Time-averaged Pressure Research Articles

Investigation of the characteristics of the heaving lock-in region of the 10:1 π-shaped section

To gain a comprehensive understanding of the intrinsic characteristics of the lock-in region of the π-shaped section, a study was conducted to investigate the influence of the Angle of Attack (AOA), damping ratio, and mass ratio on the characteristics of the two lock-in regions using a 1:50 segment model test. The Proper Orthogonal Decomposition (POD) method was employed to analyze the characteristics of the two lock-in regions, and the flow field difference between the two lock-in regions was analyzed using Computational Fluid Dynamics (CFD). The findings indicate that both the Angle of Attack (AOA) and mass ratio have comparable effects on the lock-in region. However, it is noteworthy that the AOA appears to exert a more pronounced influence on the second lock-in region, whereas the damping ratio predominantly impacts the amplitude of the lock-in region. The analysis of the distribution and jumping degree of the surface time-averaged pressure enables us to infer the key-action zone number and action position of vortices present around the given section. In the two-lock-in region, there exist two different distribution forms for the energy ratio of POD modes. The first lock-in region is primarily affected by the Karman vortex street phenomenon, whereas the second lock-in region is mainly influenced by the long tail vortex shedding.

Acoustically controlled bubble nucleation

Fundamental studies of single bubble heat transfer are crucial to improve our understanding of boiling phenomena and develop modeling and simulation tools for the design and optimization of boiling systems. However, conducting these kinds of experiments over a wide range of conditions is challenging and may involve expensive and complicated experimental techniques (e.g., lasers) to isolate and control bubble nucleation. In this work, we introduce a new technique to control nucleation with an ultrasonic beam. A low-cost and widely available piezoelectric disc driven at megahertz frequencies directed at a heated surface can increase the time-averaged pressure and suppress evaporation until the moment the transducer is turned off, allowing us to control the incipience of boiling to within 150 µs. We demonstrate this technique by measuring the physical and thermal footprint of natural and acoustically controlled bubbles with high-resolution infrared thermometry and phase detection diagnostics. We were able to raise the bubble nucleation temperature by over 30 K compared to a reference case with no acoustic control, which significantly changes bubble growth rate, microlayer formation and evaporation and departure diameter. We highlight the potential of this new low-cost and easy-to-use technique in a variety of different boiling heat transfer studies.

Fluidization behavior of stirred gas–solid fluidized beds: A combined X-ray and CFD–DEM–IBM study

Stirred gas–solid fluidized bed reactors are commercially employed in polyolefin manufacturing, but the complex gas–solid contacting dynamics pose challenges in design, scale-up, and operation. In this study, the influence of agitation on the fluidization performance of Geldart B particles was investigated experimentally by X-ray imaging and pressure drop measurements and numerically by Computational Fluid Dynamics (CFD) - Discrete Element Method (DEM) - Immersed Boundary Method (IBM). The experimentally obtained minimum fluidization curve and time-averaged pressure drop show good qualitative agreement with the simulation results. Visual observations underscore that an increase in the angular velocity of the agitator results in reduced bubble size and improved bed homogeneity, as further evidenced by reduced pressure fluctuations. Furthermore, the simulations reveal that while the impeller enhances solids agitation, a proper design study is imperative, considering that static immersed bodies, such as the stirrer shaft, can adversely impact solids motion.

Hydraulic Transport of Large Solid Particles in Inclined Pipes Under Pulsating Flow Conditions

Abstract The pressure loss due to the hydraulic transport of large solid particles should be predicted for the design of subsea mining systems. The mixture flow in a flexible jumper is expected to be unsteady during lifting operations in an actual mining system. The authors develop a one-dimensional mathematical model that predicts such pressure loss under pulsating mixture flows in a static inclined pipe assuming that the flow in the jumper is fully developed. An experiment is performed on the hydraulic transport of solid particles to obtain data for model validation. In this experiment, several kinds of solid particles are used: alumina beads, glass beads, and gravel. The experimental parameters are mixture velocity, solid concentration, pulsation period and amplitude of water velocity, and pipe inclination angle. The proposed model is validated through a comparison with experimental data. The validation confirms that the model is applicable for the prediction of the pressure loss in inclined pipes under pulsating flow conditions. Furthermore, we calculate the pressure loss due to the hydraulic transport of polymetallic sulfide ores using the proposed model. The calculation results show that the time-averaged pressure loss drastically varies with the pipe inclination angle, reaching its maximum value between the pipe inclination angles of 30 deg and 60 deg, at which the flow is inclined upward. The results also show that the amplitude of pressure loss pulsation differs little between pipe inclination angles and that the pulsation component of pressure loss should be considered in designing lifting systems.

