# Browsing by Subject "Turbulence"

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Item Anisotropic hybrid turbulence modeling with specific application to the simulation of pulse-actuated dynamic stall control(2015-12) Haering, Sigfried William; Moser, Robert deLancey; Murthy, Jayathi; Bogard, David G; Ezekoye, Ofodike A; Oliver, ToddShow more Experimental studies have shown pulse actuated dynamic stall control may provide a simple means to significantly increase the performance of lifting surfaces and expand their flight envelope. However, precise information of the complex boundary layer reattachment mechanisms are inaccessible to experimental measurements. Therefore, simulations are necessary to fully understand, optimize, and apply this method. Due to the inherent shortcomings of RANS, computational expense of LES, and deficiencies in current hybrid modeling approaches, a new hybrid modeling framework has been developed. Based in using the two-point second-order structure function to drive a local equilibrium between resolved and modeled turbulence, the new approach addresses issues associated with inhomogeneous and anisotropic grids as well as the treatment of the RANS/LES interface in hybrid simulations. Numerical studies using hybrid RANS/LES modeling approaches of a stalled airfoil with spanwise-uniform actuation regions experiencing single pulse actuated flow reattachment have been performed. The mechanism responsible for reattachment has been identified as a repeating wall-vortex interaction process. The new hybrid framework and anisotropic SGS models developed here are anticipated to be of great benefit well beyond the focus of this work with application to many challenging flow situations of pressing engineering interest.Show more Item The attenuation and reduction of a simulated hot streak due to mainstream turbulence, hot streak pitch position and film cooling(2004) Jenkins, Sean Craig; Bogard, David G.Show more This study investigated the effects of the vane and mainstream turbulence level on a simulated hot streak in a simulated three vane cascade. The effect of film cooling on the reduction of the hot streak was investigated for a fully film-cooled vane. To determine how the showerhead, suction side and pressure side coolant regions contributed to hot streak reduction, these regions were tested individually with and without the hot streak activated. The effect of mainstream turbulence level and coolant density ratio on coolant profiles and hot streak reduction was also investigated. Finally, superposition of coolant profiles and hot streak profiles was compared with measured data to evaluate the capability of additive superposition in predicting hot streak reduction due to film cooling. The effects of mainstream turbulence on the attenuation of a hot streak were found to be significant, with changes in the shape and size of the hot streak. Comparisons between the hot streak impacting the vane at the stagnation line and passing through the mid-passage showed that the peak hot streak temperature was the same for an impinging and non-impinging hot streak. Interaction with the adiabatic vane caused very sharp temperature gradients in the hot streak at the trailing edge of the vane, resulting in an increase or decrease in hot streak peak strength depending on pitch position. Additional attenuation of the hot streak occurred in the stator/rotor axial gap. Results with film cooling indicated that, while full-coverage film cooling had a substantial effect on the hot streak, this effect was primarily due to the showerhead and suction side coolant with a much lesser effect due to the pressure side. It was discovered that coolant profiles at the trailing edge could be scaled by the coolant hole exit temperature, while reduction of the hot streak was less for film cooling at low density ratio. Measurements also showed a much higher degree of coolant spreading under conditions of high mainstream turbulence. Overall, downstream of the vane using high blowing ratios, the hot streak peak was reduced by 83% compared with the peak value upstream of the vane.Show more Item CFD predictions of heat transfer coefficient augmentation on a simulated film cooled turbine blade leading edge(2011-05) Beirnaert-Chartrel, Gwennaël; Bogard, David G.; Moser, Robert D.Show more Computations were run to study heat transfer coefficient augmentation with film cooling for a simulated gas turbine blade leading edge. The realizable k-[epsilon] turbulence model (RKE) and Shear Stress Transport k-[omega] turbulence model (SST) were used for the computational simulations. RKE computations completed at a unity density ratio were confirmed to be consistent with experimental measurements conducted by Yuki et al.