Direct Flow Visualization Research Articles

Improved Glass Micromodel Methods for Studies of Flow and Transport in Fractured Porous Media

Microscale experiments can provide mechanistic insights into larger‐scale flow and transport phenomena. Studies of the microscale mechanics involved in preferential flow in general, and unsaturated fast flow paths in particular, require the development of new experimental techniques. A new method for constructing glass micromodels has been developed which permits direct visualization and quantification of flow and transport phenomena in fractured porous media. In the fracture‐matrix micromodels a sequential etching procedure was developed in order to provide the necessary contrast of depths between matrix pores and fracture apertures. This high contrast in etching depths ensures that very different capillary properties are associated with micromodel “fractures” and “matrix” blocks. Improved techniques were also developed for reducing the pore sizes of the matrix to a natural fine‐grained sandstone pore scale. The improved micromodel pattern designs allow for previously unachievable control of boundary conditions. Various saturated and unsaturated fracture flow and transport processes can be visually and quantitatively studied with these micromodels. A method for directly measuring pore‐scale flow velocity distribution through tracing trajectories of suspended fluorescent microspheres was also developed. Examples of applications include measurements of velocity profiles in fractures, imbibition, fracture‐matrix transient flow, and matrix diffusion. In general, the improved micromodel method provides a unique tool for exploring some of the previously unrecognized flow and transport processes in fractured porous media. This research is directed at providing microscale explanations to some currently unresolved flow and transport issues important in predicting the larger‐scale flow processes.

NITRIC OXIDE SYNTHASE INHIBITION DECREASES CEREBRAL BLOOD FLOW IN RATS INFUSED WITH SICKLE ERYTHROCYTES. † 914

The etiology of stroke, a major form of vaso-occlusion in hemoglobin SS(HgbSS) disease, remains unclear, but may involve red blood cell (RBC)adhesion and altered vascular tone regulation. We hypothesized that nitric oxide (NO) participates in maintaining cerebral blood flow in HgbSS disease and its inhibition would result in vaso-occlusion in an in vivo cerebral microcirculation model. Laser Doppler flow (LDF) probes were inserted into parietal skull holes in anesthetized, splenectomized adult rats to measure cerebral RBC flow. HgbSS or HgbAA-RBCs (3 mls) were infused into rats after pretreatment with either the NO synthase (NOS) inhibitor, L-nitroarginine methyl-ester (L-NAME, 40 mg/kg IV), or its inactive isomer D-NAME. Mean arterial blood pressure, end-tidal CO2, and LDF were monitored for 30 minutes after the RBC infusion. The rats treated with L-NAME + HgbSS-RBCs(LN-SS group, n=9) experienced a significant decrease in LDF (p=0.002) as compared to controls at 20 min following the RBC infusion. In addition, 56% of the LN-SS group died prior to completion of the experiment. In the control groups, L-NAME + HgbAA-RBCs (n=6), D-NAME + HgbSS-RBCs (n=5), and D-NAME + HgbAA-RBCs (n=3), all rats completed the experimental protocol with no significant change in LDF and stable hemodynamics. In order to test thelocalized effect of NOS inhibition, three rats received a topical infusion of L-NAME through a cranial window that allowed direct visualization of RBC flow in the rat cerebral microvasculature. In two of these locally treated rats (67%), total cessation of cerebral blood flow was observed following infusion with HgbSS-RBCs while maintaining stable hemodynamics. We conclude that in the presence of HgbSS-RBCs, inhibition of NOS results in cerebral vaso-occlusion (i.e. stroke) in this rat model.

NMR imaging of thermal convection patterns

Two special magnetic resonance imaging techniques were applied to the Rayleigh/Bénard problem of thermal convection for the first time. The methods were tested using a water cell with horizontal bottom and top covers kept at different temperatures with a downward gradient. Using Fourier encoding velocity imaging (FEVI) a five-dimensional image data set was recorded referring to two space dimensions of slice-selective images and all three components of the local velocity vector. On this basis, the fields of the velocity components or of the velocity magnitude were evaluated quantitatively and rendered as gray shade images. Furthermore the convection rolls were visualized with the aid of two- or three-dimensional multistripe/multiplane tagging imaging pulse sequences based on two or three DANTE combs for the space directions to be probed. Movies illustrating the fluid motions by convection in all three space dimensions were produced. It is demonstrated that the full spatial information of the convection rolls is accessible with microscopic resolution of typically 100 x 100 x 100 microns3. This resolution is effectively limited by flow displacements in the echo time, which should be well within the voxel dimension. The main perspective of this work is that the combined application of FEVI and multistripe/multiplane tagging imaging permits quantitative examinations of thermal convection for arbitrary boundary conditions and with imposed through-flow apart from the direct visualization of convective flow in the form of movies.

