Laminar Profile Research Articles

LES and URANS study on turbulent flow through 3 × 3 rod bundle with spacer grid and mixing vanes using spectral element method

In this paper, the turbulent flow in rod bundles with spacer grid and mixing vanes in nuclear reactor core was studied using spectral element method with large eddy simulation (LES) and unsteady RANS (URANS) method. Combining with the radial periodical boundary conditions, the turbulent mixing phenomena in central subchannels were investigated, especially focusing on the flow field after the spacer grid and mixing vanes. Other than setting up long bare rod bundle to establish the fully developed turbulent flow, only a 2-cm-long bare rod bundle was extruded upstream the spacer grid and mixing vanes, while the fully developed laminar velocity profile was applied on the inlet surface for velocity boundary settings. The results indicate that both of the models could predict time-averaged velocity at 0.5Dh, 1Dh, 4Dh downstream mixing vanes properly, while the URANS model underestimate the root-mean-square (RMS) value of fluctuating velocity due to the inlet treatment effect. Furthermore, turbulent kinetic energy contour with transverse velocity vector was combined to investigate the behaviors of vortices formed by split-type mixing vanes, revealing the behavior of vortices for preliminary dynamic and mechanic analysis in design process of fuel assembly.

Laminar-turbulent transition in Taylor-Couette flow from a molecule dependent transport equation

In this Letter, a previously utilized higher-order momentum transport equation obtained using a projection-perturbation formalism from the most general equation of classical transport for the N-particle distribution function, the Liouville equation, is applied to the Taylor-Couette flow, e.g. fluid flow enclosed between two concentric cylinders where the inner cylinder is rotating with some constant speed and the outer cylinder is stationary. The results of the numerical simulation of azimuthal velocity profiles are analyzed to see how an initial laminar velocity profile evolves in time into a velocity profile characteristic for turbulent flow. Although the experiments were not very accurate, they all point to the near uniformity of the velocity profiles in turbulent regime in the central region of the annulus. Using the time evolution of the velocity profiles, we further investigate possibility of observing double phase transition from a laminar to turbulent regime.

Laminar fMRI using T2-prepared multi-echo FLASH

Functional magnetic resonance imaging (fMRI) using blood oxygenation level dependent (BOLD) contrast at a sub-millimeter scale is a promising technique to probe neural activity at the level of cortical layers. While gradient echo (GRE) BOLD sequences exhibit the highest sensitivity, their signal is confounded by unspecific extravascular (EV) and intravascular (IV) effects of large intracortical ascending veins and pial veins leading to a downstream blurring effect of local signal changes. In contrast, spin echo (SE) fMRI promises higher specificity towards signal changes near the microvascular compartment. However, the T2-weighted signal is typically sampled with a gradient echo readout imposing additional T2’-weighting.In this work, we used a T2-prepared (T2-prep) sequence with short GRE readouts to investigate its capability to acquire laminar fMRI data during a visual task in humans at 7 T. By varying the T2-prep echo time (TEprep) and acquiring multiple gradient echoes (TEGRE) per excitation, we studied the specificity of the sequence and the influence of possible confounding contributions to the shape of laminar fMRI profiles. By fitting and extrapolating the multi-echo GRE data to a TEGRE = 0 ms condition, we show for the first time laminar profiles free of T2’-pollution, confined to gray matter. This finding is independent of TEprep, except for the shortest one (31 ms) where hints of a remaining intravascular component can be seen. For TEGRE > 0 ms a prominent peak at the pial surface is observed that increases with longer TEGRE and dominates the shape of the profiles independent of the amount of T2-weighting. Simulations show that the peak at the pial surface is a result of static EV dephasing around pial vessels in CSF visible in GM due to partial voluming. Additionally, another, weaker, static dephasing effect is observed throughout all layers of the cortex, which is particularly obvious in the data with shortest T2-prep echo time. Our simulations show that this cannot be explained by intravascular dephasing but that it is likely caused by extravascular effects of the intracortical and pial veins. We conclude that even for TEGRE as short as 2.3 ms, the T2’-weighting added to the T2-weighting is enough to dramatically affect the laminar specificity of the BOLD signal change. However, the bulk of this corruption stems from CSF partial volume effects which can in principle be addressed by increasing the spatial resolution of the acquisition.

