Pipe Flow Research Articles

Three-dimensional coherent structures in a curved pipe flow

Dean’s approximation for curved pipe flow, valid under loose coiling and high Reynolds numbers, is extended to study three-dimensional travelling waves. Two distinct types of solutions bifurcate from the Dean’s classic two-vortex solution. The first type arises through a supercritical bifurcation from inviscid linear instability, and the corresponding self-consistent asymptotic structure aligns with the vortex–wave interaction theory. The second type emerges from a subcritical bifurcation by curvature-induced instabilities and satisfies the boundary region equations. A connection to the zero-curvature limit was not found. However, by continuing from known self-sustained exact coherent structures in the straight pipe flow problem, another family of three-dimensional travelling waves can be shown to exist across all Dean numbers. The self-sustained solutions also possess the two high-Reynolds-number limits. While the vortex–wave interaction type of solutions can be computed at large Dean numbers, their branch remains unconnected to the Dean vortex solution branch.

Numerical Simulation of Non-Isothermal Laminar Oil Flow in an Axisymmetric Pipe

Numerical Simulation of Non-Isothermal Laminar Oil Flow in an Axisymmetric Pipe

What Is the Turbulence Problem, and When May We Regard It as Solved?

Turbulent motion of fluids is often thought of as a grand problem, but what exactly is this “turbulence problem”? Because it has often been proclaimed as very difficult and unsolved, when can we claim that it is solved? How does this situation in turbulence compare with other complex problems in physical sciences? Addressing these questions is not trivial because everyone has their favorite idea of what is required of the “solution.” The answers range from being able to calculate the pressure drop in turbulent pipe flow to being able to calculate anomalous scaling exponents to answering the regularity problem of the Navier–Stokes equations. Taking an absolute position on the basis of any of these, or other similar examples, is incomplete at best and potentially erroneous at worst. We believe that it is beneficial to have an open discussion of this topic for the advancement of the research agenda in turbulence. This article is an attempt to address the question of what constitutes the turbulence problem, its place in the scientific enterprise as a whole, and how and when one may declare it as solved.

Advancing Pressure-Based Flow Rate Soft Sensors: Signal Filtering Effects and Non-Laminar Flow Rate Determination

Precise flow measurement is crucial in fluid power systems. Especially in combination with pressure, hydraulic power can be particularly beneficial for predictive maintenance and control applications. However, conventional flow sensors in fluid power systems are often invasive, thus disrupting the flow and yielding unreliable measurements, especially under transient conditions. A common alternative is to estimate the flow rate using pressure differentials along a pipe and the Hagen–Poiseuille law, which is limited to steady, laminar, and incompressible flows. This study advances a previously introduced analytical soft sensor, demonstrating its ability to accurately determine the transient pipe flow beyond laminar conditions, without requiring a dedicated flow rate sensor. This method provides a robust and computationally efficient solution for real-world hydraulic systems by applying two pressure transducers. A key contribution of this work is the investigation of signal filtering, revealing that even a simple first-order low-pass filter with a 100 Hz cutoff frequency significantly improves accuracy, which is demonstrated for pulsation frequencies of 5, 10, and 15 Hz, where the filtered results closely match experimental data from a test rig. These findings underscore the soft sensor’s potential as a reliable alternative to traditional flow sensors, offering high accuracy with minimal computational overhead for a wide range of flow conditions.

Non-contacting laser-ultrasonic fouling detection on steel pipes.

Non-contacting laser-ultrasonic fouling detection on steel pipes.

Study on the evolution mechanism and influencing factors of CO2 corrosion product film under pipe flow

Study on the evolution mechanism and influencing factors of CO2 corrosion product film under pipe flow

An analytical, low-cost approach to determining characteristics of the vapor flow in flat heat pipes

An analytical, low-cost approach to determining characteristics of the vapor flow in flat heat pipes

Characterising slug flow in a horizontal pipe using bubble image velocimetry

Characterising slug flow in a horizontal pipe using bubble image velocimetry

Collision and clustering behavior of high-viscosity fluid-particle flow in bent pipes based on LBM-DEM

Collision and clustering behavior of high-viscosity fluid-particle flow in bent pipes based on LBM-DEM

Modeling and Design Aspects of Shallow Geothermal Energy Piles—A Case Study on Large Commercial Building Complex in Zagreb, Croatia

