Pulsating Pipe Flow Research Articles

Experimental, numerical and deep learning modeling study of heat transfer in turbulent pulsating pipe flow

Pulsating heat transfer is important in many engineering applications but is not fully understood or easily modeled. This work helps understand the phenomenon and demonstrates the efficacy and utility of deep learning modeling for pulsating heat transfer. The convective heat transfer coefficient (HTC) of pulsating flow through a copper tube was experimentally investigated at seventy-seven different operating conditions (500,000 time-series data points were collected at each operating condition) corresponding to different pulsating waveforms, frequencies, mass flow rates and pressures. The intensity of pulsation ranged from mild to strong enough to cause reverse flow. The literature on pulsating HTC is mixed, with several researchers reporting increased heat transfer and several other reporting decreased heat transfer relative to steady flow. The experimental data showed that both could be true depending on the ratio of the amplitude to mean flow velocity (amplitude ratio AR). The enhancement ratio (ER) of pulsating HTC relative to steady-flow HTC was measured to vary between 0.48 and 2.18 for AR ranging from 0.64 to14.42. A numerical study showed that this was only a (positive) correlation; fundamentally the overall pulsating HTC could be approximately explained by the variation of instantaneous Reynold’s number (Re) during the pulsating cycle. The distribution of instantaneous Re and hence the ER is primarily determined by the pulsating waveform, and this explains the disparate observations by different researchers. Long Short-Term Memory (LSTM) networks have been used to model pulsating HTC as a function of pulsating waveforms occuring across a measurement orifice. The ‘Toy Model’ concept where physics-based variables are used to increase generalization was implemented with the LSTMs for increased model robustness. These models could be used to maximize pulsating heat transfer by optimizing the waveform.

Computational Analysis of Pulsating Pipe Flow for Heat and Flow Characteristics Enhancement

To increase the performance of the thermal and flow devices with respect to the heat and flow characteristics in a pipe, different techniques are widely used such as modification in the fluid flow and modifications in the pipe geometries. Several boosting modified tubes, including a finned tube, a tube with ribs, a tube with spirally roughened walls, a corrugated tube, a fluted tube, a helical tube, an elliptical axis tube, and others, act to increase the turbulence in fluid flow and are the examples of modifications in the geometries. As compared to the modification in the pipe geometries with modification in the fluid flow, the fluid flow modifications attract more cost and most are the techniques are not so economical. In the current paper, the comparative analysis is studied between the simple pipe and pipe with dimples for heat and flow characteristics enhancements with the help of computational techniques. For pipe with dimples, the results were more satisfying as compared with the simple pipe. The maximum velocity build-up in the dimple pipe was 0.753m/s whereas in the simple pipe it was 0.604m/s, , that means there is a 24.66% increase in velocity in dimple pipe as compared with simple pipe. Also, vorticity was found to be more in the dimple pipe and which was 126000/s as compared with simple pipe 389/s , which means more than 100% increase in the vorticity. Heat transfer enhancement in the dimple pipe was observed in terms of an increase in the temperature and found to be a maximum of 418 K as compared with simple pipe only 409 K,which means that there is a 17.60% increase in the temperature in the dimple pipe. The computational study concludes that there were enhancements in the heat and flow characteristics of fluid flow in the dimple pipe as compared with the simple pipe.

Optical investigation of dense suspensions with non-Newtonian matrix under pulsating pumping

Pressure-driven Poiseuille flow of a bidisperse, non-colloidal, dense granular suspension with non-Newtonian matrix under pulsatile pumping is studied experimentally. The flow properties are recorded using a transmitted light optical system. The setup allows to derive velocity profiles with a high spatial and temporal resolution. Statements can be made about the near-wall region, which are difficult to achieve experimentally in dense suspensions. The results are compared in detail with velocity profiles based on theoretical models from the field of suspension rheology and particle migration under the conditions present in the experiment. • Spatially and temporally resolved velocity profiles of a pulsating pipe flow. • Complete comparison of experimental data with rheological and particle migration models. • Determination of wall slip coefficients and comparison with predictions of theoretical models. • Evaluation of velocity profiles during acceleration and deceleration phases (pulsating pumping). • Investigation of different particle distributions with regard to the evolving velocity profiles and wall slip.

