Time-averaged Pressure Research Articles

SEEPAGE FLOW IN A BREAKER ZONE

Seepage velocities in several breaker zones were measured with dye for visualization. Pressure gauges were also installed to obtain pore water pressure. Three cases of different incident-wave periods were treated in laboratory experiments. The time-averaged pore water pressure was higher near the shoreline, resulting in the seepage flow, the maximum velocity of which was larger as the incident-wave period was longer, generally moving from onshore to offshore. The penetrated water flowed out from the permeable seabed at a bar top or on the rather offshore side of the bar top if a bar was developed remarkably. The pore water pressure in the breaker zone showed time variation depending on the wave phases including wave breaking and bore propagation. There was a phase when the pore water pressure was locally low below the bar.

CFD Analysis and Reduced Order Modeling of Uncontrolled and Controlled Laminar Flow Over a Circular Cylinder

:The main idea of flow control is the improvement of aerodynamic characteristics of flow. This study analyzes the laminar flow over a two-dimensional (2D) circular cylinder and its control by air blowing from several slots located on the cylinder surface, computationally. The Computational Fluid Dynamics (CFD) results are validated using the experimental results available in the literature for Strouhal number, time-averaged drag coefficient and pressure coefficient distribution around the cylinder. Air blowing from four slots on the cylinder with a velocity of 50% of the free-stream velocity yields a reduction of the drag coefficient by 9%. Flow structures are separated according to their frequency content by Proper Orthogonal Decomposition (POD) method. It is seen from POD results that 99% of the total energy content can be modeled by considering only four most energetic POD modes of the flow. POD analysis of the controlled cases also show that the flow structure around the cylinder changes with the help of air blowing. For real-time flow control applications, it is important to predict the flow based on surface sensors, placed at a few discrete points. In this study, optimum sensor locations on the cylinder surface are also obtained by utilization of a one-dimensional POD analysis based on surface pressure data obtained from CFD results.

Main Gas Ingestion in a Turbine Stage for Three Rim Cavity Configurations

Experiments were carried out in a model air turbine stage to study the influence of rotor-stator rim cavity configuration on the ingestion of mainstream gas into the cavity. The three rim cavity configurations differed in their aspect ratio (height/width); the rim seal geometry remained the same. The aspect ratio was changed from the baseline ratio by installing an inner shell on the stator at an appropriate radius; this effectively introduced an axial-gap seal between the rim cavity and the cavity radially inboard. The initial step in each experiment was the measurement of time-average static pressure distribution in the turbine stage to ascertain that proper flow condition had been established. Subsequently, tracer gas concentration and particle image velocimetry techniques were employed to measure the time-average but spatially local main gas ingestion and the instantaneous velocity field in the rim cavity. At low purge air flow, regions of ingestion and egress could be identified by inspecting the instantaneous radial velocity distribution near the rim seal obtained from cavity gas velocity maps close to the stator. While the tangential velocity tended to be slightly larger for the so determined ingested gas, a more clear-cut indicator of ingestion was the strong inward gas radial velocity. Information provided by ensemble-average velocity maps was not sufficient for identifying ingestion because the averaging smeared out flow details, which varied from instant to instant. Velocity fields obtained from three-dimensional, time-dependent numerical simulation of a rim seal-cavity sector with similar dimensions qualitatively showed similar characteristics in the outer part of the cavity and provided insight into the complex flow in the seal region.

Scale effects of air-water flows in stilling basin of multi-horizontal submerged jets

A series of experiments were carried out on multi-horizontal submerged jets with four different model scales of 1:36, 1:57, 1:80, 1:200. In routine tests, scale effects have to be considered, due to complex vortex structure and strong air entrainment in stilling basin. Our focus was laid on measuring and analyzing the time-averaged pressure distribution, water depth and closed-to-bed velocity in the stilling basin of multi-horizontal submerged jets. The experiments results show that the model scale has but a slight effect on the time-averaged hydraulic characteristics in the stilling basin of multi-horizontal submerged jets, which indicates that the results of time-averaged hydraulic characteristics for a normal pressure model are reliable. However, the scale effects of air entrainment, fluctuation pressure and vortex structure call for further investigations in order to make the results of experiments serve as scientific references for practical engineering.

