Time-averaged Pressure Research Articles

Effects of Variations of Flow and Heart Rate on Intra-Aneurysmal Hemodynamics in a Ruptured Internal Carotid Artery Aneurysm During Exercise.

Background:Hemodynamics is thought to play an important role in the mechanisms responsible for initiation, growth, and rupture of intracranial aneurysms. Computational fluid dynamic (CFD) analysis is used to assess intra-aneurysmal hemodynamics.Objectives:This study aimed to investigate the effects of variations in heart rate and internal carotid artery (ICA) flow rate on intra-aneurysmal hemodynamics, in an ICA aneurysm, by using computational fluid dynamics.Patients and Methods:Computed tomography angiography (CTA) was performed in a 55 years old female case, with a saccular ICA aneurysm, to create a patient-specific geometrical anatomic model of the aneurysm. The intra-aneurysmal hemodynamic environments for three states with different flow and heart rates were analyzed using patient-specific image-based CFD modeling.Results:Results showed significant changes for the three simulated states. For a proportion of the states examined, results were counterintuitive. Systolic and time-averaged wall shear stress and pressure on the aneurysm wall showed a proportional evolution with the mainstream flow rate.Conclusion:Results reinforced the pivotal role of vascular geometry, with respect to hemodynamics, together with the importance of performing patient-specific CFD analyses, through which the effect of different blood flow conditions on the aneurysm hemodynamics could be evaluated.

Flow Control by Repetitive Discharge for Space Vehicle at High Altitude

As a new method for controlling aerodynamic characteristics of space vehicles at hypersonic speeds, repetitive pulse discharge is generated at the nose part of a hemisphere model and the flow control effect was investigated both with hypersonic wind tunnel experiments and numerical simulation based on 2-D axisymmetric Navier-Stokes equations. It was revealed that 10-kHz repetitive pulse discharge would cause bow shock oscillation, which was observed in high-speed schlieren pictures. Similar shock oscillation was also observed in the numerical simulation results where the discharge effect was replaced with repetitive temperature increase at electrodes. The averaged force acting on the hemisphere model was evaluated and it was shown that the time-averaged surface pressure is slightly less than that in case of no pulse discharge except at the surface of electrodes. The results suggest that pulse discharge generated at the stagnation area can modify the pressure distribution over a space vehicle.

Characterization and reduction of flow separation in jet pumps for laminar oscillatory flows.

A computational fluid dynamics model is used to predict the oscillatory flow through tapered cylindrical tube sections (jet pumps). The asymmetric shape of jet pumps results in a time-averaged pressure drop that can be used to suppress Gedeon streaming in closed-loop thermoacoustic devices. However, previous work has shown that flow separation in the diverging flow direction counteracts the time-averaged pressure drop. In this work, the characteristics of flow separation in jet pumps are identified and coupled with the observed jet pump performance. Furthermore, it is shown that the onset of flow separation can be shifted to larger displacement amplitudes by designs that have a smoother transition between the small opening and the tapered surface of the jet pump. These design alterations also reduce the duration of separated flow, resulting in more effective and robust jet pumps. To make the proposed jet pump designs more compact without reducing their performance, the minimum big opening radius that can be implemented before the local minor losses have an influence on the jet pump performance is investigated. To validate the numerical results, they are compared with experimental results for one of the proposed jet pump designs.

Effects of heterogeneous surface of gas diffusion layers on droplet transport in microchannels of PEM fuel cells

