Averaged Flow Field Research Articles

Microstructure of Flow-Driven Suspension of Hardspheres in Cylindrical Confinement: A Dynamical Density Functional Theory and Monte Carlo Study.

We have studied the microstructure of a flow-driven hardsphere suspension inside a cylinder using dynamical density functional theory and Monte Carlo simulations. In order to be representative of various physical conditions that may prevail in experiments, we investigate the problem using both the grand canonical (μVT) ensemble and the canonical (NVT) ensemble. In both ensembles, the hydrodynamic effect on the suspension mediated by the presence of the confining wall is implemented in a mean-field fashion by incorporating the thermodynamic work done by the inertial lift force on the particle given the average flow field. The predicted particle distribution in the μVT ensemble displays strong structural ordering at increasing flow rates due to the correspondingly higher particle concentrations inside the cylinder. In the NVT ensemble, for dilute suspensions we observe a peak in the distribution of density at a location similar to that of the Segré-Silberberg annulus, while for dense suspensions the competing effects of the inertial lift and the hardsphere interaction lead to the formation of several annuli.

Assesment of the transverse galloping stability of a railway overhead located above a railway bridge

In this paper, galloping instability of catenary overheads located at railway bridge is assessed. A first contribution of this paper is the experimental characterization, in a wind tunnel, of the flow field above the railway platform for a cross-wind protected railway bridge. From particle image velocimetry (PIV) measurements, the averaged flow field above the railway platform is obtained and estimations of the bidimensional averaged wind velocity field are reported. Another contribution of the paper is the measurement of aerodynamic coefficients of a BC-120 contact wire cross section. From both measurements, averaged PIV flow field and aerodynamic coefficients, galloping stability is assessed using a parameter based on Den Hartog criterion. A comparison with the results obtained using the aerodynamic coefficients reported by Scanlon and Stickland is also presented and discussed.

Direct numerical simulation of open-channel flow over a fully rough wall at moderate relative submergence

Direct numerical simulation of open-channel flow over a bed of spheres arranged in a regular pattern has been carried out at bulk Reynolds number and roughness Reynolds number (based on sphere diameter) of approximately 6900 and 120, respectively, for which the flow regime is fully rough. The open-channel height was approximately 5.5 times the diameter of the spheres. Extending the results obtained by Chan-Braun et al. (J. Fluid Mech., vol. 684, 2011, pp. 441–474) for an open-channel flow in the transitionally rough regime, the present purpose is to show how the flow structure changes as the fully rough regime is attained and, for the first time, to enable a direct comparison with experimental observations. Different statistical tools were used to investigate the flow field in the roughness sublayer and in the logarithmic region. The results indicate that, in the vicinity of the roughness elements, the average flow field is affected both by Reynolds number effects and by the geometrical features of the roughness, while at larger wall distances this is not the case, and roughness concepts can be applied. Thus, the roughness function is computed which in the present set-up can be expected to depend on the relative submergence. The flow–roughness interaction occurs mostly in the region above the virtual origin of the velocity profile, and the effect of form-induced velocity fluctuations is maximum at the level of sphere crests. In particular, the root mean square of fluctuations about the streamwise component of the average velocity field reflects the geometry of the spheres in the roughness sublayer and attains a maximum value just above the roughness elements. The latter is significantly weakened and shifted towards larger wall distances as compared to the transitionally rough regime or the case of a smooth wall. The spanwise length scale of turbulent velocity fluctuations in the vicinity of the sphere crests shows the same dependence on the distance from the wall as that observed over a smooth wall, and both vary with Reynolds number in a similar fashion. Moreover, the hydrodynamic force and torque experienced by the roughness elements are investigated and the footprint left by vortex structures on the stress acting on the sphere surface is observed. Finally, the possibility either to adopt an analogy between the hydrodynamic forces associated with the interaction of turbulent structures with a flat smooth wall or with the surface of the spheres is also discussed, distinguishing the skin-friction from the form-drag contributions both in the transitionally rough and in the fully rough regimes.

