Dye Visualization Research Articles

DYE VISUALIZATION OF A YAWED SLENDER DELTA WING

In the present experimental study, effects of yaw angle, β on the flow structure at high angles of attack, α with 70° sweep angle, Λ were investigated using the dye visualization technique. In the case of zero yaw angle, β a pair of leading edge vortices take place on both sides of cords axis are more or less symmetrical over the delta wing. These symmetrical vortical flow structures start deteriorating when angle yaw, β is introduced. Onset of vortex breakdown location on the windward side occurs further upstream, on the other hand, the leading edge vortex on the leeward side breaks down further downstream. A perceivable interactions are consisted between structures of vortical flows developed from pair of leading edge vortices after onset of vortex breakdowns. It can be concluded that the magnitude of the leading-edge vortices, onset of vortex breakdowns, and formation of unsteady flow structures generated after vortex breakdowns are substantially affected by yaw angle, β. Angles, θ1 and θ2 between centerline of the delta wing and both central axes of spiral vortices vary as a function of yaw angle, β.

Skin Friction Fields and Surface Dye Patterns on Delta Wings in Water Flows

This paper discusses the relationship between skin friction fields and surface dye patterns in surface luminescent dye visualizations in water flows, providing a theoretical foundation for extraction of high-resolution skin friction fields. The limiting form of the mass diffusion equation at a wall is recast as an optical flow equation connecting skin friction with the luminescent dye intensity. Snapshot solutions are obtained from a time sequence of luminescent intensity images by solving the optical flow equation via the variational method, and then a normalized skin friction field is reconstructed by averaging the snapshot solutions. An error analysis is given to identify the major error sources and the limitations of the technique. To evaluate the feasibility of this technique, surface luminescent dye visualizations on a 65 deg delta wing and a 76/40 deg double-delta wing are conducted in a water tunnel. The extracted skin friction topology on the delta wings and the velocity fields obtained by using particle image velocimetry (PIV) are discussed.

Probing Composition and Molecular Mobility in Thin Spherical Films Using Nuclear Magnetic Resonance Measurements of Diffusion

The composition and molecular mobility within thin spherical liquid films have been investigated using nuclear magnetic resonance (NMR) diffusion measurements. These films were formed either on the surface of pores inside a sponge at low saturation or by adsorbed water on the outside of urea prills during caking. Using pulsed field gradient (PFG) NMR experiments, the molecular mobility within these liquid films was determined through analysis of the conditional probability density for displacement (propagator). Molecular diffusion coefficients were determined for films in the sponge and prill systems by fitting the experimental propagators using a model for diffusion on an array of isotropically distributed infinite planes. By comparing these diffusion coefficients with bulk diffusion coefficients for a range of concentrations of urea solutions (2.1 M, 6.2 M and saturated), it was possible to optimize the PFG experimental parameters to enable accurate determination of molecular diffusion in these spherical liquid films. Determination of the diffusion coefficients for a range of urea solutions in the sponge enabled identification of the composition of the film that formed on the surface of the urea prills. Analysis of these data showed that the liquid layers are composed of saturated urea solution covering the surface of the prills, with an estimated layer thickness on the order of 10–5 m. The shape of the propagators indicated the adsorbed water was uniformly distributed over the surface of the urea prills, rather than primarily in the meniscus between particles, which agrees with dye visualization experiments on a pair of urea prills during caking. This work provides the first quantitative measurements of diffusion in thin spherical films, which is a key parameter for determining what controls the presence and rate of bonding between adjacent particle surfaces.

Absorption characteristics of falling film LiBr (lithium bromide) solution over a finned structure

In this study, an absorber design suitable for the plate-and-frame absorber configuration is introduced. The design utilizes a fin structure installed on a vertical flat plate to produce a uniform solution film and minimize its thickness and to continuously interrupt the boundary layer. Using numerical models supported by experiments employing dye visualization, the suitable fin spacing and size and surface wettability are determined. The solution flow thickness is measured using the laser confocal displacement measurement technique. The new surface structure is tested in an experimental absorption system. A significantly high absorption rate (6 × 10−3 kg/m2s at a pressure potential of 700 Pa) is achieved in comparison with the conventional absorption systems. The effect of absorber water vapor pressure, solution flow rate, solution inlet concentration, cooling water inlet temperature and solution inlet temperature on the absorption rate is investigated. The proposed design provides a potential framework for development of highly compact absorption refrigeration systems.

