Due to their efficient heat and mass transfer potential, impinging jets have received attention in various applications. Heat transfer and flow characteristics of rectangular turbulent impinging jets issued from a 24:1 aspect ratio and 24:1 contraction ratio nozzle were investigated experimentally and numerically. In the heat transfer measurements; a thin stainless-steel foil was utilized to obtain iso-flux boundary conditions on the impingement surface. The target plate was free to translate in the lateral direction and the heat transfer distributions were determined at 0 ≤ x/W ≤ 20 with the micro-thermocouples placed underneath the foil. The measurements were conducted for Re(j) = 8900 − 48600 at nozzle-to-target spacing of 0.5 ≤ H/W ≤ 12.0. Both semi and fully confined jets were investigated. Heat transfer coefficients at Re(j) = 28100, 36800, 45600 and H/W = 4.0 were determined by using adiabatic-wall temperatures and the distributions were compared with those of the wall shear stress. Off-center peaks were observed at high Re(j) and low H/W. Since the wall distributions are susceptible to nozzle-exit conditions, velocity and turbulence profiles at the nozzle-exit were measured for the velocity range of interest. Additionally, near-wall mean velocity and turbulence profiles were determined at Re(j) = 21500 and 36800 at H/W = 4.0 to have a better understanding of the secondary peaks in the wall distributions. Numerical computations were performed by using several different turbulence models (k − ω, k − ε, V 2F and Reynolds stress models). In wall-bounded turbulent flows, near-wall modeling is crucial. Therefore, the turbulence models eliminating wall functions such as the k − ω and V 2F models may be superior for modeling impingement flows. The numerical results showed reasonable agreement with the experimental data for local heat transfer and skin friction coefficient distributions. The occurrence of the secondary peaks was predicted by the k − ω and V 2F models, and for a few cases with the low-Re-k − ε models. Near-wall measurements along with the computed profiles were used to describe the “secondary peak” phenomena. It was shown that the increase in turbulence production in the wall-streamwise direction enhances turbulent momentum and heat transport in the wall-normal direction which lead to secondary peaks in the wall distributions. The possibility of improving surface heat transfer with fully-developed jets was also explored numerically as a case study.

The very first study of the development of the turbulent isothermal and incompressible air jet which issues at a constant velocity from a slender annular slot, circumnavigating an open core, into an otherwise quiescent and unbounded environment of the same density, is presented. The geometry of this source is defined by three diameters: the outer diameter of the slot $D_o$; the inner diameter of the slot $D_i$; and the diameter of the (circular) open core $D_v$. `Slender' refers to a slot for which the inner and outer diameters are approximately equal, i.e. $D_i/D_o\approx 1$. Our focus lies in understanding the development of the time-averaged flow with distance downstream and the influence of the source geometry on the development of the jet. Given the absence of information on jets issuing from the sources of interest, the investigation follows an approach reminiscent of the classic investigations into round jets. That is, it begins with the development of a nozzle and experimental set-up which are suitable for studying the slender open-core annular jet. In addition to the experimental measurements, a complementary mathematical model was developed to describe the unique near-field behaviour of the open-core jet. Measurements were acquired using flow visualisation and Particle Image Velocimetry. On examining the streamwise development of the flow, the slender almost fully open-core jet was delineated into four key regions and the characteristic scalings identified. The regions were as follows: a bounded induced-flow region; a near-source planar-jet region; a transitional region; and a far-field round-jet region. Fluid induced through the open core of the nozzle and subsequently entrained into the jet significantly enhanced the near-field dilution of the jet. Following on from this, the influence of the diameter ratio $D_i/D_o$ and ventilation ratio $D_v/D_i$ on jet coalescence was examined. Over the range of diameter ratios examined ($0.845 \leq D_i/D_o\leq 0.981$), experimental measurements and the predictions from mathematical modelling indicated that $D_i/D_o$ significantly influenced the volume flux induced through the core while the coalescing behaviour of the jet and the far-field region remained largely unchanged. Over the range of ventilation ratios examined ($0 \leq D_v/D_i\leq 0.90$), experimental measurements demonstrated that $D_v/D_i$ controlled the restriction experienced by fluid induced through the open core and significantly influenced the far-field behaviour of the jet. Our findings suggest that jet of interest is then uniquely characterised by the momentum flux $M_0$, the diameter ratio $D_i/D_o$, and the ventilation ratio $D_v/D_i$.

Jets in crossflow are used in a wide range of engineering applications and have been studied for more than 60 years. The transversal penetration and structure of a jet placed in a crossflow is known to be strongly three-dimensional. It is believed that, by using a pulsed jet inclined in the crossflow direction, the momentum transport can be controlled in two very efficient ways: the pulse can increase the jet penetration and the mixing downstream, while the inclination avoids the creation of a reverse flow at the jet exit and may extend the mixing area further downstream. Although some results are available in the literature focusing on components of this problem, none addresses the combination of these two factors. Moreover, most of these studies use elaborate flow visualizations and 2-D velocity measurement methods that may not be adequate to elucidate the complexity of such a flow. This study addresses these issues by using stereoscopic PIV measurements for a steady and fully modulated jet at a constant mean velocity ratio, V[subscript r], of 3.4. For the steady jet case, the effect of the jet Reynolds number, Re[subscript j], is investigated. For the pulsed case, the effect of a low pulsing frequency is considered as well as the pulse duty cycle. For each case, the mean three-component velocity field is examined. Proper Orthogonal Analysis (POD) of vorticity and turbulent kinetic energy are used to further evaluate the vortical and turbulent characteristics of the jet. In addition, a vortex detection algorithm, and 3D rendering of the flow streamlines are used to study the near field vortical flow structure of the jet flow. / Graduation date: 2006

