Velocity Field Measurement Technique Research Articles

Direct measurement of bed shear stress using adjustable shear plate over a wide range of Froude numbers

The objective of this study is to extend reliability and applicability of the advanced measuring device for bottom shear stress introduced in the previous paper (Park et al., 2016) so that it could be deployed in the field. The high-speed open-channel flume used for the previous study has been renovated to accommodate even higher flow velocity (up to 6 m s−1). Then we adapted a non-intrusive velocity field measurement technique to such challenging flow conditions of interest. The specific experimental conditions required high degree of adaptation and subsequent development of analysis technique. The experimental results are used to reliably estimate bottom shear stress which is then compared to the measured value using the shear plate. It is shown that the shear plate can be applied to wide range of flow conditions by adjusting elastic shear modulus of the restoring mechanism.

Effect of non-Newtonian viscosity on the fluid-dynamic characteristics in stenotic vessels

Although blood is known to have shear-thinning and viscoelastic properties, the effects of such properties on the hemodynamic characteristics in various vascular environments are not fully understood yet. For a quantitative hemodynamic analysis, the refractive index of a transparent blood analogue needs to be matched with that of the flowing conduit in order to minimize the errors according to the distortion of the light. In this study, three refractive index-matched blood analogue fluids with different viscosities are prepared—one Newtonian and two non-Newtonian analogues—which correspond to healthy blood with 45 % hematocrit (i.e., normal non-Newtonian) and obese blood with higher viscosity (i.e., abnormal non-Newtonian). The effects of the non-Newtonian rheological properties of the blood analogues on the hemodynamic characteristics in the post-stenosis region of an axisymmetric stenosis model are experimentally investigated using particle image velocimetry velocity field measurement technique and pathline flow visualization. As a result, the centerline jet flow from the stenosis apex is suppressed by the shear-thinning feature of the blood analogues when the Reynolds number is smaller than 500. The lengths of the recirculation zone for abnormal and normal non-Newtonian blood analogues are 3.67 and 1.72 times shorter than that for the Newtonian analogue at Reynolds numbers smaller than 200. The Reynolds number of the transition from laminar to turbulent flow for all blood analogues increases as the shear-thinning feature increases, and the maximum wall shear stresses in non-Newtonian fluids are five times greater than those in Newtonian fluids. However, the shear-thinning effect on the hemodynamic characteristics is not significant at Reynolds numbers higher than 1000. The findings of this study on refractive index-matched non-Newtonian blood analogues can be utilized in other in vitro experiments, where non-Newtonian features dominantly affect the flow characteristics.

Effect of pulsatile swirling flow on stenosed arterial blood flow

The existence of swirling flow phenomena is frequently observed in arterial vessels, but information on the fluid-dynamic roles of swirling flow is still lacking. In this study, the effects of pulsatile swirling inlet flows with various swirling intensities on the flow field in a stenosis model are experimentally investigated using a particle image velocimetry velocity field measurement technique. A pulsatile pump provides cyclic pulsating inlet flow and spiral inserts with two different helical pitches (10D and 10/3D) induce swirling flow in the stenosed channel. Results show that the pulsatile swirling flow has various beneficial effects by reducing the negative wall shear stress, the oscillatory shear index, and the flow reverse coefficient at the post-stenosis channel. Temporal variations of vorticity fields show that the short propagation length of the jet flow and the early breakout of turbulent flow are initiated as the swirling flow disturbs the symmetric development of the shear layer. In addition, the overall energy dissipation rate of the flow is suppressed by the swirling component of the flow. The results will be helpful for elucidating the hemodynamic characteristics of atherosclerosis and discovering better diagnostic procedures and clinical treatments.

Development of dual micro-PIV system for simultaneous velocity measurements: optical arrangement techniques and application to blood flow measurements

Blood rheological characteristics can significantly vary with the motion of red blood cells (RBCs) in plasma. Moreover, RBCs show a complicated behavior in micro vessels. Thus, the determination of either plasma or blood cell velocity distribution has been the primary objective in blood flow analysis. However, the conditions during blood flow analyses are different from the actual physiological conditions, wherein the motion of the two distinct blood phases simultaneously occurs. In this study, we used an in vitro micro-particle image velocimetry, which is a reliable velocity field measurement technique, to evaluate the velocity distribution of plasma and blood cells simultaneously. Blood flow through a rectangular microchannel was determined using special optical filter arrangements and by assuming two different hematocrit values. Using the proposed technique, the averaged parabolic velocity profiles for the RBCs and plasma were successfully obtained and compared. The developed simultaneous measurement technique can be used to predict blood cell and plasma behaviors simultaneously with high accuracy under given clinical conditions.