Effect of phase difference on the interaction of hairpin vortices induced by in-line twin synthetic jets

An experimental study is carried out to investigate the effect of operational phase difference of in-line twin synthetic jet actuators (SJAs) on the interaction of synthetic jet-induced hairpin vortices. The resulting vortex structures at four phase differences, i.e., Δϕ = 0°, 90°, 180°, and 270°, are presented and compared using both stereo dye visualization and particle image velocimetry (PIV) measurements. Three types of vortex structures are observed: one combined vortex at Δϕ = 90°, two completely separated hairpin vortices at Δϕ = 270°, and partially interacting vortex structures at Δϕ = 0° and 180°. The combined vortex is the strongest and most penetrates into the boundary layer. The completely separated hairpin vortices are the closest to the wall and hence are able to exert the most influence in the near-wall region. As for the partially interacting vortex structures, the head of one hairpin vortex interacts with legs of the other, producing complex vortex structures. Through this study it is also found that hairpin vortices issued from the upstream SJA are able to maintain their coherence more easily than their counterparts issued from the downstream SJA, regardless the phase difference. The secondary vortices captured by the PIV measurements are also compared.

Vortex patterns generated by a heaving flexible plate

We present dye visualizations of the wake behind a flexible plate flow when its leading edge is forced into harmonic heave motion. We show that, when the forcing amplitude is sufficiently large, a reverse Benard-von Karman vortex street is formed in the wake, indicative of the production of positive thrust.

PIV and LDA measurements of the wake behind a wind turbine model

In the present work we review the results of a series of measurements of the flow behind a model scale of a horizontal axis wind turbine rotor carried out at the water flume at Technical University of Denmark (DTU). The rotor is three-bladed and designed using Glauert theory for tip speed ratio λ =5 with a constant design lift coefficient along the span, CLdesign= 0.8. The measurements include dye visualization, Particle Image Velocimetry and Laser Doppler Anemometry. The wake instability has been studied in the range λ =3 – 9 at different cross-sections from the very near wake up to 10 rotor diameters downstream from the rotor. The initial flume flow was subject to a very low turbulence level with a uniform velocity profile, limiting the influence of external disturbances on the development of the inherent vortex instability. Using PIV measurements and visualizations, special attention was paid to detect and categorize different types of wake instabilities and the development of the flow in the near and the far wake. In parallel to PIV, LDA measurements provided data for various rotor regimes, revealing the existence of three main regular frequencies governing the development of different processes and instabilities in the rotor wake. In the far wake a constant frequency corresponding to the Strouhal number was found for the long-scale instabilities. This Strouhal number is in good agreement with the well-known constant that usually characterizes the oscillation in wakes behind bluff bodies. From associated visualizations and reconstructions of the flow field, it was found that the dynamics of the far wake is associated with the precession (rotation) of a helical vortex core. The data indicate that Strouhal number of this precession is independent of the rotor angular speed.

Impacts of inlet–outlet configuration, flow rate and filter size on hydraulic behavior of quasi-2-dimensional horizontal constructed wetland: NaCl and dye tracer test

The aim of this study was to assess the influences of filter size, inflow rate and inlet–outlet configuration on short-circuiting, effective volume, distribution of active/dead zones, dispersion and dye propagation in simulated constructed wetlands. A subsurface quasi two dimensional flow constructed wetland was set up for chloride tracer test and dye visualization. Three levels of filter size, inflow rate and inlet–outlet configurations were involved. Totally 81 combinations were determined. Breakthrough curves for residence time distribution and images for dye tracer propagation were obtained. Results showed that the inflow rate and inlet–outlet configurations had significant effects on hydraulic behavior. Bigger filter size and low influent decreased short-circuiting and reduced dead zones, thus increasing active area. Bigger filter size enhanced plug flow by decreasing porous water velocity, reducing preferential flow. High flow rate yielded low plug flow with lower CSTRs number caused by higher linear velocities and enlarged dispersion. Inlet–outlet configuration had most influences on flow distribution. Inflow rate had larger impacts on dead zones distribution for low inlet–high outlet configuration. For practical constructed wetland design, choice of inlet–outlet configuration and inflow rate would improve the treatment efficiency. Larger porous size, lower inflow rate with bottom inlet–top outlet configuration could help improve hydraulic behavior, thus enhancing wastewater treatment.

