On the spatial length scales of scalar dissipation in turbulent jet flames

2008 ◽  
Vol 596 ◽  
pp. 103-132 ◽  
Author(s):  
P. VAISHNAVI ◽  
A. KRONENBURG ◽  
C. PANTANO
Keyword(s):  
Thin Sheet ◽  
Measurement Techniques ◽  
Experimental Studies ◽  
Three Dimensional ◽  
Scalar Dissipation ◽  
Small Scale ◽  
Computational Domain ◽  
Length Scales ◽  
Jet Flame ◽  
True Value

Spatial length scales of the rate of dissipation, χ, of fluctuations of a conserved scalar, Z, are inferred numerically using a DNS database of a turbulent planar jet flame. The Taylor-scale Reynolds numbers lie in the range of 38 to 58 along the centreline of the simulated jet flame. Three different methods are used to study the spatial length scales associated with χ. First, analysis of the one-dimensional dissipation spectra shows an expected Reδ−3/4 (Kolmogorov) scaling with the outer-scale Reynolds number, Reδ. Secondly, thin sheet-like three-dimensional scalar dissipation structures have been investigated directly. Such structures were identified within the computational domain using level-sets of the χ-field, and their thicknesses were subsequently computed. The study shows, in accordance with experimental studies, that the captured dissipation-layer thickness also shows a Kolmogorov scaling with Reδ. Finally, spatial filters of varying widths were applied to the instantaneous Z field in order to model the averaging effect that takes place with some experimental measurement techniques. The filtered scalar dissipation rate was then calculated from the filtered scalar field. The peaks in the instantaneous filtered χ-profiles are observed to decrease exponentially with increasing filter width, yielding estimates of the true value of χ. Unlike the dissipation length scales obtained from the spectral analysis and the level-set method, the length-scale estimates from the spatial-filtering method are found to be proportional to Reδ−1. This is consistent with the small-scale intermittency of χ which cannot be captured by techniques that just resolve the conventional Batchelor/Obukhov–Corrsin scale. These results have implications when considering resolution requirements for measuring scalar dissipation length scales in experimental flows.

Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in heated coflow: a chemical explosive mode analysis

2010 ◽  
Vol 652 ◽  
pp. 45-64 ◽  
Author(s):  
T. F. LU ◽  
C. S. YOO ◽  
J. H. CHEN ◽  
C. K. LAW
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Near Field ◽  
Three Dimensional ◽  
Mode Analysis ◽  
Scalar Dissipation ◽  
Complex Flows ◽  
Jet Flame ◽  
Auto Ignition ◽  
Grid Points

A chemical explosive mode analysis (CEMA) was developed as a new diagnostic to identify flame and ignition structure in complex flows. CEMA was then used to analyse the near-field structure of the stabilization region of a turbulent lifted hydrogen–air slot jet flame in a heated air coflow computed with three-dimensional direct numerical simulation. The simulation was performed with a detailed hydrogen–air mechanism and mixture-averaged transport properties at a jet Reynolds number of 11000 with over 900 million grid points. Explosive chemical modes and their characteristic time scales, as well as the species involved, were identified from the Jacobian matrix of the chemical source terms for species and temperature. An explosion index was defined for explosive modes, indicating the contribution of species and temperature in the explosion process. Radical and thermal runaway can consequently be distinguished. CEMA of the lifted flame shows the existence of two premixed flame fronts, which are difficult to detect with conventional methods. The upstream fork preceding the two flame fronts thereby identifies the stabilization point. A Damköhler number was defined based on the time scale of the chemical explosive mode and the local instantaneous scalar dissipation rate to highlight the role of auto-ignition in affecting the stabilization points in the lifted jet flame.

Thin Sheet Metal (Membrane) Suspended Roof Structures

1995 ◽  
Vol 10 (4) ◽  
pp. 237-241
Author(s):  
P.G. Yeremeyv ◽  
D.B. Kiselev
Keyword(s):  
Thin Sheet ◽  
Experimental Studies ◽  
Three Dimensional ◽  
Strength Properties ◽  
Dimensional System ◽  
Membrane Structures ◽  
Distinctive Features ◽  
Industrial Buildings ◽  
Thin Sheet Metal ◽  
Three Dimensional System

A membrane roof is a three-dimensional system of thin metal sagging sheets fastened along the perimeter to the supporting contour. The membrane can form the roof of buildings and structures of various shapes of the surface and in-plan outlines with spans of 18 to 300 m. The longer the span the higher the economic efficiency of the membranes, the thickness of which is as low as 1 to 5 mm. The distinctive features of the structure are the most complete use of strength properties of a thin sheet in compression and the continuation in one material of load bearing and enclosure functions. Such a structure is able to resist all types of force actions, wind and seismic ones being among them. The thin-sheet roof are easy to manufacture and erect. The membrane structures are intended for civil and industrial buildings. At present a wide complex of theoretical and experimental studies of the membrane structures is being carried out.

