Although various experimental studies have confirmed a potential drag reduction of dimpled surfaces in turbulent boundary layers, the working mechanism behind the effect remains largely unresolved. The goal of this experimental study is to reveal the flow structures that could explain this drag reduction. To this end, flow visualizations over drag reducing dimpled surfaces are performed, using planar and stereoscopic Particle Image Velocimetry (PIV). The PIV measurements show that there is no significant vortex generation in the present dimples, but that instead a converger-diffuser type of flow occurs. It can be therefore concluded that it is not the generation of vortices which causes the drag reduction, in contrast to what has been proposed in literature. Based on the present measurements, a new drag reducing mechanism is proposed: the interaction between dimples causes alternating spanwise excitations of the near-wall flow which interacts with the turbulent coherent structures and leads to a reduction of the turbulent drag.
|Title of host publication||Proceeedings of the 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics|
|Subtitle of host publication||Lisbon, Portugal|
|Number of pages||21|
|Publication status||Published - 2016|
|Event||18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics - The Calouste Gulbenkian Foundation, Lisbon, Portugal|
Duration: 4 Jul 2016 → 7 Jul 2016
Conference number: 18
|Conference||18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics|
|Period||4/07/16 → 7/07/16|
- Passive flow control
- drag reduction
- PIV, turbulent boundary layers
van Campenhout, O., van Nesselrooij, M., Veldhuis, L., van Oudheusden, B., & Schrijer, F. (2016). Flow visualization over drag reducing dimpled surfaces in turbulent boundary layers using Particle Image Velocimetry. In Proceeedings of the 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics: Lisbon, Portugal Springer.