Direct numerical simulation of turbulence over systematically varied irregular rough surfaces

2019 ◽  
Vol 862 ◽  
pp. 781-815 ◽  
Author(s):  
Y. Kuwata ◽  
Y. Kawaguchi
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Root Mean Square ◽  
Drag Force ◽  
Rough Surfaces ◽  
Skin Friction Coefficient ◽  
Root Mean Square Roughness ◽  
Mean Square ◽  
Mean Velocity ◽  
Navier Stokes Equation

Lattice Boltzmann direct numerical simulation of turbulent open-channel flows over randomly distributed hemispheres at $Re_{\unicode[STIX]{x1D70F}}=600$ is carried out to reveal the influence of roughness parameters related to a probability density function of rough-surface elevation on turbulence by analysing the spatial and Reynolds- (double-) averaged Navier–Stokes equation. This study specifically concentrates on the influence of the root-mean-square roughness and the skewness, and profiles of turbulence statistics are compared by introducing an effective wall-normal distance defined as a wall-normal integrated plane porosity. The effective distance can completely collapse the total shear stress outside the roughness sublayer, and thus the similarity of the streamwise mean velocity is clearer by introducing the effective distance. In order to examine the influence of the root-mean-square roughness and the skewness on dynamical effects that contribute to an increase in the skin friction coefficient, the triple-integrated double-averaged Navier–Stokes equation is analysed. The main contributors to the skin friction coefficient are found to be turbulence and drag force. The turbulence contribution increases with the root-mean-square roughness and/or the skewness. The drag force contribution, on the other hand, increases in particular with the root-mean-square roughness whereas an increase in the skewness does not increase the drag force contribution because it does not necessarily increase the surface area of the roughness elements. The contribution of the mean velocity dispersion induced by spatial inhomogeneity of the rough surfaces substantially increases with the root-mean-square roughness. A linear correlation is confirmed between the root-mean-square roughness and the equivalent roughness while the equivalent roughness monotonically increases with the skewness. A new correlation function based on the root-mean-square roughness and the skewness is developed with the available experimental and direct numerical simulation data, and it is confirmed that the developed correlation reasonably predicts the equivalent roughness of various types of real rough surfaces.

Direct numerical simulation of turbulent channel flow up to

2015 ◽  
Vol 774 ◽  
pp. 395-415 ◽  
Author(s):  
Myoungkyu Lee ◽  
Robert D. Moser
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Direct Numerical Simulation ◽  
Turbulent Flows ◽  
Channel Flow ◽  
Turbulent Channel Flow ◽  
Streamwise Velocity ◽  
Mean Velocity ◽  
Layer Structures ◽  
High Reynolds

A direct numerical simulation of incompressible channel flow at a friction Reynolds number ($\mathit{Re}_{{\it\tau}}$) of 5186 has been performed, and the flow exhibits a number of the characteristics of high-Reynolds-number wall-bounded turbulent flows. For example, a region where the mean velocity has a logarithmic variation is observed, with von Kármán constant ${\it\kappa}=0.384\pm 0.004$. There is also a logarithmic dependence of the variance of the spanwise velocity component, though not the streamwise component. A distinct separation of scales exists between the large outer-layer structures and small inner-layer structures. At intermediate distances from the wall, the one-dimensional spectrum of the streamwise velocity fluctuation in both the streamwise and spanwise directions exhibits $k^{-1}$ dependence over a short range in wavenumber $(k)$. Further, consistent with previous experimental observations, when these spectra are multiplied by $k$ (premultiplied spectra), they have a bimodal structure with local peaks located at wavenumbers on either side of the $k^{-1}$ range.

Autocovariance functions, root-mean-square-roughness height, and autocovariance length for rough deposits of copper, silver, and gold

1982 ◽  
Vol 25 (4) ◽  
pp. 2315-2323 ◽  
Author(s):  
G. Rasigni ◽  
F. Varnier ◽  
M. Rasigni ◽  
J. P. Palmari ◽  
A. Llebaria
Keyword(s):  
Root Mean Square ◽  
Roughness Height ◽  
Root Mean Square Roughness ◽  
Mean Square

The Quantification of Microwear on Chipped Stone Tools: Assessing the Effectiveness of Root Mean Square Roughness (Rq)

2008 ◽  
Vol 33 (2) ◽  
pp. 173-189 ◽  
Author(s):  
W. James Stemp ◽  
Ben E. Childs ◽  
Samuel Vionnet ◽  
Christopher A. Brown
Keyword(s):  
Root Mean Square ◽  
Stone Tools ◽  
Root Mean Square Roughness ◽  
Mean Square ◽  
Chipped Stone

scholarly journals Manufacture of Highly Transparent and Hazy Cellulose Nanofibril Films via Coating TEMPO-Oxidized Wood Fibers

