On three-dimensional quasiperiodic Floquet instabilities of two-dimensional bluff body wakes

2003 ◽  
Vol 15 (8) ◽  
pp. L57-L60 ◽  
Author(s):  
H. M. Blackburn ◽  
J. M. Lopez
Keyword(s):  
Bluff Body ◽  
Three Dimensional ◽  
Two Dimensional

Testing Three-Dimensional Bluff-Body Models in a Low-Speed Two-Dimensional Adaptive Wall Test Section

1995 ◽  
Vol 117 (4) ◽  
pp. 546-551 ◽  
Author(s):  
D. Sumner ◽  
E. Brundrett
Keyword(s):  
Drag Coefficient ◽  
Test Section ◽  
Bluff Body ◽  
Three Dimensional ◽  
Correction Method ◽  
Blockage Ratio ◽  
Two Dimensional ◽  
Interference Effects ◽  
Low Speed ◽  
Straight Wall

Thin, sharp-edged disk models were evaluated in a low-speed two-dimensional adaptive flexible wall test section to determine the optimum adaptive wall testing environment for three-dimensional bluff-body models, by providing model testing recommendations for nominal solid blockage ratio and model span ratio. Drag coefficient measurements obtained under straight wall and adapted wall conditions showed that for a two-dimensional adaptive wall test section, the model span ratio imposes a more severe restriction upon model size than does the nominal solid blockage ratio. Minimum wall interference conditions were achieved with adapted walls for nominal solid blockage ratios less than 3 percent and model span ratios less than 21 percent, independent of the nominal test section aspect ratio, based on favorable comparison with previously-published experimental data. Data obtained under straight wall conditions confirmed that wall interference effects can only be neglected in conventional, straight-walled test sections for solid blockage ratios less than 0.5 percent and model span ratios less than 10 percent. The post-test boundary correction method of Maskell was successfully used to adjust the straight wall test section drag coefficient measurements of the larger models for wall interference effects, but no direct measurements of wall interference are used with this method. The results support the careful use of a two-dimensional wall adjustment strategy for three-dimensional nonlifting flows.

Three-dimensional transition in the wake of a transversely oscillating cylinder

2007 ◽  
Vol 577 ◽  
pp. 79-104 ◽  
Author(s):  
JUSTIN S. LEONTINI ◽  
M. C. THOMPSON ◽  
K. HOURIGAN
Keyword(s):  
Bluff Body ◽  
Oscillation Amplitude ◽  
Three Dimensional ◽  
Base Flow ◽  
Two Dimensional ◽  
Cylinder Wake ◽  
Oscillating Cylinder ◽  
Single Vortex ◽  
Spatio Temporal ◽  
Mode A

A Floquet stability analysis of the transition to three-dimensionality in the wake of a cylinder forced to oscillate transversely to the free stream has been undertaken. The effect of varying the oscillation amplitude is determined for a frequency of oscillation close to the natural shedding frequency. The three-dimensional modes that arise are identified, and the effect of the oscillation amplitude on their structure and growth rate quantified.It is shown that when the two-dimensional wake is in the 2S configuration (which is similar to the Kármán vortex street), the three-dimensional modes that arise are similar in nature and symmetry structure to the modes in the wake of a fixed cylinder. These modes are known as modes A, B and QP and occur in this order with increasing Re. However, increasing the amplitude of oscillation causes the critical Reynolds number for mode A to increase significantly, to the point where mode B becomes critical before mode A. The critical wavelength for mode A is also affected by the oscillation, becoming smaller with increasing amplitude. Elliptic instability theory is shown also to predict this trend, providing further support that mode A primarily arises as a result of an elliptic instability.At higher oscillation amplitudes, the spatio-temporal symmetry of the two-dimensional wake changes and it takes on the P + S configuration, with a pair of vortices on one side of the wake and a single vortex on the other side, for each oscillation cycle. With the onset of this configuration, modes A, B and QP cease to exist. It is shown that two new three-dimensional modes arise from this base flow, which we call modes SL and SS. Both of these modes are subharmonic, repeating over two base-flow periods. Also, either mode can be the first to become critical, depending on the amplitude of oscillation of the cylinder.The emergence of these two new modes, as well as the reversal of the order of inception of the three-dimensional modes A and B, leads to the observation that for an oscillating cylinder wake there are four different modes that can lead the transition to three-dimensionality, depending on the amplitude of oscillation. Therefore this type of flow provides a good example for studying the effect of mode-order inception on the path taken to turbulence in bluff-body wakes.For the range of amplitudes studied, the maximum Re value for which the flow remains two-dimensional is 280.

