Time-averaged Pressure Research Articles

Heat transfer in a liquid under focused ultrasonic field

The ultrasonic heat transfer enhancement has a strong dependence on the sound intensity. The higher the local sound intensity is, the better the heat transfer enhancement achieves. Therefore, a focused ultrasonic field is proposed in cavities with elliptical cross-section based on interference criterion and standing wave criterion. To design a cavity with focused ultrasonic field, sound source and heat source are purposely located at the two focus points where sound intensity could be reinforced. There is a compromise between the value of the standing wave coefficient and the time-averaged sound pressure. The profile of heat transfer coefficient is found to be spatially consistent with the simulated time-averaged sound pressure distribution. The cavities with focused ultrasonic field always present larger heat transfer coefficient of natural convection heat transfer than the ordinary ones and the cavity with standing wave coefficient k2 = 8 presents better natural convection heat transfer performance than the cavity with standing wave coefficient k2 = 6. For the sub-cooled boiling heat transfer, bubble plays an important role on the ultrasonic heat transfer enhancement effect. At larger heat flux or smaller sub-cooled temperature, bubbles gather together to form bubble cloud that presents large thermal resistance. The cavitation collapse can disperse the bubble cloud.

Positive association of serum uric acid with new-onset diabetes in Chinese women with hypertension in a retrospective analysis of the China Stroke Primary Prevention Trial.

To investigate the association of baseline serum uric acid (UA) with new-onset diabetes, and to explore the possible effect modifiers in Chinese adults with hypertension. A total of 14 943 hypertensive patients with available UA measurements and without diabetes at baseline were included from the China Stroke Primary Prevention Trial (CSPPT). Participants were randomly assigned to a double-blind daily treatment with 10 mg enalapril and 0.8 mg folic acid or 10 mg enalapril alone. The primary outcome was new-onset diabetes, defined as physician-diagnosed diabetes or use of glucose-lowering drugs during follow-up, or fasting glucose ≥7.0 mmol/L at the exit visit. Over a median follow-up of 4.5 years, 1623 participants (10.9%) developed diabetes. Overall, there was a positive association between baseline UA and new-onset diabetes in women (per SD increment; adjusted odds ratio [OR] 1.14, 95% confidence interval [CI] 1.07, 1.23), but not in men (adjusted OR 1.01, 95% CI 0.92, 1.10). Moreover, a stronger positive association between baseline UA and new-onset diabetes was found among women with lower time-averaged on-treatment systolic blood pressure during the treatment period (<140 vs. ≥140 mmHg; P-interaction = 0.024), higher baseline body mass index (<24 vs. ≥24 kg/m2 ; P-interaction = 0.012), or higher baseline waist circumference (<80 vs. ≥80 cm; P-interaction = 0.032). Our study suggested that higher baseline UA was significantly associated with increased risk of new-onset diabetes in hypertensive Chinese women, but not in men. Further prospective studies are required to validate the differential association by sex.

Numerical simulation of wind-induced mean and peak pressures around a low-rise structure

We perform Computational Fluid Dynamics (CFD) analyses of a low-rise building immersed in a turbulent boundary layer. The Silsoe 6 m cube is chosen as the main case study aiming at representing the key features of the typical wind conditions experienced by low-rise buildings. The turbulent inflow condition is generated by employing a precursor technique. A sensitivity study is conducted to investigate the effects of eight subgrid-scale (SGS) models on the flow around the cube. The incoming turbulence intensity and mesh sizing for each of the SGS models are also explored. The results in terms of time-averaged pressure fields for relatively well-resolved simulations are well compliant with available experimental data and insensitive to the selection of the SGS model. We demonstrate that, while the effect of the aforementioned factors has a limited impact on the first-order statistics of the pressure coefficients, their influence becomes much more pronounced when analysing higher-order statistics (i.e. variance, skewness and kurtosis) and extreme values. The extent of their relative importance depends on the location of the point under investigation and the quantity of interest. The current study provides guidelines on the choice of factors which can strongly affect the results obtained employing CFD simulations when the local peak pressures are of interest, for example, as in the case of cladding design.

