Increased Turbulence Levels Research Articles

Influence of Cavitation in Common-Rail Diesel Nozzles on the Soot Formation Process through Measuring Soot Emissions

The influence of cavitation in common-rail diesel nozzles on the soot formation process has been analysed experimentally. The soot formation process was characterized by measuring soot emissions in a single-cylinder engine, which was mounted on a test bench equipped with an opacimeter. In order to do this, operating conditions where the soot oxidation process was equivalent were chosen, whereby differences in the soot formation process were possible to be analysed. The results achieved confirm that cavitation provokes a soot formation process reduction. This reduction can be attributed by combining results of three effects: a reduction of the effective diameter, an increase in effective injection velocity, and an increase in turbulence level inside the nozzle orifice leading to a longer lift-off length. The three effects lead to a decrease in relative fuel/air ratio at the lift-off, therefore explaining the soot formation reduction.

The effect of an inline blockage on the formation of a turbulent free jet

In many equilibrium-based models for droplet size prediction, the fundamental parameters of turbulence are removed in favour of bulk quantities like velocity, and diameter. In this work we show that the upstream conditions of the jet are of critical importance and cannot be neglected, as they influence the turbulence field in jets in ways that are not accounted for in models based entirely on bulk parameters. This work presents the results of a single-phase investigation into the effect of upstream disturbances on a turbulent free jet.Reynolds Averaged Navier-Stokes Simulations of turbulent free jets with disturbances (in the form of orifice plates with a β ratio of 0.5) located at varying locations upstream of the pipe exit have been undertaken. Validation against experimental data for an orifice plate in a pipe found the Standard k-ε model to be the most accurate of the RANS models tested for this geometry.The additional turbulence generated from the upstream disturbances was found to be advected downstream and did not dissipate prior to the jet exit. The increased turbulence levels and altered form of the turbulence profile at exit was found to affect the evolution and observed turbulence levels of the free jet.Two distinct ‘modes’ were observed for the influence of the disturbances on the free jet. When the orifice plate is near the exit the free jet is dominated by the behaviour of semi-confined jet forming from the blockage. For the deeper set disturbances, the predicted form of the free jet was similar to that of the free jet from a straight pipe, though the turbulence levels are still elevated at the exit, affecting spreading rates and turbulence levels in the jet.Measures based on averages of TKE and TDR over hemispheres of varying radii are proposed for the development of models that do take into account the turbulent state of the exiting jet.

Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

Using flight measurements conducted between altitudes of 700 and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar–turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien–Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel are postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.

Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

Using flight measurements conducted between altitudes of 700 and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar–turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien–Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel are postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.

Effect of Surface Roughness and Inlet Turbulence Intensity on a Turbine Nozzle Guide Vane External Heat Transfer: Experimental Investigation on a Literature Test Case

Abstract Surface roughness is well known to significantly influence turbine aerodynamics and heat transfer; different studies have been undertaken in the last decades, in order to precisely characterize its effects and pursue a reliable and unified computational fluid dynamics modeling approach. Despite the effort, further research is still required to completely fulfill the goal, due to the complexity of the considered environment, with many other aspects and flow characteristics factoring into the final behavior. In this work, an experimental campaign was carried out to evaluate the heat transfer coefficient on a linear nozzle guide vane geometry. The adopted geometry has been developed and tested, at different inlet turbulence intensity, Reynolds and Mach number, at Von Kármán Institute. The results achieved on a test article with smooth surface were made available. In the present work, the effect of increased turbulence level and surface roughness was taken into account, respectively, using passive grids and conditioning the test sample surface finishing. Experiments were conducted using a transient technique by measuring the surface temperature evolution by IR thermography. The collected results integrate the existing database available in the open literature in order to support development and benchmarking of numerical approaches aimed at a reliable characterization of these aspects.

Predicting far-lee wind flow characteristics behind a 2D wedge-shaped obstacle: Experiments, numerical simulations and empirical equations

Far-lee wind flow characteristics behind 2D wedge-shaped obstacles with the front slope angle θ varying from 0° to 90° were explored through experiments and numerical simulations. The objective of this research was to study the effect of θ on wind field recovery behind the obstacle and to develop empirical equations to quantify this. Experimental results show that as θ increases, the mean wind velocity at any given height below three times the obstacle height within the far-lee region decreases while the turbulence level increases. Shifting of the shear layer center to higher elevations was also observed. Numerically, three widely used turbulence models, i.e. the Standard k-ε, RNG k-ε, and SST k-ω and the recently developed GEKO model were tested for their ability to replicate experimental results and to identify an appropriate turbulence model for subsequent simulations. A tuned version of the GEKO model was found to best predict both the mean and turbulence characteristics of wind flow in the far-lee region. Using these results, an empirical model was also proposed for predicting the mean and turbulence characteristics in the far-lee region of 2D wedge-shaped obstacles. This model is based on the concept of flow self-similarity in this region and was developed using experimental data collected herein and verified against simulated numerical data and data from other researchers, proving its validity.

