Showing papers on "Dynamic pressure published in 2016"

Overshoot of the non-equilibrium temperature in the shock wave structure of a rarefied polyatomic gas subject to the dynamic pressure

Abstract: The shock wave structure in a rarefied polyatomic gas is analyzed on the basis of non-linear extended thermodynamics with 6 independent fields (ET6); the mass density, the velocity, the temperature and the dynamic pressure, which permits us to study the shock profile also for large Mach numbers. The first result of this paper is that the shock wave structure is substantially the same as that obtained previously from the linear theory for small or moderately large Mach numbers. Only for very large Mach numbers there exist some differences in the relaxation part of the profile between the model with a non-linear production term and the one with a linear production term. The mathematical reason of this behavior is due to the fact that the non-linear differential system has the same principal part of the linear one. The classical Meixner theory of relaxation processes with one internal variable is fully compatible with the ET6 theory and this fact gives us the explicit expressions of the internal variable and the non-equilibrium temperature in the Meixner theory in terms of the 6 fields, especially, of the dynamic pressure. By using the correspondence relation, the shock wave structure described by the ET6 theory is converted into the variables described by the Meixner theory. It is shown that the non-equilibrium Meixner temperature overshoots in a shock wave in contrast to the kinetic temperature. This implies that the temperature overshoot is a matter of definition of the non-equilibrium temperature.

The Impact of Effective Buoyancy and Dynamic Pressure Forcing on Vertical Velocities within Two-Dimensional Updrafts

Abstract: This research develops simple diagnostic expressions for vertical acceleration dw/dt and vertical velocity w within updrafts that account for effective buoyancy and the dynamic pressure gradient force. Effective buoyancy is the statically forced component of the vertical gradient in the nonhydrostatic pressure field. The diagnostic expressions derived herein show that the effective buoyancy of an updraft is dependent on the magnitude of the temperature perturbation within an updraft relative to the air along the updraft’s immediate periphery (rather than relative to an arbitrary base state as in ), the updraft’s height-to-width aspect ratio, and the updraft’s slant relative to the vertical.The diagnostic expressions are significantly improved over parcel theory (where pressure forces are ignored) in their portrayal of the vertical profile of w through updrafts from a cloud model simulation and accurately diagnosed the maximum vertical velocity wmax within updrafts. The largest improvements to the ...

Shock initiated instabilities in underwater cylindrical structures

Abstract: An experimental investigation to understand the mechanisms of dynamic buckling instability in cylindrical structures due to underwater explosive loadings is conducted. In particular, the effects of initial hydrostatic pressure coupled with a dynamic pressure pulse on the stability of metallic cylindrical shells are evaluated. The experiments are conducted at varying initial hydrostatic pressures, below the critical buckling pressure, to estimate the threshold after which dynamic buckling will initiate. The transient underwater full-field deformations of the structures during shock wave loading are captured using high-speed stereo photography coupled with modified 3-D Digital Image Correlation (DIC) technique. Experimental results show that increasing initial hydrostatic pressure decreases the natural vibration frequency of the structure indicating loss in structural stiffness. DIC measurements reveal that the initial structural excitations primarily consist of axisymmetric vibrations due to symmetrical shock wave loading in the experiments. Following their decay after a few longitudinal reverberations, the primary mode of vibration evolves which continues throughout later in time. At the initial hydrostatic pressures below the threshold value, these vibrations are stable in nature. The analytical solutions for the vibration frequency and the transient response of cylindrical shell are discussed in the article by accounting for both (1) the added mass effect of the surrounding water and (2) the effect of initial stress on the shell imposed by the hydrostatic pressure. The analytical solutions match reasonably well with the experimental vibration frequencies. Later, the transient response of a cylindrical shell subjected to a general underwater pressure wave loading is derived which leads to the analytical prediction of dynamic stability.

