Showing papers on "Pipe flow published in 2020"

Patterns in Wall-Bounded Shear Flows

Abstract: Experiments and numerical simulations have shown that turbulence in transitional wall-bounded shear flows frequently takes the form of long oblique bands if the domains are sufficiently large to accommodate them. These turbulent bands have been observed in plane Couette flow, plane Poiseuille flow, counter-rotating Taylor–Couette flow, torsional Couette flow, and annular pipe flow. At their upper Reynolds number threshold, laminar regions carve out gaps in otherwise uniform turbulence, ultimately forming regular turbulent–laminar patterns with a large spatial wavelength. At the lower threshold, isolated turbulent bands sparsely populate otherwise laminar domains, and complete laminarization takes place via their disappearance. We review results for plane Couette flow, plane Poiseuille flow, and free-slip Waleffe flow, focusing on thresholds, wavelengths, and mean flows, with many of the results coming from numerical simulations in tilted rectangular domains that form the minimal flow unit for the turbulent–laminar bands.

Spectral proper orthogonal decomposition and resolvent analysis of near-wall coherent structures in turbulent pipe flows

Abstract: Direct numerical simulations, performed with a high-order spectral-element method, are used to study coherent structures in turbulent pipe flow at friction Reynolds numbers . The database was analysed using spectral proper orthogonal decomposition (SPOD) to identify energetically dominant coherent structures, most of which turn out to be streaks and quasi-streamwise vortices. To understand how such structures can be modelled, the linear flow responses to harmonic forcing were computed using the singular value decomposition of the resolvent operator, using the mean field as a base flow. The SPOD and resolvent analysis were calculated for several combinations of frequencies and wavenumbers, allowing the mapping out of similarities between SPOD modes and optimal responses for a wide range of relevant scales in turbulent pipe flows. In order to explore physical reasons behind the agreement between both methods, an indicator of lift-up mechanism in the resolvent analysis was introduced, activated when optimal forcing is dominated by the wall-normal and azimuthal components, and associated response corresponds to streaks of streamwise velocity. Good agreement between leading SPOD and resolvent modes is observed in a large region of parameter space. In this region, a significant gain separation is found in resolvent analysis, which may be attributed to the strong amplification associated with the lift-up mechanism, here understood as nonlinear forcing terms leading to the appearance of streamwise vortices, which in turn form high-amplitude streaks. For both Reynolds numbers, the observed concordances were generally for structures with large energy in the buffer layer. The results highlight resolvent analysis as a pertinent reduced-order model for coherent structures in wall-bounded turbulence, particularly for streamwise elongated structures corresponding to near-wall streamwise vortices and streaks.

Heat transfer and pressure drop investigation through pipe with different shapes using different types of nanofluids

Abstract: In the present study, the heat transfer and hydrodynamic analysis of flow through single-pipe heat exchangers of circular and square cross-sectional configurations were performed. The experimental and numerical investigations were conducted to evaluate the performance of two metallic oxides (Al2O3 and SiO2) and two carbon-based nanostructured nanofluids (KRG and GNP) in comparison with the distilled water (DW). The data obtained from the experimental runs with DW as a working fluid in both test sections were used to validate the 3-D numerical models for the square and circular pipe heat exchangers. The flow in both test sections is considered as a fully developed turbulent flow with the Reynolds number range of 6000–11,000, and both the test sections were subjected to a uniform heat flux at their outer surfaces. The concentrations of all nanofluids used in the present study were in the range of (0.025–0.1 mass%). The test rig was firstly validated during the water run by using different empirical correlations for the evaluation of pressure drop and Nusselt number and showing a very good agreement, and then, the numerical models were validated with the data obtained experimentally and the errors were less than 10% for both models. For the square tube flow, the average errors between the numerical and experimental findings of Nusselt number and pressure drop were 6.8% and 2.49%, respectively, and for the circular pipe flow, the evaluated errors were 9.34% and 5.92% for Nusselt number and pressure drop, respectively. The performance index for all the nanofluids was calculated to obtain the convective heat transfer coefficients and friction losses of the fluids in both the tubes. The results showed that the non-covalent graphene–DW is not suitable for heat transfer applications due to its higher viscosity. The results also showed a different enhancement of heat transfer for the same nanofluid in circular and square tube flows, whereas the performance index of the same nanofluid appears nearly the same for flow through both the cross sections.

Nonlinear hydrodynamic instability and turbulence in pulsatile flow.

