Showing papers on "Dynamic pressure published in 2004"

Empirical Spectral Model of Surface Pressure Fluctuations

Abstract: An empirical model of the surface pressure spectrum beneath a two-dimensional, zero-pressure-gradient boundary layer is presented that is based on the experimental surface pressure spectra measured by seven research groups. The measurements cover a large range of Reynolds number, 1.4 × 10 3 < Reθ < 2.34 × × 10 4 . The model is a simple function of the ratio of the timescales of the outer to inner boundary layer. It incorporates the effect of Reynolds number through the timescale ratio and compares well to experimental data. It is proposed that the effect of Reynolds number is more aptly described as the effect of the range of relevant scales. Spectral features of the experimental data and the scaling behavior of the surface pressure spectrum are also discussed.

Controlling a pressure transient in a well

Abstract: A system for controlling a dynamic pressure transient in a well during operations to provide fluid communication between the wellbore and adjacent formation. A method includes determining the characteristics of an adjacent formation, selecting a perforating tool for increasing fluid communication, determining a dynamic pressure transient to enhance the quality of the fluid communication and prevent damage, selecting a wellbore fluid to achieve the desired pressure transient, and performing the fluid communication process based on the selected parameters.

Magnetospheric reconnection driven by solar wind pressure fronts

Abstract: . Recent work has shown that solar wind dynamic pressure changes can have a dramatic effect on the particle precipitation in the high-latitude ionosphere. It has also been noted that the preexisting interplanetary magnetic field (IMF) orientation can significantly affect the resulting changes in the size, location, and intensity of the auroral oval. Here we focus on the effect of pressure pulses on the size of the auroral oval. We use particle precipitation data from up to four Defense Meteorological Satellite Program (DMSP) spacecraft and simultaneous POLAR Ultra-Violet Imager (UVI) images to examine three events of solar wind pressure fronts impacting the magnetosphere under two IMF orientations, IMF strongly southward and IMF Bz nearly zero before the pressure jump. We show that the amount of change in the oval and polar cap sizes and the local time extent of the change depends strongly on IMF conditions prior to the pressure enhancement. Under steady southward IMF, a remarkable poleward widening of the oval at all magnetic local times and shrinking of the polar cap are observed after the increase in solar wind pressure. When the IMF Bz is nearly zero before the pressure pulse, a poleward widening of the oval is observed mostly on the nightside while the dayside remains unchanged. We interpret these differences in terms of enhanced magnetospheric reconnection and convection induced by the pressure change. When the IMF is southward for a long time before the pressure jump, open magnetic flux is accumulated in the tail and strong convection exists in the magnetosphere. The compression results in a great enhancement of reconnection across the tail which, coupled with an increase of magnetospheric convection, leads to a dramatic poleward expansion of the oval at all MLTs (dayside and nightside). For near-zero IMF Bz before the pulse the open flux in the tail, available for closing through reconnection, is smaller. This, in combination with the weaker magnetospheric convection, leads to a more limited poleward expansion of the oval, mostly on the nightside. Key words. Magnetospheric physics (solar windmagnetosphere interactions; magnetospheric configuration and dynamics; auroral phenomena)

Sawtooth Oscillations Directly Driven by Solar Wind Dynamic Pressure Enhancements

Abstract: [1] We have examined four well-defined events of sawtooth oscillations in energetic particle flux and magnetic field at geosynchronous orbit. During all four events, nearly simultaneous energetic particle flux enhancements and magnetic field variations occurred at all MLTs for each sawtooth cycle. Geomagnetic H component data at low to middle latitude also show a global H increase simultaneously with the geosynchronous responses at all MLTs, and the northern and southern PC indices generally show increases at each sawtooth cycle. All these are what is expected if solar wind pressure enhancements impacted the magnetosphere at times appropriate to have caused the onset of each sawtooth cycle. By directly checking the solar wind data, we find that there indeed exists a series of solar wind dynamic pressure enhancements for each sawtooth event. In identifying these pressure enhancements, we have found that the relative change in the dynamic pressure is important, particularly when the magnitude of the dynamic pressure is small and that even a modest dynamic pressure enhancement can result in significant changes in the magnetosphere when the IMF stays strongly southward for a long interval. We show that each cycle of the sawtooth oscillation can be reasonably associated in timing with a corresponding solar wind dynamic pressure enhancement. On the basis of this association and the global, simultaneous geosynchronous and ground responses, we suggest that the sawtooth oscillations studied in this paper are directly driven by series of solar wind pressure enhancements and are not a repetitive internal magnetospheric response to sustained enhanced solar wind energy input.

