Peak Turbulent Kinetic Energy Research Articles

Direct numerical simulations of solid-liquid suspension in a transitional stirred tank

Solid-liquid stirred tanks are very common in industrial process, for example pharmaceutical engineering, water treatment and crystallization. In these operating units, solid-liquid two-phase flows affect the rates of momentum, mass and heat transfer as well as the chemical reactions significantly. The flow regime in most investigations of solid-liquid suspension is either laminar or turbulent, however the transitional regime is also very important especially when scaling up from bench scale to pilot scale. Therefore, it is important to investigate the hydrodynamic characteristics of solid-liquid suspension in a transitional stirred tank. In this work, the lattice Boltzmann method was used to solve the liquid flow field. The uniform, cubic grids were applied with grid spacing Δ and time step Δ t representing the lattice units in space and time respectively. The stirred tank is a cubic tank with a flat bottom, whose height and side length of the bottom are both equal to 264 Δ. The impeller is a 45° pitched-blade turbine with diameter D =189.6 Δ; width of the impeller blade W =38.4 Δ and it operates under the down-pumping configuration. The impeller-based Reynolds number is 1334, which indicates that the flows belong to transitional regime. The particles are all equally sized solid spheres with diameter d p=9.6 Δ. As for the boundary conditions in this work, the half-way bounce-back rule was used to form the no-slip boundary conditions at all tank walls including top, bottom and side walls. The immersed boundary method was applied to impose the no-slip boundary conditions at the particles surface and the impellers surface. When it comes to the collisions, a hard-sphere collision algorithm based on the two-parameters model (restitution coefficient and friction coefficient) was adopted to carry out the collisions among the particles as well as the collisions between the particles and the tank walls. The collisions between the particles and the impeller were performed by using a soft-sphere collision model. In addition, the lubrication force was applied to deal with the close-range hydrodynamic interaction between the solid surfaces. This paper mainly did the simulations for different solids volume fractions Φ =0%, 1%, 5% and 8% at impeller angle θ =30° and at the same time resolved the impeller angles to 0°, 30° and 60° for solids volume fraction Φ =5%. In terms of simulated results, at first we compare the instantaneous liquid flow field between the single-phase system and the solid-liquid two-phase system, which shows that the exist of particles clearly hindered the liquid flow field especially in the impeller-discharge region. This paper also compares the averaged hydrodynamic characteristics between the single-phase system and the solid-liquid two-phase system. The results show that the averaged liquid velocity did not change obviously with the rise of solids volume fraction from 0% to 1%, however, when the solids volume fraction rose up to 5% or even higher to 8%, the averaged liquid velocity decreased obviously in the impeller-discharge region as well as the region near the bottom of the stirred tank at impeller angles θ =30°. Finally, the turbulent kinetic energy (TKE) of the liquid is compared qualitatively and quantitatively and it shows that the TKE values decreased significantly with the rise of solids volume fraction at impeller angle θ =30°. The peak TKE values decreased more than 50% with the rise of impeller angle θ from 0° to 30° for solids volume fraction Φ =5%, however the peak TKE values did not change obviously with the impeller angle θ rising from 30° to 60°.

Effects of surface roughness on a separating turbulent boundary layer

Separating turbulent boundary layers over smooth and rough flat plates are studied by large-eddy simulations. A suction–blowing velocity distribution imposed at the top boundary of the computation domain produces an adverse-to-favourable pressure gradient and creates a closed separation bubble. The Reynolds number based on the momentum thickness and the free-stream velocity before the pressure gradient begins is 2500. Virtual sand grain roughness in the fully rough regime is modelled by an immersed boundary method. Compared with a smooth-wall case, streamline detachment occurs earlier and the separation region is substantially larger for the rough-wall case, due to the momentum deficit caused by the roughness. The adverse pressure gradient decreases the form drag, so that the point where the wall stress vanishes does not coincide with the detachment of the flow from the surface. A thin reversed-flow region is formed below the roughness crest; the presence of recirculation regions behind each roughness element also affects the intermittency of the near-wall flow, so that upstream of the detachment point the flow can be reversed half of the time, but its average velocity can still be positive. The separated shear layer exhibits higher turbulent kinetic energy (TKE) in the rough-wall case, the growth of the TKE there begins earlier relative to the separation point, and the peak TKE occurs close to the separation point. The momentum deficit caused by the roughness, again, plays a critical role in these changes.