Investigation of Turbine Shroud Distortions on the Aerodynamics of a One and One-Half Stage High-Pressure Turbine

As part of a proactive effort to investigate the ability of computational fluid dynamics tools to predict time-accurate surface-pressure histories, a combined experimental/computational investigation was performed, examining the effect of rotor shroud (casing) out-of-roundness on the unsteady pressure loading for the blade row of a full-stage turbine. The casing out-of-roundness was idealized by designing a casing ring with a sinusoidal variation. This casing ring was used to replace a flat casing for an existing turbine, and direct comparisons were made between the time-accurate pressure measurements and predictions obtained using the flat and “wavy” casings. For both casing configurations, predictions of the unsteady pressure loading for many locations on the blade and vane were obtained using Numeca’s FINE/TURBO code and General Electric’s turbine and compressor analysis (TACOMA) code. This paper will concentrate on the results obtained for the wavy casing, but the results for the flat casing are presented as a baseline case. The time-accurate surface-pressure measurements were acquired for the vane and blade of a modern, 3D, 1 and 1/2 stage high-pressure turbine operating at the design corrected speed and stage pressure ratio. The research program utilized an uncooled turbine stage for which all three airfoil rows are heavily instrumented at multiple spans to develop a full data set. The vane-blade-vane count for this machine is 38-72-38. The number of waves in the distorted shroud “wavy wall” is approximately 1.5 times the number of vanes. The resulting changes in the aerodynamic surface-pressure measurements were measurable at all blade spanwise locations. Variations in the time-averaged surface pressure of up to 5% of the flat casing values were observed. In addition, the frequency content of the time-resolved blade data for the wavy casing changed substantially from that measured using the flat casing, with changes in both amplitudes and frequencies. Imposing the casing irregularity changed the fundamental physics of the problem from a single frequency and its harmonics to a multifrequency problem with mixed harmonics. The unsteady effects of this type of problem can be addressed using the harmonic method within Numeca’s FINE/TURBO code, which is designed to handle multiple blade passing frequencies and harmonics for one blade row. A more traditional approach is included in this paper by employing the TACOMA code in a linearized mode that produces results for a single frequency. These results show that casing irregularity can have a significant influence on the blade surface-pressure characteristics. Further, it is demonstrated that the FINE/TURBO code experienced difficulty in predicting the unsteady pressure signal attributed to the wavy casing configuration, while at the same time, in capturing the unsteady signal attributed to the vane passing due to limitations in the current methodology.

Strength characteristics of the self-sustained wave in grooved channels with different groove length

The self-sustained oscillations arising in a series of grooved channels are investigated experimentally. Pressure drop, time-averaged and time-various local pressure in the grooved channels with six kinds of groove length are measured with the differential transducer and the pressure sensor, respectively, and the flow structures are visualized using the aluminum dust method. The local pressure signal shows that the self-sustained wave appears in the first or second frequency, and the Strouhal number, based on the nature frequency of the self-sustained wave, is almost equivalent for the first or second frequency in the same channel. Meanwhile, the Strouhal number for each channel decreases monotonously with the groove length. Furthermore, it is found that increasing pressure will result in higher amplitude of the self-sustained wave, this behavior is significant for the efficient heat transfer in practical engineering.