Modelling of combustion acoustics sources and their dynamics in the PRECCINSTA burner test case

A stochastic, hybrid computational fluid dynamics/computational combustion acoustics approach for combustion noise prediction is applied to the PRECCINSTA laboratory scale combustor (prediction and control of combustion instabilities in industrial gas turbines). The numerical method is validated for its ability to accurately reproduce broadband combustion noise levels from measurements. The approach is based on averaged flow field and turbulence statistics from computational fluid dynamics simulations. The three-dimensional fast random particle method for combustion noise prediction is employed for the modelling of time-resolved dynamics of sound sources and sound propagation via linearised Euler equations. A comprehensive analysis of simulated sound source dynamics is carried out in order to contribute to the understanding of combustion noise formation mechanisms. Therefrom gained knowledge can further on be incorporated for the investigation of onset of thermoacoustic phenomena. The method-inherent stochastic Langevin ansatz for the realisation of turbulence related source decay is analysed in terms of reproduction ability of local one- and two-point statistical input and therefore its applicability to complex test cases. Furthermore, input turbulence statistics are varied, in order to investigate the impact of turbulence on the resulting sound pressure spectra for a swirl stabilised, technically premixed combustor.

Experimental investigations of turbulent fragmenting stresses in a rotor-stator mixer. Part 1. Estimation of turbulent stresses and comparison to breakup visualizations

Despite large industrial relevance, the relation between rotor-stator geometry, hydrodynamics and drop breakup is poorly understood, partly since no methods for measuring the fragmenting stresses acting on drops have been established. This study attempts to bridge this gap by developing, applying and evaluating two approaches for estimating local turbulent stresses based on particle image velocimetry data: namely one traditional but indirect approach based on the dissipation rate of turbulent kinetic energy, and another more direct approach based on the spatial turbulent spectrum that has proven useful in other high-intensity emulsification processing. The approaches are evaluated in terms of validity of underlying assumptions, how they compare to breakup visualizations in the same geometry and with regard to the reliability of primary measurables.Results show three consistent regions of high stress in the rotor-stator region: in a plume extending into the stator-hole from the trailing edge, in the shear layers of the jet exiting the hole and in the macroscopic flow structure formed after the rotor blocks a stator hole. Following a drop travelling along an average velocity flow field, the measurement predict disrupting stresses exceeding the stabilizing stress at the stator hole exit, at approximately the same position where drop breakup is observed in breakup visualizations. Both methods are therefore able to predict the most likely breakup positions. It is also concluded that both methods have limitations, and that average stress alone cannot describe all aspects of the fragmentation process in rotor-stator mixers.

Comparison between 3-D-PTV and 2-D-PIV for determination of hydrodynamics of complex fluids in a stirred vessel

The capabilities of 3-D-Particle Tracking Velocimetry (PTV) to measure flow fields during the blending of Newtonian and non-Newtonian fluids in a standard baffled cylindrical vessel are assessed. The results are benchmarked against conventional 2-D Particle Image Velocimetry (PIV) data. The vessel, of diameter T=0.19m, is equipped with a 6-blade down-pumping PBT impeller of diameter, D=0.5T. Experiments in the low transitional (Re∼70), and transitional (Re∼1000) regimes have been conducted, using a range of Newtonian and non-Newtonian fluids. Turbulent flow measurements (Re>20,000) are made using Newtonian fluids. Data from both techniques are compared in terms of average flow field and, where appropriate, turbulent fluctuating velocity components. Particular emphasis is given on how comparisons can be made between the Eulerian PIV data and the Euler-Lagrangian PTV data. The overall results demonstrate the validity of the PTV technique in this application to acquire average flow fields which are in good agreement with PIV. Turbulent flow properties are less well resolved by PTV due in part to the large size of the tracer particle used. Other advantages and limitations of PTV versus PIV are also discussed.

Formation and characterization of the vortices generated by a DBD plasma actuator in burst mode

