Distinct Flow Patterns Research Articles

Free-surface flows with near-critical flow conditions

Open channel flow situations with near-critical flow conditions are often characterized by the development of free-surface instabilities (i.e., undulations). The paper develops a review of several near-critical flow situations. Experimental results are compared with ideal-fluid flow calculations. The analysis is completed by a series of new experiments. The results indicate that, for Froude numbers slightly above unity, the free-surface characteristics are very similar. However, with increasing Froude numbers, distinctive flow patterns develop. Key words: open channel flow, critical flow conditions, free-surface undulations, flow instability, undular surge, undular broad-crested weir flow, culvert flow.

UNSTEADY FLOW AND HEAT TRANSFER IN A CHANNEL WITH A BUILT-IN TANDEM OF RECTANGULAR CYLINDERS

Abstract The incompressible unsteady flow past a channel with a pair of cylinders of rectangular cross section, placed in tandem normal to the flow, is investigated by numerical simulation. The objective is to evaluate the effect of cylinder separation distance, S / H, on the flow behavior and heat transfer on the channel walls, over a range of Reynolds numbers. Above a critical Reynolds number, these flows bifurcate to a time-periodic self-sustained oscillatory state. The results reveal for S / H = 2.0 three distinct flow patterns: steady flow ( Re = 200), time-periodic oscillatory state ( 400 600.

Air movement around a worker in a low-speed flow field.

A knowledge of the air movement around a worker in a low-speed airflow is important in a number of areas: containment testing of fume cupboards; testing of personal dust samplers; testing of LEV effectiveness; and measurement of worker exposure. Measurements of velocity vectors around the upper torsos of manikins and a human in low-speed airflows have been made using a laser Doppler anemometer. Both heated and unheated manikins, as well as a 'breathing' manikin were used. The results show that quite distinctive flow patterns develop with heated and unheated bodies. Comparison of the flows around two- and three-dimensional manikins with that around a human shows that only a three-dimensional heated manikin gives good results. The unheated breathing manikin gave results which were unrepresentative of the real situation. A suitable manikin for use in sampling or testing in low-speed airflows would have a heated, rounded, three-dimensional body of reasonably human dimensions and would be non-breathing and clothed.

Early assessment by transesophageal echocardiography of left atrial appendage function after percutaneous mitral commissurotomy

Thirty-seven consecutively admitted patients with severe mitral stenosis underwent percutaneous mitral commissurotomy with a transthoracic and biplane or multiplane transesophageal echocardiographic examination before and between 24 and 48 hours after percutaneous mitral commissurotomy. Thirty patients (81%) were in sinus rhythm and 7 were in atrial fibrillation. Left atrial appendage (LAA) function was evaluated in both the transverse and the longitudinal planes by planimetry and pulsed Doppler echocardiographic interrogation at the LAA outlet. Percutaneous mitral commissurotomy resulted in a twofold increase in mitral valve area, and no severe mitral regurgitation occurred. With use of the planimetry method, there was no significant improvement in LAA ejection fraction, except in the transverse plane for patients in sinus rhythm (p = 0.03). With use of Doppler method, 3 distinct flow patterns were observed before the procedure: a “sinus pattern” in patients in sinus rhythm, and a “fibrillatory pattern” (n = 3) or a “no-flow pattern” (n = 4) in patients in atrial fibrillation. After commissurotomy, mere was a marked increase in LAA peak Doppler velocity (+62%) and in LAA velocity time integral (+31%). Of the 4 patients in atrial fibrillation with a no-flow pattern, 2 had recovery of a typical effective fibrillatory flow pattern after the procedure. The increase in peak Doppler velocity after commissurotomy was related to the decrease or regression in left atrial spontaneous echo contrast, and correlated with the increase in mitral valve area, the decrease in transmitral pressure gradient, and the increase in cardiac index; improvement in valve function after successful percutaneous mitral commissurotomy is associated with early improvement in LAA function.

