Laser Flow Visualization Research Articles

Coupled fluid-structure simulations and experiments of DBD plasma actuator for damping of plate vibrations

In the present work, an experimental and numerical study has been performed to evaluate the suitability of the dielectric barrier discharge plasma actuators to reduce free vibrations of plates. Two configurations composed of two different plexiglass plates and identical actuators were manufactured. Experimental campaigns, in terms of laser doppler anemometer velocity measurements, flow visualization and proper orthogonal decomposition were carried out to characterize the plasma actuator and to validate the numerical models. The plasma-induced virtual body force for the numerical analysis has been calibrated based on the experimental body force and the induced flow field. Then numerical studies of the plasma effect on the flow around an oscillating plate was performed. An experimental modal analysis has been also performed to understand the vibration characteristics of the plate and to support the numerical study. A reduced-order model of the system, based on the harmonic oscillator, was developed to properly design an open loop controller (system natural frequency, amplitude and timing of the attenuation force). Eventually, the work demonstrates that the control strategy based on the plasma actuation is effective in reducing the plate vibrations. However, a deeper study and a more sophisticated control strategy would be required to significantly mitigate the flutter oscillations of airfoils in real applications.

Experimental study of backflow air leakage through an opening from a depressurized enclosure

ABSTRACT In nuclear decommissioning and dismantling operations, a dynamic confinement is applied to all openings in order to prevent the transfer of pollutants outside depressurized enclosures and to insure the safety of workers. To guarantee an efficient dynamic confinement, ISO 16647 and ISO 17873 recommend to maintain a constant value for inward inflow velocity near the opening depending on the level of radioactive pollution hazard. The main purpose of this work is to identify possible conditions under which flow inversions near the opening may lead to gaseous pollutant leakage and then failure of the dynamic confinement. We aim at quantifying the amount of this backflow. Leakage from an experimental ventilated enclosure with a small opening on its frontal wall has been investigated. Laser flow visualizations and Particle Image Velocimetry (PIV) measurements showed that the presence of an additional turbulent jet flow in competition with the inward confinement flow is among the main causes leading to leakage through opening. The gas tracing technique has provided experimental data to quantify the pollutant backflow and allowed us to compare different scenarios. We conclude that a new criterion based on local aeraulic conditions near the opening is relevant to guarantee an efficient confinement.

Aerodynamic performance investigation of sweptback wings with bio-inspired leading-edge tubercles

The aerodynamic behavior of sweptback wing configurations with bio-inspired humpback whale (HW) leading-edge (LE) tubercles has been investigated through computational and experimental techniques. Specifically, the aerodynamic performance of tubercled wings with symmetric (NACA 0015) and cambered (NACA 4415) airfoils is validated against the baseline model at various angles of attack ([Formula: see text]. The [Formula: see text]/[Formula: see text] ratio of the HW flipper is strategically reduced to 0.15 for ascertaining the flow control potential of the bio-inspired wings with sweptback configuration. It is a novel effort to quantify the effect of the leading-edge protuberances on stall delay, flow separation control and distribution of streamline vortices at unique [Formula: see text]/[Formula: see text] ratio outside the thickness range of HW flipper morphology. Four tapered sweptback wing models (Baseline A, Baseline B, HUMP 0015, HUMP 4415) are used with the amplitude-to-wavelength ([Formula: see text] ratio of 0.24 and Reynolds number about [Formula: see text]. The chordwise pressure distributions are recorded at the peak, mid and trough regions of the tubercled wings through a detailed wind tunnel testing and validated with numerical analysis. Additionally, the flow characteristics over the bio-inspired surfaces have been qualitatively analyzed through the laser flow visualization (LFV) technique to reveal the influence of laminar separation bubbles (LSBs). The essential aerodynamic characteristics such as boundary layer trip delay, vortex mixing, stall delay, and flow control at different AoA are addressed through consistent experimental data. As the sweptback configuration is a primary choice for airplane wings, the improved aerodynamic characteristics of the tubercled wings can be effectively utilized for the design of novel lifting surfaces, hydroplanes and wind turbines in the near future.

