Showing papers on "Pipe flow published in 2021"

Linear instability of viscoelastic pipe flow

Abstract: A modal stability analysis shows that pressure-driven pipe flow of an Oldroyd-B fluid is linearly unstable to axisymmetric perturbations, in stark contrast to its Newtonian counterpart which is linearly stable at all Reynolds numbers. The dimensionless groups that govern stability are the Reynolds number being the Weissenberg number), marking a possible paradigm shift in our understanding of transition in rectilinear viscoelastic shearing flows. The predicted unstable eigenfunction should form a template in the search for novel nonlinear elasto-inertial states, and could provide an alternate route to the maximal drag-reduced state in polymer solutions. The latter has thus far been explained in terms of a viscoelastic modification of the nonlinear Newtonian coherent structures.

One-point statistics for turbulent pipe flow up to

Abstract: We study turbulent flows in a smooth straight pipe of circular cross-section up to friction Reynolds number using direct numerical simulation (DNS) of the Navier-Stokes equations. The DNS results highlight systematic deviations from Prandtl friction law, amounting to approximately, which would extrapolate to approximately at extreme Reynolds numbers. Data fitting of the DNS friction coefficient yields an estimated von Karman constant, which nicely fits the mean velocity profile, and which supports universality of canonical wall-bounded flows. The same constant also applies to the pipe centreline velocity, thus providing support for the claim that the asymptotic state of pipe flow at extreme Reynolds numbers should be plug flow. At the Reynolds numbers under scrutiny, no evidence for saturation of the logarithmic growth of the inner peak of the axial velocity variance is found. Although no outer peak of the velocity variance directly emerges in our DNS, we provide strong evidence that it should appear at, as a result of turbulence production exceeding dissipation over a large part of the outer wall layer, thus invalidating the classical equilibrium hypothesis.

Conductance Sensors for Multiphase Flow Measurement: A Review

Abstract: Multiphase flow is a commonly seen transient and complex dynamic system in many industrial processes. The phase fraction and velocity are two of the most important parameters for flow monitoring and measurement. Due to the advantages of simplicity in sensor structure, low fabrication costs and fast response, conductance sensors have received broad attentions in horizontal, vertical and inclined multiphase pipe flow measurement. A conductance sensor measures the multiphase mixture conductivity between two electrodes in contact with the fluid to determine the phase fraction. Combined with cross-correlation technique, the velocity can also be acquired. This paper presents a review on the basic sensing principle, different types of conductance sensors and their applications in flow monitoring, flow pattern identification, phase fraction determination and velocity measurement of multiphase flow. Finally, based on the conclusion of the disadvantages, advantages and limitations of this technique, the insight into trends for future development are given.

A new transient analytical model for heat transfer of earth-to-air heat exchangers

Abstract: In the present paper, a simple but accurate transient model to evaluate the thermal performance of earth to air heat exchangers (EAHEs) is developed Governing equations for both pipe flow and soil surrounding the pipe are derived for the transient state and are solved using Laplacian transform To validate the analytical model, the 3D numerical model of the EAHE is also simulated Results of the analytical model and numerical simulations are compared with already reported experimental data In heating period at velocity of 2 m/s, the discrepancy between the analytical model and reported experimental results is 087% and 131% for steel and PVC pipes, respectively For velocity of 5 m/s this was as low as 04% for both pipe materials For the cooling condition, a maximum discrepancy occurs for PVC pipe at 2 m/s equal to 955% Analytical results were compared with numerical simulations for both constant and time-varying inlet temperatures, and great correlation was observed Thermal diffusion into the soil was also investigated after 6 h and 120 h of continuous operation Results revealed that for the fluid velocity of 5 m/s, after 6 and 120 h of continuous operation, heat diffused up to 10 cm and 50 cm distance of the pipe axis, respectively, which showed the thermal saturation of the soil with passing the time in continuous operation mode Results show that thermal saturation of the soil should be taken into the model as ignoring it can cause an overestimation in system performance