Effects of heterogeneous surface of commercial gas diffusion layers (GDLs) which is made from carbon papers on a droplet transport in microchannels of proton exchange membrane fuel cells are investigated using the volume of fluid method. A random function is used for generating the heterogeneous surface of GDL in terms of roughness density and roughness height, which is derived from experimental data of commercial carbon paper GDLs. Surface tension, and wall adhesion forces are applied by using the volume of fluid method. The hydrophilic walls and hydrophobic GDL are considered. Results show that the heterogeneous surface of GDLs greatly affects droplet transport in the mircochannels. The movement mechanism of large droplet is translational while rolling mechanism is dominant for small droplet movement. Pressure drop and pressure fluctuation of large droplet are reduced after the formation of film flow. Generally, time-averaged pressure drop, droplet elongation, and the amount of remaining water on the GDL increase by increasing roughness density and roughness height, which is undesirable for better water management. It is also observed that the amount of adhered droplet to the hydrophilic surface of microchannel increases by increasing the roughness height and airflow velocity, which is favorable for better water removal. Average velocity of the droplet decreases by increasing roughness parameters for low airflow velocities while it increases for high airflow velocities. The detachment velocity of droplets on the rough GDL is compared with analytical solution for smooth GDLs.

Numerical and experimental research on the outlet flow field for the air multiplier

The outlet flow field of the air multiplier was simulated by a RNG k–ε model based on compressible fluid, and the simulated results were validated experimentally through a Constant Temperature Anemometer (CTA) hot-wire system and the ventilating multi-parameter testing instrument. The flow characteristics and the reason for the increase of the output flow rate relative to the turbine intake rate were revealed by investigating profiles of the time-averaged velocity and pressure. The results indicate that (1) the flow parameter profiles in the outlet flow field are not uniform, and the entire time-averaged velocity field and pressure field in near field are basically axis symmetrical in the horizontal direction instead of in the vertical direction; (2) the entrainment and suction effects resulted from the “coronary zone” of the established flow of high intense turbulence jet, enabling the air multiplier to realize “air multiplication”, and the time-averaged velocity of the upper “coronary zone” and pressures in near field make greater contributions to the increase of flow rate. (3) RNG k–ε model is reliable on predicting the time-averaged flow behavior in the outlet flow field for the air multiplier.

Jet pumps for thermoacoustic applications: Design guidelines based on a numerical parameter study

The oscillatory flow through tapered cylindrical tube sections (jet pumps) is characterized by a numerical parameter study. The shape of a jet pump results in asymmetric hydrodynamic end effects which cause a time-averaged pressure drop to occur under oscillatory flow conditions. Hence, jet pumps are used as streaming suppressors in closed-loop thermoacoustic devices. A two-dimensional axisymmetric computational fluid dynamics model is used to calculate the performance of a large number of conical jet pump geometries in terms of time-averaged pressure drop and acoustic power dissipation. The investigated geometrical parameters include the jet pump length, taper angle, waist diameter, and waist curvature. In correspondence with previous work, four flow regimes are observed which characterize the jet pump performance and dimensionless parameters are introduced to scale the performance of the various jet pump geometries. The simulation results are compared to an existing quasi-steady theory and it is shown that this theory is only applicable in a small operation region. Based on the scaling parameters, an optimum operation region is defined and design guidelines are proposed which can be directly used for future jet pump design.

Wind-driven natural ventilation for buildings with two openings on the same external wall

This study uses wind tunnel experiments to investigate the wind-driven ventilation for buildings with two openings on a single wall. The exchange rates are measured by the tracer gas decay method under different external wind speeds, directions and opening sizes. The experimental results indicate that the time-averaged pressure difference across the openings is much larger than the fluctuating pressure when the wind direction is θ=22.5–45°, and the ventilation rate can be predicted by the orifice equation. When the wind direction is θ=0° and 67.5–180°, the pressure difference across the openings is insignificant and the fluctuating pressure entrains air across the openings. The exchange rate is proportional to the root-mean-square of the pressure fluctuation. Furthermore, the dimensionless exchange rate of the shear-induced ventilation (wind is parallel to the openings) is found to be independent of the wind speed, opening area and location. The exchange rate for buildings with an internal partition is lower than that without a partition when the wind direction is θ=0–90°. A semi-empirical prediction model is proposed to calculate the exchange rate by incorporating the time-averaged pressure difference and pressure fluctuation. The application of the prediction model to compute the air change rate (ACH) is also discussed.

