This study examines the effect of varying injectant molecular weight on the penetration of transverse injection jets into a supersonic crossflow. The injectants considered here are methane (W=16.04), air (W=28.97) and carbon dioxide, (W=44.01). These results augment the previous results obtained at Virginia Tech for helium (W=4.00) injection under the same test conditions to provide a very wide range of molecular weights. Second, since shocks are ubiquitous in scramjet combustors, their influence on penetration and mixing was also studied by arranging for an oblique shock to impinge near the injection station. The cases of a shock impinging upstream and downstream of the injector were both examined. One can anticipate an important influence of molecular weight here also because of the importance of density gradients on the generation of vorticity by baroclinic torque. Increasing molecular weight was found to increase penetration in general, as well as increase the lateral spreading of the plume. The majority of the data shows a weak dependency of the jet size on molecular weight, but there are indications that under certain circumstances large changes in the flow structure may occur due to molecular weight effects. The addition of an impinging shock is found to increase mixing and decrease penetration and plume size, especially with the shock impinging downstream of the injector. / Master of Science

This study aims for mix engineers to reach a better understanding of bass and guitar mid-range in metal music. Listening tests have been conducted with two different groups of subjects, rock/metal fans and audio engineering students, regarding preferences in bass and guitar mid-range-specific attributes and their effect on mixes. Three attributes have been part of the study; “clarity”, “muddiness”, and “abrasiveness”. Multiple mixes have been created around the attributes, and these have been rated by the subjects regarding the attribute in question. The results from the study show that “clarity” is a difficult attribute to define, the high-mid is an important frequency area for a mix engineer to balance right, and the term“heaviness” is often connected with heavily distorted guitars but can also be related to the low-end of a metal mix.

Efficient mixing can be achieved in flows where turbulence is absent, if the trajectories of passively advected particles in the flow are chaotic. The chaotic nature of particle trajectories results in exponential stretching of material lines in the flow. Thus the interface along which diffusion occurs is stretched exponentially leading to efficient mixing. It has been demonstrated recently that regions in flow fields that exhibit poor mixing and non-chaotic particle trajectories can have an important bearing on the overall dynamics and transport of the entire domain. The space-time trajectories of physical stirrers or elliptic points in two dimensional flows can be classified according to braid groups. One can predict a lower bound on the topological entropy (i.e. exponential rate of stretching of material lines) of flows (h_f) by applying the Thurston-Nielsen classification theorems to these braids. This gives a reduced order model for the dynamics of transport of the entire flow field using just a few points. Recent work has shown that this methodology can be used to estimate a lower bound on h_f using the braids formed by Almost Cyclic Sets (ACS) in certain periodic Stokes' flows. These ACS are closely related to Almost Invariant Sets (AIS) which are identified using a probabilistic set oriented method that makes use of the descritised Perron-Frobenius operator of the flow map. This work extends this approach to flows at non-zero Reynolds numbers, which take into account the effects of inertia. The role of Finite Time Coherent Structures (FTCS) in the dynamics of flow fields is also investigated. Unlike ACS, the FTCS approach is more general as it can be applied to aperiodic flow fields. Further, the relationship between mixing efficiency and the topological entropy of flow fields at non-zero Reynolds numbers is also studied. / Ph. D.

Student Number : 0401422G - MSc dissertation - School of Chemical and Metallurgical Engineering - Faculty of Engineering / Mixing is an important operation that is carried out in food, paint, pharmaceutical and mineral processing industries. Ball mills are one of the many mixing vessels used in a mineral processing industry. During grinding, the mill’s efficiency depends on particle presentation to the grinding media and the adequate utilisation of the applied forces to effect breakage of particles (ore). Utilisation of applied forces is affected by how well particles and grinding media are mixed. The study of charge mixing is important as it affects the mill’s production rate and accelerates media wear, thus relevant to the cost reduction for the milling process. The kinetics of mixing in a batch ball mill were quantified both radially and axially. Experiments were conducted in a laboratory batch ball mill and two experimental programs were used to study the mixing process. Radial mixing of particles was observed to increase with increasing mill speed. For a mill used in this study, mixing of particles at Nc=90% took almost half the total time taken at Nc=75% to reach completion. A simplified mathematical model is presented, which can be used to predict the radial mixing of particles in a ball mill. Axial mixing of particles was observed to be affected by both the charge system used and segregation of particles from the grinding media. It took a minute for mixing to reach 80% completion for a mill used in the experiments. Mixing of particles was faster in a steel balls/plastic powders charge system than in a glass beads/quartz charge system. The distribution of particles in a batch mill was observed to vary along the axis of the mill. The centre of the mill was overfilled with particles, U>1, while the regions near the mill ends were underfilled, U<1. The opposite was true for the grinding media. The data reported was based on measurements of particle distribution along the mill as affected by different charge systems. The work presented in this thesis is a contribution to the continuing research on mixing of particles in ball mills.

