Introducing Perturbations into Turbulent Wall-Bounded Flow With Arrays of Long TiO2 Nanowires

2014 ◽  
Vol 137 (2) ◽  
Author(s):  
Henry A. Sodano ◽  
Aneesh Koka ◽  
Christopher R. Guskey ◽  
T. Michael Seigler ◽  
Sean C. C. Bailey
Keyword(s):  
Nanowire Array ◽  
Turbulent Channel Flow ◽  
Wall Layer ◽  
Tio2 Nanowires ◽  
Tio2 Nanowire ◽  
Wall Flow ◽  
Flow Manipulation ◽  
Turbulent Wall ◽  
Active Flow ◽  
Near Wall

A currently unexplored mechanical application of nanowires is near-wall active flow manipulation, with potential uses mixing and filtering chemicals, enhancing convective heat transfer, and reducing drag. Here, we present experimental evidence that it is possible to introduce persistent perturbations into turbulent flow with active nanowires. A TiO2 nanowire array was fabricated and installed in the bounding wall of a turbulent channel flow, and the array was oscillated by external actuation. Measurements indicated that the array increased turbulent kinetic energy throughout the entire wall layer. These findings suggest that dynamically actuated nanowires can potentially be used to implement near-wall flow control.

Instability in a viscous flow driven by streamwise vortices

2001 ◽  
Vol 432 ◽  
pp. 127-166 ◽  
Author(s):  
K. W. BRINCKMAN ◽  
J. D. A. WALKER
Keyword(s):  
Boundary Layer ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Wall Layer ◽  
External Flow ◽  
Three Dimensional Flow ◽  
Dimensional Flow ◽  
Wall Flow ◽  
Turbulent Wall ◽  
Near Wall

Unsteady separation processes at large finite, Reynolds number, Re, are considered, as well as the possible relation to existing descriptions of boundary-layer separation in the limit Re → ∞. The model problem is a fundamental vortex-driven three-dimensional flow, believed to be relevant to bursting near the wall in a turbulent boundary layer. Bursting is known to be associated with streamwise vortex motion, but the vortex/wall interactions that drive the near-wall flow toward breakdown have not yet been fully identified. Here, a simulation of symmetric counter-rotating vortices is used to assess the influence of sustained pumping action on the development of a viscous wall layer. The calculated solutions describe a three-dimensional flow at finite Re that is independent of the streamwise coordinate and consists of a crossflow plane motion, with a developing streamwise flow. The unsteady problem is constructed to mimic a typical cycle in turbulent wall layers and numerical solutions are obtained over a range of Re. Recirculating eddies develop rapidly in the near-wall flow, but these eddies are eventually bisected by alleyways which open up from the external flow region to the wall. At sufficiently high Re, an oscillation was found to develop in the streamwise vorticity field near the alleyways with a concurrent evolution of a local spiky behaviour in the wall shear. Above a critical value of Re, the oscillation grows rapidly in amplitude and eventually penetrates the external flow field, suggesting the onset of an unstable wall-layer breakdown. Local zones of severely retarded streamwise velocity are computed which are reminiscent of the low-speed streaks commonly observed in turbulent boundary layers. A number of other features also bear a resemblance to observed coherent structure in the turbulent wall layer.

Modelling smooth- and transitionally rough-wall turbulent channel flow by leveraging inner–outer interactions and principal component analysis

2019 ◽  
Vol 863 ◽  
pp. 407-453 ◽  
Author(s):  
Sicong Wu ◽  
Kenneth T. Christensen ◽  
Carlos Pantano
Keyword(s):  
Principal Component Analysis ◽  
Outer Layer ◽  
Principal Component ◽  
Turbulent Channel Flow ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Rough Wall ◽  
Smooth Wall ◽  
Wall Flow ◽  
Near Wall