(1998) and Johnston et al. (1999) whereas SST computations exhibited significant discrepancies. Moreover the effect of the density ratio on heat transfer coefficient augmentation was studied because experimental measurements of heat transfer coefficient augmentation with film cooling are generally constrained to unity density ratio tests. It was shown that heat transfer coefficient augmentation can be simulated using unity density ratio jets, but only when scaled with the momentum flux ratio of the coolant jets.Show more Item Coherent structures and two-dimensionalization in rotating turbulent flow(2005) Ruppert-Felsot, Jori Elan; Swinney, H. L., 1939-Show more We study laboratory-produced fluid turbulence under the influence of rapid rotation. Three-dimensional (3D) turbulence is generated by strong pumping through sources and sinks at the bottom of a rotating tank (48.4 cm high, 39.4 cm diameter) filled with water. This flow evolves toward quasi-two-dimensional (quasi-2D) turbulence with increasing height in the tank. Digital Particle Image Velocimetry (DPIV) measurements were taken using tracer particles illuminated by a horizontal laser light-sheet. The quasi-2D flow near the top of the tank contains many long-lived coherent vortices and jets with a wide range of sizes near the top of the tank. The vertical vorticity field exhibits complex dynamics such as vortex birth, merger, scattering, and destruction. Measurements using two synchronized cameras and vertically separated light sheets revealed coherent structures that were columnar and extended vertically throughout the tank. The effect of rotation greatly increased the vertical correlation of the flow, even for small rotation rates. A gradual decay in the correlation of increasingly vertically separated vii planes was observed, rather than a sharp transition. The discrete wavelet packet transform (DWPT) and discrete wavelet transform (DWT) were used to extract and study the dynamics of the localized coherent structures near the top of our tank. We separated the flow into a low-entropy “coherent” and a highentropy “incoherent” component by thresholding the coefficients of the DWPT and DWT of the vorticity field. Similar thresholdings using the Fourier transform and JPEG compression, the Okubo-Weiss criterion and Proper Orthogonal Decomposition (POD) were also tested. We found that the DWPT and DWT yield similar results and are much more efficient at representing the total flow than other methods. Only about 3% of the largeamplitude coefficients of the DWPT and DWT were necessary to represent the coherent component and preserve the vorticity probability density function, transport properties, and spatial and temporal correlations of the measured fields. The remaining small amplitude coefficients represent the incoherent component, which has near Gaussian vorticity PDF, contains no coherent structures, rapidly loses correlation in time, and does not contribute significantly to the transport properties of the flow. This suggests that one can faithfully describe and simulate such turbulent flow using a relatively small number of wavelet or wavelet packet modes.Show more Item Coupling Viscous Vorticity Equation (VISVE) method with OpenFOAM to predict turbulent flow around 2-D hydrofoils and cylinders(2019-06-19) Yao, Hao; Kinnas, Spyros A.Show more The VIScous Vorticity Equation (VISVE) method has already been applied to solve the laminar flow around a cylinder and a hydrofoil at low Reynolds numbers. This method is more computationally efficient and spatially compact than a viscous flow method based on primitive variables. However, the VISVE method fails at high Reynolds numbers due to the effects of turbulence. In this thesis, a synchronous coupling method was developed to couple the VISVE and a turbulence model in OpenFOAM, enabling the VISVE method to solve the turbulent flow at high Reynolds numbers in a 2-D hydrofoil case and a 2-D cylinder case. The velocity, vorticity, and pressure calculated by the coupling method agree well with the results obtained by a RANS method.Show more Item Direct numerical simulation (DNS) for incompressible turbulent channel flow at Reτ = 5200(2015-12) Lee, Myoungkyu; Moser, Robert deLancey; Biros, George; Bogard, David G.; Murthy, Jayathi; Oliver, Todd A.Show more Nearly all moving objects on Earth pass through fluids and many of them move at high speed. This makes high Re wall-bounded turbulent flows of great technological impor- tance. To study high Re wall-bounded turbulence, high spatial and temporal resolution is required due to the multi-scale nature of turbulence. Direct numerical simulation (DNS) is a technique for the study of turbulence in which the Navier-Stoke equations, the governing equations of fluid flow, are solved with sufficient resolution to represent all the scales of tur- bulence. Hence, DNS is very expensive and always limited by computational capability. To perform DNS on the most advanced high performance computing systems, extensive code optimization is required. A new turbulence DNS code, PoongBack, was developed for the studies reported here. It shows excellent performance and scalability (∼97%) on upto 786k cores on Mira at Argonne Leadership Computing Facility. We have performed DNS of turbulent channel flow using a Fourier-Galerkin method in the streamwise(x) and spanwise (z) directions and a B-Splines collocation method in the wall-normal (y) direction. The highest Reynolds number based on shear velocity (uτ = √(τw/ρ)), Reτ is approximately 5200. The simulation results exhibit a number of the char- acteristics of high Re wall-bounded turbulent flows. For example, a region where the mean velocity has a logarithmic variation is observed, with von Kármán constant κ = 0.384±0.004. There is also a logarithmic dependence of the variance of the spanwise velocity component, though not the streamwise component. A distinct separation of scales exists between the large outer-layer structures and