Local displacement efficiency in miscible floods is significantly affected by mixing taking place in the medium. Laboratory experiments usually measure flow-averaged ("cup mixed") effluent concentration histories. The core-scale averaged mixing, termed as dispersion, is used to quantify mixing in flow through porous media. The dispersion coefficient has the contributions of convective spreading and diffusion lumped together. Despite decades of research there remain questions about the nature and origin of dispersion. The main objective of this research is to understand the basic physics of solute transport and mixing at the pore scale and to use this information to explain core-scale mixing behavior (dispersion). We use two different approaches to study the interaction between convective spreading and diffusion for a range of flow conditions and the influence of their interaction on dispersion. In the first approach, we perform a direct numerical simulation of pore scale solute transport (by solving the Navier Stokes and convection diffusion equations) in a surrogate pore space. The second approach tracks movement of solute particles through a network model that is physically representative of real granular material. The first approach is useful in direct visualization of mixing in pore space whereas the second approach helps quantify the effect of pore scale process on core scale mixing (dispersion). Mixing in porous media results from interaction between convective spreading and molecular diffusion. The converging-diverging flow around sand grains causes the solute front to stretch, split and rejoin. In this process the area of contact between regions of high and low solute concentrations increases by an order of magnitude. Diffusion tends to reduce local variations in solute concentration inside the pore body. If the fluid velocity is small, diffusion is able to homogenize the solute concentration inside each pore. On the other hand, in the limit of very large fluid velocity (or no diffusion) local mixing because of diffusion tends to zero and dispersion is entirely caused by convective spreading. Flow reversal provides insights about mixing mechanisms in flow through porous media. For purely convective transport, upon flow reversal solute particles retrace their path to the inlet. Convective spreading cancels and echo dispersion is zero. Diffusion, even though small in magnitude, causes local mixing and makes dispersion in porous media irreversible. Echo dispersion in porous media is far greater than diffusion and as large as forward (transmission) dispersion. In the second approach, we study dispersion in porous media by tracking movement of a swarm of solute particles through a physically representative network model. We developed deterministic rules to trace paths of solute particles through the network. These rules yield flow streamlines through the network comparable to those obtained from a full solution of Stokes' equation. In the absence of diffusion the paths of all solute particles are completely determined and reversible. We track the movement of solute particles on these paths to investigate dispersion caused by purely convective spreading at the pore scale. Then we superimpose diffusion and study its influence on dispersion. In this way we obtain for the first time an unequivocal assessment of the roles of convective spreading and diffusion in hydrodynamic dispersion through porous media. Alternative particle tracking algorithms that use a probabilistic choice of an out-flowing throat at a pore fail to quantify convective spreading accurately. For Fickian behavior of dispersion it is essential that all solute particles encounter a wide range of independent (and identically distributed) velocities. If plug flow occurs in the pore throats a solute particle can encounter a wide range of independent velocities because of velocity differences in pore throats and randomness of pore structure. Plug flow leads to a purely convective spreading that is asymptotically Fickian. Diffusion superimposed on plug flow acts independently of convective spreading causing dispersion to be simply the sum of convective spreading and diffusion. In plug flow hydrodynamic dispersion varies linearly with the pore-scale Peclet number. For a more realistic parabolic velocity profile in pore throats particles near the solid surface of the medium do not have independent velocities. Now purely convective spreading is non-Fickian. When diffusion is non-zero, solute particles can move away from the low velocity region near the solid surface into the main flow stream and subsequently dispersion again becomes asymptotically Fickian. Now dispersion is the result of an interaction between convection and diffusion and it results in a weak nonlinear dependence of dispersion on Peclet number. The dispersion coefficients predicted by particle tracking through the network are in excellent agreement with the literature experimental data. We conclude that the essential phenomena giving rise to hydrodynamic dispersion observed in porous media are (i) stream splitting of the solute front at every pore, thus causing independence of particle velocities purely by convection, (ii) a velocity gradient within throats and (iii) diffusion. Taylor's dispersion in a capillary tube accounts for only the second and third of these phenomena, yielding a quadratic dependence of dispersion on Peclet number. Plug flow in the bonds of a physically representative network accounts for the only the first and third phenomena, resulting in a linear dependence of dispersion upon Peclet number. / text

