Closed Channel Flow Research Articles

Statistical behavior of wall-attached motions in open- and closed-channel flows via direct numerical simulation

Direct numerical simulations at two friction Reynolds numbers (550 and 1000) have been performed to explore the statistical behaviors of wall-attached motions (WAMs) in open- and closed-channel flows (OCFs and CCFs). To fully capture the largest energetic WAMs, superlarge domain sizes are employed (24–48 π h along streamwise direction with h being the height of OCFs). Based on the analysis of linear coherence spectrum, both geometric and kinematic characteristics of WAMs are investigated. Results revealed that obvious peaks could be found in spanwise coherence spectra of u (streamwise velocity) and w (spanwise velocity) at a wavelength close to 2h. Both the coherence and energy spectra density value of OCFs are greater than those of CCFs at large wavelengths, indicating that OCFs are more wall-attached. Within y/h = 0.2–0.7, the diagnosed streamwise and spanwise wavelengths of WAMs increase roughly linearly with increasing vertical position y. In the upper region ( y / h > 0.7), the diagnosed wavelengths in OCFs are smaller than those in CCFs. In addition, the inclination angle of u fluctuates around 14° within y / h = 0.4–0.6, and angles of CCFs are slightly larger than those in OCFs when y / h > 0.6. Regarding kinematic behavior, contribution fractions of WAMs to total turbulent intensity in OCFs are always larger than those in CCFs. Additionally, the wall-attached behaviors of u were found to be more sensitive to Reynolds number than w.

A quantitative study on the turbulent kinetic energy redistribution in the near free-water-surface region of open channel flows

Open channel flows (OCFs) exhibit unique characteristics compared with other wall-bounded flows due to the presence of a free water surface. One of the well-known features is the turbulence kinetic energy (TKE) redistribution phenomenon in the near free-water-surface region. Unlike previous qualitative demonstrations, the present study aims providing a quantitative investigation of this phenomenon. Specifically designed super long domain direct numerical simulations (DNSs) of OCFs and closed channel flows (CCFs) are performed at three low-to-moderate friction Reynolds numbers (Reτ = 180–1000). The numerical configurations of the two flows (i.e., temporal and spatial resolution, domain size, Reynolds number, etc.) at identical Reτ are set to be the same, with the upper boundary being the only difference between them, to allow meaningful comparisons while super long domain sizes are adopted to facilitate fully spectral comparisons of the TKE features between the two flows. With such specifically designed DNS datasets, quantitative investigations of the TKE redistribution phenomenon in OCFs have been made. It is revealed that, as Reτ increases from 180 to 1000, the streamwise and spanwise TKE components of OCFs are higher than those of CCFs by 12%–23% and 28%–17%, respectively, while in the vertical direction OCFs are lower in TKE by 44%–31%. Overall, the TKE of OCFs is higher than that of CCFs by 3%–9% as Reτ increases from 180 to 1000. The comparison of OCFs and CCFs in pre-multiplied energy spectra reveals that VLSMs play a dominant role in the TKE redistribution phenomenon in OCFs.

Direct numerical simulation of turbulent open channel flows at moderately high Reynolds numbers

Well-resolved direct numerical simulations of turbulent open channel flows (OCFs) are performed for friction Reynolds numbers up to $Re_\tau =2000$ . Various turbulent statistics are documented and compared with the closed channel flows (CCFs). As expected, the mean velocity profiles of the OCFs match well with the CCFs in the near-wall region but diverge notably in the outer region. Interestingly, a logarithmic layer with Kárman constant $\kappa =0.363$ occurs for OCF at $Re_\tau =2000$ , distinctly different from CCF. Except for a very thin layer near the free surface, most of the velocity and vorticity variances match between OCFs and CCFs. The one-dimensional energy spectra reveal that the very-large-scale motions (VLSMs) with streamwise wavelength $\lambda _x>3 h$ or spanwise wavelength $\lambda _z>0.5 h$ contribute the most to turbulence intensity and Reynolds shear stress in the overlap and outer layers (where h is the water depth). Furthermore, the VLSMs in OCFs are stronger than those in CCFs, resulting in a slightly higher streamwise velocity variance in the former. Due to the footprint effect, these structures also have significant contributions to the mean wall shear stress, and the difference between OCF and CCF enlarges with increasing $Re_\tau$ . In summary, the free surface in OCFs plays an essential role in various flow phenomena, including the formation of stronger VLSMs and turbulent kinetic energy redistribution.