Direct Numerical Simulations of Transitional and Turbulent Flow on a Turbine Airfoil

A direct numerical simulation of transitional/turbulent flow on a low-speed turbine airfoil is presented here. The nonconservative form of the Navier–Stokes equations for compressible flows is used for this simulation. The numerical method used to solve these equations is a high-order-accurate upwind-biased iterative-implicit finitedifference scheme and is also presented here. The algorithm and the simulation are extensions of earlier efforts in direct simulations of transition and turbulence on flat plates. The present investigation has additional features, such as surface curvature, an adverse pressure gradient region on the airfoil, and trailing-edge vortex shedding. The results provided in the paper include the time-averaged pressure and the Stanton number distribution on the airfoil surface and turbulence statistics and flow visualization in the transitional/turbulent regions. Comparisons with experimental and computational data obtained for flows over flat plates and in channels are provided. The results indicate that the essential features of transition and turbulence have been captured.

Numerical Simulation of Rotor-Fuselage Aerodynamic Interaction Using an Unstructured Overset Mesh Technique

Numerical simulation of unsteady flows around helicopters was conducted to investigate the aerodynamic interaction of main rotor and other components such as fuselage and tail rotor. For this purpose, a three-dimensional inviscid flow solver has been developed based on unstructured meshes. An overset mesh technique was used to describe the relative motion between the main rotor, and other components. As the application of the present method, calculations were made for the rotorfuselage aerodynamic interaction of the ROBIN (ROtor Body INteraction) configuration and for a complete UH-60 helicopter configuration consisted of main rotor, fuselage, and tail rotor. Comparison of the computational results was made with measured time-averaged and instantaneous fuselage surface pressure distributions for the ROBIN configuration and thrust distribution and available experimental data for the UH-60 configuration. It is demonstrated that the present method is efficient and robust for the simulation of complete rotorcraft configurations. Key Word : Rotor fuselage interaction, Unstructured mesh, Overset mesh technique, ROBIN, UH-60

On the effect of injection of gas in the numerical simulation of accretion flows

We investigate the effects of various ways of injection of gas at the outer boundary in the numerical simulations of non-viscous accretion flows. We study three models. In Model A, we inject material around the equatorial plane. In Models B and C, fullrange θ injection is used (we employ spherical coordinates). In all three models, the injected material has the same density distribution with polar angle θ. From the equatorial region to the polar regions, angular momentum of the injected material of Model B decreases faster than that in Model C. For all of the models, after a transient episode of infall at the beginning of the simulations, the gas piles up in the equatorial regions outside the black hole and forms a thick torus bounded by a centrifugal barrier. We find that the accretion rates of Models B and C are more than ten times higher than that in Model A. In Model A, there is weak accretion only in the torus and outflows are found on the surface of the torus. In Model B, we find strong inflows on the surface of its torus, and the accretion in the torus is weak. In Model C, strong inflows also occur on the surface of its torus, but the accretion regions are narrower and there are strong outflows in its torus. In all of our models, the time-averaged density, pressure and angular momentum in the equatorial region can be described by a radial power law, with P ∝r−3/2, P ∝r−2 and l∝r0.

S1604-1-1 感圧塗料を用いた回転ディスク表面の時間平均圧力分布計測(ヘッド・ディスク・インターフェイス,社会変革を技術で廻す機械工学)

S1604-1-1 感圧塗料を用いた回転ディスク表面の時間平均圧力分布計測(ヘッド・ディスク・インターフェイス,社会変革を技術で廻す機械工学)

Slow drag in wet-snow avalanche flow

AbstractWe report impact pressures exerted by three wet-snow avalanches on a pylon equipped with piezoelectric load cells. These pressures were considerably higher than those predicted by conventional avalanche engineering guidelines. The time-averaged pressure linearly increased with the immersion depth of the load cells and it was about eight times larger than the hydrostatic snow pressure. At the same immersion depth, the pressures were very similar for all three avalanches and no dependency between avalanche velocity and pressure was apparent. The pressure time series were characterized by large fluctuations. For all immersion depths and for all avalanches, the standard deviations of the fluctuations were, on average, about 20% of the mean value. We compare our observations with results of slow-drag granular experiments, where similar behavior has been explained by formation and destruction of chain structures due to jamming of granular material around the pylon, and we propose the same mechanism as a possible microscale interpretation of our observations.