Local Dynamic Similarity Model of Cross-Ventilation Part 1 - Theoretical Framework

Abstract: A new model has been proposed for evaluating the discharge coefficient and flow angle at an inflow opening for cross-ventilation. This model is based on the fact that the cross-ventilation flow structure in the vicinity of an inflow opening creates dynamic similarity under the condition that the ratio of the cross-ventilation driving pressure to the dynamic pressure of cross flow at the opening is consistent. It was confirmed, from a wind tunnel experiment, that the proposed model can be applied regardless of wind direction and opening position. Change of pressure along the stream tube of a cross-ventilated flow was estimated from the results of Large Eddy Simulation, and was set as the basis of model preparation. It was found that the static pressure at the opening was exhausted by the flow‘s acceleration and by turbulent kinetic energy generation during this stage.

A computational fluid dynamics (CFD) investigation of the wake closure phenomenon

Abstract: Using a computational fluid dynamics (CFD) software, the gas dynamics of the open-wake and closed-wake conditions of an annular-slit high-pressure gas atomization (AS-HPGA) nozzle were investigated to validate the predictions of a pulsatile atomization model that was recently proposed. The location of the recirculation zones, the oblique shocks and the Mach disks were analyzed for this type of closed-coupled gas atomization nozzle. The stagnation pressures located downstream of the Mach disk, in closed-wake condition, were found to be approximately twice as high as the stagnation pressure in an open-wake condition at a slightly lower atomization gas pressure. The turbulence model utilized within the CFD calculation scheme appeared to be inadequate for calculating aspiration pressure just below wake-closure pressure when the recirculation zone is extremely long and narrow. However, overall, the CFD calculation correlated well with the experimental results, showing that the aspiration pressure progressively lowers as operation pressure increases in open-wake condition, and rises as operation pressure increases in closed-wake condition.

Open-closed field line boundary position: A parametric study using an MHD model

Abstract: [1] In this paper we investigate the effect of changes in the interplanetary magnetic field (IMF), solar wind dynamic pressure, and dipole tilt angle on the position of the ionospheric projection of the open-closed field line boundary (OCB) in a magnetohydrodynamic (MHD) model We carry out a large number of steady state global MHD simulations in order to parameterize the OCB as a function of the solar wind By and Bz which we find to have the largest effect on the OCB location We interpolate between the values produced by the simulations, which allows us to evaluate the location of the OCB projection into the ionosphere for any values of ∣By∣ 0 nT We also find that the polar cap area increases with the increasing solar wind dynamic pressure The Bx component of the IMF and the dipole tilt angle are found to have relatively small effects on the location of the OCB

Influence of Fabric Mechanical Property on Clothing Dynamic Pressure Distribution and Pressure Comfort on Tight-Fit Sportswear

Abstract: Clothing pressure distribution in tight-fit sportswear was simulated by using numerical computational method. Three sets of tight-fit sportswears were simulated on the basis of their mechanical properties. In addition, a 3D human body was also simulated in order to investigate its interaction with sportswear in the wearing process. Result shows that pressure increases significantly around the waist girth until it passes through body pelvis during the wearing process. Meanwhile, pressure begins to increase for other tested body locations. Based on body contour plot, we found that pressure distribution was not uniformly distributed and high-pressure zone was concentrated around the waist girth at the end of the wearing process. The simulated pressure has no significant difference (p > 0.05) with the experimental pressure measurements. Furthermore, subjective pressure comfort rating was linearly correlated with the simulation, suggesting the simulation method can provide reliable prediction in term of pressure comfort. From the tight-fit sportswear manufacturer's point of view, simulation result would help them to satisfy consumers' physiological and psychological needs in term of pressure comfort at a lower cost.

Air-blast and the science of dynamic pressure measurements

Abstract: Considering current world events, the accurate measurement of the dynamic pressure associated with air-blast, as well as its effect on structures, is highly important. Yet, the accurate measurement of this pressure remains one on the biggest challenges that can be presented to the measurement engineer. This article attempts to provide guidance to enhance the quality of these measurements. In order to provide this guidance, a large body of the science of dynamic pressure measurement has been both summarized and organized. It is hoped that this work will also serve as a reference for other activities where the measurement of dynamic pressure has importance.