Αντικείμενο αυτής της διπλωματικής εργασίας είναι η εφαρμογή των θεμελιωδών αρχών της ψυχοακουστικής θεωρίας στο συνδυασμό ηχητικών πηγών. Ειδικότερα, εισάγεται μια νέα προσέγγιση (ψηφιακής) μίξης σημάτων (καναλιών) ήχου, η οποία βασίζεται στη αντιληπτότητα των ήχων από το ανθρώπινο από το ανθρώπινο ακουστικό σύστημα. Στόχος της εργασίας είναι να αναδείξει τα πλεονεκτήματα της νέας προσέγγισης σε σχέση με τη συμβατική μέθοδο μίξης. Για το σκοπό αυτό πραγματοποιείται ένας μεγάλος αριθμός ένθεν και ένθεν μίξεων σε ένα σύνολο από διαθέσιμα αρχεία ήχου και παρουσιάζονται σχετικά αποτελέσματα. / This diploma work applies some of the fundamental psychoacoustic principles to the combination of sound sources. A novel technique of perceptually–motivated signal dependent digital audio mixing is presented. The proposed Hierarchical Perceptual Mixing (HPM) method is implemented in the spectro–temporal domain. Its basic principle is to combine only the perceptually relevant components of the audio signals, derived after the calculation of the minimum masking threshold which is introduced in the mixing stage. Objective measures of various mixing scenarios are presented aiming to underline the advantages of HPM method over CM method (Convention Mixing).

<p>Turbulent mixing layers have been researched for many years. Currently, research is focused on studying compressible mixing layers because of their widespread applications in high-speed flight systems. While the effect of compressibility on the shear layer growth rate is well established, there is a lack of consensus over its effect on the turbulent stresses and hence warrants additional research in this area. Computational studies on compressible shear layers could provide a deep cognizance of the dynamics of fluid structures present in these flow fields which in turn would be viable for understanding the effects of compressibility on such flows. However, performing a Direct Numerical Simulation (DNS) of a highly compressible shear layer with experimental flow conditions is extremely expensive, especially when resolving the boundary layers that lead into the mixing section. The attractive alternative is to use Large Eddy Simulation (LES), as it possesses the potential to resolve the flow physics at a reasonable computational cost. Therefore the current work deals with developing a methodology to perform LES of a compressible mixing layer with experimental flow conditions, with resolving the boundary layers that lead into the mixing section through a wall model. The wall model approach, as opposed to a wall resolved simulation, greatly reduces the computational cost associated with the boundary layer regions, especially when using an explicit time-stepping scheme. An in house LES solver which has been used previously for performing simulations of jets, has been chosen for this purpose. The solver is first verified and validated for mixing layer flows by performing simulations of laminar and incompressible turbulent mixing layer flows and comparing the results with the literature. Following this, LES of a compressible mixing layer at a convective Mach number of 0.53 is performed. The inflow profiles for the LES are taken from a precursor RANS solution based on the k-ε and RSM turbulence models. The results of the LES present good agreement with the reference experiment for the upstream boundary layer properties, the mean velocity profile of the shear layer and the shear layer growth rate. The turbulent stresses, however, have been found to be underpredicted. The anisotropy of the normal Reynolds stresses have been found to be in good agreement with the literature. Based on the present results, suggestions for future work are also discussed.</p>

No description available.

