Browsing by Subject "Turbulence"

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Advanced modeling of nanoparticle nucleation: towards the simulation of particle synthesis
(2012-11) Liu, Jun
Nanotechnology holds a lot of promise for the discovery of new phenomena, and many of the envisioned processes involve nanoparticles. These particles are found in chemical sensors, drug targeting and delivery, and one important application is motivated by the need of clean renewable energy sources. Gas-to-particle conversion in the form of homogeneous nucleation within flow systems plays a significant role in a variety of natural and industrial processes of nanoparticle synthesis. In this work, nucleation processes of several metal materials and dibutyl phthalate (DBP) nanoparticles in laminar and turbulent flows are investigated via direct numerical simulations (DNS). The flows consist of condensing vapor diluted in argon or nitrogen issuing into a cooler particle-free stream. DNS facilitates probing the interactive effects of fluid dynamics and nucleation in an accurate manner. The fluid, thermal and chemical fields are obtained by solving the Navier-Stokes, enthalpy, and mass transport equations. Nucleation is simulated via calibrated classical homogeneous nucleation models. Recently developed size dependent surface tension model offers increased accuracy in predicting metal particle nucleation. This approach is attractive in that it promises to be more accurate than the classical nucleation theory while maintaining much of its simplicity when coupling with fluid dynamics. The effects of turbulence on metal nucleation are also studied using fully resolved DNS to elucidate the effects of different stages of fluid mixing on metal particle nucleation. The effects of nucleation on fluid dynamics are investigated via DNS of DBP nucleation within both laminar and turbulent jet flows. The simulations provide a demonstration of how heat release affects the interactions of fluid dynamics and nucleation at different Reynolds numbers and particle formation rates. The results provide insights into the interaction of fluid, thermal transport and nanoparticle nucleation in various flows, which stimulate development of models that will allow engineers to optimize the fluid and thermal environments for industrial nanoparticle production. For brevity, specific conclusions are provided in each chapter.
Computational Analysis of Energy Exchange Mechanisms in Turbulent Flows with Thermal Nonequilibrium
(2018-01) Neville, Aaron
The design of hypersonic vehicles is signiﬁcantly aﬀected by the state of boundary layer. Hypersonic boundary layers can be laminar or turbulent, and in chemical and vibrational nonequilibrium, each with diﬀerent length and time scales. In turbulent boundary layers, heating augmentation can be an order of magnitude or higher above laminar heating rates. The scales of the internal energy relaxation processes can be of the same order or greater than the turbulent ﬂow scales, and can interact with turbulent motion. Understanding how turbulent motion and internal energy relaxation interact is relevant to ﬂow control. Fundamental ﬂows are studied to understand how energy is exchanged between turbulent motion and internal energy relaxation. Speciﬁcally, high-ﬁdelity DNS of vibrational energy relaxation eﬀects in compressible isotropic and temporally evolving shear layers are presented. The energy exchange mechanisms are analyzed by decomposing the ﬂow into incompressible and compressible energy modes. By varying the vibrational relaxation rate, the tuning of the relaxation rate to the turbulent ﬂow is studied. Vibrational energy relaxation is demonstrated to be coupled to the turbulent ﬂow through the compressible modes of the gas. Compressions and expansions generate ﬂuctuations in the thermal state, and the vibrational energy lags behind these ﬂuctuations. Energy is then transferred to or from the vibrational energy mode at a rate proportional to the relaxation time, and the ﬂuctuations are damped. Damping of turbulent quantities are strongest when the vibrational relaxation rate is on the order of the turbulent large structure acoustic rate. Wavenumber speciﬁc damping is also observed in isotropic ﬂows when the relaxation time is on the order of the acoustic frequency of the wave. The eﬀects of vibrational relaxation are shown to increase with compressibility. However, the overall eﬀect on turbulent kinetic energy is weak due to the incompressible mode containing signiﬁcantly more energy than the compressible modes.
Copepod response behavior in turbulence
(2014-09) Krizan, Daniel
The objective of this thesis is to determine copepod response to turbulence generated by obstacles in cross flow. Mainly, flow and copepod response downstream a square fractal grid is examined but experiments downstream a cylinder provides comparison. This is done by simultaneously measuring the copepods position and velocity using 3D-PTV in a measurement volume and measuring the two dimensional three component velocity vectors of the flow using stereo PIV. These measurements are done in a way that does not elicit copepod response. Tomographic PIV is done downstream the square fractal grid without copepods to gain volumetric velocity knowledge of the flow in the measurement volume. Copepods are known to execute sudden high speed jumps (or escapes) in response to sensed hydrodynamic signals. The fractal grid was shown to elicit copepod escape, specifically directly downstream with escape frequency decreasing further downstream where turbulence levels were much lower. It was found that at a slower freestream speed copepods exhibited jumps not in reaction to flow disturbances but to reorient themselves (cruise swimming). There was almost no copepod response in the wake of a cylinder, but copepods again exhibited cruise swimming behavior at a slower freestream speed. In regions with high maximum principal strain rate (MPSR) downstream of the fractal grid, copepods were observed to exhibit multiple escapes. Moreover, copepods were observed to jump towards regions of lower turbulence and against the freestream direction. From stereo PIV, instantaneous 2D MPSR values of less than 3s^(-1) were shown to create escape in 60% of copepod escapes analyzed. Finally, it was found that on average larger MPSR resulted in larger jumps from copepods.