Chaotic advection at large Péclet number: Electromagnetically driven experiments, numerical simulations, and theoretical predictions

We present a combination of experiment, theory, and modelling on laminar mixing at large Péclet number. The flow is produced by oscillating electromagnetic forces in a thin electrolytic fluid layer, leading to oscillating dipoles, quadrupoles, octopoles, and disordered flows. The numerical simulations are based on the Diffusive Strip Method (DSM) which was recently introduced (P. Meunier and E. Villermaux, “The diffusive strip method for scalar mixing in two-dimensions,” J. Fluid Mech. 662, 134–172 (2010)) to solve the advection-diffusion problem by combining Lagrangian techniques and theoretical modelling of the diffusion. Numerical simulations obtained with the DSM are in reasonable agreement with quantitative dye visualization experiments of the scalar fields. A theoretical model based on log-normal Probability Density Functions (PDFs) of stretching factors, characteristic of homogeneous turbulence in the Batchelor regime, allows to predict the PDFs of scalar in agreement with numerical and experimental results. This model also indicates that the PDFs of scalar are asymptotically close to log-normal at late stages, except for the large concentration levels which correspond to low stretching factors.

Experimental Study of Flow Structures around Side-by-Side Spheres

The flow characteristics of a single and the two side-by-side spheres immersed in a uniform flow for Re = 5 × 103 are studied qualitatively and quantitatively employing dye visualization and a part...

Evaluating Reynolds number effects in small-scale wind turbine experiments

A series of experiments was conducted to evaluate the effect of the Reynolds number on the initial wake expansion and on the thrust coefficient in scaled wind turbine tests. Two scaled horizontal axis wind turbines were tested in a 0.6m×0.8m closed-loop water channel at Reynolds numbers that ranged from 3620 to 31,400, based on the blade tip speed and tip chord. The first rotor was a three-bladed rotor custom-designed to operate in these low Reynolds number conditions. The second rotor was a 1/4.8 scaled model of an existing two-bladed rotor. The initial wake expansion was assessed by using quantitative dye visualisation to identify the propagation of tip vortices downstream of the rotor. Thrust coefficients were determined from data obtained with a six degree-of-freedom balance. Dye visualisation indicated that the initial wake expansion downstream of the custom three-bladed rotor narrowed when the Reynolds number was lower than 20,000. Additional experiments compared the wake expansion downstream of the scaled two-bladed rotor to existing data recorded for a medium-scale rotor. The wake in the scaled two-bladed rotor was 30–60% narrower than the medium-scale rotor′s wake depending on the Reynolds number. Similarly, the thrust coefficient was reduced by 25–60% in the scaled two-bladed rotor experiments as a result of lower Reynolds numbers than in the medium-scale experiment. The results have identified that the thrust coefficient of a geometrically-scaled rotor may be degraded in a low Reynolds number environment whereas a rotor designed for the particular Reynolds number regime can generate thrust coefficients that are representative of commercial-scale wind turbines.

An experimental and computational assessment of blockage effects on wind turbine wake development

ABSTRACTAn experiment was conducted to evaluate the initial wake expansion in scaled wind turbine tests as a means to guide future wake interference studies. Five scaled wind turbine rotors with different diameters were designed for testing in a closed‐loop water channel to evaluate the effects of blockage on the initial wake expansion behind a wind turbine. The initial wake expansion was assessed by using quantitative dye visualization to identify the propagation of tip vortices downstream of the rotor. The thrust coefficient developed by the scaled models was recorded using a six‐component balance and was correlated to the downstream wake expansion. The rotors used in the experiment were operated at a tip speed ratio of 6, a Reynolds number based on the tip speed and tip chord of approximately 23,000 and resulted in blockage values that ranged from 6% to 25%. Dye visualization indicated that the initial wake expansion downstream of a rotor was narrowed and that tip vortex pairing behaviour was modified because of increasing blockage. Blockage effects were significant and resulted in a wake that was more than 50% narrower when blockage was 25% compared with the observed expansion with 10% blockage. A computational simulation was conducted with the Generalized Unsteady Vortex Particle (GENUVP) discrete vortex method code using the rotor in freestream conditions and was compared with the experiments. The magnitude of the wake expansion in the freestream computations was similar to the wake expansion in the experiment when blockage was less than 10%. Copyright © 2013 John Wiley & Sons, Ltd.

An experimental investigation of the approach flow conditions for a non-rotating, very low head water-turbine model

The adverse in-flow conditions for a very low head (VLH) water turbine have been investigated using a non-rotating turbine model. The aim of the study was to develop a better understanding of the three-dimensional flow generated upstream of the turbine by a step-like obstacle for varying turbine angles and spacings. Through flow-rate measurements, velocity-profile measurements and dye visualizations, the coupled effects between the turbine model and the varied in-flow geometry could be quantified. It was observed that the flow generated by the backward-facing step produced vortical structures that were found to corkscrew from the lower channel walls towards the mid-plane of the flow. These spanwise vortical structures were present through the interaction of the turbine model with the backward-facing step geometry for shorter cavity lengths. The coupled effects between model and step were found to be most pronounced for small turbine spacings on the order of one turbine diameter or less. Also significant velocity variations in the spanwise direction were observed in the cavity-like geometry. Finally, some preliminary results obtained from a rotating model are provided for comparison with the non-rotating model. Similar trends in terms of optimal spacing were observed, suggesting that a non-rotating model can be used for the preliminary experimental optimization of such three-dimensional cavity-like flows.

