Visualization Of Flow Structure Research Articles

Forced air flow through a rectangular channel with 3D turbulence enhancers: visualization of flow structures by laser sheet scattering

In recent times, the design of heat transfer devices is strongly evolving due to both the diffusion of additive manufacturing techniques, that use a wide variety of materials, and Computational Fluid Dynamics (CFD) -based shape optimization techniques. Considering high-aspect ratio rectangular channels, that represent the geometry of heat transfer devices used, among other applications, for batteries cooling systems and compact automotive heat exchangers, the first step of a project towards the design of optimized turbulence enhancers is the validation of the CFD methodology. In particular, since it is important to have a fast-running 3D simulation with physical meaning, several techniques of increasing complexity and computational requirements and times are currently under scrutiny, and their validation is mainly carried out by experiments that measure both local and global heat transfer enhancement. In this context, flow visualization is a powerful tool to get insights of the physical mechanisms that generate convective heat transfer enhancement. In this paper, the setup of a laser sheet scattering method to visualize turbulent structures in a Plexiglas, 1:10 aspect-ratio, rectangular channel ribbed on one of the main surfaces is presented, and results on two different cutting planes are presented for 90° and V-down squared ribs for Re = 500.

Halocline Internal Wave Attractors Visualization

The phenomenon of wave attractor, originating from ocean dynamics, in the last couple decades has become widely-studied both laboratorily and numerically. However, their discoveries in wild nature are still rare hence deepwater ones are out of technique, and attractor in smaller basins cannot form because of energy overinjection. In the current work we show that attractor may exist in shallower depths regardless the depth of the basin’s bottom. Basing on the peculiar dispersion relation typical for stratified fluid attractors can be ”trapped” in a narrow halocline layer which may facilitate their detection in the nature. Additionally, we discuss the formed flow structure visualization problems.

Visualization of post-detonation fireball flowfields and comparison to CFD modeling

Visualization of flow structures within post-detonation fireballs has been performed for benchmark validation of numerical simulations. Custom pressed PETN explosives with a 12-mm diameter hemispherical form factor were used to produce a spherically symmetric post-detonation flow with low soot yield. Hydroxyl-radical planar laser induce fluorescence (OH-PLIF) was employed to visualize the structure ranging from approximately 10μs to 35μs after shock breakout from the explosive pellet. Fireball simulations were performed using the HyBurn Computational Fluid Dynamics (CFD) package. Experimental OH-PLIF results were compared to synthetic OH-PLIF from post-processing of CFD simulations. From the comparison of experimental and synthetic OH-PLIF images, CFD is shown to replicate much of the flow structure observed in the experiments, revealing potential differences in turbulent length scales and OH kinetics. Results provide significant advancement in experimental resolution of these harsh turbulent combustion environments and validate physical models thereof.

Aerodynamic and RSM Analysis of Wingsuit Stability

A Wingsuit is a Skydiving Jumpsuit that generates more lift for longer flights. This study examined the effects of side slip angles on a beginner wingsuit at 106 Reynolds number. Experimental tests were determined by using the length of the model scale at angles of attack ranging from 0° to 40° and sideslip angles of up to 20°. Force and moment coefficients were analyzed using variations in angles of attack and sideslip. Despite the absence of significant effects of sideslip angles on the lift and drag coefficients, side force and rolling/yawing moments were highly nonlinear. Flow structure visualization and numerical simulation show that surface stalls only occur on the lower side when slip angles are lower. In individual aviation sports, wingsuits are more advantageous when they have less sideslip. With Tuft visualization on the wingsuit model, the best aerodynamic coefficient under different flight conditions was determined by comparing the Response Surface Methodology performance under different flight conditions.