Physics of unsteady cylinder-induced shock-wave/transitional boundary-layer interactions

Given the current lack of experimental data for shock waves interacting with incoming transitional boundary layers, the goal of this study was to characterize the dynamics of such an interaction to better understand the fundamental fluid physics of these complex phenomena. Here, the mean flow field and time-dependent characteristics of a three-dimensional Mach 5 cylinder-induced shock-wave/boundary-layer interaction where the upstream boundary layer is transitional, have been studied experimentally. The interactions were generated with a right circular cylinder mounted on a flat plate. Streamwise–spanwise planar laser scattering from a condensed alcohol fog and schlieren imaging were used to characterize the mean and instantaneous structure of the interaction, and fast-response wall-pressure measurements on the centreline upstream of the cylinder enabled characterization of the unsteadiness. The pressure measurements show a mean pressure profile that resembles a composite of an upstream laminar profile and a downstream turbulent profile. The upstream influence location of the transitional interaction was approximately 8.5 diameters ( $d$ ) upstream of the cylinder leading edge, which is between that of a laminar and a turbulent interaction, and is followed by a plateau region to approximately $4d$ upstream of the cylinder. The plateau region is a region with a thicker boundary layer and possible flow separation. The plateau pressure was within 7 % of the value predicted by Hill's correlation for free-interaction phenomena. Furthermore, a statistical analysis of the pressure histories suggests that the entire interaction stretches and contracts in concert. Power spectral densities of the pressure fluctuations showed unsteadiness throughout the interaction with energy content primarily centred between a region defined by a separation-length-based Strouhal number $St_{L} = 0.05\text {--}0.2$ , comparing well with other related studies of cylinder-induced interactions. Cross-correlations and coherence functions in the interaction suggest that the unsteadiness in the laminar region may be due to the entire ‘laminar’ region oscillating in response to the ‘turbulent’ unsteadiness of the intermittent region.

In-capillary (electrophoretic) digestion-reduction-separation: A smart tool for middle-up analysis of mAb

Comprehensive characterization of physicochemical properties of monoclonal antibodies (mAbs) is a critical process to ensure their quality, efficacy, and safety. For this purpose, mAb analysis at different levels (bottom-up, middle-up) is a common approach that includes rather complex multistep sample preparation (reduction, digestion). To ensure high analysis performance, the development of fully integrated methodologies is highly valuable. Capillary zone electrophoresis is a particularly well-adapted technique for the multistep implementation of analytical strategies from sample preparation to detection. This feature was employed to develop novel integrated methodologies for the analysis of mAb at the middle-up level. Multiple in-line reactions (simultaneous reduction and digestion) were performed for the first time in the separation capillary. Tris (2-carboxyethyl) phosphine hydrochloride (TCEP) was used as an effective reducing agent under a broad pH range and IdeS (Immunoglobulin degrading enzyme from Streptococcus) as a highly specific enzyme for mAb digestion. Transverse diffusion of laminar flow profile (TDLFP) was applied for reactants mixing. Both in-line sample preparation and separation parameters were optimized under non-denaturing and denaturing conditions. The developed in-line methodologies provided good reproducibility and higher peak efficiencies comparing with off-line assays. They were successfully applied to different mAbs.

Screening carbonic anhydrase IX inhibitors in traditional Chinese medicine based on electrophoretically mediated microanalysis

An electrophoretically mediated microanalysis (EMMA) method for the screening of carbonic anhydrase IX inhibitors in traditional Chinese medicine (TCM) was developed. This method combines transverse diffusion of laminar flow profiles (TDLFP) and rapid polarity switching technology to achieve rapid mixing of different reactants. Different electromigration rates of different substances make it possible that incubation, separation and detection are carried out continuously in a same fused-silica capillary. In this experiment, p-nitrophenyl acetate (pNPA) was used as the substrate for the enzyme reaction, which solved the problem that capillary electrophoresis could not detect carbonate, carbon dioxide, etc., the conventional substrates of carbonic anhydrase IX. After optimizing the enzyme reaction and separation conditions, the separation of substrate and product can be finished by baseline within 4 min. The Michaelis constant of carbonic anhydrase IX was measured to be 1.2 mM. A known inhibitor acetazolamide was used to evaluate the feasibility of this method for screening carbonic anhydrase IX inhibitors, and the half-maximal inhibitory concentration (IC50) was calculated to be 1.26 μM. Finally, 4 natural compounds of 15 traditional Chinese medicine standards were discovered to exhibit enzyme inhibitory activity, including polydatin, matrine, dauricine and cepharanthine, proving that the EMMA method is an effective means for screening carbonic anhydrase IX inhibitors. The results were supported by molecular docking study.