small inner-layer structures. At intermediate distances from the wall, the one-dimensional spectrum of the streamwise velocity fluctuation in both the streamwise and spanwise directions exhibits 1/k dependence over a short range in wavenum- ber (k). Further, consistent with previous experimental observations, when these spectra are multiplied by k (premultiplied spectra), they have a bimodal structure with local peaks located at wavenumbers on either side of the 1/k range. To study the scale dependence of the dynamics of the Reynolds stress components, we applied a spectral analysis to the terms in the Reynolds stress transport equation (RSTE). It is shown that only the turbulent transport terms show significant Re dependencies. Further- more, the turbulent transport terms can be decomposed into two parts, one that contributes to transport in the wall-normal direction and one that is responsible for transfer between length scales. The results show that the large scale motion in the outer region has direct effects on the flow in the near-wall region through transport of turbulent kinetic energy. Also, a reverse energy cascade from intermediate scales to large scales is observed in the spanwise velocity fluctuations.Show more Item Direct numerical simulation and reaction path analysis of titania formation in flame synthesis(2012-08) Singh, Ravi Ishwar; Ezekoye, Ofodike A.; Raman, VenkatShow more Flame-based synthesis is an attractive industrial process for the large scale generation of nanoparticles. In this aerosol process, a gasifi ed precursor is injected into a high-temperature turbulent flame, where oxidation followed by particle nucleation and other solid phase dynamics create nanoparticles. Precursor oxidation, which ultimately leads to nucleation, is strongly influenced by the turbulent flame dynamics. Here, direct numerical simulation (DNS) of a canonical homogeneous flow is used to understand the interaction between a methane/air flame and titanium tetrachloride oxidation to titania. Detailed chemical kinetics is used to describe the combustion and precursor oxidation processes. Results show that the initial precursor decomposition is heavily influenced by the gas phase temperature field. However, temperature insensitivity of subsequent reactions in the precursor oxidation pathway slow down conversion to the titania. Consequently, titania formation occurs at much longer time scales compared to that of hydrocarbon oxidation. Further, only a fraction of the precursor is converted to titania, and a signi cant amount of partially-oxidized precursor species are formed. Introducing the precursor in the oxidizer stream as opposed to the fuel stream has only a minimal impact on the oxidation dynamics. In order to understand modeling issues, the DNS results are compared with the laminar flamelet model. It is shown that the flamelet assumption qualitatively reproduces the oxidation structure. Further, reduced oxygen concentration in the near-flame location critically a ffects titania formation. The DNS results also show that titania forms on the lean and rich sides of the flame. A reaction path analysis (RPA) is conducted. The results illustrate the di ffering reaction pathways of the detailed chemical mechanism depending on the composition of the mixture. The RPA results corroborate with the DNS results that titania formation is maximized at two mixture fraction values, one on the lean side of the flame, and one on the rich side.Show more Item The effects of buoyancy on turbulent nonpremixed jet flames in crossflow(2005) Boxx, Isaac G.; Clemens, Noel T.Show more An experimental research study was conducted to investigate what effect buoyancy had on the mean and instantaneous flow-field characteristics of turbulent jet-flames in crossflow (JFICF). The study used an experimental technique wherein a series of normal-gravity, hydrogen-diluted propane JFICF were compared with otherwise identical ones in low-gravity. Experiments were conducted at the University of Texas Drop Tower Facility, a new microgravity science laboratory built for this study at the University of Texas at Austin. Two different diagnostic techniques were employed, high frame-rate digital cinematographic imaging and planar laser Mie scattering (PLMS). The flame-luminosity imaging revealed significant elongation and distortion of the large-scale luminous structure of the JFICF. This was seen to affect the flame-tip oscillation and burnout characteristics. Mean and root-mean-square (RMS) images of flame-luminosity were computed from the flame-luminosity image sequences. These were used to compare visible flame-shapes, flame chord-lengths and jet centerline-trajectories of the normal- and low-gravity flames. In all cases the jet-centerline penetration and mean luminous flame-width were seen to increase with decreasing buoyancy. The jet-centerline trajectories for the normal-gravity flames were seen to behave differently to those of the low-gravity flames. This difference led to the conclusion that the jet transitions from a momentum-dominated forced convection limit to a buoyancy-influenced regime when it reaches ξC ≈ 3, where ξC is the Becker and Yamazaki (1978) buoyancy parameter based on local flame chord-length. The mean luminous flame-lengths showed little sensitivity to buoyancy or momentum flux ratio. Consistent with the flame-luminosity