Development and Validation of a Turbulence Wall Model for Compressible Flows with Heat Transfer
(2016-08) Komives, Jeffrey
The computational cost to model high Reynolds number flows of engineering interest scales poorly with problem size and is excessively expensive. This fact motivates the development of turbulence wall models to lessen the computational burden. These models aim to provide accurate wall flux quantification on computational meshes that would otherwise be unable to accurately estimate these quantities. The benefit of using such an approximation is that the height of the wall-adjacent computational elements can be increased by one to two orders of magnitude, allowing for comparable increases in stable explicit timestep. This increase in timestep is critically necessary for the large eddy simulation of high Reynolds number turbulent flows. To date, most research in the application of wall models has focused on incompressible flows or flows with very weak compressibility. Very few studies examine the applicability of wall models to flows with significant compressibility and heat transfer. The present work details the derivation of a wall model appropriate for compressible flows with heat transfer. The model framework allows for the inclusion of non-equilibrium terms in the determination of wall shear and heat transfer. The model is applied to a variety of supersonic and hypersonic flows, and is studied in both Reynolds-averaged simulations and large eddy simulations. The impact of several modeling approaches and model terms is examined. The wall-modeled calculations show excellent agreement with wall-resolved calculations and experimental data. For time accurate calculations, the use of the wall model allows for explicit timesteps more than 20 times larger than that of the wall-resolved calculation, significantly reducing both the cost of the calculation and the time required converge the solution.
Direct numerical simulation and global stability analysis of boundary layer flows past roughness elements
(2022-08) Ma, Rong
Understanding wall roughness effects on flows is important in engineering applications. The objective of this dissertation is to increase understanding of roughness effects on transitional and turbulent boundary layers. The rough-wall flow is studied by performing direct numerical simulations (DNS) of the Navier-Stokes equations and global linear stability analyses. DNS of turbulent channel flow with a random-rough bottom wall is performed at friction Reynolds number $Re_{\tau}=400$ and $600$. The rough surface corresponds to the experiments of Flack et al. \cite{flack2019skin}. The computed skin friction coefficients and the roughness functions show good agreement with experimental results. The double-averaging methodology is used to investigate mean velocity, Reynolds stresses, dispersive flux, and mean momentum balance. The roll-up of the shear layer on the roughness crests is identified as a primary contributor to the wall-normal momentum transfer. The mean-square pressure fluctuations are increased within the inner layer and a phenomenological explanation is suggested by examining the dominant source terms in the pressure Poisson equation. The rapid term shows that high pressure fluctuations observed in front of and above the roughness elements are mainly due to the attached shear layer formed above the protrusions. The slow term makes a relatively smaller contribution, and is primarily increased in the troughs and in front of the roughness elements, corresponding to the occurrence of quasi-streamwise vortices and secondary vortical structures. The mean wall shear on the rough surface is highly correlated with the roughness height, and depends on the local roughness topography. Events with comparable magnitudes of the streamwise and spanwise wall-shear stress occur more frequently, corresponding to a more isotropic vorticity field in the roughness layer. The boundary layer transition due to an isolated roughness element is studied using global stability analysis and DNS. The large ratio of element height to displacement boundary layer thickness $(h/\delta^*)$ is considered to model a trip at an early location in the boundary layer. The cubical trip geometries with two aspect ratios ($\eta$) are investigated. Both steady base flows and time-averaged mean flows are able to capture the frequencies of the primary vortical structures and mode shapes. Global stability results highlight that although the varicose instability is dominant for large $h/\delta^*$, sinuous instability becomes more pronounced as $Re_h$ increases for the thin geometry ($\eta=0.5$), due to increased spanwise shear in the near-wake region. Wavemaker results indicate that $\eta$ affects the convective nature of the shear layer more than the type of instability. DNS results show that different instability mechanisms lead to different development and evolution of vortical structures in the transition process. For $\eta=1$, the varicose instability is associated with the periodic shedding of hairpin vortices, and its stronger spatial transient growth indicated by wavemaker results aids the formation of hairpin vortices farther downstream. In contrast, for $\eta=0.5$, the interplay between varicose and sinuous instabilities results in a broader-banded energy spectrum and leads to the sinuous wiggling of hairpin vortices in the near wake when $Re_h$ is sufficiently high. A sinuous mode with a lower frequency captured by dynamic mode decomposition (DMD) analysis, and associated with the ‘wiggling’ of streaks persists far downstream and promotes transition to turbulence. A new regime map is developed to classify and predict instability mechanisms based on $Re_{hh}^{1/2}$ and $d/\delta^*$ using a logistic regression model. Although the mean skin friction demonstrates different evolution for the two geometries, both of the two geometries efficiently trip the flow to turbulence at $Re_h=1100$. An earlier location of a fully-developed turbulent state is established for $\eta=1$ at $x \approx 110h$. The influence of roughness spacing on boundary layer transition over distributed roughness elements is studied using direct numerical simulation and global stability analysis, and compared to the isolated roughness element. Small spanwise spacing $\lambda_z=2.5h$ inhibits the formation of counter-rotating vortices (CVP), as a result, the hairpin vortices