Extended momentum model for assessing strut aerodynamic losses in vertical axis wind turbines

• An efficient strut design tool for vertical axis wind turbines is presented. • Thin airfoils reduce parasitic losses despite the longer chord requirements. • Blade-strut interference drag is the dominant loss mechanism in thick airfoils. • Strut losses are sensitive to scale effects, not to turbine solidity. Vertical axis wind turbines offer significant potential for urban and offshore energy, yet parasitic drag from supporting struts frequently compromises their aerodynamic efficiency. Existing strut design tools are limited, leaving a gap between simplified analytical models and computationally expensive fluid dynamics simulations. This study presents a rapid, reliable method for assessing these losses by extending the double-disk multiple streamtube (DMST) framework. The model analyzes three-dimensional realistic strut designs, integrating parasitic drag and blade-strut junction interference. Validated against computational fluid dynamic simulations, the approach was tested on an urban H-rotor turbine featuring hollow, ribbed, variable section struts with distinct airfoils. Quantitative analysis revealed that the root 30% of the strut contributes only 1% to total losses, and while the thin NACA0009 reduced the maximum power coefficient by a minimal 2.8%, the thick E863 strut caused a massive 50.3% reduction in performance, driven primarily by a 39.6% drop attributed solely to junction interference drag. While strut thickness is critical for small-scale efficiency, the relative impact of parasitic losses diminishes significantly as turbine scale and operating Reynolds numbers increase. Ultimately, the developed tool enables future research to address the full complexity of multidisciplinary strut design optimization.