Liquid-intake flow around the tip of butterfly proboscis

Butterflies drink liquid through a slender proboscis using a large pressure gradient induced by the systaltic operation of a muscular pump inside their head. Although the proboscis is a naturally well-designed coiled micro conduit for liquid uptake and deployment, it has been regarded as a simple straw connected to the muscular pump. There are few studies on the transport of liquid food in the proboscis of a liquid-feeding butterfly. To understand the liquid-feeding mechanism in the proboscis of butterflies, the intake flow around the tip of the proboscis was investigated in detail. In this study, the intake flow was quantitatively visualized using a micro-PIV (particle image velocimetry) velocity field measurement technique. As a result, the liquid-feeding process consists of an intake phase, an ejection phase and a rest phase. When butterflies drink pooled liquid, the liquid is not sucked into the apical tip of the proboscis, but into the dorsal linkage aligned longitudinally along the proboscis. To analyze main characteristics of the intake flow around a butterfly proboscis, a theoretical model was established by assuming that liquid is sucked into a line sink whose suction rate linearly decreases proximally. In addition, the intake flow around the tip of a female mosquito׳s proboscis which has a distinct terminal opening was also visualized and modeled for comparison. The present results would be helpful to understand the liquid-feeding mechanism of a butterfly.

Effect of swirling inlet condition on the flow field in a stenosed arterial vessel model

Blood flow in an artery is closely related to atherosclerosis progression. Hemodynamic environments influence platelet activation, aggregation, and rupture of atherosclerotic plaque. The existence of swirling flow components in an artery is frequently observed under in vivo conditions. However, the fluid-dynamic roles of spiral flow are not fully understood to date. In this study, the spiral blood flow effect in an axisymmetric stenosis model was experimentally investigated using particle image velocimetry velocity field measurement technique and streakline flow visualization. Spiral inserts with two different helical pitches (10D and 10/3D) were installed upstream of the stenosis to induce swirling flows. Results show that the spiral flow significantly reduces the length of recirculation flow and provokes early breakout of turbulent transition, but variation of swirling intensity does not induce significant changes of turbulence intensity. The present results about the spiral flow effects through the stenosis will contribute in achieving better understanding of the hemodynamic characteristics of atherosclerosis and in discovering better diagnosis procedures and clinical treatments.

Effect of a Bottom Gap on the Flow Characteristics behind Non-Planar Porous Fence

The flow field behind non-planar porous fence of geometric porosity ε=0.273 with various bottom gaps (G) has been investigated by hot-wire anemometer velocity field measurement technique in a wind tunnel experiment. Seven gap ratios G/H=0.000, 0.025, 0.075, 0.125, 0.150, 0.175, 0.200 of non-planar porous fence were tested in this study with the free-stream velocity fixed at 10m/s. The experimental data were analyzed and the turbulence intensity and wind reduction ratios for different gaps of the porous fence were calculated to estimate the shelter effect of a non-planar porous fence model. The results show that the gap ratio G/H=0.150 gives the best shelter effect among the seven gaps of the non-planar porous fence tested in this study, having a better mean velocity and turbulence intensity as well as wind reduction ratio in a large area behind the non-planar porous fence.

PIV를 이용한 실차 내부 환기유동의 정량적 속도장 측정

The improvement of climatic comfort is crucial not only for passenger comfort but also for driving safety. Therefore, a better understanding on the flow characteristics of ventilation flow inside the passenger compartment is essential. Most of the previous studies investigated the ventilation flow using Computational Fluid Dynamics (CFD) calculations or scale-down water-model experiments. In this study, the ventilation flow inside the passenger compartment of a real commercial automobile was investigated using a Particle Image Velocimetry (PIV) velocity field measurement technique. Under real operating conditions, the velocity fields were measured at several vertical planes for several ventilation modes. The experimental data obtained from this study can be used to understand the detailed flow characteristics in the passenger compartment of a real car and to validate numerical predictions.

Advances in digital holographic micro-PTV for analyzing microscale flows

The accurate three-dimensional (3D) velocity field measurement technique has been receiving large attention in the study of microfluidics. DHM-PTV technique was developed by combining the digital holographic microscopy and particle tracking velocimetry technique. DHM-PTV is an ideal method for measuring three-component-three-dimensional (3C-3D) velocity field in a microscale flow with a fairly good spatial resolution. The advances in the DHM-PTV technique enable the measurement of various microscale flows, such as transport of red blood cells in a microtube and 3D flows in microfluidic devices. DHM-PTV is also applied in studying the motile behavior of swimming microorganisms. DHM-PTV would play an important role in ascertaining the undiscovered basic physics in various microscale and biofluid flow phenomena. In the current study, the basic principle of the DHM-PTV technique and its typical applications to microscale flows are introduced and discussed.