are not generated and the downstream shear layer is steady. For $\lambda_z=5h$, the CVP and hairpin vortices are induced by the first row of roughness, perturbing the downstream shear layer and causing transition. Although the periodicity of the primary hairpin vortices seems to be independent on the streamwise spacing, the distributed roughness leads to lower critical $Re_h$ for instability to occur and more significant breakdown of boundary layer compared to the isolated roughness. When the streamwise spacing is comparable to the region of flow separation ($\lambda_x=5h$), the high-momentum fluid hardly moves downward into the cavities and the wake flow has little impact on the following roughness elements. The leading unstable varicose mode is associated with the central low-speed streaks along the aligned roughness elements, and its frequency is close to the shedding frequency of hairpin vortices in the isolated case. For larger streamwise spacing ($\lambda_x=10h$), two distinct modes are obtained from global stability analysis. The first mode shows varicose symmetry, corresponding to the primary hairpin vortex shedding induced by the first-row roughness. The high-speed streaks formed in the longitudinal grooves are also found to be unstable and interacting with the varicose mode. The second mode is a sinuous mode with lower frequency, induced as the wake flow of the first-row roughness runs onto the second row. It extracts most energy from the spanwise shear between the high- and low-speed streaks.
Discrete roughness effects on high-speed boundary layers
(2015-01) Iyer, Prahladh Satyanarayanan
This dissertation studies the effects of a discrete roughness element on a high-speed boundary layer using Direct Numerical Simulations (DNS) on unstructured grids. Flow past a cylindrical roughness element placed perpendicular to the flow and a hemispherical bump is studied. A compressible linear stability theory (LST) solver for parallel flows is developed based on the algorithm by Malik [33] and validated for a range of Mach numbers ranging from incompressible to Mach 10. The evolution of the perturbations from DNS is validated with the linear stability solver making the DNS algorithm suitable to study transition problems. Flow past a cylindrical roughness element at Mach 8.12 is simulated using DNS and the velocity profiles in the symmetry and wall--parallel planes are compared to the experiments of Bathel et al. [7]. The flow remains steady and laminar, and does not transition. Overall, good agreement is observed between DNS and experiments, thus validating our algorithm to study effect of roughness on high-speed flows. However, differences are observed in the separation region upstream and recirculation region downstream of the roughness. The DNS results are used to quantify possible uncertainties in the measurement technique as suggested by Danehy [20]. The effect of upstream injection (5% of the free-stream velocity) is also simulated to quantify its effects on the velocity profiles to mimic the injection of NO into air in the experiment. While the boundary layer thickness of the flow increases downstream of the injection location, its effect on the velocity profiles is small when the profiles are scaled with the boundary layer thickness.Flow past a hemispherical bump at Mach 3.37, 5.26 and 8.23 are simulated using DNS with the flow conditions matching the experiments of Danehy et al. [19] to understand the different flow features associated with the flow and the physical mechanism that causes the flow to transition to turbulence. It is observed that the Mach 3.37 and 5.26 flows transition to turbulence while the Mach 8.23 flow remains laminar downstream of the roughness element. The roughness element used in this study is large since the boundary layer thickness of the laminar boundary layer at the location of the roughness is smaller than the roughness height.The Mach 3.37 flow undergoes transition closer to the bump when compared to Mach 5.26, in agreement with experimental observations. Transition is accompanied by an increase in C_f and C_h (Stanton number). Even for the case that did not undergo transition (Mach 8.23), streamwise vortices induced by the roughness cause a significant rise in C_f until 20D downstream. Mean Van-Driest transformed velocity and Reynolds stress for Mach 3.37 and 5.26 shows good agreement with available data. The transition process involves the following key elements - Upon interaction with the roughness element, the boundary layer separates to form a series of spanwise vortices upstream of the roughness, and a separation shear layer. The system of spanwise vortices wrap around the roughness element in the form of horseshoe/necklace vortices to yield a system of counter-rotating streamwise vortices downstream of the element. These vortices are located beneath the separation shear layer and perturb it, which results in the formation of trains of hairpin-shaped vortices further downstream of the roughness for the cases that undergo transition. These hairpins spread in the span with increasing downstream distance and the flow increasingly resembles a fully developed turbulent boundary layer. A local Reynolds number based on the wall properties is seen to correlate the onset of transition for the cases considered.To assess the effect of roughness height on transition, a Mach 3.37 flow past a hemispherical bump is studied by varying the boundary layer thickness (k/delta = 2.54, 1.0, 0.25 & 0.125) where k is the roughness height and delta is the laminar boundary layer thickness at the location of the roughness. Transition occurs in all cases, and the essential mechanism of transition appears to be similar. At smaller boundary layer thickness, multiple trains of hairpin vortices are observed immediately downstream of the roughness, while a single train of hairpin vortices is observed at larger delta. This behavior is explained by the influence of the boundary layer thickness on the separation vortices upstream of the roughness element. Also, hairpin vortices that form downstream of the roughness initially scale with the height of the roughness element and further downstream, begin to scale with the boundary layer thickness, thus causing the entire boundary layer to transition. Dynamic Mode Decomposition of the pressure field for k/delta= 1 and 0.125 is used to obtain the frequency of shedding of hairpin vortices.
