The jet-in-crossflow problem has been extensively studied, mainly because of its applications in film cooling and injector designs. It has been established that in low-speed flows, pulsing the jet significantly enhances mixing and jet penetration. This work investigates the effects of pulsing on mixing and jet trajectory in high speed (compressible) flow, using Large Eddy Simulation. Jets with different density ratios, velocity ratios and momentum ratios are pulsed from an injector into a crossflow. Density ratios used are 0.55 (CH4/air), 1.0 (air/air) and 1.5 (CO2/air). Results are compared with the low speed cases studied in the past and then analyzed for high speed scaling. The simulations show that the lower density jet develops faster than a higher density jet. This results in more jet spread for the lower density jet. Scaling for jet spread and the decay of centerline jet concentration for these cases are established, and variable density scaling law is developed and used to predict jet penetration in the far field. In most non-premixed combustor systems, the fuel and air being mixed are at different initial temperatures and densities. To account for these effects, heated jets at temperatures equal to 540K and 3000K have been run. It has been observed that, in addition to the lower density of heated jets, the higher kinematic viscosity effects the jet penetration. This effect has been included and validated in the scaling law for the heated jet trajectory.

Η διδακτορική διατριβή με τίτλο «Μελέτη Διατάξεων Σταδιακής Καύσης» έχει ως αντικείμενο την περιγραφή και την ανάλυση του ροϊκού πεδίου της αλληλεπίδρασης αξονοσυμμετρικών δεσμών διαφορετικού αριθμού Reynolds. Ο έλεγχος των μηχανισμών που διέπουν το ροϊκό αυτό πεδίο είναι σημαντικό εργαλείο για την βελτιστοποίηση της ανάμιξης ρευμάτων οξειδωτικού και καυσίμου και αποτελεί τον κρισιμότερο τεχνολογικό στόχο κατά τη διερεύνηση της αποδοτικότητας σε ένα ευρύ φάσμα σχημάτων σταδιακής καύσης. Στόχος της διατριβής είναι ο χαρακτηρισμός και ο έλεγχος της μίξης κατά την διαδικασία της σύγκλισης των δεσμών, συνεισφέροντας στην κατανόηση των βασικών μηχανισμών που αναπτύσσονται στο μέσο και τυρβώδες πεδίο, αλλά και στη βελτιστοποίηση της μίξης από την οποία εξαρτάται η μείωση των περιβαλλοντικών επιπτώσεων και η αύξηση της απόδοσης της καύσης, τόσο σε συστήματα παραγωγής ενέργειας όσο και σε θαλάμους καύσης κινητήρων αεροσκαφών, τα οποία αποτελούν τις πιο «φιλικές» ως προς το περιβάλλον αλλά και τις πιο αποδοτικές πρακτικές σε συστήματα καύσης. Παρά το γεγονός αυτό, η γνώση των επιμέρους φαινομένων και μηχανισμών που κυριαρχούν στο πεδίο της αλληλεπίδρασης αξονοσυμμετρικών δεσμών χαρακτηρίζεται από σημαντικές ελλείψεις, όσο αφορά την λεπτομερή καταγραφή του μέσου και τυρβώδους πεδίου ταχυτήτων. Η Ανεμομετρία Laser Doppler αποτελεί τα βασική μετρητική διάταξη που χρησιμοποιήθηκε για την καταγραφή των μέσων και τυρβωδών όρων του πεδίου της ταχύτητας. Το πλεονέκτημα της μεθόδου, που αποτελεί ένα από τα βασικά εργαλεία για τη μέτρηση τυρβωδών χαρακτηριστικών, είναι το γεγονός ότι δεν παρεμβαίνει στη ροή, καθώς στηρίζεται στην καταγραφή της συχνότητας του φωτός που σκεδάζεται από τροχιοδεικτικά σωματίδια, τα οποία ακολουθούν πιστά τη ροή. Η μελέτη στηρίζεται στην ανάλυση των μηχανισμών που αναπτύσσονται στο μέσο και τυρβώδες πεδίο και στον χαρακτηρισμό της μίξης των συστατικών των δεσμών, όπως αυτή αντικατοπτρίζεται στις πρώτης, δεύτερης και τρίτης τάξης ροπές της ταχύτητας, αλλά και στους αδιάστατους συντελεστές λοξότητας και επιπεδότητας. Σε αυτήν την κατεύθυνση, η παρούσα εργασία έχει σαν στόχο την ανάδειξη της ανάλυσης του ροϊκού πεδίου ως την πιο κρίσιμη παράμετρο ανάπτυξης σχημάτων καύσης, καθώς τα μεγέθη και η εξέλιξή τους έτσι όπως καταγράφονται μπορούν να χρησιμοποιηθούν άμεσα ως κριτήρια για την απόδοση τέτοιων σχημάτων, προσφέροντας την δυνατότητα της επιλογής των καλύτερων λειτουργικών παραμέτρων πριν ακόμα δοκιμαστούν σε περιβάλλον καύσης. Η τυρβώδης δομή και τα ανώτερα στατιστικά των ροπών της ταχύτητας κατά την αλληλεπίδραση των δεσμών χρησιμοποιούνται αρχικά ως βασικό εργαλείο της μελέτης των συγκεκριμένων διατάξεων σταδιακής καύσης και επιπλέον χρησιμοποιούνται για να χαρακτηρίσουν τη μίξη των συστατικών των δεσμών στις διαφορετικές περιοχές της αλληλεπίδρασης, πριν, κατά τη διάρκεια και μετά την σύγκλισή τους. Τα αποτελέσματα της εργασίας δείχνουν ότι δέσμες σχετικά μικρού αριθμού Reynolds (της τάξης των λίγων χιλιάδων) μπορούν να χρησιμοποιηθούν για την παραγωγή τυρβωδών δομών, όπου η μίξη ενισχύεται μέσω της αλληλεπίδρασής τους, γεγονός που μπορεί να έχει σημαντική επίδραση στην ανάπτυξη παρόμοιων εφαρμογών σε πραγματικής κλίμακας εγκαταστάσεις. / Τhis study aims to the assessment and analysis of the flow field produced by the interaction between two axisymmetric jets with different Reynolds numbers. The scheme is considered as a generic