Multicolor dye-based flow structure visualization for seal-whisker geometry characterized by computer vision

Pinniped vibrissae possess a unique and complex three-dimensional topography, which has beneficial fluid flow characteristics such as substantial reductions in drag, lift, and vortex induced vibration. To understand and leverage these effects, the downstream vortex dynamics must be studied. Dye visualization is a traditional qualitative method of capturing these downstream effects, specifically in comparative biological investigations where complex equipment can be prohibitive. High-fidelity numerical simulations or experimental particle image velocimetry are commonplace for quantitative high-resolution flow measurements, but are computationally expensive, require costly equipment, and can have limited measurement windows. This study establishes a method for extracting quantitative data from standard dye visualization experiments on seal whisker geometries by leveraging novel but intuitive computer vision techniques, which maintain simplicity and an advantageous large experimental viewing window while automating the extraction of vortex frequency, position, and advection. Results are compared to direct numerical simulation (DNS) data for comparable geometries. Power spectra and Strouhal numbers show consistent behavior between methods for a Reynolds number of 500, with minima at the canonical geometry wavelength of 3.43 and a peak frequency of 0.2 for a Reynolds number of 250. The vortex tracking reveals a clear increase in velocity from roll-up to 3.5 whisker diameters downstream, with a strong overlap with the DNS data but shows steady results beyond the limited DNS window. This investigation provides insight into a valuable bio-inspired engineering model while advancing an analytical methodology that can readily be applied to a broad range of comparative biological studies.

Effect of oblique shock wave induced from a wavy-wall combustor surface for a splitter plate scramjet combustor

The shorter residence period of supersonic air in a scramjet combustor makes mixing and combustion challenging. Mixing augmentation occurs at the fuel-supersonic air interface. Multiple interactions between shock waves and the shear layer may significantly affect this inter surface. In this research, an attempt has been made to analyze how multiple oblique shock waves interact with the shear layer. The primary splitter plate combustor bottom wall is modified with a wavy-wall surface to ensure the development of multiple oblique shock waves. The internal flow field with and without a wavy wall surface has been analyzed by solving the two-dimensional Reynolds averaged Navier-Stokes equations and SST k-ω turbulence model. The reaction between ethylene fuel and the air is modeled with a global one-step reaction mechanism with finite rate eddy dissipation turbulence chemistry interaction. The flow disturbances with the wavy-wall surface have been evaluated by analyzing the numerical results like the flow structure, pressure, velocity, reaction rate, vortices, turbulence intensity, and interactions among shock wave, shear mixing, and boundary layers. The oblique shock waves induced from the wavy-wall surface significantly impact the mixing of fuel and air and successful reaction mechanism from the visualization of flow structure and concern results.

Control of roughness-induced transition under the influence of inflow disturbance

Control of roughness-induced transition influenced by two- or three-dimensional inflow disturbances using spanwise-uniform wall blowing is investigated from the perspective of unstable modal growth with direct numerical simulations. Visualization of flow structures for two-dimensional disturbances indicates that the counter-rotating vortex pair in the wake bends the high-shear layer, which is related to both symmetric and antisymmetric unstable modes. The interaction of these structures is found to be responsible for the initial growth of the symmetric mode, which dominates the transition process. Both upstream and downstream position wall blowing are effective for transition control. Upstream position blowing works by lifting up the boundary layer and diminishing the wake structures, resulting in inhibition of the symmetric mode. Downstream position blowing has an even better effect, attributable to reconfiguration of the streamwise vorticity field in the wake and further distortion of the high-shear layer. When the inflow disturbance is a three-dimensional wave, the antisymmetric mode becomes dominant in the near-wake region, resulting in more rapid initial growth of the symmetric mode. As oblique waves have higher growth rates in a compressible boundary layer, the background disturbance is obviously stronger. Therefore, transition occurs earlier than for two-dimensional disturbances. When upstream position blowing is introduced, the spanwise shear related to the antisymmetric mode is reduced, and the background disturbance is suppressed as the disturbed boundary layer is pushed away from the roughness. Downstream position blowing can still inhibit the symmetric mode, but the antisymmetric mode is barely influenced, since this blowing is less effective in suppressing the counter-rotating vortex pair.