Role of solar wind dynamic pressure in driving ionospheric Joule heating

Abstract: [1] We investigate the role of the solar wind dynamic pressure on the ionospheric dynamics both observationally and using a global MHD simulation. Using ACE solar wind observations we carry out a statistical superposed epoch analysis, in which we determine the ionospheric response, as characterized by the AE index, to solar wind pressure pulses identified from the ACE solar wind data. We find that under steady southward interplanetary magnetic field the AE index increases 35% in 20 min following the pressure pulse but that no such reaction is observed under steady northward IMF. Second, we determine the ionospheric Joule heating in several events using a global MHD simulation GUMICS-4. We find that the solar wind dynamic pressure and the Joule heating are positively correlated. Finally, we discuss mechanisms linking the dynamic pressure to Joule heating. We find evidence that in the simulation, increasing dynamic pressure increases field-aligned currents, which then increases ionospheric Joule heating.

Habitat friendly, multiple impellor, wind energy extraction

Abstract: A habitat friendly, wind energy system is disclosed for safely extracting usable energy from wind. Included are one or more concentrator wings that convert the dynamic pressure of wind into relatively lower static pressure and thereby induces a vacuum that draws wind into an area defined by the concentrator wings. The airflow regulation minimizes or prevents the stalling of, or the generation of a turbulent flow of wind over or between concentrator wings. Further included are multiple impellors, at least one power converter, a riser supporting these component pieces, all positioned within the area defined by the concentrator wings. Accordingly, one or more flow regulators are positioned to assist in promoting laminar flow across or between the concentrator wings and to reduce the dynamic pressure of wind on the riser to thereby increase efficiency of the system.

Flow Network Model based on Power Balance as Applied to Cross-Ventilation

Abstract: This paper reviews the flow network model based on power balance as applied to wind-induced cross ventilation in a residential building. The characteristics of wind-induced cross-ventilation with open windows in buildings are different from those of air infiltration through cracks in walls. The features of the velocity and pressure distributions are apparently different. In cross-ventilation, it seems that a kind of stream tube is formed through the building. A large part of the dynamic pressure generated at the opening is preserved within the room, and a major part of the preserved energy is directly convected outside the room. The preservation of energy is reflected as a decrease in the value of the total pressure loss coefficient, ζ. The static pressure loss does not express the total pressure loss through an opening. The flows in the building usually converge and diverge and thereby lose energy. The lost energy should also be accurately evaluated as in the total pressure loss coefficient, ζ, i...

Evaluation of the Hill-Siscoe transpolar potential saturation model during a solar wind dynamic pressure pulse

Abstract: [1] Theory, observations, and simulations have shown that saturation of the cross-polar-cap potential occurs for high solar wind electric field. One of the parameters that affect the saturation process is the solar wind dynamic pressure. Theoretical arguments predict that higher pressure levels lead to higher transpolar potentials and a slower saturation process. The question arises of how the transpolar potential will respond to sudden enhancements of solar wind pressure in the saturation regime, and if the theoretical model predictions can replicate the observations. We use ionospheric flow data from Defense Meteorological Satellite Program (DMSP) spacecraft to calculate the transpolar potential and compare it with theory before and after a sudden increase in solar wind pressure. We find that the model predictions fall below the observed values for both pressure levels but are particularly lower after the dynamic pressure enhancement. The inclusion of a “viscous” potential, or the introduction of variable ionospheric conductivity, cannot remedy the discrepancy. We suggest that modification of the existing models to include magnetotail processes might be needed to bridge the gap between theory and observations.

Epoxy-free high-temperature fiber optic pressure sensors for gas turbine engine applications

Abstract: Pressure measurements at various locations of a gas turbine engine are highly desirable to improve the operational performance and reliability. However, measurement of dynamic pressure (1psi (6.9kPa) variation superimposed on the static bias) in the operating environment of the engine, where temperatures might exceed 600°C and pressures might exceed 100psi (690kPa), is a great challenge to currently available sensors. To meet these requirements, a novel type of fiber optic engine pressure sensor has been developed. This pressure sensor functions as a diaphragm-based extrinsic Fabry-Perot interferometric (EFPI) sensor. The structure of the sensor head, composed entirely of fused silica, allows a much higher operating temperature to be achieved in conjunction with a low temperature dependence. The sensor head and the fiber tail have been packaged in a metal fitting connected to a piece of metal extension tubing, which improves the mechanical strength of the sensor and facilitates easy sensor installation. The sensor exhibited very good performance in an engine field test, demonstrating not only that the sensors' package is robust enough for engine operation, but also that its performance is consistent with that of a commercial Kulite sensor.

