The separating flow in a plane asymmetric diffuser with 8.5° opening angle: mean flow and turbulence statistics, temporal behaviour and flow structures

Abstract

The flow in a plane asymmetric diffuser with an opening angle of 8.5° has been studied experimentally using time-resolving stereoscopic particle image velocimetry. The inlet condition is fully developed turbulent channel flow at a Reynolds number based on the inlet channel height and bulk velocity of Re = 38000. All mean velocity and Reynolds stress components have been measured. A separated region is found on the inclined wall with a mean separation point at 7.4 and a mean reattachment point at 30.5 inlet channel heights downstream the diffuser inlet (the inclined wall ends 24.8 channel heights downstream the inlet). Instantaneous flow reversal never occurs upstream of five inlet channel heights but may occur far downstream the point of reattachment. A strong shear layer in which high rates of turbulence production are found is located in a region outside the separation. The static wall pressure through the diffuser is presented and used in an analysis of the balance between pressure forces and momentum change. It is demonstrated that production of turbulence causes a major part of the losses of mean flow kinetic energy. The character of the large turbulence structures is investigated by means of time-resolved sequences of velocity fields and spatial auto-correlation functions. Pronounced inclined structures are observed in the spanwise velocity and it is suggested that these are due to the legs of hairpin-like vortices.

Direct numerical simulations of turbulent flow in a conical diffuser

“…When the reference height 1 − r ref /R(x) increased, the streamwise extents of the correlation were reduced in all flows. This trend was consistent with that observed by Törnblom et al [15], who conducted PIV experiments using a planar asymmetric diffuser. The outer structures are more elongated in the diffuser flow as the pressure gradient becomes stronger.…”

“…The inclination angle was insensitive to the streamwise location. Although studies of the structures in the spanwise velocity fluctuations have not been extensively explored, the structures and the hairpin vortex were recently conjectured to be correlated [15]. As shown in figure 4 of Adrian [38], the conjecture can be supported by the coherence of the spanwise velocity fluctuations around the hairpin vortex.…”

“…A comparison with the Azad diffuser showed that the profile decreased as the opening angle decreased. Numerous studies have demonstrated the presence of a rapid increasing region and a gradually increasing region in the C p profile in both an asymmetric planar diffuser and a conical diffuser [3,14,15]. Because the rapid and gradual increases represent strongly and weakly APGs, respectively, it is difficult to distinguish the rapid regions in the case of the lowest opening angle (2ϕ = 2 • ).…”

“…In addition, the necessity of DNS at high-Reynolds-number Downloaded by [Virginia Tech Libraries] at 22:13 18 October 2014 flows was proposed as a method for validating the statistical data computed from the LES against the corresponding experimental measurement. Törnblom et al [15] studied planar diffusers experimentally using stereoscopic particle image velocimetry (PIV). Large-scale flow structures were extracted from the time-resolved sequences of the velocity fields.…”

Flutter instability in an internal flow energy harvester

“…High Reynolds number numerical and experimental studies of plane-asymmetric diffusers suggest that flow separation from the diffusing wall happens for , and is independent of Reynolds number for turbulent flows (Kaltenbach et al. 1999; Lan, Armaly & Drallmeier 2009; Törnblom, Lindgren & Johansson 2009; Chandavari & Palekar 2014). Hence, we solve (5.47) for the simplified geometry in figure 17.…”