Magnetohydrodynamic (MHD) investigations of decaying isotropic turbulence and rectangular jets (RJ) are carried out. A novel MHD lattice Boltzmann scheme that combines multiple relaxation time (MRT) parameters for the velocity field with a single relaxation time (SRT) parameter for the Maxwell’s stress tensor is developed for this study. In the MHD homogeneous turbulence studies, the kinetic/magnetic energy and enstrophy decays, kinetic enstrophy evolution, and vorticity alignment with the strain-rate tensor are evaluated to assess the key physical MHD turbulence mechanisms. The magnetic and kinetic energies interact and exchange through the influence of the Lorentz force work. An initial random fluctuating magnetic field increases the vortex stretching and forward cascade mechanisms. A strong uniform mean magnetic field increases the anisotropy of the turbulent flow field and causes inverse cascading. In the RJ studies, an investigation into the MHD effects on velocity, instability, and the axis-switching phenomena is performed at various magnetic field strengths and Magnetic Reynolds Numbers. The magnetic field is found to decelerate the jet core, inhibit instability, and prevent axis-switching. The key physical mechanisms are: (i) the exchange of energy between kinetic and magnetic modes and (ii) the magnetic field effect on the vorticity evolution. From these studies, it is found that magnetic field influences momentum, vorticity, and energy evolution and the degree of modification depends on the field strength. This interaction changes vortex evolution, and alters turbulence processes and rectangular jet flow characteristics. Overall, this study provides more insight into the physics of MHD flows, which suggests possible applications of MHD Flow Control.

Turbulent jet flows issuing from rectangular nozzles are used in many technological and practical applications. Understanding their development and mechanics is important to the design and performance improvement of these applications. Therefore, rectangular jets have been studied extensively over the past decades. Previous investigations have extensively studied the effects of various initial and boundary conditions such as nozzle type, nozzle aspect ratio, nozzle exit turbulence intensity, jet exit Reynolds number. Nozzle type (e.g. smooth contraction nozzle, channel, radially contoured nozzle) has been found to affect the development of these jets. While varying Reynolds number affects the mixing of the jet with its ambient either in the near field or in the far field. Nozzle aspect ratio is also an important boundary condition. It has been experimentally shown that aspect ratio influences the dimensionality of flow field, and hence entrainment between the jet and its ambient. Some of these investigations have used sidewalls (two parallel plates attached to the nozzle short sides) to improve the two-dimensionality of the flow fields, and/or endplate (a wall flush at the nozzle exit) to prevent mixing upstream of the nozzle. Despite this fact very few investigators have specifically considered the effect of endplate or sidewalls on jet development. In the present investigation, the effects of endplates and/or sidewalls on turbulent jets issuing from a rectangular of aspect ratio, AR= 15 have been studied. The mean streamwise and lateral velocity and turbulent characteristics of four different rectangular jets, namely, 1. jet with no endplate and no sidewalls, NENS, 2. jet with endplate and no endplate, WENS, 3. jet without endplate and with sidewalls, NEWS and 4. jet with endplate and with sidewalls, WEWS, have been measured, with x-sensor hot wire anemometry, up to an axial distance of 35 D under identical inlet conditions. Centreline measurements for the four configurations have been collected for three Reynolds number, ReD=10,000, 20,000 and 30,000. For ReD=20,000 measurements in the transverse direction were collected at 13 different downstream locations in the range of x= (0, 1, 2, 2.5, 3, 4, 5, 10, 15, 20, 25, 30 and 35) nozzle widths. The jet with no endplate and no sidewalls (NENS) and the jet with endplate and no sidewalls (WENS) produce nearly similar mean and turbulent velocity profiles indicating insignificant effect of the endplates on their development in the absence of the sidewalls. At Re=20,000, larger mean streamwise velocity values were observed at the edges of the jets (NEWS or WEWS) at distances from nozzle in the range of x/D=3-30 whereas the presence of the endplate has an insignificant effect. The presence or absence of sidewalls is also key factor determining the distributions of the lateral mean velocity component. The presence of sidewalls is associated with lower outward velocities within the edges of the jets and higher inward ones outside the edges. The absence of sidewalls makes the presence of the endplate insignificant and the lateral velocity attains outside the edges low negative values of almost the same level in both cases (NENS, WENS) The presence of an endplate has again some significance only when the sidewalls are present alleviating their effect. Absence of the endplate in jet with sidewalls, leads to