The effects of physical variables on zooplankton distributions in stratified lakes.
(2007-10) Spitael, Maria Susan
Zooplankton play a vital role in lake ecosystems. They serve as an important food source for fish, as well as being major consumers of algae, which contributes to greater water clarity. To understand the dynamics in a lake, it is necessary to understand zooplankton and how they are affected by the physical environment around them. The purpose of this research was to address the question of how turbulence and temperature stratification affect zooplankton aggregations in lakes. Laboratory experiments were performed to quantify the effects of temperature and turbulence on zooplankton distributions in a stratified tank. These measurements were designed to measure zooplankton aggregations and to provide detailed information on the physical conditions causing them. Comprehensive field measurements were taken throughout one summer, covering five 24-hour periods, in order to investigate the effects of temperature and turbulence on zooplankton aggregations in the field. A high-frequency sonar measurement device was developed to take the measurements by modifying the output of a commercial fish-finder and calibrating it to match zooplankton net counts. Our results showed that zooplankton distributions are strongly affected by temperature and turbulence, and that these effects are species-specific, and are different between day and night.
Experimental evidence for statistical scaling and intermittency in sediment transport rates
(University of Minnesota. Institute for Mathematics and Its Applications, 2009-02) Singh, Arvind; Fienberg, Kurt; Jerolmack, Douglas J.; Marr, Jeffrey D.G.; Foufoula-Georgiou, Efi
Experimental Investigation of Homogeneous Anisotropic Turbulence
(2019-07) Carter, Douglas
Motivated by the need to substantiate the existing literature on homogeneous turbulence with experimental data, a novel zero-mean homogeneous turbulence chamber is presented. Despite the anisotropy of the large scale velocity fluctuations, the experimental apparatus is found to well approximate homogeneous, shear-less turbulence over scales larger than the integral lengths of the flow for four separate cases at Reynolds numbers (based on the Taylor microscale) ranging between 154 and 412. This enables a detailed investigation of the turbulence statistics as obtained by 2D particle image velocimetry, which confirms the existence of inertial scaling ranges in both the second-order structure functions and energy spectra. It is found that the anisotropy of the flow persists down to the smallest scales, though its influence decreases with decreasing scale. The coherent structures, identified using a percolation analysis, are however isotropic in their geometry and generally collapse across cases using the Taylor microscale as a normalization length scale. Two types of scale interaction analyses are applied to the turbulent fields and indicate that there exists substantial coupling between scales small and large; challenging the classic assumption that a range of scales might emerge which is independent of the large scales. Employing the generalized Karman-Howarth-Monin equation in scale space, the energy cascade is found to move energy downscale across all cases, which is also confirmed using a filter space technique. The magnitude of the non-linear energy transfer in scale space is however found to be increasingly anisotropic for increasing large-scale RMS velocity ratio u'_1/u'_2. Using a conditional sampling procedure based on the activity of the small-scales, the non-linear energy transfer is found to have a strong dependence on the relative small-scale activity (as well as the presence of coherent structures), which causes enhanced downscale non-linear energy transfer or upscale non-linear energy transfer of moderate magnitude depending on the subset. In addition to showcasing accurate PIV measurements of homogeneous turbulence over a large range of scales, the results point to the complex nature of the energy cascade in the jet-array driven facility, with simultaneous upscale and downscale transfers at each instant as well as spatially concurrent interactions across all scales of the flow.
