Mean Flow Temperature Research Articles

Mesoscale eddies inhibit intensification of the Subantarctic Front under global warming

Abstract Oceanic mesoscale eddies are important dynamical processes in the Southern Ocean. Using high-resolution (~0.1○ for the ocean) Community Earth System Model (CESM-HR) simulations under a high-carbon emission scenario, we investigate the role of mesoscale eddies in regulating the response of the Subantarctic Front (SAF) to global warming. The CESM-HR simulates more realistic oceanic fronts and mesoscale eddies in the Southern Ocean than a coarse-resolution (~1○ for the ocean) CESM. Under global warming, the SAF is projected to intensify. The mean flow temperature advection intensifies the front, whereas the mesoscale-eddy-induced temperature advection and atmospheric dampening play primary (~67%) and secondary (~28%) roles in counteracting the effect of mean flow temperature advection. Our study suggests the importance of mesoscale eddies on inhibiting the SAF intensification under global warming and necessity of mesoscale-eddy-resolving simulations for faithful projection of future climate changes in the Southern Ocean.

Valve Vibration Induced Intake Air Flow Dynamics Analysis Using Near Valve Particle Image Velocimetry

Abstract The transient dynamics of air flow running through the moving intake valve gaps in combustion cylinders is crucial to the performance of spark-ignition direct-injection engines. However, research on the air flow behavior in the vicinity of valve exits is still limited. In this work, transient air flow characteristics of a custom-designed dual-valve system under the operating conditions of a fixed valve lift and a vibrating valve lift at two frequencies are experimentally investigated. The velocity vector field measured using planar particle image velocimetry (PIV) is first analyzed in time domain based on temporal mean and root-mean-square (RMS) values. Comparison of temporal mean flow fields reveals the difference in flow pattern while RMS represents the variation of intake air jet velocity along the inlet path of the vibration-affected jet. Instantaneous snapshots provide direct analysis of the valve vibration-induced intake air jet behavior. Furthermore, investigation in frequency domain extends insights into the dominant spectral components of flow structures. Fast Fourier transform (FFT) applied to every vector on the velocity field yields a vibration frequency affected zone (VFAZ), indicating the regions where the effect of valve vibration is significant. By employing dynamic mode decomposition (DMD) method, the spatio-temporal PIV results are decomposed into modes with specific frequencies. Reconstructed flow field using modes with the valve operating frequency visually unveils a cyclic vortex structure near the valve exit. In summary, this study elucidates the mechanism of near-valve intake air flow impingement and interaction behavior induced by the valve vibrating motion.

Cause-and-effect of linear mechanisms sustaining wall turbulence

Abstract

Multiparametric flow analysis using four-dimensional flow magnetic resonance imaging can detect cerebral hemodynamic impairment in patients with internal carotid artery stenosis.

MRI-based risk stratification should be established to identify patients with internal carotid artery stenosis (ICS) who require further PET or SPECT evaluation. This study assessed whether multiparametric flow analysis using time-resolved 3D phase-contrast (4D flow) MRI can detect cerebral hemodynamic impairment in patients with ICS. This retrospective study analyzed 26 consecutive patients with unilateral ICS (21 men; mean age, 71years) who underwent 4D flow MRI and acetazolamide-stress brain perfusion SPECT. Collateral flow via the Willis ring was visually evaluated. Temporal mean flow volume rate (Net), pulsatile flow volume (ΔV), and pulsatility index (PI) at the middle cerebral artery were measured. Cerebral vascular reserve (CVR) was calculated from the SPECT dataset. Patients were assigned to the misery perfusion group if the CVR was < 10% and to the nonmisery perfusion group if the CVR was ≥ 10%. Parameters showing a significant difference in both groups were statistically evaluated. Affected side ΔV, ratio of affected to contralateral side Net (rNet), and ratio of affected to contralateral side ΔV were significantly correlated to CVR (p = 0.030, p = 0.010, p = 0.015, respectively). Absence of retrograde flow at the posterior communicating artery was observed in the misery perfusion group (p = 0.020). Combined cut-off values of the affected side ΔV (0.18ml) and rNet (0.64) showed a sensitivity and specificity of 100% and 77.8%, respectively. Multiparametric flow analysis using 4D flow MRI can detect misery perfusion by comprehensively assessing blood flow data, including blood flow volume, pulsation, and collateral flow.

