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Showing papers on "Pipe flow published in 2015"


Journal ArticleDOI
22 Oct 2015-Nature
TL;DR: A bifurcation scenario is uncovered that explains the transformation to fully turbulent pipe flow and the front dynamics of the different states encountered in the process and is bridged between understanding of the onset of turbulence and fully turbulent flows.
Abstract: Over a century of research into the origin of turbulence in wall-bounded shear flows has resulted in a puzzling picture in which turbulence appears in a variety of different states competing with laminar background flow. At moderate flow speeds, turbulence is confined to localized patches; it is only at higher speeds that the entire flow becomes turbulent. The origin of the different states encountered during this transition, the front dynamics of the turbulent regions and the transformation to full turbulence have yet to be explained. By combining experiments, theory and computer simulations, here we uncover a bifurcation scenario that explains the transformation to fully turbulent pipe flow and describe the front dynamics of the different states encountered in the process. Key to resolving this problem is the interpretation of the flow as a bistable system with nonlinear propagation (advection) of turbulent fronts. These findings bridge the gap between our understanding of the onset of turbulence and fully turbulent flows.

175 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of roughness height and wavelength in a turbulent wall-bounded flow in both transitionally rough and fully rough regimes are investigated. But the results for the present class of surfaces show that the Hama roughness function is only marginally affected by low Reynolds numbers (or low ), and observations of outer-layer similarity (or lack thereof) show no signs of sensitivity to Reynolds number.
Abstract: Direct numerical simulations (DNS) are conducted for turbulent flow through pipes with three-dimensional sinusoidal roughnesses explicitly represented by body-conforming grids. The same viscous-scaled roughness geometry is first simulated at a range of different Reynolds numbers to investigate the effects of low Reynolds numbers and low , where is the pipe radius and is the roughness height. Results for the present class of surfaces show that the Hama roughness function is only marginally affected by low Reynolds numbers (or low ), and observations of outer-layer similarity (or lack thereof) show no signs of sensitivity to Reynolds number. Then, building on this, a systematic approach is taken to isolate the effects of roughness height and wavelength in a turbulent wall-bounded flow in both transitionally rough and fully rough regimes. Current findings show that while the effective slope (which for the present sinusoidal surfaces is proportional to ) is an important roughness parameter, the roughness function must also depend on some measure of the viscous roughness height. A simplistic linear–log fit clearly illustrates the strong correlation between and both the roughness average height (which is related to ) and for the surfaces simulated here, consistent with published literature. Various definitions of the virtual origin for rough-wall turbulent pipe flow are investigated and, for the surfaces simulated here, the hydraulic radius of the pipe appears to be the most suitable parameter, and indeed is the only virtual origin that can ever lead to collapse in the total stress. First- and second-order statistics are also analysed and collapses in the outer layer are observed for all cases, including those where the largest roughness height is a substantial proportion of the reference radius (low ). These results provide evidence that turbulent pipe flow over the present sinusoidal surfaces adheres to Townsend’s notion of outer-layer similarity, which pertains to statistics of relative motion.

