Simulations of Turbulent Flow in a Plane Asymmetric Diffuser

Abstract

Large-eddy simulations (LES) of a planar, asymmetric diffuser flow have been performed. The diverging angle of the inclined wall of the diffuser is chosen as 8.5 • , a case for which recent experimental data are available. Reasonable agreement between the LES and the experiments is obtained. The numerical method is further validated for diffuser flow with the diffuser wall inclined at a diverging angle of 10 • , which has served as a test case for a number of experimental as well as numerical studies in the literature (LES, RANS). For the present results, the subgrid-scale stresses have been closed using the dynamic Smagorinsky model. A resolution study has been performed, highlighting the disparity of the relevant temporal and spatial scales and thus the sensitivity of the simulation results to the specific numerical grids used. The effect of different Reynolds numbers of the inflowing, fully turbulent channel flow has been studied, in particular, Re b = 4,500, Re b = 9,000 and Re b = 20,000 with Re b being the Reynolds number based on the bulk velocity and channel half width. The results consistently show that by increasing the Reynolds number a clear trend towards a larger separated region is evident; at least for the studied, comparably low Reynolds-number regime. It is further shown that the small separated region occurring at the diffuser throat shows the opposite behaviour as the main separation region, i.e. the flow is separating less with higher Re b . Moreover, the influence of the Reynolds number on the internal layer occurring at the non-inclined wall described in a recent study has also been assessed. It can be concluded that this region close to the upper, straight wall, is more distinct for larger Re b . Additionally,

Direct numerical simulation of the flow around a wall-mounted square cylinder under various inflow conditions

“…It is important to note that the method described here represents an improvement with respect to the so-called recycling techniques (see for instance the work by Lund et al [38]), since the auxiliary simulation provides a sufficiently long time history of the inflow so that any unphysical behaviour due to an artificial recycling frequency is avoided. In addition to this, the boundary-layer and the main cylinder simulations are not coupled, and therefore no spatial correlations or artificial frequencies will appear (Herbst et al [39]). …”

Direct numerical simulations of turbulent flow in a conical diffuser

“…Azad used pulsed-wire skin friction probes to show that the friction velocity consistently decreased along the diffuser. However, since the mean flow detached a very short distance from the diffuser throat [1,13], the present simulation indicated that the distribution inevitably decreased at the location. The rapid decrease in the skin friction also leads to decrease in the friction Reynolds number or Kármán number, defined as Re τ (x) = u τ (x)R(x)/ν (not shown).…”

“…The near-wall streaky structures in the asymmetric planar diffuser were described by Herbst et al [1]. They reported that the structures near the inclined wall became wider as the flow propagated.…”

“…The flow in the devices can be characterized as a wall-bounded flow subject to an adverse pressure gradient (APG). The performance of diffuser-like flows in technical applications is strongly affected by the separation of the boundary layer close to the wall [1]. Thus, an understanding of the turbulent structures present in diffusers is essential for improving diffuser designs to guide the wall-bounded turbulent flows under an APG.…”

“…Building on Azad and Kassab's [5] discovery of a layer in the conical diffuser, Wu et al [14] reported observation of an internal layer located close to the upper straight wall of the planar diffuser after the throat. The influence of the Reynolds number was investigated by Herbst et al [1] for an opening angle of 8.5 • . As the Reynolds number increased, the downstream region became increasingly separated.…”