제상모드에 대한 실차 내부 환기유동의 정량적 가시화 연구

Thermal comfort inside a passenger car has been receiving large attention in automobile industries. Especially, the performance of windshield defroster is important in the design of a car to ensure passenger comport and safety. Thereby, better understanding on the ventilation flow along the vehicle windshield is essential to evaluate the performance of windshield defroster. However, most previous studies dealt with the defrost flow using CFD (computational fluid dynamics) calculations or scale-down model experiments. In this study, a real commercial automobile was used to investigate the flow discharged from the vehicle defroster and the ventilation flow along the windshield using a PIV velocity field measurement technique. The experimental data would be useful to understand the flow characteristics in detail and also can be used to validate numerical predictions.

난류 경계층에 잠긴 수직벽 주위 유동의 2차원성 연구

An experimental investigation of the flow around a vertical fence was carried out using a PIV velocity field measurement technique. The vertical fence was embedded in a turbulent boundary layer. The instantaneous velocity fields measured at cross-sectional planes reveal complex longitudinal vortices that vary in size and strength, developing from the upstream location. In the instantaneous vorticity and velocity field data, the shear flow separated from the fence top is highly turbulent and shows unsteady flow characteristics. The topography of the ensemble averaged velocity fields, especially the separation bubble formed behind the fence, shows that the spatial distributions of streamwise velocity (U) and vertical (V) are symmetric, the spanwise velocity (W) is skew-symmetric with respect to the central xy-plane(z=0).

Investigation on the flow characteristics inside an automotive HVAC system with varying ventilation mode

The ventilation flow in a heating, ventilation and air conditioning (HVAC) module of a passenger car was investigated experimentally. Three different ventilation modes with varying temperature mode were tested to study the effect of ventilation mode on the velocity field inside the HVAC module. For each mode, more than 450 instantaneous velocity fields were measured using a particle image velocimetry (PIV) velocity field measurement technique. The instantaneous velocity fields were ensemble averaged to obtain the spatial distribution of mean velocity and spanwise vorticity. The present work highlights the usefulness of the PIV technique for the analysis of the flow inside an HVAC module. The experimental results can be used not only to understand and improve the ventilation flow of an HVAC module but also to validate numerical predictions.

Evaluation of wind environment around a residential complex using a PIV velocity field measurement technique

Wind-tunnel simulations were employed to evaluate the wind environment around a tested residential area located near industrial complexes. The scaled-down geomorphological model of the test area was placed in the test section of a boundary layer wind tunnel. Particle image velocimetry (PIV) measurements were made in five vertical planes and one horizontal plane around the test area for two prevailing wind directions. The results showed that the wind speed decreased in the near surface layer and the velocity fluctuations increased in the upper region due to the presence of hills and high-rise buildings around the test area. Regions of flow separation and low-speed flow were found inside the test area for both the wind directions. The result suggests that the high-rise buildings should be well arranged with respect to the main wind directions to increase the natural ventilation inside the residential complex at the initial design stage.

Experimental study on the fluid mechanics of blood sucking in the proboscis of a female mosquito

Female mosquitoes are known to have a magnificent micro-scale pumping system that can transport small quantities of blood very effectively. To understand the dynamic characteristics of blood flow inside female mosquitoes, the measurement technique that is capable of measuring instantaneous flow fields of a biological sample at micrometer scales is required. In this study, the blood-sucking flow inside a female mosquito's food canal was measured in vivo using a micro particle image velocimetry (micro-PIV) velocity field measurement technique with high-temporal resolution. The volumetric flow rate ( Q) and the time-averaged feeding speed ( V) based on the diameter of the food canal ( D) was found to be 5.751×10 −3 mm 3/s and 0.416 cm/s, respectively. Spectral analysis on the velocity waveform shows a clear peak at 6.1 Hz, indicating distinct pulsatile blood-sucking characteristics. The Womersley number ( α) was about 0.117 and the velocity profile of the blood flow inside the proboscis has a parabolic Hagen–Poiseuille flow pattern when α is much smaller than 1.

미세 생체유동 해석을 위한 첨단 유동가시화기법

Recently microscale biofluid flows have been receiving large attention in various research areas. However, most conventional imaging techniques are unsatisfactory due to difficulties encountered in the visualization of microscale biological flows. Recent advances in optics and digital image processing techniques have made it possible to develop several advanced micro-PIV/PTV techniques. They can be used to get quantitative velocity field information of various biofluid flows from visualized images of tracer particles. In this paper, as new advanced micro-PIV techniques suitable for biofluid flow analysis, the basic principle and typical applications of the time-resolved micro-PIV and X-ray micro-PIV methods are explained. As a 3D velocity field measurement technique for measuring microscale flows, holographic micro-PTV method is introduced. These advanced PIV/PTV techniques can be used to reveal the basic physics of various microscale biological flows and will play an important role in visualizing veiled biofluid flow phenomena, for which conventional methods have many difficulties to analyze.