Optimal suppression of a separation bubble in a laminar boundary layer

By means of nonlinear optimization, we seek the velocity disturbances at a given upstream position that suppress a laminar separation bubble as effectively as possible. Both steady and unsteady disturbances are examined and compared. For steady disturbances, an informed guess based on linear analysis of transient perturbation growth leads to significant delay of separation and serves as a starting point for the nonlinear optimization algorithm. It is found that the linear analysis largely captures the suppression of the separation bubble attained by the nonlinear optimal perturbations. The mechanism of separation delay is the generation of a mean flow distortion by nonlinear interactions during the perturbation growth. The mean flow distortion enhances the momentum close to the wall, counteracting the deceleration of the flow in that region. An examination of the effect of the disturbance spanwise wavenumber reveals that perturbations maximizing the mean flow distortion also approximately maximize the peak wall pressure, which is beneficial for lowering form drag. The optimal spanwise wavenumber leading to maximal peak wall pressure is significantly larger than the one maximizing the shift in separation onset. For unsteady disturbances, the mechanism of separation delay relies on enhancing wall-normal momentum transfer by triggering instabilities of the separated shear layer. It is found that Tollmien–Schlichting waves obtained from linear stability theory provide accurate estimates of the nonlinearly optimal disturbances. Comparison of optimal steady and unsteady perturbations reveals that the latter are able to obtain a higher time-averaged peak wall pressure.

Numerical investigation of anode channel clogging of a PEMFC with a realistic droplet size distribution

In this paper, a parametric study of anode channel clogging of a proton exchange membrane fuel cell (PEMFC) is presented using the numerical volume of fluid method. The droplet size distribution in the present research is considered close to reality. Therefore, three droplets with different sizes are initially placed on the anode channel wall. The droplet sizes are obtained based on the distribution of the saturated liquid water along the whole channel under a specific working condition and the amount of condensed water in the channel, using the PEMFC code. Finally, the dynamic behavior of the droplets is investigated and compared to that of a single droplet with the same total volume. Numerical results show that, in the case of three droplets, the droplets coalesce during the removal time and form a larger droplet. Also, the droplets gradually spread over the channel walls and eventually reach the upper corners of the channel and form liquid slugs. Then, the slugs move slowly toward the channel outlet, and after a while, their shapes and positions are fixed and cause the channel clogging. Changing the number of droplets does not significantly change the time of channel clogging. The numerical results show that the time-averaged pressure drop of three droplets is about 85% less than that of a single droplet.

Hydraulic characteristics of the aeration basin in a ski-jump-step spillway

AbstractThe ski-jump-step spillway was designed using a ski-jump and an aeration basin to effectively pre-aerate flow in a stepped spillway. A new experimental study of the hydraulic characteristics of aeration basins was conducted to better understanding their pre-aeration properties and mechanisms. The plunge-pool patterns of aeration basins were classified into partially aerated, fully aerated, and vortex expelled, with increasing unit discharge. Relations between the distributions of the time-averaged pressure and the air concentration of the plunge-pools suggested that the ski-jump jet impact and the recirculating vortices are the main causes of plunge-pool air entrainment. Based on the export cross-section of the aeration basins, the bottom air concentrations remained greater than 3.0%. The sidewall air concentrations were greater than 7.5% and followed a logarithmic distribution in the vertical direction, demonstrating that the export flow attains a completely aerated state without any blackwater zones. In addition, increasing the aeration basin length was found to prevent the occurrence of a vortex expelled plunge-pool, thus promoting the appropriate pre-aeration effect under large unit discharges.

Feature Extraction of Oscillating Flow With Vapor Condensation of Moist Air in a Sonic Nozzle

The sonic nozzle is commonly used in flow measurement. However, the nonequilibrium condensation phenomenon of moist air in the nozzle has a negative effect on the measuring accuracy. To investigate this complex phenomenon, the experiments on the oscillating condensation flow of moist air were conducted by an adjustable humidification apparatus with different relative humidities (0%-100%), temperatures (30-50°C), and carrier gas pressures (1-6 bar), where the microsize pressure measuring system was designed by Bergh-Tijdeman (B-T) model. The accurate mathematical model of nonequilibrium condensation was also built and validated by the experimental data of time-averaged pressure distribution. Then, the frequency and intensity of pressure fluctuation of oscillating flow at a wide range of operation condition were obtained combining experimental data and physical simulation model. Importantly, a new semiempirical relation of dimensionless frequency deduced from dimensionless analysis was identified accurately by experimental data. Finally, the signal nonstationarity was also observed using the continuous wavelet transform (CWT). The instantaneous frequency saltation and the energy attenuation of pressure signals were observed in the condensation flow.