Molecular Tagging Velocimetry and Its Application to In-Cylinder Flow Measurements

2013 ◽  
Vol 135 (12) ◽  
Author(s):  
Ravi Vedula ◽  
Mayank Mittal ◽  
Harold J. Schock
Keyword(s):  
Internal Combustion Engines ◽  
Measurement Techniques ◽  
Quantitative Description ◽  
Experimental Studies ◽  
Small Scale ◽  
Flow Measurements ◽  
Molecular Tagging Velocimetry ◽  
Molecular Tagging ◽  
Intake Stroke ◽  
Cylinder Flows

This review article provides an overview of the experimental studies of in-cylinder flows using various flow measurement techniques with a focus on molecular tagging velocimetry. It is necessary to understand the evolution of large-scale and small-scale turbulence as prepared during the intake stroke with a cycle resolved quantitative description. Due to the difficulty in obtaining these descriptions, either by modeling or experimentally, they are often characterized with somewhat ambiguous notions of bulk swirl and tumble measurement methods. During the intake stroke, in-cylinder flows are formed in such a manner as to provide advantageous spatial and temporal behavior for mixture formation later during the compression stroke. Understanding the details of how these flows influence fuel-air mixing, the initiation of ignition, combustion, and subsequent flame propagation processes is the primary motivation for the development of the methods described in this paper. The authors provide an introduction to fundamental flow motion inside the engine cylinder and measurement techniques, e.g., hot-wire anemometry, laser Doppler anemometry, and particle image velocimetry. Furthermore, molecular tagging velocimetry is discussed in detail in terms of (i) different mechanisms, (ii) procedure and data reduction methods to obtain the desired flow properties such as velocity, vorticity, and turbulent intensities, and (iii) applications to flow studies in internal combustion engines. Finally, the significance of experimental investigations of in-cylinder flows is discussed along with possible future applications.

Visualization of Micro- and Macromixing in Liquid Mixtures of Reacting Components (Keynote Paper)

2003 ◽  
Author(s):  
Kerstin Kling ◽  
Dieter Mewes
Keyword(s):  
Chemical Reaction ◽  
Measurement Techniques ◽  
Optical Tomography ◽  
Three Dimensional ◽  
Concentration Field ◽  
Laser Induced Fluorescence ◽  
Small Scale ◽  
Fluorescence Technique ◽  
Planar Laser Induced Fluorescence ◽  
Planar Laser

Micromixing is visualized inside a stirred vessel by using two different optical measurement techniques, the optical tomography and the Planar Laser Induced Fluorescence technique (PLIF). In order to distinguish between macro- and micromixing, a mixture of two dyes is injected into the mixing vessel. One of the dyes is an inert dye whereas the second dye is undergoing a fast chemical reaction with the vessel content. The distribution of the inert dye serves as a tracer for the macromixing but does not predicate the mixing quality on the nano scale. The chemical reaction requires mixing on the molecular scale. Therefore the reacting dye, which is changing its emission characteristics during the reaction, indirectly visualizes the micromixing. The tomographical dual wavelength photometry is used to measure the three-dimensional, transient concentration fields in the whole vessel at the same time. Measurements with the Planar Laser Induced Fluorescence technique are performed in an arbitrary plane of the vessel. This restriction on a two-dimensional concentration field is recompensed with a much higher spatial resolution which allows to visualize small scale structures in the order of mm. For both techniques low Reynolds number measurements are performed in a mixing vessel equipped with a Rushton turbine. Results are presented as two- or three-dimensional concentration fields. Areas of micromixing are detected by calculating the local degree of deviation from the concentration fields. They are depending on the injection position of the dye and are mainly found in the boundary layer of the lamellas.

scholarly journals Cirrus Mammatus Properties Derived from an Extended Remote Sensing Dataset

2006 ◽  
Vol 63 (2) ◽  
pp. 712-725 ◽  
Author(s):  
Likun Wang ◽  
Kenneth Sassen
Keyword(s):  
Remote Sensing ◽  
Large Scale ◽  
Doppler Radar ◽  
Three Dimensional ◽  
Remote Sensing Data ◽  
Cloud Base ◽  
Doppler Velocity ◽  
Small Scale ◽  
Length Scales ◽  
Atmospheric Remote Sensing