Laminar drag reduction in a closed channel using bioinspired textured surfaces

Low flow drag is of great importance to a variety of engineering applications, and an effective way to achieve low drag is to use bioinspired microstructured surfaces. This work aims to reduce the skin-friction drag in closed channel flow using textured surfaces inspired by leaves of indocalamus and rice. A channel formed from a polydimethylsiloxane chunk and a silicon wafer was fabricated to study the drag reduction behavior for water or liquid paraffin oil in laminar flow. Bioinspired textures were processed on the silicon wafer surface using the deep silicon plasma etching method. The pressure drop of water or paraffin oil passing through textured channels with different velocities was measured. The maximum pressure drop reductions for the paraffin oil flow with a low velocity (Re ≈ 1) and for the water flow with a high velocity (Re < 1000) were about 5.1 and 27.3%, respectively. The contact angles of the bioinspired textured surface are also presented, and mechanisms to explain the drag reduction are proposed. Hydrophobicity leading to a change from a liquid–solid interface to a liquid–air interface is believed to provide the drag reduction for water flow, while the thin oil film formed on the textured surface due to oleophilicity helps reduce the oil flow drag.

Characteristics of very-large-scale motions in gradually varied open-channel flows upstream of a run-of-river dam

The gradually varied open-channel flows (OCF) upstream of a run-of-river (RoR) dam are ubiquitous in natural rivers. In this flow type, the velocity profile shows some similarities to that in uniform open-channel flows, but the turbulence intensity and Reynolds shear stress are slightly greater. However, the presence and properties of very-large-scale motions (VLSMs) in such flows are still unclear. To fill this research gap, time-resolved particle image velocimetry measurements were performed upstream of a modeled RoR dam in an open-channel flume. Based on pre-multiplied spectra analysis, statistical evidence of the presence of VLSMs in the flow type is reported for the first time. The results reveal that although the typical streamwise wavelength of VLSMs in such gradually varied OCF is similar to that in other flows, such as turbulent boundary layers, closed-channel flows, pipe flows, and uniform OCF, the VLSMs in the present gradually varied OCF are stronger and contribute more streamwise turbulent kinetic energy as well as Reynolds shear stress than other flows.

A comparative study on the accuracy and conservation properties of the SPH method for fluid flow interaction with porous media

Smoothed Particle Hydrodynamics (SPH) has successfully been employed in the last decade for macroscopic simulation of fluid flow in porous media. However, there are still certain inconsistencies in the existing models with regard to both the definition of the governing equations and the adopted numerical treatments. In this study, thus, in order for a comparative investigation of the existing practices, the method of spatial averaging is employed to derive two types of macroscopic conservation equations of mass and momentum through phase averaging and intrinsic phase averaging of fluid density. Consistent discretisation methods are adopted for the determination of particle's volume, in the Weakly Compressible SPH form, and velocity by which particles move; and they are used to construct three numerical models. The models are then employed to solve two different types of problems: i) a fully saturated soil medium under the closed channel flow condition, and ii) a porous medium under a free surface flow condition with varying saturation. The results of the models are compared with analytical and experimental data; and particle distribution, water surface elevation, conservation of fluid volume, and pressure distribution are investigated. It is shown that only the latter type of modelling (i.e., intrinsic phase averaging of fluid density) leads to the conservation of fluid volume for both types of the problems, while the former type of formulation (i.e., based on phased averaged density, also known as apparent density) is suitable for closed porous media flow conditions only. Comparisons with other numerical models and the author's previous study indicate that the adopted numerical treatments, especially the δ-SPH formulation, have led to an improvement in the smoothness of particles distribution and pressure field. A few recommendations are finally made about potential further improvements of the developed models and extensions of their applicability.