Abstract: Pulsating flows through tubular geometries are laminar provided that velocities are moderate. This in particular is also believed to apply to cardiovascular flows where inertial forces are typically too low to sustain turbulence. On the other hand, flow instabilities and fluctuating shear stresses are held responsible for a variety of cardiovascular diseases. Here we report a nonlinear instability mechanism for pulsating pipe flow that gives rise to bursts of turbulence at low flow rates. Geometrical distortions of small, yet finite, amplitude are found to excite a state consisting of helical vortices during flow deceleration. The resulting flow pattern grows rapidly in magnitude, breaks down into turbulence, and eventually returns to laminar when the flow accelerates. This scenario causes shear stress fluctuations and flow reversal during each pulsation cycle. Such unsteady conditions can adversely affect blood vessels and have been shown to promote inflammation and dysfunction of the shear stress-sensitive endothelial cell layer.

Extreme wall shear stress events in turbulent pipe flows: spatial characteristics of coherent motions

Abstract: This work presents a detailed analysis of the flow structures relevant to extreme wall shear stress events for turbulent pipe flow direct numerical simulation data at a friction Reynolds number . The results reveal that extreme positive wall-friction events are located below an intense sweep (Q4) event originated from a strong quasi-streamwise vortex at the buffer region. This vortex transports high streamwise momentum from the overlap and the outer layers towards the wall, giving rise to a high-speed streak within the inner region. This vortical structure also relates to regions with extreme wall-normal velocity. Consequently, the conditional fields of turbulence production and viscous dissipation exhibit peaks whose magnitudes are approximately 25 times higher than the ensemble mean quantities in the vicinity of the extreme positive events. An analysis of the turbulent inertia force reveals that the energetic quasi-streamwise vortex acts as an essential source of momentum at the near-wall region. Similarly, extremely rare backflow events are studied. An examination of the wall-normal vorticity and velocity vector fields shows an identifiable oblique vortical structure along with two other large-scale roll modes. These counter-rotating motions contribute to the formation of backflow events by transporting streamwise momentum from the inner to the outer region, creating a large-scale meandering low-speed streak. It is found that extreme events are clustered below large-scale structures of positive streamwise momentum that interact with near-wall low-speed streaks, related to regions densely populated with vortical structures. Finally, a three-dimensional model is proposed to conceptualise the flow dynamics associated with extreme events.

Experimental Study on Sand Transport Characteristics in Horizontal and Inclined Two-Phase Solid-Liquid Pipe Flow

Abstract: An experimental investigation on the hydraulic transport of sand particles in pipelines is presented in both horizontal and 30° upward inclined orientations. The pipe, with an internal diam...

Uniform-momentum zones in a turbulent pipe flow

Abstract: The characteristics and dynamics of the uniform-momentum zones (UMZ) and UMZ interfaces in a fully developed turbulent pipe flow are studied using direct numerical simulation at . The multiple UMZs detected from the probability density functions of the instantaneous streamwise velocity following de Silva et al. (J. Fluid Mech., vol. 786, 2016, pp. 309–331) showed similarities to both turbulent channel and boundary layer flows (TBL): the hierarchical structural distribution of thinner UMZs with thinner interfaces nearer the wall, accompanied with sharper and larger jumps in the streamwise velocity at the UMZ interface. The conditional average results indicate that channel and pipe are very similar quantitatively whereas pipe and TBL display significant discrepancies. The innermost UMZs in pipe flow exhibit different behaviours to the other UMZs in pipes. The contortion of the UMZ interface representing the meandering of coherent motions with high- and low-momentum streaks is examined three-dimensionally. The meandering of UMZ in both two and three dimensions intensifies away from the wall and is always wavier in the azimuthal direction than the streamwise direction. The UMZs in the near-wall region capture the small-scale velocity fluctuation of the near-wall cycle and show asymmetric modulation of ejections over sweeps. The asymmetric modulation of ejections over sweeps decreases from the wall towards the pipe centre and the opposite trend of elevated sweeps is observed for the innermost UMZs. Near the wall, the ejection regions are very spiky compared to the flat sweep regions whereas, in the pipe centre, the large-scale ejections are relatively flat and the sweep regions are spikier.

A Self-Powered and Low Pressure Loss Gas Flowmeter Based on Fluid-Elastic Flutter Driven Triboelectric Nanogenerator.