Surface correlations of hydrodynamic drag for transitionally rough engineering surfaces

ABSTRACTRough surfaces are usually characterised by a single equivalent sand-grain roughness height scale that typically needs to be determined from laboratory experiments. Recently, this method has been complemented by a direct numerical simulation approach, whereby representative surfaces can be scanned and the roughness effects computed over a range of Reynolds number. This development raises the prospect over the coming years of having enough data for different types of rough surfaces to be able to relate surface characteristics to roughness effects, such as the roughness function that quantifies the downward displacement of the logarithmic law of the wall. In the present contribution, we use simulation data for 17 irregular surfaces at the same friction Reynolds number, for which they are in the transitionally rough regime. All surfaces are scaled to the same physical roughness height. Mean streamwise velocity profiles show a wide range of roughness function values, while the velocity defect profiles show a good collapse. Profile peaks of the turbulent kinetic energy also vary depending on the surface. We then consider which surface properties are important and how new properties can be incorporated into an empirical model, the accuracy of which can then be tested. Optimised models with several roughness parameters are systematically developed for the roughness function and profile peak turbulent kinetic energy. In determining the roughness function, besides the known parameters of solidity (or frontal area ratio) and skewness, it is shown that the streamwise correlation length and the root-mean-square roughness height are also significant. The peak turbulent kinetic energy is determined by the skewness and root-mean-square roughness height, along with the mean forward-facing surface angle and spanwise effective slope. The results suggest feasibility of relating rough-wall flow properties (throughout the range from hydrodynamically smooth to fully rough) to surface parameters.

14 Peak turbulent kinetic energy assessed by cardiac magnetic resonance correlates better than aortic valve area with left ventricular parameters in aortic stenosis: Abstract 14 Table 1

Introduction Previous studies have shown only modest correlation between aortic valve area (AVA) and left ventricular mass in aortic stenosis (AS), likely due to the variable additional after-load from the aortic and systemic circulation. Cardiac magnetic resonance (CMR) has developed sequences to measure turbulent kinetic energy (TKE) in the proximal aorta, derived from time resolved three-dimensional (4D) flow imaging. It represents energy dissipation and irreversible pressure loss. Work in the same patient cohort demonstrated peak TKE is higher in bicuspid than tricuspid AS, probably due to larger ascending aortas. We assessed peak systolic TKE in patients with wide ranging AS (mild to severe) and correlated it with left ventricular (LV) parameters. Methods 22 patients with tricuspid AS (mean age 72.1 ± 8.6 years, mean indexed valve area (AVA) 0.55 ± 0.18cm 2 /m 2 ) and 20 with bicuspid AS (age 65.2 ± 8.5, indexed AVA 0.77 ± 0.42 cm 2 /m 2 ) were scanned. Peak systolic TKE was measured using specialist analysis software and systolic LV longitudinal strain with proprietary feature-tracking software. Results LV mass index was the only measure of LV function that correlated with AVA: inverse correlation (r) = −0.404; p 0.006. Peak TKE had a stronger correlation with LV mass (r 0.53; p 0.001), especially in bicuspid AS (r 0.83; p 0.0001). Peak TKE also correlated with indexed LV end diastolic volume (r 0.486; p 0.003), stroke volume (r 0.564; p 0.0004) and stroke work (r 0.58; p 0.0003). This correlation was especially strong in BAV AS (Table 1). No correlation with LV ejection fraction or systolic longitudinal strain was found. Conclusions Peak TKE is a promising tool to better understand the variable effects of AS on the LV, beyond simply severity of AS. The effect is particularly strong in bicuspid AS, who have larger aortas - the likely mechanism for increased TKE.

