Analysis of the impact of impeller geometry on the turbulent statistics inside a fan-stirred, cylindrical flame speed vessel using PIV

2012 ◽  
Vol 54 (1) ◽  
Author(s):  
Sankaranarayanan Ravi ◽  
Scott J. Peltier ◽  
Eric L. Petersen
Keyword(s):  
Flame Speed ◽  
Turbulent Statistics ◽  
The Impact ◽  
Impeller Geometry

Characterizing Surface Roughness Effects on Supersonic Turbulent Boundary Layers

2021 ◽  
Author(s):  
Rozie Zangeneh
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Practical Importance ◽  
Rough Wall ◽  
Reynolds Stresses ◽  
Roughness Effects ◽  
Mean Velocity ◽  
Modeling Tools ◽  
Turbulent Statistics ◽  
The Impact

Abstract Flight vehicles traveling at supersonic or hypersonic speeds are vulnerable to the onset of surface roughness, which can result in changes in the state of the boundary layer, ultimately affecting the performance of the vehicle. While the majority of the wetted surface area of a vehicle is relatively smooth, every vehicle will contain roughness on some level. The concept of similarity between smooth- and rough-wall flows is of great practical importance as most computational and analytical modeling tools rely on it either explicitly or implicitly in predicting flows over rough walls. While a number of important questions have yet to be answered, significant progress has been made in the understanding of flows over rough surfaces in recent years. This paper will be conducting numerical research in rough-wall-bounded turbulent flows in supersonic regimes. Wall-modeled Large Eddy Simulation (WMLES) on a flat plate with various roughness ratios will be conducted at M∞ = 2 to evaluate the boundary layer responses. These responses will be characterized in ensemble averaged mean velocity characteristics as well as turbulent intensity responses through the Reynolds Stresses. The second goal is to characterize the streamwise development of mechanical distortions in the domain. In addition, the near-wall coherent structures will be analyzed to determine the impact of roughness effects. The mean and turbulent statistics scaled by the roughness friction velocity will be compared to other results.

scholarly journals The Influence of Idealized Heterogeneity on Wet and Dry Planetary Boundary Layers Coupled to the Land Surface

2005 ◽  
Vol 62 (7) ◽  
pp. 2078-2097 ◽  
Author(s):  
Edward G. Patton ◽  
Peter P. Sullivan ◽  
Chin-Hoh Moeng
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Convective Boundary Layer ◽  
Land Surface ◽  
Surface Flux ◽  
Scalar Fields ◽  
Surface Fluxes ◽  
Convective Boundary ◽  
Turbulent Statistics ◽  
The Impact

Abstract This manuscript describes numerical experiments investigating the influence of 2–30-km striplike heterogeneity on wet and dry convective boundary layers coupled to the land surface. The striplike heterogeneity is shown to dramatically alter the structure of the convective boundary layer by inducing significant organized circulations that modify turbulent statistics. The impact, strength, and extent of the organized motions depend critically on the scale of the heterogeneity λ relative to the boundary layer height zi. The coupling with the land surface modifies the surface fluxes and hence the circulations resulting in some differences compared to previous studies using fixed surface forcing. Because of the coupling, surface fluxes in the middle of the patches are small compared to the patch edges. At large heterogeneity scales (λ/zi ∼18) horizontal surface-flux gradients within each patch are strong enough to counter the surface-flux gradients between wet and dry patches allowing the formation of small cells within the patch coexisting with the large-scale patch-induced circulations. The strongest patch-induced motions occur in cases with 4 < λ/zi < 9 because of strong horizontal pressure gradients across the wet and dry patches. Total boundary layer turbulence kinetic energy increases significantly for surface heterogeneity at scales between λ/zi = 4 and 9; however, entrainment rates for all cases are largely unaffected by the striplike heterogeneity. Velocity and scalar fields respond differently to variations of heterogeneity scale. The patch-induced motions have little influence on total vertical scalar flux, but the relative contribution to the flux from organized motions compared to background turbulence varies with heterogeneity scale. Patch-induced motions are shown to dramatically impact point measurements in a free-convective boundary layer. The magnitude and sign of this impact depends on the location of the measurement within the region of heterogeneity.