Experimental Study of the Flow Structure Stability on the Bubble Surface

The results of the flow structure visualization experiments conducted on the surface of a single bubble streamlined by uniform flow are presented. It is shown that, at certain critical values for bubble size, flow velocity, and contamination level, the axial symmetry of the surface flow loses its stability in a threshold manner, and the first instability mode in the form of two vortices appears. Below the threshold, the stationary flow on the bubble surface is impossible. The experimental results indicate that the assumption about the axial symmetry of the motion on the bubble surface containing surfactants, which is used in most theoretical and numerical studies, is invalid. Analysis of the results has revealed the most likely reason for the spiral form of the trajectory in the problem of a small rising bubble in the surrounding fluid. For the surfactant-free surface realized in the experiments with isopropyl alcohol, the rising trajectory was a straight line, and no vortex structures were observed on the bubble surface. In the experiments with water, a spiral rising trajectory was observed, and the first instability mode was formed on the bubble surface.

Mach number effect on droplet phase angular distribution behind a nozzle under near-wall liquid film outflow with co-current gas flow into vacuum

The paper is devoted to experimental study of near-wall ethanol film outflow with high-velocity co-current air flow from sonic and supersonic nozzles into vacuum. Using the developed measurement techniques (droplet phase flow structure visualization, deposition of droplets on paper substrates, spectrophotometry method), the effect of the nozzle Mach number on angular distribution of droplet phase in the central and the peripheral flow regions behind the nozzle exit is established.

Evolution of wall flow structure and measurement of shear stress issuing from supersonic jet with extended shelf

This paper reports an experimental study on the supersonic jet surface flow structure visualization and shear stress field measurement issuing from a rectangular nozzle with extended shelf. The evolution of the near-field surface flow structures with an increased Nozzle Pressure Ratio (NPR) is successfully captured by the surface oil flow, infrared detection technology, and the Shear-Sensitive Liquid Crystal Coating (SSLCC) technique. Results reveal that under smaller NPR, the wall flow structure is similar to that of a jet without the extended shelf i.e., clean jets, and this is caused by insufficient effect on the boundary layer. However, at higher amplitudes of NPR, there exists a significant effect of the boundary layer, as a near triangular separation forms on the trailing edge of the Mach stem due to the adverse pressure gradient, which is visualized for the very first time in this paper. Furthermore, the vector field of shear stress is measured quantitatively by SSLCC technique. Results shows that the magnitude of shear stress heightened with NPR increasing, and the directions of shear stress changes across the shock wave and expansion fans. In addition, surface streamlines measured by SSLCC is significantly consistent with the streamlines visualized using the oil flow technique.

Onset of intermittent flow: Visualization of flow structures

The transition from stratified to intermittent air-water two-phase flow was investigated experimentally, by flow visualization and pressure drop signals analyses, in a 30 mm ID pipe. The intermittent flow’s onset was found to be mainly dependent on the liquid superficial velocity and the pipe diameter. Plug flow, Less Aerated Slug (LAS) or Highly Aerated Slug (HAS) flows could be obtained on the gas superficial velocity grounds. The available models, compared to experiments, could not predict adequately the intermittent flow onset. The appearance of liquid slugs was revealed by peaks in the pressure drop signal. Furthermore, it was shown that the available slug frequency correlations were not valid in the zone of the onset of intermittent flow.

Visualization and measurement of supersonic jet flow field evolution issuing from a rectangular nozzle with aft-deck

An experimental study is reported of supersonic jet surface flow structure visualization and wall shear stress field measurement issuing from a rectangular nozzle with aft-deck. The near-field surface flow structures evolution from over-expansion to under-expansion with the increase of nozzle pressure ratio (NPR) are successfully captured by surface oil flow visualization and shear sensitive liquid crystal coating (SSLCC) technique. The quantitative measurement result of shear stress vector field obtained by SSLCC shows that shear stress directions change significantly across the shock wave and expansion fans, while the magnitudes of shear stress have no obvious changes. Surface streamlines calculated by SSLCC image keep great consistency with the streamlines visualized using oil flow technique, which demonstrates the accuracy and potential application of SSLCC in supersonic jet surface flow visualization.

