Rough-wall Conditions Research Articles

Influence of randomly distributed wall roughness on the flow behaviors of liquid-liquid slug flow in a co-flowing microchannel

In present study, the influence of randomly distributed wall roughness on the flow behaviors of liquid-liquid slug flow in a co-flowing microchannel has been numerically investigated, as the manufacturing tolerance of microfluidic device in engineering application is unavoidable. A circular axisymmetric microchannel model is adopted to study the effect of roughness height and correlation length on the wall roughness generation and internal laminar flow characteristics in the microchannel. The wall roughness is modeled by well-known Gaussian distribution method. The prediction of slug velocity and liquid film thickness from numerical simulations has been well validated with the empirical and analytical correlations and experimental data in the literature. The obtained results show that the rough wall condition significantly affected the internal flow behaviors, such as slug moving velocity and slug breaking-off. According to the quantitative comparison of normalized surface areas between high (larger than 0.4 m/s) and low (smaller than 0.69 m/s) velocity region, the surface area of low velocity region under rough wall condition (wall roughness: D/l=3.333 and σ/D = 0.018) is 46 % larger than smooth wall condition, which means effective flow area of microchannel is reduced. The surface area of high velocity region under rough wall condition increases around 66 % compared with smooth wall condition. This study could be used as a promising reference and provide more understanding for roughness control in industrial microfluidic device.

Spectral Theory of Turbulent Heat Transfer in the Presence of a Rough Wall.

Using the phenomenological theory of turbulence, a direct link between the Stanton number-a dimensionless number that represents the ratio of transferred heat to the thermal capacity of the fluid-and the scalar spectrum is established for both smooth wall and rough wall conditions. The effect of different scales of motion on heat transfer is demonstrated by investigating relevant limits of the scalar spectrum. It is shown that two important observations in literature-the lack of increase in heat transfer beyond a certain roughness size and the nonclassical Prandtl number scaling-are reproduced if only the viscous inertial and diffusive range of the scalar spectrum is accounted for.

Application of CFD-DSMC coupling method in H2 flow and heat transfer in microtube

In the research of porous media and the interior of microelectronic devices, fluids frequently shuttle among pores and wall cracks, therefore it is of great significance to explore the influence of media wall on fluid flow and heat transfer. In this paper, CFD-DSMC coupled iterative algorithm is first used in the research of the flow and heat transfer of gas near the wall in the porous media, which simplified into the flow and heat transfer in micron-scale tube with smooth and rough wall conditions. It is found that compared with the pure CFD method, the velocity field obtained by the coupled method is quite different in the region close to the wall, and this will affect the flow field in the mainstream region, that is, with the higher roughness of the wall, the velocity in the central region will be smaller. There is no significant difference in the temperature distribution in the microtube before and after coupling, but the temperature in the mainstream decreases with the increase of the roughness of the wall. In addition, the temperature jump was found from some of the coupling results close to the wall. The results will contribute to a deeper understanding of the influence of rough surfaces on gas flow and heat transfer near the wall, and provide guidance for the regulation of flow and heat transfer in porous media.

Evaluation of different particle-actuation modes in molecular dynamics and their impact on nanoscale flow behaviors

An accurate understanding of nanoscale gas transport mechanism is a fundamental research concern in many engineering applications, which remains as a research challenge currently. Two particle-actuation modes, namely, force-driven and pressure-driven methods, are evaluated and compared by molecular dynamics simulations of flows in nano-channels focusing on the characteristics of gas adsorption and slip velocity behaviors. The force-driven method may lead to unphysical properties when fluid inhomogeneities are significant since all fluid molecules are subjected to a same external force. By contrast, fluid molecules move forwards through the central part of the flow domain as a predominate pathway in a pressure-driven method. Results show that there is a significant difference between the two methods at smooth or small rough wall conditions, while the results tend to be consistent as roughness increases. The density distribution is uniform along the flow direction in force-driven cases, while adsorbed gas density increases in pressure-driven cases, leading to a smaller slip velocity near the outlet region. The effects of fluid wettability strength on solid surfaces and system temperature on gas adsorption/flow behaviors are also investigated and analyzed. This study is helpful for better understanding nanoscale gas dynamics and has many practical implications, such as the shale gas production.

