Large Eddy and RANS simulations of flows with adverse pressure gradients

• Autor(a) principal: Antonio Batista de Jesus
• Data de Publicação: 2015
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações do ITA
• Texto Completo: http://www.bd.bibl.ita.br/tde_busca/arquivo.php?codArquivo=3310
• topic: Turbulência; Gradientes; Dinâmica dos fluidos computacional; Modelos matemáticos; Física

Large Eddy Simulations (LES) are performed to study flows with adverse pressure gradients (APG) produced by different two-dimensional bumps profiles installed on the bottom wall of a flat channel with parallel plates. An initial study considers a bump profile for which previous direct numerical simulation (DNS) results are available at the Reynolds number Re? = 617, based on the channel inlet friction velocity. The results obtained showed that LES is an adequate methodology for that type of simulation, being capable of reproducing accurately DNS results in terms of mean flow variables, Reynolds stresses profiles and turbulent stresses balances, with refinement levels much coarser than those from DNS. Particularly, LES results reproduce the steep increase in wall friction coefficients and strong peaks in Reynolds stresses in the adverse pressure gradient regions of both bottom and top walls of the channel which are observed in DNS results and linked to instability modes of the near wall streaks. LES is used to study the same bump profile at higher Reynolds numbers, Re? = 950 and Re? = 2000, and the results obtained are very similar to those at Re? = 617, including the steep increase in wall friction coefficients and strong peaks in Reynolds stresses at the APG region for both attached and mildly separated flows. Additional bump profiles are generated by modifying the rear portion of the initial bump configuration, which is elongated in order to produce a longer adverse pressure gradient (APG) region and promote smoother curvature changes as compared to the original profile. As in previous cases, a steep increment in Reynolds stresses values is observed at the APG region along with an augmentation of skin friction values. LES results indicate that the near wall streaks instability observed in previous DNS is characteristic of adverse pressure gradient regions located downstream of an initial favorable pressure gradient (FPG). All configurations studied with LES are also analyzed by Reynolds-average Navier-Stokes(RANS) computations with various turbulence closures. In all cases analyzed with LES it is observed that in the initial FPG region the streamwise component of the Reynolds stresses is increased while the other turbulent stresses are strongly reduced which creates a pattern analogous to the pre-transitional stages of a laminar boundary layer, which becomes unstable in the APG region. This strong non-equilibrium conditions present a great challenge for RANS models which is approximately captured by models sensitized for laminar-turbulent transition prediction.