Flow Boiling of A Dilute Emulsion In the Transition Regime
(2020-05) Waikar, Ameya
This investigation investigates heat transfer of water and flow boiling of dilute emulsion in transition and turbulent regime. The gap heights for microgap of 500 and 1000 μm and nominal Reynolds number of 1600 and 2800. The emulsion in this study is an oil-in-water emulsions, where FC-72 is the oil whose droplets are suspended in water. The volume fractions for the emulsions are 1% and 2%. The heated test section is smooth. For single phase experiments, the heat transfer coefficient of water with increasing Reynolds number and decreasing the hydraulic diameter. The Nusselt number in the single-phase region is correlated to the Reynolds number, Prandtl number and aspect ratio of the channel. The Nusselt number varies linearly with ????????????ℎ.????????.????ℎ???? . In emulsion heat transfer on the smooth surfaces, the value of the heat transfer coefficient increases only for a volume fraction of 2% of the disperse component under certain conditions. Reducing the concentration to 1% provides no additional benefit and decreases heat transfer coefficient for all gap sizes and Reynolds number. The 2% emulsion has a larger overall heat transfer coefficient than that in water for lower hydraulic diameter and higher Reynolds number. The heat transfer coefficient increases with increasing wall temperature and plateaus at higher wall temperatures. The interaction between turbulence and boiling is also an area of interest in this investigation. When the emulsion boils, there is enhanced mixing in the flow, also leading to further agitation of the flow causing more turbulence. There is significant increase in pressure drop for the 2% emulsion with increasing wall temperature. Based on these observations and the previously suggested heat transfer mechanism, the following mechanisms are posited: conduction in thin film of FC-72 which reduces the heat transfer due to lower conductivity of FC-72; enhanced mixing due to boiling of FC-72 which increases heat transfer; and the boiling further increases the turbulence, enhancing the convection of the flow. These effects are quantified by correlations developed by using seven different non-dimensional parameters, and an empirical correlation is derived for calculating the heat transfer coefficient for the emulsion. The correlation is a good fit with 93.8% of data lying within ±30% of the predicted values. Further conclusions about the mechanisms involved in the flow boiling of emulsions have been made, and the data set for the flow boiling of emulsions has been further expanded into transitional and turbulent regimes.
Flow characterization on a thin film spinning apparatus
(2014-09) Alvarado, Alonso Antonio
In industrial milling operations that use comminution and wet-comminution techniques, the reduction of the particle size is usually achieved through crushing the sample with a material harder than the product. These methods are convenient when the required median particle size is above 400 um. However, to obtain post-milling particle distributions with 85% sub-micron particles (in number) is both energy intensive, and time consuming. For conventional milling machines, to have the required output in several ton/hr of a product, having a large number of particles in the micron or sub-micron sizes at an affordable rate is cumbersome.Here, a wet-comminution machine that has shown to achieve the aforementioned milestones in the laboratory scale is studied. However, when the machine is scaled to industrial processes, it was recorded that some of the product variables are difficult to scale. In these studies, we attempt to understand the mechanisms by which this machine operates in order to achieve successful scaling. The apparatus operates completely on fluid mechanics principles, it consists of two concentric cylinders, the inner cylinder that has a smaller radius than the outer, rotates while the larger is held stationary. The inner cylinder is also shorter in length than outer, hollow in the inside and has transversal holes where the shaft attaches to the apparatus. The apparatus can operate in batch condition, where the liquid volume is much less than the volume of the apparatus, typically 0.3Vt, 0.42Vt and 0.54Vt. In addition, the apparatus can operate with throughflow, which the upper plate covering the apparatus is reduced in radius.Two component Laser Doppler Velocimetry (LDV) was used to obtain even-time averaged statistics of the azimuthal and axial velocities, in the gap and underneath the impeller. Also, Flow Visualization using Kalliroscopic particles was performed as means of observing large scale structures in the gap. Moreover, single plane Particle Image Velocimetry (PIV) was used to acquire statistics of the axial and radial velocities in the gap, and both underneath as well as above the inner cylinder.It was found that at both throughflow conditions, the topology of the apparatus creates a free spinning boundary both at the bottom and above the inner cylinder. Near the bottom, the thickness of the boundary was found to decrease with Reynolds number to a limiting value, where Re; is based on gap thickness and inner cylinder tip speed. For Re > 2546, the liquid/air interface thickness is constant for a given holding volume. In the regions above and underneath the inner cylinder, corner vortices were detected; if viewing the left-hand-side, the lower one rotating counter clockwise, while the upper rotates clockwise. The thickness of these vortices was found to be constant for various axial flows at Re = 1110 and 2230. The radial length scale of the stationary vortices was found to be ~2.5d;.The flow generated inside of the gap was characterized to have Taylor vortex signatures. It was found that the length scale of the Taylor vortices in the gap is rather insensitive to Reynolds number or holding volume ratio. The average vortex pair wavelength; was found to be 3.6d. Average flow statistics in batch condition indicate that in the gap, at Re = 1110 and 2230, the azimuthal velocity is 0.5U over much of the length. Similarly, it was found that the net axial flow through the gap is close to zero.