Visualisation of the flow pattern generated by the interaction of a circular cross-section inlet and a cross-flow

Abstract A series of flow visualisation experiments of the surface flow pattern produced by a circular cross-section inlet mounted in a flat plate in a cross-stream have been conducted. The results show a distinct and repeatable surface flow pattern characterised by a nodal stagnation point behind the inlet with adjacent saddle points lying symmetrically either side of the node. These general features of the flow pattern are found over the range of inlet to free stream velocity ratios studied. However, the relative location and extent of the flow domains are affected by the change in the velocity ratio.

Nonlinear growth of the shock-accelerated instability of a thin fluid layer

Richtmyer–Meshkov instability causes spatially periodic perturbations initially imposed on a shock-accelerated, thin gas layer to develop into one of three distinct flow patterns. Planar laser-induced fluorescence imaging of the evolving layer, produced by a perturbed SF6 planar jet in air, shows an apparent flow bifurcation that is observed as mushroom-shaped or sinuous-shaped interfacial patterns. Analysis of this nonlinear instability growth, accomplished by modelling the flow field as a row of line vortices, predicts that the layer thickness grows logarithmically at later times and compares well with our measurements. Because the row of vortices is unstable, the model also provides an explanation for the appearance of the three observed interfacial patterns.

Summertime atmospheric circulation in the vicinity of a blue ice area in Queen Maud Land, Antarctica

The surface wind field is an important factor controlling the surface mass balance of Antarctica. This paper focuses on the observed atmospheric circulation during summer of an Antarctic blue ice area in Queen Maud Land. Blue ice areas are characterised by a negative surface mass balance and henceforth provide an interesting location to study the influence of meteorological processes on large local mass balance gradients. During lapse conditions, synoptic forcing determines the surface-layer flow. No significant horizontal temperature gradient with coastal stations could be detected along isobaric surfaces, indicating weak or absent thermal wind. Observations performed at the coastal stations Halley and Georg von Neumayer show the pronounced effects of synoptic forcing. The surface winds in the valley of the blue ice area could be divided into two distinct flow patterns, occurring with about equal frequency during the experiment. Flow type I is associated with cyclonic activity at the coast, resulting in strong easterly winds, precipitation and drifting snow. Flow characteristics inside and outside of the valley are similar during these conditions. Flow type II occurs when a high pressure system develops in the Weddell Sea, weakening the free atmosphere geostrophic winds. A local circulation is able to develop inside the valley of the blue ice area during these tranquil conditions. The transition from flow type II to flow type I is associated with front-like phenomena inside the valley. Some simple theoretical considerations show that surface-layer stability and the upper air geostrophic wind determine the surface flow direction in the valley. Finally, the influence of the observed circulation on the energy and mass balance of the blue ice area is discussed.

Pseudoaneurysms of the mitral—aortic intervalvular fibrosa: Dynamic characterization using transesophageal echocardiographic and Doppler techniques

