The present work attempts to study the performance of an Orthopter-type small-scale Vertical Axis Wind Turbine (O-VAWT) by means of wind tunnel experiments and numerical simulations. Experiments were conducted in an open circuit wind tunnel, where torque measurements were made at five different wind speeds for three different blade aspect ratios. A three-dimensional unsteady flow simulation utilizing the Delayed Detached Eddy Simulation (DDES), based on Spalart-Allmaras (SA) turbulence model [1], is employed for it possesses a good compromise between accuracy and computational demands. The capability of the DDES model in accurately predicting massively separated turbulent flow structures adjacent to turbine blades and performance coefficients are thoroughly assessed. Effects of wind speed, blade aspect ratio, and freestream inflow angle, on the performance of the turbine are carefully analyzed. Characteristics of turbine self-start and flow field adjacent to rotating blades with different aspect ratios and wind speeds are also investigated. (C) 2018 Elsevier Ltd. All rights reserved.