A Nonlinear Model for Prediction of Dynamic Coefficients in a Hydrodynamic Journal Bearing

Abstract: This paper investigates the variation of nonlinear stiffness and damping coefficients in a journal orbit with respect to equilibrium position. The journal orbit is obtained by the combined solution of equations of motion and Reynolds equation. In the linearized dynamic analysis, dynamic pressure is written as a perturbation of static pressure and pressure gradients at equilibrium position. However, in order to obtain nonlinear dynamic coefficients about equilibrium position, the dynamic pressure gradients in the orbit are also written as the first order perturbation of static pressure gradients and higher order pressure gradients for displacement and velocity perturbations. The dynamic coefficients are functions of bearing displacement and velocity perturbations. The higher order pressure gradients at equilibrium position are evaluated at various eccentricity ratios and L/D ratios of 0.5 and 1.0. The variation of nonlinear dynamic coefficients is analyzed for three Sommerfeld numbers of a two-axial groove journal bearing under the action of an external synchronous load along and perpendicular to the radial journal load. Results indicate that the oil film nonlinearities affect the journal motion at lower eccentricity ratios (higher Sommerfeld numbers) with wide variation in stiffness and damping coefficients.

Effect of Turn Region Treatments on the Pressure Loss Distribution in a Smooth Square Channel with Sharp 180° Bend

Abstract: An experimental investigation is carried out to study the effect of several turn treatments like single guide vane (short and long) and multiple guide vanes on the pressure drop distribution in a square cross-sectioned smooth channel with a sharp 180° bend. The sharp 180° turn is obtained by dividing a rectangular passage into two square channels using a divider wall with a rounded tip at the location where the flow negotiates the turn. The study is carried out for a divider wall thickness to hydraulic diameter ( W / D ) of 0.2 for Reynolds numbers of 13,500 and 17,000. The pressure drop distribution normalized with the mainstream fluid dynamic pressure head is presented for the outer surfaces. The results indicate that the shape and position of the guide vanes significantly affect the pressure losses associated with coolant flows through a sharp 180° bend. Properly shaped 180° vanes located in the center of the bend decrease the overall pressure drop by as much as 40–45% compared to the no guide vane within the bend.

Suppression of Open-Jet Pressure Fluctuations in the Hyundai Aeroacoustic Wind Tunnel

Abstract: Peak pressure fluctuation amplitudes in the 3 / 4 open-jet test-section of the Hyundai Aeroacoustic Wind Tunnel have been reduced from root-mean-square levels equal to 6% of the test-section dynamic pressure to levels of less than 0.5% over almost the full wind speed range of the tunnel. The improvement was accomplished using a retrofit of the test-section collector. Using an analysis of the physics of the problem, it was found that the HAWT pressure fluctuations could be accurately modeled as a resonance phenomenon in which acoustic modes of the full wind tunnel circuit are excited by a nozzle-to-collector edgetone-feedback loop. Scaling relations developed from the theory were used to design an experiment in 1/7 th scale of the HAWT circuit, which resulted in the development of the new collector design. Data that illustrate the benefit of the reduction in pressure fluctuation amplitudes on passenger-car aerodynamic force measurements are presented.

Head slider and disk drive with the same

Abstract: A head slider is designed to prevent collision against a surface of a disk due to an external impact and the like while a device is in use to avoid damages on an air bearing surface (“ABS”) of the head slider and the surface the disk. This is achieved by a structure of the head slider which produces a large positive pressure on a positive dynamic pressure generating section when the head slider comes close to the disk. More specifically, the ABS of the head slider comprises three surfaces of different positional heights, and the positive dynamic pressure generating section having a height equivalent to a mid level surface of the second highest position is provided on a portion of the head slider made of a material of high brittleness at each of locations near both side edges lateral to a tracking width of a magnetic head. In addition, a head mounting pad bearing the magnetic head is so configured that a boundary between a high level surface in the highest position and a mid level surface is asymmetrical with respect to the lateral center of the head slider in a direction of the tracking width of the magnetic head.