PIV기법을 이용한 전시용 냉장기의 내부 유동장 측정

Flow structure inside a refrigerating compartment of a scale-down display cooler model was investigated experimentally by using PIV (particle image velocimetry) method, a reliable velocity field measurement technique, in the present study. In addition, we also carried out flow visualization regarding flow structure and particle movement inside a display cooler by using a tracer method. As a result, the mean velocity field measurement shows a large scale vortical flow structure inside a refrigerating compartment due to strong entrainment flow, going through a base plate open gaps.

Temporal evolution of wake behind a rotationally oscillating circular cylinder

The temporal evolution of the near-wake behind a circular cylinder undergoing rotational oscillatory motion with a relatively high forcing frequency has been investigated experimentally using a dynamic particle image velocimetry velocity field measurement technique. Experiments were carried out varying the frequency ratio FR (=ff∕fn), the ratio of the forcing frequency ff to the natural vortex shedding frequency fn, in the range of 0.0 (stationary) to 1.0 at an oscillation amplitude of θA=30° and Reynolds number of Re=4.14×103. Depending on the forcing condition (FR), the near-wake flow showed markedly different flow regimes: stationary (FR=0.0), non-lock-on (FR=0.4) and lock-on (FR=1.0) regimes. However, at all the frequency ratios tested, the power spectral density distributions showed a single dominant peak at the same vortex shedding frequency. As the forcing frequency was increased, the rotational oscillatory motion of the cylinder decreased the length of the vortex formation region and enhanced the mutual interaction between large-scale vortices across the wake centerline. In particular, the entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. The shear-layer instability showed different mechanisms compared with the case of small oscillation amplitude. A strong vortex motion appeared in the near-wake region. In addition, the rotational oscillatory motion of the cylinder changed the local mean flow structure significantly; thereby mean velocity profiles for each frequency ratio varied markedly in the vortex formation region. The configuration of vorticity contours shrunk toward the cylinder as the frequency ratio was increased. The temporally resolved flow information extracted in the present work is useful for understanding the evolution of vortex structure and analyzing the effects of open-loop active flow control technique on the near-wake flow structure.

Flow Field and Gas-Bubble Size Analysis in Water Model for the Process of Aluminum Melt Degassing by Particle Image Velocimetry

The flow field and gas-bubble size during the process of aluminum melt degassing were investigated in water model. A Φ400mm×400mm transparent water model and an impellor degassing device were used in this study. The instantaneous velocity fields of water and bubbles under the mixture of rotary injector were measured with PIV velocity field measurement technique. Then the pictures of bubbles gained from PIV were analyzed with software to get the bubble size distribution. The results showed that bubble flow field and bubble size were influenced by rotary speed of rotor and gas flow-rate. With the increase of rotary speed of rotor, the horizontal velocity components of bubbles became larger, the stagnant time of bubbles in water increaseed correspondingly and bubble size became smaller. With the increase of gas flow-rate, the longitudinal velocity components of bubbles became larger, the stagnant time of bubbles in water decreased correspondingly, and the bubble size became larger also.

An Experimental Study on the Flow Structure inside a Display Cooler Using PIV Techniques

In the present study, the flow structure inside the refrigerating compartment of a scaleddown display cooler model was investigated experimentally using the particle image velocimetry (PIV) method, which is a reliable velocity field measurement technique. In addition, we also carried out flow visualization and computer simulations regarding the movements of thermo-fluid inside a display cooler. As a result, the velocity field measurement shows a large scale vortical flow structure inside the refrigerating compartment due to the entrained flow, thus penetrating a base plate through the open inlet gap.

Particle Image Velocimetry Measurements of the Three-Dimensional Flow in an Exhaust Hood Model of a Low-Pressure Steam Turbine

The three-dimensional flow structure inside an exhaust hood model of a low-pressure steam turbine was investigated using a particle image velocimetry (PIV) velocity field measurement technique. The PIV measurements were carried out in several selected planes under design operation conditions with simulated total pressure distribution and axial velocity profile. The mean flow fields revealed a complicated vortical flow structure and the major sources of energy loss. Vortices with different scales were observed inside the exhaust hood: a strong separation vortex (SV) behind the tip of the guide vane, a longitudinal vortex (LV) at the exhaust hood top, a large-scale passage vortex (PV) evolving throughout the flow path, and an end-wall vortex (EWV) in the region adjacent to the front end-wall. Both the SV and the large-scale PV seemed to consume large amounts of kinetic energy and reduce the pressure recovery ability. The results indicate that the steam guide vane and the bearing cone should be carefully designed so as to control the vortical flow structure inside the exhaust hood.