imaging experiments, comparison of the instantaneous PLMS flow-visualization images revealed substantial buoyancy-induced elongation and distortion of the large-scale shear-layer vortices in the flow. This effect became apparent in the JFICF at around ξy = 3.1 and grew in influence to become a dominant flow-field characteristic approximately ξy = 4.3. The PLMS images also yielded physical-insight into the nature of the fore-aft asymmetry of JFICF characteristics noted by previous researchers. Ensemble-averages of PLMS images were used to investigate centerline mixture fraction decay. Consistent with previous studies of non-reacting JICF studies, the mixture-fraction of the JFICF showed a power-law decay profile which scaled with (rd)-0.66. Over the region these measurements were made (ξy = 0 – 1.9), the mixture fraction decay scaling showed little sensitivity to buoyancy. Taken as a whole, these measurements show that buoyancy has the potential to significantly modify both the mean and instantaneous flow-field of a turbulent JFICF, even at relatively modest length-scales.Show more Item The efficiency of turbulent mixing in stratified fluids(2010-08) Ebert, Guenther Wolfgang; Swinney, H. L., 1939-; Fink, ManfredShow more Mixing is a common feature of stratified fluids. In stratified fluids the density varies with the height. This is true for the most fluids in geophysical environments, like lakes, the atmosphere or the ocean. Turbulent mixing plays a crucial role for the overall energy budget of the earth and has therefore an huge impact on the global climate. By introducing the mixing efficiency, it is possible to quantify mixing. It is defined as the ratio of gain of potential energy to the injection of mechanical energy. In the ocean energy provided by tidal forces leads to turbulence and thus highly dense water is lifted up from the deep sea to the surface. For this process, a mixing efficiency of 0.2 is estimated. Until now it is not completely understood how this high value can be achieved. Thus we measured the mixing efficiency by using a Couette-Taylor system, which can produce steady-state homogeneous turbulence. This is similar to what we find in the ocean. The Couette-Taylor system consists of two concentric cylinders that can be rotated independently. In between a stratified fluid is filled using salt as a stratifying agent. In the laboratory experiment, we obtained mixing efficiencies in the order of 0.001 as a result. Moreover we found that the mixing efficiency decreases with decreasing stratification like previous laboratory experiments have shown. As this value is two orders of magnitude smaller than what we find in the ocean, further studies will be necessary.Show more Item Electostatic plasma edge turbulence and anomalous transport in SOL plasmas(2014-08) Meyerson, Dmitry; Gentle, Kenneth W.; Waelbroeck, F.Show more Controlling the scrape-off layer (SOL) properties in order to limit divertor erosion and extend component lifetime will be crucial to successful operation of ITER and devices that follow, where intermittent thermal loads on the order of GW/m² are expected. Steady state transport in the edge region is generally turbulent with large, order unity, fluctuations and is convection dominated. Owing to the success of the past fifty years of progress in magnetically confining hot plasmas, in this work we examine convective transport phenomena in the SOL that occur in the relatively "slow", drift-ordered fluid limit, most applicable to plasmas near MHD equilibrium. Diamagnetic charge separation in an inhomogeneous magnetic field is the principal energy transfer mechanism powering turbulence and convective transport examined in this work. Two possibilities are explored for controlling SOL conditions. In chapter 2 we review basic physics underlying the equations used to model interchange turbulence in the SOL and use a subset of equations that includes electron temperature and externally applied potential bias to examine the possibility of suppressing interchange driven turbulence in the Texas Helimak. Simulated scans in E₀×B₀ flow shear, driven by changes in the potential bias on the endplates appears to alter turbulence levels as measured by the mean amplitude of fluctuations. In broad agreement with experiment negative biasing generally decreases the fluctuation amplitude. Interaction between flow shear and interchange instability appears to be important, with the interchange rate forming a natural pivot point for observed shear rates. In chapter 3 we examine the possibility of resonant magnetic perturbations (RMPs) or more generally magnetic field-line chaos to decrease the maximum particle flux incident on the divertor. Naturally occurring error fields as well as RMPs applied for stability control are known to cause magnetic field-line chaos in the SOL region of tokamaks. In chapter 3 2D simulations are used to investigate the effect of the field-line chaos on the SOL and in particular on its width and peak particle flux. The chaos enters the SOL dynamics through the connection length, which is evaluated using a Poincaré map. The variation of experimentally relevant quantities, such as the SOL gradient length scale and the intermittency of the particle flux in the SOL, is described as a function of the strength of the magnetic perturbation. It is found that the effect of