Sub-millimetre resolution laminar fMRI using Arterial Spin Labelling in humans at 7 T.

Laminar fMRI at ultra-high magnetic field strength is typically carried out using the Blood Oxygenation Level-Dependent (BOLD) contrast. Despite its unrivalled sensitivity to detecting activation, the BOLD contrast is limited in its spatial specificity due to signals stemming from intra-cortical ascending and pial veins. Alternatively, regional changes in perfusion (i.e., cerebral blood flow through tissue) are colocalised to neuronal activation, which can be non-invasively measured using Arterial Spin Labelling (ASL) MRI. In addition, ASL provides a quantitative marker of neuronal activation in terms of perfusion signal, which is simultaneously acquired along with the BOLD signal. However, ASL for laminar imaging is challenging due to the lower SNR of the perfusion signal and higher RF power deposition i.e., specific absorption rate (SAR) of ASL sequences. In the present study, we present for the first time in humans, isotropic sub-millimetre spatial resolution functional perfusion images using Flow-sensitive Alternating Inversion Recovery (FAIR) ASL with a 3D-EPI readout at 7 T. We show that robust statistical activation maps can be obtained with perfusion-weighting in a single session. We observed the characteristic BOLD amplitude increase towards the superficial laminae, and, in apparent discrepancy, the relative perfusion profile shows a decrease of the amplitude and the absolute perfusion profile a much smaller increase towards the cortical surface. Considering the draining vein effect on the BOLD signal using model-based spatial “convolution”, we show that the empirically measured perfusion and BOLD profiles are, in fact, consistent with each other. This study demonstrates that laminar perfusion fMRI in humans is feasible at 7 T and that caution must be exercised when interpreting BOLD signal laminar profiles as direct representation of the cortical distribution of neuronal activity.

Experimental study on the advective heat flux of a heat exchanger for passive cooling of spent fuel pools by temperature anemometry grid sensor

In commercial nuclear power plants spent fuel assemblies are usually stored in actively cooled water pools. The continuous decay heat release represents a potential risk in case of a station black out scenario. Thus two-phase passive heat removal systems are a key technology to enhance the safety of nuclear power plants. Such systems work only by the energy provided from the heat source, e.g. by the maintenance of a natural convection cooling. A heat transfer loop using air as an unlimited heat sink consists of a primary heat exchanger in the spent fuel pool water and a secondary heat exchanger located in ambient air. Thus the measurement of the heat flux, which gets transferred from the pool to the ambient air, is an important task. If one would measure heat flux, flow rates and temperatures in many positions by help of local probes, the natural flow would get strongly disturbed. For that reason we introduce a heat flux measurement around the secondary heat exchanger located in ambient air, which applies temperature and velocity measurement by an anemometric principle.A 6.5m long flow channel with an electrical heated finned tube heat exchanger was set up at the TOPFLOW facility at HZDR. Since the tubes of a heat exchanger would be tilted in a passive heat removal system, i.e. to allow drainage of the condensed heat transfer medium, different tiled angles were adjusted to 0° (horizontal), 20°,30° and 40°. The frontal velocity was varied between 0.5ms and 4ms and three thermocouples were placed up- and downstream of the heat exchanger respectively. A Temperature Anemometry Grind Sensor (TAGS) was located downstream the heat exchanger. It consists of a wire grid with platinum resistance elements, which are placed in the small sub-channels of a flow straightener to generate laminar flow profiles. Two methods were used to calculate the heat flux: arithmetical average and weighting of the flow area. The results of velocity was compared with the average velocity measured by the volume flow control and out of the velocity and temperature the heat flux was calculated and compared with electrical supplied heat flux. The calculated average velocity measured by the TAGS corresponds well with the velocity measured by the volume flow controller up to approximately 3ms with a maximum deviation of ±5%, but underestimates the velocity measured by the volume flow controller at higher velocities. The heat flux was calculated by five methods, 1.) from the three thermocouples up- and downstream of the heat exchanger, 2.) from the average temperatures measured by the TAGS, 3.) from the weighted temperature measured by the TAGS, 4.) from the average temperature and velocity measured by the TAGS and 5.) from the weighted temperature and velocity measured by the TAGS. In this order the accuracy of methods increases compared to the electrical supplied heat flux. For the last method the maximum deviation was 6.5% for all tilt angles. This measurement concept determines the heat flux without disturbing the flow in the loop.

