Turbulence structures over irregular rough surfaces

Abstract

Turbulent flow in a channel with irregular two-dimensional rough surfaces is analysed\ud through wall-resolving large eddy simulation (LES). Both walls of the channel are\ud roughened through the superimposition of sinusoidal functions having random amplitude\ud and four different wavelengths. The downward shift of the velocity profile in the\ud log region due to the roughness, known as roughness function, is well captured in the\ud simulations. The spanwise and wall-normal turbulence intensities are found to increase\ud with the roughness height, while the streamwise component decreases. The analysis\ud of the Reynolds stress anisotropy tensor highlights a tendency towards isotropisation,\ud confirmed by the vorticity rms. The analysis of the statistics shows that the effects of\ud the roughness on the turbulent flow are greatly related to the increase of the height\ud of the maximum peaks of the corrugations. Although the inner layer is dramatically\ud affected by the wall irregularities, the outer layer appears not affected by the specific\ud wall shape and a collapse of the turbulence statistics in smooth- and rough-wall conditions\ud is observed. Much of the present results are consistent with the observations\ud made over regular roughness, nevertheless the turbulence structures and the turbulent\ud kinetic energy production analysis shows that the turbulence dynamics are directly\ud affected by the local shape of the irregularities. The spatial inhomogeneities of the\ud height of the roughness peaks and of the cavity region locally modifies the structures\ud introducing variable length-scales. Overall the irregular roughness destroys the coherent\ud pattern of the flow selectively, depending on the spatial distribution of the higher\ud roughness peaks

Parametric forcing approach to rough-wall turbulent channel flow

“…Most studies report a decrease of the near wall anisotropy by the presence of the roughness elements (Antonia & Krogstad 2001;Smalley et al 2002; . Far away from the wall a reasonably good collapse onto the smooth wall case is observed in most studies (at least for turbulent channel flow), and the collapse seems to occur later for stronger roughnesses (DeMarchis et al 2010). These observations are all mirrored in our data.…”

“…With increasing roughness height the changes in the velocity profile extend increasingly towards the middle of the channel resulting in a lower centreline velocity (see figure 6). Similar observations were made in numerical simulations where the height of the roughness elements was varied systematically (DeMarchis et al 2010). The effect of increasing the roughness factor compared to increasing h + is mainly an increased reduction of the near wall velocity.…”

“…The streamwise velocity shows in these respects for small to moderate roughness heights similar behaviour to the fully resolved direct numerical simulations discussed above. However, one significant difference can be observed: the negative minimum of the spanwise correlation the streamwise velocity increases with increasing strength of the roughness term whereas it decreases or remains approximately constant for rough wall flows (DeMarchis et al 2010;Birch & Morrison 2011) indicating that the alternating positive-negative pattern in the streamwise velocity field is weakened (Leonardi et al 2004). This difference is probably due to the damping nature of the roughness term which does not reproduce the detailed interaction of the flow with the roughness elements, leading rather to a coarsening of the structure of the velocity field than an increased level of small scale structures as one would expect for a fully resolved rough surface (see for example figure 15 in Bhaganagar et al (2004)).…”

“…In general a decrease of the streamwise correlation of the velocity field is observed in the roughness sublayer and an increase of the spanwise correlation (Ashrafian & Andersson 2006;Bhaganagar et al 2004;DeMarchis et al 2010;Leonardi et al 2004).…”

Reynolds-number dependence of the near-wall flow over irregular rough surfaces

“…For example, average streak length is estimated to be l + streak ≈ 1000 and spacing ≈100 (Smith & Metzler 1983;Chernyshenko & Baig 2005). In a rough-wall flow, the roughness will disrupt the streak-like structures and thus significantly reduce their length (Orlandi & Leonardi 2009;De Marchis et al 2010). In figure 20, contours of the wall-normal component of the instantaneous vorticity field are shown for a plane in the upper part of the canopy.…”