Time Derivative Of Pressure Research Articles

Effects of hydrogen addition on the propagation and autoignition of methane/oxygen/inert mixtures under engine-relevant conditions

The confined spherical expanding flame method was used to measure laminar flame speeds and ignition delay times of blends of methane and hydrogen at engine-relevant conditions. Laminar flame speeds were measured for hydrogen addition to methane of up to 40 % on a molar basis, for pressures ranging from 8 to 30 atm, and for unburned mixture temperatures ranging from 420 to 530 K. It was found that the laminar flame speed increases with the hydrogen addition, as expected, and that the trend of the data is consistent with predictions resulting from using recently developed kinetic models. For properly chosen conditions, autoignition of the end gas was induced during the compression stage of the outward flame propagation, and ignition delay times were determined using the rapid changes in the pressure-time derivatives. The autoignition experiments were carried out for blends of methane, n-pentane, and hydrogen. The use of n-pentane was essential in order to achieve autoignition within the time scales available in the existing spherical facility. In the presence of n-pentane, a two-stage ignition behavior was observed, and the addition of hydrogen was found to reduce the ignition delay times. All data were modeled, and insight was provided into the mechanisms controlling flame propagation and the two stages of ignition. These results are expected to be of direct relevance to ongoing efforts in hydrogen utilization toward the decarbonization of the transportation sector.

Ongoing analysis of far-field acoustical measurements during the Artemis-I launch

As the global space industry expands, impacts from super heavy-lift launch vehicle noise on payloads, communities, and natural habitats are better understood with improved source models. To support model development, this paper discusses ongoing analyses of far-field acoustical measurements made during the NASA Space Launch System (SLS) Artemis-I mission. Fifteen acoustical measurement stations were deployed prior to the launch, including ten autonomous stations within Kennedy Space Center and five manned stations off-Center, up to 50 km from the pad. This paper presents a brief summary of data analyzed from all 15 stations, including A-weighted and unweighted sound pressure levels and spectra. Additionally, the skewness of the pressure time derivative is shown as a quantifier of the noise’s crackling component. Finally, source directivity and sound power analyses are presented. [Work supported in part by the NSF REU Program and the Utah Space Grant Consortium.]

Error detection and filtering of incompressible flow simulations for aeroacoustic predictions of human voice.

The presented filtering technique is proposed to detect errors and correct outliers inside the acoustic sources, respectively, the first time derivative of the incompressible pressure obtained from large eddy simulations with prescribed vocal fold motion using overlay mesh methods. Regarding the perturbed convective wave equation, the time derivative of the incompressible pressure is the primary sound source in the human phonation process. However, the incompressible pressure can be erroneous and have outliers when fulfilling the divergence-free constraint of the velocity field. This error is primarily occurring for non-conserving prescribed vocal fold motions. Therefore, the method based on a continuous stationary random process was designed to detect rare events in the time derivative of the pressure. The detected events are then localized and treated by a defined window function to increase their probability. As a consequence, the data quality of the non-linearly filtered data is enhanced significantly. Furthermore, the proposed method can also be used to assess convergence of the aeroacoustic source terms, and detect regions and time intervals, which show a non-converging behavior by an impulse-like structure.

On the possibility of nonlinear reflections in shock-containing noise near a high-performance military aircraft

The skewness of the pressure time derivative, or derivative skewness, is a useful indicator of crackle perception in jet noise. Near a military aircraft, derivative skewness values are greater at ground-based measurements relative to nearby off-ground locations. As proposed in Vaughn et al. [J. Acoust. Soc. Am. 149(4), 2403–2414 (2021)], nonlinear reflections are a possible explanation for the increased derivative skewness values at the ground. For an ensemble of windowed shock events, the propagation angle, apparent source location, and angle of incidence relative to the ground are estimated using a two-point cross correlation. The shocks are then classified into shock reflection regimes based on their combination of angle of incidence and measured shock strength. A significant number of shocks measured along the ground array fall in the nonlinear reflection regime. A nonlinear shock reflection results in a pressure increase at the shock greater than twice the free-field pressure, which causes an increase in the derivative skewness value. Therefore, measurements at the ground may overpredict the crackle perception of a standing observer.

