Use Of Flow Visualization Research Articles

Improvement and validation of a flow model for conical vortices

Separation bubble and conical vortices on a large-span flat roof were observed in this study through the use of flow visualization. The results indicated that separation bubble occurred when the flow was normal to the leading edge of the flat roof. Conical vortices that occur under the cornering flow were observed near the leading edge, and their appearance was influenced by the wind angle. When the wind changed from along the diagonal to deviating from the diagonal of the roof, the conical vortex close to the approaching flow changed from circular to be more oblong shaped. Based on the measured velocities in the conical vortices by flow visualization, a proposed two-dimensional vortex model was improved and validated by simplifying the velocity profile between the vortex and the potential flow region. Through measured velocities and parameters of vortices, the intensities of conical vortices and separation bubble on a large-span flat roof under different wind directions were provided. The quasi-steady theory was corrected by including the effect of vortices. With this improved two-dimensional vortex model and the corrected quasi-steady theory, the mean and peak suction beneath the cores of the conical vortices and separation bubble can be predicted, and these were verified by measured pressures on a larger-scale model of the flat roof.

WHY MAKING USE OF FLOW VISUALIZATION?

The methods of flow visualization serve for visualizing the motion of fluids that are normally transparent and invisible to the naked eye. Following a postulation of Hermann von Helmholtz, visual perception of the processes under study supports the understanding of the seen or recorded phenomena, here: developing a physical model of the flow processes. This postulation requires a link between the eye and brain where a sufficient knowledge from the past must be stored in order to interpret the visible information. Among all possible sensations, seeing is the most significant source for "constructing a picture of the world" (Ernst Mach). Therefore, quantitative data on physical processes, obtained by measurements or calculations, are most frequently displayed in a visible form in order to facilitate the understanding of their meaning. A flow picture includes an abundance of quantitative data; a number of fundamental flow phenomena unknown before have been discovered by appropriate visualization, e.g., the existence of a supersonic flow, or the formation of coherent structures in turbulent flow.

Clean seeding for flow visualization and velocimetry measurements

Flow visualization, particle image velocimetry (PIV), and laser Doppler velocimetry (LDV) are among the most useful tools available for experimental aerodynamics studies. Implementation of these techniques, however, requires that seed material be introduced into the flow. The undesirable qualities of the seeding material often prevent the use of flow visualization and velocimetry techniques in many test environments. This is particularly true for large-scale, closed-circuit tunnels where facility operators must weigh the risks of facility contamination, sensor damage, and safety concerns that might result from the introduction of seed particles. Identification of a practical clean seeding material that minimizes or eliminates these concerns would enable flow visualization and velocimetry techniques to be deployed in these facilities. Here, we demonstrate two seeding systems that have the potential to provide such a solution. The first system is a new concept which uses liquid carbon dioxide that can be made to form discrete particles as it expands from a high-pressure tank. PIV measurements are demonstrated in several flows, including supersonic and subsonic tunnels, using these residue-free seed particles. The second system utilizes a combination of steam and liquid nitrogen to produce an aerosol or fog that serves as flow seeding. Water- or steam-based seeding has been previously demonstrated for flow visualization in subsonic tunnels; here however, we utilize this seed material for PIV and LDV measurements as well as for flow visualization in a large supersonic tunnel.

Condensation Flow Mechanisms in Microchannels: Basis for Pressure Drop and Heat Transfer Models