Using artificial neural network and quadratic algorithm for minimizing entropy generation of Al2O3-EG/W nanofluid flow inside parabolic trough solar collector

Abstract Entropy generation minimization approach, quadratic optimization algorithm and artificial neural network (ANN) have been applied to find optimal condition of the turbulent Al2O3-60:40% EG/W nanofluid flow inside the absorber tube of a parabolic trough solar collector (PTSC). A three-input ANN has been employed for predicting optimal volume fraction ( ϕ opt ). The process is carried out for optimizing nanoparticle concentration, nanoparticle diameter, nanofluid average flow temperature and Reynolds number. Results show that the rate of the entropy generation decreases by decreasing volume fraction, increasing particle diameter and increasing average flow temperature. Adding the nanoparticles to the base-fluid increases frictional entropy generation and decreases thermal entropy generation. It causes an improvement in heat transfer but an increase in viscous irreversibility too. Finally, it was observed that for each particle sizes and average flow temperatures, there is a specific amount of optimal volume fraction, ϕ opt ; which is not dependent on the Re number. There is an optimal volume fraction for all Re numbers at constant particle size and mean flow temperature. Also, the optimum values of nanoparticle size, nanofluid average flow temperature and Reynolds number are found to be 90 nm, 360 K and 4000, respectively.

Topologies of velocity-field stagnation points generated by a single pair of magnets in free-surface electromagnetic experiments.

The velocity fields generated by a static pair of magnets in free-surface electromagnetically forced flows are analyzed for different magnet attitudes, ionic currents, and brine depths. A wide range of laminar velocity fields is obtained despite the forcing simplicity. The velocity fields are classified according to their temporal mean flow topology, which strongly depends on the forcing geometry but barely on its strength, even through the bifurcation to unsteady regimes. The mean flow topology possesses a major influence on the critical Reynolds number Rec under which the steady velocity fields remain stable. The qualitative comparison of the dependence of Rec on the topology is in agreement with previous works. The unsteady configurations evidence the advection of smaller flow structures by the largest scales, commonly known as "sweeping."

Adjoint-Based Linear Analysis in Reduced-Order Thermo-Acoustic Models

This paper presents the linear theory of adjoint equations as applied to thermo-acoustics. The purpose is to describe the mathematical foundations of adjoint equations for linear sensitivity analysis of thermo-acoustic systems, recently developed by Magri and Juniper (J. Fluid Mech. (2013), vol. 719, pp. 183--202). This method is applied pedagogically to a damped oscillator, for which analytical solutions are available, and then for an electrically heated Rijke tube with a mean-flow temperature discontinuity induced by the compact heat source. Passive devices that most affect the growth rate / frequency of the electrical Rijke-tube system are presented, including a discussion about the effect of modelling the mean-flow temperature discontinuity.

Role of Turbulence and Particle Exposure on Entrainment of Large Spherical Particles in Flows with Low Relative Submergence

Laboratory experiments of sediment entrainment were carried out in a flume for different flow configurations with particle image velocimetry (PIV) used to measure the flow velocity. The experiments were done in a low submergence condition. Measurements revealed that the vertical distribution of streamwise flow velocity at the instant of entrainment is significantly different from the temporal mean flow profile. The role of turbulence for a given flow depth is found to decrease with an increase in exposure of the particle. An expression for threshold velocity is proposed that accurately represents the enhanced near-bed turbulence during entrainment. A Shields-type parameter, expressed in terms of critical near-bed flow velocity rather than shear velocity, is used to define the sediment transport threshold.