156 citations


Journal ArticleDOI
TL;DR: In this paper, a hybrid RANS/large eddy simulation of a non-premixed, high-pressure laboratory combustor that produces self-excited longitudinal instabilities is presented.
Abstract: The unsteady gas dynamic field in a closed combustor is determined by the nonlinear interactions between chamber acoustics, hydrodynamics, and turbulent combustion that can energize these modes. These interactions are studied in detail using hybrid RANS/large eddy simulations (RANS = Reynolds Averaged Navier-Stokes) of a non-premixed, high-pressure laboratory combustor that produces self-excited longitudinal instabilities. The main variable in the study is the relative acoustic length between the combustion chamber and the tube that injects oxidizer into the combustor. Assuming a half-wave (closed-closed) combustion chamber, the tube lengths approximately correspond to quarter-, 3/8-, and half-wave resonators that serve to vary the phasing between the acoustic modes in the tube and the combustion chamber. The simulation correctly predicts the relatively stable behavior measured with the shortest tube and the very unstable behavior measured with the intermediate tube. Unstable behavior is also predicted for the longest tube, a case for which bifurcated stability behavior was measured in the experiment. In the first (stable) configuration, fuel flows into the combustor uninterrupted, and heat release is spatially continuous with a flame that remains attached to the back step. In the second (unstable) configuration, a cyclic process is apparent comprising a disruption in the fuel flow, subsequent detachment of the flame from the back step, and accumulation of fuel in the recirculation zone that ignites upon arrival of a compression wave reflected from the downstream boundary of the combustion chamber. The third case (mixed stable/unstable) shares features with both of the other cases. The major difference between the two cases predicted to be unstable is that, in the intermediate length tube, a pressure wave reflection inside the tube pushes unburnt fuel behind the back step radially outward, leading to a post-coupled reignition mechanism, while in the case of the longest tube, the reignition is promoted by vortex convection and combustor-wall interaction. Other flow details indicated by the simulation include the relative phase between flow resonances in the tube and the combustor, increased mixing due to baroclinic torque, and the presence of an unsteady triple flame.

119 citations


Journal ArticleDOI
TL;DR: In this paper, the authors presented a unique investigation of boundary layer flow at very high Reynolds numbers, including mean velocities, streamwise turbulence variances, and moments up to 10th order.
Abstract: Measurements are presented in zero-pressure-gradient, flat-plate, turbulent boundary layers for Reynolds numbers ranging from to ( ). The wind tunnel facility uses pressurized air as the working fluid, and in combination with MEMS-based sensors to resolve the small scales of motion allows for a unique investigation of boundary layer flow at very high Reynolds numbers. The data include mean velocities, streamwise turbulence variances, and moments up to 10th order. The results are compared to previously reported high Reynolds number pipe flow data. For , both flows display a logarithmic region in the profiles of the mean velocity and all even moments, suggesting the emergence of a universal behaviour in the statistics at these high Reynolds numbers.

111 citations


Journal ArticleDOI
TL;DR: In this article, a direct numerical simulation of a turbulent pipe flow at a high Reynolds number of Reτ = 3008 over a long axial domain length (30R) was performed.
Abstract: A direct numerical simulation of a turbulent pipe flow at a high Reynolds number of Reτ = 3008 over a long axial domain length (30R) was performed. The streamwise mean velocity followed the power law in the overlap region (y+ = 90–300; y/R = 0.03–0.1) based on the power law indicator function. The scale separation of the Reynolds shear stresses into two components of small- and large-scale motions (LSMs) revealed that the LSMs in the outer region played an important role in constructing the constant-stress layer and the mean velocity. In the pre-multiplied energy spectra of the streamwise velocity fluctuations, the bimodal distribution was observed at both short and long wavelengths. The kx−1 region associated with the attached eddies appeared in λx/R = 2–5 and λx/y = 18–160 at y+ = 90–300, where the power law was established in the same region. The kz−1 region also appeared in λz/R = 0.3–0.6 at y+ = 3 and 150. Linear growth of small-scale energy to large-scale energy induced the kx−1 region at high Reyno...

96 citations


Journal ArticleDOI
TL;DR: In this paper, a simulation of a heated turbulent pipe flow with a fluid at supercritical pressure is performed at a Reynolds number of Re τ = 360, based on pipe diameter and friction velocity at the inlet.

90 citations


Journal ArticleDOI
TL;DR: A comprehensive literature survey on the thermal-hydraulic performance of liquid flow and heat transfer in pipes with internal integral-fins, twisted tape inserts, corrugations, dimples, and compound enhancement techniques is conducted in this paper.

85 citations


Journal ArticleDOI
TL;DR: In this paper, the results of eddy-resolving simulations and supporting flow visualizations are investigated for the turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel.
Abstract: The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder’s base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).