Synthesizing ocean bottom pressure records including seismic wave and tsunami contributions: Toward realistic tests of monitoring systems

Abstract: The present study proposes a method for synthesizing the ocean bottom pressure records during a tsunamigenic earthquake Firstly, a linear seismic wave simulation is conducted with a kinematic earthquake fault model as a source Then, a nonlinear tsunami simulation is conducted using the sea bottom movement calculated in the seismic wave simulation By using these simulation results, this method can provide realistic ocean bottom pressure change data, including both seismic and tsunami contributions A simple theoretical consideration indicates that the dynamic pressure change caused by the sea bottom acceleration can contribute significantly until the duration of ~90 s for a depth of 4000 m in the ocean The performance of a tsunami monitoring system was investigated using the synthesized ocean bottom pressure records It indicates that the system based on the hydrostatic approximation could not measure the actual tsunami height when the time does not elapse enough The dynamic pressure change and the permanent sea bottom deformation inside the source region break the condition of a simple hydrostatic approximation A tsunami source estimation method of tFISH is also examined Even though the synthesized records contain a large dynamic pressure change, which is not considered in the algorithm, tFISH showed a satisfactory performance when 5 min after the earthquake occurrence The pressure records synthesized in this study, including both seismic wave and tsunami contributions, are more practical for evaluating the performance of our monitoring ability, whereas most tsunami monitoring tests neglect the seismic wave contribution

Dynamic Pressure-Sensitive Paint Demonstration in AEDC Propulsion Wind Tunnel 16T

Abstract: Unsteady aerodynamics has been difficult to measure since the beginning days of wind tunnel testing. Distributed dynamic loads have been determined by using special (dynamic) transducers that measure pressure fluctuations at a few to several hundred points on a wind tunnel model and then integrating the pressure fluctuations over a specified area. Typical data acquisition systems that capture the pressure fluctuations record at rates up to 50,000 samples/channel/second and can generate many terabytes of raw data. Processing the large amount of data generated typically cannot be accomplished during wind tunnel tests, so selected data are processed for evaluation during testing. Another area of concern over the last several decades has been weapons bay acoustics and their effect on store separation. The U.S. Air Force (USAF) needs to understand the aerodynamics of payloads separating from aircraft to support weapons system design and certification. Customers to the Air Force such as Lockheed Martin Aeronautics Company have been developing computational fluid dynamics (CFD) models of stores separation in order to reduce the amount of wind tunnel and flight testing required for the certification process on each weapon and platform. Currently CFD validation can only be performed at point locations where dynamic pressure transducers are installed. Engineers at the Arnold Engineering Development Complex (AEDC) have utilized a steady-state pressure-sensitive paint (PSP) capability in the Propulsion Wind Tunnel (PWT) 16T to acquire surface pressure data on wind tunnel models. A logical next step was to extend this capability to measure fluctuating pressures with PSP. Innovative Scientific Solutions, Inc. (ISSI) has developed a fast-responding PSP under USAF and NASA Phase II small business innovation research (SBIR) grants that can detect pressure fluctuations up to 20kHz. This was made possible by new developments in high-speed camera technology and brighter light emitting diode (LED) technology. However, the test section in 16T places the test article approximately 8-11 feet away from cameras and LEDs. Detecting the fluorescence emitted by the fast PSP with exposure times in the hundreds of micro-seconds would be a challenge. In support of the Air-Delivered Weapon Certification Cost Reduction program, ISSI was awarded a grant to demonstrate the fast PSP capability in 16T with AEDC and Lockheed as partners. ISSI would provide the technical assistance, equipment and PSP, Lockheed would perform CFD computations and provide the test article, and AEDC would develop data acquisition and image processing software and perform the wind tunnel testing in 16T. Power spectral density (PSD) comparisons are made between the fast PSP and conventional dynamic pressure transducers. In addition, the complete spatial distribution of the sound pressure level (SPL) at selected frequencies are presented to aid understanding of the data and provide additional insight. Proper Orthogonal Decomposition is applied to the data for identification of relevant flow structures.

