Flow characteristics around a square cylinder with changing chamfer dimensions

Abstract  

It is known that for a square cylinder subjected to uniform flow, the drag force changes with the angle of attack. To clarify the flow characteristics around a square cylinder with corner cutoffs, we measured the drag coefficient and the Strouhal number for changing chamfer dimensions. We analyzed the flow around a square cylinder with corner cutoffs by applying the RNG k–ε turbulent model, and investigated the surface flow pattern using visualization by means of the oil film and mist flow method. From these results, we obtained the surface flow patterns by the oil film method and numerical analysis. The numerical results agreed well with the experimental values. The drag coefficient of the square cylinder with corner cutoffs decreased suddenly at an angle of attack of about α = 0°– 10° when compared with the drag coefficient for a square cylinder. The minimum value of the drag coefficient for the square cylinder with corner cutoffs decreased by about 30% compared with that for the square cylinder. The drag coefficient of the square cylinder with 10% corner cutoffs was found to be smallest, since the wake area of this square cylinder was smaller compared with that of the other square cylinder.     

Numerical simulation and flow visualization using soap film of the self-organized vortex structure in the wake of an array of cylinders

Abstract  

With the aid of computational fluid dynamics (CFD) and simple flow visualization technique using flowing soap-film, we present here the wake structures behind an array of cylinders for Reynolds numbers corresponding to both laminar and turbulent flow regimes. The image results illustrate interesting vortex interactions past these equally spaced cylinders; for low Reynolds number flow, well-organized wake pattern persists and manifests unsteadily to different symmetry states. An increase of free stream flow velocity causes the wake transition, resulting in the formation of asymmetric flow wake with chaotic mixing at the far wake. Observations from both the numerical simulations and soap-film are in good agreement at least qualitatively.     

Spatial hot-wire visualization of the Λ-structure transformation into the turbulent spot on the smooth flat plate surface and riblet effect on this process

Abstract  

Evolution of the Λ-vortex and its transformation into the turbulent spot in a flat plate boundary layer are investigated experimentally. Extensive measurements visualizing the Λ-structure transformation into the turbulent spot on the smooth and ribbed surfaces of the flat plate are presented. The flow behavior in the course of spatial evolution of the Λ-structure and turbulent spot is discussed. Specific features of the downstream evolution of Λ-structure and turbulent spot on the smooth and ribbed surfaces are demonstrated, such as suppression by riblets of the Λ-vortex transformation into the turbulent spot, appearance of the coherent structures of Λ-vortex within ensemble-averaged turbulent spot, and oblique waves generation both by the Λ-vortex and turbulent spot.     

Flow visualization and scanning PIV measurement of three-dimensional structure in near field of strongly buoyant jet

Abstract  

The near-field structure of strongly buoyant jet issuing from a square nozzle at low Froude and Reynolds numbers is studied by using LIF flow visualization and time-resolved scanning PIV. These experimental techniques allow the visualization of unsteady three-dimensional flow phenomenon occurring in the near-field of strongly buoyant jet. It is found that the buoyant jet is unstable to the positive buoyancy forces, which promote the inflow motion near the nozzle exit. The surrounding low temperature fluid moves into the nozzle inside along the nozzle corner and mixes with the high temperature fluid deep into the nozzle. Then, the flow pattern inside the nozzle becomes highly complex to promote the laminar to turbulent transition of the jet. The statistical flow characteristics of the strongly buoyant jet are evaluated from the scanning PIV measurement, and the result indicates the presence of axisymmetric distributions of mean flow and velocity fluctuations in the circle of diameter equal to the square side of the nozzle.     

