investigation of drag coefficient at subcritical and critical reynolds number region for circular cylinder with helical grooves

Drag reduction of an object is the major concern in many engineering applications. experimental studies have been carried out on circular cylinder with helical grooves in a subsonic wind tunnel. different cases of helical grooves with different pitches, helical groove angles and number of starts of helical groove on circular cylinder are tested. experimental results show the drag coefficient is sensitive with reynolds number and decreases at critical reynolds number and at subcritical and supercritical or transcritical reynolds number the drag coefficient increases as compared with smooth cylinder. the longitudinal grooves over the cylinder surface are tested and showed that drag coefficient much decreases at the subcritical and critical reynolds number region. the experimental results are validated with available literature and obtained good agreement.