scholarly journals Photoionization feedback in turbulent molecular clouds

2020 ◽  
Vol 500 (2) ◽  
pp. 1833-1843
Author(s):  
Nina S Sartorio ◽  
Bert Vandenbroucke ◽  
Diego Falceta-Goncalves ◽  
Kenneth Wood
Keyword(s):  
Molecular Clouds ◽  
Source Parameters ◽  
Density Structure ◽  
Star Forming ◽  
Turbulent Statistics ◽  
Model Cloud ◽  
H Ii Region ◽  
A Minor ◽  
2D And 3D ◽  
The Impact

ABSTRACT We present a study of the impact of photoionization feedback from young massive stars on the turbulent statistics of star-forming molecular clouds. This feedback is expected to alter the density structure of molecular clouds and affect future star formation. Using the AMUN-Rad code, we first generate a converged isothermal forced turbulent density structure inside a periodic box. We then insert an ionizing source in this box and inject photoionization energy using a two-temperature pseudo-isothermal equation of state. We study the impact of sources at different locations in the box and of different source luminosities. We find that photoionization has a minor impact on the 2D and 3D statistics of turbulence when turbulence continues to be driven in the presence of a photoionizing source. Photoionization is only able to disrupt the cloud if the turbulence is allowed to decay. In the former scenario, the presence of an H ii region inside our model cloud does not lead to a significant impact on observable quantities, independent of the source parameters.

Hybrid RANS/LES Simulation of a Lean Premixed Swirl Burner Based on a Turbulent Flame Speed Closure

2009 ◽  
Author(s):  
Guido Ku¨nne ◽  
Christian Klewer ◽  
Johannes Janicka
Keyword(s):  
Hybrid Methods ◽  
Turbulent Flame ◽  
Computational Cost ◽  
Flame Speed ◽  
Model Parameters ◽  
Detached Eddy Simulation ◽  
Combustion Modeling ◽  
Turbulent Flame Speed ◽  
Eddy Simulation ◽  
The Impact

In this work, simulations of a strongly swirled premixed flame at atmospheric pressure were carried out using classical RANS-methods as well as different hybrid RANS/LES approaches. In the context of RANS, a large number of simulations using the k-ε-model were performed to study the impact of sensitivities related to boundary conditions and model parameters. For the transient simulations, the hybrid methods, DES (Detached Eddy Simulation) and SAS (Scale Adaptive Simulation) as implemented in ANSYS-CFX, were employed. These methods were used to avoid the prohibitive computational cost of LES in boundary layers but to resolve the detached eddies to capture the flame turbulence interaction. Combustion modeling in CFX is based on a transport equation for the progress variable combined with a turbulent flame speed closure to treat the chemical source term. In addition, isothermal LES was performed in advance to identify the coherent structures, such as precessing vortex cores, which were observed experimentally.

Investigation of Turbulence Models Applied to Premixed Combustion Using a Level-Set Flamelet Library Approach

2004 ◽  
Vol 126 (4) ◽  
pp. 701-707 ◽  
Author(s):  
Ulf Engdar ◽  
Per Nilsson ◽  
Jens Klingmann
Keyword(s):  
Level Set ◽  
Turbulence Intensity ◽  
Turbulent Flame ◽  
Turbulence Models ◽  
Flame Speed ◽  
Premixed Combustion ◽  
Chemical Mechanism ◽  
Turbulent Flame Speed ◽  
The Mean ◽  
The Impact

Most of the common modeling approaches to premixed combustion in engineering applications are either based on the assumption of infinitely fast chemistry or the flamelet assumption with simple chemistry. The level-set flamelet library approach (FLA) has shown great potential in predicting major species and heat release, as well as intermediate and minor species, where more simple models often fail. In this approach, the mean flame surface is tracked by a level-set equation. The flamelet libraries are generated by an external code, which employs a detailed chemical mechanism. However, a model for the turbulent flame speed is required, which, among other considerations, depends on the turbulence intensity, i.e., these models may show sensitivity to turbulence modeling. In this paper, the FLA model was implemented in the commercial CFD program Star-Cd, and applied to a lean premixed flame stabilized by a triangular prism (bluff body). The objective of this paper has been to investigate the impact on the mean flame position, and hence on the temperature and species distribution, using three different turbulent flame speed models in combination with four different turbulence models. The turbulence models investigated are: the standard k-ε model, a cubic nonlinear k-ε model, the standard k-ω model and the shear stress transport (SST) k-ω model. In general, the computed results agree well with experimental data for all computed cases, although the turbulence intensity is strongly underestimated at the downstream position. The use of the nonlinear k-ε model offers no advantage over the standard model, regardless of flame speed model. The k-ω based turbulence models predict the highest turbulence intensity with the shortest flame lengths as a consequence. The Mu¨ller flame speed model shows the least sensitivity to the choice of turbulence model.  