This paper presents an overview of the use of flow visualization in micro- and mini-channel geometries for the development of pressure drop and heat transfer models during condensation of refrigerants. Condensation flow mechanisms for round, square, and rectangular tubes with hydraulic diameters in the range of 1–5 mm for 0 < x < 1, and 150 kg/m2-s and 750 kg/m2-s were recorded using unique experimental techniques that permit flow visualization during the condensation process. The effect of channel shape and miniaturization on the flow regime transitions was documented. The flow mechanisms were categorized into four different flow regimes: intermittent flow, wavy flow, annular flow, and dispersed flow. These flow regimes were further subdivided into several flow patterns within each regime. It was observed that the intermittent and annular flow regimes become larger as the tube hydraulic diameter is decreased, and at the expense of the wavy flow regime. These maps and transition lines can be used to predict the flow regime or pattern that will be established for a given mass flux, quality, and tube geometry. These observed flow mechanisms, together with pressure drop measurements, are being used to develop experimentally validated models for pressure drop during condensation in each of these flow regimes for a variety of circular and noncircular channels with 0.4 < Dh < 5 mm. These flow regime-based models yield substantially better pressure drop predictions than the traditionally used correlations that are primarily based on air-water flows for large diameter tubes. Condensation heat transfer coefficients were also measured using a unique thermal amplification technique that simultaneously allows for the accurate measurement of the low heat transfer rates over small increments of refrigerant quality and high heat transfer coefficients characteristic of microchannels. Models for these measured heat transfer coefficients are being developed using the documented flow mechanisms and the corresponding pressure drop models as the basis.

The most profitable use of flow visualization in the elimination of thrombus from a monopivot magnetic suspension blood pump.

The purpose of this study was to eliminate fluid dynamic causes of thrombus formation for the monopivot magnetic suspension centrifugal pump under development with the aid of flow visualization as an indirect measurement tool for animal experiments. The formation of thrombus observed in early animal experiments was successfully overcome by combining the multiple washout holes at the center into a single hole, optimizing the hole diameter, and eliminating the pivot gap. Flow visualization was used to optimize the washout hole diameter influencing the flow around the pivot. In animal experiments flow visualization showed that the contours of thrombus corresponded to shear rates of 300 s(-1) or 1300-1700 s(-1). It was found that flow visualization is a useful technique to predict locations where low shear thrombi form and to optimize the pump design in the development stage.

Sound Generation by a Centrifugal Pump at Blade Passing Frequency

This paper reports the results of an experimental study of the pressure pulsations produced by a centrifugal volute pump at its blade passing frequency and their amplification by acoustic resonance in a connected piping system. Detailed measurements were made of the pressure fluctuations in the piping as a function of pump speed and flow rate. A semi-empirical model was used to separate acoustic standing waves from hydraulic pressure fluctuations. The effects of modifying the cut-water geometry were also studied, including the use of flow visualization to observe the flow behavior at the cut-water. The results suggest that the pump may act as an acoustic pressure or velocity source, depending on the flow rate and the cut-water geometry. At conditions of acoustic resonance, the pump acted as an open termination of the piping, i.e., as a node in the acoustic pressure standing waves. Rounding the cut-water had the effect of reducing the amplitude of acoustic resonance, apparently because of the ability of the stagnation point to move and thereby reduce the vorticity generated.

The use of Flow Visualization and Computatlonal-Fluid-Oynamics (CFD) as optimization tools for duct geometry design In artificial heart valves

The use of Flow Visualization and Computatlonal-Fluid-Oynamics (CFD) as optimization tools for duct geometry design In artificial heart valves

Effects of handrails on separated shear flow and vortex-induced oscillation

The effects of a simplified handrail on the separated shear flow near leading edge and on the vortex-induced oscillation are investigated by several wind tunnel experiments, by use of flow visualization, to observe aerodynamic behavior and pressure distribution around the deck surface. The handrail is composed of vertical posts and a horizontal bar whose depth and height from deck surface are varied. The flow pattern near the leading edge is greatly influenced by the depth and height of the horizontal bar. Different responses for several handrails are clarified by the flow patterns and unsteady pressure properties.

Parameterisation of horizontal leaf filters and optimisation for the filtration of beer

For process design it is necessary to analyse the flow phenomena in precoat filters. This is achieved with the use of flow visualization in models of horizontal leaf filters in different dimensions. The parameterisation of the hydrodynamics in the filter embraces variations in the construction and operating parameters. The results are then transferred to a pilot plant. This subsequently leads to an optimised filter apparatus with improved filtration performance. The channelling of the suspension flow, combined with the optimal Reynolds number and flow velocity, leads to a homogeneous flow to the filter's surface with a homogeneous build-up of the filter cake. For the filtration of beer the optimised apparatus has a larger filtrate volume per run, reduced differential pressure rise, reduced specific consumption of filter aid, water, energy and detergents.