The Effects of Upstream Wall Roughness on the Spatio-Temporal Characteristics of Flow Separations Induced by a Forward-Facing Step

Abstract Separating and reattaching turbulent flows induced by a forward-facing step submerged in thick oncoming turbulent boundary layers (TBL) developed over smooth and rough upstream walls were investigated using time-resolved particle image velocimetry. The examined upstream walls resulted in smooth, transitionally rough, and fully rough wall conditions. The upstream boundary layer thicknesses were 4.3 and 6.7 times the step height in the smooth and rough wall cases, respectively. The Reynolds number based on the step height and freestream velocity was 7800. The effects of upstream wall roughness on the mean flow characteristics, Reynolds stresses defined in both Cartesian and curvilinear coordinate systems, as well as the unsteadiness of the turbulent separation bubbles were critically examined. The results show that upstream wall roughness increases the boundary layer thickness and turbulence intensity and consequently, promotes early mean flow reattachment over the step. Distinct regions of significantly elevated vertical Reynolds normal stress and Reynolds shear stress were observed upstream of the step in the fully rough wall case compared to the smooth wall case. Proper orthogonal decomposition (POD) and the reverse flow area over the step were employed to investigate the unsteadiness of the separation bubbles. The first POD mode coefficient and the reverse flow area over the step were strongly correlated and exhibited the same dominant frequency.

Effect of sandpaper-like small wall roughness on deflagration-to-detonation transition in a hydrogen–oxygen mixture

Deflagration-to-detonation transition (DDT) in a stoichiometric hydrogen–oxygen mixture was experimentally investigated using a channel with a sandpaper-like rough wall as a small blockage. The combustion channel was 486 mm long, 12 mm wide, and 10 mm high in the inner cross-section, and the top and bottom walls were covered with a sand cloth with surface roughness of 1000 µm by Rz and 100 µm by Ra (rough wall condition). The channel wall without the sand cloth (polished wall condition) was also tested for comparison. The entire process from the flame propagation following spark ignition to the detonation transition was visualized through optical windows on the side walls by high-speed schlieren photography. Although only the slow subsonic flame was observed in the polished wall condition, the wall roughness greatly enhanced flame acceleration and the detonation transition occurred at 120 mm downstream from the ignition. In the rough wall condition, the reaction front along the channel wall, which might propagate in the unreacted gas in the many cavities between the flame edge and the rough wall, was observed. This reaction front finally developed to form the high-speed tulip flame. The prominent reaction front near the channel wall and the accumulation of compression waves (the precompression zone), which increased the pressure up to 10–15 times the initial pressure, were observed immediately ahead the tulip flame, and this triggered the detonation onset. The estimated temperature in the precompression zone of 600–700 K was not high enough to induce instantaneous self-ignition. The present observation might indicate the experimental evidence for the possible mechanism of the final detonation onset, which was local spontaneous flame acceleration coupled with the compression wave immediately ahead of the flame front; which was suggested for highly reactive mixtures (Liberman et al. 2010).

Old and new concepts based on coherent structures for understanding sediment transport

Coherent structures develop due to an instability process on smooth beds under turbulent flow conditions and they are relevant for the incipient motion of fine sand sediment. Similar flow structures were found to be relevant for the sediment transport under rough wall conditions, but an instability process could not explain them. In this article we show that rough walls produce a new class of Λ-vortices, these vortices develop from separations on the grains, and are new relevant flow structures.