On the Unsteady Behavior of the Flow around NACA 0012 Airfoil with Steady External Conditions at Re=1000

Even for the stationary airfoils, due to the boundary and shear layer interactions of upper and lower surface of the airfoils, alternating vortex patterns form and the flow becomes time dependent. In the current study, the unsteady behavior of the flow around a symmetric airfoil is considered as incidence angle increases. The flow patterns are presented for wide range of angles of attack values. The vortex pattern generated is analyzed numerically for different angles of attack at Re=1000 around NACA 0012 airfoil. At this Reynolds number, the flow is laminar and boundary layers are quite thick. Flow separation and unsteady vortex shedding is observed even at low angles of attack. For NACA 0012 airfoil, the unsteady vortex pattern is observed at about 8° angle of attack for Re=1000. Spectral analysis is performed for angles of attack ranging from 0° to 90°. It is presented that amplitude spectrum of lift coefficient (C1) start to shows a peak at 8° for NACA 0012 and the aerodynamic forces presents oscillatory behavior afterwards. The effect of angle of attack to wake pattern and instantaneous and mean aerodynamic coefficients are discussed. The time-averaged streamlines, pressure and skin friction coefficients are analyzed to observe the vortex formation and separation from the airfoil upper surface at this low Reynolds number.

A non-invasive assessment of cardiopulmonary hemodynamics with MRI in pulmonary hypertension

PurposeWe propose a method for non-invasive quantification of hemodynamic changes in the pulmonary arteries resulting from pulmonary hypertension (PH). MethodsUsing a two-element Windkessel model, and input parameters derived from standard MRI evaluation of flow, cardiac function and valvular motion, we derive: pulmonary artery compliance (C), mean pulmonary artery pressure (mPAP), pulmonary vascular resistance (PVR), pulmonary capillary wedge pressure (PCWP), time-averaged intra-pulmonary pressure waveforms and pulmonary artery pressures (systolic (sPAP) and diastolic (dPAP)). MRI results were compared directly to reference standard values from right heart catheterization (RHC) obtained in a series of patients with suspected pulmonary hypertension (PH). ResultsIn 7 patients with suspected PH undergoing RHC, MRI and echocardiography, there was no statistically significant difference (p<0.05) between parameters measured by MRI and RHC. Using standard clinical cutoffs to define PH (mPAP>25mmHg), MRI was able to correctly identify all patients as having pulmonary hypertension, and to correctly distinguish between pulmonary arterial (mPAP>25mmHg, PCWP<15mmHg) and venous hypertension (mPAP>25mmHg, PCWP>15mmHg) in 5 of 7 cases. ConclusionsWe have developed a mathematical model capable of quantifying physiological parameters that reflect the severity of PH.

Launch Vehicle Base Flow Analysis Using Improved Delayed Detached-Eddy Simulation

During the ascent of carrier rockets, the massively separated flow in the base region generates highly unsteady wall pressure fluctuations. The resulting dynamic loads can induce a response of the launcher structure called buffeting. To study the characteristics of the base flow behind three different configurations of the Ariane 5 launcher, improved delayed detached-eddy simulation in combination with unstructured grids is used. The objective of this work is to demonstrate the ability of improved delayed detached-eddy simulation to describe the impact of the connecting struts between central stage and boosters and to analyze the efficiency of a buffeting reduction device in the base region. Comparisons of the numerical results with time-resolved and time-averaged surface pressure data and particle-image-velocimetry measurements obtained from the German–Dutch High Speed Wind Tunnel are carried out in good agreement.

Long-term renal outcomes in a cohort of 1814 Chinese patients with biopsy-proven lupus nephritis.

In the present study, we observed the renal outcomes in a cohort of 1814 Chinese patients with biopsy-proven lupus nephritis (LN) and evaluated the risk factors associated with poor renal prognosis. The 5 -, 10 -, 15 - and 20-year renal survival rates were 93.1%, 87.9%, 81.0% and 68.3%, respectively. Gender, LN duration, mean arterial pressure (MAP), proteinuria, serum creatinine, haemoglobin and pathological classification at the time of biopsy were independent risk factors for end-stage renal disease (ESRD). The long-term renal outcomes of patients with class II LN were unfavorable as opposed to those with class V. Additionally, the time-average proteinuria (TA-Pro) and the time-average mean arterial pressure (TA-MAP) during the follow-up were important risk factors for ESRD, with better predictive values than the baseline proteinuria and MAP. The results underscore the need for proteinuria and blood pressure control during follow-up in patients with LN; proteinuria levels should be controlled at least to < 1.0 g/24 h, and optimally to < 0.5 g/24 h; MAP should not exceed 96.5 mmHg. More attention should be paid to class II LN and emphasis should be placed on recurrence prevention of class II LN.

