Quantitative Flow Visualization Technique Research Articles

Free-Surface Velocity Measurement Using Direct Sensor Orientation-Based STIV.

Particle image velocimetry (PIV) is a quantitative flow visualization technique, which greatly improves the ability to characterize various complex flows in laboratory and field environments. However, the deployment of reference objects or ground control points (GCPs) for velocity calibration is still a challenge for in situ free-surface velocity measurements. By combining space-time image velocimetry (STIV) with direct sensor orientation (DSO) photogrammetry, a laser distance meter (LDM)-supported photogrammetric device is designed, to realize the GCPs-free surface velocity measurement under an oblique shooting angle. The velocity calibration with DSO is based on the collinear equation, while the lens distortion, oblique shooting angle, water level variation, and water surface slope are introduced to build an imaging measurement model with explicit physical meaning for parameters. To accurately obtain the in situ position and orientations of the camera utilizing the LDM and its embedded tilt sensor, the camera’s intrinsic parameters and relative position within the LDM are previously calibrated with a planar chessboard. A flume experiment is designed to evaluate the uncertainty of optical flow estimation and velocity calibration. Results show that the proposed DSO-STIV has good transferability and operability for in situ measurements. It is superior to propeller current meters and surface velocity radars in characterizing shallow free-surface flows; this is attributed to its non-intrusive, whole-field, and high-resolution features. In addition, the combined uncertainty of free-surface velocity measurement is analyzed, which provides an alternative solution for error assessment when comparing measurement failures.

Characterization of swirling-flow behavior in complex pipeline using bubble trajectory method with stereo particle tracking/image velocimetry

To understand the swirling-flow behavior in a complex pipeline geometry comprising an elbow and orifice, a quantitative flow visualization technique is introduced into the flow-field observation using a bubble trajectory method combined with stereo particle tracking/image velocimetry. For the chosen bubble sizes of approximately 2 mm un a pipe of 56 mm in diameter, their motion is correlated with the mean swirling flow, and when analyzed in a straight swirling flow, the bubbles cluster close to the point of maximum axial velocity for swirl intensities larger than 0.3. Subsequently, this experimental technique is applied to the characterization of a swirling flow in the Mihama pipeline model, for which the streamwise variation of the location of the maximum axial velocity and its magnitude are measured over the full range of the pipeline model. A spiral swirling flow is observed downstream of the elbow owing to the combined effect of the swirling and secondary flows in the elbow. The spiral swirling flow persists in the following straight pipe down to the orifice, where a highly accelerated flow with a high turbulence is generated towards the top wall of the pipe. This result agrees closely with the observation of pipe-wall thinning in the prototype Mihama pipeline.

Shock wave interactions within concave cavities

The interaction of a plane shock wave in air with concave profiles has been used in the past mainly to understand the nature of shock wave focusing. The current study examines the complex two-dimensional flow field resulting from the interaction of a plane shock wave entering a symmetrical cavity with curved walls. Of particular interest are the development of reflection patterns of the incident shock wave at the profile wall and the process of gas dynamic focus. These principal flow features are examined across a wide range of different reflector shapes. This includes a review of previously studied profiles such as cylindrical and parabolic, and also of a number of additional profiles, including compound profiles, where an inlet profile merging with that of the main cavity is shown to have major effects on the focusing mechanism and pressures. The various reflector shapes were specified by varying the shape of the profile and the depth-to-aperture ratio. The strength of the incident plane shock wave was limited between Mach numbers of 1.04 and 1.45. The principal flow features were established and examined experimentally using a variety of qualitative and quantitative flow visualization techniques, supplemented with numerical results. Time-resolved high-speed imaging was used to capture the interaction providing the unique ability to track the various transient flow features over the course of the interaction. The three primary factors that influence the maximum pressure amplification at focus, and the focus mechanism, are the incident shock strength, the depth-to-aperture ratio of the profile and an inlet profile leading into the main cavity, if present. An inlet profile results in higher-pressure amplifications for corresponding shock strengths and depth-to-aperture ratios. Increases in the depth-to-aperture ratio increase the maximum pressure amplification observed at focus. This occurs due to a combination of factors including: the strengthening of the individual shock waves involved in focus; the duration of focus; and the strengthening of a compressive flow field that develops adjacent to the shock system during focus. The compressive flow field adjacent to the shock system at focus is shown to be of great importance to the focus process.