Nanomaterials ◽  
2019 ◽  
Vol 9 (1) ◽  
pp. 107 ◽  
Author(s):  
Weisheng Yang ◽  
Liang Jiao ◽  
Wei Liu ◽  
Hongqi Dai
Keyword(s):  
Root Mean Square ◽  
Low Cost ◽  
Inorganic Nanoparticles ◽  
Root Mean Square Roughness ◽  
Mean Square ◽  
Cellulose Nanofibril ◽  
Wood Fibers ◽  
Excellent Thermal Stability ◽  
Blending Method ◽  
Coating Method

Traditionally, inorganic nanoparticles (SiO2, TiO2) have been utilized to tune the optical haze of optoelectronic devices. However, restricted to complex and costly processes for incorporating these nanoparticles, a simple and low-cost approach becomes particularly important. In this work, a simple, effective, and low-cost method was proposed to improve optical haze of transparent cellulose nanofibril films by directly depositing micro-sized 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized wood fibers (“coating” method). The obtained films had a high total transmittance of 85% and a high haze of 62%. The film samples also showed a high tensile strength of 80 MPa and excellent thermal stability. Dual sides of the obtained films had different microstructures: one side was extremely smooth (root-mean-square roughness of 6.25 nm), and the other was extremely rough (root-mean-square roughness of 918 nm). As a reference, micro-sized TEMPO-oxidized wood fibers and cellulose nanofibrils were mixed to form a transparent and hazy film (“blending” method). These results show that hazy transparent films prepared using the “coating” method exhibit superior application performances than films prepared using the “blending” method.

On the Nature of O-Rings in Contact With Rough Surfaces

1988 ◽  
Vol 110 (4) ◽  
pp. 632-637 ◽  
Author(s):  
W. E. Warren ◽  
J. G. Curro ◽  
D. E. Amos
Keyword(s):  
Theoretical Analysis ◽  
Root Mean Square ◽  
Contact Pressure ◽  
Contact Force ◽  
Rough Surfaces ◽  
Fundamental Importance ◽  
Elastic Behavior ◽  
Surface Irregularity ◽  
Mean Square ◽  
Irregular Surfaces

This work provides a theoretical analysis of the elastic behavior of an O-ring compressed between two rigid plates with irregular surfaces. Relations between deflection, contact force and contact pressure are obtained. The contact pressure, which is of fundamental importance in establishing criteria for effective sealing, is dependent upon both the amplitude and wavelength of the surface irregularity. This analysis suggests that surfaces in contact with O-ring seals should be characterized by the root mean square slope Δq in addition to the usual Ra which depends on amplitude only.

Time-Resolved Velocity Measurements in a Matched Refractive Index Facility of Randomly Packed Spheres

2018 ◽  
Author(s):  
Ethan Kappes ◽  
Mateusz Marciniak ◽  
Andrew Mills ◽  
Robert Muyshondt ◽  
Stephen King ◽  
...  
Keyword(s):  
Root Mean Square ◽  
Experimental Studies ◽  
Reynolds Numbers ◽  
Index Of Refraction ◽  
Mean Square ◽  
Reynolds Stresses ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Time Resolved ◽  
Wall Boundary

Complex geometries and randomly connected void spaces within packed beds have hindered efforts to characterize the underlying transport phenomena occurring within. In this communication, we present our experimental studies on a facility of randomly packed spheres that can be a representative of sections within a reactor core in a nuclear power plant. The results of high-fidelity velocity measurements can be seen using Time-Resolved Particle Image Velocimetry (TR-PIV) at the pore scales and near the wall boundary in the Matched Index of Refraction (MIR) facility. The MIR approach allows for a non-invasive analysis of the flow within packed spheres at the microscopic scales with high temporal and spatial resolution. Flow characteristics obtained from the TR-PIV measurements at various Reynolds numbers are presented. The results include the first- and second-order flow statistics, such as mean velocity, root-mean-square fluctuating velocity and Reynolds stresses. Effects of the wall boundary and Reynolds numbers on flow patterns are currently being investigated. Comparisons of the mean velocities, root-mean-square fluctuating velocities, and Reynolds stress components show the increase of flow mixing and turbulent intensities within the gaps between spheres in the packed bed. Sizes of recirculation regions, however, seem to be independent of the increase of Reynolds numbers.

Sign in / Sign up

Export Citation Format

Share Document