Experimental investigation on heat transfer augmentation of transverse twisted tapes in a confined rectangular channel

This study investigates the influence of twisted tape inserts on a flow pathway in transverse direction of a flow situation. An array of twisted tapes was mounted on the confined channel to disturb the flow to enhance the heat transfer. The test section had 20 twisted tapes of 35-mm height, 10-mm width and a twist angle of 180° mounted on a silica gel–coated plate. Experiments were conducted on the fabricated experimental setup for different discharge conditions. The Reynolds number considered for the study ranged from 2300 to 3500, and the heat inputs varied from 50 to 250 W, with an interval of 50 W. To visualize the presence of twisted tapes on the flow path, the rectangular channel was visualized by laser flow visualization method, which reported the impact of the twisted tapes on the flow. The existence of twisted tape affects the flow, and it forms a swirl that provides a proper mixing of fluid to enhance heat transfer. The outcome of the present investigation provides a solution to enhance heat transfer and proposes the use of twisted tapes instead of using segmented fins in design of fins.

Study the Flow behind a Semi-Circular Step Cylinder (Laser Doppler Velocimetry (LDV) and Computational Fluid Dynamics (CFD))

Laser Doppler Velocimetry (LDV) measurements, flow visualizations and unsteady Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) simulations have been carried out to study the turbulent wake that is formed behind a semi-circular step cylinder at a constant flow rate. The semi-circular cylinder has two diameters, a so-called step cylinder. The results from the LDV measurements indicate that wake length and vortex shedding frequency varies with the cylinder diameter. This implies that a step cylinder can be used to attract fish of different size. By visualizations of the formation of a recirculation region and the well-known von Kármán vortex street behind the cylinder are disclosed. The simulation results predict the wake length and shedding frequency well for the flow behind the large cylinder but fail to capture the dynamics of the flow near the step in diameter to some extent and the flow behind the small cylinder to a larger extent when compared with measurements.

Multiple Jet/Wall/Crossflow Interactions

This paper summarizes thirty years of research on ground vortex and upwash flows generated by single and multiple impinging jets in a crossflow with relevance to vertical/Short Take-Off and Landing Aircrafts (V/STOL). Laser Doppler measurements and flow visualization are presented for turbulent circular jets emerging into a low-velocity cross-stream and, then, impinging on a flat surface perpendicular to the geometrical jet-nozzle axis. The experiments were performed for single, twin side-by-side., twin in tandem, and three jets through a crossflow for velocity ratios between the jet exit and the crossflow VR =Vj /Uo of 7.5 to 90. The multiple jets configurations include interject spacings of S=5D and L=6D. The impinging heights were varied from 5 to 20 jet diameters and include mean and turbulent velocity characteristics along the two normal directions parallel to the nozzle axis. The largest velocity ratios are characterized by a large penetration of the impinging jets, giving rise to a ground vortex due to the collision of the radial wall jet and the crossflow that wraps around the impinging point like a scarf. The results help to understand the flow around a V/STOL aircraft operating in ground vicinity with front wind or small forward movement that may result in enhanced negative pressures in the underside of the aircraft causing a pitching moment and suction down force towards the ground.

Free Convection in Asymmetrically Heated Vertical Channels With Opposing Buoyancy Forces

Laser interferometry and flow visualization were used to study free convective heat transfer inside a vertical channel. Most studies in the literature have investigated buoyancy forces in a single direction. The study presented here investigated opposing buoyancy forces, where one wall is warmer than the ambient and the other wall is cooler than the ambient. An experimental model of an isothermally, asymmetrically heated vertical channel was constructed. Interferometry provided temperature field visualization and flow visualization was used to obtain the streamlines. Experiments were carried out over a range of aspect ratios between 8.8 and 26.4, using temperature ratios of 0, −0.5, and −0.75. These conditions provide a modified Rayleigh number range of approximately 5 to 1100. In addition, the measured local and average Nusselt number data were compared to numerical solutions obtained using ANSYS FLUENT. Air was the fluid of interest. So the Prandtl number was fixed at 0.71. Numerical solutions were obtained for modified Rayleigh numbers ranging from the laminar fully developed flow regime to the turbulent isolated boundary layer regime. A semi-empirical correlation of the average Nusselt number was developed based on the experimental data.