Prediction of Drag Reduction Effect by Machine Learning toward Optimization of Pulsating Control in Turbulent Pipe Flow

Machine learning based on experimental data was performed to predict the drag reduction effects of various pulsating pipe flows. In the experiment, the voltages input to the pumps were changed over time to generate 2448 different pulsating flows. Pulsating flows with higher pressure gradients than in previous study was investigated with high responsive pumps. Using machine learning model specialized for time series prediction based on experimental data, time series data of flow velocity and differential pressure from the voltages applied to the pumps to calculate the drag reduction rates. It was confirmed that the model could predict the drag reduction effect of complex pulsating flows with the almost same accuracy as in the previous study. Optimization of the voltage waveforms for pulsating control was also performed using the learned model. As a result, the drag reduction rate of optimal voltage waveform within the conditions equivalent to those of the experiment is predicted to be 42%. This is a larger value than the maximum drag reduction rate of 39% which is measured in the experiment.

Analytical model for electromagnetic induction in pulsating ferrofluid pipe flows

Energy harvesting processes using ferrofluidic induction—a process that generates voltage via the pulsation of a ferrofluid (iron-based nanofluid) through a solenoid—have received increasing attention over the past decade. In this paper an analytical model is proposed to predict the induced electromotive force (EMF) based on the flow behavior and magnetic properties of a pulsating ferrofluid energy harvesting device. The ferrofluid is treated as an idealized series of discrete magnetic ‘slugs’ within passing through the solenoid, and the model identifies key parameters for describing and optimizing ferrofluidic induction in pulsating pipe flows. The resulting expression for induced EMF as a function of slug position relative to the solenoid is numerically integrated to determine the root mean square (RMS) of EMF during one pulsation cycle. Data from a previously documented study using an experimental induced EMF test rig is analyzed to find corresponding measured values of EMF RMS and used to validate the analytical model. Experimental and analytical results both show an increase in induced EMF with higher pulse frequency, increased number of bias magnets, and reduced spacing between the magnet and solenoid. At low magnetic flux densities and pumping frequencies, the EMF is negligible; with 0.3 mT magnetic flux density and 9 and 18 Hz pumping frequencies the measured RMS is nominally zero. For a 9 Hz pumping frequency the RMS value of the induced EMF is less than 1 μV at all magnetic flux densities. For the highest pumping frequency examined, 30 Hz, the induced EMF RMS is greater than 1 μV for all magnetic flux densities greater than 0.7 mT. As the experimentally measured values of EMF lie within the uncertainty bounds of the model, induced EMF can be predicted within an order of magnitude for magnetic fluid-induced energy harvesting systems.

Prediction of the drag reduction effect of pulsating pipe flow based on machine learning

Prediction of drag reduction effect caused by pulsating pipe flows is examined using machine learning. First, a large set of flow field data is obtained experimentally by measuring turbulent pipe flows with various pulsation patterns. Consequently, more than 7000 waveforms are applied, obtaining a maximum drag reduction rate and maximum energy saving rate of 38.6% and 31.4%, respectively. The results indicate that the pulsating flow effect can be characterized by the pulsation period and pressure gradient during acceleration and deceleration. Subsequently, two machine learning models are tested to predict the drag reduction rate. The results confirm that the machine learning model developed for predicting the time variation of the flow velocity and differential pressure with respect to the pump voltage can accurately predict the nonlinearity of pressure gradients. Therefore, using this model, the drag reduction effect can be estimated with high accuracy.

Wall Shear Stress in Pulsating Pipe Flow at Resonance Conditions

Hydraulic vibrations in liquid-filled pipelines may cause unwanted operational problems. Wall shear stress and consequential damping can reduce the harmful effects of vibrations close to resonance. A large-scale pipeline apparatus at Deltares, Delft, The Netherlands, has been used for pulsating pipe flow tests. The apparatus consists of a constant-head tank, a horizontal 206 mm diameter 49 meters long steel pipe and an oscillating valve at the downstream end. Wall shear stress has been measured by a number of hot-film sensors. Tests have been performed with an average flow Reynolds number of about 22,000. Results of a hydraulic resonance case with oscillating frequency fex = 5 Hz are presented. Pipe wall vibrations for this case are small. The shape of the velocity profile at resonance conditions is a typical unsteady-state velocity profile with reverse flow near the pipe wall. The CFD study in an axisymmetric domain was conducted to better understand the pulsating flow phenomena. Different settings of boundary conditions, based on the experimental investigations, were used. The CFD results show the unsteady character of wall shear stress at resonance. This phenomenon has not been observed in the measured results to such an extent. The measured shear stress resembles quasi-steady behaviour.