Noise exposure of the inner ear during robotic drilling.

Preserving the cochlear structures and thus hearing preservation, has become a prominent topic of discussion in cochlear implant (CI) surgery. Various approaches and soft surgical techniques have been described when approaching the inner ear. Robot-assisted cochlear implant surgery (RACIS) reaches the round window in a minimally invasive manner by following a trajectory of minimal trauma. This involves the drilling of a keyhole trajectory to the round window, through the facial recess, with no need for a complete mastoidectomy. It involves less drilling, less drilling time and less structural damage. A lot of attention has been paid to the structural traumatic causes of hearing loss but acoustic trauma during the exposure of the inner ear appears to be neglected topic. The aim was to measure the noise exposure of the inner ear during the robotic drilling of the mastoid and bony overhang of the round window. The results were compared with the milling in conventional cochlear implantation surgery. RACIS on fresh frozen human cadavers. The equivalent frequency-weighted and time-averaged sound pressure level LAF in dB and the noise dose in % derived from a noise damage model, both obtained during RACIS. The robotic drilling of 6 trajectories towards the inner ear were performed, including 4 trajectories through round window access and 2 trajectories through cochleostomy. The results were compared with the data of 7 cases of conventional CI surgery that have been described in literature. The induced equivalent sound pressure level LAF was determined via an accelleration sensor at the zygomatic arch and a calibration according to bone conduction audiometry. A noise dose for the whole procedure was calculated from the equivalent sound pressure level LAF and the exposure time using a noise damage model. A noise dose of 100% is considered a critical exposure limit and values above are considered potentially harmful, with the risk of hearing impairment. The maximum LAF was 82 dB during fiducial screw placement; 87 dB during middle ear access; 95 dB for the accesses through the round window and 88 dB for the accesses through cochleostomy. The noise dose due to the HEARO®-procedure was always far below the critical value of 100%. There was no acoustic trauma of the inner ear in all cases with the noise dose being smaller than 0.1% in five out of the six cases. The maximum LAF in the seven cases of conventional CI surgery was 118 dB with a maximum cumulative noise dose of 172.6%. The critical exposure limit of 100% was exceeded in three cases of conventional CI surgery. RACIS provokes significantly less acoustic trauma than conventional mastoid surgery in our findings. There were no observable differences in noise exposure levels between a cochleostomy or a round window approach where the bony overhang needed to be drilled.

Study on the Fluctuating Load Characteristics of the Submerged Radial Gate in the High-Head Flood Discharge Outlet

The fluctuating pressure acting on the radial gate in the high-head flood discharge outlet is the main excitation source of flow-induced vibration. Therefore, this paper delves into the distribution characteristics of fluctuating pressure on the panel of the high-head submerged radial gate based on hydraulic model tests. Hydraulic tests were first conducted to obtain the distribution patterns of time-averaged pressure and the root mean square (RMS) of fluctuating pressure on the radial gate panel. Secondly, the improved complete ensemble empirical mode decomposition with adaptive noise (ICEEMDAN) and HHT method was employed to identify the causes of the fluctuating pressure on the gate panel. Finally, the ICEEMDAN-SSA (the sparrow search algorithm)–LSTM (long short-term memory) method was utilized to achieve accurate prediction of the fluctuating pressure on the gate panel. The results show that the time-averaged pressure in the middle of the gate panel is higher than that at the top and near the bottom edge, which differs significantly from the static pressure distribution. The RMS of the fluctuating pressure near the bottom edge is higher than that in the middle and at the top. The fluctuating pressure acting on the gate panel in the time domain can be regarded as a stationary process. The fluctuating pressure on the gate panel is caused by the combined diffusion and random mixing of multi-scale vortices in the turbulent eddy structure. The ICEEMDAN-SSA-LSTM combined method significantly improves the prediction accuracy of fluctuating pressure on the gate panel compared to the LSTM and ICEEMDAN-LSTM methods.

