Direct numerical simulations have been conducted to investigate a zero-pressure-gradient boundary layer flow over a single deep circular dimple with a depth $d$ of $0.1D$ and a Reynolds number ${\text{Re}}_D$ (based on the dimple diameter $D$ and freestream velocity $U$) of 20 000. This is a sequel to the earlier study on flow over a significantly shallower dimple with $d/D=0.05$ [Zhu et al., Phys. Fluids 36, 023608 (2024)]. The effect of inlet boundary layer thickness $\delta$ is explored by considering values ranging from $0.07D$ to $0.1D$. The flow features a tornado-like vortex pair within the dimple, followed by a quasiperiodically shedding hairpin vortex street downstream of the dimple. The peak frequency of the vortex shedding increases from $1.05U/D$ to $1.41U/D$ as $\delta /D$ decreases. The tornado-like vortex pair exhibits a transition from a symmetric pattern to an asymmetric pattern and finally to a quasiperiodic spanwise meandering pattern as $\delta /D$ decreases to 0.07. Both sinuous and varicose streak instabilities emerge downstream, with sinuous instabilities showing notably lower frequencies. It is found that the formation of the primary hairpin vortex downstream of the dimple is initiated by the varicose instability, which triggers the shedding of the shear layer near the downstream edge of the dimple. This shed shear layer then connects with the tornado-like vortex pair, forming a complete hairpin vortex. The sinuous instability results in visible periodic meandering of the tornado-like vortex pair at $\delta /D=0.07$, leading to an early transition to turbulence far downstream of the dimple, manifested by hairpin vortex packets.

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• Boundary layers
• Flow control
• Flow instability
• Low Reynolds number flows
• Vortex flows