is also of major importance. Local supersonic flow conditions may lead to severe lifetime degradation.

Numerical analysis of static and dynamic wind turbine airfoil characteristics in transonic flow

“…This turbine is strategically engineered using industry input to play a significant role in driving the ongoing evolution of next-generation wind turbine technologies. A recent study shows the potential occurrence of transonic flow under normal operational conditions, especially at cut-out wind speed, when the blade is pitched to large negative angles of attack, with shedding power in excess of rated conditions of the generator [8]. As the wind speed increases beyond rated wind speed, the blades adjust their pitch in order to maintain a rated power.…”

“…As the wind speed increases beyond rated wind speed, the blades adjust their pitch in order to maintain a rated power. [8], showed that close to cut out wind speed, the blades of the IEA 15WM reached high negative angles of attack. Figure 2 shows the operational conditions, in terms of inflow Mach number and angle of attack, in which transonic flow can occur for a given wind speed.…”

“…The first set serves to establish the transonic flow threshold using URANS. This analysis is carried out for pairs of angles of attack and Mach numbers in the vicinity of the conditions where transonic flow was predicted in [8], as shown in Figure 2. Two Reynolds numbers are considered: 1.8 and 9 million.…”

“…While earlier research recognized the potential impact of compressibility on large wind turbines due to density variations, it was only very recently that De Tavernier and von Terzi [8] demonstrated that the next-generation wind turbines could face transonic flow. They analyzed the operational condition of the IEA-15MW RWT using the software OpenFast and compressibility corrections for the airfoil polars.…”

“…The approach presented in [8] allows to identify where and when transonic flow might occur when operating a wind turbine. Nevertheless, there are uncertainties in the results due to the application of compressibility corrections on airfoil polars, as previously indicated by [2].…”