Investigation of the dynamics in separated turbulent flow

Abstract

Dynamical behavior of the turbulent channel flow separation induced by a wall-mounted twodimensional bump is studied, with an emphasis on unsteadiness characteristics of vortical motions evolving in the separated flow. The present investigations are based on an experimental approach and Direct Numerical Simulation (Dns). The main interests are devoted to give further insight on mean flow properties, characteristic scales and physical mechanisms of low-frequencies unsteadiness. The study also aims to clarify the Reynolds number effects. Results are presented for turbulent flows at moderate Reynolds-number Re τ ranging from 125 to 730 where Re τ is based on friction velocity and channel half-height. A large database of time-resolved two-dimensional Piv measurements is used to obtain the velocity distributions in a region covering the entire shear layer and the flow surrounding the bump. An examination of both high resolved velocity and wall-shear stress measurements showed that for moderate Reynolds numbers, a separated region exists until a critical value. Under this conditions, a thin region of reverse flow is formed above the bump and a large-scale vortical activity is clearly observed and analyzed. Three distinct self-sustained oscillations are identified in the separated zone. The investigation showed that the flow exhibits the shear-layer instability and vortex-shedding type instability of the bubble. A low-frequency self-sustained oscillation associated with a flapping phenomenon is also identified. The experimental results are further emphasized using post-processed data from Direct Numerical Simulations, such as flow statistics and Dynamic Mode Decomposition. Physical mechanisms associated with observed self-sustained oscillations are then suggested and results are discussed in the light of instabilities observed in a laminar regime for the same flow configuration.

Outer layer turbulence dynamics in a high-Reynolds-number boundary layer up to recovering from mild separation

“…The size of the recirculation region seems to be the relevant length scale for the frequency normalisation. Indeed, Dandois, Garnier & Sagaut (2007) summarised several previous works in the literature and found the Strouhal number to be around St = 0.12-0.18 (with St = fL R /U ∞ , where f is the frequency, L R is the mean recirculation length, and U ∞ is the free stream velocity upstream of separation), a result that is also confirmed for instance in the recent work of Fadla et al (2019).…”

“…The second mode is the shedding mode associated with the vortex pairing in the shear layer. According to Hasan (1992) and the recent numerical and experimental results of Fadla et al (2019), its frequency seems to scale with the step/ramp height such that St H = 0.2. The last mode is the flapping mode.…”

“…Turbulent structures may complicate the identification of roll-up vortices due to turbulent diffusion. However, they have still been observed both numerically and experimentally as, for instance, in the works of Na & Moin (1998), Song & Eaton (2004) and Fadla et al (2019).…”

Flows over surface-mounted bluff bodies with different spanwise widths submerged in a deep turbulent boundary layer

“…Turbulent flow separation has also been exploited in some applications to enhance heat transfer, improve flow-mixing properties and stabilize combustion processes (Meinders, Hanjalic & Martinuzzi 1999;Abu-Mulaweh 2009). Motivated by these practical and fundamental considerations, flows around prototypical two-dimensional (2-D) geometries including a forward-facing step (FFS), forward-backward-facing step (FBFS) and bump (Wilhelm, Härtel & Kleiser 2003;Pearson, Goulart & Ganapathisubramani 2013;Fadla et al 2019;, as well as three-dimensional (3-D) surface-mounted bluff bodies such as a cube (Castro & Robins 1977;Hussein & Martinuzzi 1996;Hearst, Gomit & Ganapathisubramani 2016) have been the subject of intense research over the past decades. Nonetheless, the present understanding of the spatio-temporal dynamics of flow separations induced by surface-mounted bluff bodies with spanwise dimensions encompassing 3-D and 2-D configurations is deficient.…”

Superhydrophobic surfaces to reduce form drag in turbulent separated flows

“…In absence of SHSs, the bump is known to make the flow separate, creating a strong shear layer emanating from the separation point and producing a recirculating region [33][34][35] . The configuration, with no SHSs has been extensively investigated using classical statistical tools of turbulence theory [36][37][38][39][40][41] .…”