Surface and Gas Temperature Measurements Along a Film Cooled Wall

Film cooling is widely used to protect critical sections of propulsion devices from extreme heat loads in the internal flowpath. Many studies have been conducted to formulate scaling laws characterizing the effectiveness of film cooling in a variety of configurations for the development of engineering correlations. There is an emerging demand for detailed measurements near film-cooled surfaces to support computational model development of these complex wall-bounded flows. In this study a unique hot wind tunnel facility was used to perform detailed measurements in a canonical two-dimensional thermally stratified wall-jet configuration. Wall temperature, adiabatic effectiveness, and turbulent flow measurements were performed over a range of carefully controlled inlet conditions. Far-field laboratory effectiveness measurements compare well with previously developed scaling laws; however, the effect of the ratio of mainstream to cooling stream temperature is significantly underpredicted. The micro-thermocouple probe used in this study provided mean flow temperatures and detailed turbulent statistics characterizing the nearwall mixing behavior. The mean flow temperature measurements in the far field demonstrate self-similar behavior whereas fluctuating temperatures indicate fully developed turbulence in the wall layer. The turbulent statistics provided in this study are especially useful for development and validation of computational models of film-cooled surfaces.

Velocity and transport of the Makassar Strait throughflow

Analyses of ocean current measurements from two moorings in the Makassar Strait (MAK‐1, December 1996 to July 1998, and MAK‐2, December 1996 to February 1998) with newly processed acoustic Doppler current profiler (ADCP) data provide a new estimate of transport and vertical velocity structure for this important passageway of the Indonesian Throughflow. The 7‐month record of the MAK‐1 ADCP and the 3‐month record of the MAK‐2 ADCP, nominally set at depths of 150 m, are extrapolated to the end of the mooring period using a surface layer relationship to the shallowest current meter at 200 m and to the regional winds. The southward transport within Makassar Strait is confined for the most part to the upper 750 m, above the blocking topographic sill of Makassar Strait. The transport maximum occurs within the thermocline (100–300 m). After the temporal mean flow has been removed, the vertical structure of the along channel flow in the upper 750 m is decomposed using the empirical orthogonal function (EOF) method. The first two modes contain 91% of the total variance. For the entire mooring period, the total depth‐integrated transport was 8.1 ± 1.5 Sv (Sv = 1 × 106 m3/s), with 7.9 ± 1.2 Sv for calendar year1997. During the peak of 1997/1998 El Niño from September 1997 to mid‐February 1998, the first mode time series displays northward flow, enhancing the vertical shear within the thermocline and reducing the mean throughflow to 4.6 ± 0.9 Sv.

Wide-range robust control of combustion instability

This paper presents the concept and design of a two-layer robust control system for suppression of combustion instabilities over a wide range of operation. The control law is synthesized based on a multiple-time-scale model of combustion dynamics. Control actions on the fast-time scale are provided by secondary fuel injection, and are gain-scheduled according to the variations of mean-flow temperature and velocity on the slow-time scale. A linear parameter varying (LPV) L 2-gain control law is formulated in the setting of differential game theory. Simulation experiments have been conducted to evaluate the control law under wide-range operation of a generic combustor in terms of the trade-off among: (1) fuel injection rate and pressure oscillation; (2) transient and steady responses; and (3) stability robustness and performance.