Dynamic responses of the Earth's radiation belts during periods of solar wind dynamic pressure pulse based on normalized superposed epoch analysis

Abstract: Using the electron flux measurements obtained from five satellites (GOES-15 and POES 15, 16, 18, and 19), we investigate the flux variations of radiation belt electrons during forty solar wind dynamic pressure pulses identified between September 2012 and December 2014. By utilizing the mean duration of the pressure pulses as the epoch timeline and stretching or compressing the time phases of individual events to normalize the duration by means of linear interpolation, we have performed normalized superposed epoch analysis to evaluate the dynamic responses of radiation belt energetic electrons corresponding to various groups of solar wind and magnetospheric conditions in association with solar wind dynamic pressure pulses. Our results indicate that by adopting the timeline normalization we can reproduce the typical response of the electron radiation belts to pressure pulses. Radiation belt electron fluxes exhibit large depletions right after the Pdyn peak during the periods of northward IMF Bz and are more likely to occur during the Pdyn pulse under southward IMF Bz conditions. For the pulse events with large negative values of (Dst)min, radiation belt electrons respond in a manner similar to those with southward IMF Bz, and the corresponding post-pulse recovery can extend to L ~ 3 and exceed the pre-pulse flux levels. Triggered by the solar wind pressure enhancements, deeper earthward magnetopause erosion provides favorable conditions for the prompt electron flux dropouts that extend down to L ~ 5, and the pressure pulses with longer duration tend to produce quicker and stronger electron flux decay. In addition, the events with high electron fluxes before the Pdyn pulse tend to experience more severe electron flux dropouts during the course of the pulse, while the largest rate of electron flux increase before and after the pulse occurs under the pre-conditioned low electron fluxes. These new results help us understand how electron fluxes respond to solar wind dynamic pressure pulses and how these responses depend on the solar wind and geomagnetic conditions and on the preconditions in the electron radiation belts.

CFD Simulations of Pressure Drop and Velocity Field in a Cyclone Separator with Central Vortex Stabilization Rod

Abstract: A problem of cyclone separators is the low grade efficiency of small particles. Therefore, a high efficiency cyclone separator has been developed and successfully tested in former work. In this cyclone separator, a vortex stabilizer is used to suppress the vortex core precession. In this article, the pressure and flow field in this cyclone separator are calculated by means of computational fluid dynamics using the commercial software Ansys Fluent 13. The position of the vortex core is tracked in these simulations by searching the position of minimal dynamic pressure and the centre of moment of the horizontal velocity components as function of the axial coordinate. The results are compared with experimental data. It is demonstrated that when using a stabilizer, the vortex is kept in position. Furthermore the maximum of the tangential velocity is found to be larger, which is known to have a positive effect on the separation of small particles in the inner solid body rotation vortex.

Shock-Initiated Buckling of Carbon/Epoxy Composite Tubes at Sub-Critical Pressures

Abstract: A comprehensive investigation on the implosion of composite cylinders subjected to a nearby explosion is performed. Experiments are conducted in a large pressure vessel, designed to provide constant hydrostatic pressure throughout the event. Carbon fiber/epoxy filament-wound tubes are studied with constant hydrostatic pressure and varying charge standoff distances to determine the effect of the explosive loading on the mechanisms of collapse. 3-D Digital Image Correlation (DIC) is used to capture the full-field displacements and velocities during the implosion event, and to characterize the initial dynamic response of the tube. Dynamic pressure transducers measure the shock waves generated by the explosive and also the pressure pulse generated by the collapse. Results show that different magnitudes of explosive loading produce drastic differences in the way implosions are initiated, and in the extent of damage to the structure. Experiments with strong explosive loading show immediate collapse of the tube upon the arrival of shock wave. Relatively smaller explosive loading result in collapses due to the additional bubble pulse loading, or after accumulating damage for extended periods of time.