staged combustion configuration. In this frame, the identification and control of the mechanisms that dominate the flow field is one of the most significant issues towards the enhancement of mixing, thus the increasing of combustion operational and environmental efficiency. The scope of the study is to characterize and control the mixing process during the interaction and merging of the jets. The study contributes to the understanding of the dominating mechanisms occurring in the flow, through the assessment of the mean and turbulent flow features. The initial conditions used in the experiments diverge from the marginal conditions that refer to the interaction of equal momentum jets and the combining of a “strong” and a “weak” jet. In addition, due to the particularly low, although turbulent, Reynolds numbers used at jets’ exits, the capability of the jets to produce and maintain strongly turbulent structures during and after their interaction is also evaluated. During the experiments included in this thesis, a two dimensional Laser Doppler Anemometry (LDA) was used to measure the main components of the velocity vector (i.e. in the axial and the radial or horizontal directions). A pair of similar optical systems were used to produce two ellipsoidal control volumes and collect the light scattered by tracer particles, that faithfully represent the flow. Axisymmetric turbulent jets constitute the main component regarding the realization of staged combustion configurations based on multiple jets arrangements. Jet flow fields have been extensively studied during the last decades, mainly focusing on the self-similar region, i.e. the region where the mean and turbulent features are dynamically preserved. Recently, most of the studies are related to the identification of divergences arising from the different initial conditions and specific parameters, such as the shape of the orifice, the type of the confinement imposed at the exit, the jet density etc. The effects of these features are briefly summarized within the context of the thesis. In particular, the low Reynolds number effect is under consideration, as the jets recorded in the experiments are compared to typical previous studies devoted to the self-similarity region of jets with significantly higher Reynolds numbers. In the first part of the experiments, a jet of reference is monitored within an area covering the initial development and extends to the boundaries of the self-similarity region. Measurements include axial and radial distributions of the mean and turbulent axial and radial velocity components, turbulent shear stresses and terms that represent the turbulent transport of the Reynolds stresses. The budget of the turbulent kinetic energy is presented at the final measurement station, showing the particular mechanisms occurring in the flow. Measurements on the jets’ interaction field are presented afterwards. The distributions are systematically presented in comparison to the distributions recorded during the sole action of the primary jet Characteristic profiles are presented at locations within the merging and the mixing zones. In the early stages of development, the patterns of both jets can be identified. Within the merging region, besides the absorption of the secondary jet, the measurements indicate a spatial suppression of primary jet’s characteristics. Further downstream, the profiles resemble to those of a standalone jet. Higher values of the mean and turbulent terms are observed while the profiles extend over a wider region, in accordance with the secondary jet’s Reynolds number. Results of the present study demonstrate that skewness and flatness factors can be used as indicators of small scale mixing. The field of the interaction is further analyzed and discussed for a third pair of initial conditions and the proper decomposition of the mean and turbulent profiles using three different types of seeding conditions. In the early stages, the measurements confirm the independent action of the jets, although the tendency of the secondary jets’ pattern to be deformed. Within the merging zone, the distributions referring to the different types of seeding are characterized by humps and increased turbulent features in the secondary jet’s development region. The shape, the intensity and the extent of the humps depends on the seeding type. Most of the characteristics observed in this region are related to the probability density function distributions, which are composed by samples that correspond to tracers emanating from the individual sources. However, within the mixing zone, the profiles are nearly similar for the individual types of seeding, leading to the conclusion that all the samples contributing to measurements have been adapted by the flow field due to the small scale mixing, which is continuously improved further downstream.