Transitional pulsatile flows with stenosis in a two-dimensional channel

Abstract: Although blood flows are mostly laminar, transition to turbulence and flow separations are observed at curved vessels, bifurcations, or constrictions. It is known that wall-shear stress plays an important role in the development of atherosclerosis as well as in arteriovenous grafts. In order to help understand the behavior of flow separation and transition to turbulence in post-stenotic blood flows, an experimental study of transitional pulsatile flow with stenosis was carried out using time-resolved particle image velocimetry and a microelectromechanical systems wall-shear stress sensor at the mean Reynolds number of 1750 with the Womersley number of 6.15. At the start of the pulsatile cycle, a strong shear layer develops from the tip of the stenosis, increasing the flow separation region. The flow at the throat of the stenosis is always laminar due to acceleration, which quickly becomes turbulent through a shear-layer instability under a strong adverse pressure gradient. At the same time, a recirculation region appears over the wall opposite to the stenosis, moving downstream in sync with the movement of the reattachment point. These flow behaviors observed in a two-dimensional channel flow are very similar to the results obtained previously in a pipe flow. We also found that the behavior in a pulsating channel flow during the acceleration phase of both 25% and 50% stenosis cases is similar to that of the steady flow, including the location and size of post-stenotic flow separation regions. This is because the peak Reynolds number of the pulsatile flow is similar to that of the steady flow that is investigated. The transition to turbulence is more dominant for the 50% stenosis as compared to the 25% stenosis, as the wavelet spectra show a greater broadening of turbulence energy. With an increase in stenosis to 75%, the accelerating flow is directed toward the opposite wall, creating a wall jet. The shear layer from the stenosis bifurcates as a result of this, one moving with the flow separation region toward the upper wall and the other with the wall jet toward the bottom wall. Low wall-shear stress fluctuations are found at two post-stenotic locations in the channel flow – one immediately downstream of the stenosis over the top wall (stenosis side) inside the flow separation region, and the other in the recirculation region on the bottom wall (opposite side of the stenosis).

Fast Equivalent Micro-Scale Pipe Network Representation of Rock Fractures Obtained by Computed Tomography for Fluid Flow Simulations

Abstract: Fractures in rocks often provide preferential fluid migration pathways and their geometrical properties are the main factors influencing the permeability of the rock mass. By taking into account the complex geometries of rock fractures, including tortuous features, highly variable fracture apertures, rough fracture wall surfaces and complex fracture intersections, a highly effective approach is proposed in this work to investigate the behaviour of fluid flows in laboratory-scale fractured rocks. The computed tomography (CT) scanning was used to capture the micro-structures of non-planar fractures in a sandstone specimen. An image processing method was then developed to extract the three-dimensional fracture network. The fractures and fracture network were represented using an equivalent pipe network model, taking into consideration the complex geometries mentioned above. The nonlinear flow is incorporated into the model through the consideration of a friction factor in the pipe flow method (PFM). The absolute difference of the derived permeability between PFM and finite volume method (FVM) is 3.45%, but the FVM needs 15 times more CPU computation time. Therefore, the proposed approach is better than FVM in terms of computational efficiency. In addition, the use of the friction factor was demonstrated to be effective and efficient to model nonlinear flow within the fracture network, where the flow nonlinearity is caused by high flow velocity and the formation of eddies in certain parts of the fracture network, leading to a decrease in the overall apparent permeability and an increase in the flow tortuosity. The proposed method was further validated against flow test data covering a wide range of linear and nonlinear flow regimes.

Scale-wise analysis of upward turbulent bubbly flows: An experimental study

Abstract: In this work, we investigate the upward turbulent bubbly pipe flow according to relevant length scales, to understand the bubble-induced turbulence modulation of the liquid-phase flow. Using two-phase particle image velocimetry, the bubbly flow fields in a vertical pipe (diameter of 40 mm) were obtained for Reynolds numbers (ReD) of 5300 and 44 000, while increasing the volume void fraction (bubble size of 2.5–4.5 mm) to 1.8% and 1.5%, respectively. The turbulent bubbly flow is visualized and further quantified from the decomposed flow fields (according to the length scales) using the discrete wavelet transform. At ReD = 5300, flow structures of the energy-containing scales (larger than the Taylor microscale) are energized by the disturbance from bubble surface deformation and near wake (whose scales are located close to the integral scale), resulting in the turbulence enhancement across the entire pipe. The interaction between wake vortices, sized between the integral scale and Taylor microscale, apart from the near wake also contributes to the turbulence enhancement. At higher ReD, despite the similar bubble size and void fraction, the scales of flow agitation owing to the bubble surface and the shed wake vortices become smaller, having a negligible effect on turbulence modification. However, the scale of the near wake is still large enough to enhance the turbulence at the pipe center (although it is significantly reduced toward the wall). With the highest void fraction among tested, on the other hand, the mean liquid velocity gradient becomes milder near the pipe wall, which causes a turbulence suppression locally near the wall.