The present study reports the formation and evolution characteristics of the continuously generated vortical structure and resulting flow field in quiescent air induced by a dielectric-barrier-discharge (DBD) plasma actuator in burst mode operation. A starting vortex is formed during the initial actuation period, which disappears after a small time interval for continuous mode operation of the DBD plasma actuator. A burst input signal to the actuator generates a train of self-similar vortices. The behaviour of vortices and the average flow field induced by the actuator has been studied using high speed schlieren visualization and particle image velocimetry technique for different actuation amplitude and duty cycle parameters. These repeating vortices travel faster than the starting vortex, and the vortex core velocity of these repeating vortices increases with increase in duty cycle parameter. Fuller u-velocity profile, higher v-velocity near the edge of the outer shear layer region, and higher growth of the wall jet thickness is observed due to enhanced entrainment by repeating vortices for burst mode operation. The repeating vortices travel at an angle of 21° relative to the wall surface for duty cycle parameter of 90.9% in comparison to 31° for the starting vortex. Self-similarity of the velocity profile is delayed in the streamwise direction for burst mode operation in comparison to that for the continuous mode of operation. This can be attributed to delay in attaining the maximum velocity of the wall jet profile and presence of coherent structures for the burst mode operation. The non-dimensional vortex core location and size for repeating vortices follow power law fit similar to the starting vortex with difference in value of the power law exponent. The phase difference between the input voltage and current drawn is in the range of π12 to π9 (in radians) for both continuous and burst mode operation indicating identical electrical behaviour of the actuator in both modes of operation. Overall, the present study illustrates the effectiveness of a DBD plasma actuator as a vortex generator while operating in the burst mode. The train of vortices generated during the burst mode operation shows overall similarity with the starting vortex, and the difference in behavior is attributed to the interaction between the vortices shed during consecutive bursting cycle, which is a function of the duty cycle parameter.

Rotor-Stator Phase-Locked and Ensemble Averaged Flow Fields in a Multi-Stage Transonic Axial Compressor

Rotor-Stator Phase-Locked and Ensemble Averaged Flow Fields in a Multi-Stage Transonic Axial Compressor

CONTROL OF SELF-SUSTAINED OSCILLATIONS OF A THREE-DIMENSIONAL WATER JET IN A SLENDER CHANNEL

This work focuses on control of an oscillatory main jet submerged in a slender channel of small thickness to width ratio, by two lateral opposite and normal injections arranged at the same height, on the end sidewalls of the channel right above the main jet exit. The objective of this work is about the control of the jet deviation and its oscillations frequency, according to the flow rate of the two lateral injections. The validation confirms that second-order turbulence models predict more accurately such flows than first-order models. Due to Coanda effect, jet oscillations have been highlighted for several Flow rate ratios (β). Two flow behaviours of time averaged flow fields corresponding to two ranges of β are detected. It appears that the key question of this flow is also the two oscillatory motions of the main and the lateral jets.

Large-scale motions in turbulent boundary layers subjected to adverse pressure gradients

It is known that large-scale streamwise velocity-fluctuating structures ($u^{\prime }$) are frequently observed in the log region of a zero pressure gradient turbulent boundary layer, and that these motions significantly influence near-wall small-scale $u^{\prime }$-structures by modulating the amplitude (Hutchins & Marusic, J. Fluid Mech., vol. 579, 2007, pp. 1–28; Mathis et al., J. Fluid Mech., vol. 628, 2009, pp. 311–337). In the present study, we provide evidence that the spatial organization of large-scale structures in the log region is significantly influenced by the strength of adverse pressure gradients in turbulent boundary layers based on a direct numerical simulation dataset. For a mild adverse pressure gradient boundary layer flow, groups of hairpin vortices are coherently aligned in the streamwise direction to form hairpin vortex packets, and streamwise merging events of the induced large-scale $u^{\prime }$-structures create a larger streamwise length scale of structures than that for a zero pressure gradient boundary layer flow. As the pressure gradient strength increases further, however, the formation of hairpin packets is continuously suppressed, and large-scale motions are consequently not concatenated to create a longer motion, resulting in a significant reduction of the streamwise coherence of large-scale structures in the log layer. Although energy spectrum maps for $u^{\prime }$-structures show that the large-scale energy is continuously intensified above the log layer with an increase in the pressure gradient, amplitude modulation of the near-wall small-scale motions is dominantly induced by log region large-scale structures for adverse pressure gradient flows. Conditional averaged flow fields with large-scale Q2 and Q4 events indicate that large-scale counter-rotating roll modes play an important role in organizing the flows under the pressure gradients, and the large-scale roll modes associated with Q4 events are more enhanced in the outer layer than those associated with Q2 events, reducing the streamwise coherence of the vortices in a packet.