Stable channel flow with spanwise heterogeneous surface temperature

Recent studies have demonstrated that large secondary motions are excited by surface roughness with dominant spanwise length scales of the order of the flow's outer length scale. Inspired by this, we explore the effect of spanwise heterogeneous surface temperature in weakly to strongly stratified closed channel flow (at$Ri_\tau =120$, 960;$Re_\tau = 180$, 550) with direct numerical simulations. The configuration consists of equally sized strips of high and low temperature at the lower and upper boundaries, while an overall stable stratification is induced by imposing an average temperature difference between the top and bottom. We consider the influence of the width of the strips (${\rm \pi} /8 \leq \lambda /h \leq 4{\rm \pi} $), Reynolds number, stability and upper boundary condition on the mean flow structure, skin friction and heat transfer. Results indicate that secondary flows are excited, with alternating high- and low-momentum pathways and vortices, similar to the patterns induced by spanwise heterogeneous surface roughness. We find that the impact of the surface heterogeneity on the outer layer depends strongly on the spanwise heterogeneity length scale of the surface temperature. Comparison to stable channel flow with uniform temperature reveals that the heterogeneous surface temperature increases the global friction coefficient and reduces the global Nusselt number in most cases. However, for the high-Reynolds cases with$\lambda /h \geq {\rm \pi} /2$, we find a reduction of the friction coefficient. At stronger stability, the vertical extent of the vortices is reduced and the impact of the heterogeneous temperature on momentum and heat transfer is smaller.

Experimental Study on the Effect of Triangular Plate Barrier Height on Flow Velocity Distribution in Open Channels

Open and closed channel flow flows are distinct, the flow in the channels will constantly change. The flow will also be altered if the water level or flow velocity changes. It was found that employing the Pitot Tube Portable yielded findings identical to those calculated by other methods. A water transfer system is made up of natural or artificial structures via which water is moved from one site to another. The carrier building may be open or closed, depending on whether you want to utilize it as a shipping or receiving location. An open channel with a relatively narrow opening at the top is known as an open conduit. The speed data collection technique is carried out vertically, with a review point as illustrated above. The velocity of the flow was measured using a Pitot Tube Portable Automatic tool. The results are entered into the Froude number (fr) after each review point to determine the type of velocity flow at each Review point. The normal flow velocity distribution pattern emerges at the measurement sites of 450 cm, 500 cm, and 550 cm.

On the scaling of large-scale structures in smooth-bed turbulent open-channel flows

This paper investigates the existence and scaling of the so-called large-scale and very-large-scale motions (LSMs and VLSMs) in non-uniform turbulent open-channel flows developing over a smooth bed in a laboratory flume. A laser Doppler anemometry system was employed to measure vertical profiles of longitudinal and bed-normal velocity statistics over a wide range of hydraulic conditions. Pre-multiplied spectra of the longitudinal velocity fluctuations revealed the existence of two peaks occurring at wavelengths consistent with those associated with LSMs and VLSMs as detected in the past literature pertaining to wall turbulence. However, contrary to so-called canonical wall flows (i.e. flat-plate boundary layers, pipe and closed-channel flows), the LSM and VLSM peaks observed in the open-channel flows investigated herein are detectable over a much larger extent of the wall-normal coordinate. Furthermore, the VLSM peak appears at von Karman numbers $Re_{\unicode[STIX]{x1D70F}}$ as low as 725, whereas in other wall flows much higher values are normally required. Finally, as conjectured by a recent study on uniform rough-bed open-channel flows, the present paper confirms that LSM wavelengths scale nicely with the flow depth, whereas the channel aspect ratio (i.e. the ratio between channel width and flow depth) is the non-dimensional parameter controlling the scaling of VLSM wavelengths. The intensity and wavelengths of the VLSM peaks were also observed to depend on the spanwise coordinate. This result suggests that VLSMs might be dynamically linked to secondary currents, as these are also known to vary in strength and size across the channel width.

Experimental Study on the Friction Drag Reduction of Superhydrophobic Surfaces in Closed Channel Flow

Experimental Study on the Friction Drag Reduction of Superhydrophobic Surfaces in Closed Channel Flow

Transport and deposition of weakly inertial particles in closed channel flows at low Reynolds number