An experimental investigation of flow past a dual step cylinder

Flow development in the wake of a dual step cylinder has been investigated experimentally using Laser Doppler Velocimetry and flow visualization. The dual step cylinder model is comprised of a large diameter cylinder (D) mounted at the mid-span of a small diameter cylinder (d). The experiments have been performed for a Reynolds number (ReD) of 1,050, a diameter ratio (D/d) of 2, and a range of large cylinder aspect ratios (L/D). The results show that the flow development is highly dependent on L/D. The following four distinct flow regimes can be identified based on vortex dynamics in the wake of the large cylinder: (1) for L/D ≥ 15, three vortex shedding cells form in the wake of the large cylinder, one central cell bounded by two cells of lower frequency, (2) for 8 < L/D ≤ 14, a single vortex shedding cell forms in the wake of the large cylinder, (3) for 2 < L/D ≤ 6, vortex shedding from the large cylinder is highly three-dimensional. When spanwise vortices are shed, they deform substantially and attain a hairpin shape in the near wake, (4) for 0.2 ≤ L/D ≤ 1, the large cylinder induces vortex dislocations between small cylinder vortices. The results show that for Regimes I to III, on the average, the frequency of vortex shedding in the large cylinder wake decreases with L/D, which is accompanied by a decrease in coherence of the shed vortices. In Regime IV, small cylinder vortices connect across the large cylinder wake, but these connections are interrupted by vortex dislocations. With decreasing L/D, the frequency of dislocations decreases and the dominant frequency in the large cylinder wake increases toward the small cylinder shedding frequency.

Research of Aerodynamic Characteristics about Minibus

To research the aerodynamic characteristics of minibus, the simplified model is studied with simulation and wind tunnel test. The validated method was adopted in the numerical simulation, the wind tunnel test were carried out in order to validate the results of numerical simulation. The tests include aerodynamic drag measure, filament method, and laser flow visualization. Through the contrasted research the results of simulation and test, the aerodynamic characteristics of minibus was analyzed and the characteristic of field around minibus was discovered. The conclusions provide the theoretical foundation for styling optimization of minibus, which are very helpful to develop new vehicle styling.

Critical Reynolds Number in Constant-Acceleration Pipe Flow From an Initial Steady Laminar State

The transition to turbulence in a constant-acceleration pipe flow from an initial laminar state was investigated in a custom-made apparatus permitting visual access to the water flow in the pipe. The apparatus allowed both laser Doppler velocimetry measurements and flow visualization using a tracer. The experiment was carried out by accelerating the flow from a steady laminar state to a steady turbulent state. The relation between the critical Reynolds number for transition to turbulence and the acceleration was found to be similar to that in a constant-acceleration pipe flow started from rest. In addition, with increased acceleration, the turbulent transition was found to be delayed to higher Reynolds numbers using flow visualization with simultaneous laser Doppler velocimetry measurements.

Liquid sheet disintegration at high pressure: An experimental approach

The primary atomization was studied in a 300 μm thickness water sheet, generated by a planar airblast atomizer. The research novelty consisted in increasing the airflow absolute pressure from atmospheric conditions to 6 bar. The experimental techniques employed included Oscillometry by Laser Intensity Reflexion (ORIL), Laser Doppler Velocimetry (LDV) and flow visualization by fast video camera. The atomization mechanisms, described in the literature at atmospheric environments, were observed at high pressure conditions, for a constant momentum flux ratio. Furthermore, a new atomization mechanism was observed at high values of this ratio. Finally, dimensionless relations have been proposed for the global oscillation frequency, minimum air oscillation velocity, break-up distance and transversal wavelength. To cite this article: V.G. Fernandez et al., C. R. Mecanique 337 (2009).

Aerodynamic Characteristics of Asymmetric Bluff Bodies

The wake of asymmetric bluff bodies was experimentally measured using particle imaging velocimetry, laser Doppler anemometry, load cell, hotwire, and flow visualization techniques at Re=2600–8500 based on the freestream velocity and the characteristic height of the bluff bodies. Asymmetry is produced by rounding some corners of a square cylinder and leaving others unrounded. It is found that, with increasing corner radius, the flow reversal region is expanded, and the vortex formation length is prolonged. Accordingly, the vortex shedding frequency increases and the base pressure rises, resulting in a reduction in the mean drag as well as the fluctuating drag and lift. It is further found that, while the asymmetric cross section of the cylinder causes the wake centerline to shift toward the sharp corner side of the bluff body, the wake remains globally symmetric about the shifted centerline. The near wake of asymmetric bluff bodies is characterized in detail, including the Reynolds stresses, characteristic velocity, and length scale, and is further compared with that of the symmetric ones.