Molecular Tagging Velocimetry in Superfluid Helium-4: Progress, Issues, and Future Development

Helium-4 in the superfluid phase (He II) is a two-fluid system that exhibits fascinating quantum hydrodynamics with important scientific and engineering applications. However, the lack of high-precision flow measurement tools in He II has impeded the progress in understanding and utilizing its hydrodynamics. In recent years, there have been extensive efforts in developing quantitative flow visualization techniques applicable to He II. In particular, a powerful molecular tagging velocimetry (MTV) technique, based on tracking thin lines of He$^*_2$ excimer molecules created via femtosecond laser-field ionization in helium, has been developed in our lab. This technique allows unambiguous measurement of the normal-fluid velocity field in the two-fluid system. Nevertheless, there are two limitations of this technique: 1) only the velocity component perpendicular to the tracer line can be measured; and 2) there is an inherent error in determining the perpendicular velocity. In this paper, we discuss how these issues can be resolved by advancing the MTV technique. We also discuss two novel schemes for tagging and producing He$^*_2$ tracers. The first method allows the creation of a tagged He$^*_2$ tracer line without the use of an expensive femtosecond laser. The second method enables full-space velocity field measurement through tracking small clouds of He$^*_2$ molecules created via neutron-$^3$He absorption reactions in He II.

Hydrodynamic study of freely swimming shark fish propulsion for marine vehicles using 2D particle image velocimetry.

Two-dimensional velocity fields around a freely swimming freshwater black shark fish in longitudinal (XZ) plane and transverse (YZ) plane are measured using digital particle image velocimetry (DPIV). By transferring momentum to the fluid, fishes generate thrust. Thrust is generated not only by its caudal fin, but also using pectoral and anal fins, the contribution of which depends on the fish’s morphology and swimming movements. These fins also act as roll and pitch stabilizers for the swimming fish. In this paper, studies are performed on the flow induced by fins of freely swimming undulatory carangiform swimming fish (freshwater black shark, L = 26 cm) by an experimental hydrodynamic approach based on quantitative flow visualization technique. We used 2D PIV to visualize water flow pattern in the wake of the caudal, pectoral and anal fins of swimming fish at a speed of 0.5–1.5 times of body length per second. The kinematic analysis and pressure distribution of carangiform fish are presented here. The fish body and fin undulations create circular flow patterns (vortices) that travel along with the body waves and change the flow around its tail to increase the swimming efficiency. The wake of different fins of the swimming fish consists of two counter-rotating vortices about the mean path of fish motion. These wakes resemble like reverse von Karman vortex street which is nothing but a thrust-producing wake. The velocity vectors around a C-start (a straight swimming fish bends into C-shape) maneuvering fish are also discussed in this paper. Studying flows around flapping fins will contribute to design of bioinspired propulsors for marine vehicles.

Precise measurement of three-dimensional positions of transparent ellipsoidal particles using digital holographic microscopy.

The dynamic motions of various particles suspended in microscale flows are essential phenomena in the scientific and engineering fields. These motions can be precisely measured by using 3D quantitative flow visualization techniques. Moreover, most cells and particles in nature possess a nonspherical shape. Digital holographic microscopy (DHM) is employed to measure the 3D positional information of transparent ellipsoidal particles. Both in-plane and out-of-plane positional information are obtained by analyzing the distinctive light scattering from the microsized ellipsoidal particles. The performance of the 3D position measurement method is experimentally verified for ellipsoidal particles seeded in a planar surface and a microtube. This DHM technique exhibits promising potential in the dynamic analysis of ellipsoidal particles and cells suspended in various microscale fluid flows.

A Quantitative Flow Visualization Technique for On-site Sport Aerodynamics Optimization

Aerodynamics plays a crucial role in many speed sports, where races are often won by fractions of a second. A thorough understanding of the flow field around an athlete is of paramount importance to optimize the athletes’ posture, garment roughness and equipment shape to achieve the minimum aerodynamic drag and maximum velocity. To date, aerodynamic measurements are typically conducted in wind tunnels, using balances or pressure sensors. As a consequence, no information on the flow field responsible of the aerodynamic loads is gathered. Furthermore, the use of steady models yields a flow field that may differ significantly from that encountered during a race, where the athlete is in motion. The present paper proposes to use large-scale tomographic PIV for sport aerodynamic investigation. Helium-filled soap bubbles (HFSB) as flow tracers allow velocity measurements in a volume exceeding 10,000 cm3, from which the aerodynamic loads can be computed. The technique is suitable for conducting on-site aerodynamic measurements via the ring-of-fire concept: the measurements are carried out during the athletes’ training in a velodrome, thus reproducing the same flow conditions met during races.