the chaos is to broaden the profile of the sheath-loss coefficient, which is proportional to the inverse connection length. That is, the SOL transport in a chaotic field is equivalent to that in a model where the sheathloss coefficient is replaced by its average over the unperturbed flux surfaces. Both fully chaotic and the flux-surface averaged approximation of RMP application significantly lower maximum parallel particle flux incident on the divertor.Show more Item Erosion of a sharp density interface by homogeneous isotropic turbulence(2019-05) Lagade, Joel Albano; Johnson, Blair AnneShow more Desalination, commonly used for potable water production, generates brines that are ultimately released back into the environment. Desalination brines discharged into coastal regions with weak currents and mild bathymetry, such as the Gulf of Mexico, do not necessarily mix with surrounding natural waters and remain stably stratified (Hodges et al., 2011). Because dense immobile saline layers from these discharges can cause hypoxia and threaten local ecosystems, we are conducting an experimental study to investigate the effect of turbulence on a sharp density interface to identify mechanisms of turbulence that promote and/or inhibit interfacial erosion. There remains a gap in the literature regarding the interaction of mean shear free homogeneous isotropic turbulence with a sharp density interface, a critical component in understanding dynamics across a stably stratified system. To address this fundamental question, we use randomly actuated synthetic jet arrays (RASJA - Variano & Cowen (2008)) to generate homogeneous isotropic turbulence, absent mean shear, above a dense fluid layer. The Richardson number is varied to ascertain the thresholds at which the density interface erodes and mixing between the stratified layers occurs. As in Johnson & Cowen (2018), who characterized the mean shear free turbulent boundary layer at solid and sediment beds, particle image velocimetry is used to complete a statistical analysis of the turbulent flow field at and above density interface. Simultaneous laser induced fluorescence measurements are obtained to capture erosion, sharpening, and mixing. Statistical metrics of the turbulence are coupled with the evolution of concentration profiles and mixing, which is determined by measuring temporally resolved isopycnal displacements. In the current work, we provide the first experimental data to test quantifying entrainment across stratified fluids as described and applied in direct numerical simulation studies by Zhou et al. (2017). By examining the interplay between mean shear free homogeneous isotropic turbulence and a sharp density gradient, we aim to deduce under what environmental conditions it is sustainable to discharge brine into relatively quiescent flows, considering key factors such as ambient turbulence and relative salinity variance between the brine and surrounding waters.Show more Item Experimental investigation of overall effectiveness and coolant jet interactions on a fully cooled C3X turbine vane(2013-05) McClintic, John W; Bogard, David G.Show more This study focused on experimentally measuring the performance of a fully cooled, scaled up C3X turbine vane. Experimental measurements focused on investigating row-to-row interactions of coolant jets and the contributions of external film cooling and internal impingement cooling to overall cooling effectiveness. Overall effectiveness was experimentally measured using a thermally scaled, matched Biot number vane model featuring a realistic internal impingement scheme and had normalized surface temperatures that were representative of those found on engine components. A geometrically identical vane was also constructed out of low conductivity polystyrene foam to measure the normalized adiabatic wall temperature, or adiabatic effectiveness of the film cooling configuration. The vanes featured a full coverage film-cooling scheme with a five-row showerhead and 13 total rows of holes containing 149 total coolant holes. This study was the first study to make highly detailed measurements of overall effectiveness on a fully-cooled vane model and expands on previous studies of adiabatic and overall effectiveness on the showerhead and single rows of holes on a matched Biot vane by considering a fully cooled configuration to determine if the results from these previous studies also hold for a fully cooled configuration. Additionally, velocity and thermal fields were measured just upstream of two different suction side rows of holes in order to study the effect of introducing upstream coolant injection. The effects of mainstream turbulence and span-wise location were examined and at the downstream row of holes, the contributions of different rows of holes to the approach flow were compared. This study was the first to measure mean and fluctuating velocity data on the suction side of a turbine vane with upstream coolant injection. Understanding the effects of how upstream injection affects the performance of downstream rows of holes is critical to understanding the film cooling performance on a fully cooled turbine airfoil.Show more Item Experimental investigation of the performance of a fully cooled gas turbine vane with and without mainstream flow and experimental analysis supporting the redesign of a wind tunnel test