Validation and Analysis of Nonlinear System of Centrifugal Compressor using Hot Wire Flow Measurement and Jacobian Matrix

ABSTRACTA mathematical method is developed to predict surge and instability inception in a centrifugal compressor map based on Greitzer and Moore model. The method uses phase plane and Jacobian matrix to determine stable and unstable zone of centrifugal compressor and to extract limit cycle in unstable zone. In order to evaluate accuracy of analytical method, a compressor test rig facility is designed and tested in order to be compared with theoretical results. Accurate Honeywell piezoelectric pressure transducers are used to measure time averaged pressure at inlet and outlet ducts of the compressor. In order to record the flow fluctuation precisely, hot wire sensors are installed in entrance and exit of impeller. The results represent instability inception near true stall point but different from measured data. The discrepancy mostly originates from inlet and outlet ducts which causes measure stall point of compressor deviates from true one. An analytical method which is able to predict surge and instability in a compressor is highly significant for instrumentation and controlling of system.

Numerical investigation of pulsed fluidized bed using CFD-DEM: Insights on the dynamics

Computational Fluid Dynamics and Discrete Element Method are used to study the dynamics of a pseudo-2D pulsed fluidized bed, consisting of Geldart B particles. The focus is on the response of the system to variations in the mean gas velocity, VM, and the amplitude of oscillations, VA, at an imposed frequency of 3.5 Hz. Distinctive flow patterns are mapped out based on observation of simulations, pressure fluctuations and bubble granular temperature profiles. A regime transition is observed when VA is varied under constant VM. Such transition is underlined by: a peak in time-averaged pressure drop and coordination number; a change in the dynamics of the solid phase; an increase in velocity of the void fronts traversing the particle bed; and a decrease in minimum average height attained by the solid phase. This is related to the suppression of void layers at higher VA, occurring more easily at lower VM.

Effect of Two Bogie Cavity Configurations on the Underbody Flow and Near Wake Structures of a High-Speed Train

A time-dependent simulation method, DES (detached eddy simulation), combined with Realizable k-ε turbulence model, has been adopted to study the underbody flow and near wake structures of a high-speed train with two bogie cavity configurations laid on the stationary ground. The numerical data, including time-averaged aerodynamic drag forces and pressure coefficients, were compared with experimental results from previous wind tunnel tests. A detail comparison of the instantaneous flow structures, mean velocity vector contours, velocity and pressure profiles under the train bottom in the symmetry plane and velocity contours overlaid with streamlines in the wake has been conducted in the two configurations. Also the aerodynamic drag coefficients for the two cases are discussed herein. The two cases show that the bogie cavity configurations contribute to the differences of velocity and pressure distributions in each bogie region, as well as the complex vortex structures around the bogie regions. Compared to the inclined bogie cavity configuration, the train with straight plates experiences a lower drag force by 2.8% for a three-car model in the stationary ground. Thus, an effective simplification criterion for the train model will contribute to an accurate prediction of forces of trains in simulations.