Rest versus Exercise Hemodynamics for Middle Cerebral Artery Aneurysms: A Computational Study

Exercise is an accepted method of improving cardiovascular health; however, the impact of increases in blood flow and heart rate on a cerebral aneurysms is unknown. This study was performed to simulate the changes in hemodynamic conditions within an intracranial aneurysm when a patient exercises. Rotational 3D digital subtraction angiograms were used to reconstruct patient-specific geometries of 3 aneurysms located at the bifurcation of the middle cerebral artery. CFD was used to solve for transient flow fields during simulated rest and exercise conditions. Inlet conditions were set by using published transcranial Doppler sonography data for the middle cerebral artery. Velocity fields were analyzed and postprocessed to provide physiologically relevant metrics. Overall flow patterns were not significantly altered during exercise. Across subjects, during the exercise simulation, time-averaged WSS increased by a mean of 20% (range, 4%-34%), the RRT of a particle in the near-wall flow decreased by a mean of 28% (range, 13%-40%), and time-averaged pressure on the aneurysm wall did not change significantly. In 2 of the aneurysms, there was a 3-fold order-of-magnitude spatial difference in RRT between the aneurysm and surrounding vasculature. WSS did not increase significantly during simulated moderate aerobic exercise. While the reduction in RRT during exercise was small in comparison with spatial differences, there may be potential benefits associated with decreased RRT (ie, improved replenishment of nutrients to cells within the aneurysmal tissue).

Determining Time-Average Sound Pressure Level from Sampled Sound Levels without Time Delay Effect Caused by Experimental Time Weighting

Determining Time-Average Sound Pressure Level from Sampled Sound Levels without Time Delay Effect Caused by Experimental Time Weighting

Traveling void wave in horizontal two-phase flow

In the framework of pattern dynamics approach, the discrete bubble model was developed for simulating inherent fluctuation of void fraction in a horizontal two-phase flow. Then flow patterns were identified based on the statistical properties of void fraction fluctuation, and the flow pattern map agreed with the experimental observation of high-pressure two-phase flow of CO 2 in horizontal tubes. The time-averaged pressure drop and the void fraction obtained in the simulation agreed reasonably with the existing correlations. Thus the horizontal flow version of the discrete bubble model demonstrates its relevance in simulating inherent fluctuation of two-phase flow.

Pressure Measurements and Comfort of Foam Safety Cushions for Confined Seating

Glider flights may require the pilot to sit for many hours in a cramped cockpit that allows little movement. Experiments were undertaken to evaluate the performance of different seat cushions in a glider simulator. Subjects were male glider pilots with a maximum height of 1.85 m (6.07 ft) who participated in simulated glider flights lasting 1.5 h. A pressure-mapping device was used to determine cushion performance. By analyzing 15 subjects we calculated the pressure threshold for comfort, above which fidgeting provided objective evidence of discomfort. To determine cushion performance relative to that threshold, 20 other pilots then sat on 5 different viscoelastic foam cushions in the simulator. The time-averaged peak pressure below which no discomfort-induced fidgeting occurred was 8.8 kPa (1.28 psi). The highest peak pressure at which discomfort could be relieved by fidgeting was 11.0 kPa (1.6 psi). Of the five cushions tested, pressure remained below the discomfort threshold for almost all subjects for only one type of cushion. The best-performing cushion had a layered structure made up of approximately 25 mm of Confor C47 foam with an overlay of approximately 13 mm of Confor C45. The other types of energy-absorbing cushions tested, either with or without a softer top layer, are unlikely to provide comfortable seating solutions for most pilots. We conclude that satisfactory cushions are available for this application and that they can be objectively evaluated using this technique.