section(2013-12) Mosberg, Noah Avram; Bogard, David G.Show more This study focused on experimentally determining the cooling performance of a fully cooled, scaled-up model of a C3X turbine vane. The primary objective was to determine the differences in overall effectiveness in the presence and absence of a hot mainstream flowing over the vane. Overall effectiveness was measured using a thermally scaled matched Biot number vane with an impingement plate providing the internal cooling. This is the first study focused on investigating the effect of removing the mainstream flow and comparing the contour and laterally-averaged effectiveness data in support of the development of an assembly line thermal testing method. It was found that the proposed method of factory floor testing of turbine component cooling performance did not provide comparable information to traditional overall effectiveness test methods. A second experiment was performed in which the effect of altering the angle of attack of a flow into a passive turbulence generator was investigated. Measurements in the approach flow were taken using a single wire hot-wire anemometer. This study was the first to investigate the effects such a setup would have on fluctuating flow quantitates such as turbulence intensity and integral length scale rather than simply the mean quantities. It was found that both the downstream turbulence intensity and the turbulence integral length scale increase monotonically with approach flow incidence angle at a specified distance downstream of the turbulence generator.Show more Item Extending the active model split to compressible flows(2022-08-11) Pederson, Clark Curtis; Moser, Robert deLancey; Oliver, Todd; Goldstein, David; Bogard, David; Ezekoye, OfodikeShow more Hybrid RANS/LES models have consistently shown superior accuracy over RANS in predictions of massively separated flows. However, these models have experienced difficulties with modeled stress depletion, smooth-wall separated flow, and shock-separated flows. The Active Model Split (AMS) hybridization was created to address some of the fundamental shortcomings of traditional hybrid RANS/LES models. While previous work considered incompressible flows, this thesis demonstrates how the AMS framework can be extended to address compressible flows. AMS-based hybrid models for the turbulent heat flux and turbulent transport terms in the total energy equation are presented. This compressible hybrid RANS/LES framework is demonstrated on a subsonic channel, a supersonic channel, an axisymmetric transonic bump, and an impinging shock boundary-layer interaction. Some features, such as shock location and separation length, are seen to improve with the AMS framework. The AMS framework is shown to be resistant to modeled stress depletion, though modeling errors in the mean stress can still occur. In general, skin friction is predicted less accurately with the AMS framework than with steady RANS models. Possible explanations for this are given, and model corrections and future lines of research are suggested.Show more Item Hamiltonian description of Hall and sub-electron scales in collisionless plasmas with reduced fluid models(2018-09-14) Miloshevich, George; Morrison, Philip J.; Hazeltine, Richard; Breizman, Boris; Fitzpatrick, Richard; Gamba, Irene MShow more In MHD magnetic helicity has been shown to represent Gauss linking numbers of magnetic field lines by Moffatt and others; thus it is endowed with topological meaning. The noncanonical Hamiltonian formulation of extended MHD models (that take two-fluid effects into account) has been used to arrive at their common mathematical structure, which manifests itself via the existence of two generalized helicities and two Lie-dragged 2-forms. The helicity invariants play an important role in the second part of thesis dedicated to understanding the directionality of turbulent cascades. Generally speaking, invariants (such as energy) can flow in two directions in a turbulent cascade: forward (towards small scales, leading to dissipation) and inverse (towards large scales), leading to the formation of a condensate. This directionality in extended MHD models is estimated using analytical considerations as well as tests involving 2D numerical simulations. The cascade reversal (transition) of the square magnetic vector potential is found, viz. when the forcing wavenumber exceeds the inverse electron skin depth the square magnetic vector potential starts to flow towards large wavenumbers, as opposed to the typical MHD behavior. In addition, the numerics suggest a simultaneous transition to the inverse cascade of energy in this inertial MHD regime. This is accompanied by the appearance of large scale structures in the velocity field, as opposed to the magnetic field as in the MHD case. Final chapters of the thesis are devoted to devising the action principle for the relativistic extended MHD. First the special relativistic version is discussed, where the covariant noncanonical Poisson bracket is found. This is followed by a short recourse towards describing relativistic collisionless reconnection mediated by the electron thermal inertia (purely relativistic effect). Next, 3+1 splitting inside the Poisson bracket is performed, while only non-relativistic terms are retained. Thus one arrives at nonrelativistic extended MHD bracket with