Aerodynamic behaviour of bodies in the wakes of other bodies

1971 ◽  
Vol 269 (1199) ◽  
pp. 425-437 ◽  
Keyword(s):  
Bluff Body ◽  
Three Dimensional ◽  
Future Research ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Limited Information ◽  
Irrotational Flow ◽  
Two Bodies

Wakes of two-dimensional bluff bodies are described, with emphasis on the properties of the wake which influence the loads on other bodies placed in the wake. The unsteady irrotational flow outside the true wake is included in the discussion. Some limited information on the wakes of three-dimensional bluff bodies is also considered. The interaction between two bodies is subdivided into two categories: (i) when the bodies are close together and the upstream body is influenced by the downstream one and (ii) when the bodies are so far apart that only the downstream body is affected. Experiments are described in which the load on an aerofoil in the wake of a two-dimensional bluff body was measured. The results are presented in the form of an aerodynamic admittance and these experiments are used to illustrate the type of problem associated with the determination of the loads on a bluff body in a wake. Experiments are also described which show the large variation of time-averaged load which can be developed on a body which is part of a closely packed complex of bodies, as the orientation of the complex to the wind is varied. Finally, some ideas for future research are outlined.

Computational and Neuro-Fuzzy Study of the Effect of Small Objects on the Flow and Thermal Fields of Bluff Bodies

2006 ◽  
Author(s):  
Ahmed F. Abdel Gawad
Keyword(s):  
Drag Reduction ◽  
Bluff Body ◽  
Three Dimensional ◽  
The Body ◽  
Bluff Bodies ◽  
Small Object ◽  
Two Dimensional ◽  
Neuro Fuzzy ◽  
Body Cooling ◽  
Two Dimensional Flows

The aim of the present study is to find computationally the optimum parameters that affect the drag reduction of bluff bodies using a small object (obstacle). These parameters include the size of the obstacle as well as the gap between the obstacle and the bluff body. Two- and three-dimensional bodies were investigated in turbulent flow fields. The research was focused on the cases of the rectangular-section obstacle. Four values of the obstacle size were studied, namely: 4%, 10%, 35%, and 100% of the size of the bluff body. The effect of the obstacle on the thermal field of the two-dimensional body was also studied. Comparisons were carried out with the available experimental measurements. A proposed neuro-fuzzy approach was used to predict the drag reduction of the entire system. Results showed that system drag reductions up to 62% (two-dimensional flows) and 48% (three-dimensional flows) can be obtained. Also, enhancement of the body cooling up to 75% (two-dimensional flows) may be achieved. Generally, useful comments and suggestions are stated.

Characteristics of Bluff Body Stabilized Turbulent Premixed Flames

2011 ◽  
Author(s):  
Alejandro M. Briones ◽  
Balu Sekar ◽  
Hugh Thornburg
Keyword(s):  
Bluff Body ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reacting Flows ◽  
Chemical Mechanism ◽  
Reacting Flow ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Aspect Ratios ◽  
Trailing Edges