Impact of turbulence on the coherent flame dynamics in a bluff-body stabilized flame

Coherent flame oscillations generated by large-scale flow instabilities have been shown to significantly influence combustor performance and thermoacoustic instability. This study examines the influence of turbulence intensity on the large-scale dynamics of rod-stabilized flames. Instability in the flow field of a bluff-body stabilized flame, which is a function of mean shear in the flow field, turbulence intensity of the incoming flow, and the location of the flame with respect to the shear layer, manifests as coherent vortex shedding. However, this vortex shedding is not self-excited if the flow is globally stable, as is often the case in reacting flows. In this experiment, time-resolved, three-component velocity measurements from high-speed stereoscopic particle image velocimetry are taken at three turbulent inlet flow conditions and at three bulk flow velocities. To identify whether these instabilities occur, the velocities fields are filtered using a wavelet transform around select spectral bands and then decomposed using proper orthogonal decomposition to extract the most energetic motion in the flow; this filtering method retains any time-dependent, intermittent behavior. The results show that vortex shedding is intermittent and the degree of intermittency is dependent on the in-flow turbulence level. Although all the cases tested were determined to be globally stable, variations in the flow velocity change the structure of the flame and flow, which alters the receptivity of the flow to turbulent perturbations. As a result, the strength and regularity of vortex shedding increase with increasing turbulence level and increased receptivity, indicating that the system is a noise-forced globally-stable oscillator.

Görtler vortices and streaks in boundary layer subject to pressure gradient: excitation by free stream vortical disturbances, nonlinear evolution and secondary instability

Abstract

Dry powder inhaler aerosol deposition in a model of tracheobronchial airways: Validating CFD predictions with in vitro data.

Effective drug delivery into the lungs plays an important role in management of pulmonary diseases that affect millions all around the world. The main objective of this investigation is to study airflow structure, as well as transport and deposition of micron-size particles at different inhalation flow rates in a realistic model of human tracheobronchial airways. The airway model was developed based on computed tomography (CT) images of a healthy 48-years-old female, which includes extrathoracic, trachea, and bronchial airways up to fourth generations. Computational fluid dynamics (CFD) simulations were performed to predict transport and deposition of inhaled particles and the results were compared to our previous in vitro experiments. Airflow structure was studied through velocity contours and streamlines in the extrathoracic region, where the onset of turbulence, reverse flow and subsequently vortex formation, and laryngeal jet are found to be critical phenomenons in the formation of airflow and deposition patterns. The deposition data was presented by deposition efficiency (DE) and deposition fraction (DF) against impaction parameter and Stokes number. At all of the inhalation flow rates, highest values of deposition fractions were devoted to the mouth-throat (MT), tracheobronchial tree (TB), and trachea (Tra), respectively (At 60L/min: MT=6.7%, TB=5.3%, Tra=1.9%). The numerical deposition data showed a good agreement with the experimental deposition data in most of the airway regions (e.g. less than 10% difference between the deposition fractions in the tracheobronchial region). Enhancing inhalation flow rate in all of the airway regions led to an uptrend in deposition rate due to the increase of particles inertia and turbulence level. In addition, the increase of particle deposition with enhancing inhalation flow rate in all of the sections including extrathoracic, trachea, and tracheobronchial tree suggesting that inertial impaction is the dominant deposition mechanism due to the increase of inertial force. In conclusion, the validated CFD model provided an opportunity to cover the limitations of our previous experimental investigation on aerosol deposition of commercial inhalers and became an efficient method for further studies.