High-fidelity unstructured overset simulation of complex turbulent flows
(2023-05) Morse, Nicholas
The goal of this dissertation is to provide insight into the underlying physics of two sets of complex flows: (i) the axisymmetric and appended DARPA SUBOFF and (ii) tabbed jets in crossflow. The accurate simulation of the flow around marine vessels such as the DARPA SUBOFF is critical for maneuvering predictions, which are inherently challenging due to the characteristically large Reynolds numbers, the complex geometries of the hull, appendages, and propeller, and the unsteady flow-fields, which consist of turbulent boundary layers with pressure gradients, curvature, junction flows, and separations. The understanding of jets in crossflow (JICFs) is important for a variety of applications, and there has been significant interest in designing passive devices to control the mixing and penetration characteristics of the jet, although the specific effects of these devices are not well understood. The unstructured overset method of Horne and Mahesh [1,2] provides the flexibility to perform large-eddy simulations (LES) and direct numerical simulations (DNS) to extract valuable physical insights from these flows.First, wall-resolved LES is performed to study flow about the axisymmetric DARPA SUBOFF hull at a Reynolds number of 1.1×10^6 based on the hull length and free stream velocity. To gain an understanding of the streamline curvature and pressure gradient effects of the hull’s turbulent boundary layer (TBL), the axisymmetric Reynolds-averaged Navier-Stokes equations are derived in an orthogonal coordinate system aligned with streamlines, streamline-normal lines, and the plane of symmetry. Analysis in this frame of reference provides a new perspective on curved TBLs, and has numerous practical benefits, including the orthogonality of the streamline-normal coordinate to the hull surface and to the free stream velocity far from the body, which is critical for studying bodies with concave streamwise curvature. In the potential flow outside the boundary layer, the momentum equations in the streamline coordinate frame naturally reduce to the differential form of Bernoulli’s equation and the s-n Euler equation for curved streamlines. In the curved laminar boundary layer at the front of the hull, the streamline momentum equation represents a balance of the streamwise advection, streamwise pressure gradient, and viscous stress, while the streamline-normal equation is a balance between the streamline-normal pressure gradient and centripetal acceleration. At the mid-hull TBL, the curvature terms and streamwise pressure gradient are negligible, and the results conform to traditional analysis of flat plate boundary layers. Finally, the thick stern TBL causes the curvature and streamwise pressure gradient terms to reappear to balance the turbulent and viscous stresses. This balance is used to explain the characteristic variation of static pressure observed for thick boundary layers at the tails of axisymmetric bodies. Next, trip-resolved LES of the DARPA SUBOFF is performed to investigate the extent to which the details of tripping affect the development of TBLs in model-scale studies, which are limited to moderate Reynolds number TBLs. In particular, four cases are studied at length-based Reynolds numbers of 1.1×10^6 and 1.2×10^6: the bare hull and appended SUBOFF with a resolved experimental trip wire geometry, and the same cases tripped using a simple numerical trip (wall-normal blowing), which serves as an example of artificial computational tripping methods often used in practice. When the trip wire height exceeds the laminar boundary layer thickness, LES reveals that shedding from the trip wire initiates transition, and the near field is characterized by an elevation of the wall-normal Reynolds stress and a modification of the turbulence anisotropy and mean momentum balance. This trip height also induces a large jump in the boundary layer thickness, which affects the rate at which the TBL grows and how it responds to pressure gradients and curvature. In contrast, a trip wire height shorter than the laminar boundary layer thickness is shown to initiate transition at the reattachment point of the trip-induced recirculation bubble. The artificial trip reasonably replicates the resolved trip wire behavior. For each case, the inner layer collapses rapidly in terms of the mean profile, Reynolds stresses, and mean momentum balance. This is followed by the collapse of the Reynolds stresses in coordinates normalized by the local momentum thickness, which proves to be a more robust outer scale than the 99% thickness due to its lower sensitivity to the over-tripped wake at the edge of the boundary layer. The importance of tripping the model appendages is also highlighted, due to their lower Reynolds numbers and susceptibility to laminar separations. Finally, DNS of a JICF with a triangular tab at two positions are performed at jet-to- crossflow velocity ratios of R = 2 and 4 with a jet Reynolds number of 2000 based on the jet’s bulk velocity and exit diameter. DNS and dynamic mode decomposition reveal that a tab on the upstream side of the jet produces Lambda-shaped streamwise vortices in the upstream shear layer (USL), while a tab placed 45 degrees from the upstream side produces a tertiary vortex for R = 4, which is not present at R = 2. For the upstream tab, the presence of streamwise vortices curled around the spanwise USL vortices provides an explanation for the improvements in mixing and spreading associated with an upstream tab. This streamwise vortex structure shows remarkable similarities to the ‘strain- oriented vortex tubes’ observed for disturbed plane shear layers. In contrast, the tab placed 45 degrees from the upstream position produces significantly different effects. At R = 4, the jet cross-section is significantly skewed away from the tab and a tertiary vortex is formed, as observed in past experiments on round JICFs at relatively high R and low Reynolds numbers. The 45 degree tab produces asymmetric effects in the wake of the jet at R = 2, but the effect on the jet cross-section is much smaller, highlighting the sensitivity of jets at high R to asymmetric perturbations.
