Direct numerical simulation has been performed in order to study ressure-driven turbulent flow in a rod-roughened channel at Reynolds number Reτ = 400 (based on the mean pressure gradient). Square rods were attached to both channel walls and protruded only 0.034 of the channel’s half-height into the flow. Roughness elements were spaced at 7 heights, which corresponded to the so-called “k–type” laboratory roughness.The classical logarithmic variation of the mean velocity was found to be maintained in the rough-wall channel flow. The only effect roughness had was to shift the log-profile downwards, the magnitude of which was about 7.1. This, corresponded to the upper limit of the transitionally rough region, based on the associated equivalent sand-grain roughness height. Within the layer of thickness about 3-5 times roughness height (roughness sublayer ), the dependency of the mean velocity and turbulence properties on the streamwise location with respect to the rods was revealed.Instead of viscous sublayer, an intensive shear layer was formed emanated from the crest of roughness elements. It was observed that the wall-ward transport of the kinetic energy was substantially increased very close to the wall while the transport of the kinetic energy away from the wall was relatively reduced at just about the edge of the roughness sublayer. Visualizations of the fluctuating velocities and vortices in this region revealed the presence of elongated streaky structures very similar to those routinely observed in the structure of the smooth-wall turbulence, with much shorter coherence in the streamwise direction and less organization in the spanwise direction. The intensity of the vorticity fluctuations in the roughness sublayer were increased whereas in the outer layer, they remained unaffected. The anisotropy invariant maps for the smooth and rough cases clearly showed that the state of the near-wall turbulence for the two cases were substantially different, whereas in the regions away from the wall, the two cases exhibited similarities. Generally, the results obtained from this study supported the classical wall similarity hypothesis.

An experimental and numerical study of channel flow with rough walls

DNS of turbulent flow in a rod-roughened channel

A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impi...

https://web.iitd.ac.in/~adewan/Dewan_2012_HTE_Rabijit.pdf

Recent Trends in Computation of Turbulent Jet Impingement Heat Transfer

Modeling turbulent flow over fractal trees with renormalized numerical simulation

Microbubble-based methods, in recent times, have been widely used for purification of water and wastewater. Microbubbles have several physicochemi- cal properties, which make them eminently suitable for wastewater treatment. In this review, these properties have been analyzed in detail from the perspective of appli- cation. Various types of microbubble generators and their operation principles have been discussed. The transport of gas into the aqueous phase has been explained, and the correlations to predict the volumetric mass transfer coef- ficient have been presented. Many practical applications using ozone, oxygen and air microbubbles, some of which are currently at various stages of commercialization, have been presented. Other important uses of microbubbles for wastewater treatment, namely, removal of fine solid particulate matter and oil, have also been discussed. In addition, directions for future research of microbubble technology and their potential applications have been identified.

Microbubble-aided water and wastewater purification: a review

Turbulent channel flows where there are small two-dimensional rods of square cross-section regularly arranged along one wall, normal to the average flow, are simulated without using any model. The other wall is assumed to be flat because of the limitations on computer memory. Sparsely distributed ribs modulate the flow near the wall substantially, but when the ribs are densely distributed the property of flat-wall flow is retained to a large extent. The scales of time and length used to normalize near-wall turbulence are normally wall scales based on total wall drag and viscosity. However, it is thought that these scales may not be adequate, in view of the modification of the turbulence generation mechanism. On the other hand, the flow in the layer away from the wall has been confirmed to be little affected by roughness, and scaled by total wall drag and the distance normalized by the channel half-width. It has also been confirmed that the property of turbulence away from the wall in the case of a flat wa...

Numerical simulation of channel flow with a rib-roughened wall

A Direct Numerical Simulation (DNS) is conducted for a turbulent flow in a channel bounded by one rough and another smooth wall. Rough wall is modeled by roughness elements represented by zero-volume wall-normal lines extending from the wall surface to around 15-30 in wall unit, which generate Stokes drag to the local flow. Global Reynolds number based on the mean friction velocities on two walls and channel half width is 400. It is inteded here to demonstrate the validity of the roughness model, comparing the obtained flow with available experimental results. It was confirmed that the model reproduces the experimentally obtained mean flow satisfactorily and that turbulence modification by roughness in the layer above the top of roughness elements is qualitatively coincident with the experimental results.

A DNS of a Turbulent Flow in a Rough-Wall Channel Using Roughness Elements Model

In order to develop an efficient jet mixing method, direct numerical simulations of combined jets are carried out. The Reynolds number defined with a nozzle diameter is Re = 1 500. Spatial discretization is performed by adopting a hybrid scheme of a sixth order compact scheme in the streamwise direction and Fourier series in the cross section. The distance between two jets is fixed at six times the jet diameter, and the inclination angle of the jets is changed from 45 to 70 deg. The results reveal that the turbulence intensity increases with a decrease in the inclination angle and that the jet width increases via jet excitation. These findings suggest that the diverse requirements of jet mixing control can be satisfied by a flexible combination of jets.

Direct Numerical Simulation of Jet Mixing Control Using Combined Jets

The effects of the contraction area ratio CR on the flow characteristics of the free jet issued from orifice nozzle are examined and made clear experimentally. The large vortex structure of submerged orifice water jet is visualized by tracer method and examined the effects of CR on it. The mean and fluctuating velocities of orifice air jet are measured by a single probe and hot-wire anemometry and the effects of CR on them are also examined. CR was changed from 1.00 to 0.11. As a result, the following and others are made clear newly. The centerline maximum velocity uc/um can be presented by uc/um=1.9CR3-3.55CR2+1.38CR+1.53. The maximum of uc/um=1.9CR3-3.55CR2+1.38CR +1.53 occurs at CR=0.27 and it is uc/um=1.7.

Flow Analysis of Orifice Free Jet : Effects of Contraction Ratio(Fluilds Engineering)

The effects of bubble size on an upward gas-liquid (CO2-water) flow in a vertical pipe after an abrupt expansion is investigated visually and experimentally in the present work. The mean bubble size varies from small scale (db=0.3mm) to relatively large scale (db=4.5mm). Extensive visualization experiments and PIV analysis show different flow patterns downstream of the expansion of flows containing bubbles of different sizes. The effect of bubble size is also investigated measuring the pressure distribution along the pipe and the drag of the expansion and its difference under different bubble sizes is calculated and compared with that of single-phase flow. The fluctuation phenomena occurring downstream are also investigated. The experiments are conducted under constant Reynolds number (Re=1.0×104) and volumetric gas flow rate ratio (αv=0∼10%). The present work gives valuable information about how the bubble size affects the flow characteristics even under steady flow conditions, and explains the differences between results reported by other authors investigating under similar conditions.

Koichi Tsujimoto

Papers

Numerical simulation of channel flow with a rib-roughened wall

A DNS of a Turbulent Flow in a Rough-Wall Channel Using Roughness Elements Model

Direct Numerical Simulation of Jet Mixing Control Using Combined Jets

Flow Analysis of Orifice Free Jet : Effects of Contraction Ratio(Fluilds Engineering)

Investigation of Bubble Size Effect on Vertical Upward Bubbly Two-Phase Pipe Flow Consisted With an Abrupt Expansion