Abstract: A self-powered and low pressure loss gas flowmeter is presently proposed and developed based on a membrane’s flutter driven triboelectric nanogenerator (TENG). Such a flowmeter, herein named “TENG flowmeter”, is made of a circular pipe in which two copper electrodes are symmetrically fixed and a nonconductive, thin membrane is placed in the middle plane of the pipe. When a gas flows through the pipe at a sufficiently high speed, the membrane will continuously oscillate between the two electrodes, generating a periodically fluctuating electric voltage whose frequency can be easily measured. As demonstrated experimentally, the fluctuation frequency (fF) relates linearly with the pipe flow mean velocity (Um), i.e., fF ∝ Um; therefore, the volume flow rate Q (=Um × A) = C1fF + C2, where C1 and C2 are experimental constants and A is the pipe cross-sectional area. That is, by the TENG flowmeter, the pipe flow rate Q can be obtained by measuring the frequency fF. Notably, the TENG flowmeter has several advantages over some commercial flowmeters (e.g., vortex flowmeter), such as considerable lower pressure loss, higher sensitiveness of the measured flow rate, and self-powering. In addition, the effects of membrane material and geometry as well as flow moisture on the flowmeter are investigated. Finally, the performance of the TENG flowmeter is demonstrated.

Using uncertainty to improve pressure field reconstruction from PIV/PTV flow measurements

Abstract: A novel pressure reconstruction method is proposed to use the uncertainty information to improve the instantaneous pressure fields from velocity fields measured using particle image velocimetry (PIV) or particle tracking velocimetry (PTV). First, the pressure gradient fields are calculated from velocity fields, while the local and instantaneous pressure gradient uncertainty is estimated from the velocity uncertainty using a linear-transformation-based algorithm. The pressure field is then reconstructed by solving an overdetermined linear system which involves the pressure gradients and boundary conditions. This linear system is solved with generalized least squares (GLS) which incorporates the previously estimated variances and covariances of the pressure gradient errors as inverse weights to optimize the reconstructed pressure field. The method was validated with synthetic velocity fields of a 2D pulsatile flow, and the results show significantly improved pressure accuracy. The pressure error reduction by GLS was 50% with 9.6% velocity errors and 250% with 32.1% velocity errors compared to the existing baseline method of solving the pressure Poisson equation (PPE). The GLS was more robust to the velocity errors and provides greater improvement with spatially correlated velocity errors. For experimental validation, the volumetric pressure fields were evaluated from the velocity fields measured using 3D PTV of a laminar pipe flow with a Reynolds number of 630 and a transitional pipe flow with a Reynolds number of 3447. The GLS reduced the median absolute pressure errors by as much as 96% for the laminar pipe flow compared to PPE. The mean pressure drop along the pipe predicted by GLS was in good agreement with the empirical estimation using Darcy–Weisbach equation for the transitional pipe flow.

Electron spin-vorticity coupling in low and high Reynolds number pipe flows

Abstract: Spin hydrodynamic coupling is a recently discovered method to directly generate electricity from an electrically conducting fluid flow in the absence of Lorentz forces. This method relies on a collective coupling of electron spins - the internal quantum mechanical angular momentum of the electrons - with the local vorticity of a fluid flow. In this work, we experimentally investigate the spin hydrodynamic coupling in circular and non-circular capillary pipe flows and extend a previously obtained range of Reynolds numbers to smaller and larger values, 20

On the wake flow behind a sphere in a pipe flow at low Reynolds numbers

Abstract: Numerical simulations are carried out to investigate the flow around a stationary sphere in a pipe. Seven sphere diameters d = 0.1D–0.9D (D is the diameter of the pipe) are chosen to investigate the effects of the blockage ratio on the flow characteristics. Three series of simulations are conducted. The first series of simulations is based on a fixed pipe flow Reynolds number Rep = 1250 (based on the inlet mean velocity and D) and the sphere Reynolds number Res (based on the sphere cross-sectional mean velocity and d) that is varying in the range of 249 ≤ Res ≤ 1360. The second series of simulations is based on a fixed Res = 500 and Rep varying between 460 ≤ Rep ≤ 678. The third series of simulations is based on a fixed Reg = 500 (based on the mean velocity of flow through the gap between the sphere and the pipe wall and d) and Rep varying between 113 ≤ Rep ≤ 773. The instantaneous vortical structures are presented to show different flow patterns behind the spheres with different d. For the sphere with the small diameter (d ≤ 0.5D), the vortex shedding in the wake flow behind the sphere is similar to that with the sphere subjected to a uniform flow. However, for the sphere with a larger diameter (d ≥ 0.7D), the flow behind the sphere is different from the sphere subjected to a uniform flow. At Rep = 1250, the large-scale vortex shedding behind the sphere is suppressed for d ≥ 0.8D and strong small-scale vortical structures are formed behind the sphere. At Res = 500, different behaviors of wake flow are observed with the increasing d. It is found that the vortex shedding is stabilized for 0.4D ≤ d ≤ 0.7D due to the confinement of the pipe wall, while the wake vortices become chaotic for d ≥ 0.8D due to the interaction between the wake flow and the pipe wall boundary layer. The vortex shedding is suppressed for d ≥ 0.9D. At Reg = 500, the wake flow behind the sphere is stabilized with the increasing d. The combined effects of the blockage ratio and Reynolds number on the flow pattern in the wake region, the hydrodynamic quantities of the sphere, and the power spectra of the velocities at different detection points are discussed in detail. Furthermore, in addition to power spectral analysis, sparsity-promoted Dynamic Mode Decomposition (SPDMD) is used to analyze the dominant flow modes in the wake region for different blockage ratios. The dominant flow characters associated with the hairpin vortex shedding, the Kelvin–Helmholtz instability, and the low-frequency modulation of the wake flow can be captured by the DMD modes, and their spatial structures are revealed by the mode shapes.