Evidence for nonlinear reflections in shock-containing noise near high-performance military aircraft

Skewness values for the pressure time derivative are greater at ground-based measurements near a tactical aircraft than they are at nearby off-ground locations. A possible explanation for this phenomenon is the occurrence of nonlinear, irregular shock reflections at the ground. Propagation angle, source location, and corresponding angle of incidence relative to the ground are estimated using a two-point cross correlation of windowed shock events. Nonlinear reflections are likely to occur based on the combination of angles of incidence and measured shock strengths and cause a pressure increase at the shock that is greater than twice the free-field pressure. The associated pressure increase at the shocks appears to enhance shock-related metrics at the ground compared to off-ground locations.

Decoupled nonlinear adaptive control of position and stiffness for pneumatic soft robots

This article addresses the problem of simultaneous and robust closed-loop control of joint stiffness and position, for a class of antagonistically actuated pneumatic soft robots with rigid links and compliant joints. By introducing a first-order dynamic equation for the stiffness variable and using the additional control degree of freedom, embedded in the null space of the pneumatic actuator matrix, an innovative control approach is introduced comprising an adaptive compensator and a dynamic decoupler. The proposed solution builds upon existing adaptive control theory and provides a technique for closing the loop on joint stiffness in pneumatic variable stiffness actuators. Under a very mild assumption involving the inertia and actuator matrices, the solution is able to cope with uncertainties of the model and, when the desired stiffness is constant or slowly varying, also of the pneumatic actuator. Position and stiffness decoupling is achieved by the introduction of a first-order differential equation for an internal state variable of the controller, which takes into account the time derivative of pressure in the stiffness dynamics. A formal proof of the stability of the position and stiffness tracking errors is provided. An appealing property of the approach is that it does not require higher derivatives of position or any derivatives of stiffness. The solution is validated with respect to several use-cases, first in simulation and then via a real pneumatic soft robot with McKibben muscles. A comparison with respect to existing techniques reveals a more robust position and stiffness tracking skill.

Prediction of mechanical-hysteresis behavior and complex moduli using the phase field crystal method with modified pressure controlled dynamic equation

Damping materials have been used in numerous engineering applications. The important property that plays a key role in this type of material is a damping capacity which is related to mechanical-hysteresis behavior of viscoelasticity. During the last decade, the phase-field-crystal (PFC) model has emerged as a robust tool to predict various material phenomena. This density-functional-type model has the advantage over a conventional phase-field model in terms of atomistic resolution and molecular dynamics in terms of computational expense. In this work, we propose a method to predict mechanical-hysteresis behavior and its related complex moduli parameters using a modified-pressure-controlled dynamics (MPCD) equation incorporating with the PFC model, denoted as PFC-MPCD method, in a three-dimensional solid structure. We modify the previously proposed pressure-controlled dynamics (PCD) equation by introducing the pressure-time derivative term which allows us to produce the results consistent with the standard linear solid model (SLS) at the broader frequency range. We apply sinusoidal pressure oscillation and compare the results predicted by both models. The results demonstrate that mechanical-hysteresis behavior and complex moduli parameters predicted by PFC-MPCD method are in good agreement with those of SLS model and consistent with numerous experimental observations whereas the results produced by original PCD equation tends to exhibit inaccurate results at the very frequency. We expect that this new PFC-MPCD method can be extended to a more complex system and still be capable to exhibit the accurate mechanical-hysteresis behavior and complex moduli parameters which results in predicting more reliable damping capacity parameter in damping material design.