arbitrary ion to electron mass ratio. In conclusion, it is outlined how the Hamiltonian 3+1 formalism can be developed for general relativistic Hall MHD using canonical Clebsch parametrization and some comments are added on possible issues regarding the quasi-neutrality assumption in the model that is used throughout the chapter.Show more Item Investigation of buoyancy effects on turbulant nonpremixed jet flames by using normal and low-gravity conditions(2003-12) Idicheria, Cherian Alex, 1977-; Clemens, Noel T.Show more Item Numerical discretization effects in large eddy simulation of turbulence(2022-06-15) Yalla, Gopal Robert; Moser, Robert deLancey; Engquist, Björn, 1945-; Ghattas, Omar; Heimbach, Patrick; Oliver, Todd; Willcox , KarenShow more Large eddy simulation (LES) is now over half a century old and while it has become more widely used as computational capabilities have expanded, its adoption as an engineering tool has arguably been limited by the shortcomings of subgrid models. Most current subgrid models are formulated under the assumption that the subgrid scales are approximately isotropic, and that other complications, such as numerical discretization and inhomogeneous resolution, are negligible. This limits the fidelity of the models when applied in complex flows. For LES to become a robust engineering tool, subgrid models applicable to more complex scenarios will be required. In particular, the effects of numerical discretization must be considered. In this thesis we develop several analytical and computational tools for identifying the characteristics of an LES introduced by numerical discretization and filtering. First, the effects of numerical dispersion error on the turbulent energy cascade are explored. It is shown that dispersion error due to convection by a large mean velocity causes a decoherence of the phase relationship among interacting Fourier modes, resulting in a reduction of the energy transfer rate from large to small scales. Nonlinear dispersion error due to convection from turbulent fluctuations is also explored through the development of an eddy-damped quasi-normal markovian (EDQNM) type of analysis that is applicable to the filtered turbulence in an LES. EDQNM is shown to be a useful tool for exploring dispersion effects because it exposes the relaxation rate of the third-order velocity correlations. An explicit filtering formulation based on the properties of the underlying numerics is developed to remove the highly dispersive wavemodes in an LES. Further, the EDQNM LES theory is also used to determine the a priori properties of the subgrid stress needed to recover an inertial range spectrum in the presence of non-spectral numerics and non-cutoff explicit filters. Second, the convection of turbulence through nonuniform grids is explored. This introduces additional challenges due to so-called commutation error, or neglect of the commutator of the filtering and differentiation operators. We employ a multiscale asymptotic analysis to investigate the characteristics of the commutator. Further, we show how commutation error manifests in simulation and demonstrate its impact on the convection of homogeneous isotropic turbulence through a coarsening grid. A connection is made between the commutation error and the propagation properties of the underlying numerics. A framework for modeling this commutator is proposed that accounts for properties of the discretization. The forcing of turbulence convecting through a refining grid is also explored and a formulation based on divergence-free wavelets is proposed. Results in isotropic turbulence suggest this formulation may be effective at energizing newly resolvable scales and therefore allowing for sharper grid transitions to finer resolved regions. There are several additional challenges to formulating more broadly applicable subgrid models for LES and we expect that the techniques developed here will also be useful for addressing these wide range of issues.Show more Item On characteristics of stable boundary layer flow fields and their influence on wind turbine loads(2011-08) Park, Jinkyoo; Manuel, Lance; Basu, SukantaShow more Fourier-based stochastic simulation of wind fields commonly used in wind turbine loads computations is unable to account for contrasting states of atmospheric stability. Flow fields in the stable boundary layer (SBL), for instance, have characteristics such as enhanced wind shear and veering wind direction profiles; the influence of such characteristics on utility-scale wind turbine loads has not been studied. To investigate these influences, we use large-eddy simulation (LES) to generate inflow wind fields and to estimate load statistics for a 5-MW wind turbine model. In the first part of this thesis, we describe a procedure employing LES to generate SBL wind fields for wind turbine load computations. In addition, we study how large-scale atmospheric conditions affect the characteristics of wind fields and turbine loads. Next, in the second part, we study the contrasting characteristics of LES-SBL and stochastic NBL flow fields and their influences on wind turbine load statistics by isolating effects of the mean wind (shear) profile and of variation in wind direction and turbulence levels over the rotor sept area. Among large-scale atmospheric conditions, the geostrophic wind speed and surface cooling