Objectives. The aim of this study was to provide a detailed description of echocardiographic and Doppler features of pseudoaneurysms involving the mitral-aortic intervalvular fibrosa and to compare echocardiographic and aortographic findings.Background. Infection of the aortic valve may spread to the aortic annulus, resulting in ring abscesses or pseudoaneurysms, or both, of the intervalvular fibrosa, which can alter patient management and prognosis.Methods. The echocardiographic and Doppler findings of 20 patients with pseudoaneurysms or ring abscesses, or both, were reviewed and compared with surgical and aortographic results.Results. A total of 23 lesions were identified, of which 16 were intervalvular pseudoaneurysms, and 7 were ring abscesses. Transthoracic echocardiography detected 43% of the lesions, whereas transesophageal echocardiography identified 90% (p < 0.01). The most distinct feature of the pseudoaneurysms was marked pulsatility, with systolic expansion and diastolic collapse (mean systolic area [±SD] 4.1 ± 3.4 cm2vs. diastolic mean area 1.8 ± 2.2 cm2, p < 0.05). Using color Doppler, two types were identified: unruptured pseudoaneurysms (n = 9), which communicated only with the left ventricular outflow tract and had a distinct flow pattern, and ruptured pseudoaneurysms (n = 7), which, in addition, communicated with the left atrium or aorta. Compared with pseudoaneurysms, ring abscesses were smaller and nonpulsatile and showed either no flow or continuous systolic and diastolic flow, the site of paravalvular aortic insufficiency. In 10 patients who underwent aortography, three lesions were identified, and findings were concordant with echocardiography. However, in seven patients aortographic findings were normal, whereas echocardiography identified intervalvular pseudoaneurysms, all of which were documented at operation.Conclusions. Intervalvular pseudoaneurysms are more frequently detected by transesophageal echocardiography than by aortography or transthoracic examination and exhibit distinct dynamic features and Doppler patterns that can further help characterize cavitary lesions in the aortic root and guide appropriate surgical intervention.

NUMERICAL STUDY OF BOILING AND NATURAL CONVECTION IN CAPILLARY POROUS MEDIA USING THE TWO-PHASE MIXTURE MODEL

Abstract A newly developed two-phase mixture model is applied, in conjunction with a control-volume-based finite difference method, to numerically investigate boiling with thermal convection in a porous layer heated from below. The numerical procedure employs a fixed grid and avoids tracking explicitly the moving interface between the liquid and two-phase regions. Numerical results are obtained to shed light on the intricate interactions between boiling and natural convection as well as to explain experimental observations. Four distinct flow patterns that were observed in previous experiments are predicted. A quantitative comparison of the predicted and measured vapor volume fraction in the porous bed shows good agreement. The numerical results also agree with published linear stability results. In addition, the present study documents the effects of important parameters such as Rayleigh number, bottom heat flux, and aspect ratio.

Ultrafast Contrast Enhanced Magnetic Resonance Imaging of Congenital Hydronephrosis in a Rat Model

Since new ultrafast magnetic resonance imaging (MRI) might offer unique advantages for evaluating renal blood flow, anatomy and urinary excretion, we used this technique to characterize a rat model with congenital partial ureteropelvic junction obstruction. MRI of 9 rats from an inbred colony with unilateral congenital (nonsurgical) hydronephrosis was compared with the contralateral nonhydronephrotic kidney serving as control. Our new imaging technique consisted of a 1-minute ultrafast gradient recalled imaging sequence during the first minute (64 images per imaging time 960 milliseconds) after contrast bolus injection with gadolinium-diethylenetri-aminepentaacetic acid for assessment of renal blood flow followed by a 30-minute period with image acquisition every 30seconds to study contrast distribution and excretion. Signal intensities were analyzed continuously over selected, different regions of interest. Anatomic analysis of MRI noncontrast studies showed precise delineation of the hydronephrotic pelvis and cortico-medullary junction. After contrast gadolinium-diethylenetriaminepentaacetic acid injection signal intensity from the region of interest from hydronephrotic kidneys differed from nonhydronephrotic kidneys by showing less cortical decrease, suggesting decreased blood flow, less medullary decrease and delayed contrast excretion. Clear contrast distribution among the cortex, medulla and collecting system allowed selective estimation of different regions of interest and excellent anatomic evaluation. Renal anatomy and renal pelvic pressures were confirmed after scans were completed.Ultrafast contrast enhanced MRI allows simultaneous assessment of renal morphology, blood flow and function. In hydronephrotic partially obstructed kidneys distinct flow and excretion patterns measured with contrast enhanced MRI allow differentiation between the obstructed and nonobstructed kidney on physiological rather than purely anatomic means. This imaging technique may provide a useful method of evaluating congenital hydronephrosis obviating the need for multiple different diagnostic procedures.