Radius of Investigation in Pressure Transient Testing

The spatial domain of interpretation in pressure transient and formation testing is constrained by its radius of investigation. A classical descriptor for this radius relies on the time derivative of pressure within a bounded domain becoming time independent. Another approach quantifies the radius of investigation based on the distance at which the pressure change is greater than the characteristic noise or resolution in the measurement system. In this paper, we postulate that the correct measure of radius of investigation is the distance at which the inversion of noisy test data is able to discern an anomaly within a given tolerance. This paper details the design requirements for a testing and measurement system suitable for a given investigation distance, measurement noise, and the confidence requirements for the inferred distance. The approach is demonstrated with two specific models. The first is a formation with a sealing fault in an infinite medium, and the second is a formation with a single-layer boundary vertically displaced from a production probe. With normally distributed random and correlated noise, computed results show that the measure of distance investigated is highly dependent upon the background over which the anomaly is imposed. Although the methodology is illustrated with particular models, the principles are applicable in general.

Large-break LOCA analysis with modified boiling heat-transfer model in TRACE code

Numerical analyses were conducted to replicate several tests for simulating a double-ended cold-leg large break loss-of-coolant accident (LBLOCA) in the Loss-of-Fluid Test (LOFT) using the TRACE (version 5/patch level 4) code. Analytical results by the original TRACE code were so conservative that especially a first peak of cladding temperature was estimated higher than the experimental data at the blowdown phase and subsequent temperature drop corresponding to the temporal quench was not seen. We were interested in minimum film boiling temperature (Tmin) as a heat transfer model factor estimating the quench at the moment, investigated correlation equations for Tmin in previous studies and especially focused on ones given as a function of coolant mass flow because the complicated flow transient and decompression in the core region at the blowdown phase was interpreted as having an influence on the cladding temperature behavior. There are several correlations meeting the above condition but it was revealed that they are insufficient to apply for high pressure especially. Therefore, a new term including an effect of mass flow flux and time derivative of pressure was defined and added with a proportional coefficient hypothetically to the current correlation in the TRACE code for modification. The LOFT analyses were conducted again using the modified TRACE code, and it was shown by applying roughly the same proportional coefficient to all the cases of LOFT analyses that estimation of the cladding temperature behavior was improved more precisely at the blowdown phase. Also, the transition during the phase was explained phenomenologically with the wall heat transfer mode and boiling curve.

Unified solver for fluid dynamics and aeroacoustics in isentropic gas flows

The high computational cost of solving numerically the fully compressible Navier–Stokes equations, together with the poor performance of most numerical formulations for compressible flow in the low Mach number regime, has led to the necessity for more affordable numerical models for Computational Aeroacoustics. For low Mach number subsonic flows with neither shocks nor thermal coupling, both flow dynamics and wave propagation can be considered isentropic. Therefore, a joint isentropic formulation for flow and aeroacoustics can be devised which avoids the need for segregating flow and acoustic scales. Under these assumptions density and pressure fluctuations are directly proportional, and a two field velocity-pressure compressible formulation can be derived as an extension of an incompressible solver. Moreover, the linear system of equations which arises from the proposed isentropic formulation is better conditioned than the homologous incompressible one due to the presence of a pressure time derivative. Similarly to other compressible formulations the prescription of boundary conditions will have to deal with the backscattering of acoustic waves. In this sense, a separated imposition of boundary conditions for flow and acoustic scales which allows the evacuation of waves through Dirichlet boundaries without using any tailored damping model will be presented.

Handling of spatially truncated source fields based on pressure time derivatives in hybrid computational aeroacoustics

In hybrid computational aeroacoustics, the acoustic computation domain is oftentimes larger than the domain of the flow simulation. Consequently, discontinuities of the source field occur on the acoustic mesh, which can diminish the quality of the results. A common way to handle truncated source fields is spatial windowing. The present work focuses on aeroacoustic approaches that use time derivatives of the incompressible base flow pressure as acoustic source terms and their distinctive character with respect to source domain truncation. Especially for those approaches, a novel alternative to spatial windowing is proposed, which uses a simplified pressure field continuation beyond the source domain. Numerical results display that spatial windowing and simplified pressure field continuation can both treat truncated source fields effectively. The windowing approach is computationally cheaper but exhibits issues for small flow computation domains. The simplified pressure field continuation can overcome these and other issues yielding very good results while adding little computational cost.