Wind Tunnel to Atmospheric Mapping for Static Aeroelastic Scaling

Abstract: Wind tunnel to Atmospheric Mapping (WAM) is a methodology for scaling and testing a static aeroelastic wind tunnel model. The WAM procedure employs scaling laws to define a wind tunnel model and wind tunnel test points such that the static aeroelastic flight test data and wind tunnel data will be correlated throughout the test envelopes. This methodology extends the notion that a single test condition - combination of Mach number and dynamic pressure - can be matched by wind tunnel data. The primary requirements for affecting this extension are matching flight Mach numbers, maintaining a constant dynamic pressure scale factor and setting the dynamic pressure scale factor in accordance with the stiffness scale factor. The scaling is enabled by capabilities of the NASA Langley Transonic Dynamics Tunnel (TDT) and by relaxation of scaling requirements present in the dynamic problem that are not critical to the static aeroelastic problem. The methodology is exercised in two example scaling problems: an arbitrarily scaled wing and a practical application to the scaling of the Active Aeroelastic Wing flight vehicle for testing in the TDT.

Apparatus and methods for dynamically pressure testing an article

Abstract: The dynamic pressure tester includes a first cylinder and a first piston movable relative to the cylinder. The piston may have one or more weights applied thereto. The cylinder is vibrated by a shaker table and pressure pulses in the fluid are transmitted to a stationary housing and to a second piston in the housing. The opposite side of the piston contacts a corrosive or caustic fluid also in contact with the sensor face of an article being tested. The pressure pulses are transmitted by the second piston and corrosive fluid to the sensor face, enabling dynamic pressure testing in the corrosive fluid.

Evaluation of dynamic performance of pressure sensors using a pressure square-like wave generator

Abstract: In this study, we measured the dynamic characteristics of pressure sensors using a pressure square-like wave generator (PSWG). With high excitation energy, the PSWG can measure dynamic pressure and work more effectively in a high frequency range. Under the same experimental parameters (10 bar, 600 Hz), the performance of six pressure sensors of dissimilar design and structure was evaluated. The experimental results indicate that they all exhibited extremely different dynamic characteristics. The dynamic pressure sensors based on quartz plates and crystals possess larger overshoot, greater gain margin and shorter rise time in comparison with other sensors based on strain gauge and piezoresistive materials. Compared with other traditional methods, such as the hydraulic control method, the PSWG proves to be superior in that it can be employed to evaluate the dynamic performances of pressure sensors at high frequency of above 10 kHz.

Pressure transducer for measuring low dynamic pressures in the presence of high static pressures

Abstract: A sensor is described, which basically consists of a leadless high sensitivity differential transducer chip which responds to both static and dynamic pressure. Located on the transducer are two sensors. One sensor has a thicker diaphragm and responds to both static and dynamic pressure to produce an output indicative of essentially static pressure, the static pressure being of a much higher magnitude than dynamic pressure. The other sensor has a thinner diaphragm and has one side or surface responsive to both static and dynamic pressure. The other side of the differential sensor or transducer structure has a long serpentine reference tube coupled to the underneath of the diaphragm. The tube only allows static pressure to be applied on the underside of the diaphragm and because of the natural resonance frequency of the tube, the dynamic pressure is suppressed and does not, in any manner, interface with the sensor or transducer having a thinned diaphragm. Thus, the thinned diaphragm differential unit provides an output which is indicative of the dynamic pressure, as the static pressure applied to both the top and bottom surfaces of the transducer sensor is cancelled.