Abstract The first quantitative and statistical evaluation of cirrus mammatus clouds based on wavelet analysis of remote sensing data is made by analyzing the University of Utah Facility for Atmospheric Remote Sensing (FARS) 10-yr high-cloud dataset. First, a case study of cirrus mammata combining a high-resolution lidar system and a W-band Doppler radar is presented, yielding an assessment of the thermodynamic environment and dynamic mechanisms. Then, 25 cirrus mammatus cases selected from the FARS lidar dataset are used to disclose their characteristic environmental conditions, and vertical and length scales. The results show that cirrus mammata occur in the transition zone from moist (cloudy) to dry air layers with weak wind shear, which suggests that cloud-induced thermal structures play a key role in their formation. Their maximum vertical and horizontal length scales vary from 0.3 to 1.1 km and 0.5 to 8.0 km, respectively. It is also found that small-scale structures develop between the large-scale protuberances. The spectral slopes of the lidar-returned power and mean radar Doppler velocity data extracted from the cirrus cloud-base region further indicate the presence of developed three-dimensional, locally isotropic, homogeneous turbulence generated by buoyancy. Finally, comparisons of anvil and cirrus mammata are made. Although both are generated in a similar environment, cirrus mammata generally do not form fallout fronts like their anvil counterparts, and so do not have their smooth and beautiful outlines.

A Combined Experimental and Computational Study for the Design of a Low Water Consumption Cooling Tower

2014 ◽  
Author(s):  
Burak Dogan ◽  
Ibrahim Yilmaz ◽  
Ozgur Polat ◽  
Oytun Karabulut ◽  
Ahmet Ural ◽  
...  
Keyword(s):  
Water Consumption ◽  
Cooling Tower ◽  
Computational Study ◽  
Experimental Studies ◽  
Three Dimensional ◽  
Small Scale ◽  
Computational Fluid Dynamics Cfd ◽  
Set Up ◽  
Multi Phase ◽  
Alternative Designs

In this study, a combined experimental and computational study for the design of a low water consumption cooling tower is performed. The purpose of the study is to reduce the water consumption without decreasing the efficiency of a typical cooling tower. To achieve this aim, it is necessary to enhance the homogeneity of mist/air mixture. For this purpose, firstly, an experimental set-up including a small scale wind tunnel is installed which provides opportunity to examine different inlet and outlet configurations easily. Computational Fluid Dynamics (CFD) is used extensively to examine the effects of different configurations before experimental studies. Simulations of different inlet and outlet configurations are performed using only air. Several turbulators are designed and simulated to increase the turbulence levels. A three dimensional multi-phase CFD model is utilized to design a nozzle-turbulator system for the cooling tower. As results of the computational and experimental studies, the most efficient inlet and outlet configurations are specified and turbulators are selected from the alternative designs.

Milli-Scale Junction Flow Experiments

2010 ◽  
Author(s):  
Evan C. Lemley ◽  
Willy L. Duffle ◽  
Jesse K. Haubrich ◽  
Andrew W. Henderson
Keyword(s):  
Reynolds Number ◽  
Laminar Flow ◽  
Experimental Studies ◽  
Three Dimensional ◽  
Small Scale ◽  
Energy Losses ◽  
Turbulent Regime ◽  
Pressure Losses ◽  
Inlet Tube ◽  
Flow Experiments

Laminar flow is increasingly important area of study as it dominates microscale and milliscale applications in devices such as microvalves, pumps, and turbines and in biomedical applications such as stents and biological flows. Studies of pressure losses in junctions have mostly been focused on turbulent flow conditions that exist in larger scale piping systems. There is a need for laminar flow studies of energy losses in junctions so that engineers can better predict, design, and analyze flow in microscale and other small scale systems. Unlike in the turbulent regime, Reynolds number plays a dominant role in energy losses for laminar flow, so new studies should document the effects of Reynolds number. This paper documents laminar flow experiments in a milliscale junction. This work builds on previous experience of the authors in computational fluids dynamics simulations of junctions. The planar junction under study consists of a circular tubes with two outlets and one inlet. A general technique has been developed to produce computer and physical models of junctions in which the inlet tube size is set, but the outlets are allowed to vary in size and angle relative to the inlet tube. A generalized algorithm has been implemented to create three-dimensional models of the junctions for both computational and experimental studies. The junction test sections for experiments are milled from cast acrylic in two pieces to match three-dimensional computer models. The test sections are placed in a system that provides steady-state flow of water to test sections and has been designed to measure pressure losses and flow rates through the test section.

scholarly journals Numerical Investigation of Very-Large-Scale Motions in a Turbulent Boundary Layer for Different Roughness

Energies ◽  
2020 ◽  
Vol 13 (3) ◽  
pp. 659 ◽  
Author(s):  
Hehe Ren ◽  
Shujin Laima ◽  
Hui Li
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Three Dimensional ◽  
Flow Structures ◽  
Small Scale ◽  
Computational Domain ◽  
Low Speed ◽  
Instantaneous Flow ◽  
Gradual Process ◽  
Good Agreement

