Effects of irregular two-dimensional and three-dimensional surface roughness in turbulent channel flows

Abstract

a b s t r a c tWall-resolved Large Eddy Simulation of fully developed turbulent channel flows over two different rough surfaces is performed to investigate on the effects of irregular 2D and 3D roughness on the turbulence. The two geometries are obtained through the superimposition of sinusoidal functions having random amplitudes and different wave lengths. In the 2D configuration the irregular shape in the longitudinal direction is replicated in the transverse one, while in the 3D case the sinusoidal functions are generated both in streamwise and spanwise directions. Both channel walls are roughened in such a way as to obtain surfaces with statistically equivalent roughness height, but different shapes. In order to compare the turbulence properties over the two rough walls and to analyse the differences with a smooth wall, the simulations are performed at the same Reynolds number Re s = 395. The same mean roughness height h = 0.05d (d the half channel height) is used for the rough walls.The roughness function obtained with the 3D roughness is larger than in the 2D case, although the two walls share the same mean height. Thus, the considered irregular 3D roughness is more effective in reducing the flow velocity with respect to the 2D roughness, coherently with the literature results that identified a clear dependence of the roughness function on the effective slope (see Napoli et al. (2008)), higher in the generated 3D rough wall. The analysis of higher-order statistics shows that the effects of the roughness, independently on its two-or three-dimensional shape, are mainly confined in the inner region, supporting the Townsend's wall similarity hypothesis. The tendency towards the isotropization is investigated through the ratio between the resolved Reynolds stress components, putting in light that the 3D irregular rough wall induces an higher reduction of the anisotropy, with respect to the 2D case.

Numerical observations of turbulence structure modification in channel flow over 2D and 3D rough walls

“…In fully rough regime, even though 2D and 3D rough walls are characterised by the same value r þ (% 20), the roughness function obtained with the 3D roughness is significantly higher than that obtained in the 2D case (DU þ ¼ 9:1 for 3D case and DU þ ¼ 7:6 for 2D one). This result was already pointed out by De Marchis and Napoli (2012), who concluded that the effects of 3D roughness on the mean velocities are higher than those of 2D roughness. Conversely, in transitional rough regime the velocity profiles achieved over 2D and 3D rough walls collapse each other, thus reaching the same value of roughness function DU þ ¼ 4:0.…”

“…3(b). The profiles, as pointed out by De Marchis and Napoli (2012), show that the effects of both 2D and 3D wall roughness are mainly confined in the inner layer, whereas in the most of the outer layer all profiles are quite similar.…”

Characterization of irregularly micro-structured surfaces related to their wetting properties

“…As for the irregularly structured surface, the surface roughness was characterized by three-dimensinal irregular geometry [12]. To understand the influence of surface 3D morphology on the wetting behavior, the volume ratio χ was introduced to characterize the topography of irregularly micro-structured surface (Fig.…”

“…Although our previous study has investigated the characterization of the regularly micro-grooved surface relating to its anisotropic wetting properties [11], its fabrication was costly and time-consuming. Because of the easy fabrication of irregular structures, the irregularly micro-structured surfaces with wetting properties allow for new opportunities in controlling the microflows of fluid in micro-devices [12].…”

On scaling pipe flows with sinusoidal transversely corrugated walls: analysis of data from the laminar to the low-Reynolds-number turbulent regime

“…(2008),Flack (2009) andNapoli (2012) are included in the plot. The results are presented for 0 ES 0.38 at the highest Reynolds number and are found to show a trend similar to other observations.…”