Assessment of left atrial appendage function by biplane transesophageal echocardiography in patients with nonrheumatic atrial fibrillation: Identification of a subgroup of patients at increased embolic risk

Objectives. This study was conducted to identify a subgroup of patients with nonrheumatic atrial fibrillation with an increased risk for cardiogenic embolism by assessing left atrial appendage function.Background. Patients with nonrheumatic atrial fibrillation have an increased risk for thromboembolic complications. The left atrial appendage is the most likely source for thrombus formation. It is likely that the appendage function (contraction, filling dynamics) is related to the pathogenesis of thrombus formation.Methods. Twenty-nine patients with nonrheumatic atrial fibrillation (group I) underwent biplane transesophageal echocardiography. The maximal and minimal areas during a cardiac cycle and the peak emptying and filling velocities of the appendage were measured in both scan planes. For comparison, two additional groups were also analyzed. Group II consisted of 12 patients with chronic atrial fibrillation due to significant mitral stenosis, and group III consisted of 30 patients who were in sinus rhythm.Results. Patients with nonrheumatic atrial fibrillation showed two distinct appendage flow patterns: either well defined peak filling and emptying waves (≥25 cm/s) with visible fibrillatory contractions of the appendage wall (“high flow profile”) or irregular, very low, peak filling and emptying waves (<25 cm/s) associated with almost no visible appendage contractions (“low flow profile”). The left atrial appendage function in the first subgroup resembles that seen in patients with sinus rhythm, whereas the appendage function in the latter subgroup resembles more the “static pouch” seen in patients with rheumatic atrial fibrillation. Events suggestive of cardiogenic embolism occurred in six patients from group I, five of whom were in the low flow profile subgroup (p < 0.05). The spontaneous echo contrast phenomenon was observed in 80% of the low flow profile subgroup but in only 5% in the high flow profile subgroup (p < 0.05). Three thrombi confined to the left atrial appendage were detected by transesophageal echocardiography in group I; all three of the patients were in the low flow profile subgroup.Conclusions. The assessment of left atrial appendage function by transesophageal echocardiography may be helpful to identify subgroups of patients with nonrheumatic atrial fibrilation with an increased risk of thrombus formation.

Evaluation of the Basilar Artery Flow Velocity by TCD Examination

We measured flow velocities of the basilar artery (BA) via the suboccipital window by TCD in 115 subjects to investigate BA hemodynamics. Twenty one of the subjects had posterior circulation pathology, 37 had anterior circulation pathology, and the remaining 57 cases had no lesions. The vascular lesion was confirmed by cerebral angiography in all cases. The characteristic findings of the BA flow velocity by the site of vascular lesion were as follows. 1) Normal: signal obtainable from a depth of 80 mm to depths beyond 95 mm. Timeaveraged mean velocity was between 20 and 45 cm/s and velocity variations occurred at different depths<16 cm/s. 2) BA occlusion: a) Total occlusion: no signal or signal only from the site adjacent to the vertebrobasilar junction. b) Distal occlusion: continuous low velocity (<20 cm/s) .c) Proximal occlusion: continuous retrograde flow. 3) Stenosis of the BA: sudden increase in flow velocity at the stenotic portion or continuous low velocity proximal to the site of severe stenosis. 4) Occlusion of the bilateral vertebral arteries (VA) : flow from the anterior spinal artery detectable. 5) Unilateral occlusion of the VA: no distinctive flow pattern. 6) Internal carotid artery occlusion or vascular rich tumor in the posterior lobe: increased flow velocity along the BA. 7) Other anterior circulation pathology: no characteristic signal.