Numerical investigation of tip clearance effects on propulsion performance and pressure fluctuation of a pump-jet propulsor

Rotor-stator interaction and tip clearance between rotor blade tip and shroud of a pump-jet propulsor can induce strong pressure fluctuations and vibrations, which may cause damage of the propulsion system at some severe working conditions. This paper investigates numerically the effects of tip clearance on propulsion performance and pressure fluctuations in a pump-jet propulsor. The shear-stress transport (SST) k–omega turbulence model together with the sliding mesh technique are adopted to simulate the three-dimensional unsteady flow. The accuracy of the computation results is verified through available experimental data of propulsion performance with a tip clearance of 1 mm. The comparison of hydrodynamic performance for pump-jet propulsor with tip clearance of 0, 1 mm, 2 mm and 4 mm shows that the thrust and propulsion efficiency can be reduced significantly due to the increase of tip clearance sizes. Further results indicate that the decrease of thrust and torque of the pump-jet propulsor with tip clearance is mainly caused by the change of time-averaged pressure distributions on the rotor blade in the range of 0.8R–1.0R (where R is the rotor diameter). In addition, a method based on Statistics is applied to reveal the characteristics of pressure fluctuations at all grid nodes on the whole blade surface, and the result shows that the amplitudes of pressure fluctuations increase significantly on the blade surface when the tip clearance size is enlarged, especially on the blade tip, blade root and the leading edge. Moreover, it is found that the tip clearance effects do not change the main frequency of thrust and torque fluctuations, while it has a significant impact on the amplitudes of fluctuations, especially in the higher frequency band.

Impact of Different Nose Lengths on Flow-Field Structure around a High-Speed Train

In this study, the time-averaged and instantaneous slipstream velocity, time-averaged pressure, wake flows, and aerodynamic force of a high-speed train (HST) with different nose lengths are compared and analyzed using an improved delayed detached-eddy simulation (IDDES) method. Four train models were selected, with nose lengths of 4, 7, 9, and 12 m. To verify the accuracy of the numerical simulation results, they were compared with wind tunnel test results. The comparison results show that the selection of the numerical simulation method is reasonable. The research results show that with increasing nose length, the peak values of the time-averaged slipstream velocity of the trackside position (3 m from the center of track and 0.2 m from the top of rail) and the platform position (3 m from the center of track and 0.2 m from the top of rail) decrease continuously, and show a trend of rapid reduction at first, and then a slow decrease. As the nose length increased from 4 to 12 m, the time-averaged slipstream velocity at the trackside position and platform position are decreased by 57% and 19.5%, respectively. At a height of 1.6 m from the top of the rail, ΔCP max (maximum pressure coefficient), |ΔCP min| (the absolute value of minimum pressure coefficient), and ΔCP (pressure change coefficient) decrease with increasing nose length, which is similar to the peak value of time-averaged slipstream velocity, decreasing rapidly at first and then slowly. As the nose length increased from 4 to 12 m, decreases of ΔCP max, |ΔCP min|, and ΔCP by 26.5%, 58.5%, and 44.8% were shown, respectively. Different nose lengths also have a significant impact on wake flow.

Visual Experimental Study on Liquid-Nitrogen Cavitating Flow on NACA 66 Hydrofoil

Cavitation is one of the main causes of failure in rocket engine cryopumps. Based on a visual experimental platform for liquid-nitrogen () cavitating flow over a NACA 66 hydrofoil, morphology evolutions of the cavity cloud for different cavitation numbers are observed with a high-speed camera. The time-averaged pressure distributions in the cavitation zone are also measured, and the maximum temperature drop is derived from the pressure data by assuming the thermodynamic equilibrium in the cavitation zone. The attached cavitation length is dependent on the cavitation number, and the relationship is investigated by image analysis using the standard-deviation method. The shedding characteristics of the cavitation cloud of and room-temperature water are compared, and the mechanism is qualitatively clarified. The research results provide valuable experimental data, which are rare in this field, and help to understand the influence of cryogenic cavitation.

On the Prediction of Pressure Fluctuations and Pressure Drop Caused by Confined Bubble Growth During Flow Boiling in a Rectangular Mini/Micro-Channel

Heat-sinks based on flow boiling in microchannels have the potential to mitigate temperature rise in high heat flux devices such as electronic equipment. One of the major challenges is the instability associated with pressure fluctuations caused due to the confined bubble growth during boiling in a microchannel. This paper presents a comprehensive model to estimate the dynamic pressure fluctuations in a channel of rectangular cross-section and compares with the experimental results. The time-averaged pressure drop obtained from the transient model has been verified with the experimental results and the steady two-phase flow pressure drop correlations. Effects of various parameters such as heat flux and mass flux on the time-averaged pressure drop and the transient pressure fluctuations are presented. The paper also provides an approximate methodology to estimate the nucleation frequency derived from the waiting period of the bubble. The calculated frequency is validated qualitatively with the experimental observation available in the literature. The current work also demonstrates that the transient pressure drop models, which, with suitable inputs, predict the local pressure fluctuations during confined bubble growth, can also be used to predict the steady- flow pressure drop.

