671 Thermophysics and Aeromechanics, 2016, Vol. 23, No. 6 DOI: Flow structure due to hexagonal cavities and bumps on a plate surface U. Butt 1* and C. Egbers 2 1 The University of Lahore, Pakistan 2 BTU Cottbus, Cottbus, Germany E-mail: mech.usmanbutt@gmail.com * (Received July 12, 2015; in revised form December 29, 2015) We present the results of flow visualization and velocity measurements on a hexagonal structured surface. Several configurations with concave and convex hexagonal structures are investigated. Each hexagonal structure is 2.7 mm deep and 33 mm wide (width between flats) and has a height to diameter ratio of 0.05 based on equivalent diameter. Considered are flow velocities 19 m/s, 24 m/s, and 27 m/s. The flow bifurcates on the leading edge of the concave configuration into two counter rotating vortices and propagates further in streamwise direction. The circulating regions are identified by the peaks in r.m.s velocity curves. In case of concave configuration, the flow splits up into counter rotating vortical structures in a vertical plane parallel to the flow. The lower vortex rotating in the opposite direction of the flow cause the oil film fringes to drift upstream. Complex circulating regions similar to the arrangement of slices in an orange can be observed on the trailing edge of the concave hexagonal structure. Key words: flow visualization, oil film interferometry, flow control. Introduction Passive flow control methods such as bringing shallow dimples or micro riblets on the surface have intensively been investigated in the last decades. A relatively new method — the hexagonal structures — have also proved to be an effective way of influencing the flow in a similar way [1]. It is well recognized that hexagonal structures reduce overall form drag (in a similar way as shallow dimples) to give significantly lower drag coefficients. This occurs because the hexagonal structures cause the boundary layer on the cylinder or sphere to transit to turbulence further upstream, creating a small region of separated flow on the downstream portion of the cylinder. As a result, towers with structured surfaces have less aerodynamic drag caused by high velocity cross winds. The benefits of hexagonal depressions are more evident at high Reynolds numbers. In spite of these and other applications, nothing is known about the flow structure produced by individual hexagonal structures on the surfaces exposed to external, cross flows. No studies have yet been conducted to investigate the detailed, dynamic flow structure. Only limited studies are available which give some information on the flow structures produced by shallow spherical dimples. The authors of [2] describe symmetric and nonsymmetrical streamlines on flow patterns produced by dimple cavities with a variety of sizes. Cells of fluid  U. Butt and C. Egbers, 2016