Experimental and numerical study on onset of inflow in near field of buoyant jet at low Froude number

Abstract  

In the present paper, the onset of inflow in the near field of a vertical buoyant jet issuing from a square duct is studied by experimental flow visualization and numerical simulation. The experimental critical condition for the onset of inflow is obtained from the scanning LIF visualization in the near field of the buoyant jet at various combinations of Froude numbers and Reynolds numbers. The experimental result shows that the critical Froude number increases with an increase in Reynolds number of the buoyant jet. The critical condition is also examined by numerical simulation based on the Navier-Stokes equation and energy conservation equation, under the assumption that the flow separation occurs at the duct exit. The main feature of the inflow observed by experiment is well reproduced in the numerical results.     

Investigation of the pseudo-shock wave in a two-dimensional supersonic inlet

Abstract  

This paper describes experimental and numerical investigations into the multiple shock waves/turbulent boundary layer interaction in a supersonic inlet. The test model has a rectangular shape with an asymmetric subsonic diffuser of 5°. Experiments were conducted to obtain the visualization images and static pressure data by using supersonic wind tunnel. Numerical simulation was performed by solving the RANS equations with the Menter’s SST turbulent model. The inflow condition was a free-stream Mach number of 2.5 and a unit Reynolds number of 7.6 × 10⁷/m. Numerical results showed good agreement with the experimental results. Based on this agreement, the flow characteristics which are often very difficult to obtain experimentally alone were analyzed with the aid of numerical simulation. The structures, pressure and velocity distributions, and total pressure loss of the pseudo-shock wave in the supersonic inlet were presented in detail from flow visualization images and static pressures.     

An experimental study on swirling flow in a cylindrical annuli using the PIV technique

Abstract  

An experimental investigation was conducted to study the characteristics of turbulent swirling flow in an axisymmetric annuli. The swirl angle measurements were performed using a flow visualization technique using smoke and dye liquid for Re = 60,000–80,000. Using the two-dimensional particle image velocimetry method, this study found the time-mean velocity distribution and turbulent intensity in water with swirl for Re = 20,000, 30,000, and 40,000 along longitudinal sections. There were neutral points for equal axial velocity at y/(R − r) = 0.7–0.75, and the highest axial velocity was recorded near y/(R − r) = 0.9. Negative axial velocity was observed near the convex tube along X/(D − d) = 3–23.     

Analysis of formation and breakup mechanisms in rotary atomization through spray visualization

Abstract  

The spray characteristics of rotary atomization for small gas turbines were investigated using a high-speed camera and Nd:YAG laser flash visualization. To analyze the breakup process of the liquid column and liquid film, spray visualization tests were performed under varied test conditions. Visualization was conducted at rotational speeds from 5,000 to 40,000 rpm and fuel flow rates from 15 to 90 kg/h. From the test results, we discovered that a varicose wave appeared on the surface of the liquid column at rotational speeds below 10,000 rpm. As the rotational speed increases, the column forms a liquid film via a dilatational wave and breaks up via a sinuous wave. Under a gradual increase in rotational speed, the breakup of the liquid column is observed, and the formation of a liquid film is ultimately observed. We conclude that liquid film formation is essential for high spray quality in rotary atomization systems.     

Study on flow distribution for room air conditioner using visualization technique

Abstract  

Flow distributions of a room air conditioner (RAC) have been analyzed by a visualization technique such as a particle image velocimetry (PIV) in this study. Flow structures have been investigated inside and outside the RAC to improve efficiencies. Accuracies of the measured velocities by the PIV have been confirmed by a Pitot tube at important locations around the RAC. Then, a numerical analysis has been performed by developed computer programs to design parts for the RAC with improved efficiencies. The design guideline has been proposed based on the analyzed results to reduce the condensation problem and increase the flow rate. Finally, shapes of a fan with high flow rates and an outlet with reduced condensation problems have been obtained based on the developed experimental and numerical methods in this study.     