Hydraulic characteristics of stepped spillway dropshafts for urban deep tunnel drainage systems: the case study of Chengdu city

Cities in southwestern China experience urban drainage and overflow pollution after extreme rainfall events, which are major problems. In this study, a type of stepped spillway dropshaft suitable for drainage by deep tunnels in Chengdu was proposed and the hydraulic characteristics were investigated experimentally. The results showed that the nappe flow and strong turbulent deflected jet flow in the stepped spillway allowed the dropshaft to greatly dissipate energy. According to the distribution of the time-averaged pressure on the steps, the flow on the steps could be divided into a recirculating region, a wall-impinging region and a mixing region. The time-averaged pressure on the outside of the step was higher than that on the inside due to the centrifugal force effect of the water. The fluctuating pressure distribution of the step approximated the normal distribution. It was acceptable to calculate the minimum pressure with 3 times the root mean square (RMS). The vibration of the flow on the stepped spillway did not resonate with the step. When the outflow tunnel was under submerged outflow conditions, the aeration in the stepped spillway was exhausted through air holes and only a small amount of air entered the outflow tunnel, thereby avoiding an air explosion.

High-Throughput Triggered Merging of Surfactant-Stabilized Droplet Pairs Using Traveling Surface Acoustic Waves.

We present an acoustofluidic device for fluorescently triggered merging of surfactant-stabilized picoliter droplet pairs at high throughput. Droplets that exceed a preset fluorescence threshold level are selectively merged by a traveling surface acoustic wave (T-SAW) pulse. We characterize the operation of our device by analyzing the merging efficiency as a function of acoustic pulse position, duration, and acoustic pressure amplitude. We probe droplet merging at different droplet rates and find that efficient merging occurs above a critical acoustic power level. Our results indicate that the efficiency of acoustically induced merging of surfactant stabilized droplets is correlated with acoustic streaming velocity. Finally, we discuss how both time-averaged and instantaneous acoustic pressure fields can affect the integrity of surfactant layers. Our technique, by allowing the merging of up to 105 droplets per hour, shows great potential for integration into microfluidic systems for high-throughput and high-content screening applications.

Comparison Between Fluidic Oscillators and Steady Jets for Separation Control

An experimental study was conducted to compare performance and efficiency between fluidic oscillators and steady jets with respect to commonly used flow control coefficients for momentum, mass flow, and energy in support of the development of active flow control technology. Different actuation modules were tested on a model of the NASA hump geometry. Time-averaged pressure measurements were conducted with high spatial resolution, and stereoscopic particle image velocimetry was performed downstream of the actuation location to investigate the performance differences in detail. Surface flow visualization in a quiescent environment was employed to assess the spreading angle of the various actuators. Results show that fluidic oscillators are almost always superior to steady jets with respect to any flow control coefficient at a given spacing. The lone outlier occurs for tightly spaced actuators in which both perform similarly with respect to the energy coefficient. This general superiority is in part due to a larger jet spreading angle, the associated spanwise coverage, and the existence of stronger coherent streamwise vortices in comparison to steady jets.

Effect of the Area Contraction Ratio on the Hydraulic Characteristics of the Toothed Internal Energy Dissipaters