Large-Eddy Simulation of Supersonic Round Jets: Effects of Reynolds and Mach Numbers

Large-eddy simulations of supersonic turbulent jets are performed for Reynolds numbers of for the purpose of understanding the effects of Reynolds numbers and the Mach number . The subgrid terms in large-eddy simulations are modeled using a combination of the dynamic Smagorinsky (“General Circulation Experiments with the Primitive Equations. Part I, Basic Experiments,” Monthly Weather Review, Vol. 54, No. 1, 1963, pp. 99–164) and Yoshizawa (“Statistical Theory for Compressible Turbulent Shear Flows, with the Application to Subgrid Modelling,” Physics of Fluids, Vol. 54, No. 1, 1986, pp. 2152–2164) models. Simulations are performed for supersonic jets having Reynolds numbers of 1500, 3700, and 7900, and Mach numbers of 1.4 and 2.1. Two of the simulations are validated with experimental data. The Reynolds number value is observed to play a role in the transition to turbulence but, once transition is achieved, it has a subdued effect above a threshold value; that is, as seen experimentally for supersonic flows, a similarity is found here. This similarity occurs for Reynolds number values that are relatively small compared to those typical of the fully turbulent regime. The turbulent structures in the transition region are more coherent, and the potential core is longer when the Mach number is larger, which leads to a slower downstream velocity decay. The root-mean-square velocities are biased in the axial direction, as expected. In the fully turbulent regions, the computed Reynolds stress is higher for a larger-Mach-number jet. Peak pressure fluctuations occur at about half a jet diameter, radially away from the centerline of the jet, and this location is independent of both the Reynolds number and Mach number values. The pressure–velocity correlations and the turbulent kinetic energy profiles are investigated along the centerline and radial directions, and it is found that the peak turbulent kinetic energy occurs at the same location as the maximum pressure fluctuations.

Two-equation and multi-fluid turbulence models for Rayleigh–Taylor mixing

This paper presents a new, improved version of the K–L model, as well as a detailed investigation of K–L and multi-fluid models with reference to high-resolution implicit large eddy simulations of compressible Rayleigh–Taylor mixing. The accuracy of the models is examined for different interface pressures and specific heat ratios for Rayleigh–Taylor flows at initial density ratios 3:1 and 20:1. It is shown that the original version of the K–L model requires modifications in order to provide comparable results to the multi-fluid model. The modifications concern the addition of an enthalpy diffusion term to the energy equation; the formulation of the turbulent kinetic energy (source) term in the K equation; and the calculation of the local Atwood number. The proposed modifications significantly improve the results of the K–L model, which are found in good agreement with the multi-fluid model and implicit large eddy simulations with respect to the self-similar mixing width; peak turbulent kinetic energy growth rate, as well as volume fraction and turbulent kinetic energy profiles. However, a key advantage of the two-fluid model is that it can represent the degree of molecular mixing in a direct way, by transferring mass between the two phases. The limitations of the single-fluid K–L model as well as the merits of more advanced Reynolds-averaged Navier–Stokes models are also discussed throughout the paper.

Turbulent kinetic energy in the ascending aorta is greater in bicuspid than tricuspid aortic valve stenosis

Background The main determinant of the haemodynamic significance of aortic stenosis (AS) is the irreversible pressure loss that is created by the stenosis. The majority of the pressure loss is caused by conversion/dissipation of turbulent kinetic energy (TKE) to heat. Recent developments in cardiac magnetic resonance 4D flow imaging have allowed the noninvasive assessment of TKE. Bicuspid aortic valve disease is known to be associated with a larger ascending aorta and disordered flow patterns, and we hypothesised that peak TKE would be higher in bicuspid AS than tricuspid AS. Methods 15 patients with bicuspid AS (mean age 63.6 years; mean aortic valve area 1.4cm 2 ; mean dimension of the ascending aorta at the level of the pulmonary artery 3.9cm) and no more than mild other valve disease were compared with 22 patients with tricuspid AS (mean age 72.9 years; mean aortic valve area 1.2cm 2 ; mean dimension of the ascending aorta at the level of the pulmonary artery 3.2cm) and no more than mild other valve disease. All subjects underwent time resolved, three dimensional cine magnetic resonance flow imaging at 3 Tesla, for the assessment of peak TKE. The peak TKE was obtained by integrating the TKE per voxel across the ascending aorta at each time frame of the cardiac cycle. Results