Aerodynamic studies over a manoeuvring UCAV 1303 configuration

AbstractThe flow past an unmanned combat air vehicle (UCAV) 1303 was investigated in a water tunnel in an effort to understand the complex flow field. The UCAV 1303 is characterised by a non slender, 47° swept-leading-edge delta wing, with a cranked trailing edge, a cropped wing tip and a fuselage. It also has no vertical tail. Dye visualisation pictures and five-axis strain gage load data were obtained on a 1:72 scale model in both steady flow and while the model executed preprogrammed manoeuvres. From the strain gage data, body axis force and moment data were derived. In addition to some similarities, comparison of the flow features observed with those of more well known slender delta wing flows showed some remarkable differences such as tip-stall, the presence of two like-sense vortices on the same side of the wing, their simultaneous spiral and bubble bursting. This paper predominantly discusses the flow visualisation results and attempts to correlate the load data to some of the observed flow features.

Flow structures around an equilateral triangle arrangement of three spheres

This paper represents the results of an experimental study on the flow structure around a single sphere and three spheres in an equilateral-triangular arrangement. Flow field measurements were performed using a Particle Image Velocimetry (PIV) technique and dye visualization in an open water channel for a Reynolds number of Re=5×103 based on the sphere diameter. The distributions and flow features at the critical locations of the contours of the velocity fluctuations, the patterns of sectional streamlines, the vorticity contours, the turbulent kinetic energy, the Reynolds stress correlations and shedding frequency are discussed. The gap ratios (G/D) of the three spheres were varied in the range of 1.0⩽G/D⩽2.5 where G was the distance between the sphere centers, and D was the sphere diameter which was taken as 30mm. Due to the interference of the shedding shear layers and the wakes, more complex features of the flow patterns can be found in the wake region of the two downstream spheres behind the leading sphere. For G/D=1.25, a jet-like flow around the leading sphere through the gap between the two downstream spheres occurred, which significantly enhanced the wake region. It was observed that a continuous flow development involving shearing phenomena and the interactions of shedding vortices caused a high rate of fluctuations over the whole flow field although most of the time-averaged flow patterns were almost symmetric about the two downstream spheres.

Experimental investigation of freely falling thin disks. Part 1. The flow structures and Reynolds number effects on the zigzag motion

AbstractThis paper describes an experimental investigation of the dynamics of a freely falling thin circular disk in still water. The flow patterns of the disk zigzag motion are studied using dye visualization and particle image velocimetry. Time-resolved disk motions with six degrees of freedom are obtained with a stereoscopic vision method. The flow separation and vortex shedding are found to change with the Reynolds number, $\mathit{Re}$. At high Reynolds numbers a new dipole vortex is shed that is significantly different from Kármán-type vortices. The vortical structures are mainly composed of leading-edge vortices, a counter-rotating vortex pair and secondary trailing-edge vortices. The amplitude of the horizontal oscillation is also dependent on the Reynolds number with a critical Reynolds number ${\mathit{Re}}_{cr} \approx 2000$, where the oscillatory amplitude is proportional to $\mathit{Re}$ for $\mathit{Re}\lt {\mathit{Re}}_{cr} $, but becomes invariant for $\mathit{Re}\gt {\mathit{Re}}_{cr} $. Three-dimensional dipolar vortices were also observed experimentally.

The effect of angle of attack on the flow structure over the nonslender lambda wing

The aim of the current study at first stage is to demonstrate the general flow structure qualitatively using the dye visualization technique. Secondly, the instantaneous flow data taken by a Stereoscopic Particle Image Velocimetry (stereo-PIV) over a stationary nonslender lambda wing is used to determine the time-averaged flow topology in order to provide detailed information about crucial events like formation of leading edge vortices and vortex breakdown in plan-view planes and cross-flow planes. The flow structure close to the lambda wing surface and development of the vortex breakdown are investigated as functions of angles of attack within the range of 7∘⩽α⩽17∘. Experimental analyses are composed of time-averaged patterns of streamlines, vorticity contours, transverse and streamwise velocity components, Reynolds-stress correlations, distribution of fluctuating velocities, and turbulent kinetic energy. Results show that the angle of attack has a substantial influence on the flow behavior on the nonslender lambda wing surface.