Initial stress and nonlinear material behavior in patient-specific AAA wall stress analysis

Rupture risk estimation of abdominal aortic aneurysms (AAA) is currently based on the maximum diameter of the AAA. A more critical approach is based on AAA wall stress analysis. For that, in most cases, the AAA geometry is obtained from CT-data and treated as a stress free geometry. However, during CT imaging, the AAA is subjected to a time-averaged blood pressure and is therefore not stress free. The aim of this study is to evaluate the effect of neglecting these initial stresses (IS) on the patient-specific AAA wall stress as computed by finite element analysis. Additionally, the contribution of the nonlinear material behavior of the AAA wall is evaluated. Thirty patients with maximum AAA diameters below the current surgery criterion were scanned with contrast-enhanced CT and the AAA's were segmented from the image data. The mean arterial blood pressure (MAP) was measured immediately after the CT-scan and used to compute the IS corresponding with the CT geometry and MAP. Comparisons were made between wall stress obtained with and without IS and with linear and nonlinear material properties. On average, AAA wall stresses as computed with IS were higher than without IS. This was also the case for the stresses computed with the nonlinear material model compared to the linear material model. However, omitting initial stress and material nonlinearity in AAA wall stress computations leads to different effects in the resulting wall stress for each AAA. Therefore, provided that other assumptions made are not predominant, IS cannot be discarded and a nonlinear material model should be used in future patient-specific AAA wall stress analyses.

A Method for Estimating the Influence of Time-Dependent Vane and Blade Pressure Fields on Turbine Rim Seal Ingestion

A method of estimating the turbine rim seal ingestion rates was developed using the time-dependent pressure distributions on the hub of turbines and a simple-orifice model. Previous methods use the time-averaged pressure distribution downstream of the vanes to estimate seal ingestion. The present model uses the pressure distribution near the turbine hub, obtained from 2D time-dependent stage calculations, and a simple-orifice model to estimate the pressure-driven ingress of gas-path fluid into the turbine disk cavity and the egress of cavity fluid to the gas path. The time-dependent pressure distribution provides the influence of both the vane wakes and the bow wave from the blade on the pressure difference between the hub pressure at an azimuthal location and the cavity pressure. Results from the simple-orifice model are used to determine the effective Cd that matches the cooling effectiveness at radii near the rim seal with the amount of gas-path-ingested flow required to mix with the coolant flow. Cavity ingestion data from rim seal ingestion experiments in a 1.5-stage turbine and numerical simulations for a 1 vane, 2-blade sector of the 16-vane, 32-blade turbine were used to evaluate the method. The experiments and simulations were performed for close-spaced and wide-spaced half stages between both the vane and blade and between the blade and a trailing teardrop-shaped strut. The comparison of the model with a single Cd for axial gap seals and the experiments showed a reasonable agreement for both close- and wide-spaced stages.

Impact of Statin Therapy on Central Aortic Pressures and Hemodynamics

Statins reduce the risk of cardiovascular events in people with hypertension. This benefit could arise from a beneficial effect of statins on central aortic pressures and hemodynamics. The Conduit Artery Function Evaluation-Lipid-Lowering Arm (CAFE-LLA) study, an Anglo-Scandinavian Cardiac Outcomes Trial (ASCOT) substudy, investigated this hypothesis in a prospective placebo-controlled study of treated patients with hypertension. CAFE-LLA recruited 891 patients randomized to atorvastatin 10 mg/d or placebo from 5 centers in the United Kingdom and Ireland. Radial artery applanation tonometry and pulse-wave analysis were used to derive central aortic pressures and hemodynamic indices at repeated visits over 3.5 years of follow-up. Atorvastatin lowered low-density lipoprotein cholesterol by 32.4 mg/dL (95% confidence interval [CI], 28.6 to 36.3) and total cholesterol by 35.1 mg/dL (95% confidence interval, 30.9 to 39.4) relative to placebo. Time-averaged brachial blood pressure was similar in CAFE-LLA patients randomized to atorvastatin or placebo (change in brachial systolic blood pressure, -0.1 mm Hg [95% CI, -1.8 to 1.6], P=0.9; change in brachial pulse pressure, -0.02 mm Hg [95% CI, -1.6 to 1.6], P=0.9). Atorvastatin did not influence central aortic pressures (change in aortic systolic blood pressure, -0.5 mm Hg [95% CI, -2.3 to 1.2], P=0.5; change in aortic pulse pressure, -0.4 mm Hg [95% CI, -1.9 to 1.0], P=0.6) and had no influence on augmentation index (change in augmentation index, -0.4%; 95% CI, -1.7 to 0.8; P=0.5) or heart rate (change in heart rate, 0.25 bpm; 95% CI, -1.3 to 1.8; P=0.7) compared with placebo. The effect of statin or placebo therapy was not modified by the blood pressure-lowering treatment strategy in the factorial design. Statin therapy sufficient to significantly reduce cardiovascular events in treated hypertensive patients in ASCOT did not influence central aortic blood pressure or hemodynamics in a large representative cohort of ASCOT patients in CAFE-LLA.