Right Heart Pressure Transients and Pulmonary Venous Pressure

tricular (RV) ejection phase indices were limited in part because the complex shape and geometry of the right ventricle limited any reasonable calculation of volume and wall stress. Likewise, methods to assess the RV ejection fraction were in their infancy. By contrast, RV and pulmonary artery (PA) pressure measurements were relatively easy to obtain and it was thought that RV pressure transients and the isovolumic indices of contractility might be applied to the assessment of RV and perhaps even global cardiac contractility. This idea matured during deliberations about the independent function of the right ventricle and the fact that RV failure occurring secondary to LV failure was not easily explained on the basis of the relatively small pressure load imposed by elevated pulmonary venous pressures. Recognizing the distinct architectural and anatomic differences between the right and left ventricles, further discussions led to considerations of ventricular interaction and the possibility that the normal left ventricle directly assists the right ventricle in its pumping function while the dysfunctional left ventricle was limited in this regard. In the early 1970s, cardiologists exhibited a considerable interest in the relative merits of the isovolumic and ejection phase indices of left ventricular (LV) contractility. The isovolumic indices were derived from high-fidelity measurements of LV pressure prior to aortic valve opening; thus, LV pressure and the time derivative of pressure (dP/dt) were used to calculate ‘contractile element’ velocity during the early systole when ventricular volume was nearly constant. Because the pressure transients occurred prior to aortic valve opening, the isovolumic indices were considered to be independent of changes in aortic pressure and LV afterload. The ejection phase indices were derived from measurements of LV dimensions or volume during ejection; thus, dimension or volume strain (ejection fraction) and strain rates were determined. With the incorporation of pressure data, wall stresses were calculated, myocardial length-force-shortening relations were plotted, and afterload-ejection fraction assessed [1–4] . During this period, interest in the right ventricle was limited, mainly due to technical reasons. The right venReceived: April 28, 2016 Accepted after revision: May 9, 2016 Published online: June 10, 2016

Effect of Stagger on the Vibroacoustic Loads from Clustered Rockets

The effect of stagger startup on the vibroacoustic loads that form during the end-effects regime of clustered rockets is studied using both full-scale (hot-gas) and laboratory-scale (cold-gas) data with vehicle geometry. Both configurations comprise three nozzles with thrust-optimized parabolic contours that undergo free-shock separated flow and restricted-shock separated flow as well as an end-effects regime before flowing full. Acoustic pressure waveforms recorded at the base of the nozzle cluster are analyzed using various statistical metrics as well as time-frequency analysis. The findings reveal a significant reduction in end-effects regime loads when engine startups are staggered. However, regardless of stagger, both the skewness and kurtosis of the acoustic pressure time derivative elevate to the same levels, thereby demonstrating the intermittence and impulsiveness of the acoustic waveforms during the end-effects regime.

Thermoacoustic modeling and uncertainty analysis of two-dimensional conductive membranes

A model for two-dimensional graphene-based thermoacoustic membranes is investigated analytically and numerically validated using Bayesian statistics in this study. The temperature and the pressure variables are first analytically determined in one-dimension by noticing that the magnitude of the pressure time derivative is small in the heat transfer equations and by taking advantage of the large disparity between the length scales. The one-dimensional findings are then extended to three-dimensions, where pressure fluctuation produced by the surface temperature variation is determined using an acoustic piston model. Through the one and three-dimensional model analysis, the dependence of acoustic pressure as a function of frequency is studied. The acoustic response with respect to the frequency shows different characteristics when assuming Dirichlet (temperature) or Neumann (heat flux) boundary conditions. The thermoacoustic model is validated with a graphene-on-paper loudspeaker using Bayesian statistical methods and a Delayed Rejection Adaptive Metropolis algorithm to identify model parameters and their uncertainty. The findings provide insights into the heat transport mechanisms associated with sound generation from thermally cycling thin conductive membranes at high frequencies.