Experimental study of flow characteristics of an oblique impinging jet

An experimental study has been performed by using particle image velocimetry to investigate the effect of nozzle-to-plate separation distance (L) and jet impingement angle on the flow characteristics of an obliquely inclined submerged water jet. Measurements were taken for L = 1D, 2D, 4D and 6D (where D is the diameter of the nozzle) for the jet impingement angle (θ) of 45° and 26°, and the flow characteristics in the uphill and the downhill regions are investigated at Reynolds number of 2600 (based on the nozzle diameter D and the jet velocity $$U_{{\text{o}}}$$). It is observed that surface spacing has an opposite effect in the uphill and the downhill regions in terms of wall jet flow. In the uphill region, the tendency of the wall jet to grow increases with increase in L/D. However, in the downhill region, the jet velocity and its thickness are observed to reduce as the separation distance is increased. The distance between the stagnation point and the geometric centre is observed to decrease with increase in L/D because of jet–ambient fluid interaction in the uphill region. The jet width is observed to grow for θ = 45° in the downstream of the plate due to enhanced jet–ambient fluid interaction. Flow at θ = 26° shows that after impingement the entire jet deviates towards the downhill side, which indicates the existence of a critical impingement angle below which there is no flow of the jet in the uphill region. RMS velocity fluctuations and shear stress show an increased turbulence level downstream of the plate in the downhill region for smaller impinging distance implying higher jet–ambient fluid interaction and increased jet width. They, along with the negative turbulence production term, reveal the region of flow separation and reattachment. The decrease in the peak value of Nusselt number can be related to the drop in the jet momentum at the stagnation point with increase in the surface spacing.

Dynamics of Fast Electrons in an Inhomogeneous Plasma with Plasma Beam Instability

The results of numerical simulation of the one-dimensional propagation of fast electrons in a plasma with Langmuir turbulence are presented. The resonant interaction of fast electrons with Langmuir waves and Coulomb collisions with particles of the background plasma are taken into account. It is shown that an increase in plasma density along the direction of the propagation of the electron beam leads to the transformation of the Langmuir turbulence spectrum into a region of high phase velocities and “spreading” of the velocity distribution function of accelerated electrons. For a Gaussian electron beam with an energy of 30 keV at its peak and a characteristic scale of an increase in plasma density of 107 cm, the electron spectrum expands; some of the electrons are accelerated to 1.3 of their initial velocity. In addition, a narrow (~105 cm) layer with an increased turbulence level forms in the plasma. As the electrons move deeper into the plasma, Coulomb collisions restore the inverse over velocity part of the electron distribution function, which leads to the formation of a turbulent layer at a distance from the plasma boundary of ~107 cm–108 cm; however, the energy density of Langmuir turbulence in this layer is ~(10–6–10–7) of the plasma thermal energy.

Transitional shock wave boundary layer interaction over a flexible panel

A three-dimensional transitional shock boundary layer interaction (SWBLI) over a finite-span flexible panel is investigated by performing direct numerical simulations (DNS). The laminar inflow is at Mach 2, on which an oblique shock of turn angle 5.62∘ is imposed. The shock impinges at the mid-chord length of the panel, giving rise to flow separation due to the induced adverse pressure gradient. The flow separation leads to streamline curvature in the laminar boundary layer, initiating a centrifugal instability similar to Görtler instability; where the nominally two-dimensional and steady SWBLI becomes three-dimensional and unsteady at a critical Reynolds number. The flow transition is characterized by the presence of counter-rotating streamwise-oriented Görtler-like vortices, engendering spanwise undulations for increasing Görtler number. The transitional SWBLI is analyzed over both the rigid and flexible panels by performing proper orthogonal decomposition (POD) in order to examine the onset of transition as well as the modal response of the flexible panel. The presence of flexible panel promotes flow transition, resulting in lower values of the critical Reynolds and Görtler numbers. Furthermore, at a post-critical Reynolds number, the interaction results in increased turbulence levels and exchange of energy at the low and high wavenumber modes.

Effects of Turbulence Intensity and Biogas Composition on the Localized Forced Ignition of Turbulent Mixing Layers

ABSTRACTThree-dimensional compressible Direct Numerical Simulations (DNS) have been used to investigate the localized forced ignition of statistically planar mixing layers of biogas/air mixtures for different levels of turbulence intensity and biogas composition. The biogas is represented by a CH/CO mixture and a two-step mechanism involving incomplete oxidation of CH to CO and HO, and an equilibrium between the CO oxidation and the CO dissociation has been used. This two-step mechanism captures the variation of the unstrained laminar flame speed with equivalence ratio and CO dilution with sufficient accuracy when compared with detailed chemistry results. A successful ignition of CH/CO/air mixing layer initially gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilized on the stoichiometric mixture fraction iso-surface. Long after the energy deposition has ended, the lean branch may merge with the diffusion flame, but edge flame propagation along the stoichiometric mixture fraction iso-surface is observed at all stages. The highest heat release rate (HRR) is obtained on the rich premixed branch irrespective of the turbulence intensity and biogas composition, but its magnitude decreases with increasing CO dilution. The most probable edge flame displacement speed decreases in time and converges to its theoretical value for laminar cases. As the turbulence level increases, the edge flame displacement speed assumes a larger range of values, although its mean is consistently lower than the corresponding laminar one. Furthermore, in the turbulent cases, the probability of finding negative values of the edge flame displacement speed has been found to be non-zero. This probability is larger than the one of finding positive values for the cases failing to reach a self-sustained flame propagation without the energy deposition assistance. This becomes more probable as the turbulence intensity and CO dilution increase. This is reflected in the diminished burning extent, quantified in terms of burned gas volume, with increasing turbulence intensity and CO dilution.