Pulsating pipe flow instability perturbed by a trip wire

This study is concerned with the phenomenon of flow instability in the entrance region of a pulsating pipe flow perturbed by a trip wire at the inlet, where the flow was uniform at the core with a thin boundary layer on the wall. First of all, it is illustrated by a case without the installation of a trip wire that small disturbance developed in the accelerating phase of pulsation, which was well explained by the inflection-point instability mechanism. Subsequently, one of the cases studied with the installation of a trip wire indicates that the inflection-point instability mechanism still played the key role, but the instability was promoted further upstream. On the other hand, there were cases of which the flow instability development was rather due to the viscous effect mainly, which was decaying in a certain downstream region, followed by a renewal growth with irregular fluctuations.

Transition to turbulence in pulsating pipe flow

Fluid flows in nature and applications are frequently subject to periodic velocity modulations. Surprisingly, even for the generic case of flow through a straight pipe, there is little consensus regarding the influence of pulsation on the transition threshold to turbulence: while most studies predict a monotonically increasing threshold with pulsation frequency (i.e. Womersley number, $\unicode[STIX]{x1D6FC}$), others observe a decreasing threshold for identical parameters and only observe an increasing threshold at low $\unicode[STIX]{x1D6FC}$. In the present study we apply recent advances in the understanding of transition in steady shear flows to pulsating pipe flow. For moderate pulsation amplitudes we find that the first instability encountered is subcritical (i.e. requiring finite amplitude disturbances) and gives rise to localized patches of turbulence (‘puffs’) analogous to steady pipe flow. By monitoring the impact of pulsation on the lifetime of turbulence we map the onset of turbulence in parameter space. Transition in pulsatile flow can be separated into three regimes. At small Womersley numbers the dynamics is dominated by the decay turbulence suffers during the slower part of the cycle and hence transition is delayed significantly. As shown in this regime thresholds closely agree with estimates based on a quasi-steady flow assumption only taking puff decay rates into account. The transition point predicted in the zero $\unicode[STIX]{x1D6FC}$ limit equals to the critical point for steady pipe flow offset by the oscillation Reynolds number (i.e. the dimensionless oscillation amplitude). In the high frequency limit on the other hand, puff lifetimes are identical to those in steady pipe flow and hence the transition threshold appears to be unaffected by flow pulsation. In the intermediate frequency regime the transition threshold sharply drops (with increasing $\unicode[STIX]{x1D6FC}$) from the decay dominated (quasi-steady) threshold to the steady pipe flow level.

Numerical analysis of the drag reduction for turbulent pulsating pipe flows based on large eddy simulations

Numerical analysis of the drag reduction for turbulent pulsating pipe flows based on large eddy simulations

An investigation into inflection-point instability in the entrance region of a pulsating pipe flow.

This paper investigates the inflection-point instability that governs the flow disturbance initiated in the entrance region of a pulsating pipe flow. Under such a flow condition, the flow instability grows within a certain phase region in a pulsating cycle, during which the inflection point in the unsteady mean flow lifts away from the viscous effect-dominated region known as the Stokes layer. The characteristic frequency of the instability is found to be in agreement with that predicted by the mixing-layer model. In comparison with those cases not falling in this category, it is further verified that the flow phenomenon will take place only if the inflection point lifts away sufficiently from the Stokes layer.

Flow visualization of simple pipe and channel flows obtained by MRI time-slip method

Herein, time-resolved magnetic resonance imaging, a noninvasive medical diagnostic imaging technique, was evaluated as a noncontact measurement tool for intuitively understanding fluid machineries. Simple pipe flows and channel flows are investigated by the 2D time–spatial labeling inversion pulse (2D time–SLIP) method, which can track a labeled water mass and visualize it using two-dimensional images. In this article, moving water masses of steady and pulsating pipe flows in a straight single pipe and a double cylindrical pipe (which are often seen in fluid machines and heat exchangers) are described. Then, abruptly contracting and expanding channels were tested and compared with particle image velocimetry (PIV) measurements or numerical simulations to evaluate their validity. In addition, as a feasibility test, a rotating water wheel and a fluidic diode with a strong swirling flow were tested to estimate this method’s applicability to fluid machines. The results suggest that the time-SLIP method of tracking a labeled water mass is sufficiently accurate for use in simple fluid machinery under low Re number conditions.