The hyporheic zone is the reactive interface between surface water and groundwater found beneath streams and rivers, where chemical gradients and an abundant biological presence allow beneficial attenuation of contaminants. Such attenuation often requires reactants from surface water and groundwater to mix, but few studies have explored the controls on mixing of upwelling groundwater water in the hyporheic zone and its potential to foster mixing-dependent reactions. The goals of this dissertation are therefore to evaluate the effects of (1) hydraulic controls and (2) reaction kinetic controls on hyporheic mixing and mixing-dependent reactions, and (3) use two-dimensional visualization techniques to quantify patterns of hyporheic mixing and mixing-dependent reactions. These objectives were addressed by hyporheic zone simulations using a laboratory sediment mesocosm and numerical models. In the laboratory, a hyporheic flow cell was created to observe both conservative dye mixing and abiotic mixing-dependent reaction. The numerical models MODFLOW and SEAM3D were then used to simulate the experimental data to better understand hydraulic and transport processes underlying laboratory observations and provide sensitivity analysis on hydraulic and reaction kinetic parameters. Visualization techniques showed a distinct mixing zone developing over time for both conservative and reactive conditions. Mixing zone thickness in both conditions depended on surface water head drop and the ratio of boundary inflows of surface water and groundwater (inflow ratios). The abiotic reaction caused the mixing zone to shift even under steady-state hydraulics indicating that hyporheic zone mixing-dependent reactions affect the location of mixing as chemical transformations take place. The numerical model further showed the production zone to be thicker than the mixing zone and located where reactants had already been depleted. Finally, mapping of two-dimensional microbial respiration (i.e., electron acceptor utilization) patterns in streambed sediments using dissolved oxygen and carbon dioxide planar optodes showed that coupling multiple such 2D chemical profiles can enhance understanding of microbial processes in the hyporheic zone. Temporal dynamics for these chemical species revealed development of spatial heterogeneity in microbial respiration and hence microbial activity. Our results show key hydrologic and biogeochemical controls on hyporheic mixing and mixing-dependent reactions. These reactions represent a last opportunity for attenuation of groundwater borne contaminants prior to entering surface water. / Doctor of Philosophy / The boundary between surface water and groundwater beneath streams and rivers is known to have an abundant biological presence that allows for beneficial reduction of contaminants when chemicals combine. This combination of chemicals due to mixing of the waters is an important characteristic of the boundary area (defined as the hyporheic zone). However, controls on mixing and the impact on contaminant reduction are not fully understood. Therefore, the goals of this dissertation are to evaluate (1) the effects of varying water level and flow and (2) the effects of the rates of the reaction on mixing of chemicals and chemical transformation, and (3) use two-dimensional visualization processes to quantify the reactions and mixing occurring at the boundary area of surface water and groundwater. We used both laboratory and numerical model simulations to study mixing at the boundary area. The two-dimensional visualization in both laboratory and numerical models show distinct regions where mixing occurred between the surface water and groundwater. The extent of the mixing (mixing thickness) was most dependent on the flow ratio between the upward groundwater and downward surface water. The observations were made with non-varying surface and groundwater flow rates but changes on the mixing thickness and location were seen throughout the duration of the experiments revealing that chemical reaction dynamics have an influence on the mixing process. Ultimately, these types of reactions represent a last opportunity for attenuation of groundwater borne contaminants prior to entering surface water.

Turbulent mixing enhancement has received a great deal of attention in the fluid mechanics community in the last few decades. Generally speaking, mixing enhancement involves the increased dispersion of the fluid that makes up a flow. The current work focuses on mixing enhancement of an axisymmetric jet via high amplitude fluidic pulses applied at the nozzle exit with high aspect ratio actuator nozzles. The work consists of small scale clean jet experiments, small scale micro-turbine engine experiments, and full scale laboratory simulated core exhaust experiments using actuators designed to fit within the engine nacelle of a full scale aircraft. The small scale clean jet experiments show that mixing enhancement compared to the unforced case is likely due to a combination of mechanisms. The first mechanism is the growth of shear layer instabilities, similar to that which occurs with an acoustically excited jet except that, in this case, the forcing is highly nonlinear. The result of the instability is a frequency bucket with an optimal forcing frequency. The second mechanism is the generation of counter rotating vortex pairs similar to those generated by mechanical tabs. The penetration depth determines the extent to which this mechanism acts. The importance of this mechanism is therefore a function of the pulsing amplitude. The key mixing parameters were found to be the actuator to jet momentum ratio (amplitude) and the pulsing frequency, where the optimal frequency depends on the amplitude. The importance of phase, offset, duty cycle, and geometric configuration were also explored. The experiments on the jet engine and full scale simulated core nozzle demonstrated that pulse fluidic mixing enhancement was effective on realistic flows. The same parameters that were important for the cleaner small scale experiments were found to be important for the more realistic cases as well. This suggests that the same mixing mechanisms are at work. Additional work was done to optimize, in real time, mixing on the small jet engine using an evolution strategy.