Measurement of pressure field around a NACA0018 airfoil from PIV velocity data

In solving these equations numerically, Eqs. (1) and (2) are transformed into general coordinates and they are solved by SOR scheme to obtain the time-averaged static pressure field around the airfoil. Note that the numerical grids are given by O type in the present study and the grid spacing is controlled to be fine near the airfoil and outer boundary, which is set at 5 times of the chord length C of the airfoil from the airfoil center. The number of grids is 600 x 300 in circumferential and radial direction, respectively. In solving these equations, all velocities on the right hand side of Eq. (1) are evaluated from the experimental velocity data measured by PIV. 3. Experiment The velocity field around a NACA0018 airfoil is measured using a standard PIV system, which consists of Nd:YAG laser (532 nm, 30 mJ/pulse), CCD camera with frame straddling function (1280 x 1024 pixels with 12 bits in gray level) and the pulse controller. The experiment is carried out in an { } ( ) j i j u u

Measuring the Gas-Solids Distribution in Fluidized Beds -- A Review

In gas-solid fluidized beds, the distribution of the particles typically varies both in time and space. Since the gas-solids distribution and its variation have a strong influence on the performance of a fluidized bed for a given process, it is very important to accurately measure the gas-solids or voidage distribution. This paper reviews techniques for measuring the voidage distribution in gas-solid fluidized beds, with a focus on the developments during the last ten years. We will treat subsequently direct visualisation, tomography, optical probes, capacitance probes, and pressure measurements.Dense gas-solids flows are typically opaque to visible light. This makes optical techniques only of limited use in dense gas-solids flow. However, direct visualization can be useful for very dilute systems, pseudo 2-D beds, and the outer layer of dense, 3-D systems. Tomography is frequently used to obtain the voidage distribution in a horizontal cross-section of the bed. Electric capacitance tomography is fast, but its spatial resolution is limited and image reconstruction is still troublesome. Although some steps forward have been made, research is continuing at this point. For nuclear (X-ray and gamma-ray) tomography, the image reconstruction is much easier and the spatial resolution better, but its temporal resolution is typically much lower. Therefore, research efforts for nuclear tomography are mainly aimed at increasing the measurement frequency. Optical probes determine the voidage as a function of time in a small measurement volume, either by the degree of reflection or by the degree of transmission of a light bundle. Capacitance probes determine the voidage as a function of time in a small measurement volume by measuring the dielectric permittivity of the gas-solids suspension in the measurement volume. Both optical and capacitance probe techniques are reasonably well-developed; the current research effort spent at improving them seems limited, especially for capacitance probes. Time-averaged pressure measurements are commonly used to determine the average bed density and bed height. By sampling the local pressure at a sufficiently high frequency (typically in the order of 200 Hz), much more information can be obtained about the fluidized bed hydrodynamics. However, obtaining quantitative voidage data from pressure fluctuations measurement remains a difficult task; in-bed pressure fluctuation (and acoustic) measurements are mostly used to determine changes in the voidage dynamics and distribution.