Non-reacting and reacting flows past typical flameholders are modeled with URANS and LES. The continuity, momentum, energy, species, and turbulence governing equations are solved using two- and three-dimensional configurations. Either 2-step global or 44-step reduced chemical mechanism for C3H8-air combustion, accounting for turbulence-chemistry interaction, and with temperature- and species-dependent thermodynamic and transport properties is utilized. For square and rectangular bluff bodies the flow separates at the leading edges, whereas for triangular bluff body separation occurs only at the trailing edges. These bluff bodies exhibit two shear layers at the trailing edges that shed asymmetric vortices. For rectangular bluff bodies with aspect ratios (AR) less than 2.3 there is backflow from the wake. With increasing AR from unity, backflow is gradually diminished, and the von Ka´rma´n Strouhal number (StvK) decreases. For 2.0<AR<2.3, StvK jumps to a higher value and separation again occurs at the trailing edges for AR = 2.3. Further increase in AR decreases StvK again. The simulations with URANS qualitatively and quantitatively match experimental results for StvK vs. AR. Quantitative discrepancies are, however, found for AR≥2.3. In addition, two-dimensional non-reacting flows with URANS are sufficient to predict StvK. Moreover, two-dimensional simulations of reacting flow indicate that the flame promotes static and dynamic stability for AR = 1.0 and 2.3. The flame is dynamically unstable for AR = 2.0, exhibiting a von Ka´rma´n flow pattern. Stable flames anchored at the most downstream separation location (e.g., the flame anchored at AR = 1.0 is attached to the leading edge, whereas that of AR = 2.3 is attached to the trailing edge). Realizable k-ε URANS and LES simulations for the triangular cylinder closely match the experimental StvK for both non-reacting and reacting flows. Nonetheless, LES predicts a smaller recirculation length than k-ε URANS. LES predicts a flow field in which Be´rnard/von Ka´rma´n (BvK) instability is suppressed, whereas URANS predicts a competition between the Kelvin-Helmholtz (KH) instability and BvK.

scholarly journals Stabilization of absolute instability in spanwise wavy two-dimensional wakes

2013 ◽  
Vol 727 ◽  
pp. 346-378 ◽  
Author(s):  
Yongyun Hwang ◽  
Jinsung Kim ◽  
Haecheon Choi
Keyword(s):  
Vortex Shedding ◽  
Bluff Body ◽  
High Efficiency ◽  
Three Dimensional ◽  
Base Flow ◽  
Absolute Instability ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Flow Modification ◽  
Wake Instability

AbstractControlling vortex shedding using spanwise-varying passive or active actuation (namely three-dimensional control) has recently been reported as a very efficient method for regulating two-dimensional bluff-body wakes. However, the mechanism of how the designed controller regulates vortex shedding is not clearly understood. To understand this mechanism, we perform a linear stability analysis of two-dimensional wakes, the base flow of which is modified with a given spanwise waviness. Absolute and convective instabilities of the spanwise wavy base flows are investigated using Floquet theory. Two types of base-flow modification are considered: varicose and sinuous. Both of these modifications attenuate absolute instability of two-dimensional wakes. In particular, the varicose modification is found to be much more effective in the attenuation than the sinuous one, and its spanwise lengths resulting in maximum attenuation show good agreement with those in three-dimensional controls. The physical mechanism of the stabilization is found to be associated with the formation of streamwise vortices from tilting of two-dimensional Kármán vortices and the subsequent tilting of these streamwise vortices by the spanwise shear in the base flow. Finally, the sensitivity of absolute instability to spanwise wavy base-flow modification is investigated. It is shown that absolute instability of two-dimensional wakes is much less sensitive to spanwise wavy base-flow modification than to two-dimensional modification. This suggests that the high efficiency observed in several three-dimensional controls is not due to the sensitive response of the wake instability to the spanwise waviness in the base flow.

High Resolution Microscopy of Multi-Layered Biological Crystals

1982 ◽  
Vol 40 ◽  
pp. 80-83
Author(s):  
H.A. Cohen ◽  
T.W. Jeng ◽  
W. Chiu
Keyword(s):  
High Resolution ◽  
Low Dose ◽  
Three Dimensional ◽  
Data Interpretation ◽  
Two Dimensional ◽  
Biological Objects ◽  
Density Maps ◽  
Density Map ◽  
Complex Crystal ◽  
High Resolution Microscopy

This tutorial will discuss the methodology of low dose electron diffraction and imaging of crystalline biological objects, the problems of data interpretation for two-dimensional projected density maps of glucose embedded protein crystals, the factors to be considered in combining tilt data from three-dimensional crystals, and finally, the prospects of achieving a high resolution three-dimensional density map of a biological crystal. This methodology will be illustrated using two proteins under investigation in our laboratory, the T4 DNA helix destabilizing protein gp32*I and the crotoxin complex crystal.