68 citations


Journal ArticleDOI
TL;DR: In this article, the adaptive Neuro fuzzy group method of data handling is used to develop new empirical formulae for the prediction of longitudinal dispersion coefficients in pipe flow using 233 experimental case studies of dispersion coefficient with a R = 0.
Abstract: Longitudinal dispersion in pipelines leads to changes in the characteristics of contaminants. It is critical to quantify these changes because the contaminants travel through water networks or through chemical reactors. The essential characteristics of longitudinal dispersion in pipes can be described by the longitudinal dispersion coefficient. This paper presents the application of the adaptive Neuro fuzzy group method of data handling to develop new empirical formulae for the prediction of longitudinal dispersion coefficients in pipe flow using 233 experimental case studies of dispersion coefficient with a R e range of 900 to 500,000 spanning laminar, transitional and turbulent pipe flow. The NF-GMDH network was improved using particle swarm optimization based evolutionary algorithm. The group method data handling is used to develop empirical relations between the longitudinal dispersion coefficient and various control variables, including the Reynolds number, the average velocity, the pipe friction coefficient and the pipe diameter. GMDH holds advantage in the case of small data samples due to the optimal choice of the model complexity with automatic adaptation to an unknown level of the data uncertainties. Sensitivity analysis is performed on the developed model and the weight and importance of each control variable is presented. The results indicate that the proposed relations are simpler than previous numerical solutions and can effectively evaluate the longitudinal dispersion coefficients in pipe flow.

68 citations


Journal ArticleDOI
TL;DR: In this paper, the friction factor for a fully developed pipe flow is examined at high Reynolds numbers up to ReD = 1.8 × 107 with high accuracy using the high Reynolds number actual flow facility at AIST, NMIJ.
Abstract: The friction factor for a fully developed pipe flow is examined at high Reynolds numbers up to ReD = 1.8 × 107 with high accuracy using the high Reynolds number actual flow facility “Hi-Reff” at AIST, NMIJ. The precise measurement of the friction factor is achieved by the highly accurate measurement of the flow rate, and the measurement uncertainty is estimated to be approximately 0.9% with a coverage factor of k = 2. The result examined here is obviously different from the Prandtl equation and the experimental results from the superpipe at Princeton University. The deviation of the present result from the Prandtl equation in the lower Reynolds number region is approximately 2.5% and −3% at the higher Reynolds number. For ReD 2.0 × 105, and it increases with the Reynolds number and reaches −6% at ReD = 1.0 × 107. The Karman constant estimated by the measured fric...

63 citations


Journal ArticleDOI
TL;DR: Based on the experimentally determined flow properties of the coal gangue-fly ash (CGF) slurry, a pipe flow model for predicting the flow behavior of the CGF slurry in the pipe loop is developed in this article.

Journal ArticleDOI
TL;DR: In this article, the axial induction of swirling flow was investigated both numerically and experimentally using a commercial CFD package for axial Reynolds numbers less than 2000, and the results showed that swirl flow leads to much higher friction factors compared with theoretical values for non-swirl (i.e. purely axial) flow.

Journal ArticleDOI
TL;DR: In this article, a dual-plane snapshot POD analysis of turbulent pipe flow at a Reynolds number of 104,000 was presented, where high-speed PIV data were simultaneously acquired in two planes, a crossstream plane (2D-3C) and a streamwise plane on the pipe centerline.
Abstract: A dual-plane snapshot POD analysis of turbulent pipe flow at a Reynolds number of 104,000 is presented. The high-speed PIV data were simultaneously acquired in two planes, a crossstream plane (2D-3C) and a streamwise plane (2D-2C) on the pipe centerline. The cross-stream plane analysis revealed large structures with a spatio-temporal extent of 1-2R, where R is the pipe radius. The temporal evolution of these large-scale structures is examined using the timeshifted correlation of the cross-stream snapshot POD coefficients, identifying the low energy intermediate modes responsible for the transition between the large-scale modes. By conditionallyaveraging based on the occurrence/intensity of a given cross-stream snapshot POD mode, a complex structure consisting of wall-attached and detached large-scale structures is shown to be associated with the most energetic modes. There is a pseudo-alignment of these large structures, that together create structures with a spatio-temporal extent of about 6R, which appears to explain the formation of the very large scale motions previously observed in pipe flow.