With ambitious targets set by the EU for the reduction of emissions from the energy sector by 2030, there is a need to design and develop more building projects using renewable energy sources. Even though in Europe, heating and cooling share from renewable resources is increasing, and in 2021, the total share in this sector in Croatia was at 38%, the share of heat production by heat pumps is rather low. One possibility to increase this share is to install energy piles when constructing a building, which is becoming an increasingly common practice. This case study focuses on such a system designed for a large, non-residential building in Zagreb, Croatia. The complex was designed as 13 separate dilatations, with central heating and cooling of all facilities, covered by 260 energy piles (130 pairs in serial connection), with a length of the polyethylene pipe of 20 m in a double loop inserted within the pile. The thermo-technical system was designed as a bivalent parallel system, with natural gas covering peak heating loads and a dry cooler covering cooling peak loads when the loads cannot be covered only by ground-source heat pumps. In the parallel bivalent system, the geothermal source will work with a much higher number of working hours at full load than is the case for geothermal systems that are dimensioned to peak consumption. Therefore, the thermal response test was conducted on two energy piles, connected in series, to obtain thermogeological parameters and determine the heat extraction and rejection rates. The established steady-state heat rate defines the long-term ability to extract heat energy during constant thermal load, with the inlet water temperature from the pile completely stabilized, i.e., no significant further sub-cooling is achieved in the function of the geothermal field operation time. Considering the heating and cooling loads of the building, modeling of the system was performed in such a manner that it utilized renewable energy as much as possible by finding a bivalent point where the geothermal system works efficiently. It was concluded that the optimal use of the geothermal field covers total heating needs and 70% for cooling, with dry coolers covering the remaining 30%. Additionally, based on the measured thermogeological parameters, simulations of the thermal response test were conducted to determine heat extraction and rejection rates for energy piles with various geometrical parameters of the heat exchanger pipe and fluid flow variations.

Prediction of hydraulic gradient for backward erosion piping in river levees considering flow regime and pipe geometry

Prediction of hydraulic gradient for backward erosion piping in river levees considering flow regime and pipe geometry

Time for polymers: Onset of drag reduction in the Reynolds–Weissenberg-plane

The flow of dilute aqueous polyacrylamide PAAm solutions with varying polymer molecular weight was studied in turbulent pipe flow. Scans in Reynolds number Re were performed in Newtonian turbulent flow [pre drag reduction (DR) onset] and intermediate DR flow. Polymer degradation shifted the onset of DR to higher Re. Polymer relaxation times derived from molecular weight and viscosity contribution of the polymer combined with the shear rate of the flow were used to calculate the Weissenberg number Wi. Fanning friction factors f derived from experimental data were fitted as a function of Re and flow distance to obtain flow properties for any state of polymer degradation and at any Re. From this fit function the onset of DR for varying polymer molecular weight was derived in the Re–Wi plane and the flow regimes accessed with increasing Re at constant molecular weight were identified. Trends in DR onset strongly depend on the shear rate used to calculate Wi. The elasticity number El was only independent of Re when Wi was determined from volume shear rate instead of the wall shear rate. Our observations indicate that DR is dominated by processes in the volume rather than those occurring at the wall.

Stability of N‐front and N‐back solutions in the Barkley model

ABSTRACTIn this article, we establish for an intermediate Reynolds number domain the stability of ‐front and ‐back solutions for each corresponding to traveling waves, in an experimentally validated model for the transition to turbulence in pipe flow proposed in [Barkley et al., Nature 526(7574):550‐553, 2015]. We base our work on the existence analysis of a heteroclinic loop between a turbulent and a laminar equilibrium proved by Engel, Kuehn and de Rijk in Engel, Kuehn, de Rijk, Nonlinearity 35:5903, 2022, as well as some results from this work. The stability proof follows the verification of a set of abstract stability hypotheses stated by Sandstede in [SIAM Journal on Mathematical Analysis 29.1 (1998), pp. 183‐207] for traveling waves motivated by the FitzHugh–Nagumo equations. In particular, this completes the first detailed analysis of Engel, Kuehn and de Rijk in [Engel, Kuehn, de Rijk, Nonlinearity 35:5903, 2022] leading to a complete existence and stability statement that nicely fits within the abstract framework of waves generated by twisted heteroclinic loops.

Analyzing Geothermal Heat Exchangers Using Comsol Non-Isothermal Pipe Flow Model

Earth–air heat exchangers (EAHE) hold great promise for reducing typical air conditioning systems’ energy consumption whilst preserving high indoor comfort. The present analyses a 3D model using COMSOL Multiphysics software for a geothermal heat exchanger to examine the thermal behavior along the piping system. The experimental findings of the real EAHE in Baghdad City during January and June are transferred into the nonisothermal pipe flow interface for modeling temperature, velocity and pressure distributions along the piping system. The temperature variation of subsurface soil and the radial temperature distribution around the pipe are modeled into the heat transfer interface. The magnitude of heat flux is also computed in different times along the pipe. The effects of the continuous operation of EAHE on the output temperature and on the soil temperature around the pipe are also considered. The model’s output demonstrates that the air temperature rise in January is 10 whereas the air temperature drop is 14 in June. The effect of extracted/absorbed heat transfer from/to air in pipes is extended up to 0.7 m in the radial direction of the soil surrounding pipes due to continuous airflow in EAHE.