Three-dimensional temporal reconstruction and analysis of plume images

An experiment with two subsonic jets generating a cross-flow was conducted as part of a study of the structural features of temporal reconstruction of plume images. The flow field structure was made visible using a direct injection flow visualization technique. It is shown that image analysis and temporal three-dimensional visualization can provide new information on the vortical structural dynamics of multiple jets in a cross-flow. It is expected that future developments in image analysis, quantification and interpretation, and flow visualization of rocket engine plume images may provide a tool for correlating the engine diagnostic features by interpreting the evolution of the structures in the plume.

Diagnosis of patent ductus arteriosus in adults by biplane transesophageal color Doppler flow mapping

In infants and children, direct visualization of patent ductus arteriosus and shunt flow is often successful using transthoracic 2-dimensional 1 and color Doppler echocardiography. 2 In adults, however, transthoracic echocardiography may fail to yield diagnostic information on patent ductus arteriosus because the ductus is located far from the transducer on the chest wall. 3 Since the recent development of a transesophageal transducer with the capabilities of color-coded Doppler flow imaging, both single- and biplane transesophageal echocardiography have been extensively used in the assessment of patients with cardiovascular diseases. 4–8 Transesophageal approach affords consistent high-quality 2-dimensional images of the arch and descending aorta without being restricted by lung tissue or ribs. This study evaluates the diagnostic usefulness of transthoracic and biplane transesophageal color Doppler echocardiography in adolescent and adult patients with patent ductus arteriosus.

A Flow Visualization Study of the Cathode Geometry Effect in Magnetoelectrolytic Cells

Natural convection at various solid‐liquid interfaces in an electrolytic system was studied via a direct flow visualization technique using an analytical indicator. Copper cathodes of various shapes but comparable size were used to provide different geometries of the solid‐liquid interface: a sphere, a hemisphere, a horizontal cylinder, a single cone, a vertical cylinder, a double cone, and one‐half of a double cone were employed. Experimental results indicate clearly that highly specific propagation patterns and locally strong anisotropies can be created when magnetic fields are imposed on the electric field, but at present, the complexity of interplaying phenomena allows a mainly qualitative analysis.

Color Doppler evaluation of aortic dissection.

Color Doppler studies were performed in 16 adult patients with proven DeBakey type I and III aortic dissection. Simultaneous opacification of both aortic lumina with oppositely directed flow was noted by color Doppler in at least one aortic segment in 14 of 16 patients (12 type I, two type III). In two patients (one type I, one type III), flow was seen in one lumen only, with clot demonstrated in the other lumen in one of them. Of 12 patients in whom communication between two aortic dissection channels was shown by angiography/surgery, color Doppler correctly identified them in nine patients (four ascending aorta, two aortic arch, and three descending aorta), either by direct visualization of flow moving from one lumen into the other (six patients) or indirectly by analyzing differences in timing of opacification of the two lumina and flow direction (three patients). Also, color Doppler correctly diagnosed aortic regurgitation as severe (aortic regurgitation jet occupying more than 75% of left ventricular outflow) in three patients and moderate in four patients. Color Doppler provides comprehensive evaluation of flow dynamics in aortic dissection.

Color flow mapping in obstetrics.

Blood flow studies are now used extensively in evaluating fetal peripheral circulation. By using continuous and pulsed Doppler ultrasonic equipment the raw Doppler shifts caused by blood moving in the vessels can be analyzed in different ways. Spectral data can be used to estimate blood velocity and volume in the vessel, and pulsatility characteristics and turbulence. All these data are obtained from very restricted areas within the body so that aiming of the probe is critical, and obtaining data from an area requires long and tedious effort. Color flow mapping which has recently been introduced into clinical practice provides an overview of flow velocities and directions within an area. Color coded flow can be displayed by applying the moving target identification principle (as in radar) to an ultrasonic diagnostic instrument where sound waves are used instead of radio waves. The value of flow mapping has already been recognized in the diagnosis of congenital and acquired heart disease in adults and children. Direct visualization of intracardiac flow provides for a fast and accurate diagnosis of various cardiac defects, such as the ventricular septal defect, without cardiac catheterization. In the present study we have investigated the potential of color flow mapping in obstetrics. 211 pregnant women were examined between the 15th and 40th weeks of pregnancy. 193 of them had normal pregnancies; 18 were abnormal, including 11 cases of insulin dependent diabetes, 2 cases of Rh-immunization, 2 cases of nonimmunologic fetal hydrops, 2 cases of fetal cardiac structural defects and 1 case of intrauterine growth retardation. Intracardiac flow was clearly seen in 78% (35/45) of cases at the gestational age between the 20th and 24th weeks. After the 24th week the heart flow visualization rate gradually decreased to 31% (11/36) before term. In 2 cases of congenital heart abnormality flow mapping enabled clear visualization of reverse flow through the tricuspid valve and diagnosis of valvular insufficiency. The best results in studies of flow in peripheral vessels were obtained from umbilical vessels which were seen in all cases after the 15th week. The flow visualization rate was much lower when the fetal aorta, intrahepatic umbilical vein and internal carotid artery were studied. The comparison between the diameter of the umbilical vein measured on the B-scan and flow width in the same vessel showed no significant difference (t = 0.26; p less than 0.01; N = 209).(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of Taylor vortex structure in combined axial and rotating flow