Performance enhancement of BPSK-SIM- and DPSK-SIM-based FSO downlink over atmospheric turbulence using aperture averaging and receiver diversity

In this paper, we evaluate the bit error rate (BER) performance of a free-space optical satellite downlink by considering the atmospheric turbulence effects using binary shift keying subcarrier intensity modulation and differential phase shift keying subcarrier intensity modulation (DPSK-SIM). The performance of the link is enhanced using aperture averaging and receiver diversity. The closed form mathematical expressions of BER for BPSK-SIM and DPSK-SIM schemes are derived and analyzed. It is observed that on varying the turbulence level, the performance of the link degrades when the turbulence level increases. The improved BER of 10−12 and 10−10 at signal-to-noise ratio of 30 dB of the link for both BPSK-SIM and DPSK-SIM is obtained by using aperture averaging (aperture diameter, D = 10 cm) and receiver diversity with optimal combining.

Future technology on the flight deck: assessing the use of touchscreens in vibration environments

Use of touchscreens in the flight deck has been steadily increasing, however, their usability may be severely impacted when turbulent conditions arise. Most previous research focusses on using touchscreens in static conditions; therefore, this study assessed touchscreen use whilst undergoing turbulent representative motion, generated using a 6-axis motion simulator. Touchscreens were tested in centre, side and overhead positions, to investigate how turbulence affected: (1) error rate, movement times and accuracy, (2) arm fatigue and discomfort. Two touchscreen technologies were compared: a 15” infra-red and a 17.3” projected capacitive touchscreen with force sensing capability. The potential of the force sensing capability to minimise unintentional interactions was also investigated. Twenty-six participants undertook multi-direction tapping (ISO 9241; ISO 2010) and gesture tasks, under four vibration conditions (control, light chop, light turbulence and moderate turbulence). Error rate, movement time and workload increased and usability decreased significantly, with screen position and increasing turbulence level.Practitioner Summary: This study evaluated the use of infra-red and projected capacitive touchscreen technologies using multi-directional tapping and gesture tasks, whilst being subjected to different levels of turbulence representative motion. Performance degraded significantly with increasing turbulence level and touchscreen location. This has implications for future flight deck design.

A novel type of semi-active jet turbulence grid

This article describes a novel approach to generate increased turbulence levels in an incoming flow. It relies on a cost-effective and robust semi-active jet grid, equipped with flexible tubes as moving elements attached onto tube connections placed at the intersections of a fixed, regular grid. For the present study, these flexible tubes are oriented in counter-flow direction in a wind tunnel. Tube motion is governed by multiple interactions between the main flow and the jets exiting the tubes, resulting in chaotic velocity fluctuations and high turbulence intensities in the test section. After describing the structure of the turbulence generator, the turbulent properties of the airflow downstream of the grid in both passive and active modes are measured by hot-wire anemometry and compared with one another. When activating the turbulence generator, turbulence intensity, turbulent kinetic energy, and the Taylor Reynolds number are noticeably increased in comparison with the passive mode (corresponding to simple grid turbulence). Furthermore, the inertial subrange of the turbulent energy spectrum becomes wider and closely follows Kolmogorov's -5/3 law. These results show that the semi-active grid, in contrast to passive systems, is capable of producing high turbulence levels, even at low incoming flow velocity. Compared to alternatives based on actuators driven by servo-motors, the production and operation costs of the semi-active grid are very moderate and its robustness is much higher.