Structural Development and Geomorphic Relationship in Wadi Atyaruh, Al Jabal Al Akhdar, NE Libya

The present study has been carried out to analyze the relationship between faulting and geomorphology of the Wadi Atyaruh to reveal the effect of structures on the morphology and distribution of the different karst features. Geographical Information System (GIS) and Remote Sensing (RS) techniques were applied to investigate morphological and topographic characteristics of Wadi Atyaruh, based on ASTER Global Digital Elevation Model (ASTER GDEM) Version 3 (V3) data. Dextral strike-slip fault is the main faulting type in the valley. Conjugate faults system has been found in the study area, which reveals the orientations of the principal stresses. Wadi Atyaruh consists mainly of Dernah formations (Eocene) and Quaternary deposits. Two types of Karren have been recognized, they are Rillen karren and solution basis which are well distributed in Darnah formation. Caves are found in the Darnah formation, the passages and chambers of these caves show a phreatic bedding plane, elliptical shape, laminar profiles, and rectangular and rounded top vadose profiles. The drainage system of this valley is sub-parallel drainage pattern to dendritic drainage pattern, indicating that the area has affected by strike-slip movement (Dextral), joints and it has steep slopes. interpretation of DEM of study area indicate moderate and high relief, low run off and high infiltrations due to the nature of the fracture carbonate rock, the basin have early mature stage of erosion development. Geomorphic parameters such as hill shade, slop, aspect, area shaded and elevation maps, was produced to describe geomorphic forms and processes of the Wadi Atyaruh. The complete morphometric analysis of drainage basin indicates that the given area is having good groundwater prospect.

Direct Numerical Simulation of Turbulent Heat Transfer at Low Prandtl Numbers in Planar Impinging Jets

DNS simulations of planar impinging jets are performed to provide reference data to validate RANS models for low-Prandtl turbulent heat transfer. In that configuration, a hot plane jet is blown from a slot at the top wall of a channel and impinges on the cooler bottom wall. The present DNS simulations are carried out at Re=4000 and 5700, based on the jet width and velocity, and for Pr down to 0.01. Two cases are investigated: the case of a laminar uniform jet profile and the case of a fully turbulent inlet profile coming from an auxiliary channel flow simulation running in parallel. The DNS results show how the heat transfer in this configuration evolves from the turbulence-dominated case at Pr=1 to the molecular diffusion-dominate case at Pr=0.01. The temperature field at Pr=0.01 is much smoother than at higher Prandtl number because the much larger heat diffusivity quickly diffuses the temperature fluctuations. The comparison between the laminar and the fully-developed turbulent inflows also show different heat transfer characteristics in the vicinity of the stagnation point, depending on the presence of velocity fluctuations in the bottom-wall boundary layers.

Effect of hydrogen addition on the combustion characteristics of premixed biogas/hydrogen-air mixtures

The current work presents computational investigations of biogas-hydrogen blends' combustion characteristics for various thermo-physical conditions relevant for practical combustor applications. The numerical simulations were performed using 0-D SENKIN and 1-D PREMIX codes, whereas chemical kinetics is modelled using the GRI Mech 3.0 mechanism. Biogas composition represented by 60CH4/40CO2% (v/v) is blended with different H2 concentration varying from 0 to 50% (v/v) in the fuel. The effect of initial temperature and pressure on ignition delay, laminar flame speed (Su), flame temperature (Tad), species concentrations (Ci), and heat release rate profiles is investigated. The results showed that GRI-Mech 3.0 predictions are in good agreement with the available experimental ignition delay data for CH4/CO2-air mixture for stoichiometric ambient condition (P = 0.1 MPa and φ = 1). For the high-temperature oxidation, GRI-Mech 3.0 has accurately predicted Su available from literature for pure biogas (RH = 0.0) under a wide range of equivalence ratios (φ = 0.7–1.4) for ambient conditions. The computational analysis was then extended for RH ranging from 0.0 to 0.5 over a wide range of equivalence ratio (φ) = 0.7–1.4, initial temperatures (T) = 300–600 K and initial pressures (P) = 0.1–7.0 MPa. These results obtained were used to develop a new analytical correlation for the laminar flame speed (Su = Suo(φ,RH)TTuα(φ)PPuβ(φ,RH)) in terms of operating conditions. The results show that hydrogen addition to the biogas improves both the flame speed and ignition delay. The extent of reduction in flame speed due to elevation of initial pressure is a linear function of hydrogen addition to biogas. Furthermore, the sensitivity analysis was studied to assess the influence of hydrogen added to the biogas using laminar flame speed sensitivity coefficient (σ). The understanding of these mixtures’ combustion characteristics at given initial conditions leading to feasibility conformity of optimal blends of biogas/hydrogen mixtures for the design improvement of practical combustors is discussed.