Journal ArticleDOI
TL;DR: In this paper, a two-fluid model is presented for the simulation of fully-suspended liquid-solid slurry flows in horizontal pipes, and the main improvements concern the use of a new wall boundary condition for the solid phase, which allows accounting for particle shape.

Journal ArticleDOI
TL;DR: In this article, the local wavelet energy (LWE) coefficients were extracted through continuous wavelet decomposition of the phase fraction history of a two-phase pipe flow with water holdup fluctuations provided by a set of conductivity and capacitance sensors.

Book ChapterDOI
01 Jan 2015
TL;DR: In this article, the velocity and Reynolds number calculations for pipe flow are introduced as well as the use of the Reynolds number in classifying liquid in terms of pressure and how it is measured.
Abstract: In this chapter, we discuss pressure and how it is measured. The velocity and Reynolds number calculations for pipe flow are introduced as is the use of the Reynolds number in classifying liquid. Existing methods of calculating the pressure drop because of friction in a pipeline using the Darcy–Weisbach equation are discussed. The importance of the Moody diagram is explained. Also, the trial and error solutions of friction factor from the Colebrook–White equation are covered. The use of Hazen–Williams and MIT pressure drop equations are discussed. Minor losses in pipelines from valve, fitting, pipe enlargement, and pipe contractions are analyzed. The concept of drag reduction as a means of reducing frictional head loss is also introduced. Also, we introduce the various methods of calculating the pressure drop in a gas. The more commonly used equations for pressure drop in gas pipelines are discussed. The effect of elevation changes is explained and the concept of Reynolds number, friction factor, and transmission factor are introduced. The use of pipeline efficiency in comparing various equations is illustrated. The average velocity of gas flow and the limiting value of erosional velocity are discussed.

Journal ArticleDOI
TL;DR: The mystery attributed to the breakdown process of the Osborne Reynolds pipe transition can be partially resolved with a direct, spatially evolving simulation that carries weakly but finitely perturbed laminar inflow through gradual rather than abrupt transition arriving at the fully developed turbulent state.
Abstract: The precise dynamics of breakdown in pipe transition is a century-old unresolved problem in fluid mechanics. We demonstrate that the abruptness and mysteriousness attributed to the Osborne Reynolds pipe transition can be partially resolved with a spatially developing direct simulation that carries weakly but finitely perturbed laminar inflow through gradual rather than abrupt transition arriving at the fully developed turbulent state. Our results with this approach show during transition the energy norms of such inlet perturbations grow exponentially rather than algebraically with axial distance. When inlet disturbance is located in the core region, helical vortex filaments evolve into large-scale reverse hairpin vortices. The interaction of these reverse hairpins among themselves or with the near-wall flow when they descend to the surface from the core produces small-scale hairpin packets, which leads to breakdown. When inlet disturbance is near the wall, certain quasi-spanwise structure is stretched into a Lambda vortex, and develops into a large-scale hairpin vortex. Small-scale hairpin packets emerge near the tip region of the large-scale hairpin vortex, and subsequently grow into a turbulent spot, which is itself a local concentration of small-scale hairpin vortices. This vortex dynamics is broadly analogous to that in the boundary layer bypass transition and in the secondary instability and breakdown stage of natural transition, suggesting the possibility of a partial unification. Under parabolic base flow the friction factor overshoots Moody’s correlation. Plug base flow requires stronger inlet disturbance for transition. Accuracy of the results is demonstrated by comparing with analytical solutions before breakdown, and with fully developed turbulence measurements after the completion of transition.