An analytical approach is developed to show how particles sparsely distributed in steady and laminar channel flows can be transported for long distances or conversely deposited inside the channel due to the relative importance between the flow-induced drag and gravity forces. More precisely, we establish a rather simple particle trajectory equation which demonstrates that when particles’ inertia is negligible, their behavior is characterized by the channel geometry and by a dimensionless number W that relates the ratio of the particles sedimentation terminal velocity to the flow mean velocity. The proposed particle trajectory equation is verified by comparing its predictions to particle tracking numerical simulations taking into account particle inertia and fully resolving equations. The equation is shown to be valid under the conditions that flow inertial effects are limited. Based on this trajectory equation, we build a regime diagram that can predict the behavior of particles entering closed channel flows. This diagram, by relating W to the ratio of the channel mean aperture to its total length, enables to forecast if the particles entering the flow will be either deposited or transported along the channel. The influence of the channel geometry on the particle behavior is then investigated by considering channels with straight and sinusoidal walls. In particular, the effect of the corrugation amplitude, of the asymmetry and of the phase lag between the walls on the extent of the transport and deposition zones is evaluated and verified against numerical experiments. Firstly, it is shown that the regime diagram for straight channels can be used for wavy channels with in-phase walls. Secondly, it is found that increasing the phase lag between the two walls and/or the walls corrugation amplitude leads to an increase of both the transport and sedimentation zones. Finally, it is demonstrated that increasing the lower wall corrugation amplitude relatively to the upper wall corrugation enhances particle transport.

Enhancing Capillary-Driven Flow for Paper-Based Microfluidic Channels.

Paper-based microfluidic devices have received considerable interest due to their benefits with regards to low manufacturing costs, simplicity, and the wide scope of applications. However, limitations including sample retention in paper matrix and evaporation as well as low liquid flow rates have often been overlooked. This paper presents a paper-based capillary-driven flow system that speeds up flow rates by utilizing narrow gap geometry between two parallel surfaces separated by a spacer. The top surface is hydrophobic, while the bottom surface is a hydrophobic paper substrate with a microfluidic channel defined by a hydrophilic pathway, leaving sides of the channel open to air. The liquid flows on the hydrophilic path in the gap without spreading onto the hydrophobic regions. The closed-channel flow system showed higher spreading distances and accelerated liquid flow. An average flow rate increases of 200 and 100% were obtained for the nanoparticle-coated paperboard and the blotting papers used, respectively. Fast liquid delivery to detection zones or reaction implies rapid results from analytical devices. In addition, liquid drying and evaporation can be reduced in the proposed closed-channel system.

Experimental study of the solid-liquid interface in a yield-stress fluid flow upstream of a step

We present an experimental study investigating the transition zone between a liquid-like unyielded region and a solid-like yielded region in a yield-stress fluid. The configuration consists of a rectangular closed-channel flow disturbed by the presence of a step. Upstream of the step, a solid-liquid interface between a dead zone and a flow zone appears. In this study, we use a model fluid, namely polymer micro-gel Carbopol, which exhibits Herschel-Bulkley viscoplastic rheology. Exploiting the fluid transparency, the flow is monitored by particle image velocimetry using an internal visualization technique. The main outcome of this study is to show that, except in a thin transition layer close to the solid-liquid interface, the flow behaves as an apparent Poiseuille flow with an apparent slip condition at the base. The slip frontier is found to be almost independent of the flow rate while the corresponding slip velocity increases with the flow rate.

Röthlisberger channels with finite ice depth and open channel flow

AbstractThe theoretical basis of subglacial channel dynamics can be traced back to the work of Röthlisberger (1972) and Nye (1953). Röthlisberger (1972) considered the channels’ behaviour to be governed by a mix between water friction melting back the channel walls and the viscous closure of the surrounding ice; Nye (1953) derived a viscous closure rate for the ice. While their modelling is evidently well constructed, two aspects of their work have gone undeveloped. The first is the consideration of a finite glacier depth within the viscous closure law, instead of the assumption of an infinite glacier depth. The second is the allowance of a region of open channel flow, so that a channel’s water may transition from a region of closed channel flow to one where the water is exposed to the atmosphere. This paper helps close these two gaps, showing how Nye’s equation for the rate of ice closure can be modified, and how the point of transition between closed and open channel flow may be determined.

Fluid Drag Reduction with Shark‐Skin Riblet Inspired Microstructured Surfaces

AbstractEngineering marvels found throughout living nature continually provide inspiration to researchers solving technical challenges. For example, skin from fast‐swimming sharks intrigue researchers since its low‐drag riblet microstructure is applicable to many low drag and self‐cleaning (antifouling) applications. An overview of shark skin related studies that have been conducted in both open channel (external) and closed channel (internal) flow experiments is presented. Significant work has been conducted with the open channel flow, and less with closed channel flow. The results provide design guidance when developing novel low drag and self‐cleaning surfaces for applications in the medical, marine, and industrial fields. Experimental parameters include riblet geometry, continuous and segmented configurations, fluid velocity (laminar and turbulent flow), fluid viscosity (water, oil, and air), closed channel height dimensions, wettability, and scalability. The results are discussed and conceptual models are shown suggesting the effect of viscosity, coatings, and the interaction between vortices and riblet surfaces.