고 받음각 ONERA 70도 삼각날개 와류 유동의 압력 섭동 분석

Delayed Detached-Eddy Simulation was conducted to investigate surface pressure coefficient distribution and surface pressure fluctuation over an ONERA 70-degree delta wing at a high angle of attack. Time-averaged surface pressure distribution is directly affected by the primary vortices, whereas the pressure fluctuation is influenced by the unsteady fluctuating boundary layer over the surface. And pressure coefficient, velocity, pressure fluctuation, and turbulent kinetic energy were analyzed along the vortex core in order to investigate the process of vortex breakdown. Consequently, strong pressure fluctuations were found where the vortex breakdown was occurred at x~620 mm. The turbulent kinetic energy abruptly increased and followed after the vortex breakdown.

Validation of filtered two-fluid models for gas–particle flows against experimental data from bubbling fluidized bed

Predictions of simulations based on filtered Two-Fluid Models (TFMs) with constitutive relations for filtered fluid–particle drag coefficient and filtered stresses proposed by Igci and Sundaresan [Ind. Eng. Chem. Res. 50 (2011) 13190–13201] and Milioli et al. [AIChE J. 59 (2013) 3265–3275] were compared against experimental data from a bubbling fluidized bed challenge problem put forward by the National Energy Technology Laboratory and Particulate Solids Research Inc. It is found that the most important correction to filtered models is a modification to the drag, and filtered stresses play a secondary role at best. As expected, coarse grid simulations using the kinetic-theory based TFM over-predicted the gas–particle drag force, yielding unphysical bed expansion. The filtered fluid–particle drag model proposed by Igci and Sundaresan that classifies the inhomogeneity in sub-filter scale flow structures using filter size and filtered particle volume fraction as markers also predicted unphysical bed expansion. Refined filtered drag models proposed by Milioli et al. based on filtered fluid–particle slip velocity as an additional marker led to good agreement with experimental data on bed expansion and the time-averaged gas pressure gradient. It was also observed that inadequate grid resolution in the region between gas distributor and the adjacent cylindrical wall of the test unit could lead to spurious asymmetric gas–particle flow predictions. With the inclusion of adequate inflation layer elements in that region, flow predictions became nearly symmetric with little to no effect on bed expansion predictions. However, it dramatically and qualitatively altered the details of gas–particle structures in the bed.

Experimental and numerical investigation on steady blowing flow control within a compact inlet duct

A combined experimental and numerical investigation of flow control actuation in a short, rectangular, diffusing S-shape inlet duct using a two-dimensional tangential control jet was conducted. Experimental and numerical techniques were used in conjunction as complementary techniques, which are utilized to better understand the complex flow field. The compact inlet had a length-to-hydraulic diameter ratio of 1.5 and was investigated at a free-stream Mach number of 0.44. In contrast to the baseline flow, where the flow field was fully separated, the two-dimensional control jet was able to eliminate flow separation at the mid-span portion of the duct and changed considerably the three-dimensional flow field, and ultimately, the inlet performance. A comparison between the baseline (no actuation) and forced flow fields showed that secondary flow structures dominated both flow fields, which is inevitably associated with total pressure loss. Contrary to the baseline case, the secondary flow structures in the forced case were established from the core flow stagnating on the lower surface of the duct close to the aerodynamic interface plane. High fidelity spectral analysis of the experimental results at the inlet’s exit plane showed that the baseline flow field was dominated by pressure fluctuations corresponding to a Strouhal number based on hydraulic diameter of 0.26. Not only did the two-dimensional tangential control jet improve the time-averaged pressure recovery at the inlet exit plane (13.3% at the lower half of the aerodynamic interface plane), it essentially eliminated the energy content of the distinct unsteady fluctuations which characterized the baseline flow field. This result has several implications for the design of a realistic engine inlet; furthermore, it depicts that a single non-intrusive static pressure measurement at the surface of the duct can detect flow separation.