Journal ArticleDOI
TL;DR: The main part of this contribution to the special issue of EJM-B/Fluids dedicated to Patrick Huerre outlines the problem of the subcritical transition to turbulence in wall-bounded flows in its historical perspective with emphasis on plane Couette flow, the flow generated between counter-translating parallel planes.
Abstract: The main part of this contribution to the special issue of EJM-B/Fluids dedicated to Patrick Huerre outlines the problem of the subcritical transition to turbulence in wall-bounded flows in its historical perspective with emphasis on plane Couette flow, the flow generated between counter-translating parallel planes Subcritical here means discontinuous and direct, with strong hysteresis This is due to the existence of nontrivial flow regimes between the global stability threshold Re g , the upper bound for unconditional return to the base flow, and the linear instability threshold Re c characterized by unconditional departure from the base flow The transitional range around Re g is first discussed from an empirical viewpoint (Section 1 ) The recent determination of Re g for pipe flow by Avila et al (2011) is recalled Plane Couette flow is next examined In laboratory conditions, its transitional range displays an oblique pattern made of alternately laminar and turbulent bands, up to a third threshold Re t beyond which turbulence is uniform Our current theoretical understanding of the problem is next reviewed (Section 2 ): linear theory and non-normal amplification of perturbations; nonlinear approaches and dynamical systems, basin boundaries and chaotic transients in minimal flow units; spatiotemporal chaos in extended systems and the use of concepts from statistical physics, spatiotemporal intermittency and directed percolation, large deviations and extreme values Two appendices present some recent personal results obtained in plane Couette flow about patterning from numerical simulations and modeling attempts

Journal ArticleDOI
TL;DR: This novel Dean-flow-coupled elasto-inertial microfluidic device could offer a continuous, sheathless, and high throughput (>10 000 s(-1)) 3D focusing performance, which may be valuable in various applications from high speed flow cytometry to cell counting, sorting, and analysis.
Abstract: In this paper, 3D particle focusing in a straight channel with asymmetrical expansion–contraction cavity arrays (ECCA channel) is achieved by exploiting the dean-flow-coupled elasto-inertial effects. First, the mechanism of particle focusing in both Newtonian and non-Newtonian fluids was introduced. Then particle focusing was demonstrated experimentally in this channel with Newtonian and non-Newtonian fluids using three different sized particles (3.2 μm, 4.8 μm, and 13 μm), respectively. Also, the effects of dean flow (or secondary flow) induced by expansion–contraction cavity arrays were highlighted by comparing the particle distributions in a single straight rectangular channel with that in the ECCA channel. Finally, the influences of flow rates and distances from the inlet on focusing performance in the ECCA channel were studied. The results show that in the ECCA channel particles are focused on the cavity side in Newtonian fluid due to the synthesis effects of inertial and dean-drag force, whereas the particles are focused on the opposite cavity side in non-Newtonian fluid due to the addition of viscoelastic force. Compared with the focusing performance in Newtonian fluid, the particles are more easily and better focused in non-Newtonian fluid. Besides, the Dean flow in visco-elastic fluid in the ECCA channel improves the particle focusing performance compared with that in a straight channel. A further advantage is three-dimensional (3D) particle focusing that in non-Newtonian fluid is realized according to the lateral side view of the channel while only two-dimensional (2D) particle focusing can be achieved in Newtonian fluid. Conclusively, this novel Dean-flow-coupled elasto-inertial microfluidic device could offer a continuous, sheathless, and high throughput (>10 000 s−1) 3D focusing performance, which may be valuable in various applications from high speed flow cytometry to cell counting, sorting, and analysis.

Journal ArticleDOI
TL;DR: In this article, a volume-filtered Euler-Lagrange large eddy simulation methodology is used to predict the physics of turbulent liquid-solid slurry flow through a horizontal periodic pipe.

Journal ArticleDOI
TL;DR: In this paper, the effect of pipe inclination angle on transition boundaries between flow patterns is investigated comprehensively, showing that non-stratified flows such as bubbly and slug flows are dominant flow patterns in the upward flows and stratified flows in the downward flows.