Variations in turbulent kinetic energy at a point source submarine groundwater discharge in a reef lagoon

The influence of sea level variations due to tides and wave setup on turbulent kinetic energy (TKE) was observed at a point source submarine groundwater discharge in a fringing coral reef lagoon. Tidal and wave setup variations modulated speed, TKE, TKE dissipation, and water temperature and salinity at the buoyant jet. The primary driver of jet TKE and speed variations was tides, while wave setup was a minor contributor. An inverse relationship between surface elevation and TKE was explained with an exponential equation based on sea level variations. During low tides, peak jet speeds (up to 0.3 m s−1) and TKE per unit mass (up to 0.4 m2 s−2) were observed. As high tide approached, the jet produced minimum TKE of ~0.003 m2 s−2 and TKE dissipation ranged from 2 to 8×10−4 m2 s−3. This demonstrated the sensitivity of the jet discharge to tides despite the small tidal range (<20 cm). Jet temperatures and salinities displayed semidiurnal oscillations with minimum salinity and temperature values during maximum discharge. Jet salinities increased throughout low tides while temperatures decreased. This pattern suggested the jet conduit was connected to a stratified cavity within the aquifer containing cool fresh water over cool salty water. As low tides progressed, jet outflow increased in salinity because of the mixing within the conduit, while lower jet temperatures suggested water coming from further or deeper in the aquifer. The presence of such a cavity has been recently confirmed by divers.

Hemodynamic Impact of Transcatheter Aortic Valve Deployment Configuration.

Transcatheter aortic valve (TAV) replacement holds promise for a large number of patients who otherwise have limited or no treatment options. However, it also poses various challenges, due to its unique disease treatment mechanism. Successful TAV deployment and function are heavily reliant on the tissue-stent interaction [1,2]. For patients with aortic stenosis, heavy calcium deposition on the valve leaflets and the aortic root can also cause distortion of TAV geometries, resulting in a valve of an elliptical shape [3–5] instead of a nominal circular shape. In a recent study by Schultz et al. [5], the geometry and apposition of the TAV after implantation in 30 patients with aortic stenosis were evaluated using multislice computed tomography. The results indicated that none of the TAV frames reached exactly nominal designed dimensions. The difference between the orthogonal smallest and largest diameters of TAV cross section at the ventricular end was 4.4 mm, which was clearly an asymmetric elliptical shape [5]. In this study, we sought to develop computational models of valve hemodynamics under clinically relevant deployment scenarios.The geometry of a “generic” TAV has been described in detail [6]. It is a thin pericardial bioprosthetic valve with similar design features to TAVs used commercially. This model has been extended previously to the development of elliptical valves, with eccentricities of 0.3, 0.5, and 0.68 and alignment of either the major or minor axis of the ellipse to a native commissure [7]. In the current study, the open configuration for each of these elliptical models was obtained using abaqus finite element simulation (Fig. 1(a)). The leaflets were attached to a circular mounting ring (Fig. 1(b)) and then sealed in a 38-mm tube (Fig. 1(c)) in preparation for import into computational fluid dynamics (CFD) software using Hypermesh (Altair Engineering, Troy, MI).The CFD software star-CCM+ was used, with a polyhedral mesh and boundary conditions of 30 liters per minute (LPM) mass flow at the inlet and 100 mmHg pressure at the outlet. A Newtonian fluid was used, with a viscosity of 0.0035 Pa-s and a density of 1056 kg/m3. Turbulence was modeled by specifying intensity and length scale for the k-ε model.The sealed geometry was also used to fabricate physical models through additive manufacturing, which were then used for experimental validation using steady forward flow at 30 LPM. A centerline probe was advanced upstream, measuring pressure at 1-mm spacing.For the baseline circular valve, a peak transvalvular pressure of 10.92 mmHg was simulated (Fig. 2), which correlated to an effective orifice area (EOA) of 2.93 cm2. In a long-axis view, the peak centerline velocity was 2.16 m/s and the peak turbulent kinetic energy (TKE) was 0.30 J/kg. We observed that EOA dropped for more elliptical conditions, particularly when the minor axis of the ellipse was aligned with the TAV commissures. The changes in flow were also observed as higher transvalvular pressure, higher peak velocity, and higher TKE as compared with the baseline circular condition.Experimental pressure versus axial position curves can be used to validate steady-flow models. Best turbulence modeling was obtained for 10% turbulent intensity and a length scale of 4.6 mm. TAV ellipticity tended to cause increased EOA with higher peak velocity. These models may be used in the future to investigate more complicated clinical scenarios.The authors gratefully acknowledge the funding provided by the NSF, NIH, and Edwards Lifesciences, Inc.