High-speed imaging of shock wave focusing in concave profiles using shearing interferometry and direction-indicating colour schlieren

In this study both qualitative and quantitative flow visualization techniques are used to examine the complex flow field developed during the interaction of a plane shock wave with a compound profile. Shearing interferometry and direction-indicating colour schlieren are implemented in conjunction with high-speed time-resolved imaging. Comparisons between the results provided by the two flow visualization systems are made at incident shock strengths of Mach 1.28 and Mach 1.35. The results from both systems indicate the complex nature of the transient compression and expansion systems developed during the interaction behind the various shock fronts. The direction-indicating results prove valuable in eliminating the ambiguity with respect to the sign of the high-order fringes found in the shearing interferometry results.

Experimental Investigation of Operating Room Air Distribution in a Full-Scale Laboratory Chamber Using Particle Image Velocimetry and Flow Visualization

Room air distribution in hospital operating rooms (OR) is critical to the effective functioning of surgical procedures, but the air distribution patterns are governed by complex physics that are currently not well understood. Both qualitative and quantitative flow visualization techniques were used to evaluate the room air distribution in a full-scale chamber designed to simulate a hospital operating room. A laser sheet illumination technique was used to identify key features of the room air distribution, and particle image velocimetry (PIV) was used to measure the velocity field in a plane crossing the surgical site. Hospital operating rooms require the use of ASHRAE Group E diffusers in an array above the surgical table, providing downward, unidirectional, non-aspirating air flow across the sterile region of the room. The supply air jet is characterized by complex physics, including annular shape, impingement, buoyancy, a large jet to room aspect ratio, and recirculation. The large diameter of the jet relative to the room size makes the overall room air distribution highly sensitive to the parameters of the supply air. The air distribution pattern in the room was found to have relatively low velocity and turbulence near the supply air diffuser, but increasing velocity and turbulence in the shear region at the edge of the supply air jet. Flow visualization and PIV methods both demonstrated an angle of the shear layer inwards towards the center of the jet. This flow feature reduces the overall coverage area for the sterile air flow and may pose a risk to the protection of the surgical patient.

Thermal imaging study of scalar transport in shallow wakes

The thermal imaging technique relies on the usage of infrared signal to detect the temperature field. Using temperature as a flow tracer, thermography is used to investigate the scalar transport in the shallow-water wake generated by an emergent circular cylinder. Thermal imaging is demonstrated to be a good quantitative flow visualization technique for studying turbulent mixing phenomena in shallow waters. A key advantage of the thermal imaging method over other scalar measurement techniques, such as the Laser Induced Fluorescence (LIF) and Planar Concentration Analysis (PCA) methods, is that it involves a very simple experimental setup. The dispersion characteristics captured with this technique are found to be similar to past studies with traditional measurement techniques.

Comparison of flow structures in the downstream region of a cylinder and sphere

An experimental investigation of flow structures downstream of a circular cylinder and sphere immersed in a free-stream flow is performed for Re = 5000 and 10,000 using qualitative and quantitative flow visualization techniques. The obtained results are presented in terms of time-averaged velocity vectors, patterns of streamlines, vorticity, Reynolds stress correlations and turbulent kinetic energy distributions. Flow data reveal that the size of wake flow region, the location of singular and double points, the peak values of turbulence quantities, such as Reynolds stress correlations, vorticity fluctuations and turbulent kinetic energy vary as a function of models’ geometry and Reynolds Numbers. The concentration of small scale vortices is more dominant in the wake of the sphere than that of the cylinder. The maximum value of turbulent kinetic energy (TKE) occurs close to the saddle point for the cylinder case while two maximum values of TKE occur along shear layers for the sphere one because of the 3-D flow behavior.