Materials to re-create Direct Observations of Coastally-Generated Near-Inertial Waves During a Wind Event
(2024-10-22) Kelly, Samuel; smkelly@d.umn.edu; Kelly, Samuel; University of Minnesota Duluth, Large Lakes Observatory
A recent manuscript examined near inertial currents and internal waves in Lake Superior using shipboard observations. This submission includes the Matlab scripts and functions to analyze the observations and recreate the figures in the manuscript.
Microalgal swimming in fluid environments: experimental and numerical investigations
(2013-09) Chengala, Ahammed Anwar
The objective of this research was to examine the effects of small-scale fluid motion on the kinetic behavior and some key physiological aspects of Dunaliella primolecta Butcher (D. primolecta / Dunaliella). D. primolecta, a fast growing microalga, is a promising organism for alternative energy production because of its capability to accumulate significant amount of "lipids", a major prerequisite for commercial production of microalgal oil-derived biofuel. For kinetic response studies of Dunaliella, flow visualization and quantification techniques such as Particle Image Velocimetry (PIV) and Digital Holographic microscopy were employed. The two-dimensional PIV results showed that Dunaliella were influenced by the fluid flow as soon as the local (or ambient) flow velocities surrounding the cells exceeded the individual (flow subtracted) swimming velocity of Dunaliella. Further inspection of the swimming characteristics of Dunaliella under shear flow in a three-dimensional holography revealed that Dunaliella preferred to swim cross-stream (i.e. also the direction of local vorticity) when the shear flow exceeded a critical value, and this resulted in Dunaliella dispersing in a thin two-dimensional horizontal layer. The cell body rotation was absent during this display in shear flow, although the cell body rotation was evident while swimming in stagnant fluid. A physical model was developed that provided a possible explanation for the cell orienting and swimming in the cross-stream direction in a shear flow while cell body remained irrotational. The experimental swimming data also showed good agreement with the computational results. In order to investigate the biochemical composition and some physiological aspects in Dunaliella under different flow conditions, a laboratory bioreactor equipped with speakers was utilized. The fluid flow velocities in the proximity of the cells generated by the speaker bioreactor are observable in natural water ecosystems. The results showed that the flow condition with the highest turbulence investigated favored the growth and lipid accumulation in Dunaliella.
Numerical modeling of turbulent flows in arbitrarily complex natural streams.
(2010-08) Kang, Seok Koo
An efficient and versatile numerical model is developed for carrying out high-resolution simulations of turbulent flows in natural meandering streams with arbitrarily complex, albeit fixed, bathymetry and instream hydraulic structures. The numerical model solves the three-dimensional, unsteady, incompressible Navier-Stokes and continuity equations in generalized curvilinear coordinates. This model can handle the arbitrary geometric complexity of natural streams by using the sharp-interface curvilinear immersed boundary (CURVIB) method. To enable efficient simulations on grids with tens of millions of nodes in long and shallow domains typical of natural streams, the algebraic multigrid method (AMG) is used to solve the Poisson equation for pressure. Free-surface is treated either with the rigid-lid approach or modeled using a two-phase flow approach implemented using level-sets. Depending on the desired level of resolution and available computational resources, the numerical model can either simulate turbulence via direct numerical simulation (DNS), large-eddy simulation (LES) or unsteady Reynolds-averaged Navier-Stokes (URANS) simulation. The numerical model is validated by simulating several test cases for which good quality laboratory data or benchmark simulations are available in the literature. The potential of the model as a powerful tool for simulating energetic coherent structures in turbulent flows in natural river reaches is demonstrated by applying it to carry out LES and URANS simulations in a field scale natural-like meandering stream, Outdoor StreamLab, at resolution sufficiently fine to capture vortex shedding from cm-scale roughness elements on the bed. Comparisons between the simulated mean velocity and turbulence kinetic energy fields with field-scale measurements are reported and show that the numerical model can capture all features of the measured flow with high accuracy. Furthermore, the simulated flowfields are analyzed to elucidate the multi-faceted physics of the flow in a natural stream with pool-riffle sequences and to uncover the underlying physical mechanisms. The simulations provide new insights into the role of large-scale roughness in flow through riffles and elucidate the three-dimensional structure, interactions and governing mechanisms of the inner and outer bank secondary flow cells and recirculation zones in the pools. Moreover, the simulations underscore the role of turbulence anisotropy throughout the stream and suggest important links between stream hydrodynamics and morphodynamics. Calculations are also carried out for the same meandering stream with an instream structure installed along its outer bank to demonstrate the utility of the model as a powerful tool for developing science-based design guidelines for stream restoration.
Numerical Simulation Of The Atmospheric Boundary Layer Over Complex Topography: A Modern Approach To A Classical Problem
(2020-05) Andersen, Noah
Numerical methods were developed and validated to simulate the atmospheric boundary layer (ABL) using large eddy simulation (LES). This framework captures the topography of the Earth’s surface rather than modeling it. To robustly simulate the ABL, four unique capabilities (temperature transport, topographic data, immersed boundary method with wall modeling, and turbulent inflow generation) were added to a traditional finite difference computational fluid dynamics code. The accuracy of each capability was analyzed individually using validation tests. Then, a full scale simulation of the ABL over a tidal inlet was conducted. It was found that the resolved topography of the Earth’s surface had a significant effect on the flow field. Furthermore, it was found that the results from LES are more accurate than mesoscale simulations. Lastly, it was found that the errors in the present simulation are a result of the roughness model used over the sea surface.