Unsteady flow and heat transfer for cylinder pairs in a channel

The incompressible, two-dimensional, unsteady flow past a pair of cylinders of square cross-section, placed in tandem normal to the flow in a channel, has been investigated by dye visualization and direct numerical simulation. The objective has been to evaluate the effect of cylinder separation distance, λ, on the flow behavior and heat transfer, for cylinder diameter ratios, D d , of 1 and 2 over a range of Reynolds numbers 200 ⩽ Re ⩽ 1600, based on the larger (downstream) cylinder diameter. A comparison between the experimental and numerical results for cylinders of equal cross-section dimensions shows very good agreement. The results for D d = 1 reveal three distinct flow patterns as a function of λ and Re which, apparently, have not been previously reported: (1) For 0.25 ⩽ λ ⩽ 4 with Re ⩽ 200, the inter-cylinder flow consists of a pair of steady counter-rotating eddies which do not exchange fluid with the surrounding flow and eddy shedding is observed only for the downstream cylinder. (2) For 0.25 ⩽ λ ⩽ 1.0 with 400 ⩽ Re ⩽ 1600, vertical flow oscillations arise in the inter-cylinder space, and the periodic ingestion of backward-jetting fluid from the top and bottom walls of the downstream cylinder into the inter-cylinder space is observed. For fixed λ the unsteadiness increases with Re but only the downstream cylinder sheds large eddies. (3) At a critical inter-cylinder spacing related to Re according to λ c ∼ Re − 2 3 , the shedding of large eddies also occurs at the upstream cylinder and this results in a highly mixed inter-cylinder flow. For a cylinder diameter ratio of D d = 2 , with the smaller cylinder located upstream of the larger heated cylinder, it is shown that for Re fixed an optimal location exists for the upstream cylinder such that the heat transfer from the downstream cylinder is maximized. The optimal location corresponds to a spacing smaller than the critical inter-cylinder spacing.

Flow patterns induced by substrata and body morphologies of benthic organisms, and their roles in determining availability of food particles

A physical model, supported by field experiments, predicts that body and substratum morphology of benthic organisms can determine the particle composition encountered by the organisms. The model is based on the velocity gradient of boundary-layer flows, its resultant particle distribution, and the flow pattern induced by bluff bodies (benthic organisms and substrata). The various body morphologies of benthic organisms can be divided into two categories of slenderness ratio (SR—height to width ratio of the body plane normal to the flow) that induce two distinct ambient flow patterns. Ascending flow is induced by low-SR body morphologies and diverging flow by high-SR morphologies. The model predicts, based on the induced flow pattern, that high-SR bodies encounter higher fluxes of fine suspended particles, whereas low-SR bodies encounter higher fluxes of bedload particles. We suggest, therefore, that high-SR species are suspension feeders, and low-SR species are coarse particle or bedload feeders.

Flow patterns and endothelial cell morphology in a simplified model of an artificial ventricle.

The aim of this study was to delineate the flow patterns in a non-unidirectional flow field inside a ventricle-shaped cell culture chamber, and examine the resulting morphology and integrity of the endothelium in select regions of the monolayer. The chamber was perfused by pulsatile flow, and the coherent motion of the fluid was studied using flow visualization aided by image analysis. Four distinct flow patterns were discerned and examined: central jet, flow impingement, flow separation, and recirculating eddies. The influence of these patterns on endothelial cell morphology was assessed after 20 h of exposure to flow. There were no signs of damage to the endothelium in the jet region nor was there evidence of cell alignment with the flow. Yet, there were changes in cell morphology and cytoskeletal architecture as compared to control. By contrast, within the eddies where the flow was highly disturbed, there was apparent damage to the endothelium. Thus, exposure of cells to random velocity fluctuations in regions of quasi-static flow compromises the integrity of the monolayer. Identification of such sites and acquisition of the knowledge necessary to protect the cells from denudation will be valuable for the endothelialization efforts of cardiac prostheses.

Instability growth patterns of a shock-accelerated thin fluid layer.