Visit-to-Visit Systolic Blood Pressure Variability Independently Predicts Cardiovascular Events in a Kidney Transplant Recipients' Cohort.

The predictive role of blood pressure variability for all-cause mortality and fatal and nonfatal cardiovascular events has been described in the general population and in patients with diabetes, independently of mean BP. Although systolic blood pressure variability has been proposed as an informative measure for predicting clinical outcomes in patients with chronic kidney disease, its role in kidney transplant recipients is still debatable. We performed a retrospective, observational, monocentric analysis of all kidney transplant recipients in follow-up at the outpatient Nephrology Clinic of San Martino Hospital from January 1, 2016 to December 31, 2016, who underwent kidney transplantation >12 months. The primary outcome was a fatal or nonfatal cardiovascular event (myocardial infarction, unstable angina, stroke, and hospitalization for heart failure). Visit-to-visit systolic blood pressure variability was expressed as the SD of systolic blood pressure values recorded at baseline and 3 months up to 18 months. Among the 272 patients (mean age, 64±13; 63% men) included in the present analyses, for each increase of 2.7 mm Hg in systolic blood pressure SD, the risk for events increased 3-fold (hazard ratio [HR], 3.1 [95% CI, 1.19-7.88]; P=0.02), and patients in the highest tertile of systolic blood pressure SD showed a 4-fold increased risk (HR, 4.1 [95% CI, 1.34-12.43]; P=0.01). This relationship was maintained even after incremental adjustment for time-averaged pulse pressure, age, diabetes, and prior cardiovascular event (HR, 3.2 [95% CI, 1.1-10.0]; P=0.04). Long-term blood pressure variability represents a risk factor for cardiovascular events in kidney transplant recipients, even independently by several confounding factors including blood pressure load.

Reconstruction Of Time-Averaged 3D Pressure Fields In The Wake Of An Ahmed Body With Pressure From PIV

In this contribution, we present and compare time-averaged pressure fields reconstructed from scanned stereoscopic 2D3C-PIV data and volumetric 3D-PTV data (Shake-The-Box) with classic pointwise surface pressure tap measurements. Measurements were conducted in the near-wake region of an one-quarter scaled Ahmed body (Ahmed 1984) with 25° slant angle at height-based Reynolds numbers of Re = 5.06 · 10^4 and 1.13 · 10^5, respectively, i.e. free-stream velocities of 10.5 m/s and 23.5 m/s. The measurements are a continuation of previous measurements (Gericke et al. 2023, Ladwig et al. 2023) with a focus on the more complex flow topology concerning the wake flow structure. Measurements were conducted in a low-speed wind tunnel of Göttinger design with a semi-closed test section at Bochum University of Applied Sciences. The investigated flow field in the near wake region is dominantly characterized by a strongly three-dimensional flow field based on a mutually influenced vortex system in the wake, which challenges the utilized pressure reconstruction algorithm based on the Reynolds-averaged Navier-Stokes equations. The present study addresses challenges in pressure reconstruction due to the presence of highly transient rotational and shearing flow in the context of a practical application. The results show good agreement between reconstructed PIV/PTV-based pressure fields and reference pressure measurements using classic wall pressure measurements. The global characteristics of the pressure distributions are correctly reproduced. The agreement is particularly good in areas of unidirectional flow and local smaller velocity gradients. In shear zones as well as separation and, e.g., recirculation areas, larger deviations are clearly visible, as the velocity gradients present near the surface are not sufficiently captured at all.