Internal shear layers and edges of uniform momentum zones in a turbulent pipe flow

Abstract: This paper provides an experimental investigation on the internal shear layers and the edges of the uniform momentum zones (UMZs) in a turbulent pipe flow. The time-resolved stereoscopic particle image velocimetry data are acquired in the cross-section of the pipe, and span the range of Reynolds number Reτ=340--1259. In the first part of the study, internal shear layers are detected using a three-dimensional detection method, and both their geometry as well as their fingerprint in the flow statistics are examined. Three-dimensional conditional mean flow analysis revealed a strong low-speed region beneath the average shear layers. This low-speed region is associated with positive wall-normal fluctuations, and it is accompanied by two swirling motions having opposite signs on either side in the azimuthal direction. Moreover, the shear layers are stretched by the two opposite azimuthal motions. In the second part of the study, the shear layers are treated as the continuous edges of the UMZs, which are detected using the histogram method following Adrian et al. (J. Fluid Mech., vol. 422, 2000, pp. 1–54) and de Silva et al. (J. Fluid Mech., vol. 786, 2016, pp. 309–331). For this part, two different orientation of the planes are used, i.e. the wall-normal–streamwise plane and the wall-normal–spanwise plane (cross-section of the pipe). Comparison of the detected structures shows that the shear layers mostly overlap with a UMZ edge (in either plane).

Transient flow of liquid in plastic pipes

Abstract: Nowadays, plastic pipes play a key role in fluid conveyance, for example, in urban water supply systems. Dynamic analysis of designed water pipe networks requires the use of numerical methods that allow for solving basic equations that describe transient flows occurring in plastic pipes. In this paper, the main equations were formulated with pressure (p) and velocity (v) as the principal variables. A novel simplified retarded strain solution is used to properly model the pipe-wall viscoelasticity effect during transient flow process. Unsteady friction is calculated with the use of a filtered weighting function (with three exponential terms). The proposed numerical method enables the efficient modelling of transient flow in plastic pressurized pipes. Extensive simulations are performed and compared with experimental results known from three different European research centres (London, Cassino, and Lyon), with the aim of demonstrating the impacts of plastic pipe properties and frequency-dependent hydraulic resistance on transient pipe flow oscillations. The effectiveness of the proposed method for determining the creep functions of plastic pipes is also examined and discussed.

Upper edge of chaos and the energetics of transition in pipe flow

Abstract: In the past two decades, our understanding of the transition to turbulence in shear flows with linearly stable laminar solutions has greatly improved. Regarding the susceptibility of the laminar flow, two concepts have been particularly useful: the edge states and the minimal seeds. In this nonlinear picture of the transition, the basin boundary of turbulence is set by the edge state's stable manifold and this manifold comes closest in energy to the laminar equilibrium at the minimal seed. We begin this paper by presenting numerical experiments in which three-dimensional perturbations are too energetic to trigger turbulence in pipe flow but they do lead to turbulence when their amplitude is reduced. We show that this seemingly counterintuitive observation is in fact consistent with the fully nonlinear description of the transition mediated by the edge state. In order to understand the physical mechanisms behind this process, we measure the turbulent kinetic energy production and dissipation rates as a function of the radial coordinate. Our main observation is that the transition to turbulence relies on the energy amplification away from the wall, as opposed to the turbulence itself, whose energy is predominantly produced near the wall. This observation is further supported by the similar analyses on the minimal seeds and the edge states. Furthermore, we show that the time evolution of production-over-dissipation curves provides a clear distinction between the different initial amplification stages of the transition to turbulence from the minimal seed.