Non-sinusoidal waveform effects on heat transfer performance in pulsating pipe flow

In the present paper, an unsteady motion of fluid flow in a pulsating pipe is studied to determine the effect of non-sinusoidal waveforms on the heat transfer performance. Three non-sinusoidal waveforms, namely sawtooth, square and triangular waveforms have been considered. Explicit analytical expressions for a periodic laminar flow describing the flow and heat transfer at small and large times with sawtooth and square pressure waveforms have been derived using Bessel transform technique. The heat transfer performance of periodic flow at sawtooth and square pressure waveforms has been compared with the published result for triangular waveform [1]. The temperature performance for a triangular waveform pressure is very different from the sawtooth and square pressure waveforms.

Wall friction and velocity measurements in a double-frequency pulsating turbulent flow

Wall shear stress measurements employing a hot-film sensor along with laser Doppler velocimetry measurements of the axial and tangential velocity and turbulence profiles in a pulsating turbulent pipe flow are presented. Time-mean and phase-averaged results are derived from measurements performed at pulsation frequencies ${\it\omega}^{+}={\it\omega}{\it\nu}/\bar{u}_{{\it\tau}}^{2}$ over the range of 0.003–0.03, covering the low-frequency, intermediate and quasi-laminar regimes. In addition to the base case of a single pulsation imposed on the mean flow, the study also investigates the flow response when two pulsations are superimposed simultaneously. The measurements from the base case show that, when the pulsation belongs to the quasi-laminar regime, the oscillating flow tends towards a laminar state in which the velocity approaches the purely viscous Stokes solution with a low level of turbulence. For ${\it\omega}^{+}<0.006$, the oscillating flow is turbulent and exhibits a region with a logarithmic velocity distribution and a collapse of the turbulence intensities, similar to the time-averaged counterparts. In the low-frequency regime, the oscillating wall shear stress is shown to be directly proportional to the Stokes length normalized in wall units $l_{s}^{+}~(=\sqrt{2/{\it\omega}^{+}})$, as predicted by quasi-steady theory. The base case measurements are used as a reference when evaluating the data from the double-frequency case and the oscillating quantities are shown to be close to superpositions from the base case. The previously established view that the time-averaged quantities are unaffected by the imposition of small-amplitude pulsed unsteadiness is shown to hold also when two pulsations are superposed on the mean flow.

Experimental Study on Effect of Pulsating Pipe Flow upon Pressure Loss

Experimental Study on Effect of Pulsating Pipe Flow upon Pressure Loss

Pulsating pipe flow with large-amplitude oscillations in the very high frequency regime. Part 2. Phase-averaged analysis

AbstractThis paper is the follow-up of a previous study (Manna, Vacca & Verzicco,J. Fluid Mech., vol. 700, 2012, pp. 246–282) that numerically investigated the effects of a harmonic volume forcing on the turbulent pipe flow at a bulk Reynolds number of$\simeq 5900$. There, the investigation was focused on the time- and space-averaged statistics of the first- and second-order moments of the velocity, the vorticity fluctuations and the Reynolds stress budgets in order to study the changes induced on the mean current by the oscillating component. The amplitude of the latter was used as a parameter for the analysis. However, as the flow is inherently unsteady, the phase-averaged statistics are also of interest, and this is the motivation and subject of the present study. Here, we show the variability of the above quantities during different phases of the flow cycle and how they are affected by the amplitude of the oscillation. It is observed that when the ratio of the oscillating to the time-constant velocity component is of the order of one (${\it\beta}\simeq O(1)$), the phase-averaged profiles are appreciably influenced by the pulsation, although only small deviations of the time-averaged counterparts have been documented. In contrast, when that ratio is increased by one order of magnitude (${\it\beta}\simeq O(10)$) the phase- and cycle-averaged quantities differ considerably, especially during the decelerating part of the cycle. In more detail, the amplitude and the phase of all turbulence statistics show significant variations with${\it\beta}$. This variability has important implications in the dynamics and modelling of these flows. Since the data have been obtained by direct numerical simulations and validated by comparisons with experimental studies, the results could be used for validation of codes, testing of turbulence models or measurement procedures.