Unsteady wake and heat transfer characteristics of three tandem circular cylinders in forced and mixed convection flows

In natural convection (high Richardson number Ri), a high Prandtl number (Pr) leads to thinner thermal boundary layers, enlarging the thermal gradient and hence the enhancement of buoyancy effect. In forced convection (low Ri), a high Pr introduces thicker velocity boundary layers. In mixed convection scenarios, where both forced and natural convection are significant, the interaction between Pr and Ri determines the resultant flow pattern and heat transfer characteristic. Three tandem circular cylinders with an identical spacing ratio of 4.0 in both forced and mixed convection flows were numerically investigated by using finite element method. The computations were carried out in the range of Pr = 5–50 and Ri = 0–2 at a low Reynolds number of Re = 150. The results of the squared strain rate and the vorticity shed light on the enstrophy transfer process. Thermal plume structures in the far wake originate from the upper dispersed vortices due to the high superimposed buoyancy at low Pr, while they are suppressed at high Pr. The increase in Pr plays a role as the flow stabilization, while the growth of Ri plays the reverse role. The time-averaged velocity, pressure coefficient, and temperature become more asymmetrical at high Ri. The Nusselt number of the upstream cylinder is approximately equal to the empirical result without the consideration of thermal buoyancy. Due to the thermal buoyancy, the migration of shear layers along the cylinder surface leads to the frequency alteration and harmonic frequency in the drag, lift, and Nusselt coefficients.

Wake transitions of flow past two tandem semi-circular cylinders near a moving wall

Wake transitions of flow past two tandem semi-circular cylinders near a moving wall were numerically investigated using the lattice Boltzmann method at the Reynolds number of 150 with various gap ratios (G/D, where G and D are the spacing between the cylinder and wall and the cylinder diameter, respectively) and spacing ratios (L/D, where L is the distance between the cylinder centers). The analysis aims to clarify the effects of L/D and G/D on wake structures, hydrodynamic forces, Strouhal number, and spectral energy of the flow exerted on both cylinders. This study reveals five distinct flow regimes in the L/D-G/D space, such as overshoot, continuous reattachment, pair-wise, quasi-coshedding, and coshedding. These regimes are identified using plots of vorticity contour, time history of drag and lift coefficients (CD and CL), power spectral density, and proper orthogonal decomposition of the vorticity fluctuation into deterministic spatial structures. Furthermore, the flow regime maps and diagrams of time-averaged pressure coefficients on the surface of the cylinders are given to analyze the influence of the moving wall. A significant change in CD and CL is observed for both cylinders, depending on the L/D and G/D ratios. The time-averaged drag coefficient of the downstream cylinder is remarkably lower than that of the upstream cylinder. A significant increase in the time-averaged lift coefficient of the upstream cylinder is observed when the G/D is small due to near-moving wall effects. The root-mean-squared value of the lift and drag coefficients of the downstream cylinder is lower than that of the upstream cylinder as a result of the proximity effect. Meanwhile, the Strouhal number for both cylinders remains mostly the same.

Transient spray combustion characteristics in a gas–liquid pintle rocket engine under acoustic excitation

In order to investigate the acoustic oscillation characteristics of gas–liquid pintle rocket engines and elucidate the path by which spray combustion process provides energy to the combustor pressure oscillation, a LOX/GCH4 pintle engine with rectangular combustor was designed. By adding transverse velocity disturbance for the first time, the acoustic response of spray combustion process was simulated, and the effect of excitation amplitude on acoustic response was researched. Numerical results show that the adopted transverse velocity disturbance can excite the first-order transverse acoustic oscillation with same excitation frequency in the engine combustor. The acoustic response maintenance mechanism under extrinsic excitation is summarized for pintle engines. Besides, the temperature distribution inside the engine combustor tends to be uniform, and the low-frequency oscillation caused by the flame transverse swing gradually disappears. The amplitude of combustor pressure oscillation is dominated by excitement amplitude and phase difference between the pressure and heat release in combustion reaction region. In addition, the time-averaged combustor pressure can be amplified mainly by transverse velocity disturbance. The research work can provide a reference for related fire tests on the acoustic response of a subscale gas–liquid pintle engine.

