Abstract
Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.
Highlights
Laminar-turbulent transition in boundary layer flows is, despite its high relevance in many aerodynamic applications, still far from being fully understood and it remains an activeFlow, Turbulence and Combustion (2020) 104:533–552 research topic within the aerodynamics community
We focus on the dynamics of boundary layer transition in the incompressible flow regime when it is promoted by an isolated three-dimensional roughness element
We studied the wake dynamics of a micro-ramp vortex generator immersed in a quasiincompressible (M∞ = 0.2) boundary layer at supercritical roughness Reynolds number (Rehh = 463) conditions
Summary
Laminar-turbulent transition in boundary layer flows is, despite its high relevance in many aerodynamic applications, still far from being fully understood and it remains an activeFlow, Turbulence and Combustion (2020) 104:533–552 research topic within the aerodynamics community. Laminar-turbulent transition in boundary layer flows is, despite its high relevance in many aerodynamic applications, still far from being fully understood and it remains an active. Surface roughness can either accelerate or delay the transition process [4, 5], which is of interest in flow control applications. We focus on the dynamics of boundary layer transition in the incompressible flow regime when it is promoted by an isolated three-dimensional roughness element. Transition predictions for this class of problems generally rely on the roughness Reynolds number, which is defined as
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