Not all flows over dimpled geometries can lead to a columnar vortex generation. Based on the experimental and modeling information published up to the present time, five major flow regimes can be specified (Leontiev et al., 2002):

• Laminar regime with parallel flow threads
  
  
• Laminar regime with flow attachment inside the cavity
  
  
• Laminar regime with no flow attachment in the cavity
  
  
• Turbulent regime with flow attachment inside the cavity
  
  
• Turbulent regime with no flow attachment in the cavity

The regime oflaminar flow with parallel flow threads can be reached only at extremely low velocity or extremely high viscosity of the flowing medium, so it has limited practical significance. However, the other four flow regimes can be successfully applied to the real-life applications ranging from chemical reforming and process intensification equipment to gas turbine recuperators and waste heat recovery systems.

The flow regime depends on the flow velocity and the dimple geometry (h/D), where h is the dimple depth and D is the dimple diameter. Three types of spherical dimple geometry can be conditionally distinguished:

• shallow dimples (h/D < 0.1)
  
  
• deep dimples (h/D > 0.3)
  
  
• moderate depth dimples (h/D = 0.1–0.3)