Alleviation of SWBLI over the payload of a launch vehicle by change of nose shape

Wind tunnel tests on a typical heavy-lift launch vehicle configuration featuring a core vehicle flanked by two strap-on boosters showed very high levels of unsteady pressures over the payload region at transonic Mach numbers when the nose had a cone-cylinder shape. This was due to very high levels of Shock Wave Boundary Layer Interactions (SWBLI) associated with the conical shape, which caused alternating flow featuring the formation of a unsteady λ-shock system over the payload region and a pair of counter-rotating vortices. The phenomenon was found to strongly depend on the proximity of the noses of the strap-on boosters to the boat tail downstream of the payload region. When the conical nose was replaced by an ogive-shaped nose, significant reduction in SWBLI occurred, with dramatic reduction of shock oscillations, pressure fluctuations, replacement of alternating flow by steady flow and disappearance of the vortex-pair. Time-averaged pressure fluctuations indicate reduction in pressure gradients due to a more gradual expansion of flow over the ogive nose compared to conical nose, an observation also supported by CFD simulations. The alleviation of SWBLI with ogive nose was so effective that the presence or absence of strap-on boosters had practically no effect on pressure fluctuations over the payload region. Unlike the conical shape, the ogive nose cone shape seems to meet both the shape and pressure parameter-criteria of NASA to ensure buffet-free environment.

#2251 Long-term systolic blood pressure variability as an independent risk factor for cardiovascular events in kidney transplant recipients

Abstract Background and Aims Cardiovascular (CV) events represent the main cause of death in patients undergoing renal transplantation. Several studies have described the role of blood pressure variability (BPV) in predicting all-cause mortality and fatal and non-fatal CV events in the general population and in diabetic patients. Systolic blood pressure variability (SBPV) has recently been proposed as a predictor of clinical outcomes in patients with chronic kidney disease. However, evidence is still scarce about the prognostic role of BPV in the setting of renal transplantation. The aim of the study was to evaluate the role of long-term SBPV in the development of fatal and non-fatal CV events (myocardial infarction, unstable angina or revascularization, stroke and hospitalization for heart failure) in subjects undergoing kidney transplantation. Method We performed a single-center retrospective observational analysis in a cohort of 272 kidney transplant patients (mean age 64 ± 13 years, 66% men), with a transplant duration ≥12 months and with at least 6 blood pressure measurements at defined intervals (3 months ±15 days) over an assessment period of 18 months. BPV was expressed as standard deviation of ambulatory systolic blood pressure (SBP) and patients were accordingly classified in tertiles (low <9.3 mmHg, medium 9.3-13 mmHg, high >13 mmHg). The role of SBPV was analyzed by Kaplan-Meier (logrank) and its independent effect by Cox Regression. Results Over a mean follow-up of 58.7 ± 17.2 months, 27 subjects (9.9%) developed CV events. For each increase of 2.7 mmHg in SBP standard deviation, the risk for fatal or non-fatal CV events increased almost 3-folds (P = 0.02) and patients in the highest tertile of SBP standard deviation showed a 4-fold increased risk (P = 0.01) (Fig. 1). This relationship was maintained even after incremental adjustment for time-averaged pulse pressure, age, diabetes, prior CV event, and dialysis vintage (HR 3.2, 95% CI 1.1–10.0, p 0.04) (Table 1). Conclusion Long-term BPV represents an independent risk factor for CV events in kidney transplant patients. It is therefore essential to seek for stable long-term blood pressure levels in this population in order to minimize the residual CV risk.

LONG-TERM SYSTOLIC BLOOD PRESSURE VARIABILITY AS AN INDEPENDENT RISK FACTOR FOR CARDIOVASCULAR EVENTS IN KIDNEY TRANSPLANT RECIPIENTS