EXPERIMENTAL INVESTIGATION ON INITIAL DISTURBANCES IN PULSATING PIPE FLOWS

Growth of the initial disturbances in pulsating pipe flows was examined in this study from the point of view of Rayleigh instability. Experimental results obtained show that the instability is initiated in the developing region during the deceleration phase when an inflection point is obviously present in the velocity profile. Moreover, the results obtained are able to unveil the process regarding the disturbances get amplified and re-laminarized with respect to the phase angle and the radial location.

On the boundary conditions of cerebral aneurysm simulations

Cerebral aneurysms are bulges appearing on brain arteries. The development, growth and rupture has been intensively studied using numerical tools. We present a series of studies covering many of the open questions related to the boundary conditions used in numerical simulations, namely the inlet, outlet and the wall. The effect of boundary conditions cannot be overestimated; yet it received much less attention than it would have deserved.An analytical formula of the pulsating pipe flow with rigid walls is used to study the effects of the varying parameters of the cardiac cycle. Both idealised and real geometries are used to study the propagation of the boundary effects, and to analyse the effects of distensible boundaries.Our results provide useful criteria for the necessary inlet and outlet length. We found that the deformability of wall boundaries has a significant influence; however, simulations with rigid walls are still able to capture the main hemodynamic properties of the flow.The results can be effectively used in the simulation of cerebral aneurysm and provide a firmer ground in a broader sense for hemodynamic simulations.

Development length of sinusoidally pulsating laminar pipe flows in moderate and high Reynolds number regimes

Recent studies on laminar, fully-developed, sinusoidally pulsating pipe flows have revealed the existence of a unique signature map that can be used for the measurement of the arbitrary time-varying, instantaneous mass flow rate from the recorded axial pressure gradient data and vice versa. This measuring technique is, however, valid for the hydrodynamically fully-developed flow of an incompressible fluid. The present study, therefore, deals with the numerical evaluation of the required development length as functions of the mean Reynolds number, the amplitude of mass flow rate pulsation and the pulsation frequency in the moderate and high Reynolds number regimes. The investigation shows that in the low-frequency, quasi-steady regime, the instantaneous variations of L/D can be predicted by the steady-state results for corresponding instantaneous Reynolds numbers. On the other hand, at higher pulsation frequencies, considerable deviation from the pure sinusoidal signal occurs for the development length and its amplitude decreases with increase in pulsation frequency. Finally, using the results of the present simulations, a simple correlation is proposed that can be used in order to predict the maximum development during a cycle as functions of ReM,m˙A∗ and F.

Pulsating pipe flow with large-amplitude oscillations in the very high frequency regime. Part 1. Time-averaged analysis

AbstractThis paper numerically investigates the effects of a harmonic volume forcing of prescribed frequency on the turbulent pipe flow at a Reynolds number, based on bulk velocity and pipe diameter, of 5900. The thickness of the Stokes layer, resulting from the oscillatory flow component, is a small fraction of the pipe radius and therefore the associated vorticity is confined within a few wall units. The harmonic forcing term is prescribed so that the ratio of the oscillating to the mean bulk velocity ($\ensuremath{\beta} $) ranges between 1 and 10.6. In all cases the oscillatory flow obeys the Stokes analytical velocity distribution while remarkable changes in the current component are observed. At intermediate values $\ensuremath{\beta} = 5$, a relaminarization process occurs, while for $\ensuremath{\beta} = 10. 6$, turbulence is affected so much by the harmonic forcing that the near-wall coherent structures, although not fully suppressed, are substantially weakened. The present study focuses on the analysis of the time- and space-averaged statistics of the first- and second-order moments, vorticity fluctuations and Reynolds stress budgets. Since the flow is unsteady not only locally but also in its space-averaged dynamics, it can be analysed using phase-averaged and time-averaged statistics. While the former gives information about the statistics of the fluctuations about the mean, the latter, postponed to a subsequent paper, shows how the mean is affected by the fluctuations. Clearly, the two phenomena are connected and both of them deserve investigation.