Abstract
The mean and instantaneous flow separation of two different three-dimensional asymmetric diffusers is analysed using the data of large-eddy simulations. The geometry of both diffusers under investigation is based on the experimental configuration of Cherry et al. (Int J Heat Fluid Flow 29(3):803–811, 2008). The two diffusers feature similar area ratios of mathrm{AR}=4.8 and mathrm{AR}=4.5 while exhibiting differing asymmetric expansion ratios of mathrm{AER}=4.5 or mathrm{AER}=2.0, respectively. The Reynolds number based on the averaged inlet velocity and height of the inlet duct is approximately {textit{Re}}=10{,}000. The time-averaged flow in both diffusers in terms of streamwise velocity profiles or the size and location of the mean backflow region are validated using experimental data. In general good agreement of simulated results with the experimental data is found. Further quantification of the flow separation behaviour and unsteadiness using the backflow coefficient reveals the volume portion in which the instantaneous reversal flow evolves. This new approach investigates the cumulative fractional volume occupied by the instantaneous backflow throughout the simulation, a power density spectra analysis of their time series reveals the periodicity of the growth and reduction phases of the flow separation within the diffusers. The dominating turbulent events responsible for the formation of the energy-containing motions including ejection and sweep are examined using the quadrant analysis at various locations. Finally, isourfaces of the Q-criterion visualise the instantaneous flow and the origin and fate of coherent structures in both diffusers.
Highlights
A diffuser is a gradual or abrupt expansion or enlargement of the cross-sectional area of a duct, pipe or canal whose main purpose is to reduce a fluid flow’s kinetic energy
The present paper investigates the influence of the aspect ratio on the development and the hydrodynamic behaviour of the flow separation within two geometrically similar asymmetric rectangular diffusers
Inflection points consist of localised loss of momentum in the velocity profile transferred from a nearby low-velocity region
Summary
A diffuser is a gradual or abrupt expansion or enlargement of the cross-sectional area of a duct, pipe or canal whose main purpose is to reduce a fluid flow’s kinetic energy. Wu et al.’s numerical study (2006) provides the structural features of the internal layer located at the flat wall of the diffuser, it is generated by the low-frequency of the turbulent fluctuation Another planar diffuser with 8◦ diverging angle was researched by Törnblom et al (2009) in which the time-averaged flow field was monitored and the energy spectra, instantaneous flow and auto-correlations were analysed to reveal the existence of large-scale hairpin vortices. This experimental study subsequently led Herbst et al (2007) to studying the influence of the Reynolds number on the mean-flow separation size and location. The typology and location of coherent structure motions responsible for the detachment and separation of the flow is identified using the Q criterion and quadrant analysis
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