Objective: Cardiovascular (CV) events represent the main cause of death in patients undergoing renal transplantation. Several studies have described the role of blood pressure variability (BPV) in predicting all-cause mortality and fatal and non-fatal CV events in the general population and in diabetic patients. Systolic blood pressure variability (SBPV) has recently been proposed as a predictor of clinical outcomes in patients with chronic kidney disease. However, evidence is still scarce about the prognostic role of BPV in the setting of renal transplantation. The aim of our study was to evaluate the role of long-term SBPV in the development of fatal and non-fatal CV events (myocardial infarction, unstable angina or revascularization, stroke and hospitalization for heart failure) in subjects undergoing kidney transplantation. Design and method: We performed a single-center retrospective observational analysis in a cohort of 272 kidney transplant patients (mean age 64±13 years, 66% men), with a transplant duration beyond 12 months and with at least 6 blood pressure measurements at defined intervals (3 months ± 15 days) over an assessment period of 18 months. BPV was expressed as standard deviation of ambulatory systolic blood pressure (SBP) and patients were accordingly classified in tertiles (low <9.3 mmHg, medium 9.3-13 mmHg, high >13 mmHg). The role of SBPV was analyzed by Kaplan-Meier (logrank) and its independent effect by Cox Regression. Results: Over a mean follow-up of 58.7±17.2 months, 27 subjects (9.9%) developed CV events. For each increase of 2.7 mmHg in SBP standard deviation, the risk for fatal or non-fatal CV events increased almost 3-folds (P=0.02) and patients in the highest tertile of SBP standard deviation showed a 4-fold increased risk (P= 0.01) (Figure 1). This relationship was maintained even after incremental adjustment for time-averaged pulse pressure, age, diabetes, prior CV event, and dialysis vintage (HR 3.2, 95% CI 1.1–10.0, p 0.04) (Table 1). Conclusions: Long-term BPV represents an independent risk factor for CV events in kidney transplant patients. It is therefore essential to seek for stable long-term blood pressure levels in this population in order to minimize the residual CV risk.

Fully developed pipeline flow fast simulation model and application to a rod bundle subchannel

For the fully developed pipeline flow, if the radial time-averaged velocity and radial pressure gradient can be ignored, the time-averaged Navier-Stokes equations can be simplified into a diffusion equation. Based on this, the Fully Developed Model (FDM) was proposed. The adiabatic FDM was applied to the simulation of the flow field in a rod bundle subchannel and got results consistent with that from the commercial CFD code. The short time calculation of FDM makes its application in system analysis codes becoming possible.

Effect of Strouhal number on propulsion of tandem flapping foils

Motivated by the energy efficient locomotion by flock of birds and school of fish to develop biomimetic systems, the current study studies the propulsive performance of the tandem configuration of two-foil system. The effects of the kinematic motion parameters such as heave and pitch amplitudes and flapping frequency on the propulsive performance of the tandem two-foil system is yet to be explored comprehensively due to the complex nature of the wake-foil interaction. Here, we numerically investigate these effects by utilizing a finite element based moving mesh framework to model the flapping foil motion subjected to fluid flow. The performance of the upstream foil is found to be similar to that of an isolated single foil. However, for the downstream foil, a beating phenomenon is observed in the thrust response when the flapping frequency is different for both the foils. This feature can help tune propulsive performance of the foil system. Furthermore, through a control volume analysis, we show that sole reliance on the time-averaged streamwise velocity may not yield correct trends for the mean thrust forces, which has been typically followed in the literature. Variation in the time-averaged pressure in the wake is significant in predicting correct thrust forces.

In silico parametric analysis of femoro-jugular venovenous ECMO and return cannula dynamics

Background: Increasingly, computational fluid dynamics (CFD) is helping explore the impact of variables like: cannula design/size/position/flow rate and patient physiology on venovenous (VV) extracorporeal membrane oxygenation (ECMO). Here we use a CFD model to determine what role cardiac output (CO) plays and to analyse return cannula dynamics. Methods: Using a patient-averaged model of the right atrium and venae cava, we virtually inserted a 19Fr return cannula and a 25Fr drainage cannula. Running large eddy simulations, we assessed cardiac output at: 3.5–6.5 L/min and ECMO flow rate at: 2–6 L/min. We analysed recirculation fraction (Rf), time-averaged wall shear stress (TAWSS), pressure, velocity, and turbulent kinetic energy (TKE) and extracorporeal flow fraction (EFF = ECMO flow rate/CO). Results: Increased ECMO flow rate and decreased CO (high EFF) led to increased Rf (R = 0.98, log fit). Negative pressures developed in the venae cavae at low CO and high ECMO flow (high CR). Mean return cannula TAWSS was >10 Pa for all ECMO flow rates, with majority of the flow exiting the tip (94.0–95.8 %). Conclusions: Our results underpin the strong impact of CO on VV ECMO. A simple metric like EFF, once supported by clinical data, might help predict Rf for a patient at a given ECMO flow rate. The return cannula imparts high shear stresses on the blood, largely a result of the internal diameter.