Online Combustor Stability Margin Assessment Using Dynamic Pressure Data

Abstract: This paper describes a strategy for determining a combustor’s dynamic stability margin. Currently, when turbines are being commissioned or simply going through day to day operation, the operator has no idea how the dynamic stability of the system is affected by changes to fuel splits/operating conditions unless, of course, pressure oscillations are actually present. We have developed a methodology for ascertaining stability margin from passive monitoring of the acoustic pressure. This method consists of signal processing and analysis that determines a real-time measure of combustor damping. When the calculated damping is positive, the combustor is stable. When the damping goes to zero, the combustor approaches its stability boundary. Changes in the stability margin of each of the combustor’s stable modes due to tuning, aging or environmental changes can then be monitored through online analysis of the pressure signal. This paper outlines the basic approach used to quantify acoustic damping and demonstrates the technique on combustor test data.Copyright © 2004 by ASME

Portable system for measuring dynamic pressure in situ and method of employment therefor

Abstract: A dynamic pressure testing or calibration system packaged as a portable unit for characterizing pressure sensors, such as transducers. Embodiments are packaged for carry on the body, are battery-operated, compatible with existing transducer mounts, and quickly learned and easily used by a single operator. The system supplies a pre-specified impulse (pressure pulse) of fluid, preferably a benign gas, such as air, or an inert gas such as helium or nitrogen. In select embodiments, the gas pulse has a fast rise time and its amplitude may be varied over a pre-specified dynamic range. For example, the rise time may emulate that of an impulse created during an explosion by a resultant pressure wave, i.e., less than 100 microseconds. Embodiments also incorporate a data acquisition capability that accurately captures and records both the supplied impulse and the response of the sensor under test.

A Fundamental Study on the Air Flow Structure of Outflow Openings

Abstract: A Local Dynamic Similarity Model, applicable to dynamic similarity of cross-ventilation, has been applied to outflow openings. Cross-ventilation performance at the openings on the outflow side has been evaluated, and the structure of air flows around the outflow openings has been studied by LES and wind tunnel experiments. It was found that LES reproduces the wind tunnel experiment results fairly well, such as the extensive increase of discharge coefficient in a small region where dimensionless room pressure, PR*, is low. The evaluation of the pressure field by LES revealed that the remainder of the dynamic pressure in the air flows and the change of the pressure field around the outflow openings have a strong influence on the discharge coefficient. Furthermore, by identifying the configuration of the stream tube of the ventilation air flow, it was found that the discharge coefficient is changed depending on how the air flows exit. In general, dynamic pressure, Pt, tangential to the wall surface a...

Dynamic pressure thrust bearing part and method of manufacturing dynamic pressure thrust bearing part

Abstract: To manufacture a dynamic pressure bearing part at a low cost, capable of making consumption power small by reducing a load of rotation and having remarkable durability while ensuring small size and high performance. Dynamic pressure generating projection parts 21 are formed on the surface of an annular ring, which is a material of the dynamic pressure bearing part having a profile shape of a herringbone type having a first belt-like part 11 L and a second belt-like part 11 R which are branched out from a tip end 21 S to a rear end so as to form a double fork shape, and widths of which are made to be approximately fixed widths. The material of the dynamic pressure bearing part is subjected to a magnetic barrel polish processing to obtain the dynamic pressure thrust bearing part.

Dynamic pressure bearing and rotation machine employing same

Abstract: A dynamic pressure bearing in which prolongation of its useful life is realized by holding a magnetic fluid by a magnetic field in the stationary state and holding it by the centrifugal force during rotation thereby preventing leakage and scattering of the magnetic fluid due to instable pumping of the dynamic pressure bearing without using a magnetic fluid having a high saturation magnetization value. The dynamic pressure bearing is characterized in that a sleeve (3) having a protruding tubular part is fitted relatively rotatably on a shaft (1) having a thrust plate (2) formed closely in the axial direction, an annular cover (4) is formed axially outside the thrust plate (2),the radial inner periphery of the annular cover (4) serves as an opening part, at least two air gap parts serve as reservoirs (10) for holding the magnetic fluid, and a magnetomotive force member (15) for concentrating the lines of magnetic force is provided radially outside the end part (13) of the magnetic fluid (12).

Method and device for identifying malfunctioning of a compressed air consumer circuit in an electronic compressed air system for a vehicle

Abstract: To detect a failure of a compressed air consumer circuit in a compressed air system for vehicles, pressure is continuously measured in compressed air consumer circuits and evaluated in an electronic control unit which compares the pressure values and/or determined negative pressure gradients of the compressed air consumer circuits with a respective threshold value and shuts off an air-consumer circuit if the pressure values and/or negative pressure gradients thereof satisfy a circuit-failure criterion. The circuit-failure criterion is satisfied when the pressure values and/or pressure gradients are below the respective threshold value for a time equal to or longer than the time of a dynamic pressure change or of a dynamic pressure collapse.