Toothed internal energy dissipaters (TIED) are a new type of internal energy dissipaters, which combines the internal energy dissipaters of sudden reduction and sudden enlargement forms with the open-flow energy dissipation together. In order to provide a design basis for an optimized body type of the TIED, the effect of the area contraction ratio (ε) on the hydraulic characteristics, including over-current capability, energy dissipation rate, time-averaged pressure, pulsating pressure, time-averaged velocity, and pulsating velocity, were studied using the methods of a physical model test and theoretical analysis. The main results are as follows. The over-current capability mainly depends on ε, and the larger ε is, the larger the flow coefficient is. The energy dissipation rate is proportional to the quadratic of Re and inversely proportional to ε. The changes of the time-averaged pressure coefficients under each flow are similar along the test pipe, and the differences of the time-averaged pressure coefficient between the inlet of the TIED and the outlet of the TIED decrease with the increase of ε. The peaks of the pulsating pressure coefficient appear at 1.3 D after the TIED and are inversely proportional to ε. When the flow is 18 l/s and ε increases from 0.375 to 0.625, the maximum of time-averaged velocity coefficient on the line of Z/D = 0.42 reduces from 2.53 to 1.17, and that on the line of Z/D = 0 decreases from 2.99 to 1.74. The maximum values of pulsating velocity on the line of Z/D = 0.42 appear at 1.57D and those of Z/D = 0 appear at 2.72D, when the flow is 18 l/s. The maximum values of pulsating velocity decrease with the increase of ε. Finally, two empirical expressions, related to the flow coefficient and energy loss coefficient, are separately presented.

Natural circulation two-phase flow characteristics in a rod bundle channel under stable and rolling motion conditions

Natural circulation two-phase flow characteristics are studied under stable and rolling motion conditions in a rod bundle channel. The mass quality, flow rate and frictional pressure drop variation results with increasing heat flux are given and the effects of system pressure and inlet subcooling are analyzed. The flow rate decline is observed under high mass quality conditions since the frictional pressure drop is large enough. The flow instability characteristics for different stages with increasing heat flux are given. The main reason for the flow instability characteristics variation is that the dominant factor changes from subcooled boiling to the pressure drop oscillation. The experimental results indicate that increasing the system pressure and the inlet resistance of the test section, including moving the compressible volume to a downstream position, can delay the onset of the flow instability. The time-average flow parameters under rolling motion conditions are mainly influenced by the stable mass quality and the rolling parameters. For high stable mass quality conditions, the time-average two-phase flow parameters decrease a little compared to stable results. With the stable mass quality decreasing, the time-average mass quality gradually increases with the rolling frequency. For further lower stable mass quality conditions, the time-average values are even larger than stable values. For the frictional resistance calculation, a few correlations are evaluated and recommended. Additionally, new correlations are fitted for stable and time-average two-phase frictional pressure drop prediction in high mass quality conditions.

Predictive Value of Resting Pd/Pa for Fractional Flow Reserve Assessed with Monorail Pressure Microcatheter in Real-World Practice

Background and Objectives: The aims of this study were (1) to examine the diagnostic accuracy of resting time-averaged distal coronary pressure (Pd) to mean aortic pressure (Pa) ratio to predict hyperemic fractional flow reserve (FFR) and (2) to identify a resting Pd/Pa value that can preclude the need for hyperemic FFR assessed with use of a monorail pressure catheter. Methods: A total of 191 stenoses were assessed. After exclusions, 157 FFR data sets from 103 patients were analyzed. Results: Resting Pd/Pa showed poor agreement with hyperemic FFR (r=0.619, P&lt;0.001). The receiver operating characteristic curve for resting Pd/Pa with reference to hyperemic FFR of 0.80 or less showed an area under the curve of 0.800 (95% confidence interval 0.732–0.868, P&lt;0.001), with the greatest diagnostic accuracy of 74.5% for resting Pd/Pa of less than 0.85. Resting Pd/Pa of 0.96 or greater had a sensitivity of 100% and a negative predictive value of 100%, and resting Pd/Pa of 0.82 or less had a specificity of 98.9% and a positive predictive value of 94.1% to predict abnormal FFR of 0.80 or less. These results were consistent regardless of the vessels studied, the location of lesions, and the severity of stenosis. Conclusions: Resting Pd/Pa showed poor agreement with hyperemic FFR assessed with use of a monorail pressure microcatheter. However, resting Pd/Pa of 0.96 or greater had excellent sensitivity and negative predictive value to predict normal hyperemic FFR, and resting Pd/Pa of 0.82 or less had excellent specificity and positive predictive value to predict abnormal hyperemic FFR.