Numerical simulations of high speed turbulent jets in crossflow.
(2012-08) Chai, Xiaochuan
On variable-density subgrid effects in turbulent flows
(2018-11) GS, Sidharth
Eulerian mass density variations in a flow relate to compressibility and material inhomogeneities in the fluid. These variations can be caused due to high flow speeds, heat transfer, thermo-chemical reactions and/or phase change. From a local perspective, density gradient in space affects the velocity gradient dynamics due to variable inertia, in the presence of pressure-gradient driven acceleration, and therefore indirectly, the dissipation rate of kinetic energy and enstrophy. In turbulent flows, density variations and their effects on the velocity field influences the interscale interactions. Of particular interest is the turbulent dynamics in the presence of large vorticity generation by baroclinic torque. Although these effects are usually transient (in space or time) as turbulent mixing homogenizes the density field, the deviation from constant-density dynamical evolution can be statistically significant, particularly in instability-dominated flows with high sensitivity to initial/boundary conditions. In unsteady reacting flows, sustained chemi-acoustic interactions result in turbulent vorticity dynamics that is markedly different from the well-studied incompressible constant-density turbulence. Large-eddy simulations of high Reynolds number variable-density flows require adequate representation of unresolved small-scale variable-density effects. The present work is an effort to understand subgrid-scale (SGS) variable-density effects to improve the fidelity and accuracy of our simulations in these regimes. The thesis focuses on Reynolds-filtered governing equations to compute the large-scale vorticity dynamics more precisely. A novel equation set for coarse-grained mass, momentum and energy is derived that employs only second order moment based closures, and allows explicit representation of subgrid-scale compressibility and inertial effects. The new form of the filtered equations has terms that represent the SGS mass flux, pressure-gradient acceleration, and velocity-dilatation correlation. We attempt to quantify the dynamical significance of these terms with direct numerical and large eddy simulations.
Settling Velocity of Snow With Varying Morphology In the Atmospheric Turbulence
(2020-06) Lim, Kaeul
Here we present the field measurements of snow morphology as well as its corresponding settling velocity based on the data collected from multiple deployments at EOLOS Wind Energy Research Field Station at Rosemount, MN, USA. All the deployments were conducted at night, allowing us to implement the in situ large-scale particle image velocimetry (PIV) and particle tracking velocimetry (PTV) to quantify the turbulent flow field and snow particle settling velocity in a sampling area on the order of 10 m as reported by Nemes et al. [J. Fluid Mech. 2017] and Heisel et al. [J. Fluid Mech. 2018]. The general micrometeorological conditions were provided by a 130 m meteorological tower (met-tower) equipped with 4 sonic anemometers and 6 low frequency humidity and temperature sensors at the field site, and the turbulence characteristics were measured using both the met-tower sonics and in situ PIV. In addition, the snow particle size and morphology were captured using digital in-line holography (DIH). Snow particle terminal velocity in quiescent flow is calculated corresponding to the snow morphology based on the modified drag coefficient empirical formulas. The settling velocity of snow particles was captured using in situ PIV or PTV during each deployment. The turbulence conditions from the available deployments varied by an order of magnitude in the Taylor-scale Reynolds number, covering a range of Stokes number and different cases of snow particle-turbulence interaction phenomenology. The snow morphological effect on snow fall speed is quantitatively observed corresponding to the snow particle classification. The comparisons of snow particle fall speed and settling velocity quantify settling velocity enhancement by turbulence. Overall, the presented study focuses on a better understanding of the effects of snow morphology and atmospheric turbulence on the settling velocity of snow in nature.
Shipboard turbulence and temperature profiles from the Near Inertial Coastal Experiment (NICE), 2017-2019
(2023-01-27) Kelly, Samuel M; smkelly@d.umn.edu; Kelly, Samuel M; University of Minnesota Duluth, Large Lakes Observatory
The Near Inertial Coastal Experiment (NICE) observed the physical properties of western Lake Superior from 2016-2020. The aim was to observe near-inertial internal waves and vertical mixing due to turbulence. Using the R/V Blue Heron as an observing platform during the summers of 2017, 2018, and 2019, a Rockland Scientific VMP-250 recorded 5,813 profiles of temperature, salinity, fluorescence, turbidity, and the turbulent kinetic energy dissipation rate, and an RBR fastDuet recorded 3,974 profiles of temperature. These profiles are archived here. Additional underway data from the R/V Blue Heron is available at the Rolling deck 2 Repository (R2R; https://www.rvdata.us/search/vessel/Blue%20Heron). Moored data from the NICE experiment was documented by Austin and Elmer (2022).

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