The Effect of Free Surface on the Vortex Shedding From Inclined Circular Cylinders

Spectral analysis of laser Doppler velocimetry measurements and flow visualization are employed to study the wake of inclined cylinders piercing a free surface. The results indicate that the free surface affects vortex shedding almost two to three cylinder diameters below the free surface. Variation of the vortex axis inclination and cells of shedding were visually observed. These effects induce a phase difference in the vortex-shedding mechanism. Streamwise vortical structures are formed along the primary vortex axis. Furthermore, for the case of the highest Froude number, evidence is provided that vortex shedding is suppressed in the region near the free surface.

Fluid Dynamics Study on a Cold Model of Fixed-Bed Fire-Tube Heating Pyrolysis Reactor for Ejection of Solid Char

To innovate an easy and economic way for removal of solid char product from a fixed-bed fire-tube heating pyrolysis reactor the fluid dynamics studies on its cold model are essential. Thus, two types of fluid dynamics experiments have been carried out on a cold model of the pyrolysis reactor: (i) to determine the ejection pressure of the solid char from the reactor, which was carried out with the help of an air compressor and artificial solid char and (ii) to investigate the flow pattern inside the reactor chamber during ejection of char that was conducted by laser Doppler velocimetry (LDV) measurement and flow visualization test. The experimental results show that variation of char ejection pressure with respect to superficial air velocity always follows a polynomial of second-degree. The normalized pressure should be reached to a value about 1.7 with a corresponding normalized superficial air velocity nearly to 1.0, to be started the char product blown out from the reactor model properly. The LDV measurements and visualization test ensure that the spiral char exit port is not able to create a rotary motion inside the reactor chamber during the removal of solid char.

TURBULENT SWIRLING NATURAL GAS FLAMES: STABILITY CHARACTERISTICS, UNSTEADY BEHAVIOR AND VORTEX BREAKDOWN

Laser diagnostic and flow visualization techniques have previously been applied to several reacting and isothermal swirling jets. Resolved flow features include the presence of vortex breakdown and unsteady behavior. As such, turbulent flames stabilized on the present (laboratory scale) burner bear similitude to those in larger (industrial) swirl combustors. With this in mind, the impact of vortex breakdown and unsteady behavior on flame stability continues to require further study. Such understanding can be gained through applying non-intrusive laser diagnostics to flow conditions covering a broad range of flame stability characteristics. This paper presents the results of one such investigation. Laser Doppler Velocimetry (LDV) measurements are acquired (along the centreline) to ascertain the presence of time periodicity and downstream recirculation. Unsteady behavior is identified through the spectra of axial velocity data whilst negative axial velocities delineate vortex breakdown. This information is augmented with observations of visible flame length and discussed in relation to established flame stability characteristics. Findings indicate that swirl numbers, over which improvements in flame stability occur, coincide with conditions leading to unsteady behavior and shorter flames. Because downstream flow reversal is not resolved at all such conditions, information available indicates flow unsteadiness is a clearer contributing factor, compared to vortex breakdown, on the observed stability characteristics. Shorter flames coupled with improvements in flame stability have significant implications on the design and operation of swirl combustors.

Typical flow between enclosed corotating disks and its dependence on Reynolds number

The flow in an enclosed co‐rotating disk pair is investigated by Laser Doppler Velocimetry (LDV) measurements and flow visualizations. First, the typical flow structure at Re = 5.25 × 105 and S = 0.09 is clarified. The flow fields in the r – θ and the r – z planes are both investigated and then divided into several flow regions based on the distinct flow types observed. The flow regions found in the two different planes are also compared and integrated. Second, with S fixed, the dependence of the flow field structure upon the Reynolds number is discussed. Three regimes of the r – θ plane flow with different Reynolds numbers are identified based on the measured mean velocity and spectral intensity. When Re < 1.6 × 105, no solid body region is found and the flow is in a laminar regime. In the range 1.6 × 105 ≤ Re ≤ 2.0 × 106, the solid body region and the outer region vortices coexist, and an empirical equation is developed to estimate the number of vortices. When Re > 2.0 × 106, the flow becomes turbulent. As Re increases from 9.3 × 104 to 5.25 × 105, the spectral intensity initially increases and then decreases before increasing again to an even higher level, resulting in an increasing sawtooth pattern.