Spacer-filled channels are employed in membrane modules in many industrial applications where feed-flow spacers (employed to separate membrane sheets and create flow channels) tend to enhance mass transport characteristics, possibly mitigating fouling and concentration polarization phenomena. In this work direct numerical simulation was performed for the flow in the spacer-filled channels to obtain a better understanding of fluid flow and mass transfer phenomena in these channels. A solute with a Schmidt number of 1 at Reynolds numbers of 300, 500 and 800 (based on the bulk velocity and spacer diameter) was considered. The effect of spacer location was also studied for three different configurations, spacer at the centre of the channel, at off-centre location, and attached to the wall. Instantaneous velocity fields and flow structures such as separation of boundary layer on the walls and on the cylinder, eddies on the walls, recirculation regions and vortex shedding were investigated. A Fourier analysis was carried out on the time series velocity data. Using this analysis the Strouhal number was calculated and the development of the flow towards a broader turbulent state at higher Reynolds number was captured. Other statistical characteristics such as time-averaged velocities and wall shear rates are obtained and discussed. The average pressure loss which represents the operation cost of membrane modules was calculated for the channels and found to be highest for spacer at the centre of the channel and lowest for spacer attached to the wall. Scalar transport equation is directly solved along with Navier-Stokes equation to get the concentration field. Local Sherwood number is obtained on the walls and the relationship between shear stress, vortex shedding, and mass transfer enhancement was explored. The overall Sherwood number and Stanton number of the channels, which indicate the mass transfer performance of the channels, are obtained. It was observed that as spacer approaches the wall mass transfer rate is decreasing. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-11-30 11:44:07.479

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The steady flow aerodynamics of a cascade of compressor airfoils is computed using a two-dimensional thin layer Navier-Stokes flow solver. The Dhawan and Narasimha transition model and Mayle‟s transition length model were implemented in this flow solver so that transition from laminar to turbulent flow could be included in the computations. A method to speed up the convergence of the fully turbulent calculations has been introduced. In addition, the effect of turbulence production formulations and including streamline curvature correction in the Spalart-Allmaras turbulence model on the transition calculations is studied. These transitional calculations are correlated with the low and high incidence angle experimental data from the NASA-GRC Transonic Flutter Cascade. Including the transitional flow showed a trendwise improvement in the correlation of the computational predictions with the pressure distribution experimental data at the high incidence angle condition where a large separation bubble existed in the leading edge region of the suction surface.

An experimental investigation into fully developed high aspect ratio channels was undertaken. A review of the literature reveals that there is a need for accurate measurement of the inner peak value of streamwise turbulence intensity despite the large number of studies already completed. The scattered data on this subject could be attributed either to insufficient channel size (aspect ratio or length) or to hot-wire spatial filtering. A new, high quality, channel flow facility was designed and constructed, considering the most recent geometric limitation provided in the literature. To obtain accurate results, data were acquired using hot-wire probes with constant viscous-scale sensing length and were corrected using the most recent correction formula proposed by Smits et al. (2011). The results show dependence of inner peak value on Reynolds number in channels flow - its magnitude increasing with increasing Reynolds number.