Optimize combustion of compressed natural gas engine by improving in-cylinder flows

In order to solve the problem of slow flame propagation in a spark-ignition engine fueled with compressed natural gas (CNG), the influence of in-cylinder flows on combustion process was investigated in CA6SE3-21E4N CNG-engine by means of numerical simulation and experiment. The status of in-cylinder flows from intake to expansion stroke was described by computational fluid dynamic tool, which revealed that the in-cylinder flows were one of the main reasons of slow burning rate. Therefore, a special-shaped combustion chamber called Cross was used to improve the in-cylinder flows. The results showed that peak turbulent kinetic energy of Cross was 43.9% higher than that of original combustion chamber called Cylinder during the late compression period at 1450 rpm 100% load. The combustion parameters, brake specific fuel consumption (BSFC) and regulated emissions were obtained by means of experiment. At 1450rpm 25%, 50%, 75% and 100% load conditions, the ignition delay of Cross was longer than that of Cylinder, moreover, the Cross produced averagely 5.75°CA shorter combustion duration. The BSFC of Cross was on an average of 4.3% reduction at 1450 rpm as well as the HC and CO emissions were reduced whereas the NOx emissions were significantly increased.

Measurements and modeling of open-channel flows with finite semi-rigid vegetation patches

The hydrodynamics of flows through a finite length semi-rigid vegetation patch (VP) were investigated experimentally and numerically. Detailed measurements have been carried out to determine the spatial variation of velocity and turbulence profiles within the VP. The measurement results show that an intrusion region exists in which the peak Reynolds stress remains near the bed. The velocity profile is invariant within the downstream part of the VP while the Reynolds stress profile requires a longer distance to attain the spatially invariant state. Higher vegetation density leads to a shorter adjustment length of the transition region, and a higher turbulence level within the VP. The vegetation density used in the present study permits the passing through of water and causes the peak Reynolds stress and turbulence kinetic energy each the maximum at the downstream end of the patch. A 3D Reynolds-averaged Navier–Stokes model incorporating the Spalart–Allmaras turbulence closure was employed subsequently to replicate the flow development within the VP. The model reproduced transitional flow characteristics well and the results are in good agreement with the experimental data. Additional numerical experiments show that the adjustment length can be scaled by the water depth, mean velocity and maximum shear stress. Empirical equations of the adjustment lengths for mean velocity and Reynolds stress were derived with coefficients quantified from the numerical simulation results.