An overview of a Lagrangian method for analysis of animal wake dynamics

The fluid dynamic analysis of animal wakes is becoming increasingly popular in studies of animal swimming and flying, due in part to the development of quantitative flow visualization techniques such as digital particle imaging velocimetry (DPIV). In most studies, quasi-steady flow is assumed and the flow analysis is based on velocity and/or vorticity fields measured at a single time instant during the stroke cycle. The assumption of quasi-steady flow leads to neglect of unsteady (time-dependent) wake vortex added-mass effects, which can contribute significantly to the instantaneous locomotive forces. In this paper we review a Lagrangian approach recently introduced to determine unsteady wake vortex structure by tracking the trajectories of individual fluid particles in the flow, rather than by analyzing the velocity/vorticity fields at fixed locations and single instants in time as in the Eulerian perspective. Once the momentum of the wake vortex and its added mass are determined, the corresponding unsteady locomotive forces can be quantified. Unlike previous studies that estimated the time-averaged forces over the stroke cycle, this approach enables study of how instantaneous locomotive forces evolve over time. The utility of this method for analyses of DPIV velocity measurements is explored, with the goal of demonstrating its applicability to data that are typically available to investigators studying animal swimming and flying. The methods are equally applicable to computational fluid dynamics studies where velocity field calculations are available.

PIV measurements of flow through a sudden contraction

Flow through a sudden contraction occurs in a variety of engineering applications including injection moulding, extrusion, thermoforming and many polymer melt processing applications. Instabilities observed during polymer melts processing can have an important limiting factor affecting production rates in many industrial area. Analysis of contraction flow is a useful process to identify these defects. In this study, the flow of a Newtonian fluid in an axisymmetrical 4:1 sudden contraction geometry is studied using quantitative flow visualization and Particle Image Velocimetry (PIV) technique. The flow is driven by the effect of gravity under atmospheric conditions in room temperature. Study of the Newtonian entrance problem is useful precursor to the study of the entrance behaviour of viscoelastic fluids. Time averaged and instantaneous velocity vector maps of the flow field, time-averaged velocity and vorticity contours as well as axial and radial velocity profiles right above the contraction for different Reynolds numbers are presented.

An In Vitro Study of Changing Profile Heights in Mitral Bioprostheses and Their Influence on Flow

In vitro assessment of different profiles for prosthetic mitral valves can result in better understanding of the physics of transmitral flow for each design. It has been postulated that decreasing the profile height of the mitral bioprosthetic valve has potential clinical benefit. In the present study, we compared the atrial and ventricular flow characteristics in different conditions using Carpentier-Edwards Perimount mitral valves with various profile heights. Each valve was placed at the intersection of the left ventricle, made of transparent silicone rubber, and the left atrium in Caltech's left heart pulsed flow simulator system. Digital particle image velocimetry has been used as the quantitative flow visualization technique. With the intention of studying the blood wash out around each valve, circulation and particle residence time were computed based on the vorticity and velocity fields around each valve, respectively. Results show that by increasing the profile's height at the atrial side of the valve, the magnitude of circulation near the atrial side of the valve decreases while particle residence time increases. However, extreme reduction of profile height in the ventricular side may increase the magnitude of circulation around the valve and decrease the particle residence time.

CHANGING PROFILE HEIGHTS IN PERIMOUNT??? MITRAL VALVE: AN IN-VITRO QUANTITATIVE FLOW VISUALIZATION STUDY

In vitro assessment of different profiles for prosthetic mitral valves could result in better understanding of the physics of transmitral flow for each design. It has been postulated that decreasing the profile height of the mitral bioprosthetic valve has potential clinical benefit. In the present study, we have compared the atrial and ventricular flow characteristics in different conditions using Carpentier-Edwards PERIMOUNT™ mitral valve with various profile heights. Each valve was placed at the intersection of the left ventricle, made of transparent silicone rubber, and the left atrium in Caltech’s left heart pulsed flow simulator system. Digital particle image velocimetry (DPIV) has been used as the quantitative flow visualization technique. With the intention of studying the blood wash out, circulation was calculated based on the vorticity field around each valve. Results show that by increasing the profile’s height at the atrial side of the valve, the magnitude of circulation near the atrial side of the valve decreases. On the other hand, extreme reduction of profile’s height in ventricular side would increase the magnitude of circulation distal to the valve.

Shear Evaluation by Quantitative Flow Visualization Near the Casing Surface of a Centrifugal Blood Pump.