Comparison of Flow Characteristics of Different Sphere Geometries Under the Free Surface Effect

Comparison of the experimental results of turbulent flow structures between a smooth sphere and a sphere with a vent hole, roughened, and o-ring is presented in the presence of a free-surface. Dye visualization and particle image velocimetry (PIV) techniques were performed to examine effects of passive control methods on the sphere wake for Reynolds number Re = 5000 based on the sphere diameter with a 42.5mm in an open water channel. Instantaneous and time-averaged flow patterns in the wake region of the sphere were examined from point of flow physics for the different sphere locations in the range of 0≤h/D ≤2.0 where h was the space between the top point of the sphere and the free surface. The ratio of ventilation hole to sphere diameter was 0.15, o-ring was located at 55 o with a 2 mm from front stagnation point of the sphere and roughened surface was formed by means of totally 410 circular holes with a 3 mm diameter and around 2 mm depth in an equilateral triangle arrangement. The flow characteristics of instantaneous velocity vectors, vorticity contours, time-averaged streamline patterns, Reynolds stress correlations and streamwise and cross-stream velocity fluctuations for both the smooth and passively controlled sphere were interpreted.

A Targeted Approach to Sentinel Lymph Node Biopsies in the Parotid Region for Head and Neck Melanomas

Cutaneous melanoma is on the rise in the United States, and the head and neck region is the primary site in 20% of patients. Lymph node status is the best indicator of prognosis for melanoma. In the head and neck, sentinel lymph node (SLN) biopsy presents particular challenges, with the parotid region posing difficulties that include locating the lymph nodes, less frequent visualization of blue dye, and the possibility of higher morbidity because of the proximity of lymph nodes to important neurovascular structures. Surgical approaches to the SLN dissection in the parotid region are variable, and may include superficial or total parotidectomies. Parotid-sparing SLN biopsies for head and neck melanomas were evaluated to determine rates of local recurrence. The charts of 301 patients from the Yale Melanoma Unit who underwent resection of their head and neck melanoma were reviewed. The location of the primary melanoma was noted, and the sentinel lymph node dissections from the operative reports were documented. Demographic and outcome data were recorded, including course of melanoma management, local recurrence, and postoperative course. Fifty-eight patients underwent SLN biopsy of lymph nodes in the parotid region. Parotid-sparing SLN biopsies comprised 94.8% of total surgical approaches for SLN biopsies in the parotid region. Of the remaining patients who underwent SLN biopsies in the parotid region, 5.17% had a superficial parotidectomy and none had a total parotidectomy. Sentinel lymph nodes were found in all depth layers of the parotid, and LNs were dissected out successfully without the need to remove the parotid in the most cases. The parotid region recurrence rate was 0% for SLN biopsies that either included or spared the parotid gland. There were no localized complications from the sentinel lymph node biopsies. The parotid-sparing SLN biopsy was performed without any local recurrence in the parotid region. The parotid-sparing SLN biopsy can be carried out in a safe, efficient manner without affecting the rate of local recurrence or postoperative complication. This less-invasive SLN biopsy procedure precludes the complications associated with parotidectomies and may reduce the morbidity for patients with melanomas of the head and neck.

Suppression of asymmetric flow behavior downstream of two side-by-side circular cylinders with a splitter plate in shallow water

The flow downstream of a pair of circular cylinder in a side-by-side arrangement normal to the free stream is known to exhibit intermittently bistable structure for the range of G/D=1.2–2.2 where G is the center-to-center distance between the cylinders and D is the diameter of the cylinder. Eventually, the wake downstream of one of the two cylinders can be wider or narrower than the one downstream of the other cylinder depending on the direction of gap-flow deflection. In the present study, such an asymmetric flow behavior downstream of two side-by-side cylinders, which were vertically located in shallow water, was passively controlled with a splitter plate with a length of L (1⩽L/D⩽5). The center of splitter plate was just coincided with the mid-height of the gap between centers of the cylinders. The investigations were carried out in a water channel using dye visualization and particle image velocimetry, PIV for qualitative and quantitative measurements, respectively. The diameter of the cylinder, D was 40mm while the depth of water was 20mm so that the shallow flow condition was provided through the experiments. The Reynolds number, Re based on D was 5000 and the cylinder’s center to center spacing to the cylinder diameter ratio (G/D) was equal to 1.25. The results demonstrated that the deflection of the wake and thereby the bistability of the wake was considerably prevented with the presence of the splitter plate for L/D⩾3 which resulted in two well symmetric, stable wakes having approximately the same order of magnitudes of vortex shedding frequencies around the cylinders.