Swirl and Cross-Flow Effects on Vitiated Jet Flames

A burner was developed to accommodate extensive limits of firing for lean and rich fuel/air mixtures via efficient vitiation of the reactive streams by a pilot premixed flame jet under swirl and cross-flow effects. The stability and combustion characteristics of the corresponding normal and inverse triple flames were addressed for vitiating a double-swirled flow through a concentric vane swirler by a pilot stream swirled due to a tangential entry. Increasing the vitiating stream heat input ratio to 0.5 increased the stability limit to eight times the laminar flame speed as the inner swirl number increased to 2.0. Increasing the cross-flow air momentum flux ratio from 1.0 to 3.0 further increased the normalized stability limit to 8.5, while employing double coswirl and triple coswirl increased such limit to respective values of 10.2 and 11.9. Vitiating a double-swirled rich fuel/air mixture stream by a tangential entry-swirled lean fuel/air mixture stream enlarged the stoichiometric zone to pronounce a peak turbulent kinetic energy of 3.5 m2/s2. Increasing the fuel mass fraction gradient increased the stability limit only with a vitiating lean mixture, while increasing the swirled portion of the vitiating stream increased the normalized stability limit to 19.6. Allocating the counterswirl across the vitiated stream was more efficient to increase the normalized stability limit to 26.7 as the relative swirl angle reached 120°. Increasing the cross-flow opposing jet strain rate enhanced the H and H2 preferential diffusion, increased the temperature homogeneity, and reduced the flame length. There was a corresponding reduction in NOx emissions to a minimum of 38 ppm as the strain rate increased to 13000 s−1, while the normalized stability limit increased to 27.3. Decreasing the longitudinal spacing between the cross-flow stages from 12.0 to 4.0 cm decreased the NOx emissions further to 18 ppm and increased the normalized stability limit to 30.5.

Magnetic Resonance Measurement of Turbulent Kinetic Energy for the Estimation of Irreversible Pressure Loss in Aortic Stenosis

The authors sought to measure the turbulent kinetic energy (TKE) in the ascending aorta of patients with aortic stenosis and to assess its relationship to irreversible pressure loss. Irreversible pressure loss caused by energy dissipation in post-stenotic flow is an important determinant of the hemodynamic significance of aortic stenosis. The simplified Bernoulli equation used to estimate pressure gradients often misclassifies the ventricular overload caused by aortic stenosis. The current gold standard for estimation of irreversible pressure loss is catheterization, but this method is rarely used due to its invasiveness. Post-stenotic pressure loss is largely caused by dissipation of turbulent kinetic energy into heat. Recent developments in magnetic resonance flow imaging permit noninvasive estimation of TKE. The study was approved by the local ethics review board and all subjects gave written informed consent. Three-dimensional cine magnetic resonance flow imaging was used to measure TKE in 18 subjects (4 normal volunteers, 14 patients with aortic stenosis with and without dilation). For each subject, the peak total TKE in the ascending aorta was compared with a pressure loss index. The pressure loss index was based on a previously validated theory relating pressure loss to measures obtainable by echocardiography. The total TKE did not appear to be related to global flow patterns visualized based on magnetic resonance-measured velocity fields. The TKE was significantly higher in patients with aortic stenosis than in normal volunteers (p < 0.001). The peak total TKE in the ascending aorta was strongly correlated to index pressure loss (R(2) = 0.91). Peak total TKE in the ascending aorta correlated strongly with irreversible pressure loss estimated by a well-established method. Direct measurement of TKE by magnetic resonance flow imaging may, with further validation, be used to estimate irreversible pressure loss in aortic stenosis.