Construction of one-step H2/O2 reaction mechanism for predicting ignition and its application in simulation of supersonic combustion

This study developed a method to establish a one-step global reaction mechanism for predicting the ignition delay time. The activation energy database applied in 850–1800K temperature, 0.1–100 atm pressure and a wider excess air coefficient within the range of flammable limit was constructed and the one-step reaction mechanism was then obtained for a specified chemical reaction state through a look-up or Lagrange interpolation technology. The present one-step global mechanism is able to predict the accurate ignition delay time compared with the detailed mechanism, and is superior to the other available reduced mechanisms. Coupled to a high-order in-house Computational Fluid Dynamics (CFD) code, the current reaction mechanism is successfully applied to numerical simulations of supersonic chemically reacting mixing layers under different flow conditions. The numerical predictions of supersonic combustion process, ignition position as well as averaged flow fields, using the present mechanism are comparable to the results using the detailed mechanism. Therefore, this simplified reaction mechanism is suitable for fast simulation-based designs in industrial.

Reconstructing three-dimensional wake topology based on planar PIV measurements and pattern recognition analysis

The present study presents a new technique for reconstructing the salient aspects of three-dimensional wake topology based on time-resolved, planar, two-component particle image velocimetry data collected in multiple orthogonal planes. The technique produces conditionally averaged flow field reconstructions based on a pattern recognition analysis of velocity fields. It is validated on the wake of a low-aspect ratio dual step cylinder geometry, consisting of a large diameter cylinder (D) with small aspect ratio (L/D) attached to the mid-span of a small diameter cylinder (d). For a dual step cylinders with D/d = 2, and L/D = 1, numerical and experimental data are considered for ReD = 150 (laminar wake) and for ReD = 2100 (turbulent wake). The results show that the proposed technique successfully reconstructs the dominant periodic wake vortex interactions and can be extended to a wide range of turbulent flows.

Characterisation of cross-flow above a railway bridge equipped with solid windbreaks

The flow field above a two dimensional model of a railway bridge equipped with solid windbreaks is analysed in a wind tunnel. Particle image velocimetry (PIV) is used to measure the flow velocity in planes perpendicular to the bridge span. The mean velocity components, the two-component turbulent kinetic energy, the turbulence intensities of the velocity fluctuation components and the Reynolds shear stress above the bridge deck are presented. The flow patterns based on the streamlines of the average flow field are analysed. The inclusion of a windbreak produces a separation bubble, that is locked to the bridge deck due to presence of the leeward fence. Special attention is paid to the analysis of the flow field characteristics along the vertical profiles above the railway tracks. The inclusion of the windbreak leads both to an increase of the mean velocity and the turbulence intensity around the catenary contact wires. On the other hand, the flow in the region close to the bridge deck is slowed-down. The effect of the size of the final interrogation window used in the PIV analysis is considered, more particularly on the determination of the mean velocity and turbulence intensity. The results show that a decrease of the final interrogation window leads to an increase of the turbulence intensity when there are no wind protection devices installed on the bridge.

Data-enabled prediction of streak breakdown in pressure-gradient boundary layers

Streaks in pre-transitional boundary layers are analysed and their properties are extracted from direct numerical simulation data. Streaks that induce breakdown to turbulence via secondary instability are shown to differ from the remainder of the population in various attributes. Conditionally averaged flow fields establish that they are situated farther away from the wall, and have a larger cross-sectional area and higher peak amplitude. The analysis also shows that the momentum thickness acts as a similarity parameter for the properties of the streaks. Probability density functions of the streak amplitude, area, and shear along the streaks, collapse among the various pressure gradients when plotted as a function of the momentum thickness. A prediction scheme for laminar–turbulent transition based on artificial neural networks is presented, which can identify the streaks that will eventually induce the formation of turbulent spots. In comparison to linear stability theory, the approach achieves a higher prediction accuracy at considerably lower computational cost.