Experimental Investigation on Underwater Buckling of Thin-Walled Composite and Metallic Structures

Abstract: An experimental study on the underwater buckling of composite and metallic tubes is conducted to evaluate and compare their collapse mechanics. Experiments are performed in a pressure vessel designed to provide constant hydrostatic pressure through the collapse. Filament-wound carbon-fiber/epoxy, glass/polyester (PE) tubes, and aluminum tubes are studied to explore the effect of material type on the structural failure. Three-dimensional digital image correlation (DIC) technique is used to capture the full-field deformation and velocities during the implosion event. Local pressure fields generated by the implosion event are measured using dynamic pressure transducers to evaluate the strength of the emitted pressure pulse. The results show that glass/PE tubes release the weakest pressure pulse and carbon/epoxy tubes release the strongest upon collapse. In each case, the dominating mechanisms of failure control the amount of flow energy released.

Dynamic pressure generator for dynamic calibrations at different average pressures based on a double-acting pneumatic actuator

Abstract: The ability to conduct dynamic calibrations of pressure sensors at different average pressures is very important because the dynamic properties of pressure sensors also depend on the value of the average pressure during the pressure pulsations. The main limitation of periodic dynamic pressure generators is the difficulty in achieving sinusoidal pulsations at higher amplitudes and frequencies in a gaseous medium due to the nonlinearities that result from the physical gas dynamics. This paper presents a newly developed dynamic pressure generator based on a double-acting pneumatic actuator in which the piston is driven by an electrodynamic shaker. In order to minimize the static preload of the shaker, which is due to the pressure acting on the piston, and therefore to enable the generation of larger pressure pulsations at different average pressures with the same excitation force, the cylinder chambers above and below the piston are pressurized with the same initial static gas pressure. As the dynamic characteristics of the dynamic pressure generator are also defined by the properties of the piston and both the cylinder chambers, the actuator dimensions were optimized with the help of mathematical modeling. In order to demonstrate the capabilities and limitations of the developed dynamic pressure generator, both mathematical and experimental analyses were performed.

Magnetospheric vortices and their global effect after a solar wind dynamic pressure decrease

Abstract: Using multipoint data from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites, we report a magnetospheric flow vortex driven by a negative solar wind dynamic pressure pulse. The observed vortex rotated in a direction opposite to that associated with positive solar wind dynamic pressure pulses. The vortex was moving tailward, as confirmed by a global magnetohydrodynamics (MHD) simulation. In addition, the equivalent ionospheric currents (EICs) deduced from ground magnetometer station data reveal that a current vortex in the ionosphere near the foot point of the satellites has a rotation sense consistent with that observed in the magnetosphere. The field-aligned current (FAC) density estimated from three THEMIS satellites is about 0.15nA/m(2), and the total FAC of the vortex is about 1.5-3x10(5)A, on the order of the total FAC in a pseudobreakup, but less than the total FAC in most moderate substorms, 10(6)A. Key Points

Gust Load Alleviation with Robust Control for a Flexible Wing

Abstract: Traditional methods for gust alleviation of aircraft are mostly proposed based on a specific flight condition. In this paper, robust control laws are designed for a large flexible wing with uncertainty in Mach number and dynamic pressure. To accurately describe the aeroelastic model over a large flight envelope, a nonlinear parameter-varying model is developed which is a function of both Mach number and dynamic pressure. Then a linear fractional transformation is established accordingly and a modified model order reduction technique is applied to reduce the size of the uncertainty block. The developed model, in which the statistic nature of the gust is considered by using the Dryden power spectral density function, enables the use of -synthesis procedures for controller design. The simulations show that the controller can always effectively reduce the wing root shear force and bending moment at a given range of Mach number and dynamic pressure.

RK-SIMPLER: Explicit Time-Accurate Algorithm for Incompressible Flows

Abstract: The SIMPLE family of algorithms has popularized the pressure-based schemes for incompressible flows and is the basis of many commercial codes. The influence of pressure on velocity is of primary importance in incompressible flows. The continuity equation implicitly dictates the pressure field, yet pressure is not a variable of the mass conservation equation. The pressure term that appears in the momentum equations is often treated as a source term. Thus, there is no explicit conservation equation for pressure. This predicament, and its remedy well known by the name “pressure–velocity coupling,” is resolved in SIMPLE and its variants by obtaining an approximate pressure correction field, which is used iteratively to correct the velocity field and/or the pressure field, seeking an overall satisfaction of the conservation equations. The approximate nature of the pressure correction equation often causes convergence issues. A new algorithm is presented here, eliminating the need for the pressure correction eq...