Laminar Profile of Auditory Steady-State Response in the Auditory Cortex of Awake Mice.

ObjectiveAuditory steady-state response (ASSR) is a gamma oscillation evoked by periodic auditory stimuli, which is commonly used in clinical electroencephalographic examination to evaluate the neurological functions. Though it has been suggested that auditory cortex is the origin of ASSR, how the laminar architecture of the neocortex contributes to the ASSR recorded from the brain surface remains unclear.MethodsWe used a 16-channel silicon probe to record the local field potential and the single-unit spike activity in the different layers of the auditory cortex of unanesthetized mice. Click-trains with a repetition rate at 40-Hz were present as sound stimuli to evoke ASSR.ResultsWe found that the LFPs of all cortical layers showed a stable ASSR synchronizing to the 40-Hz click stimuli, while the ASSR was strongest in the granular (thalamorecipient) layer. Furthermore, time-frequency analyses also revealed the strongest coherence between the signals recorded from the granular layer and pial surface.ConclusionOur results reveal that the 40-Hz ASSR primarily shows the evoked gamma oscillation of thalamorecipient layers in the neocortex, and that the ASSR may be a biomarker to detect the cognitive deficits associated with impaired thalamo-cortical connection.

Fast in-line bottom-up analysis of monoclonal antibodies: Toward an electrophoretic fingerprinting approach.

For their characterization and quality control, monoclonal antibodies are frequently analyzed at the bottom-up level to generate specific fingerprints that can be used to tackle post-translational modifications or ensure production consistency between lots. To circumvent time-consuming and labor-intensive off-line sample preparation steps, the implementation of integrated methodologies from sample preparation to separation and detection is highly valuable. In this perspective, capillary zone electrophoresis appears as a choice technique since the capillary can subsequently be used as a vessel for sample preparation and electrophoretic discrimination/detection of the reaction products. Here, a fast in-line methodology for the routine quality control of mAbs at the bottom-up level is reported. Simultaneous denaturation and reduction (pretreatment step) were conducted with RapiGest® surfactant and dithiothreitol before in-line tryptic digestion. Reactant mixing was realized by transverse diffusion of laminar flow profile under controlled temperature. In-line digestion was carried out with a resistant trypsin to autolysis. The main parameters affecting the digestion efficiency (trypsin concentration and incubation conditions) were optimized to generate mAb electrophoretic profiles free from trypsin interferences. An acidic MS-compatible BGE was used to obtain high resolution separation of released peptides and in-line surfactant cleavage. The whole methodology was performed in less than two hours with good repeatability of migration times (RSD = 0.91%, n = 5) and corrected peak areas (RSD = 9.6%, n = 5). CE-fingerprints were successfully established for different mAbs and an antibody-drug conjugate.

An analytic probability density function for partially premixed flames with detailed chemistry

Laminar premixed flame profiles of methane/air free flames and strained flames at different fuel/air ratios and strain rates are analyzed using detailed chemistry with Lewis numbers equal to one. It is shown that the detailed chemistry flame profiles of progress variables CO2 + CO and H2O + H2 in canonically stretched coordinates can be fitted accurately by a slight generalization of recently proposed analytical presumed flame profiles over a wide range of fuel/air ratios through adaptation of a single model parameter. Strained flame profiles can be reproduced using an additional linear coordinate transformation, emulating the compression of the preheat zone by strain as predicted by premixed flame theory. The model parameter can alternatively be determined using only the laminar flame speeds and the fully burnt temperatures from the laminar flame calculations. The stretch factor of the coordinate transformation is proportional to cp/lambda, which drops by a factor up to 4 across the laminar flame. It is shown how the non-constant cp/lambda modifies the laminar flame probability density function (pdf) and a polynomial fit to cp/lambda as a function of the progress variable allows analytical results for the laminar flame pdf and the mean value of the progress variable and of the reaction source term. An analytic pdf for partially premixed flames is proposed based on Bayes's theorem as a combination of a beta pdf for the mixture fraction and the laminar flame pdf's evaluated at the respective fuel/air ratio.