A laboratory investigation into the influence of a rigid vegetation on the evolution of a round turbulent jet discharged within a cross flow

The study of buoyant jets, those between pure jets and plumes, has been carried out with ever greater frequency over recent years due to its application in different practical engineering fields, i.e. appropriate design of outfalls for the disposal of municipal and industrial waste waters. The dispersion of waste and the related dilution of pollutants are governed by the mean-flow and turbulence characteristics of the resulting jets, which themselves depend on environmental conditions.The present study deals with how a uniform cross-stream with a channel bed surface covered by rigid emergent stems affects the behaviour of a circular turbulent buoyant jet. The time-averaged temperature and velocity fields are investigated in order to understand jet diffusion and penetration within the ambient fluid.The examination and comparison of the measured scalar and vector quantities show that the presence of emergent vegetation in the receiving environment affects both the average flow field and the jet structure, reducing the mean channel velocity, with a notable increase in jet penetration height and dilution compared to the test case without vegetation. This result is confirmed by the several vertical profiles of the mean scalar concentration and the normalized vertical velocity component along the channel centre plane. Moreover, the rigid emergent vegetation and its driven instabilities promote a distortion of the mean concentration and normalized axial velocity component profiles in the trajectory-based coordinate system.

A tomographic particle image velocimetry investigation of the flow development over dual step cylinders

This experimental study focuses on the near wake development of a dual step cylinder geometry consisting of a long base cylinder of diameter d to which a larger diameter (D) cylinder of length L is attached coaxially at mid-span. The experiments cover a range of Reynolds numbers, 2000 ≤ ReD ≤ 5000, diameter ratios, 1.33 ≤ D/d ≤ 2.0 and large cylinder aspect ratios, 0.5 ≤ L/D ≤ 5 using Tomographic particle image velocimetry. Distinct changes in wake topology are observed varying the above parameters. Supporting previous experimental studies on the same geometry involving flow visualization and planar measurements, four distinct flow regimes are identified to which a distinct three-dimensional wake topology can be associated. The vortex-dominated wake dynamical behaviour is investigated with Proper Orthogonal Decomposition (POD) and conditional averaging of three-dimensional velocity fields is used to exemplify the different shedding regimes. The conditionally averaged flow fields are shown to quantitatively resolve flow features equivalent to those obtained from a reduced order model consisting of the first ten to twenty POD modes, identifying the dominant vortex shedding cells and their interactions.

PIV measurements of the flow at the inlet of a turbocharger centrifugal compressor with recirculation casing treatment near the inducer

Turbocharging reciprocating engines is a viable solution in order to meet the new regulations for emissions and fuel efficiency in part because turbochargers allow to use smaller, more efficient engines (downsizing) while maintaining power. A major challenge is to match the flow range of a dynamic turbomachine (the centrifugal compressor in the turbocharger) with a positive displacement pump (the engine) as the flow range of the latter is typically higher. The operating range of the compressor is thus of prime interest. At low mass flow rate (MFR), the compressor range is limited by the occurrence of surge. To control and improve it, numerous and varied methods have been used. Yet, an automotive application requires that the solution remains relatively simple and preferably passive. A common feature that has been demonstrated to improve the surge line is the use of flow recirculation in the inducer region through a circumferential bleed slot around the shroud, also called “ported shroud”, similar to what has been developed for axial compressors in the past. The compressor studied here features such a device. In order to better understand the effect of the recirculation slot on the compressor functioning, flow measurements were performed at the inlet using particle image velocimetry and the results were correlated with pressure measurements nearby. Measurements were taken on a compressor with and without recirculation and across the full range of normal operation and during surge using a phase-locking method to obtain average flow fields throughout the entire surge cycle. When the recirculation is blocked, it was found that strong backflow develops at low MFR perturbing the incoming flow and inducing significant preswirl. The slot eliminated most of the backflow in front of the inducer making the compressor operation more stable. The measurements performed during surge showed strong backflow occurring periodically during the outlet pressure drop and when the instantaneous MFR is near 0 or negative. The flow motion at the inlet is highly three dimensional as flow enters in the center of the inducer at all times, even when the instantaneous flow rate is negative, compared to the reversed flow observed in the entire inlet for surging axial compressors.