On the Development and Application of the Fast-Response Aerodynamic Probe System in Turbomachines—Part 1: The Measurement System

This contribution gives an overview of the current state, performance, and limitations of the fast-response aerodynamic probe measurement system developed at the Turbomachinery Lab of the ETH Zurich. In particular, the following topics are addressed: • Probe technology: Miniature probes with tip diameter ranging from 0.84 to 1.80 mm (one-sensor and three-sensor probes, respectively) have been developed. New technologies derived from microelectronics and micromechanics have been used to achieve an adequate packaging of the microsensor chips used. Both the sensor packaging and the sensor calibration (time-independent and time-dependent) are crucial issues for the DC accuracy of any measurement. • Aerodynamic probe calibration: The methods used for the sensor calibration and the aerodynamic probe calibration, the pertinent automated test facilities, and the processing of the output data are briefly presented. Since these miniature probes are also capable of measuring the mean flow temperature, aspects related to the effective recovery factor and the self-heating of the probe tip are treated and some recommendations related to sensor selection are given. • Measurement system and data evaluation: The early measurement chain described in Gossweiler et al. (1995) has evolved into the fast-response aerodynamic probe system. This automatic system incorporates dedicated measurement concepts for a higher accuracy and a more efficient operation in terms of time and failures. An overview of the data evaluation process is given. The fast-response aerodynamic probe system has been tested in real-sized turbomachines under industrial conditions within the temperature limits of 140°C imposed by the sensor technology (axial-flow turbofan compressor, axial-flow turbine, centrifugal compressor). These applications confirmed the potential of the system and encouraged its further development. Now, the system is routinely used in the facilities of the Turbomachinery Lab and in occasional measurement campaigns in other laboratories. Part 2 of this contribution (Roduner et al.) will focus on the application of the fast-response aerodynamic probe system in a transonic centrifugal compressor of the ETH Turbomachinery Laboratory, while Part 3 (Ko¨ppel et al.) treats more sophisticated data analysis methods. [S0889-504X(00)01003-5]

Quasi-40-day oscillation and its teleconnection structure together with the possible dependence on conversion of barotropic unstable energy of temporal mean flow

A study is made of the distribution of the diagnostic quantity vector\(\overrightarrow E \) and the teleconnection structure of 30–50 (quasi-40) day oscillation, together with the dependence on the conversion of barotropic unstable energy of mean flow in terms of ECWMF daily 500 hPa grid data in winter, indicating that the energy transportation is closely associated with the westerly jet position, with zonal (meridional) propagation in the strong (weak) wind region, that considerable conversion of barotropic energy occurs at the jet exit region where low-frequency oscillation gains energy from the mean flow, leading to maximum kinetic energy for the oscillation observed there, which is marked by evident barotropy in striking contrast to the baroclinicity at low latitudes and that the teleconnection core is related to the center of action in the atmosphere and bound up with the pattern of the west wind.

Cerebral blood flow velocity: The influence of myocardial contractility on the velocity waveform of brain supplying arteries

Indices of Doppler-derived velocity waveforms of arteries perfusing the brain are used as relative measures of neonatal brain blood flow. Using a dog model, we investigated the influence of changes in myocardial contractility, induced by dobutamine, on the blood flow velocity waveform of the vertebral artery. The following indices of the velocity waveform were investigated during control states and during 5 or 10 μg/kg/min dobutamine infusion: peak systolic flow velocity (PSFV), temporal mean flow velocity (TMFV), end-diastolic flow velocity (EDFV) and acceleration time (ACC-time). PSFV and ACC-time of the vertebral artery showed a strong relationship with myocardial contractile state. These results indicate that PSFV of an artery supplying the brain or indices which combine PSFV with MFV or EDFV should not be used for noninvasive assessment of brain blood flow or cerebral vascular resistance. ACC-time may prove to be very useful in assessing changes in myocardial contractile state.

Cerebral Blood Flow Velocity and Cardiac Output in Infants of Insulin-Dependent Diabetic Mothers

Cardiac output and cerebral blood flow velocity in the anterior cerebral and internal carotid arteries were investigated in eight large-for-date infants of insulin-dependent diabetic mothers and 12 healthy term infants during the first four days of life using two-dimensional/pulsed Doppler ultrasound. Temporal mean flow velocity was used as an indicator of changes in cerebral blood flow. Six of the eight infants of diabetic mothers had ventricular septal hypertrophy with reduced cardiac outputs and stroke volumes. Mean flow velocity in both cerebral vessels showed a comparable pattern in both groups throughout the study period and was independent of mean arterial pressure, suggesting unaltered cerebral hemodynamics in the infants of diabetic mothers.