Bioinspired micro/nanostructured surfaces for oil drag reduction in closed channel flow

Living nature contains amazing structures and materials which provide inspiration for bioinspired solutions that can solve many of mankind's greatest challenges. Two such challenges include reducing fluid drag and promoting self-cleaning. With inspiration from promising flora and fauna, a systematic study has been conducted using a combination of actual and replica samples. These include rice leaf, butterfly wing, fish scale, and shark skin samples, as well as laser etched riblets. In order to mimic properties from actual rice leaves, select replica and riblet samples received a nanostructured coating. Drag is determined by pressure drop measurements from sample lined rectangular closed channels with laminar oil flow. Oleophilic surfaces with appropriate micro/nanostructured surfaces may allow the formation of a thin oil film believed to reduce drag. A conceptual model is presented describing the role of surface structures allowing the formation of such a drag reducing thin oil film.

Shark skin inspired low-drag microstructured surfaces in closed channel flow

Living nature is the inspiration for many innovations and continues to serve as an invaluable resource to solve technical challenges. Skin from fast swimming sharks intrigue researchers since its low-drag riblet structure is applicable to many engineering applications. In this study, riblet-lined closed channel (rectangular duct) internal flow was examined since its effect is less understood than with open channel external flow. With one experimental setup and two fluids, this study examines various dimensional aspects of microstructured riblets. Experimental parameters include riblet geometry, fluid velocity (laminar and turbulent flow), fluid viscosity, riblet combinations, channel size, wettability, and scalability. For direct comparison, the sample flow channel was fabricated to accommodate multiple samples with water and air in various flow conditions, where drag is characterized by measuring pressure drop. Results are discussed and conceptual models are shown suggesting the interaction between vortices and the riblet surfaces.

A family of explicit algebraic models for Reynolds stresses and passive scalar fluxes

A family of explicit algebraic models for Reynolds stresses and passive scalar fluxes is proposed and a few variants are discussed, according to the selected set of model constants. Validation is attempted against direct numerical simulation in a steady closed-channel flow for two values of the Reynolds number, indicating the ability of the model to reasonably predict the anisotropy tensor and the scalar flux components when considering wall functions to account for near-wall effects. The production of turbulent kinetic energy is well predicted, while the neutral turbulent Prandtl number has values in agreement with the literature. The proposed model is then compared with an experiment on a heated cylinder with slight temperature variations, with satisfactory results. The proposed model can be applied to various 2D simply-sheared flows involving a passive scalar, i.e. if stratification effects are negligible.

On the connection between reaction efficiency and interface structure in open laminar flows

We investigate numerically the steady-state properties of a mixing-controlled reaction between segregated reactants continuously inflowing–outflowing the annular region between counter-rotating coaxial cylinders in the Stokes regime. This system provides a prototypical example of an open nonchaotic flow generating steady-state kinematic mixing structures of arbitrarily fine lengthscales which can be characterized analytically. The kinematic interface structure is compared and contrasted to that of the reaction interface associated with the presence of a small but finite diffusivity. Results of accurate numerical simulations show that reaction efficiency and mixing performance are only weakly correlated with the global measure of both the kinematic and the reaction interface. Relevant information on reaction regimes at low diffusivity can instead be obtained from the analysis of the spatial distribution of the kinematic interface, which controls the localization properties of unreacted species. The generality of the results is tested for several prototypical open and closed channel flows.

MHD Effects on Heat Transfer in a Molten Salt Blanket

Heat transfer in closed channel flows of molten salts (MS)s, such as FLiBe or FLiNaBe, has been considered under specific reactor conditions. MHD effects have been accessed for two blanket concepts: self-cooled MS blanket, and dual-coolant MS blanket. The effect of heat transfer degradation due to turbulence reduction by a magnetic field in the First Wall channels of the self-cooled blanket was analyzed with the K-ε model of turbulence. In the dual-coolant blanket, the MS flow is laminar. A 2-D MHD code was used to calculate the laminar velocity profile first. Then, the temperature field was calculated using a 3-D temperature code. Reasonable interface temperatures below the material limit of 550°C, and low heat escape from the breeder zone have been demonstrated. Model limitations and the ways of their improvement are also discussed.