Quantitative measurement of dynamic flow induced by <Emphasis Type="Italic">Tetrahymena pyriformis</Emphasis> (<Emphasis Type="Italic">T. pyriformis</Emphasis>) using micro-particle image velocimetry

Abstract  

In-depth quantitative visualization studies are required to understand the flow induced by swimming micro-organisms and find potential applications. The present study visualized the flow induced by Tetrahymena pyriformis of size 45–50 μm, which swam freely and via stimulation by galvanotaxis in a PDMS micro-chamber using a micro-particle image velocimetry system. The results showed that the maximum velocity of the induced flow was around 430 μm/s for free swimming and 700 μm/s for galvanotactic-controlled swimming. Due to the applied electric field, the electro-osmosis flow led to increased velocity of roughly 135 μm/s at 3 V/mm. The increased velocity stems from the increased motility of the cell under the electric field. Therefore, it was demonstrated that galvanotaxis can control the swimming direction and increase the induced velocity.     

Separated flow around a rotating gyroball

Abstract  

In baseball, a gyroball is known as a pitched ball which has its rotation axis oriented towards the catcher, i.e., in flight direction, and therefore does not create a lift force. The purpose of this study was to clarify what effect the seams of such a rotating gyroball have on the drag force acting on the ball. Two typical seam patterns, one with two and one with four seams, were selected. First, pitching experiments were carried out to capture the trajectories of various breaking balls. From the obtained trajectories the drag coefficients were estimated. Flow visualization was applied to a heated flying gyroball with the help of the schlieren technique to investigate the flow separation area. To verify the results obtained in the pitching tests, corresponding wind-tunnel experiments were also conducted with a device which allowed the ball to rotate freely in the tunnel. Drag measurements and flow visualization by fog were performed on a rotating gyroball. Both in the pitching and wind-tunnel tests, the drag coefficient of the two-seam gyroball was smaller than that of the four-seam one by 0.04 or approximately 13%. The flow visualization revealed that the flow-separated area of the two-seam gyroball was smaller due to flow reattachment made possible by a more energetic boundary layer. This observation can well explain the drag difference between two- and four-seam gyroballs.     

Entrained droplets in underexpanded gas jet in water

Abstract  

A visualization study was performed to investigate the flow of an underexpanded nitrogen gas jet injected into water. The stagnation pressure was varied in the range 0.5–8.0 MPa. The gas jet length and expansion angle were obtained from time-averaged images captured using a high-speed camera. The gas jet length and expansion angle increased approximately linearly with increasing stagnation pressure. The entrainment velocity and the velocity of entrained water droplets in the gas jet were obtained by particle image velocimetry.     

Design,Implementation, Simulation,and Visualization of a Highly Efficient RIM Microfluidic Mixer for Rapid Freeze-Quench of Biological Samples

Abstract  

Rapid freeze-quench (RFQ) trapping of short-lived reaction intermediates for spectroscopic study plays an important role in the characterization of biological reactions. Recently, there has been considerable effort to achieve sub-millisecond reaction deadtimes. We present here a new, robust, high-velocity microfluidic mixer that enables such rapid freeze-quenching. It is a based on the mixing method of two impinging jets commonly used in reaction injection molding of plastics. This method achieves efficient mixing by inducing chaotic flow at relatively low Reynolds numbers (Re = 140). We present the first mathematical simulation and microscopic visualization of mixing in such RFQ micromixers, the results of which show that the impinging solutions efficiently mix within the mixing chamber. These tests, along with a practical demonstration in an RFQ setup that involves copper wheels, show that this new mixer can in practice provide reaction deadtimes as low as 100 μs.     

Unsteady surface signature of a pulsed vortex generator jet

Abstract  

Porous pressure-sensitive paint (PSP) is employed as a visualization technique for unsteady flow features on a low-pressure turbine blade. Recognizing that the measurement of high-frequency pressure fluctuations in unsteady flows—especially in turbomachinery—has proven to be difficult, recent advancements in the development of porous PSP have enabled the high-resolution measurement of pressure fields with frequency content of at least 20 kHz. In this work, PSP is applied to an L1A low-pressure turbine blade section (Re = 20,000 based on axial chord) to visualize the surface dynamics of a vortex generator jet (VGJ) pulsed at 10.6 Hz with nitrogen gas. Intensity-based, time-resolved PSP measurements reveal the development and the surface structure of the VGJ as well as the spanwise variation in the blowing profile.     