To clarify the correlation between high-shear flow and hemolysis in blood pumps, detail shear velocity distribution was quantified by an experimental method with a model centrifugal blood pump that has a series data of hemolysis tests and computational fluid dynamic analyses. Particular attention was paid to the shear velocity near the casing surface in the volute where the high shear causes in circumferentially wide region that is considerable to cause high hemolysis. Three pump models were compared concern with the radial gap width between the impeller and casing (the radial volute width) also with the outlet position whereas the impeller geometry was identical. These casing geometries were as follows: model 1-the gap width is standard 3mm and the outlet locates to make a smooth geometrical connection with the volute, model 2-the gap width is small 0.5mm and the outlet locates to make the smooth geometrical connection with the volute, and model 3-the gap width is small 0.5mm and the outlet locates to hardly make the smooth geometrical connection with the volute but be similar radial position with that of model 1. Velocity was quantified with a particle tracking velocimetry that is one of the quantitative flow visualization techniques, and the shear velocity was calculated. Results showed that all large shear velocity existed within the layers of about 0.1mm from the casing surface and that those layers were hardly affected by a vane passage even if the gap width is 0.5mm. They also showed that the maximum shear velocity appeared on the casing surface, and the shear velocities of models 2 and 3 were almost twice as large as that of model 1. This finding is in full corresponding with the results of hemolysis tests which showed that the hemolysis levels of both models 2 and 3 were 1.5 times higher than that of model 1. These results suggest that detailed high-shear evaluation near the casing surface in the volute is one of the most important keys in estimating the hemolysis of a centrifugal blood pump.

Quantitative Visualization of Flow Through a Centrifugal Blood Pump: Effect of Washout Holes

To clarify the effect of washout holes on the flow in a centrifugal blood pump to prevent blood stagnation, a quantitative flow visualization technique was applied to compare flows in models with and without washout holes. A scaled-up model of a prototype pump and a high speed video camera were used for the flow visualization, and images were processed by particle tracking velocimetry. Particular attention was paid to the flow through the gaps behind and in front of the impeller. The results showed that in the gap behind the impeller, washout holes caused not only an inward flow, but also an increase in the tangential velocities. In the gap in front of the impeller, washout holes caused an outward flow and a decrease in the tangential velocities. Head flow characteristics were little affected by the washout holes in this initial design for which the flow through the washout holes was set to be approximately 10% of the flow in the external circuit. These results suggest that the flow through washout holes is significant in the prevention of blood stagnation in 2 ways. First, the inward radial velocity behind the impeller and outward velocity in front of the impeller result in fluid exchange, and second, a tangential velocity increase reduces fluid stagnation behind the impeller.

Dense gas simulation flows over rough surfaces

Mixing within a gravity current head, passing over a rough surface, is assessed using a two-dimensional quantitative full-field flow visualization technique. The method combines salt-water modelling, laser-induced dye fluorescence and video image enhancement. The mixing mechanisms that occur internal to the head structure are shown to play a major role in the overall dilution process and to be significantly affected by surface roughness. The effect of surface roughness is consequential for the accurate prediction of heavier-than-air gas dispersions, for instance. Flows with Reynolds numbers of 500, 1000 and 1500 and density differences of 0.5, 1.0 and 1.5% are considered. Results are presented for flows over two surface types — smooth and rough with a relative roughness element height of 0.35 and frontal obstruction of 50% based on roughness element diameter.

Quantitative visualization of entrapped phase dissolution within a horizontal flowing fracture

An experiment was conducted to demonstrate the utility of quantitative fracture flow visualization techniques in the study of entrapped fluid phase (air) dissolution into a flowing phase (water) within a horizontal, transparent, analog rough‐walled fracture. The fracture aperture field and phase occupancy were measured using light transmission techniques and then combined to calculate bulk water‐phase saturation within the fracture as a function of time. Fracture relative permeability as a function of water‐phase saturation showed a smooth power law behavior during dissolution. Periodic step pulses of clear water within the dyed water inflow yielded dye concentration fields that demonstrate channeling induced by the entrapped air phase. Clusters of the entrapped air‐phase exhibited three types of dissolution behavior: general shrinkage, interfacial recession along cluster appendages, and cluster splitting. Locations for the advance of the wetting phase (water) into a nonwetting entrapped air cluster on its dissolution are not always correlated with either zones of high mass transfer rate (as inferred from gradients in the pulsed dye concentration fields) or with narrow apertures where the wetting phase has been thought to most easily invade. These results suggest that within an individual cluster of the entrapped phase, fluid pressure is at equilibrium and that the path of cluster shrinkage may be controlled primarily by capillary forces resulting from the full three‐dimensional curvature that minimizes surface energy of the phase interface.

Optical flow visualization by deflection mapping using a single Ronchi grating

A new quantitative optical flow visualization technique is presented. In general, it is based on mapping the deflection angles using a single transmission Ronchi grating. The performance of the method has been illustrated and verified using numerical simulations and an actual experiment.