Instrumentation For Quantitative Microscopy

1977 ◽  
Vol 35 ◽  
pp. 206-209
Author(s):  
B. Ralph ◽  
A.R. Jones
Keyword(s):  
Volume Fraction ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Automatic Data ◽  
Phase Volume Fraction ◽  
Statistical Confidence ◽  
Resultant Data ◽  
Automatic Data Collection ◽  
Third Dimension ◽  
Evolution Of Microstructure

In all fields of microscopy there is an increasing interest in the quantification of microstructure. This interest may stem from a desire to establish quality control parameters or may have a more fundamental requirement involving the derivation of parameters which partially or completely define the three dimensional nature of the microstructure. This latter categorey of study may arise from an interest in the evolution of microstructure or from a desire to generate detailed property/microstructure relationships. In the more fundamental studies some convolution of two-dimensional data into the third dimension (stereological analysis) will be necessary.In some cases the two-dimensional data may be acquired relatively easily without recourse to automatic data collection and further, it may prove possible to perform the data reduction and analysis relatively easily. In such cases the only recourse to machines may well be in establishing the statistical confidence of the resultant data. Such relatively straightforward studies tend to result from acquiring data on the whole assemblage of features making up the microstructure. In this field data mode, when parameters such as phase volume fraction, mean size etc. are sought, the main case for resorting to automation is in order to perform repetitive analyses since each analysis is relatively easily performed.

A linearity test for thick specimens imaged by HVEM over a 180° angular range

1991 ◽  
Vol 49 ◽  
pp. 552-553
Author(s):  
Yu Liu
Keyword(s):  
Phase Contrast ◽  
Three Dimensional ◽  
Angular Range ◽  
Two Dimensional ◽  
Back Projection ◽  
Line Integral ◽  
Exponential Law ◽  
Transmission Electron ◽  
Absorption Law ◽  
Linearity Test

The image obtained in a transmission electron microscope is the two-dimensional projection of a three-dimensional (3D) object. The 3D reconstruction of the object can be calculated from a series of projections by back-projection, but this algorithm assumes that the image is linearly related to a line integral of the object function. However, there are two kinds of contrast in electron microscopy, scattering and phase contrast, of which only the latter is linear with the optical density (OD) in the micrograph. Therefore the OD can be used as a measure of the projection only for thin specimens where phase contrast dominates the image. For thick specimens, where scattering contrast predominates, an exponential absorption law holds, and a logarithm of OD must be used. However, for large thicknesses, the simple exponential law might break down due to multiple and inelastic scattering.

An Image Reconstruction System Applied to High Voltage Microscopy

1975 ◽  
Vol 33 ◽  
pp. 292-293
Author(s):  
D. E. Johnson
Keyword(s):  
High Voltage ◽  
Prior Information ◽  
Block Diagram ◽  
Three Dimensional ◽  
Real Space ◽  
Two Dimensional ◽  
Efficient Manner ◽  
Fourier Methods ◽  
Tilt Series ◽  
Methods Of Reconstruction

Increased specimen penetration; the principle advantage of high voltage microscopy, is accompanied by an increased need to utilize information on three dimensional specimen structure available in the form of two dimensional projections (i.e. micrographs). We are engaged in a program to develop methods which allow the maximum use of information contained in a through tilt series of micrographs to determine three dimensional speciman structure.In general, we are dealing with structures lacking in symmetry and with projections available from only a limited span of angles (±60°). For these reasons, we must make maximum use of any prior information available about the specimen. To do this in the most efficient manner, we have concentrated on iterative, real space methods rather than Fourier methods of reconstruction. The particular iterative algorithm we have developed is given in detail in ref. 3. A block diagram of the complete reconstruction system is shown in fig. 1.

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