Laser-induced fluorescence imaging of a shock-accelerated thin gas layer, produced by a planar ${\mathrm{SF}}_{6}$ jet in air, shows multiple flow evolutions. Richtmyer-Meshkov instability causes spatially periodic perturbations initially imposed on the jet to develop into one of three distinct flow patterns, indicating nonlinear instability growth. Slight differences in the vorticity distribution deposited on the air-${\mathrm{SF}}_{6}$ interfaces by the shock interaction produce a bifurcated flow, observed as mushroom-shaped or sinuous-shaped interfacial patterns.

Experimental studies of pulsatile flow and endothelial cell adaptation in ventricle shaped cell culture chambers.

The authors' long-term research goal is to minimize the risk of thromboembolic complications in cardiac prostheses by lining blood contacting surfaces with a functional monolayer of autologous endothelial cells. These cells recognize changes in hemodynamics and can adapt effectively to experimentally manipulated flow conditions. By implication, the morphology of endothelial cells, in conjunction with their function, might serve as an indicator of the flow patterns in a particular location. It was hypothesized that, by understanding flow patterns at a given site, the local morphology and function of the endothelial cells in such a region could be predicted. To test this hypothesis, a series of ventricle shaped flow chambers were designed and perfused with pulsatile flow. The flow field in the chambers was studied by computer aided dye visualization and nuclear scintigraphy. The results showed that the large scale motion of the fluid in the cavity was highly coherent and consisted of distinct flow patterns. The temporal and spatial characteristics of the flow patterns, and their implications with respect to endothelial cell endurance in this in vitro environment, were examined in detail.

An experimental study of oscillatory thermocapillary convection in cylindrical containers

An experimental study of oscillatory thermocapillary convection in small cylindrical containers with a heating wire placed along the center axis is performed by investigating the flow structures and temperature distributions under various conditions. To supplement the flow visualization the surface is scanned using an infrared imager. Here 2 cS viscosity (Pr=27) silicone oil is used as the test fluid. It is observed that beyond a certain temperature difference between the container wall and the heating wire, a distinctive unsteady flow pattern appears. This unsteady phenomenon is identified as oscillatory thermocapillary. After the onset of oscillations the flow structure becomes nonaxisymmetric and wave motion is observed at the free surface. It is shown that the critical temperature difference is independent of container dimensions if the aspect ratio is fixed.

Doppler color-flow images from a stenosed arterial model: Interpretation of flow patterns

The capability of the recently introduced Doppler color-flow mapping devices to accurately detect flow patterns in the region of an arterial stenosis was evaluated by use of an in vitro flow model. Pulsatile flow simulating that in a low-resistance vessel was induced through a straight acrylic tube, which alternatively contained axisymmetric stenoses of 0%, 20%, 40%, 60%, and 80% diameter reduction. Doppler color-flow mapper images were taken in realtime along the tube midplane from 0 to 8 diameters downstream of each stenosis. Comparison of the Doppler color-flow mapping results with similarly recorded flow visualization (hydrogen bubble) images showed a close correspondence of key features of the flow, including detection of a high-velocity, centerline jet and nearwall separated flow zones. Distinctive flow patterns exist with each stenotic case, and these should be of considerable value in diagnosing clinical disease conditions.

Unsteady three-dimensional natural convection in a fluid-saturated porous medium

Natural convection in a cube of fluid-saturated porous medium having a constant temperature top and bottom is studied numerically. In the first of two special cases considered, the vertical sides are insulated. In this case, the numerical simulations indicate permanently unsteady regularly and irregularly fluctuating convective states at Rayleigh numbers (R*) above 550. The regularly fluctuating convective state defined by simply periodic oscillations in the Nusselt number begins at R* between 550 and 560. The frequency of the oscillations appears to depend on R* approximately as f∝ (R*)3.6. The irregularly fluctuating convective state defined by random variations in the Nusselt number begins at R* between 625 and 640. In the second case, heat is transferred through the vertical sides of the cube. Three distinct flow patterns are identified depending on the rate of heat transfer and the Rayleigh number. For all runs in the range of Rayleigh numbers studied, the transition from the first to the second flow pattern occurs abruptly.