Direct numerical simulations (DNS) of turbulent channel flow over rough surfaces, formed from hexagonally packed arrays of hemispheres on both walls, were performed at friction Reynolds numbers $Re_{\unicode[STIX]{x1D70F}}=200$, $400$ and $600$. The inner normalized roughness height $k^{+}=20$ was maintained for all Reynolds numbers, meaning all flows were classified as transitionally rough. The spacing between hemispheres was varied within $d/k=2$–$4$. The statistical properties of the rough-wall flows were contrasted against a complementary smooth-wall DNS at $Re_{\unicode[STIX]{x1D70F}}=400$ and literature data at $Re_{\unicode[STIX]{x1D70F}}=2003$ revealing strong modifications of the near-wall turbulence, although the outer-layer structure was found to be qualitatively consistent with smooth-wall flow. Amplitude modulation (AM) analysis was used to explore the degree of interaction between the flow in the roughness sublayer and that of the outer layer utilizing all velocity components. This analysis revealed stronger modulation effects, compared to smooth-wall flow, on the near-wall small-scale fluctuations by the larger-scale structures residing in the outer layer irrespective of roughness arrangement and Reynolds number. A predictive inner–outer model based on these interactions, and exploiting principal component analysis (PCA), was developed to predict the statistics of higher-order moments of all velocity fluctuations, thus addressing modelling of anisotropic effects introduced by roughness. The results show excellent agreement between the predicted near-wall statistics up to fourth-order moments compared to the original statistics from the DNS, which highlights the utility of the PCA-enhanced AM model in generating physics-based predictions in both smooth- and rough-wall turbulence.

Dynamical Significance of Turbulent Wall Layer Streaks

1990 ◽  
Vol 43 (5S) ◽  
pp. S219-S226 ◽  
Author(s):  
P. S. Bernard ◽  
R. A. Handler
Keyword(s):  
Reynolds Stress ◽  
Turbulence Modeling ◽  
Strong Association ◽  
Turbulent Channel Flow ◽  
Wall Layer ◽  
Low Speed ◽  
Process Fluid ◽  
Low Speed Streaks ◽  
Turbulent Wall

The role of low speed streaks in the dynamical processes leading to the generation of Reynolds stress is investigated using ensembles of computed particle paths obtained from a direct numerical simulation of turbulent channel flow. Simultaneous visualization of appropriate Eulerian fields and trajectories of fluid particles which are most indicative of Reynolds stress production are given. These graphically illustrate the occurrence of ejection events at a series of discrete locations along low speed streaks. A strong association between streamwise vortices and the ejecting fluid is found. In particular, visualization of the ejecting fluid shows the presence of vortices which drive fluid from the sides up and over the low speed regions. As part of this process fluid from within the streaks appears to be entrained outward from the wall. Some of the implications of these results for turbulence modeling will be described.

Indium Nanoparticles Capped TiO2 Nanowire Array for Boosted Photosensing Adoption

2020 ◽  
pp. 1-1
Author(s):  
Vinit Kumar Yadav ◽  
Amitabha Nath ◽  
Abhijit Das ◽  
Julaiba Mazumder ◽  
Mitra Barun Sarkar
Keyword(s):  
Nanowire Array ◽  
Tio2 Nanowire ◽  
Indium Nanoparticles

Simulating flow separation from continuous surfaces: routes to overcoming the Reynolds number barrier

2009 ◽  
Vol 367 (1899) ◽  
pp. 2885-2903 ◽  
Author(s):  
Michael Leschziner ◽  
Ning Li ◽  
Fabrizio Tessicini
Keyword(s):  
Reynolds Number ◽  
Turbulent Flows ◽  
Major Elements ◽  
Wall Layer ◽  
Navier Stokes ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Rans Models ◽  
High Reynolds ◽  
Near Wall

This paper provides a discussion of several aspects of the construction of approaches that combine statistical (Reynolds-averaged Navier–Stokes, RANS) models with large eddy simulation (LES), with the objective of making LES an economically viable method for predicting complex, high Reynolds number turbulent flows. The first part provides a review of alternative approaches, highlighting their rationale and major elements. Next, two particular methods are introduced in greater detail: one based on coupling near-wall RANS models to the outer LES domain on a single contiguous mesh, and the other involving the application of the RANS and LES procedures on separate zones, the former confined to a thin near-wall layer. Examples for their performance are included for channel flow and, in the case of the zonal strategy, for three separated flows. Finally, a discussion of prospects is given, as viewed from the writer's perspective.

Sign in / Sign up

Export Citation Format

Share Document