Estimating Laminar Flame Speed and Ignition Delay for a Series of Natural Gas Mixtures at IC Engine-Relevant Conditions

2019 ◽  
Vol 142 (6) ◽  
Author(s):  
Kelsey Fieseler ◽  
Taylor Linker ◽  
Mark Patterson ◽  
Daniel Rem ◽  
Timothy J. Jacobs
Keyword(s):  
Natural Gas ◽  
Ignition Delay ◽  
Laminar Flame ◽  
Initial Conditions ◽  
Initial Pressure ◽  
Fuel Mixture ◽  
Flame Speed ◽  
Laminar Flame Speed ◽  
Ic Engine ◽  
The Impact

Abstract Two equations are developed to estimate laminar flame speed and ignition delay for different alkane mixtures at a range of engine-relevant conditions. Fuel mixtures of methane, ethane, propane, butane, and pentane were selected by analyzing the natural gas composition in a natural gas pipeline located in the Midwestern United States. The laminar flame speed and ignition delay were calculated for each mixture at each set of conditions using Cantera, a chemical kinetics solver. The range of initial conditions for laminar flame speed includes temperatures from 300 to 700 K, pressures from 1 to 40 bar, equivalence ratios from 0.4 to 1.2, and residual fractions from 0% to 20%. These data were then fit to a non-linear regression. The range of initial conditions for the ignition delay equation includes temperatures from 1100 to 2000 K, pressures from 1 to 40 bar, equivalence ratios from 0.4 to 1.15, and residual fractions from 0% to 20%. These data were fit to a previously developed equation. Sensitivity studies were conducted on each equation to quantify the impact of the independent variables on the target variable. This showed that, for laminar flame speed, the initial pressure, temperature, and equivalence ratio had the largest impact, with fuel composition having a lesser impact. For ignition delay, the temperature and pressure were shown to have the largest impact. There is a room for improvement, namely, increasing the fuel mixture variability and range of initial conditions, and developing a better fit to the data.

scholarly journals Assessment of the impact of nanosecond plasma discharge on the combustion of methane air flames

2020 ◽  
Vol 197 ◽  
pp. 10001
Author(s):  
Donato Fontanarosa ◽  
Ghazanfar Mehdi ◽  
Maria Grazia De Giorgi ◽  
Antonio Ficarella
Keyword(s):  
Plasma Discharge ◽  
Research Area ◽  
Chemical Kinetic ◽  
Flame Speed ◽  
Chemical Effect ◽  
Stoichiometric Ratio ◽  
Pulsed Plasma ◽  
Plasma Flame ◽  
Active Particles ◽  
The Impact

At present, development of plasma assisted ignition and combustion is a very promising research area due to its wide applications in the field of aeronautical engines and power sector. Plasma discharge can improve the combustion because it produces large number of chemically active particles which affects the chemical reaction. Simulation is an effective tool to analyze the interaction between the plasma and the flame through the implementation of plasma-assisted combustion. This study focused on three main objectives. Initially a microscopic plasma model with detailed kinetic plasma mechanisms was developed, then the validation of these mechanisms in air/methane mixture has been performed. Finally, the effects of nano pulsed plasma discharge on combustion have been investigated. In order to accomplish the above task, two numerical tools Chemical Kinetic Solver (CHEMKIN) and Plasma Kinetic Solver (ZDPlasKin) are used. It was found that the kinetic model of plasma provides good overall agreement with experimental data and identify key processes for species (e.g. O atom) generation and decay. The results showed that with the increase of reduced electric field, active particles and intermediate species/radicals (in particular ozone) are increased. ZDPlasKin results were incorporated in CHEMKIN to investigate and compare the flame speed, thermal and chemical effect by using a GRI-Mech scheme modified with the addition of ozone reactions. It has been found that with the adding of plasma flame speed was increased up to 26% at stoichiometric ratio. The chemical heat release also showed increment at low temperatures that confirmed the combustion enhancement. Furthermore, ignition delay timings were significantly reduced with the plasma excitation.