Revisiting tin melting for phase change model verification

Model verification is necessary before numerical models can be applied to produce meaningful results. For solid-liquid phase change modelling involving convection, pure gallium and tin melting have been widely used as reference for verification. It was later found that contrasting observations have been reported on the flow structure of both metals in the liquid region during the phase change process. Some researchers have reported monocellular while others reported multicellular structures in past works. In this work, tin melting problem was revisited by extending the results to flow structure visualization with Line Integral Convolution (LIC) plots to confirm the flow structure for tin melting thus pure metals in general. Enthalpy-porosity formulation coupled with Finite-Volume Method (FVM) was used to solve the set of governing equations which represented the problem at Prandtl Number = 0.02, Stefan Number = 0.01 and Rayleigh Number = 2.5 x 105. The location of solid-liquid interface and LIC plots at different times were presented. At initial state, the solid-liquid interface was closely similar for all grid sizes but as time progresses, finer grids provided improved solutions as expected. Reasonable fine grid size must be selected for solid-liquid phase change models to ensure complete physics of the problems are captured and eventually yield acceptable numerical results. The LIC plots confirmed that the flow structure is multicellular. Future phase change models referring to pure metal melting problem for verification should obtain similar flow structure to be considered acceptable.

The Scale Characteristics and Formation Mechanism of Aeolian Sand Streamers Based on Large Eddy Simulation

AbstractAeolian sand transport near the surface in the field often occurs in the form of sand streamers, which is very important to understand in‐depth the physics of aeolian transport and accurately predict the sand transport rate. In this paper, sand streamers were successfully reproduced by combining the Lagrangian trajectory simulation of particles with large eddy simulation of turbulence. The simulated sand streamers are hill‐like. Their characteristic scales are extracted by the two‐point correlation of sand concentration. It is found that sand streamers corresponding to the correlation threshold of are 2.0–4.5 m long and 0.3–0.4 m wide, while 30–100 m long and 1.2–2.4 m wide for , which are substantially consistent with the existing field observation results and increases with particle size. Conditional averaging method provided an ensemble‐mean visualization of flow structures that are responsible for the formation of streamers. It is found that sand streamers on a flat and erodible surface exist in the near‐surface wake area of large/very large scale turbulent motions (LSMs/VLSMs), indicating that sand streamers are the footprint of LSMs/VLSMs. Moreover, the characteristic scales of sand streamers will be affected by the splashing process of sand saltation. On a nonerodible surface, sand streamers trailing from upstream sandy surface become narrow though it can propagate downstream for a long distance.

An experimental study of the influence of the type of turbulent wake on the flow around models of wings of various shapes

This paper presents the results of experimental studies aimed at studying the influence of the type of turbulent wake on the flow around models of various shapes. The patterns of visualization of flow structure for each flow regime were obtained on all models. The structure of the turbulent wake in space for two sources of disturbances is investigated. A comparison was made between the results. It was found that, depending on the type of turbulent wake, the separation region can significantly decrease in size.

Measurement of multi-dimensional distribution of local bubble parameters in a vertical annulus under subcooled boiling conditions

In this study, an experimental investigation was performed to measure the local bubble parameters in steam-water subcooled boiling flow in a vertical channel at near-atmospheric pressure. The test section was a vertical concentric annulus in which the electric heater rod was installed at the center of the flow channel. The effective heated section was 2 m in length, and inner and outer diameters of the annulus were 10 mm and 30 mm, respectively. The optical fiber probes were used to measure various local bubble parameters in the subcooled flow boiling. The local void fraction, bubble arrival frequency, bubble velocity, interfacial area concentration (IAC), and Sauter mean diameter were measured at four elevations, and their radial profiles were obtained at 10 points by traversing the sensing tip of optical fiber probes. The high-speed camera was also applied for visualization of flow structures at three elevations. In the experiments, the flow conditions were set for the heat flux of 198 ~ 377 kW/m2, the mass flux of 376 ~ 680 kg/m2s and the inlet subcooling of 12.3 ~ 15.8 K at a pressure ranging from 115 to 202 kPa. It is expected that the obtained datasets will be useful for assessment of boiling models implemented in a CFD-scale analysis code.