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a direct method for testing the frequency response of various hot-wire anemometry systems to very high frequency velocity fluctuations (up to 50 kHz).
Abstract: Based on the need to characterise the accuracy of hot-wire anemometry (HWA) in high Reynolds number wall-bounded turbulence, we here propose a novel direct method for testing the frequency response of various systems to very high frequency velocity fluctuations (up to 50 kHz). A fully developed turbulent pipe flow is exploited to provide the input velocity perturbations. Utilising the unique capabilities of the Princeton Superpipe, it is possible to explore a variety of turbulent pipe flows at matched Reynolds numbers, but with turbulent energy in different frequency ranges. Assuming Reynolds number similarity, any differences between the appropriately scaled energy spectra for these flows should be indicative of measurement error. Having established the accuracy of this testing procedure, the response of several anemometer and probe combinations is tested. While these tests do not provide a direct or definitive comparison between different anemometers (owing to non-optimal tuning in each case), they do provide useful examples of potential frequency responses that could be encountered in HWA experiments. These results are subsequently used to predict error arising from HWA response for measurements in wall-bounded turbulent flows. For current technology, based on the results obtained here, the frequency response of under- or over-damped HWA systems can only be considered approximately flat up to 5–7 kHz. For flows with substantial turbulent energy in frequencies above this range, errors in measured turbulence quantities due to temporal resolution are increasingly likely.

Journal ArticleDOI
Jin Lee1, Junsun Ahn1, Hyung Jin Sung1
TL;DR: In this article, statistical measures of turbulence intensities in turbulent pipe and channel flows at a friction Reynolds number of Reτ ≈ 930 were explored by a population of large-scale motions (LSMs) and very-large-scale motion (VLSMs).
Abstract: Statistical measures of turbulence intensities in turbulent pipe and channel flows at a friction Reynolds number of Reτ ≈ 930 were explored by a population of large-scale motions (LSMs) and very-large-scale motions (VLSMs) Although the statistical measures characterizing these internal turbulent flows were similar in the near-wall region, the extents of the mean streamwise velocities and cross-stream components of the turbulence intensities differed in the core region The population density of VLSMs/LSMs decreased/increased significantly in the core region of the pipe flow The survival time of VLSMs of the pipe flow was shorter than that of the channel flow The area fractions of the VLSMs displayed similar trends to the population density The wall-normal and spanwise turbulence intensities in the pipe flow increased in the core regions due to the high-speed large-scale structures and associated motions above the structures The large-scale structures increased the streamwise intensity and the Reynold

Journal ArticleDOI
TL;DR: In this paper, a new empirical closure relationship is proposed as a function of viscosity (viscous) and Froude (inertia) dimensionless numbers to predict the gas fraction in slugs for high-viscosity liquid (180-587mPa) and gas flow in horizontal pipes.

Journal ArticleDOI
TL;DR: In this article, the impact force of a cantilevered pipe conveying fluid interacting with two support walls on both sides is investigated, and the nonlinear equations of motion are discretized via Galerkin's method and solved by using a fourth-order Runge-Kutta method.
Abstract: In this paper, the nonlinear dynamics of a cantilevered pipe conveying fluid interacting with two support walls on both sides is first investigated. The main goal of this study is to explore how the dynamics of a cantilevered pipe will perform in the presence of two support walls along the pipe axis. The interacting force is defined as impact in order to simulate the impacting effects for a pipe with various flow velocities. The impact force is modeled either by a cubic spring or by a trilinear spring. The nonlinear equations of motion are discretized via Galerkin’s method, and the discretized equations are solved by using a fourth-order Runge–Kutta method. Results show that the pipe would periodically impact the walls when the flow velocity is just beyond the critical value. When the flow velocity is sufficiently higher, however, the pipe may behave different patterns of contacting the walls, such as point contact and segments contact. Periodic, quasi-periodic motions, as well as chaotic oscillations are observed in such a pipe system.

Journal ArticleDOI
TL;DR: An approximate one-dimensional model is developed to study the scattering of acoustic waves in a straight duct with varying wall impedance, and several situations are characterized to show the importance of negative energy waves, strong interactions between acoustic and hydrodynamic modes, or asymmetric scattering.
Abstract: The acoustic propagation in lined flow duct with purely reactive impedance at the wall is considered. This reacting liner has the capability to reduce the speed of sound, and thus to enhance the interaction between the acoustic propagation and the low Mach number flow ( M≃0.3). At the lower frequencies, there are typically four acoustic or hydrodynamic propagating modes, with three of them propagating in the direction of the flow. Above a critical frequency, there are only two propagating modes that all propagate in the direction of the flow. From the exact two-dimensional formulation an approximate one-dimensional model is developed to study the scattering of acoustic waves in a straight duct with varying wall impedance. This simple system, with a uniform flow and with non-uniform liner impedance at the wall, permits to study the scattering between regions with different wave characteristics. Several situations are characterized to show the importance of negative energy waves, strong interactions between acoustic and hydrodynamic modes, or asymmetric scattering.