rate have the greatest influence on flow field characteristics and, thus, on wind turbine loads. Higher geostrophic winds lead to increased mean and standard deviation values of the longitudinal wind speed at hub height. Increased surface cooling rates lead to steeper shear profiles and appear to also increase fatigue damage associated with out-of-plane blade root moments. In summary, our studies suggest that LES may be effectively used to model wind fields in the SBL, to study characteristics of turbine-scale wind fields, and to assess turbine loads for conditions that are not typically examined in design standards.Show more Item Probing giant-planet forming zones around Solar-like stars with CO(2017-08) Yu, Mo, Ph. D.; Kraus, Adam L.; Evans, Neal J.; Dodson-Robinson, Sarah; Willacy, Karen; Lacy, John; Jaffe, Daniel; Bergin, EdwinShow more Protoplanetary disks are dusty disks around young stars where planets are formed. The evolution and composition of protoplanetary disks determine the time, environments and materials available for planet formation. However fundamental properties of protoplanetary disks such as mass, composition, and the angular momentum transfer mechanism are poorly constrained by observations. In this dissertation, we discuss the thermal and chemical evolution of protoplanetary disks around Solar-type stars, and evaluate methods to measure two key parameters - disk mass and turbulent velocity in the framework of an evolving disk system. We first build a chemical evolution model based on an MRI-active disk around a Solar-type star, and discuss the chemical depletion of CO due to the formation of complex organic molecules (Chapter 2). We then investigate the challenges one faces when measuring disk masses with CO due to the chemical depletion of CO and optical depth effects (Chapter 3). We propose strategies to correct for the CO depletion effect and constrain the disk mass within factor of a few accuracy. We also investigate the possibility of constraining turbulent velocities with CO line profiles in Chapter 4. Peak-to-trough ratios of CO rotational lines have been proposed as a robust probe for turbulent velocity. However we show that the peak-to-trough ratio could vary by $25\%$ due uncertainties in effects of CO depletion. One would underestimate the degree of turbulence if the chemical depletion of CO is not properly accounted for.Show more Item Quantitative measurements of ablation-products transport in supersonic turbulent flows using planar laser-induced fluorescence(2015-08) Combs, Christopher Stanley; Clemens, Noel T.; Danehy, Paul M; Ezekoye, Ofodike A; Raja, Laxminarayan; Varghese, Philip LShow more A recently-developed experimental technique based on the sublimation of naphthalene, which enables imaging of the dispersion of a passive scalar using planar laser-induced fluorescence (PLIF), is applied to a Mach 5 turbulent boundary layer and a NASA Orion capsule flowfield. To enable the quantification of naphthalene PLIF images, quantitative fluorescence and quenching measurements were made in a temperature- and pressure-regulated test cell. The test cell measurements were of the naphthalene fluorescence lifetime and integrated fluorescence signal over the temperature range of 100 K to 525 K and pressure range of 1 kPa to 40 kPa in air. These data enabled the calculation of naphthalene fluorescence yield and absorption cross section over the range of temperatures and pressures tested, which were then fit to simple functional forms for use in the calibration of the PLIF images. Quantitative naphthalene PLIF images in the Mach 5 boundary layer revealed large-scale naphthalene vapor structures that were regularly ejected out to wall distances of approximately y/δ = 0.6 for a field of view that spanned 3δ to 5δ downstream of the trailing edge of the naphthalene insert. The magnitude of the calculated naphthalene mole fraction in these structures at y/δ = 0.2 ranged from approximately 1-6% of the saturation mole fraction at the wind tunnel recovery temperature and static pressure. An uncertainty analysis showed that the uncertainty in the inferred naphthalene mole fraction measurements was ± 20%. Mean mole fraction profiles collected at different streamwise locations were normalized by the mole fraction measured at the wall and a characteristic height of the scalar boundary layer, causing the profiles to collapse into one “universal” mole fraction profile. Two-dimensional fields of naphthalene mole fraction were also obtained simultaneously with velocity by using particle image velocimetry (PIV) and PLIF. The images show large-scale naphthalene vapor structures that coincide with regions of relatively low streamwise velocity. The covariance of naphthalene mole fraction with velocity indicates that an ejection mechanism is transporting low-momentum, high-scalar-concentration fluid away from the wall, resulting in the protrusions of naphthalene vapor evident in the instantaneous PLIF images. Lastly, naphthalene PLIF was used to visualize the dispersion of gas-phase ablation products on a scaled Orion capsule model at four different angles of attack at Mach 5. High concentrations of scalar were imaged in the capsule recirculation region. Additionally, intermittent turbulent structures were visualized on the heat shield surface, particularly for the 12° and 52° AoA cases.Show more