Prediction of bubbly flow and flow regime development in a horizontal air-water pipe flow with a morphology-adaptive multifluid CFD model

Prediction of bubbly flow and flow regime development in a horizontal air-water pipe flow with a morphology-adaptive multifluid CFD model

Discussion of “Random Walk Particle Tracking to Model Dispersion in Steady Laminar and Turbulent Pipe Flow”

Discussion of “Random Walk Particle Tracking to Model Dispersion in Steady Laminar and Turbulent Pipe Flow”

Thermally fully developed pipe flows of active liquids

Active matter laden active liquids define a unique class of liquids, whose extremely rich dynamics can be captured only by appropriately considering the contribution of active matter induced stresses. In this paper, we develop analytical solutions for studying the effect of a background active fluid flow in the temperature distribution and the Nusselt number in a thermally fully developed pipe flow with constant surface heat flux. Specifically, we consider the flow of an active liquid consisting of active particles demonstrating vortex defects: consequently, in the presence of an axial gradient in activity, there occurs an induced pressure-driven flow in a pipe that has a profile different from the Hagen-Poiseuille flow profile in non-active pressure-driven flow. We find that the Nusselt number for the case of the background active flow is 3.83, which is smaller than the classical value of 4.36 observed for the non-active liquids with background pressure-driven flow (with constant heat flux). We justify this decrease by noting that for the case where the activity gradient (for active flows) is identical to the pressure gradient (for non-active pressure-driven flows), the overall flow strength is smaller for the active flows: such reduced flow strength causes a reduced convective heat transfer triggering a decrease in the Nusselt number for the case of the background active flows. This reduced convective heat transfer also causes a smaller temperature away from the wall (for the case where the wall temperature is greater than the mean temperature) for the case of the background active flows.

Turbulent transport in annular Poiseuille flow with axial rotation

Annular Poiseuille flow, which is the axial Poiseuille flow between two concentric circular cylinders, is an important canonical system for the study of wall turbulence. In the present work, the variation of momentum and heat transport in the turbulent annular Poiseuille flow under the influence of axial system rotation is investigated. We performed direct numerical simulation under radius ratio η=0.5, friction Reynolds number Reτ=200, and Prandtl number Pr=1. The intensity of rotation effect is characterized by the rotation number, which varies from 0 to 128 in the present work. Our results show a counter-intuitive phenomenon: when the rotation increases from zero, the heat transfer exhibits a monotonic increase, while the axial flow rate has a monotonic decrease, which are both opposite to the trends in a rotating pipe flow. We also reveal the main effect of the Coriolis force on the mean velocity profiles and flow structures. A more thorough investigation is conducted by studying the budget terms of Reynolds stresses and turbulent heat flux, which explores the enhancement effect of the Coriolis force on the turbulent radial transport of axial momentum and heat, along with its redistribution mechanism on turbulent kinetic energy. Finally, energy spectra demonstrate the emergence of large-scale structures in the radial-azimuthal plane under strong rotation.

Laboratory study of orifice jets from a pressurized pipe

Orifice jet through a small opening or orifice into a surrounding fluid exhibits intricate fluid dynamics, encompassing the interaction of the jet with the fluid. Although the dynamics of simple jets has been extensively studied, there are still unresolved issues that need to be addressed. In this study, we experimentally investigated the characteristics of non-buoyant jets emerging from a sharp-edged orifice located in the wall of a pipe with a constant flow rate. Specifically, we examined how the ratio between the variable jet flow rate to the constant pipe flow rate upstream of the orifice influences not only the jet trajectory but (for the first time to the authors' knowledge) also the jet velocity-related features. Particle image velocimetry measurements were performed to explore the velocity fields in the near and intermediate regions of the jet and data were analyzed in terms of fields of time-averaged velocity components, turbulent intensities, Reynolds shear stresses, vorticity, and Q-criterion. The transversal velocity profiles were shown and their Gaussianity investigated via skewness and kurtosis. Thus, second-order statistics of turbulence of the velocity fluctuations were carried out. Moreover, a spectral analysis was performed after the Taylor hypothesis verification was proved and a net scale separation was guaranteed to develop the inertial sub-range. The influence of variable jet flow rate and the constant pipe flow rate on the jet behavior is shown and, in particular, how, as this parameter decreases, the jet moves away from the typical axisymmetric features of a round simple jet. In addition, the spectra analysis is used to identify the jet potential core and the large-scale organization zones, highlighting the effect of variable jet flow rate and the constant pipe flow rate on the extension of the potential core region and on the mixing layer.

Similarity learning with neural networks.

In this work, we introduce a neural network algorithm designed to automatically identify similarity relations from data. By uncovering these similarity relations, our network approximates the underlying physical laws that relate dimensionless quantities to their dimensionless variables and coefficients. Additionally, we develop a linear algebra framework, accompanied by code, to derive the symmetry groups associated with these similarity relations. While our approach is general, we illustrate its application through examples in fluid mechanics, including laminar Newtonian and non-Newtonian flows in smooth pipes, as well as turbulent flows in both smooth and rough pipes. Such examples are chosen to highlight the framework's capability to handle both simple and intricate cases, and further validate its effectiveness in discovering underlying physical laws from data.