Bubble characteristics in three-phase systems used for pulp and paper processing

Experiments are conducted to study the hydrodynamic behavior of three-phase pulp slurry systems at consistencies ranging from 0.1 to 1.0% (weight percent pulp). The pulp phase is a low-density fibrous material which swells to several times its original volume in water. The overall hydrodynamic behavior, including the regime transitions, of a multi-bubble, three-phase pulp slurry is studied by measuring the overall gas holdup and by using a light transmittance probe to determine the bubble characteristics. The results demonstrate that the hydrodynamic behavior of a three-phase pulp slurry varies from the behavior of a gas-liquid system. The deviation in the behavior becomes more pronounced as the pulp consistency increases. The bubble characteristics of bubbles injected through a single bubble injector are studied in a small rectangular column through the use of flow visualization and a particle image velocimetry technique. The measured change in the behavior of single bubbles injected into the pulp slurry through a single injector, is the basis for a proposed mechanism for the increased bubble coalescence in a multi-bubble system.

Some experimental observations of secondary motions in a confined vortex flow

Three decades have passed since vortex breakdown was first identified as a natural fluid flow phenomenon. Three key theories have been proposed to explain the phenomenon: hydrodynamic instability, conjugate states and flow stagnation. Despite a considerable amount of theoretical and experimental investigation, there is still nothing approaching a completely satisfactory theory of vortex breakdown. In addition, there is no agreement on a complete physical description of the structure of vortex breakdown. The present experimental investigation may substantiate a few earlier conjectures. We discuss an experimental finding that might help clarify the phenomenon through the use of flow visualization and laser-Doppler velocimetry. Experimental measurements substantiate earlier measurements and theoretical calculations of the velocity field. The evidence suggests that there is a connection between criticality and instability.

Laminar-Turbulent Transition on a Swept Wing.

Three-dimensional boundary-layer transition experiments are currently being conducted on a 45° swept wing in the Arizona State University Unsteady Wind Tunnel. Crossflow-dominated transition is produced via a model with contoured end liners to simulate infinite swept-wing flow. Fixed-wavelength, stationary and traveling crossflow vortices are observed. The frequencies of the most amplified traveling waves are in agreement with linear-stability theory; however, traveling waves at frequencies an order of magnitude higher than predicted are also observed near transition. Boundary-layer profiles measured at several spanwise locations show streamwise disturbance profiles characteristic of the crossflow instability. Near the transition location, severe distortions of the steady boundary-layer profiles are observed in the form of multiple inflection points. This distorted boundary layer is due to the stationary crossflow vortex and is subject to a Rayleigh-type instability in the stream direction. As a result, a high-frequency secondary instability is detected in the transition region, and spatial relations of the process are well documented by the coupled use of flow visualization and hot-wire measuremests. In all cases, we observe a transition due to this high-frequency Rayleigh instability induced by the stationary crossflow vortex.

Interaction of a vortex couple with a free surface

The characteristics of three-dimensional flow structures (scars and striations) resulting from the interaction between a heterostrophic vortex pair in vertical ascent and a clean free surface are described. The flow features at the scar-striation interface (a constellation of whirls or coherent vortical structures) are investigated through the use of flow visualization, a motion analysis system, and the vortex-element method. The results suggest that the striations are a consequence of the short wavelength instability of the vortex pair and the helical instability of the tightly spiralled regions of vorticity. The whirls result from the interaction of striations with the surface vorticity. The whirl-merging is responsible for the reverse energy cascade leading to the formation and longevity of larger vortical structures amidst a rapidly decaying turbulent field.

Study of a circulation control airfoil with leading/trailing-edge blowing

An experimental study of the flow past a two-dimensional circulation control airfoil under steady leading/trailing edge blowing was conducted. The effect of varying jet momentum on the flow field was determined through the use of flow visualization and airfoil force measurements. The flow patterns associated with leading edge blowing, alone and simultaneously with trailing edge blowing, are described and correlated with the lift measurements.

