Unsteadiness in a large turbulent separation bubble

Abstract

The unsteady behaviour of a massively separated, pressure-induced turbulent separation bubble (TSB) is investigated experimentally using high-speed particle image velocimetry (PIV) and piezo-resistive pressure sensors. The TSB is generated on a flat test surface by a combination of adverse and favourable pressure gradients. The Reynolds number based on the momentum thickness of the incoming boundary layer is 5000 and the free stream velocity is$25~\text{m}~\text{s}^{-1}$. The proper orthogonal decomposition (POD) is used to separate the different unsteady modes in the flow. The first POD mode contains approximately 30 % of the total kinetic energy and is shown to describe a low-frequency contraction and expansion, called ‘breathing’, of the TSB. This breathing is responsible for a variation in TSB size of approximately 90 % of its average length. It also generates low-frequency wall-pressure fluctuations that are mainly felt upstream of the mean detachment and downstream of the mean reattachment. A medium-frequency unsteadiness, which is linked to the convection of large-scale vortices in the shear layer bounding the recirculation zone and their shedding downstream of the TSB, is also observed. When scaled with the vorticity thickness of the shear layer and the convection velocity of the structures, this medium frequency is very close to the characteristic frequency of vortices convected in turbulent mixing layers. The streamwise position of maximum vertical turbulence intensity generated by the convected structures is located downstream of the mean reattachment line and corresponds to the position of maximum wall-pressure fluctuations.

Measurements of pressure and velocity fluctuations in a family of turbulent separation bubbles

“…This shows that the first maximum of c p is in fact the result of two separate phenomena: on the one hand, the signature of the low-frequency breathing motion, mostly apparent at the position of maximum adverse pressure gradient, and on the other hand the increase in high-frequency pressure fluctuations caused by the turbulent structures submitted to an APG and their subsequent lift-off from the wall. This reconciles the DNS results of Na & Moin (1998b) and Abe (2017) with the conclusions of Weiss et al (2015) and Mohammed-Taifour & Weiss (2016) obtained from observations of the Large TSB only.…”

“…In all three cases, the streamwise distribution of wall-pressure fluctuations features two maxima, the first close to the position of maximum adverse pressure gradient (x/L p = 0) and the second at the end of the region of intermittent back flow (x/L p 0.75). The first (upstream) maximum was shown to be caused by the superposition of two separate phenomena occurring at approximately the same streamwise position: first, the signature of a low-frequency contraction and expansion (breathing) of the complete separation bubble, mostly noticeable at the position of maximum adverse pressure gradient, and documented by Mohammed-Taifour & Weiss (2016) in the case of the Large TSB. The amplitude of this low-frequency breathing was further shown to increase with the size of the separation bubble.…”

“…Experiments were performed in the TFT boundary-layer wind tunnel already described in detail in Mohammed-Taifour et al (2015) and Mohammed-Taifour & Weiss (2016). Briefly, the low-speed wind tunnel is of blow-down type, with a test section 3 m in length and 0.6 m in width, as illustrated in figure 1.…”

Spatio-temporal dynamics of turbulent separation bubbles

“…The thin solid vertical line marks the high-frequency f h = 0.0025 Uo/θo, the blue dashed vertical line represents the low-frequency f l = 0.001 Uo/θo, the dotted-dash vertical line marks fm = 0.002 Uo/θo. than some previous studies, because this motion agrees phenomenologically with the "breathing" phenomenon associated with the low-frequency motion in earlier studies of incompressible SBs (Pauley et al 1990;Spalart & Strelets 2000;Weiss et al 2015;Mohammed-Taifour & Weiss 2016).…”

Input-Output Analysis of a Separated Flow Past a Flat Plate

“…1. This configuration has been widely studied as a proxy for separation in the flow past a wing at high angle of attack, because it isolates the effects of separation from other geometric effects such as wall curvature [18]. The numerical simulation of the three-dimensional incompressible Navier-Stokes equations is performed with a second-order finite difference code ViCar3D [20].…”