Turbulent channel flow of generalized Newtonian fluids at a low Reynolds number

Abstract: Several studies concerning the turbulent pipe flow of generalized Newtonian (GN) fluids may be found in the literature, but not for channel flow, although that has been extensively studied for other types of non-Newtonian fluids, such as those with viscoelastic effects. Direct numerical simulations corresponding to statistically converged turbulent channel flow of GN fluids at a low frictional Reynolds number have been performed. The shear-dependent viscosity is introduced through the Carreau fluid model, and results corresponding to the Newtonian fluid case are compared to those of moderate shear-thickening and shear-thinning fluid behaviour. The different statistics studied reveal that shear-dependent fluid rheology appears mainly to affect the flow within the inner layer region and with shear-thinning behaviour; suppressing near-wall structures such as quasi-streamwise vortices and low-speed streaks, inhibiting turbulence generating events and leading to different drag reduction features. These include: enhancement of streamwise turbulence intensity and suppression of the other cross-sectional intensities, decrease of the Reynolds shear stress (leading to a lessening in turbulent production), decrease in energy redistribution between individual components of the Reynolds stress tensor through the velocity–pressure gradient term and overall increase in turbulence anisotropy at both small and large scales. In particular, it is noted that at the channel centre ‘rod-like’ turbulence states, a known low-Reynolds-number behaviour, are more clearly seen with shear-thinning fluid rheology.

Role of pulsatility on particle dispersion in expiratory flows

Abstract: Expiratory events, such as coughs, are often pulsatile in nature and result in vortical flow structures that transport respiratory particles. In this work, direct numerical simulation (DNS) of turbulent pulsatile jets, coupled with Lagrangian particle tracking of micron-sized droplets, is performed to investigate the role of secondary and tertiary expulsions on particle dispersion and penetration. Fully developed turbulence obtained from DNS of a turbulent pipe flow is provided at the jet orifice. The volumetric flow rate at the orifice is modulated in time according to a damped sine wave, thereby allowing for control of the number of pulses, duration, and peak amplitude. Thermodynamic effects, such as evaporation and buoyancy, are neglected in order to isolate the role of pulsatility on particle dispersion. The resulting vortex structures are analyzed for single-, two-, and three-pulse jets. The evolution of the particle cloud is then compared to existing single-pulse models. Particle dispersion and penetration of the entire cloud are found to be hindered by increased pulsatility. However, the penetration of particles emanating from a secondary or tertiary expulsion is enhanced due to acceleration downstream by vortex structures.

Oil Fraction Measurement of Nonuniform Dispersed Oil–Water Two-Phase Flow Based on Ultrasonic Attenuation

Abstract: Oil volume fraction determination is important for the transportation and separation of oil–water dispersed flow in petroleum industry, for which ultrasound method is suitable with the advantages of good penetration and low cost. The existing ultrasonic attenuation methods are based on homogeneous media for the determination of phase fraction without considering the effect of inhomogeneity on attenuation. In this article, the fractal approach is introduced to modify the acoustic attenuation model by characterizing the inhomogeneity of the dispersed phase in pipe flow, which establishes the relationship between multi-frequency attenuation and droplet size distribution. Then, an improved covariance matrix adaptive evolutionary strategy (CMA-ES) based on the established attenuation model is proposed to estimate oil volume fraction, which combines restart strategy with local search strategy to enhance the search performance and avoid falling into local optimum. Moreover, a demodulation method based on swept-frequency chirp is designed for the time-varying dispersed oil–water pipe flow to quickly obtain the multi-frequency attenuation. The effectiveness of the fractal-based modified attenuation model and the improved CMA-ES is verified by numerical simulations and flowing experiments. The experimental results at different oil volume fractions and flow rates indicate that the fractal-based modified attenuation model can effectively reduce the impact of nonuniform distribution of dispersed phases on the attenuation measurement, and thus effectively improve the measurement accuracy of the oil volume fraction, especially for the dispersed flow in the horizontal pipe.

Energetic motions in turbulent partially filled pipe flow

Abstract: Turbulent partially filled pipe flow was investigated using stereoscopic particle imaging velocimetry in the cross-stream plane for a range of flow depths at a nominally constant Reynolds number of 30 000 (based on the bulk velocity and hydraulic diameter) Unlike full pipe flow, which is axisymmetric, the turbulent kinetic energy exhibits significant azimuthal (and radial) variation Proper orthogonal decomposition (POD) of the fluctuating velocity field indicates that the leading-order POD modes occupy the “corners” where the free surface meets the pipe wall and that these modes, which are closely linked to the instantaneous cellular structure, contribute nearly a quarter of the overall turbulent kinetic energy Spatial distributions of the large- and very-large-scale motions (LSMs/VLSMs) estimated from pseudo-instantaneous three-dimensional velocity fields reveal a preference for the sides (in close proximity to the free surface) and bottom quadrant of the pipe That the LSMs and VLSMs are shown to populate a region spanning the width of the free surface, as well as the corners, strongly suggests that there is a dynamical connection between LSMs/VLSMs and the instantaneous cellular structures in turbulent partially filled pipe flow, which can explain the spatial redistribution of the turbulent kinetic energy