lowest spread rate compared to the case with endplates (comparing jet with no endplate and with sidewalls, NEWS and jet with endplate and with sidewalls, WEWS). The presence of an endplate in the absence of sidewalls has again insignificant effect on the jet’s spread rate (comparing jet with no endplate and no sidewalls, NENS and jet with endplate and no sidewalls, WENS). The effects of endplates and/or sidewalls on the decay rates have been investigated for ReD=10,000, 20,000 and 30,000. The presence of the endplate in jets with no sidewalls has insignificant effect on decay rate as was observed for all Reynolds numbers. The presence of the endplate in jet with sidewalls, leads to higher decay rates for all Re tested compared to all cases. The decay rates of the jets with no sidewalls in the presence and absence of the endplate (NENS, WENS) seem to reach their asymptotic values at around ReD=20,000. But the jets with sidewalls have not reached an asymptotic behaviour even at the highest Re tested. The implementation of sidewalls has been found to lead to a decrease of the streamwise turbulent velocity, u′, at the exit shear layer. Current results indicate that the presence of an endplate has an insignificant effect and the estimates for the root mean square of the streamwise velocity fluctuation u′, at the exit shear layer can be grouped according to the absence or presence of sidewalls (3.8% of the mean centreline velocity for NENS and WENS, and 3.1% for NEWS and WEWS). For all turbulent terms, the presence of the endplate has an effect in the presence of sidewalls and the nondimensional streamwise turbulent velocity attains always higher values in its presence (comparing WEWS with NEWS). The profiles of the streamwise turbulent velocity, u′ profiles for the jet with no endplate and no sidewalls, (NENS) and the jet with endplate and no sidewalls, (WENS) are almost indistinguishable in the range x/D=15-35 but the profiles of the jet with no endplate and with sidewalls, (NESWS) and jet with endplate and with sidewalls, (WEWS) indicate a monotonic increase of the values in the central area of the jets. This indicates that the observed trends are mainly due to the presence of the sidewalls and presence or absence endplate has insignificant effect. / Οι επίπεδες τυρβώδεις δέσμες εκροής ανήκουν στην κατηγορία των απλών ροών που παρουσιάζουν μεγάλο ενδιαφέρον για τη ρευστομηχανική και για το λόγο αυτό αποτέλεσαν θέμα μελέτης και έρευνας εδώ και πολλά χρόνια με θεωρητικά, πειραματικά και υπολογιστικά εργαλεία. Οι αρχικές διερευνήσεις επικεντρώθηκαν στη μελέτη του μέσου και τυρβώδους πεδίου της ταχύτητας ή κάποιου παθητικού ρυπαντή προσπαθώντας να αναδείξουν κυρίως τα κοινά χαρακτηριστικά αυτοομοιότητας που αναπτύσσουν οι δέσμες εκροής κυρίως στο απομακρυσμένο πεδίο. Κατά τις τελευταίες δεκαετίες και με δεδομένη τη σημασία που έχουν οι τυρβώδεις ορθογωνικές δέσμες εκροής σε πολλές πρακτικές εφαρμογές (συστήματα καύσης, ψύξη επιφανειών, έλεγχος άντωσης, έλεγχος θορύβου κλπ) η έρευνα προσανατολίστηκε στην επίδραση των αρχικών και συνοριακών συνθηκών (γεωμετρικά χαρακτηριστικά και διαμόρφωση του ακροφυσίου εκροής, αριθμό Reynolds, ένταση τύρβης στην έξοδο) στη διαμόρφωση του εγγύς και του μέσου πεδίου. Σε αρκετές από τις προηγούμενες διερευνήσεις έχουν χρησιμοποιηθεί πλευρικά τοιχώματα (sidewalls) που επεκτείνουν τις στενές πλευρές του ακροφυσίου στη διεύθυνση της ροής ή/και τοίχωμα εκροής (endplate) με στόχο την επίτευξη διδιάστατης ροής. Η σημασία της παρουσίας αυτών των τοιχωμάτων, η οποία έχει μελετηθεί ελάχιστα στο παρελθόν, αποτελεί το στόχο διερεύνησης της παρούσας διατριβής. Η επίδραση των πλευρικών τοιχωμάτων και του τοιχώματος εκροής μελετήθηκαν πειραματικά με τη χρήση ενός ακροφυσίου ομαλής σύγκλισης με λόγο πλευρών 15 με τη χρήση διατάξεων που υλοποιούσαν τέσσερεις τύπους συνοριακών συνθηκών και συγκεκριμένα: 1. Ελεύθερη δέσμη εκροής χωρίς τοίχωμα εκροής - χωρίς πλευρικά τοιχώματα (No endplate - no sidewalls, NENS), 2. Δέσμη εκροής με τοίχωμα εκροής - χωρίς πλευρικά τοιχώματα (With endplate - no sidewalls, WENS), 3. Δέσμη εκροής χωρίς τοίχωμα εκροής - με πλευρικά τοιχώματα (No endplate - with sidewalls, NEWS), 4. Δέσμη εκροής με τοίχωμα εκροής - με πλευρικά τοιχώματα (With endplate - with sidewalls, WEWS). Η πειραματική διερεύνηση έγινε με την χρήση ανεμομετρίας θερμού σύρματος (Hot wire anemometry, HWA) με αισθητήρα τύπου Χ για τη μέτρηση της διαμήκους και της εγκάρσιας συνιστώσας της ταχύτητας. Πραγματοποιήθηκαν μετρήσεις και για τις τέσσερεις διατάξεις επάνω στον άξονα εκροής για τρείς διαφορετικούς αριθμούς Reynolds, ReD = 10,000, 20,000 και 30,000 σε αποστάσεις μέχρι 35 πλάτη ακροφυσίου, D, από το σημείο εκροής. Για ReD = 20000 καταγράφηκαν επίσης οι εγκάρσιες κατανομές των δύο συνιστωσών της ταχύτητας σε δεκατρείς σταθμούς σε αποστάσεις από την έξοδο x/D = 0, 1, 2, 2.5, 3, 4, 5, 10, 15, 20, 25, 30 και 35. Τα αποτελέσματα του μέσου όσο και του τυρβώδους πεδίου καταδεικνύουν ότι η παρουσία των πλευρικών τοιχωμάτων έχει σημαντική επίδραση στην ανάπτυξη του ροϊκού πεδίου. Αντίθετα η παρουσία του τοιχώματος εκροής διαφοροποιεί τα αποτελέσματα μόνο στην περίπτωση που υπάρχουν ήδη τα πλευρικά τοιχώματα.