Impact of ground and wheel boundary conditions on numerical simulation of the high-speed train aerodynamic performance

In this paper, the aerodynamic performance of a high speed train with moving ground and rotating wheels (MG&RW) conditions has been investigated using Computational Fluid Dynamic (CFD). The numerical simulations under the condition of stationary ground and moving ground are also compared and discussed. To validate the accuracy of the mesh resolution and methodology, the CFD results are compared with the wind tunnel test results. The aerodynamic forces, unsteady and time average flow fields around the high-speed train are analyzed. The main aim of this study is to investigate how the moving ground and wheel conditions influence the numerical simulation results. The moving ground condition eliminates the effects of boundary layers of ground and rail track, which results in the velocity under the train being faster than in the stationary ground condition. As a result, the drag of every bogie and the pressure on the bottom surface calculated under moving ground conditions is found to be higher than that under the stationary ground condition. The wheel rotation boundary condition has little effect on the force distribution on the bottom surface of the train. However, at the bogie regions, it increases the velocity of airflow around and behind the wheel, causing a change of the pressure distribution and an increment of the wheels’ drag. Consequently, the total drag of the train in moving ground with rotating wheel condition is nearly equal to that of condition with the moving ground considered. In addition, the total drag of bogies just takes up 10.4% in all in the stationary ground condition, and it accounts for 12.7% in the moving ground condition and 15.1% in the moving ground with rotating wheel condition.

Investigation of the effects of quarl and initial conditions on swirling non-premixed methane flames: Flow field, temperature, and species distributions

Detailed measurements are presented of the turbulent flow field, gas species concentrations and temperature field in a non-premixed methane swirl flame. Attention is given to the effect of the quarl geometry on the flame structure and emission characteristics due to its importance in gas turbine and industrial burner applications. Two different quarls were fitted to the burner exit, one a straight quarl and the other a diverging quarl of 15° half cone angle. Stereoscopic Particle Image Velocimetry (SPIV) was applied to obtain the three components of the instantaneous velocity on a vertical plane immediately downstream of the quarl exit. Temperature and gaseous species measurements were made both inside and downstream of the quarls, using a fine wire thermocouple and sampling probe, respectively. This work provides experimental verification by complementary techniques. The results showed that although the main flame structures were governed by the swirl motion imparted to the air stream, the quarl geometry, fuel loading and air loading also had a significant effect on the flow pattern, turbulence intensity, mixture formation, temperature distribution, emissions and flame stabilization. Particularly, in the case of the straight quarl flame, the flow pattern leads to strong, rapid mixing and reduces the residence time for NO formation within the internal recirculation zone (IRZ). However, for the diverging quarl flames, the recirculation zone is shifted radially outward, and the turbulent interaction between the central fuel jet and the internal recirculation zone IRZ induces another small vortex between these two flow features. Less mixing near the diverging quarl exit is observed, with a higher concentration of NO and CO in the post-combustion zone. The instantaneous flow field for both flames showed the existence of small scale vortical structure near the shear layers which were not apparent in the time averaged flow field. These structures, along with high levels of turbulence present in the shear layer, result in the enhancement of the mixing, especially with straight quarl. This article outlines results that yield deeper insight into the combustion of swirling flames stabilized within the quarl and to establish an experimental data base for modelers.

Numerical study of high speed jets in crossflow

Large-eddy simulation (LES) and dynamic mode decomposition (DMD) are used to study an underexpanded sonic jet injected into a supersonic crossflow and an overexpanded supersonic jet injected into a subsonic crossflow, where the flow conditions are based on the experiments of Santiago & Dutton (J. Propul. Power, vol. 13 (2), 1997, pp. 264–273) and Beresh et al. (AIAA J., vol. 43, 2005a, pp. 379–389), respectively. The simulations successfully reproduce experimentally observed shock systems and vortical structures. The time averaged flow fields are compared to the experimental results, and good agreement is observed. The behaviour of the flow is discussed, and the similarities and differences between the two regimes are studied. The trajectory of the transverse jet is investigated. A modification to Schetz et al.’s theory is proposed (Schetz & Billig, J. Spacecr. Rockets, vol. 3, 1996, pp. 1658–1665), which yields good prediction of the jet trajectories in the current simulations in the near field. Point spectra taken at various locations in the flowfield indicate a global oscillation for the sonic jet flow, wherein different regions in the flow oscillate with a frequency of $St=fD/u_{\infty }=0.3$. For supersonic jet flow, no such global frequency is observed. Dynamic mode decomposition of the three-dimensional pressure field obtained from LES is performed and shows the same behaviour. The DMD results indicate that the $St=0.3$ mode is dominant between the upstream barrel shock and the bow shock for the sonic jet, while the roll up of the upstream shear layer is dominant for the supersonic jet.