The application of biomimicry to a mechanical valve design for the abatement of flow instabilities

Studies have suggested that the anterior leaflet of the native mitral valve aid in the formation of an asymmetric vortex ring similar to that seen in healthy left ventricles. The physiological vortex structure in the left ventricle is vital to the smooth redirection of the filling jet at the annulus to the aorta with minimal loss of energy. However, the implantation of artificial prostheses especially mechanical valves leads to the disruption of this flow field with an observed increase in turbulence levels. In light of this, we seek to investigate if this physiological left ventricular vortex can be replicated solely by the inclusion of an “anterior leaflet-like” feature within a mechanical valve design and its subsequent effects on downstream turbulence. In vitro experiments involving 2D3C particle image velocimetry was done on a newly designed mechanical valve consisting solely of a curved anterior leaflet and the results compared against that of the well-established Hancock II tissue bio-prosthesis, with the latter serving as a control. Our findings suggest that hemodynamic performance can be improved by solely mimicking the geometric feature of the anterior leaflet in a mechanical valve design, potentially resulting in lower thrombosis.

Modified Lewis Number and Buoyancy Ratio Effects on Turbulent Double-Diffusive Convection in Porous Media Using the Thermal Nonequilibrium Model

This work presents a study of double-diffusive free convection in a porous square cavity under turbulent flow regime and with aiding drive. The thermal nonequilibrium model was employed to analyze the energy and mass transport across the enclosure. Governing equations were time- and volume-averaged according to the double-decomposition concept. Analysis of a modified Lewis number, Lem, showed that for porous media, this parameter presents opposite behavior when varying the thermal conductivity ratio or the Schmidt number, while maintaining the same value for Lem. Differently form free flow, the existence of the porous matrix contributes to the overall thermal diffusivity of the medium, whereas mass diffusivity is only effective within the fluid phase for an inert medium. Results indicated that increasing Lem through an increase in Sc reduces flow circulation inside porous cavities, reducing Nuw and increasing Shw. Results further indicate that increasing the buoyancy ratio N promotes circulation within the porous cavity, leading to an increase in turbulence levels within the boundary layers. Partial contributions of each phase of the porous cavity (solid and fluid) to the overall average Nusselt number become independent of n for higher values of the thermal conductivity ratio, ks/kf. Further, for high values of ks/kf, the average Nusselt number drops as N increases.

Exhaust of turbulence cloud at the tongue shaped deformation event

Exhaust of turbulence cloud at the tongue-shaped deformation which triggers MHD bursts is observed in the Large Helical Device in the low density plasma with significant contribution of trapped particles injected by perpendicular neutral beam injection. The exhaust of turbulence cloud is characterized by the abrupt large increase of turbulence amplitude in the frequency range of 150–500 kHz measured with Doppler reflectometer at the edge region of the plasma (). The increase of turbulence amplitude is significantly large and is by one order of magnitude. This abrupt increase of turbulence level is transient and disappears within one milli-second (typically ∼600 μs). In contrast, the turbulence level slightly inside the plasma edge () decreases by a factor of 2 after the MHD bursts.

Numerical study of heat transfer and flow behavior in a circular tube fitted with varying arrays of winglet vortex generators

Winglet vortex generators (WVGs) mounted inside a circular tube, generate longitudinal vortices and offer increased turbulence level with a comparatively lower pressure penalty for a higher heat exchange performance. In this study, the thermal enhancement and flow structure arising from radially-arrayed winglets mounted at different attack and inclination angles, as well as different winglet lengths, are analyzed by CFD simulation. The flow and heat transfer behaviors are presented in the turbulent flow region for air flow, with Reynolds number ranging from 6000 to 27,000. Vortex generators are arranged inside the tube as a series of four rings, with each ring having 4 WVGs on the inner surface of a circular tube. The present research investigates the characteristics of WVGs which include four winglet inclination angles (α = 0°, 10°, 20° and 30°), five winglet attack angle (β = 0°, 10°, 20°, 30°, and 45°), and three winglet lengths (L = 10 mm, 15 mm, 20 mm). The data shows that winglets in a tube result in a considerable enhancement of Nusselt number and friction factor. It is found that Nusselt number and friction factor augment with the increase of attack angle or length, yet declines with the increase of inclination angle. The maximum of Nu is 86.88, at L = 20 mm, PR = 4.8, α = 0°, and β = 45°. The largest Nusselt number ratio, Nu/Nu0, defined as a ratio of augmented Nusselt number to the Nusselt number for fully developed smooth flow is 136%, which was obtained at a lower Reynolds number (Re = 6000). The results also illustrated that WVGs could generate longitudinal and transverse vortices to induce both impingement flow and recirculation zone leading to higher heat transfer and comparatively lower pressure drop. Nusselt number contours reveal that the wake zone behind WVGs displays the intensity of heat transfer along the tube.