scholarly journals Mass Transport and Turbulent Statistics within Two Branching Coral Colonies

Fluids ◽  
2020 ◽  
Vol 5 (3) ◽  
pp. 153
Author(s):  
Md Monir Hossain ◽  
Anne E. Staples
Keyword(s):  
Mass Transport ◽  
Turbulent Flow ◽  
Reynolds Stress ◽  
Transport Processes ◽  
Transport Mechanisms ◽  
Local Flow ◽  
Turbulent Statistics ◽  
The Mean ◽  
Mass Transport Mechanisms ◽  
The Impact

Large eddy simulations were performed to characterize the flow and mass transport mechanisms in the interior of two Pocillopora coral colonies with different geometries, one with a relatively loosely branched morphology (P. eydouxi), and the other with a relatively densely branched structure (P. meandrina). Detailed velocity vector and streamline fields were obtained inside both corals for the same unidirectional oncoming flow, and significant differences were found between their flow profiles and mass transport mechanisms. For the densely branched P. meandrina colony, a significant number of vortices were shed from individual branches, which passively stirred the water column and enhanced the mass transport rate inside the colony. In contrast, vortices were mostly absent within the more loosely branched P. eydouxi colony. To further understand the impact of the branch density on internal mass transport processes, the non-dimensional Stanton number for mass transfer, St, was calculated based on the local flow time scale and compared between the colonies. The results showed up to a 219% increase in St when the mean vortex diameter was used to calculate St, compared to calculations based on the mean branch diameter. Turbulent flow statistics, including the fluctuating velocity components, the mean Reynolds stress, and the variance of the velocity components were calculated and compared along the height of the flow domain. The comparison of turbulent flow statistics showed similar Reynolds stress profiles for both corals, but higher velocity variations, in the interior of the densely branched coral, P. meandrina.

Formation of Lunar Craters and Bright Rays as a Result of Meteorite Impacts

1962 ◽  
Vol 14 ◽  
pp. 415-418
Author(s):  
K. P. Stanyukovich ◽  
V. A. Bronshten
Keyword(s):  
Explosive Charge ◽  
The Moon ◽  
Meteorite Impacts ◽  
The Earth ◽  
Lunar Craters ◽  
The Impact

The phenomena accompanying the impact of large meteorites on the surface of the Moon or of the Earth can be examined on the basis of the theory of explosive phenomena if we assume that, instead of an exploding meteorite moving inside the rock, we have an explosive charge (equivalent in energy), situated at a certain distance under the surface.

The Geosciences Applied to Lunar Exploration

1962 ◽  
Vol 14 ◽  
pp. 169-257 ◽  
Author(s):  
J. Green
Keyword(s):  
Lunar Surface ◽  
Probable Mechanism ◽  
Point Of View ◽  
Lunar Exploration ◽  
Geological Model ◽  
Apply Geophysics ◽  
Surface Features ◽  
The Impact

The term geo-sciences has been used here to include the disciplines geology, geophysics and geochemistry. However, in order to apply geophysics and geochemistry effectively one must begin with a geological model. Therefore, the science of geology should be used as the basis for lunar exploration. From an astronomical point of view, a lunar terrain heavily impacted with meteors appears the more reasonable; although from a geological standpoint, volcanism seems the more probable mechanism. A surface liberally marked with volcanic features has been advocated by such geologists as Bülow, Dana, Suess, von Wolff, Shaler, Spurr, and Kuno. In this paper, both the impact and volcanic hypotheses are considered in the application of the geo-sciences to manned lunar exploration. However, more emphasis is placed on the volcanic, or more correctly the defluidization, hypothesis to account for lunar surface features.

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