Subcritical and supercritical bifurcations in axisymmetric viscoelastic pipe flows

Abstract: Axisymmetric viscoelastic pipe flow of Oldroyd-B fluids has been recently found to be linearly unstable by Garg et al. (Phys. Rev. Lett., vol. 121, 2018, 024502). From a nonlinear point of view, this means that the flow can transition to turbulence supercritically, in contrast to the subcritical Newtonian pipe flows. Experimental evidence of subcritical and supercritical bifurcations of viscoelastic pipe flows have been reported, but these nonlinear phenomena have not been examined theoretically. In this work, we study the weakly nonlinear stability of this flow by performing a multiple-scale expansion of the disturbance around linear critical conditions. The perturbed parameter is the Reynolds number with the others being unperturbed. A third-order Ginzburg–Landau equation is derived with its coefficient indicating the bifurcation type of the flow. After exploring a large parameter space, we found that polymer concentration plays an important role: at high polymer concentrations (or small solvent-to-solution viscosity ratio is sufficiently large. The present analysis provides a theoretical understanding of the recent studies on the supercritical and subcritical routes to the elasto-inertial turbulence in viscoelastic pipe flows.

Integral measures of the zero pressure gradient boundary layer over the Reynolds number range 0≤Rτ<∞

Abstract: A recently developed mixing length model of the turbulent shearing stress has been shown to generate a universal velocity profile that provides an accurate approximation to incompressible pipe flow velocity profiles over a wide Reynolds number range [B. J. Cantwell, “A universal velocity profile for smooth wall pipe flow,” J. Fluid Mech. 878, 834–874 (2019)]. More recently, the same profile was shown to accurately approximate velocity profiles in channel flow, the zero pressure gradient boundary layer, and the boundary layer in an adverse pressure gradient [M. A. Subrahmanyam, B. J. Cantwell, and J. J. Alonso, “A universal velocity profile for turbulent wall flows,” AIAA Paper No. 2021-0061, 2021 and M. A. Subrahmanyam, B. J. Cantwell, and J. J. Alonso, “A universal velocity profile for turbulent wall flows including adverse pressure gradient boundary layers,” J. Fluid Mech. (unpublished) (2021)] The universal velocity profile is uniformly valid from the wall to the free stream at all Reynolds numbers from zero to infinity. At a low Reynolds number, the profile approaches the laminar channel/pipe flow limit. The primary measure of the Reynolds number used in this work is the friction Reynolds number Rτ=uτδ/ν. It is a little unusual to use Rτ for the boundary layer since it requires that the velocity profile be cutoff using an arbitrarily defined overall boundary layer thickness, δ. Because of the slow approach of the velocity to the free stream, different conventions used to define the thickness lead to different values of Rτ assigned to a given flow. It will be shown in this paper that, through its connection to channel/pipe flow, the universal velocity profile can be used to define a practically useful, unambiguous, measure of overall boundary layer thickness, called here the equivalent channel half height, δh. For Rτ>≈5000, the universal velocity profile defines a Reynolds number independent shape function that can be used to generate explicit expressions for the infinite Reynolds number behavior of all the usual integral boundary layer measures; displacement thickness, momentum thickness, energy thickness, overall boundary layer thickness, and skin friction. The friction coefficient Cf(Rδ2) generated by the universal velocity profile accurately approximates data over a wide range of momentum thickness Reynolds numbers collected by Nagib et al. [“Can we ever rely on results from wall-bounded turbulent flows without direct measurements of wall shear stress?,” AIAA Paper No. 2004-2392, 2004]. The universal velocity profile is used to integrate the von Kaŕman boundary layer integral equation [T. von Karman, “Uber laminaire und turbulente reibung,” Z. Angew. Math. Mech. 1, 233–252 (1921)] in order to generate the various thicknesses and friction velocity as functions of the spatial Reynolds number, Rx=uex/ν.