Bayesian multipoint velocity encoding for concurrent flow and turbulence mapping

An approach to efficiently measure three-dimensional velocity vector fields and turbulent kinetic energy of blood flow is presented. Multipoint phase-contrast imaging is used in combination with Bayesian analysis to map both mean and fluctuating velocities over a large dynamic range and for practically relevant signal-to-noise ratios. It is demonstrated that the approach permits significant spatiotemporal undersampling to allow for clinically acceptable scan times. Using numerical simulations and in vitro measurements in aortic valve phantoms, it is shown that for given scan time, Bayesian multipoint velocity encoding provides consistently lower errors of velocity and turbulent kinetic energy over a larger dynamic range relative to previous methods. In vitro, significant differences in both peak velocity and turbulent kinetic energy between the aortic CoreValve (150 cm/s, 293 J/m3) and the St. Jude Medical mechanical valve (120 cm/s, 149 J/m3) were found. Comparison of peak turbulent kinetic energy measured in a patient with aortic stenosis (950 J/m3) and in a patient with an implanted aortic CoreValve (540 J/m3) revealed considerable differences relative to the values detected in healthy subjects (149±12 J/m3) indicating the potential of the method to provide a comprehensive hemodynamic assessment of valve performance in vivo.

Concentric elliptical jet diffusion flames with co- and cross-flows

The characteristics of concentric elliptical jet diffusion flames were addressed with co- and cross-flow streams for pollution reduction and more flexible industrial firing. The peak turbulent kinetic energy was maximized at the major axis tips to pronounce increased jet entrainment by 478% in comparison to concentric circular jets. Aided by a fuel jet Schmidt number less than 1.0, increasing the outer to inner velocity ratio to 1.6 recorded peak NOx emissions of 24ppm. Increasing the aspect ratio from 1.5 to 3.3 nearly duplicated the peak turbulent kinetic energy and pronounced 9% increase in the combustion efficiency with 25% HC reduction. As dominated by the peak turbulent energy merging, the optimum angular positioning of the two jets was 35° for higher inner jet velocity. For lower inner jet velocity, the best angular positioning was 45° and 30° for higher and lower inner aspect ratio, respectively. By introducing radially opposing air jets, the optimum angular positioning decreased to 25° as the flame was shortened by 43%. The opposing flow increased the elliptical jet peak turbulent energy to 25 times, while the stability limits were extended by 45%. The multi-stage air cross-flow revealed more aerodynamic features. Using less number of stages and increasing the cross-flow admission distance to 20cm at increased cross-flow percent to 70% minimized the average HC and NOx emissions to 0.15% and 10ppm, respectively. The optimum performance was thus found when lifted flame premixing features were coupled with the impinging cross-flow stabilizing effects.

Multidimensional turbulence spectra – identifying properties of turbulent structures

Development of models for several phenomena occurring in turbulent single and multiphase flows requires improved description and quantification of the turbulent structures. This is needed since often the phenomena are very fast or nonlinear. Previously the authors have presented experimental measurements that show that the breakup of bubbles and drops in turbulence is due to interaction with single turbulent vortices. Hence, it is not sufficient to use average turbulence properties when developing models for CFD simulation of engineering applications. In this paper the results from analysis of individual turbulent structures are presented. Results from analysis of the turbulent kinetic energy in turbulent structures, using Eulerian vortex identification methods, are presented. The amount of turbulent kinetic energy associated with a coherent vortex defined using different vortex identification methods is quantified. It is shown that the peak turbulent kinetic energy is located near the edge of the region identified as coherent, making the analysis challenging and development of models difficult. However, detailed analysis of a small number of coherent vortices from LES of turbulent pipe flow reveals new information about their life history. The growth (i.e. entrainment of the surrounding liquid), enstrophy, lifetime, and energy of a specific coherent vortex are tracked over time.

Fire Whirl Evolution Observed during a Valley Wind‐Sea Breeze Reversal

This observational study documented the atmospheric environment of a prescribed fire conducted in a narrow valley when a small fire whirl developed during a mesoscale wind reversal. Based on analysis of in situ meteorological measurements, it is hypothesized that the fire whirl formed due to the presence of strong vertical wind shear caused by the interaction of a sea breeze front with a weaker up‐valley wind. Vorticity generated by the interaction of the wind shear and the fire front was estimated to be ~0.2 s−1. Peak turbulence kinetic energy was caused by the wind shear rather than the buoyancy generated by the fire front. It was also found that the convective Froude number itself may not be sufficient for fire whirl prediction since it is less relevant to the near‐surface boundary‐layer turbulence generated by environmental wind shear. Observations from this case study indicate that even low‐intensity prescribed fires can result in the formation of fire whirls due to mesoscale changes in the ambient atmospheric environment.