Journal ArticleDOI
TL;DR: In this article, a detailed comparison between pipe flows and boundary layers at friction Reynolds numbers of \({Re}}_\tau \approx 5000-20,000) reveals subtle differences.
Abstract: Recent experiments in high Reynolds number pipe flow have shown the apparent obfuscation of the \(k_x^{-1}\) behaviour in spectra of streamwise velocity fluctuations (Rosenberg et al. in J Fluid Mech 731:46–63, 2013). These data are further analysed here from the perspective of the \(\log r\) behaviour in second-order structure functions, which have been suggested as a more robust diagnostic to assess scaling behaviour. A detailed comparison between pipe flows and boundary layers at friction Reynolds numbers of \({{Re}}_\tau \approx\) 5000–20,000 reveals subtle differences. In particular, the \(\log r\) slope of the pipe flow structure function decreases with increasing wall distance, departing from the expected \(2A_1\) slope in a manner that is different to boundary layers. Here, \(A_1 \approx 1.25\), the slope of the log law in the streamwise turbulence intensity profile at high Reynolds numbers. Nevertheless, the structure functions for both flows recover the \(2A_1\) slope in the log layer sufficiently close to the wall, provided the Reynolds number is also high enough to remain in the log layer. This universality is further confirmed in very high Reynolds number data from measurements in the neutrally stratified atmospheric surface layer. A simple model that accounts for the ‘crowding’ effect near the pipe axis is proposed in order to interpret the aforementioned differences.

Journal ArticleDOI
TL;DR: In this article, the authors combine the simultaneous two-phase PIV (S2P-PIV) technique and wave field measurements to investigate the flow dynamics of both phases, e.g. velocity profiles, turbulence profiles, distribution of turbulent structures, as well as the wave statistics, of the exact same stratified flow pattern.

Journal ArticleDOI
TL;DR: In this paper, the origin of the swirl switching phenomenon, which is a large scale oscillation of the flow after the bend, has been studied for different bend curvature ratios, and a classification of the phenomenon into a high and a low frequency switching with two distinct physical origins was proposed.
Abstract: Turbulent flow through 90° pipe bends, for four different curvatures, has been investigated using large eddy simulations. In particular, the origin of the so-called swirl switching phenomenon, which is a large scale oscillation of the flow after the bend, has been studied for different bend curvature ratios. A classification of the phenomenon into a high and a low frequency switching, with two distinct physical origins, is proposed. While the high frequency switching stems from modes formed at the bend, and becomes increasingly important for sharp curvatures, the low frequency switching originates from very-large-scale motions created in the upstream pipe flow.

Journal ArticleDOI
TL;DR: In this article, the authors studied the drag reduction performance of polymeric drag reducers induced by high Reynolds number flows in tubes and used three kinds of polymers: two flexible and one rigid.
Abstract: This paper studies the loss of efficiency of polymeric drag reducers induced by high Reynolds number flows in tubes. The overall pressure was fixed and the apparatus was built so as to minimize the polymer degradation. We used three kinds of polymers: two flexible and one rigid. We conducted our tests to take into account the drag reduction (DR) for a wide range of concentrations of each polymer. The main results are displayed for the DR as a function of the number of passes through the apparatus. The mechanism of the loss of efficiency for the Xanthan Gum (XG) solutions (the rigid one) seems to be completely different from that observed for Poly (ethylene oxide) (PEO) and Polyacrylamide (PAM) (the flexible materials). While the PEO and PAM mechanically degrade by the action of the turbulent flow, the XG seems to remain intact, even after many passes through the pipe flow apparatus. From the practical point of view, it is worth noting that the PAM solutions are clearly more efficient than the PEO and XG. ...