Estimation of laminar BOLD activation profiles using deconvolution with a physiological point spread function

BackgroundThe specificity of gradient echo (GE)-BOLD laminar fMRI activation profiles is degraded by intracortical veins that drain blood from lower to upper cortical layers, propagating activation signal in the same direction. This work describes an approach to obtain layer specific profiles by deconvolving the measured profiles with a physiological Point Spread Function (PSF). New methodIt is shown that the PSF can be characterised by a TE-dependent peak to tail (p2t) value that is independent of cortical depth and can be estimated by simulation. An experimental estimation of individual p2t values and the sensitivity of the deconvolved profiles to variations in p2t is obtained using laminar data measured with a multi-echo 3D-FLASH sequence. These profiles are echo time dependent, but the underlying neuronal response is the same, allowing a data-based estimation of the PSF. ResultsThe deconvolved profiles are highly similar to the gold-standard obtained from extremely high resolution 3D-EPI data, for a range of p2t values of 5–9, which covers both the empirically determined value (6.8) and the value obtained by simulation (6.3). -Comparison with Existing Method(s) Corrected profiles show a flatter shape across the cortex and a high level of similarity with the gold-standard, defined as a subset of profiles that are unaffected by intracortical veins. ConclusionsWe conclude that deconvolution is a robust approach for removing the effect of signal propagation through intracortical veins. This makes it possible to obtain profiles with high laminar specificity while benefitting from the higher efficiency of GE-BOLD sequences.

In vitro performance of echoPIV for assessment of laminar flow profiles in a carotid artery stent.

.Purpose: Detailed blood flow studies may contribute to improvements in carotid artery stenting. High-frame-rate contrast-enhanced ultrasound followed by particle image velocimetry (PIV), also called echoPIV, is a technique to study blood flow patterns in detail. The performance of echoPIV in presence of a stent has not yet been studied extensively. We compared the performance of echoPIV in stented and nonstented regions in an in vitro flow setup.Approach: A carotid artery stent was deployed in a vessel-mimicking phantom. High-frame-rate contrast-enhanced ultrasound images were acquired with various settings. Signal intensities of the contrast agent, velocity values, and flow profiles were calculated.Results: The results showed decreased signal intensities and correlation coefficients inside the stent, however, PIV analysis in the stent still resulted in plausible flow vectors.Conclusions: Velocity values and laminar flow profiles can be measured in vitro in stented arteries using echoPIV.

A near-exact analytic solution of progress variable and pdf for single-step Arrhenius chemistry

A new chemical reaction rate source term in the transport equation of a single progress variable is proposed and implications for premixed flame modelling are discussed. This surrogate source term approximates the Arrhenius one over a large range of activation energies and density ratios almost perfectly. An analytic, invertible flame profile and the corresponding 1D laminar flame pdf are derived. The laminar flame Eigenvalue is evaluated analytically for unity and non-unity Lewis number and compared to classical results. A method to evaluate the progress variable pdf from DNS data is introduced. It is based on regularized delta functions and a transformation of the DNS progress variable field with the inverse of the 1D laminar flame profile, which behaves like a signed distance function. The pdf factorizes into the flat flame pdf, a geometrical wrinkling factor and a correction factor accounting for changes in the inner flame structure. The pdfs of RANS-like filter volumes and their ingredients are evaluated from DNS datasets of statistically flat turbulent flames featuring different turbulence intensity levels. For all DNS cases the wrinkling and correction factors vary little with progress variable. The wrinkling factor rises with turbulence intensity. The pdf is similar to that of a 1D laminar flame pdf, scaled by the wrinkling factor. Opportunities for improvements to premixed turbulent flame modelling are discussed.