An Optimal Numerical Strategy for Intake in Crosswind Conditions

Abstract Crosswind can reduce the operability of an aeroengine significantly at static or near static operating conditions. Computational fluid dynamics predictions of the flow at crosswind conditions will play an important part in future designs, however, accurate numerical predictions of the flow within the intake remain challenging even for simulations of cases with intake only. The main objective of this paper is to demonstrate the importance of numerical setup and to determine an optimal computational model for crosswind investigations that can be used by other researchers. By considering the flow to be inherently unsteady, the influence of inlet and exit boundary conditions, and grid sensitivity is studied by using unsteady Reynolds-average Navier–Stokes (URANS) simulations. Numerical predictions of the time-averaged intake pressure recovery (IPR) and the motion of the vortex on the ground are compared against the existing experimental data. The results show that for an intake under crosswind, the ground vortex that forms under the intake and the in-duct separation, when present, exhibit unsteady behavior that become stronger as the crosswind velocity is increased. The steady-state simulation is only representative at lower crosswinds. The intake flow separation and ground vortex predictions are influenced by the inlet boundary layer profiles. Moreover, acoustic reflection was observed at the intake exit boundary which propagates upstream, creating artificial unsteady frequencies in IPR. The reflection is generated from the use of uniform boundary conditions at the intake exit and is not dissipated in the grid due to the long wavelength; this can be mitigated by using a choked nozzle at the intake exit. Acoustic reflection was also observed at the far-field exit boundary. These reflections are caused from interaction of trailing vortex and far-field boundary.

Rotating detonation combustor performance informed through a novel megahertz-rate stagnation pressure measurement

An experimental stagnation pressure measurement technique is presented for a rotating detonation combustor (RDC). Schlieren imaging enables rotating detonation wave passage to be correlated with oscillations observed in the under-expanded exhaust plume. By measuring the spatiotemporal variation in exhaust plume divergence angle, stagnation pressure measurements of the RDC were acquired at a rate of 1 MHz. Combustor mass flux was varied between 202 and 783 kg/m2s, producing equivalent available pressures (EAPs) in the range of 3.42–13.5 bar. Time-averaged stagnation pressure measurements gathered using this technique were in agreement with the measured EAP within ±1.5%. Time-resolved stagnation pressure measurements allow for the pressure ratio produced across detonation wave cycles to be determined. For the conditions tested, detonation pressure ratios and wave speeds decreased while increasing the mean operating pressure of the combustor. Numerical modeling of the conditions tested indicates that the decrease in pressure ratio and wave speed is a result of elevated levels of combustion prior to the detonation wave arrival (i.e., “preburning”). Simultaneous OH* chemiluminescence measurements within the combustion chamber show an increase in preburned heat release relative to detonative heat release for increasing operating pressures of the RDC, in agreement with the results of the numerical model. Modeled chemical kinetic timescales decrease by approximately the same magnitude by which the preburning mass fraction increased in the range of operating pressures tested, suggesting that the faster reaction rates associated with higher pressure combustion may be the reason for increased preburning within the combustor.

An in vitro experimental investigation of oscillatory flow in the cerebral aqueduct

This in vitro study aims at clarifying the relation between the oscillatory flow of cerebrospinal fluid (CSF) in the cerebral aqueduct, a narrow conduit connecting the third and fourth ventricles, and the corresponding interventricular pressure difference. Dimensional analysis is used in designing an anatomically correct scaled model of the aqueduct flow, with physical similarity maintained by adjusting the flow frequency and the properties of the working fluid. The time-varying pressure difference across the aqueduct corresponding to a given oscillatory flow rate is measured in parametric ranges covering the range of flow conditions commonly encountered in healthy subjects. Parametric dependences are delineated for the time-averaged pressure fluctuations and for the phase lag between the transaqueductal pressure difference and the flow rate, both having clinical relevance. The results are validated through comparisons with predictions obtained with a previously derived computational model. The parametric quantification in this study enables the derivation of a simple formula for the relation between the transaqueductal pressure and the stroke volume. This relationship can be useful in the quantification of transmantle pressure differences based on non-invasive magnetic-resonance-velocimetry measurements of aqueduct flow for investigation of CSF-related disorders.