Physical and Computer Modelling of Blood Flow in a Systemic-to-pulmonary Shunt

The aim of this work was the application of computer and physical in vitro simulation methods for estimating surgery procedure hemodynamics. The modified Blalock-Taussig (mB-T) palliative surgical procedure is performed to increase the pulmonary blood flow in children with congenital heart defects. Such a systemic-to-pulmonary shunt yields substantial modification in the blood flow within the large blood vessels. The objective of the present study was to investigate basic characteristics of the flow, flow pattern and pressure-flow efficiency, before and after opening of the mB-T graft. The model was based on the vessel geometry obtained from the Visible Human Project and included the arch of aorta, the three arteries branching from the arch, the pulmonary trunck, and the left and right pulmonary arteries. The graft was added between the left subclavian artery and the left pulmonary artery. The glass model of the vessels was produced and investigated in a physical model of the cardiovascular system with an artificial ventricular device as the blood pump. Flow rate and hydrostatic pressure were measured at the inlet to and outlets from the glass model and in a few points within the system. Laser flow visualization was also performed. Computer simulations were done using the boundary conditions from the physical model. The opening of the mB-T graft changed flow distribution in all branches (including inflow). A complex flow pattern with large eddies and channelling of the flow in the vicinity of the graft and within it was observed in flow visualization and in computer simulations. Because of that complexity the local measurements of hydrostatic pressure at the vessel wall could not predict the average flow rate. The reversed flow in the graft was observed during the systole. The complex flow pattern developed in the physical model of the mB-T graft. The channelling of the flow and the formation of large eddies may yield high shear stress and modify blood properties. The rigid wall model can describe only some flow characteristics observed in vivo. Computer simulation is a very fast and accurate method which permits earlier qualification of cardiac surgeons on how to change cardiac vascular blood flow after operations.

Reynolds number and impeller diameter effects on instabilities in stirred vessels

AbstractFlow instability phenomena in stirred vessels were studied with laser anemometry and flow visualization. The effects of fluid density and viscosity, impeller Reynolds number (Re), impeller design, diameter, and off‐bottom clearance were investigated in order to quantify the frequencies (f) of the macroinstabilities stemming from precessional motions. The instabilities are characterized by two frequencies, one present at low Re, and another at high Re values. For intermediate Re values, both frequencies were present. In all cases, f was proportional to the impeller speed (N). The parameter f′ = f/N was shown to be linearly related to the impeller diameter in the low Re range; f′ was not affected by impeller clearance. At constant Re, a change in fluid density and viscosity did not affect f′. The energy contained in the instability frequencies was shown to vary across the vessel. The implications of the results mixing practice are discussed. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2050–2063, 2004

CFD simulations of 3D flow in a pump stator with a spherical surface

A stator in which the crossover has a spherical surface is a new concept in the design of stators for multistage pumps. In this article from Poland, Professor Janusz Plutecki and Dr Janusz Skrzypacz of the Dept of Mechanical Engineering at Wroclaw University of Technology discuss the creation of a numerical model of the flow in this innovative pump stator. The computational results obtained using Fluent 6.0 are compared with experimental data from measurements of pressure and velocity by laser Doppler anemometry and flow visualization. The possibilities of using CFD code in industrial applications are discussed.

Laser flow visualization and velocity fields by particle image velocimetry in an electrostatic precipitator model

Although improving electrostatic precipitator (ESP) collection of fine particles (micron and submicron sizes) remains of interest, it is not yet clear whether the turbulent flow patterns caused by the presence of electric field and charge in ESPs advance or deteriorate fine particle precipitation process. In this paper, results of the laser flow visualization and Particle Image Velocimetry (PIV) measurements of the particle flow velocity fields in a wire-to-plate type ESP model with seven wire electrodes are presented. Both experiments were carried out for negative and positive polarity of the wire electrodes. The laser flow visualization and PIV measurements clearly confirmed formation of the secondary flow (velocity of several tens of cm/s) in the ESP model, which interacts with the primary flow. The particle flow pattern changes caused by the strong interaction between the primary and secondary flows are more pronounced for higher operating voltages (higher electrohydrodynamic number NEHD) and lower primary flow velocities (lower Reynolds number Re). The particle flow patterns for the positive voltage polarity of the wire electrodes are more stable and regular than those for the negative voltage polarity due to the nonuniformity of the negative corona along the wire electrodes (tufts).