A Mathematical Model for the Laminar Entrance Region of a Newtonian Fluid in a Cylindrical Pipe

AbstractThis paper develops equations for velocity, pressure drop, and wall shear stress in the entrance or development region of a cylindrical pipe. The model quantifies the velocity and wall shear stress contributions to the entrance region pressure drop and illustrates how data are used to determine the numerical values of parameters needed to complete the model. It assumes a Newtonian fluid, laminar flow, steady-state, and a constant mass density fluid. The fluid axial velocity profile at the entrance region inlet is modeled by an equation that is close to a flat axial velocity and drops off to zero as the radius approaches the wall. The fluid velocity at the entrance region exit is modeled as the axial, fully developed, laminar flow parabolic velocity profile. The inlet velocity profile is multiplied by a decaying function F(x) that is unity at the entrance region inlet and decreases to zero at the entrance region exit. The exit velocity profile is multiplied by a growing function G(x) that is zero at the entrance region inlet and increases to unity at the entrance region exit. The pressure drop through the entrance region is expressed in terms of the wall viscous friction and the change in axial momentum of the fluid. Two mathematical models for F(x) and G(x) are presented. One is more advantageous when pressure drop data and a few centerline velocity data points are available, and the second is more advantageous when only velocity data are available.

Effects of phase regression on high-resolution functional MRI of the primary visual cortex

High-resolution functional MRI studies have become a powerful tool to non-invasively probe the sub-millimeter functional organization of the human cortex. Advances in MR hardware, imaging techniques and sophisticated post-processing methods have allowed high resolution fMRI to be used in both the clinical and academic neurosciences. However, consensus within the community regarding the use of gradient echo (GE) or spin echo (SE) based acquisition remains largely divided. On one hand, GE provides a high temporal signal-to-noise ratio (tSNR) technique sensitive to both the macro- and micro-vascular signal while SE based methods are more specific to microvasculature but suffer from lower tSNR and specific absorption rate limitations, especially at high field and with short repetition times.Fortunately, the phase of the GE-EPI signal is sensitive to vessel size and this provides a potential avenue to reduce the macrovascular weighting of the signal (phase regression, Menon 2002). In order to determine the efficacy of this technique at high-resolution, phase regression was applied to GE-EPI timeseries and compared to SE-EPI to determine if GE-EPI's specificity to the microvascular compartment improved. To do this, functional data was collected from seven subjects on a neuro-optimized 7 T system at 800 μm isotropic resolution with both GE-EPI and SE-EPI while observing an 8 Hz contrast reversing checkerboard. Phase data from the GE-EPI was used to create a microvasculature-weighted time series (GE-EPI-PR). Anatomical imaging (MP2RAGE) was also collected to allow for surface segmentation so that the functional results could be projected onto a surface. A multi-echo gradient echo sequence was collected and used to identify venous vasculature.The GE-EPI-PR surface activation maps showed a high qualitative similarity with SE-EPI and also produced laminar activity profiles similar to SE-EPI. When the GE-EPI and GE-EPI-PR distributions were compared to SE-EPI it was shown that GE-EPI-PR had similar distribution characteristics to SE-EPI (p < 0.05) across the top 60% of cortex. Furthermore, it was shown that GE-EPI-PR has a higher contrast-to-noise ratio (0.5 ± 0.2, mean ± std. dev. across layers) than SE-EPI (0.27 ± 0.07) demonstrating the technique has higher sensitivity than SE-EPI. Taken together this evidence suggests phase regression is a useful method in low SNR studies such as high-resolution fMRI.

Generation of Excess Noise by Jets with Highly Disturbed Laminar Boundary-Layer Profiles

The generation of noise by jets with highly disturbed laminar boundary-layer profiles at the nozzle exit, also referred to as initially nominally laminar jets in the literature, is investigated using large-eddy simulation and linear stability analysis. Four jets at a Mach number of 0.9 and a Reynolds number of , one with a nonlaminar boundary-layer profile and three others with laminar profiles, are considered for exit peak turbulence intensities equal to 6% in the nonlaminar case and to 9% in the laminar ones. The jets with laminar boundary-layer profiles all radiate greater sound pressure levels than the jet with a nonlaminar profile but weaker initial disturbances. This particularly appears at high frequencies for the jets with a thinner boundary layer compared with the nonlaminar case. These results are shown to be related to the dependence on the shape of the boundary-layer profile of the most unstable frequencies downstream of the nozzle. For a laminar profile, these frequencies are similar to those obtained downstream in the mixing-layer profiles, whereas they are higher for a nonlaminar profile. Despite larger nozzle-exit flow disturbances, this leads to longer-term persistence of coherent large-scale structures in the shear layers, hence stronger velocity fluctuations and noise levels, for the present initially nominally laminar jets than for the other one.