Flow visualization for an unconventional forebody

Some results of flow visualization are described for an unconventional fuselage with Erickson-like forebody. The experiment includes force measurement, surface oil flow visualization, and laser sheet flow visualization. Some results are also obtained from a CFD code for solving laminar Navier-Stokes equations. Although there are some differences between the model in the experiment and the model in the computation, the location of vortex on the models exhibits good agreements. The investigation in this paper shows that varying the forebody shape of fuselage can change the flow characteristics greatly and the Erickson-like forebody has potential to improve the lateral-direction stability of the aircraft at high angles of attack.     

Color Schlieren imaging of high-pressure overexpanded planar nozzle flow using a simple, low-cost test apparatus

Abstract  

Complex flow features within rocket nozzles can exert significant influence on both the dynamics and safety of rockets during flight. Specifically, under over-expanded flow conditions, during, low altitude flight, random, often large side loads can appear within nozzles. While significant research has focused on this classical problem, due to the high nozzle pressure ratios (NPR) extant across rocket nozzles, most experimental work: (1) has focused on measuring wall pressure distributions under conditions when side loads appear, (2) has been carried out in large government or industrial test facilities, and (3) has only provided limited, though crucially important, visualization data. This short paper describes the construction and operation of a very simple, low cost test apparatus that allows imaging of flow features within planar nozzles, under the high NPR conditions characteristic of medium-to-large rockets. Representative color Schlieren images of flow shock structure obtained within the test apparatus are also presented and briefly described.     

Stereoscopic story visualization in literary works demonstrated by Shakespeare’s plays

Abstract  

A stereoscopic method of identifying story patterns in literary works is newly developed. The pattern is extracted from textual information by the detection of thematically assigned keywords, and depicted as visual imageries. The applicability of the method is demonstrated in several of Shakespeare’s plays. The complex scenario patterns in Shakespeare’s tragedies are successfully captured with applying the method for two different themes in each play. As the result, the organization of story accompanying multiple themes in a single play has been obtained as a pair of visual imageries, i.e. stereoscopic story visualization. This approach, in combination with a quadrant analysis of the plots, allows us in interpretation further complexity of human psychology in the characters and scene-by-scene transitions in each play.     

Electrohydrodynamic flow in a wire-plate non-thermal plasma reactor measured by 3D PIV method

This work was aimed at measurements of the electrohydrodynamic (EHD) secondary flow in a non-thermal plasma reactor using three-dimensional particle image velocimetry (3D PIV) method. The wide-type non-thermal plasma reactor used in this work was an acrylic box with a wire discharge electrode and two plate collecting electrodes. The positive DC voltage was applied to the wire electrode through a 10 MΩ resistor. The collecting electrodes were grounded. The voltage applied to the wire electrode was 28 kV. Air flow seeded with a cigarette smoke was blown along the reactor duct with an average velocity of 0.6 m/s. The 3D PIV velocity fields measurements were carried out in four parallel planes stretched along the reactor duct, perpendicularly to the wire electrode and plate electrodes. The measured flow velocity fields illustrate complex nature of the EHD induced secondary flow in the non-thermal plasma reactor.     

A robust security framework for 3D images

Abstract  

Three-dimensional (3D) visualization of spatial and non-spatial data is a well-established practice having numerous applications. The cheapest and the most efficient way to 3D visualization is 3D images/Anaglyphs. 3D images contain 3D information of the objects present in the image. These images are easily obtained by superimposing left and right eye images in different color in a single image. In this paper, a novel security framework, viz., watermarking scheme, is presented to ensure their security. The proposed security framework is employed in fractional Fourier transform domain of secret color channel followed by the embedding using singular value decomposition. The secret channels (SEC) are obtained by applying reversible integer transform on the RGB channels. The experimental results prove the robustness and imperceptibility of the proposed watermarking scheme.