Efficient and non-reflecting far-field boundary conditions for incompressible flow calculations

Traditionally, the artificial compressibility (AC) method of Chorin is used for simulation of low-speed and incompressible flows. In this method, the difficulty of continuity and momentum equations decoupling is removed by adding an artificial time derivative of pressure to the continuity equation. For the first time, a fully two-dimensional upwind scheme was presented for AC equations by using characteristic structure of equations by the authors. In this paper, a new remote boundary calculation method is presented by using the idea of characteristics for these equations. Instead of simple boundary conditions which usually are employed for incompressible flows, the flow quantities at the far-field boundaries are evaluated by compatibility relations of characteristic equations. This is implemented by assuming a row of ghost cells outside of the far-field boundary and using the flux calculation method based on characteristics similar to the inside of computational domain. The idea in conjunction with multidimensional characteristic based scheme was tested for incompressible flow around circular cylinder in comparison with conventional far-field boundary condition and showed good improvements in the terms of accuracy and convergence speed.

Large eddy simulation of crackle noise in hot supersonic jets

Crackle noise from heated, supersonic jets is investigated through high-fidelity large eddy simulation (LES). Simulations of a military-style nozzle reveal that N-shaped waves responsible for crackle noise emerge directly from the supersonic jet turbulence, and thus do not depend solely on nonlinear acoustic propagation effects. Conditional averaging using a backtracking algorithm furthermore reveals intermittent large-scale flow structures embedded in the jet turbulence as a likely source of the N-shaped waves. The skewness of pressure (Ffowcs Williams et al., 1975) and the skewness of the time derivative of pressure (Gee et al., 2007) are evaluated from full-field simulation data, and assessed as metrics of crackle. We test the hypothesis that the crackle level depends solely on the jet velocity (Ffowcs Williams et al., 1975) by comparing three simulations sharing the same geometry, but having different operating conditions so that the velocity and temperature are varied independently.

Reconstructing the South Pacific Convergence Zone Position during the Presatellite Era: A La Niña Case Study

Abstract Sixty-four southwest Pacific island meteorological stations were used to develop ENSO-related austral warm season (November–April) rainfall climatologies for 1961–90. Historical data for the same station set were then examined for 1955/56, one of the strongest La Niñas of the twentieth century. The pattern for that event was typical of a well-coupled protracted La Niña, with above-normal rainfall (>150%) observed for 17 stations in the case study. Tropical cyclone tracks were also used to illustrate the proximal effects of historical storms on station rainfall. Several stations that were located along the northern edge of the tropical cyclone swarm, but south of a region of anomalously low rainfall, were selected as possible candidates having been close to the mean South Pacific convergence zone (SPCZ) location. Linear interpolations between those stations highlighted a likely SPCZ position for the 1955/56 event. The reconstructed SPCZ location indicated a position southwest of normal, consistent with many La Niñas observed during the satellite era. The 1955/56 SPCZ reconstruction compared favorably with NOAA Cooperative Institute for Research in Environmental Sciences (CIRES) twentieth-century reanalysis (20CR) positions of maximum atmospheric water content and omega (time derivative of pressure that illustrates vertical velocity, and a proxy for convection and SPCZ location) at 500 hPa. Comparisons between Global Precipitation Climatology Project (GPCP) warm season rainfall composite plots for several satellite era La Niñas and one well-coupled La Niña event that occurred during 2010/11 to omega at 500 hPa and maximum rainfall amounts suggest that the 1955/56 reconstruction from in situ rainfall measurements captured the SPCZ location. These findings suggest that use of the 20CR could help to identify hemispheric-scale atmospheric circulation features like the SPCZ and improve understanding of ENSO dynamics prior to the satellite era.