Comparative analysis of different micro-pressure sensors using comsol multiphysics

Abstract: Amongst MEMS technology, pressure measuring technology is one of the most important parameter, which is measure of force per unit area. In this work a comparative analysis of different pressure sensor mechanisms is carried out. Capacitive pressure sensors work on the principle of electrostatic transduction mechanism, piezoresistive pressure sensor employs a change in resistance values of piezoresistive elements placed on the surface of the diaphragm. The piezoelectric pressure sensor works based on piezoelectric effect, where the sensing element is stressed due to the applied pressure, positive electric charge is generated which in turn results to induced voltage. The simulation and modeling of proposed pressure sensors is done using COMSOL Multiphysics. Based on simulation results, capacitive pressure sensor exhibits non-linear response. Piezoresistive pressure sensor provides high linearity and better sensitivity. Piezoelectric pressure sensors achieve high sensitivity but are applicable to only dynamic pressure conditions.

Measurement of unsteady transition on a pitching airfoil using dynamic pressure sensors

Abstract: Unsteady boundary layer transition on a pitching OA209 airfoil in a wind tunnel was detected by using pressure fluctuation measurement at different oscillation frequency. Thirty Kulite dynamic pressure transducers flush-mounted on the airfoil surface recorded pressure signatures, and root mean square of pressure signatures were calculated. Results indicated that the criterion of transition for static airfoil defined as the peak of root mean square of pressure fluctuation was still suitable for detection of transition on a pitching airfoil. Fixed transition experiment for pitching airfoil was performed to validate the conclusion. Effect of oscillation frequency on transition was investigated. For small reduced frequency, the hysteresis loop is larger near leading edge. With increasing in the oscillation frequencies, the transition was promoted and relaminarization was enhanced.

Investigations of rotating instability and fluctuating tip clearance flow in a low-speed axial compressor:

Abstract: To investigate the relationship between rotating instability and fluctuating tip clearance flow, experimental tests as well as numerical studies are conducted on a 1.5 stage low-speed axial compressor rig. The rotating instability is detected through casing mounted dynamic pressure sensors. The phase-locked and root-mean-square pressure contours show that the interaction of tip clearance flow with the neighboring blade is the most likely cause of casing flow field fluctuation. From the calculated results of entire-annulus cascade, it is found that the tip clearance vortex structures oscillate periodically at low flow rate conditions. That leads to intense fluctuation of the casing flow field. Further, phase lag of pressure fluctuations exists between two neighboring passages, which induces a rotating pressure pattern of multiple frequencies. The frequency characteristics of this rotating pressure pattern caused by fluctuating tip clearance flow is coincident with the frequency of rotating instability. Thi...

Stall-Warning Approach Based on Aeroacoustic Principle

Abstract: In a fan/compressor, the pass of the blade is a periodic event and this periodicity deteriorates when rotating stall is gradually approached. The present work studies this periodicity of the pressure signal and explains when and why this periodicity deterioration happens from the perspective of the aeroacoustic principle. A theoretical analysis shows that the change of blade circulation has a close association with rotating stall and can be reflected on the periodicity deterioration of pressure signals sampled by a near-field dynamic pressure sensor located on the casing wall. A defined parameter Rc, which can revalue this periodicity, has the potential to be a stall-warning criterion. Then, a stall-warning approach is developed in terms of this feature. The values of Rc are calculated by the pressure signals of several blade pitches based on the current and the previous shaft periods. Statistical estimations are conducted on Rc within statistical windows by calculating the probabilities of Rc less than a...

Relationship between solar wind dynamic pressure and amplitude of geomagnetic sudden commencement (SC)

Abstract: The local time variation of geomagnetic sudden commencements (SCs) has not been taken into account in the Siscoe’s linear relationship which connects the SC amplitude with the corresponding dynamic pressure variation of the solar wind. By considering the physical background of SC, we studied which local time is best to extract the information of the solar wind dynamic pressure and concluded that the SC amplitude at 4–5 h local time of middle- and low-latitude stations most directly reflects the dynamic pressure effect. This result is used to re-check the order of magnitude of the largest 3 SCs observed since 1868.