Indomethacin-induced changes in renal blood flow velocity waveform in premature infants investigated with color Doppler imaging

Renal dysfunction has been recognized as an adverse effect of indomethacin treatment and is probably secondary to impairment of renal blood flow. We therefore evaluated renal artery blood flow velocity in 15 premature infants with a symptomatic ductus arteriosus before and during the first 12 hours after a single intravenous dose of 0.1 mg/kg of indomethacin. Renal artery blood flow velocity was measured serially by color-Doppler flow imaging and used as a qualitative measure of true renal blood flow. Indomethacin administration led to a sharp decrease in peak systolic flow velocity and temporal mean flow velocity of the renal artery. This effect was maximal at 10 minutes after indomethacin dosing; the flow velocities showed a slow recovery, reaching baseline values again at 2 hours after indomethacin dosing. We conclude that indomethacin can affect renal blood supply in the premature infant for a period of at least 1 hour after indomethacin treatment.

Effect of indomethacin on superior mesenteric artery blood flow velocity in preterm infants

In 15 preterm infants with symptomatic patent ductus arteriosus, blood flow velocity changes in the superior mesenteric artery were investigated with Doppler ultrasound just before and during the first 12 hours after a single dose of indomethacin. Indomethacin administration led to an instantaneous decrease in all infants of temporal mean flow velocity in the superior mesenteric artery, which was maximal 10 minutes after administration of indomethacin, followed by a more sustained recovery, slightly greater than baseline values, 12 hours after indomethacin treatment. Simultaneously determined temporal mean flow velocity of the anterior cerebral artery, used as an indicator of changes in cerebral blood flow, had a similar pattern as in the mean flow velocity in the superior mesenteric artery (r = 0.49; p less than 0.001). Our data suggest that indomethacin lowered blood supply to the bowel, similar to its action on cerebral blood flow.

Cerebral Blood Flow Velocity Changes in Preterm Infants After a Single Dose of Indomethacin: Duration of Its Effect

Indomethacin decreases cerebral blood flow velocity and blood flow in the preterm infant. The duration of this negative effect has not been established. Cerebral blood flow velocity was evaluated in 24 preterm infants with symptomatic patent ductus arteriosus before and during the first 12 hours after a single intravenous dose of indomethacin, 0.1 mg/kg. Cerebral blood flow velocity was estimated by serial Doppler investigations of the anterior cerebral arteries. Indomethacin administration led to an instantaneous decrease of peak systolic flow velocity, temporal mean flow velocity, and end-diastolic flow velocity of the anterior cerebral arteries in all infants, which was maximal between 2 and 40 minutes after indomethacin administration and was followed by a more sustained recovery of all velocities to baseline values. Temporal mean flow velocity was not different from pre-indomethacin values at 3 hours after the administration. It is concluded that indomethacin can impact the cerebral circulation of the preterm infant for at least 2 hours. This may have consequences in preterm infants with unstable hemodynamics and pulmonary function.

The behaviour of a steam slug in upward water flow in an isothermal vertical pipe at a reduced pressure.

We investigated the behaviour of a steam slug which was rising in an isothermal vertical pipe at a reduced pressure. When the water temperature around the steam slug was higher than the saturated temperature, the slug grew upon rising in the pipe. The temperature difference between the steam slug and the surrounding water was increased, when water velocity in the pipe was increased or a static pressure to the system was decreased. The increase in velocity of the leading edge of a rising steam slug was caused by the slug growth. But the trailing edge velocity of a steam slug was equal to that of an air slug without growth in water. As the water temperature around the tip of the steam slug was locally higher than the mean flow temperature, it was regarded that the liquid vaporization, which caused the slug growth, occured at the front face of the slug.