Effect of pressure gradients on the different stages of roughness induced boundary layer transition

• Published in: The International journal of heat and fluid flow 2020-12, Vol.86, p.108688, Article 108688
• Main Authors: Suryanarayanan, Saikishan, Goldstein, David B., Berger, Alexandre R., White, Edward B., Brown, Garry L.
• Format: Article
• Language: English
• Subjects: Boundary layers; Laminar to turbulent transition; Roughness; Vorticity dynamics
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2020.108688

•Roughness induced boundary layer transition studied using direct numerical simulations.•Pressure gradients were applied over specific streamwise extents of the flow evolution.•Favorable pressure gradient (FPG) increases vortex line compression and reduces lift-up.•FPG stabilizes the secondary (modal) instabilities.•FPG disrupts the turbulent wedge spreading cycle. The physical mechanisms of roughness-induced transition (RIT) in pressure gradient boundary layers are studied using direct numerical simulations. Recent investigations have examined RIT processes in zero-pressure-gradient boundary layers (Suryanarayanan et al., 2019). The present study uses a vorticity dynamics point of view to examine how these processes are altered by a locally accelerating or decelerating flow that strains the vorticity field and creates a net vorticity flux at the wall. Flow acceleration is imposed on specific streamwise extents of the flow. This provides an understanding about how the fundamental mechanisms in different stages of RIT are affected by pressure gradients. The present results suggest that both lift-up and subsequent amplification of the unsteady perturbations are mitigated by flow acceleration. The effect on lift-up is explained by the compression (i.e. large negative value of the stretching term) of the wall-normal vorticity by negative dv/dy. Consistent with earlier experimental observations on spots and wedges, favorable pressure gradients reduce turbulent wedge spreading and nearly arrest the spreading when sufficiently strong. This result is also explained in terms of vorticity dynamics.