Wall-model large eddy simulations (WMLES) are conducted to investigate the spatial features of large-scale and very-large-scale motions (LSMs and VLSMs) in turbulent boundary flow in different surface roughnesses at a very high Reynolds number, O (106–107). The results of the simulation of nearly smooth cases display good agreement with field observations and experimental data, both dimensioned using inner and outer variables. Using pre-multiplied spectral analysis, the size of VLSMs can be reduced or even disappear with increasing roughness, which indirectly supports the concept that the bottom-up mechanism is one of the origins of VLSMs. With increases in height, the power of pre-multiplied spectra at both high and low wavenumber regions decreases, which is consistent with most observational and experimental results. Furthermore, we find that the change in the spectrum scaling law from −1 to −5/3 is a gradual process. Due to the limitations of the computational domain and coarse grid that were adopted, some VLSMs and small-scale turbulence are truncated. However, the size of LSMs is fully accounted for. From the perspective of the spatial correlation of the flow field, the structural characteristics of VLSMs under various surface roughnesses, including three-dimensional length scales and inclination angles, are obtained intuitively, and the conclusions are found to be in good agreement with the velocity spectra. Finally, the generation, development and extinction of three-dimensional VLSMs are analyzed by instantaneous flow and vorticity field, and it shows that the instantaneous flow field gives evidence of low-speed streamwise-elongated flow structures with negative streamwise velocity fluctuation component, and which are flanked on each side by similarly high-speed streamwise-elongated flow structures. Moreover, each of the low-speed streamwise-elongated flow structure lies beneath many vortices.

scholarly journals Controlled experiment of underwater vision-based mapping: A preliminary evaluation

2021 ◽  
Vol 925 (1) ◽  
pp. 012054
Author(s):  
F Muhammad ◽  
Poerbandono ◽  
H Sternberg
Keyword(s):  
Light Propagation ◽  
Three Dimensional ◽  
Ground Truth ◽  
Controlled Experiment ◽  
Small Scale ◽  
Preliminary Evaluation ◽  
True Value ◽  
Localization And Mapping ◽  
Underwater Vision ◽  
Web Camera

Abstract Underwater vision-based mapping (VbM) constructs three-dimensional (3D) map and robot position simultaneously out of a quasi-continuous structure from motion (SfM) method. It is the so-called simultaneous localization and mapping (SLAM), which might be beneficial for mapping of shallow seabed features as it is free from unnecessary parasitic returns which is found in sonar survey. This paper presents a discussion resulted from a small-scale testing of 3D underwater positioning task. We analyse the setting and performance of a standard web-camera, used for such a task, while fully submerged underwater. SLAM estimates the robot (i.e. camera) position from the constructed 3D map by reprojecting the detected features (points) to the camera scene. A marker-based camera calibration is used to eliminate refractions effect due to light propagation in water column. To analyse the positioning accuracy, a fiducial marker-based system –with millimetres accuracy of reprojection error– is used as a trajectory’s true value (ground truth). Controlled experiment with a standard web-camera running with 30 fps (frame per-second) shows that such a system is capable to robustly performing underwater navigation task. Sub-metre accuracy is achieved utilizing at least 1 pose (1 Hz) every second.

Large-Eddy Simulation of the Onset of the Sea Breeze

2007 ◽  
Vol 64 (12) ◽  
pp. 4445-4457 ◽  
Author(s):  
M. Antonelli ◽  
R. Rotunno
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Large Scale ◽  
Three Dimensional ◽  
Sea Breeze ◽  
Small Scale ◽  
Computational Domain ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Les Model

Abstract This paper describes results from a large-eddy simulation (LES) model used in an idealized setting to simulate the onset of the sea breeze. As the LES is capable of simulating boundary layer–scale, three-dimensional turbulence along with the mesoscale sea-breeze circulation, a parameterization of the planetary boundary layer was unnecessary. The basic experimental design considers a rotating, uniformly stratified, resting atmosphere that is suddenly heated at the surface over the “land” half of the domain. To focus on the simplest nontrivial problem, the diurnal cycle, effects of moisture, interactions with large-scale winds, and coastline curvature were all neglected in this study. The assumption of a straight coastline allows the use of a rectangular computational domain that extends to 50 km on either side of the coast, but only 5 km along the coast, with 100-m grid intervals so that the small-scale turbulent convective eddies together with the mesoscale sea breeze may be accurately computed. Through dimensional analysis of the simulation results, the length and velocity scales characterizing the simulated sea breeze as functions of the externally specified parameters are identified.

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