Time asymptotics of non-darcy flows controlled by total flux on the boundary

We study the long term asymptotics of the diffusive capacity, the integral characteristic of a domain with respect to a nonlinear Forchheimer flow in porous media. Conditions on the boundary are given in terms of the total flux and constraints on the pressure trace on the boundary. We prove that, if the total flux is stabilizing, then the difference between the pressure average inside the domain and the pressure average on the boundary is stabilizing as well. This result can be used for calculating the productivity index of the well, an important characteristic of the well performance. To obtain the main theorem, a refined comparison of the fully transient pressure with the pseudo-steady state pressure (the time derivative of pressure is constant) was performed. These results can be effectively used in reservoir engineering and can also be applied to other problems modeled by nonlinear diffusive equations. Bibliography: 26 titles.

Taxol, a Microtubule Stabilizer, Improves Cardiac Functional Recovery during Postischemic Reperfusion in Rat in Vitro

Microtubule disruption contributes to cellular and organic dysfunction, and is implicated in ischemia/reperfusion (I/R) injury. The purpose of this study was to explore the effects of taxol, a microtubule stabilizer, on cardiac functional recovery during reperfusion. Left ventricular developed pressure, left ventricular end-diastolic pressure, maximal time derivatives of pressure and the severity of ventricular arrhythmias were analyzed in isolated rat heart. Microtubule structure was immunohistochemically measured. Apoptosis and necrosis was identified with TUNEL or TTC staining, respectively. Mitochondrial permeability transition pore (mPTP) mRNA expression was examined by real-time polymerase chain reactions. mPTP opening, reactive oxygen species (ROS), and oxidative enzyme activities were measured with fluorometric or spectrophotometric techniques. Intracellular calcium concentration ([Ca(2+) ](i) ) and Ca(2+) transients were examined by Fura-2-AM and Fluo-3-AM, respectively. Cytosolic cytochrome c, sarcoplasmic reticulum Ca(2+) -ATPase (SERCA2), ryanodine receptors (RyR), phospholamban (PLB), and PLB phosphorylation were analyzed by Western blot. Effective refractory period (ERP) and afterpotential-mediated activity were detected using microelectrode. Taxol improved the functional recovery of post-I/R. Taxol preserved the intact microtubule structure in reperfusion. mPTP mRNA expression was unchanged while the mPTP opening was reduced by taxol, and this effect was accompanied by the decreased ROS level caused by oxidative enzymes activities' changes. Taxol reduced apoptosis and the level of cytosolic cytochrome c in reperfusion. Taxol also promoted rapid recovery of [Ca(2+) ](i) , prevented reduction of the amplitude of Ca(2+) transients and shortened the decay time of Ca(2+) transients. The protein expression of SERCA2, RyR, and PLB remained unchanged in reperfusion. Taxol prevented the increase of Phospho-Thr17-PLB and Phospho-Ser16-PLB in reperfusion. In addition, taxol facilitated rapid recovery of ERP and counter-acted afterpotential-mediated activity. Taxol may effectively improve cardiac functional recovery during reperfusion via inhibiting mPTP opening, ameliorating abnormal calcium homeostasis, and reducing the substrates associated with arrhythmias.

Statistical analysis of a characteristic shock formation distance for high-amplitude noise

Previous research involved investigating a characteristic shock formation distance for Gaussian, finite-amplitude noise propagating in a cylindrical plane wave tube [Muhlestein and Gee, POMA 12, (in press)]. In particular, the evolution of the probability density function of the pressure and the first-order time derivative of pressure along with the skewness of the pressure derivative were experimentally studied. It was concluded that a constant-factor modification to the nonlinear distortion length defined by Gurbatov and Rudenko may yield a suitable characteristic shock formation distance in a statistical sense. Additional Gaussian noise data with a broader frequency range have now been taken, and the effects of boundary layer dispersion considered. Furthermore, noise with other statistical distributions and pressure statistics mimicking high velocity jet noise have been examined. These data are analyzed statistically as before using probability density function estimates and the skewness of the pressur...