Visualization of flow structures inside a conical spouted bed by electrical capacitance volume tomography

Electrical capacitance volume tomography (ECVT) was conducted to measure instantaneous gas–solid flow structures in a conical spouted bed. The sensor has 24 electrodes/channels and was used to make three-dimensional measurements of the solid volume fraction. Flow structures in the conical spouted bed were analyzed using quantitative ECVT images. Three-dimensional images of gas–solid flow structures and time-averaged vertical and radial solid volume fraction profiles are presented. Different fluidization regimes in spouted beds were characterized by analyzing pressure fluctuation signals and ECVT images. ECVT was shown to be robust and powerful for the three-dimensional visualization and quantitative measurement of a gas–solid fluidization system.

Visualization of flow structures in a rotary type energy recovery device by PIV experiments

The rotary type energy recovery device (RERD) has been widely employed in the Reverse Osmosis membrane technology to decrease the system electricity consumption. Understanding of the flow dynamic in the rotor duct is beneficial to design a high-performance of a RERD, but visualization experiments have hardly been carried out due to the high-speed rotation of the rotor duct. Hence, a visualization apparatus of a RERD with a single stationary duct and two rotary endcovers was proposed in this study. A two-dimensional particle image velocimetry (PIV) measurement was applied to visualize the flow structure in the duct. The phase-locking method and the time-averaged processing method were utilized to analyze the vortex dynamics and the flow characteristics. From experimental results, an “entrance effect” can be clearly identified at the duct entrance, which caused the vortex formation. At the duct entrance, the unstable flows structure demonstrated a three-dimensional structure. The liquid piston, a stable flow with uniform velocity, also can be found in the middle section of the duct, effectively preventing the over mixing between the seawater stream and the brine reject stream. These experiment results would be beneficial to researchers in better understanding the flow dynamic in the duct of a RERD.

Development of videogrammetry as a tool for gas-particle fluidization research

Many industries use fluidization of solid particles for energy efficiency or environmental friendly process development, and this paper introduces research techniques developed for investigating gas-particle systems At present there is plenty of room for refining gas-particle fluidization process. With the rapidly rising application of mathematical modelling, real time visualization of processes will be widely used for validation of those models in the near future. In presented research, photogrammetry, as a part of close range vision metrology, has been expanded to allow dynamic space and time analysis of the phase concentration distribution inside fluidization devices. A novel videogrammetry method was created with additional stochastic process analysis for detailed frequency and amplitude characteristics. Videogrammetry was used for the assessment of flow regimes, which were held in various types of fluidization apparatuses. Classic bubbling, jet-spouted and fast circulating fluidization processes were explored under the investigation. Videogrammetry is non-invasive flow regime recognition method, which enables detailed research of gas-particle fluidization phenomena. Until now, there were no comparative studies for three different types of fluidization processes with the use of one complex approach. Developed videogrammetric method consists of the flow structure visualization and dynamic image analysis. The analysed feature is the grey level of the image in time domain, and grey level signals were analysed with the use of autocorrelation function and power density function. The results are presented as images, plots and a flow map. Efficiency of the method was tested by comparison of real observed flow structures to the reconstructed flow structures and the recognition accuracy reached 92%.

Visualization of flow structures and mixing characteristics induced by forced vibration in supersonic mixing layer

Visualization of flow structures and mixing characteristics induced by forced vibration in supersonic mixing layer