Evolution of waves in a horizontal pipe propagating on a surface of a liquid film sheared by gas

Abstract: Different wavy regimes in stratified air–water pipe flow are determined for a wide range of gas and liquid flow rates in a 10 m long horizontal pipe with a diameter of 24 mm. Three sub-regions of wavy stratified flow are identified: ripples, roll waves, and pre-annular wavy flow. Statistical parameters, such as local mean film thickness and its higher moments (root-mean-square, skewness, excess kurtosis) as well as wave characteristics (mean heights and wave height distributions, lengths, propagation velocities, etc.), are measured and analyzed. It is demonstrated that ripples are essentially linear waves and their propagation velocities are described reasonably well by linear wave theory. High amplitude roll and pre-annular waves are substantially nonlinear, and their propagation velocities differ significantly from that of ripples. Transition to roll waves causes a sharp increase in higher statistical moments. Evolution of wave and statistical parameters characterizing each sub-region of stratified gas–liquid pipe flow is studied. Simplified models describing roll waves are presented; the model predictions are verified by experiments.

Nonlinear evolutions of streaky structures in viscoelastic pipe flows

Abstract: The dynamics of streaky structures in a viscoelastic pipe flow of FENE-P fluids is studied numerically. The values of Weissenberg number ( W i , polymer relaxation time over flow turn-over time) and viscosity ratios are varied to evaluate the effects of viscoelasticity in the pipe. For Reynolds numbers 3000 ∼ 3500 (based on pipe radius and maximum laminar velocity) near the transition boundary, we select finite-amplitude two-dimensional rolls as initial conditions and add infinitesimal three-dimensional perturbations to trigger the transition. The 2D streamwise rolls are highly unstable to small 3D disturbances, which helps to better understand the evolution of large scale structures. The stronger temporal intermittency in viscoelastic flows makes it more difficult to identify a successful transition; we found that quantities directly related to viscoelasticity can serve as better indicators of transition to turbulence. The role of polymer stress is studied through the kinetic energy budget and it is found that polymer torque opposes the vorticity and become more dominant for higher W i , consistent with previous studies. The results show that the secondary instability of the streaky structures can be enhanced by viscoelasticity at low W i , while at high W i viscoelasticity delays the transition to turbulence, sometimes leading to relaminarization.

Hydrothermal analysis of conventional and baffled geothermal heat exchangers in transient mode

Abstract: In this paper, the idea of using the baffles to improve the performance of the coaxial geothermal heat exchanger (CGHE) has been proposed. Thermal and hydrodynamic analysis of two types of CGHE during heat injection and thermal recovery processes was investigated. A transient numerical model and turbulent model have been developed to study the CGHE. In this study, for both cases, the active borehole length was 165 m and the flow rate and the injected heat were 0.58 L s−1 and 6.4 kW, respectively. Augmentation of the heat transfer from the ground to the annular flow and reduction in the heat transfer rate between the annular flow and central pipe flow were the primary goals of this study. For this purpose, 40 baffles were considered inside the annular portion (over the central pipe). Comparisons between the obtained results and available data showed that the simulations were valid. The outlet (and inlet) temperatures of the baffled CGHE were 13.2% lower than those for the conventional CGHE. Also, the baffled CGHE had better thermal performance than the conventional CGHE and caused a quicker recovery of temperature, which is useful for improving the performances of the CGHE. Also, the pressure drop of the annular flow for the baffled CGHE was higher than the conventional CGHE, while the pumping powers for two CGHEs were not significantly different.

Transport of particles in a turbulent rough-wall pipe flow

Abstract: Dense, small particles suspended in turbulent smooth-wall flow are known to migrate towards the wall. It is, however, not clear if the particle migration continues in a rough-wall flow and what the responsible mechanism is, especially with changing roughness parameters. Here, we address this using direct numerical simulation of a turbulent pipe flow of a fixed friction Reynolds numbers and changing the roughness size as well as the Stokes number of the particles. The transport and deposition mechanisms of particles are segregated into three different regimes dictated by the Stokes number. Particles with small Stokes number follow the carrier fluid and are affected by the turbulent structures of the rough wall. Flow separation in the wake of the roughness and stagnant flow in the trough of the roughness causes these particles to be trapped in the roughness canopy. Particles with very large Stokes number, on the other hand, are attracted to the wall due to turbophoresis and collide with the rough wall where the frequency of wall collision increases with increasing Stokes number. These ballistic particles are unaffected by the turbulent fluctuations of the flow and their trajectory is determined by the roughness topography. At intermediate Stokes numbers, the transport of the particles is influenced by both the wall collisions and also the turbulent flow. Particles in this range of Stokes number occasionally collide with the wall and are entrained by the turbulent flow. In this regime, the particles may have a mean streamwise velocity that is larger than the bulk flow rate of the fluid. Finally, we observe that bulk particle velocity scale better with a time scale based on the roughness elements rather than the usual viscous time scale.