Whereas streamwise effective slope ( $ES_{x}$ ) is accepted as a key topographical parameter in the context of rough-wall turbulent flows, the significance of its spanwise counterpart ( $ES_{y}$ ) remains largely unexplored. Here, the response of turbulent channel flow over irregular, three-dimensional rough walls with systematically varied values of $ES_{y}$ is studied using direct numerical simulation. All simulations were performed at a fixed friction Reynolds number 395, corresponding to a viscous-scaled roughness height $k^{+}\approx 65.8$ (where $k$ is the mean peak-to-valley height). A surface generation algorithm is used to synthesise a set of ten irregular surfaces with specified $ES_{y}$ for three different values of $ES_{x}$ . All surfaces share a common mean peak-to-valley height and are near-Gaussian, which allows this study to focus on the impact of varying $ES_{y}$ , since roughness amplitude, skewness and $ES_{x}$ can be eliminated simultaneously as parameters. Based on an analysis of first- and second-order velocity statistics, as well as turbulence co-spectra and the fractional contribution of pressure and viscous drag, the study shows that $ES_{y}$ can strongly affect the roughness drag penalty – particularly for low- $ES_{x}$ surfaces. A secondary observation is that particular low- $ES_{y}$ surfaces in this study can lead to diminished levels of outer-layer similarity in both mean flow and turbulence statistics, which is attributed to insufficient scale separation between the outer length scale and the in-plane spanwise roughness wavelength.
Measurements of a turbulent boundary-layer developing over systematically generated roughness are acquired for friction Reynolds numbers ranging between 3000 < Re τ < 6000. A set of near-Gaussian surfaces with matched amplitude parameters and specified effective slopes in streamwise and spanwise directions are synthesised using a roughness generation algorithm. Three cases are considered: (i) an isotropic surface with equal streamwise (ES x = 0.34) and spanwise effective slope (ES y = 0.34); (ii) an anisotropic spanwise elongated surface with ES x = 0.34 and ES y = 0.17, and (iii) an anisotropic streamwise elongated surface with ES x = 0.17 and ES y = 0.34. The surfaces are manufactured from square sheets of acetal copolymer using an in-house CNC router. Note that surface (iii) is obtained by simply rotating surface (ii) by 90 degrees. The principal interest here is to quantify the sensitivity of the Hama roughness function to systematic changes in surface anisotropy. To this end, hot-wire anemometry measurements are acquired at three different freestream velocities under zero-pressure gradient conditions for each surface. Relative to the isotropic case, an increase in the turbulence intensity is seen in the near-wall region for the anisotropic cases. As expected, decreasing ES x leads to a lower mean momentum deficit which confirms the findings of many previous experimental and numerical studies. However, results also suggest that ES y plays an important role. Even for the mildly anisotropic case considered here, the roughness function is seen to vary by up to 15% as ES y is reduced while ES x is held constant. In addition, regions of high streamwise dispersive velocity are seen to extend further into the flow field as ES y reduces. These observations suggest that existing models for drag prediction need to be modified to account for surface anisotropy.
Wall reflections in particle image velocimetry (PIV) measurements is one of the limiting factors in obtaining velocity information in the vicinity of rough walls. This work tests two different techniques to suppress wall reflections for time-resolved, two-dimensional particle tracking velocimetry (2D-PTV) and stereoscopic-PIV (stereo-PIV). The 2D-PTV was performed for a turbulent boundary layer developed on a rough wall in a water towing tank facility. To measure velocity in the vicinity of the rough wall, the near-wall region on the viewing window was covered with tinted papers to mitigate the wall reflection and maintain a visible reference plane of the towed wall. Using this technique allowed detection of particles at a wall-normal distance of approximately 300 μm (10 wall units) from the roughness valleys for an imaging system with 1 m standoff distance. To suppress the wall refection for the stereo-PIV over a rough wall performed in a wind tunnel, the roughness is coated with fluorescent paint to shift the reflected light to a higher wavelength. A narrow band-pass filter was then used on the imaging lenses, allowing only particle images to be recorded while extinguishing the majority of the reflected laser light from the surface. Reliable velocity measurements with this technique were obtained up to 500 μm (5.0 wall units) from the roughness valleys.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.