The Reynolds number dependency of the steady and unsteady loading on a slightly rough circular cylinder: From subcritical up to high transcritical flow state

Simultaneous measurements of the spanwise-integrated unsteady aerodynamic forces and time-averaged local surface pressures on a 2D slightly rough circular cylinder were carried out over a wide range of Reynolds numbers in the high-pressure wind tunnel in Göttingen. The Reynolds numbers of 15×103≤Re≤12×106 spanned the known flow state regimes from subcritical up to high transcritical. The surface of the cylinder had a mean relative roughness of ks/D=1.2×10−3. The results demonstrated that especially in the critical flow regime the spanwise flow was strongly three-dimensional. In this regime two discontinuities at two closely spaced Reynolds numbers were observed, coupled with critical fluctuations in the lift force and the formation of a separation bubble at each side of the cylinder. The narrowing of the wake led to a sharp decrease of the drag coefficient and a steep increase of the values for the minimum pressure coefficients and the base pressure coefficient. In the upper transition state the boundary layer separation wandered upstream, resulting in an increase of the drag coefficient and a decrease of the base pressure coefficient and the Strouhal number. All measured flow field properties reached intermediate steady plateaus in the transcritical state for Re≥1.3×106. It was also shown that the surface roughness had a strong effect on the cylinder flow; in particular the disappearance of the supercritical regime and the subsequent strong recovery of the drag coefficient in the upper transition.

Effect of rolling motion on two-phase frictional pressure drop of boiling flows in a rectangular narrow channel

In order to investigate the two-phase frictional pressure drop characteristics of boiling flow in a rectangular narrow channel under rolling motion, a series of experiments and theoretical analysis are performed. The results demonstrate that the total additional pressure drop fluctuation has the same period with the rolling motion, and the fluctuation amplitude increases with the increase of rolling amplitude and rolling frequency. The time average additional pressure drop is 2–3 orders of magnitude smaller than that of frictional pressure drop in the boiling region. The fluctuation amplitude of the two-phase frictional pressure drop increases with increasing rolling amplitude, rolling period and heat flux, while it decreases with the increase of system pressure. Compared with the additional pressure drop in two-phase regions the outlet quality of channel and the space variation of the experimental loop are the main reasons that induce the fluctuation of two-phase frictional pressure drop. The mass flux fluctuation varies with the fluctuation of two-phase frictional pressure drop, and the fluctuation amplitude of mass flux increases with the increase of rolling amplitude and rolling period. The phase lag between the fluctuation of mass flux and frictional pressure drop is approximately equal to 1/4 rolling period.

Single drop breakup experiments in stirred liquid–liquid tank

The breakup process has been investigated by single drop experiments in a stirred liquid–liquid tank. The time taken to deform, stretch and produce two or more fragments was measured. For each of the observed breakup events, the number of drops in the resulting population was registered. The occurrence of equal and unequal breakup events was also noticed. Four oil-in-water systems were studied: toluene, petroleum, n-dodecane, and octanol. The mean energy dissipation rate of the tank was 1.14m2/s3 and the diameter of the observed mother drops that breakup were mainly in the range of 1–4mm. Multiple breakup events were the most frequent outcome in the experiments. It was observed that the number of daughter drops produced in a breakup was dependent on the mother drop size. The breakup time increased with the mother drop size and was in the approximate range of 10–100ms. A large uncertainty in the measurements is associated with the determination of the final breakup time, particularly in the cases of high order breakup events. Most breakup events took place in the large velocity field close to the radial impeller. It is not evident whether the breakup events were due to the time average shear or pressure and velocity fluctuations.