For many decades, turbulence has been the subject of extensive numerical research and experimental work. A bottleneck problem in turbulence research has been to detect and characterize the energetic, space and time-dependent structures and give a mathematical definition to each topology. This research presents a fundamental study of coherent structures, embedded in turbulent flows, by use of Proper Orthogonal Decomposition (POD). The target is to detect dominant features which contain the largest fraction of the total kinetic energy and hence contribute more to a turbulent flow. POD is proven to be a robust methodology in multivariate analysis of non-linear problems. This method also helps to obtain a low-dimensional approximation of a high-dimensional process, like a turbulent flow. This manuscript-based dissertation consists of five chapters. The first chapter starts with a brief introduction to turbulence, available simulation techniques, limitations and practical applications. Next, POD is introduced and the step-by-step approach is explained in detail. Three submitted manuscripts are presented in the subsequent chapters. Each chapter starts with introducing the study case and explaining the contribution of the study to the whole topic and also has its topic-relevant literature review. Each article consists of two parts: flow simulation and verification of the results at the onset, followed by POD analysis and reconstruction of the turbulent flow fields. For flow simulation, Large Eddy Simulation (LES) was performed to obtain databases for POD analysis. The simulations were validated by making comparison with available experimental and numerical studies. For each case, coherent topologies are characterized and the contribution of kinetic energy for each structure is determined and compared with previous literature. The first manuscript focused on investigating the large-scale dynamics in the wake of an infinite square cylinder. This case is the first step towards the targeting study case of this research, i.e. flow over rib roughened walls. The main purpose the first step is to establish a benchmark for comparison to the more complicated cases of a square cylinder with a nearby wall and flow over a rib-roughened surface. For POD analysis, the three-dimensional velocity field is obtained from LES of the flow around an infinite square cylinder at a Reynolds number of Re = 500. The POD algorithm is examined and the total energy of the flow is found to be well captured by only a small number of eigenmodes. From the energy spectrum, it is learned that each eigenmode represents a particular flow characteristic embedded in the turbulent wake, and eigenmodes with analogous characteristics can be bundled as pairs. Qualitative analysis of the dominant modes provided insight as to the spatial distribution of dominant structures in the turbulent wake. Another outcome of this chapter is to develop physical interpretations of the energetic structures by examining the temporal coefficients and tracking their life-cycle. It was observed that the paired temporal coefficients are approximately sinusoidal with similar order of magnitude and frequency and a phase shift. Lastly, it was observed that the turbulent flow field can be approximated by a linear combination of the mean flow and a finite number of spatial modes. The second manuscript analyses the influence of a solid wall on the wake dynamics of an infinite square cylinder. Different cases have been studied by changing the distance between the cylinder and the bottom wall. From the simulation results, it is learned that the value of drag and lift coefficients can be significantly affected by a nearby solid wall. From the energy decay spectrum it is observed that the energy decay rate varies for different gap ratios and accordingly a physical explanation is developed. Visualization of coherent structures for each case shows that for larger gaps, although the structures are distorted and inclined away from the wall, the travelling wave characteristic persists. Lastly, it is observed that as the gap ratio gets smaller, energetic structures originated by the wall begin to appear in the lower index modes. The last manuscript presents a numerical study of the structures in turbulent Couette flow with roughness on one wall, which as mentioned earlier, is the targeting study case of this research. Flow over both roughened and smooth surfaces was examined in a single study. Comparison was made with experiments and other numerical studies to verify the LES results. The mean velocity distribution across the channel shows that the rib roughness on the bottom wall has a strong effect on the velocity profile on the opposite wall. The energetic coherent dynamics of turbulent flow were investigated by the use of POD. The energy decay spectrum was analysed and the influence of a roughened wall and each roughness element on formation of those structures was investigated. Coherent POD modes on a spanwise sampling plane are detected. A secondary swirling motion is visualized, for the first two modes and counter-rotating cells are observed in the lower region of the channel above the rough wall for the higher modes. At the end, a quantitative analysis of the POD temporal coefficients was performed, which characterize the life-cycle of each coherent dynamic. A motivating outcome of this analysis is to decompose the time trace curves into quasi-periodic and fluctuations curves and to detect a linkage between these life cycles and physical meaning and location of each energetic pattern. At the end, in a closuring chapter, concluding remarks of this research work are presented in more detail and some potential extensions have been proposed for future researchers.

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