Astrophysikalische Jets sind ausgedehnte, kollimierte Massenausflüsse von verschiedenen astronomischen Objekten. Zeitabhängige magnetohydrodynamische (MHD) Simulationen der Jet-Entwicklung müssen den Akrretionsprozess in der Scheibe berücksichtigen, da der Jet aus der Scheibenmaterie gespeist wird. Allerdings ist die simultane Berechnung der Entwicklung von Scheibe und Jet schwierig, da die charakteristischen Zeitskalen unterschiedlich sind. Selbstähnliche Modelle zeigten, daß eine Beschreibung der Jetentstehung aus einer Akkretionsscheibe durch rein magnetische Prozesse möglich ist. / In this thesis the magnetohydrodynamic jet formation and the effects of magnetic diffusion on the formation of axisymmetric protostellar jets have been investigated in three different simulation sets. The time-dependent numerical simulations have been performed, using the magnetohydrodynamic ZEUS-3D code.

Thesis (Ph. D.)--University of Hong Kong, 1993.

Dans le cadre des jets en impact, les géométries d'injecteur ont une influence importante sur les transferts de chaleur, en affectant le profil de vitesse du jet, ce qui peut modifier le comportement des structures tourbillonnaires. De plus, même si le nombre de Reynolds d'injection est le critère principal d'influence d'un jet, la taille de l'injection peut jouer sur les transferts au travers d'effets de compressibilité. Nos travaux ont donc porté sur l'étude expérimentale aérodynamique et thermique de l'impact de jets d'air.Vélocimétrie par images de particules (PIV) a été utilisée pour des mesures de vitesse.Pour des mesures thermiques, une méthode basée sur l'emploi simultané d'un film chauffant et de la thermographie infrarouge a été utilisée. Les résultats aérodynamiques sont utilisés pour expliquer les phénomènes thermiques observés. La première partie a permis d'étudier et de comparer six différentes géométries d'injecteur : injection ronde, en «croix» et en « pétales », perforées sur une surface plate et hémisphérique avec une même section de passage libre. Les résultats montrent que la surface hémisphérique amène à de meilleurs transferts de chaleur que la surface plate mais l'effet diminue progressivement avec l'augmentation de la distance d'impact. L'injection ronde sur hémisphère apporte le meilleur transfert de chaleur par rapport aux autres injections. La deuxième partie a permis d'examiner l'effet de la compressibilité des jets d'air en impact (pour des nombres de Mach allant jusqu'à 0.9). Les résultats montrent que l'effet de la compressibilité sur le nombre de Nusselt se limite à la région d'impact.ABSTRACT / In the context of an impinging jet, nozzle geometry markedly impacts heat transfer between jet and plate by affecting the velocity profile at the jet exit and thereby potentially modifying the behavior of the jet's vortex structures. Moreover, even if Reynolds number is the main influence criterion of a jet, the injection diameter can play on heat transfer through compressibility effects. Our work bas therefore focused on aerodynamic and thermal experimental study of impinging air jets. Particle image velocimetry (PIV) was used for velocity measurements. For thermal measurements, a method based on the simultaneous use of a heating film and the infrared thermography was used. Aerodynamic results are used to explain the observed thermal phenomena. The first part allowed us to study and compare six different injection geometries : round orifice, cross-shaped orifice and daisy orifice, perforated on fiat and hemispherical surface with the same free area. The results show that the hemispherical surface leads to better heat transfer than the flat surface, but the effect decreases progressively with jet-to-plate distance increasing.The round injection on hemisphere brings the best heat transfer in comparison with the other injections. The second part allowed us to examine the compressibility effect of impinging air jets (for Mach numbers up to 0.9). The results show that the effect of the compressibility on Nusselt number is limited to the stagnation region.

An axisymmetric, turbulent water jet with several very slow, coflowing external streams was measured with a frequency shifted laser Doppler anemometer. The objective was to approximate a jet submerged in an ambient fluid of infinite domain by using a confined jet in a uniform coflow. The coflow prevents flow reversal outside the jet, but if the coflow velocity is not small compared to the jet velocity, the jet will no longer be self-preserving. Thus, the objective is reached in the limit as the coflow approaches zero, but in the absence of reverse flow. In the present study, a jet with several slow coflows was examined to investigate this behavior, the data was extrapolated to the limit (U_s/U_j) to obtain the free jet results and reduce uncertainty in earlier data. The Reynolds number based on the jet diameter and exit velocity was 32100. Conservation of momentum of the jet was demonstrated up to the measurement limit of x/d = 100. Its distribution suggests that the near fleld axial pressure variation has significant effects on the momentum flux. The results also indicate that momentum flux measurements require accurate data to the edge of the jet. The similarity of mean and rms velocity profiles suggest the existence of a region of self-preservation. The entrainment rate, centerline velocity decay rate and spreading rate of the jet were determined and compared to previous measurements with and without a coflowing stream. The variation of these jet parameters with respect to the veIocity ratios was obtained. The limiting values of the jet parameters were determined by extrapolation to zero velocity ratio. This study indicates that a slow coflowing stream is an ideal way to eliminate the recirculating zone present outside jets without coflows. By reducing the coflow to a negligible velocity with constant Craya-Curtet number, researchers can greatly reduce the wide experimental variation in jet entrainment and spreading rates found in different facilities. The results also indicated that a confined jet with a very slow coflow without recirculation can asymptotically approach the conditions of a free jet. An estimate of the variation of the duct size versus the velocity ratio is obtained. It suggests that it is not possible to reduce the velocity ratio to an arbitrarily small value without backflow because the duct would become impractically large. / Ph. D.

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Reproduction de : Thèse de doctorat : Dynamique des fluides : Toulouse, INPT : 2005. / Titre provenant de l'écran-titre. Bibliogr. 78 réf.