Turbulence structures over irregular rough surfaces

Turbulent flow in a channel with irregular two-dimensional rough surfaces is analysed through wall-resolving large eddy simulation (LES). Both walls of the channel are roughened through the superimposition of sinusoidal functions having random amplitude and four different wavelengths. The downward shift of the velocity profile in the log region due to the roughness, known as roughness function, is well captured in the simulations. The spanwise and wall-normal turbulence intensities are found to increase with the roughness height, while the streamwise component decreases. The analysis of the Reynolds stress anisotropy tensor highlights a tendency towards isotropisation, confirmed by the vorticity rms. The analysis of the statistics shows that the effects of the roughness on the turbulent flow are greatly related to the increase of the height of the maximum peaks of the corrugations. Although the inner layer is dramatically affected by the wall irregularities, the outer layer appears not affected by the specific wall shape and a collapse of the turbulence statistics in smooth- and rough-wall conditions is observed. Much of the present results are consistent with the observations made over regular roughness, nevertheless the turbulence structures and the turbulent kinetic energy production analysis shows that the turbulence dynamics are directly affected by the local shape of the irregularities. The spatial inhomogeneities of the height of the roughness peaks and of the cavity region locally modifies the structures introducing variable length-scales. Overall the irregular roughness destroys the coherent pattern of the flow selectively, depending on the spatial distribution of the higher roughness peaks.

Statistical structure of turbulent-boundary-layer velocity–vorticity products at high and low Reynolds numbers

The mean wall-normal gradients of the Reynolds shear stress and the turbulent kinetic energy have direct connections to the transport mechanisms of turbulent-boundary-layer flow. According to the Stokes–Helmholtz decomposition, these gradients can be expressed in terms of velocity–vorticity products. Physical experiments were conducted to explore the statistical properties of some of the relevant velocity–vorticity products. The high-Reynolds-number data (Rθ≃O(106), where θ is the momentum thickness) were acquired in the near neutrally stable atmospheric-surface-layer flow over a salt playa under both smooth- and rough-wall conditions. The low-Rθdata were from a database acquired in a large-scale laboratory facility at 1000 >Rθ> 5000. Corresponding to a companion study of the Reynolds stresses (Priyadarshana & Klewicki,Phys. Fluids, vol. 16, 2004, p. 4586), comparisons of low- and high-Rθas well as smooth- and rough-wall boundary-layer results were made at the approximate wall-normal locationsyp/2 and 2yp, whereypis the wall-normal location of the peak of the Reynolds shear stress, at each Reynolds number. In this paper, the properties of thevωz,wωyanduωzproducts are analysed through their statistics and cospectra over a three-decade variation in Reynolds number. Hereu,vandware the fluctuating streamwise, wall-normal and spanwise velocity components and ωyand ωzare the fluctuating wall-normal and spanwise vorticity components. It is observed thatv–ωzstatistics and spectral behaviours exhibit considerable sensitivity to Reynolds number as well as to wall roughness. More broadly, the correlations between thevand ω fields are seen to arise from a ‘scale selection’ near the peak in the associated vorticity spectra and, in some cases, near the peak in the associated velocity spectra as well.

Effects of Wall Roughness on Particle Velocities in a Turbulent Channel Flow

Abstract Experimental measurements using a laser Doppler anemometer (LDA) system have been performed on 150μm dense glass particles in a fully developed downward channel flow in air. Tests were conducted in smooth, rough development, and fully rough wall conditions with a channel Reynolds number of 13,800, corresponding to a centerline gas phase velocity of 10.5m∕s with a dilute loading of particles of 15% by mass fraction. Velocities were measured and statistics compared to see the nature of the effects of the wall roughness in a rebuilt channel facility originally used for important works including Kulick, Fessler, and Eaton, (1994, “Particle Response and Turbulence Modification in Fully-Developed Channel flow,” J. Fluid Mech., 277, pp. 109–134) and Paris (2001, “Turbulence Attenuation in a Particle-Laden Channel Flow,” Ph.D. thesis, Stanford University, Stanford, CA). Wall roughness has a substantial impact on gas phase mean velocities across most of the channel width, except very near the wall. The turbulence intensity of the gas phase is enhanced across the entire channel in the presence of fully rough walls. The rough walls have an even greater impact on the particle phase. Streamwise particle velocities are reduced up to 40%, and become quite uniform across the channel. Particle fluctuating velocities are nearly doubled near the channel centerplane. Profiles appear uniform, due in large part to strong transverse mixing induced by particle-wall collisions. Much of the data of Kulick and Paris is shown here to be strongly influenced by wall conditions with poorly defined roughness in the development region, followed by rapid flow recovery in a relatively smooth test section.