Spatial and temporal structures of turbulent bubble-driven flows in a rectangular water tank

The spatial and temporal structures of turbulent water flows driven by air bubbles in a rectangular water tank were investigated. The time-resolved particle image velocimetry (PIV) technique was adopted for quantitative visualization. Flow rates of compressed air were changed from 2 to 4 l/min at 0.5 MPa, and the corresponding range of bubble-based Reynolds number ranged from 6,740 to 13,220. The dynamics of flow structures was further investigated by the time-resolved proper orthogonal decomposition (POD) analysis technique. When the flow rate was increased, the main vortex core moved to the side and bottom wall. Locations of peak turbulent kinetic energy regions depended on the bubble Reynolds number. Both spatial and temporal modes were quite different with respect to the flow rates. The first temporal mode was harmonized with the second temporal mode, with small oscillations in the case of the lowest Reynolds number. However, temporal modes oscillate with higher frequencies when the Reynolds number increases. Based on the result of the FFT analysis of each temporal mode, we conjectured that low-frequency oscillation was attributed to the recirculating flow, whereas a higher dominant frequency was related to the vibration of the free surface that interacts with the rising bubbles.

Investigation of temporal mixing layer in shallow water with BGK algorithm

Shallow water flows are common in natural or man-made hydraulic systems, for example, in the wake of islands and in the downstream of headlands and other protrusions. Experiments, numerical simulations and theoretical analysis are applied to investigate the phenomenon. The present work is devoted to the comparison of nonlinear models and linear stability analysis results. In previous work, nonlinear models focus on the spatial growth of perturbations and linear stability analysis focuses on the temporal growth of perturbations. Comparisons are then performed with Gaster’s transformation, which maps the temporal growth of perturbations to the spatial growth. However, Gaster’s transformation only works well near the neutral curve, at which the flows are marginally unstable. The paper circumvents the use of Gaster’s transformation by comparing linear temporal analysis with nonlinear temporal simulations, both focusing on the temporal growth of perturbations. It is found that within a short time the linear stability analysis underestimates the peak turbulent kinetic energy and the wavenumber at which the flow attains the peak kinetic energy. Over time, the peak energy and the corresponding wavenumber obtained from the linear stability analysis are consistent with the nonlinear simulation results. Also, an initial energy drop is observed in the nonlinear simulations but not in the linear stability analysis results. Such drop may be due to 1.) the initial transient growth which is not captured with the normal-mode based linear stability analysis, or 2.) some energy in the cross-stream velocity perturbations is converted to the potential energy, resulting in a change in the water depth, which is not captured with the rigid-lid assumption in the linear stability analysis.

Aerodynamic performance of a bypass engine with fan nozzle exit area change by warped chevrons

Variation of the bypass nozzle exit area enables optimization of the turbofan engine operating cycle over a wider range of operational conditions resulting in improved thrust and/or fuel consumption. Two mechanisms for varying the nozzle area have been investigated. The first uses an array of chevrons which when closed, form a full body of revolution and when warped/curved, increase the exit area while forming a serrated trailing edge. The second technique incorporates an axially translating section of the nacelle shroud and uses the change in the nozzle boat-tail radial location with the axial location as a means to vary the nozzle exit area. To analyse the effects on a typical rotor/stator stage, computational fluid dynamics simulations of the NASA Rotor 67, Stator 67A stage integrated into a custom-built nacelle were performed. Nozzles with 8, 12, and 16 chevrons were simulated to evaluate the impact of the variation in geometry upon the nacelle wake and local forces. Gross thrust of the nacelle and the turbulent kinetic energy (TKE) variation through the wake is compared. The chevron nozzle attains a nearly 2 per cent maximum thrust improvement over the translating nozzle technique. The chevron nozzle also has significantly lower (nearly 8 per cent) peak TKE levels in the jet plume.