Performance comparison of jet pumps with rectangular and circular tapered channels for a loop-structured traveling-wave thermoacoustic engine

Abstract Gedeon streaming can considerably deteriorate the thermal efficiency of a traveling-wave thermoacoustic engine employing a loop configuration. Introducing a jet pump into the loop configuration is one of the effective ways to suppress Gedeon streaming. This paper focuses on the performance comparison between the jet pumps with rectangular and circular tapered channels, by numerically simulating and analyzing the time-averaged pressure drop induced by the jet pumps. Three parameters, i.e., the coefficient of time-averaged resistance, the coefficient of overall resistance, and the coefficient of effectiveness, are proposed to evaluate the performance of a jet pump. The emphasis is put on the effects of cross-sectional shape, thickness-to-diameter ratio, and rounding radius of the tapered channels. In absence of rounding the edge of the tapered channel, the jet pump with a rectangular tapered channel is more efficient to produce the time-averaged pressure drop at a large thickness-to-diameter ratio, while the jet pump with a circular tapered channel is more efficient at a small thickness-to-diameter ratio. However, it should be noted that the directions of the induced time-averaged pressure drops in these two cases are opposite. Rounding the edge of the small opening of the tapered channel can effectively improve jet pump’s performance. Upon rounding, the jet pump with a circular tapered channel can produce a higher time-averaged pressure drop and work more efficiently than the one with a rectangular tapered channel.

Performance of subgrid-scale models in coarse large eddy simulations of a laminar separation bubble

The flow over many blades and airfoils at moderate angles of attack and Reynolds numbers ranging from 104 to 105 undergoes separation due to the adverse pressure gradient generated by surface curvature. In many cases, the separated shear layer then transitions to turbulence and reattaches, closing off a recirculation region—the laminar separation bubble. An equivalent problem is formulated by imposing suitable boundary conditions for flow over a flat plate to avoid numerical and mesh generation issues. Recent work demonstrated that accurate large eddy simulation (LES) of such a flow is possible using only O(1%) of the direct numerical simulation (DNS) resolution but the performance of different subgrid-scale models could not be properly assessed because of the effects of unquantified numerical dissipation. LES of a laminar separation bubble flow over a flat plate is performed using a pseudo-spectral Navier-Stokes solver at resolutions corresponding to 3% and 1% of the chosen DNS benchmark by Spalart and Strelets (2000). The negligible numerical dissipation of the pseudo-spectral code allows an unambiguous assessment of the performance of subgrid-scale models. Three explicit subgrid-scale models—dynamic Smagorinsky, σ, and truncated Navier-Stokes (TNS)—are compared to a no-model simulation (under-resolved DNS) and evaluated against benchmark DNS data focusing on two quantities of critical importance to airfoil and blade designers: time-averaged pressure (Cp) and skin friction (Cf) predictions used in lift and drag calculations. Results obtained with explicit subgrid-scale models confirm that accurate LES of laminar separation bubble flows is attainable with as low as 1% of DNS resolution, and the poor performance of the no-model simulation underscores the necessity of subgrid-scale modeling in coarse LES with low numerical dissipation.

A numerical investigation on the vortex formation and flow separation of the oscillatory flow in jet pumps.

A two-dimensional computational fluid dynamics model is used to predict the oscillatory flow through a tapered cylindrical tube section (jet pump) placed in a larger outer tube. Due to the shape of the jet pump, an asymmetry in the hydrodynamic end effects will exist which will cause a time-averaged pressure drop to occur that can be used to cancel Gedeon streaming in a closed-loop thermoacoustic device. The performance of two jet pump geometries with different taper angles is investigated. A specific time-domain impedance boundary condition is implemented in order to simulate traveling acoustic wave conditions. It is shown that by scaling the acoustic displacement amplitude to the jet pump dimensions, similar minor losses are observed independent of the jet pump geometry. Four different flow regimes are distinguished and the observed flow phenomena are related to the jet pump performance. The simulated jet pump performance is compared to an existing quasi-steady approximation which is shown to only be valid for small displacement amplitudes compared to the jet pump length.