AbstractThe sequential form of the axial displacement of inclined Taylor vortices and their progressive degeneration to horizontal streaks due to the increasing dominance of axial flow is demonstrated via an analytical indicator‐based direct flow visualization technique applied to a concentric cylinder cell with the inner cylinder rotating.

An experimental investigation of convection patterns in flow past an obstacle

The propagation of liquid flow in a cylindrical tube past solid obstacles of various shapes and sizes has been studied at Reynolds numbers up to about 6,600 via a direct electrochemical flow visualization technique where the obstacles were connected as cathodes and the colour change of a suitable analytical indicator was employed for the marking of convecting flow. Typical propagation patterns are shown by photographs and the effect of the obstacle on local velocities at the core of tube is analyzed.

A flow visualization technique using analytical indicators: Theory and some applications

The fundamental concepts of a direct flow visualization technique, based on chemically and electrochemically induced local pH variations is described. Sharp changes in indicator colour in liquid media permit a highly reproducible observation of convection patterns, illustrated by five applications.

The direct visualization of blood flow by real-time ultrasound: clinical observations and underlying mechanisms.

The observation of spontaneous low-amplitude echoes from the blood in the larger blood vessels has not been satisfactorily explained. In vitro experiments were performed to study flowing and stationary blood by ultrasound. In both cases numerous low-level echoes were seen in blood and suspensions of washed red cells. No echoes were observed in plasma or solutions of hemoglobin. It was concluded that the red blood cells is the most likely scattering agent responsible for the contrast effect observed clinically.

In vivo microscopy of the gastric microcirculation.

A method for observing the gastric microcirculation directly in the living animal and the results of its application to the study of the rat gastric microcirculation are described. The exteriorized stomach of an anesthetized rat was transilluminated by a cool light transmitted from a high intensity light source by means of a quartz rod. Illumination was adequate for direct microscopic visualization of blood flow through the muscle layer. After a portion of muscle and serosa had been dissected carefully, blood flow could be traced through the rich submucosal network of arterioarterial anastamoses down to the mucosa and back through collecting veins, emerging from the mucosa to the submucosal venovenous anastamotic network. By everting and transilluminating the exteriorized stomach, the superficial mucosa was visualized and blood flow could be traced through the hexagonally distributed capillaries surrounding the gastric glands to the collecting veins. The distribution of vessels in each of these layers is described in detail. Despite a careful search, no arteriovenous anastamoses were found, either in the submucosa or in the superficial mucosa.

Airway resistance and lung volume in the newborn infant.

Extract: The total body plethysmograph method of DuBois et al. [7] for measurement of lung volume and airway resistance was adapted to the study of newborn infants. A Rendell-Baker mask directly controlled by the operator proved by trial and error to minimize the chances of pressure leaks and undesirable pressure on the nose and allowed the mandible to be pulled forward to minimize pharyngeal obstruction. Use of an oscilloscope for direct visualization of flow and pressure tracings was essential in order to detect immediately pressure leaks, artifacts, and abnormal patterns during measurements of both lung volume and airway resistance. Interpretation of the measurements of airway resistance and of the relation of conductance to lung volume over a wide range of age and lung volumes requires that the flow rates at which measurements are made be recorded, along with lung volumes. In 51 newborn infants, an airway resistance value of 19.2±5.6 cm H2O/l/s was observed. Thoracic gas volume at functional residual capacity was 89.9±15.5 ml. The resulting mean value for conductance (reciprocal of airway resistance), 0.052 l/s/cm H2O, fell above the calculated regressions of conductance versus lung volume determined by Briscoe and DuBois [3], who used data obtained from older children and adults. Direct comparison is limited since airway resistance in the older subjects was usually obtained using panting flow rates of 0.5 l/s, while the infants in this study had flow rates of approximately 0.1 l/s at the time airway resistance was measured. Speculation: Confirmation of the presence of the major expiratory flow-limiting segment in the nasal airway of newborn and small infants is needed. This location suggests a physiologic means for slowing expiratory flow and thus maintaining expiratory intraluminal pressure in the tracheobronchial tree, which might protect the compliant trachea and major bronchi from compression by positive intrapleural pressure in obstructive pulmonary disease. It is possible that there is an airflow-lung volume-airway resistance relation throughout life; however, a wide range of control data will be required to establish such a relation.