Investigation of a fast transition from pump mode to generating mode in a model scale reversible pump turbine

Abstract: Pumped storage power plants are an efficient way to store energy at a large scale. In the last years, the changes between pump and turbine mode have become more and more frequent and the necessity of fast changes has increased. This paper analyses the flow in a model scale pump turbine during a fast transition from pump mode to generating mode by means of CFD. Results will be compared between two different mesh sizes and between simulation and measurement. A linear variation of rotational speed over time is chosen. A time-dependent flow rate through the machine is prescribed at the inlet. Due to the varying conditions, a fully transient analysis is carried out using the open-source code OpenFOAM®. The state of the machine at certain points of time during the transient is compared to the results for steady state simulations with identical boundary conditions. To characterize the phenomena in the guide vane channels, torque on selected guide vanes is evaluated as well as pressure at predefined locations. In the runner, pressure sensors are evaluated near the leading edge on pressure and suction side. In the draft tube, four dynamic pressure sensors in a plane below the runner are analysed. Frequencies and amplitudes are compared to simulation.

Dynamic pressure roadway support physical model test apparatus and dynamic pressure roadway support physical model test method

Abstract: The invention discloses a dynamic pressure roadway support physical model test apparatus and a dynamic pressure roadway support physical model test method. The apparatus includes an inner frame and an outer frame. The inner frame is provided with a top static load oil cylinder and a top vibration oil cylinder. The outer frame is provided with a bottom vibration oil cylinder. The inner frame and the outer frame are respectively provided with roller wheel sets and guide rails, so that the whole inner frame can vibrate vertically inside the outer frame under the effect of the bottom vibration oil cylinder. The basic principle of the test is that the mining pressure on a roadway is decomposed into two parts including static load and dynamic load, which are directly applied to a model roadway, wherein the static load is the influence of lead abutment pressure and is loaded by means of the top static load oil cylinder; and the dynamic load is the vibration effect due to roof breaking and collapse and the like in a mining area and is loaded by means of the top and the bottom vibration oil cylinders. Because the whole inner frame can vibrate vertically inside the outer frame, the apparatus is free of addition of a large-area vibration bench, thereby reducing occupied space of the apparatus. In addition, simulation on the mining pressure by advancing a working face is unnecessary, so that the model is reduced in size and the test is reduced in labor intensity and cost. The apparatus and the method can also be used for quantitatively researching the relationship between the mining pressure and roadway deformation damage.

Experimental investigation of unsteady characteristics of ventilated cavitation flow around an under-water vehicle:

Abstract: The objective of this article is to investigate the flow structure of the ventilated cavitation around an under-water vehicle. In the experiments, a high-speed camera system is used to observe cavity evolution of the unsteady cavitation flow, the velocity field is measured by the particle image velocimetry technique, and dynamic pressure measurement systems are used to measure pressure fluctuations under different cavitation numbers. We seek to investigate the mechanism of the re-entrant flow and shock wave phenomenon during the cavity evolution. The study concludes that the ventilated cavity is insufficient to overcome the re-entrant jet intrusion and the re-entrant jet moves upstream straightly as well as curvilinearly. Then, the vortex structure rotating clockwise forms on the vehicle surface and the cavity area with low velocity represents the vortex. The re-entrant jet rolls back after the re-entrant jet reaches the front of the cavity. The experimental results also show that the pressure signals at ...

Reducing pressure oscillations in discrete fluid power systems

Abstract: Discrete fluid power systems featuring transmission lines inherently include pressure oscillations. Experimental verification of a discrete fluid power power take off system for wave energy converters has shown the cylinder pressure to oscillate as force shifts are performed. This article investigates how cylinder pressure oscillations may be reduced by shaping the valve opening trajectory without the need for closed loop pressure feedback. Furthermore the energy costs of reducing pressure oscillations are investigated.