A study of three-dimensionally oscillating natural convection over a heated horizontal rectangular plate installed in a container.

The aim of this study is to clarify the existence and performance of a three-dimensionally oscillating natural convection of a standing wavetype over a horizontal heated plate of a rectangular cross section in an air container. First, by use of flow visualization and fluid temperature measurements, it was shown that the oscillation motion occurred under a certain specified condition. The main parameter which controls the wave length of the standing wave is the distance between the two side walls of the container supporting the heated plate. Second, the time-mean heat transfer coefficient of the upper surface of the heated plate was found to be about 26% higher than that without oscillation in the case of Gγ≒105.

Thick airfoil at low Reynolds number and high incidence

Flows, Flow Visualization III, edited by W. J. Yang, Hemisphere, Washington DC, 1985, pp. 123-127. McGregor, I., The Vapour-Screen Method of Flow Visualization, Journal of Fluid Mechanics, Vol. 11, Dec. 1961, pp. 481-511. Nelson, R. C., The Use of Flow Visualization for Vortex Trajectory Mapping, Flow Visualization III, edited by W. J. Yang, Hemisphere, Washington DC, 1985. Brand, A. G., Komerath, N. M., McMahon, H. M., Results from Laser Sheet Visualization of a Periodic Rotor Wake, AIAA Paper 88-0192, Jan. 1988. Brand, A. G., McMahon, H. M., Komerath, N. M., Wind Tunnel Data From a Rotorwake/Airframe Interaction Study, School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA. Contract No. DAAG29-82-K-0094, July 1986.

Use of flow visualization and tracer gas studies for designing an InP/InGaAsP OMVPE reactor

Studies of gas flow using smoke particle visualization and a tracer gas measurement technique were used to provide design information about growth tubes for organometallic vapor phase epitaxy (OMVPE) of InP and InGaAsP alloys. It is shown that convective cells may form when a gas of a different density than that of the gas already flowing in the tube is rapidly switched into the reactor, even when the vent and run lines of the switching manifold are pressure and flow balanced. This effect, reported here for the first time, could severely degrade the material quality of InP/InGaAsP heterojunctions. This transient convection after gas switching is particularly acute for a vertical growth tube with a large cross-sectional area (≈ 80 cm 2), such as the vertical reactor tube studied has a rectangular tube with a high aspect ratio (7.6/1.6) and a susceptor surface parallel to the gas flow. The cross-sectional area (≈ 10 cm 2) is greatly reduced and for the same volumetric flow rate the gas velocities are increased, which eliminated convection, even for density changes considerably greater than those encountered in InP/InGaAsP growth. Susceptor rotation in this geometry is expected to average growth nonuniformities which result from gas phase depletion. At high rotation rates (> 200 rpm), the susceptor motion induces vortices which are likely to produce nonuniform growth and graded interfaces. However, at ≈ 100 rpm there is no significant perturbation of the gas flow with mean gas velocities of >15 cm/s. This rotation rate is sufficient to achieve considerable averaging at typical OMVPE growth rates.

円筒形絞りの噴流領域における振動流の挙動 Ｉ 流れの可視化実験

Cylindricacl hokes are widely used as a component of oil hydraulice quipment. s The dynamic characteristics between flowrate and pressure drop across the cylindrical chokes were discussed by the authors from a practical point of view, and they obtained the mathematical model which can represent the high frequency characteristics of the chokes. In the model, they assumed no pressure recovery at the downstream neighborhood of the choke. However, the oscillatory jetflow from outlet of cylindrical chokes shows very complex behavior which are quite different from the steady jetflow and it is not clarified quantitatively. In order to utilize the chokes as a flowmeter, it is indispensable to discuss the estimation of the dynamics of pressure drop in the downstream jetflow region of cylindrical chokes. In this research, the dynamic behavior of the jetflow in the downstream region of cylindrical chokes are investigated precisely by making use of flow visualization.