Critical Point for Bifurcation Cascades and Featureless Turbulence

Abstract: In this Letter we show that a bifurcation cascade and fully sustained turbulence can share the phase space of a fluid flow system, resulting in the presence of competing stable attractors. We analyze the toroidal pipe flow, which undergoes subcritical transition to turbulence at low pipe curvatures (pipe-to-torus diameter ratio) and supercritical transition at high curvatures, as was previously documented. We unveil an additional step in the bifurcation cascade and provide evidence that, in a narrow range of intermediate curvatures, its dynamics competes with that of sustained turbulence emerging through subcritical transition mechanisms.

Experiment Study of the Failure Mechanism and Evolution Characteristics of Water–Sand Inrush Geo-Hazards

Abstract: Water–sand inrush disasters are frequently encountered during underground engineering construction in karst terrain. The objective of this paper is to study the failure mechanism and evolution characteristics of water–sand inrush caused by the instability of filling medium in karst cavity, as well as the impacts of soil compactness, hydraulic pressure and confining pressure on the instability process. In response to this purpose, a stress-controlled seepage test apparatus in consideration of particle loss was designed, and a series of seepage tests were performed correspondingly. The test results indicate that: (1) Based on the nonlinear feature analysis of water-outflow pattern, the water–sand inrush process can be divided into the “slow flow” stage, “transition flow” stage and “pipe flow” stage by Transition Point I, II. (2) The decreasing soil compactness and increasing hydraulic pressure both exponentially facilitate the seepage-erosion process by increasing the particle-erosion ability; the increasing confining pressure extends the “slow flow” stage and shortens the duration of the “transition flow” stage, ultimately advancing the occurrence of the “pipe flow” stage; the existence of critical hydraulic pressure for the seepage-erosion progress is confirmed, the occurrence of the “pipe flow” stage is significantly advanced once the hydraulic pressure over the critical value. (3) The particle loss caused by the seepage-erosion process is the internal mechanism of water–sand inrush, the variation characteristics of water-outflow pattern are crucial external manifestations correspondingly. Therefore, with the monitoring of water-outflow pattern variation tendency as indicators, the critical status of water–sand inrush can be near-real-time identified, which offers experimental foundation for the early warning and forecast of the occurrence of water–sand inrush.

Internal Flow Analysis of a Heat Transfer Enhanced Tube with a Segmented Twisted Tape Insert

Abstract: A heat exchanger is a device that transfers unneeded heat from one region to another, and transferred heat may be fully reused, thus improving energy efficiency. To augment this positive process, many studies and investigations on automation technologies have been performed. Inserts are widely used in pipe flow for heat transfer enhancement, since they can break the boundary layer and promote the heat exchange. Segmented twisted tape, which is applicable in 3D printing, is a novel insert and has potential in heat transfer enhancement. To clarify its advantages and disadvantages, this research presents a numerical investigation of vortex flow and heat enhancement in pipes containing one segmented twisted element. Flow state and heat transfer behaviour are obtained by simulation under constant wall temperature with different Reynolds numbers, ranging from 10,000 to 35,000. The effects of geometric parameters, including twist ratio (P/D = 2.0, 3.3 and 4.6) and length ratio (L/P = 0.3, 0.5 and 0.7), on the Nusselt number (Nu) and friction factor (f) are investigated. Streamline and temperature distribution are presented. Meanwhile, local and overall heat transfer performance is compared with those of a smooth tube, and the overall performance is evaluated by performance evaluation factor (η). The results indicate that the twist ratio (P/D) plays a dominant role in heat transfer enhancement while the length ratio (L/P) also has considerable influence. It is shown that a segmented tape insert can increase the overall heat transfer rate by 23.5% and the friction factor by 235%, while local improvement along the tube can be 2.8 times more than the plain tube.

An Oldroyd-B solver for vanishingly small values of the viscosity ratio: Application to unsteady free surface flows

Abstract: This work is concerned with time-dependent axisymmetric free surface flows of Oldroyd-B fluids for any value of β, the ratio of solvent to total viscosities The Oldroyd-B constitutive equation is dealt with by employing a novel technique to calculate the conformation tensor while an EVSS transformation allows the solution of the momentum equations coupled with the free surface stress conditions: this avoids numerical instabilities that can arise when using small values of β The convergence of this new methodology is verified on pipe flow and also by comparing results from the literature for the time-dependent impacting drop problem This approach is then used to predict the time-dependent free surface flow after a viscoelastic drop impacts a solid surface for β values in the range [0, 1] The impacting drop problem is investigated for polymer solutions containing a small solvent contribution ( β ⟶ 0 ) or without any solvent viscosity ( β = 0 ) In addition, a study of the bouncing drop problem for different values of β, Weissenberg and Reynolds numbers is undertaken