1142 粗面を有する二次元丘陵地まわりの乱流構造

Roughness effects on the turbulence structures over a two-dimeansional rough hill are investigated experimentally and numerically in this study. The hill concerned has a maximum slope of about 32゜ with a seperation bubble formed at its leeside. The results of smooth wall condition and rough wall condition (roughness density 45%) are compared to make clear the roughness effects. On the rough wall condition, the reattachment length is getting longer, and the wake flow is influenced by the oncoming turbulence within a longer region. The power spectrum of the velocity fluctuation measured at the wake region decreases in the low frequency regime and increases in high frequency regime much more than on the rough wall condition.

HEAT TRANSFER IN VERTICAL GRANULAR FLOWS

The effect of shearing on the eonvective heat transfer from a heater immersed in a vertical granular flaw in a channel was investigated experimentally. Comparisons were made with previous experimental and semiempirical results. Experiments were performed with smooth and rough wall conditions in order to examine the influence of boundaries on the shear rate, medium density, and particle agitation. The results indicated that the medium density adjacent to the wall played a critical role in determining the overall heat transfer.

Interaction of turbulent coherent structures and small scale structures

In this paper experimental research is carried out for the existence of large and small scale structures in turbulent boundary layer and the interactions between these structures. Based on the experimental results, a new method is suggested to describe the coherent structures, in which the interactions between the coherent and small-scale structures are considered. Using this method a new pattern recognition method is suggested and used for the coherent structures in turbulent boundary layer in smooth and rough wall conditions. The results show that the suggested method is reasonable.

Experimental investigation on secondary currents in the turbulent flow through a straight conduit

In continuation to an earlier publication, experiments have been made in the turbulent flow through a conduit of rectangular cross-section with large aspect ratio. One of the long walls has been made rough, except for a strip, located centrally. As shown in the earlier paper, secondary currents will occur in the regions of transition from smooth to rough wall-condition. The main purpose of the investigation was to check the admissibility of the simplifying assumptions made to the mechanical-energy balance equation. The results of the measurements indeed justified the neglect of unimportant terms of this equation, leading to the following rule. When in a localized region the production is much greater (smaller) than the viscous dissipation, there must be a secondary current that transports turbulence-poor fluid into (outwards) this region and turbulence-rich fluid outwards (into) the region.

Momentum transfer in climbing film flow in annular duct

AbstractThe characteristics of upward gas–liquid flow in a vertical annular duct were investigated. The flow regime studied was the climbing film regime in which water flowed as a film up the inner core of the annulus while air flowed in the annular space, the outer wall of the annulus remaining dry. Friction losses, air velocity distributions, and film characteristics were studied, the latter by photographing the climbing film through the transparent outer tube. Friction loss and film thickness were correlated with Lockhart–Martinelli parameters, X, Φ, and RL. The presence of the climbing film caused the point of maximum velocity of the air profile to move toward the outer tube, indicating that the film created a rough wall condition. The inner portion of the velocity profile was correlated by Nikuradse's rough tube equation, while the outer portion was correlated by a logarithmic equation which previous workers have reported for single‐phase flow. Kapitza's theory of wave formation was applied to the climbing film and was found to predict reasonable values for the mean film thickness. However, it failed to predict reliable values for the wavelength of surface waves.