Negative Pressure Coefficient Research Articles

Experimental Study of Pressure Fluctuation in Stilling Basins

Stilling basins dissipate energy to form hydraulic jumps and rotational flows. Hydraulic jump and rotational current phenomenon produce pressure fluctuation at the bottom of stilling basins. In the present study, pressure fluctuations and their locations have been studied in a physical model of Namrod Dam. Results showed that fluctuations in presence of jump in the basin are high and, therefore, the fluctuation factors are, respectively, high. In positive pressure coefficient (C ), it is evident that when a jump is present, the turbulence and disturbance factors increase and, therefore, the pressure fluctuations go up, respectively. In negative pressure coefficients (C ), as is expected from positive pressure coefficients, the maximum pressure fluctuations occurred at Q/Q max = 0.47 with regard to forming a complete hydraulic jump at this discharge. Regarding available empirical equations, the thickness of slab for different hydraulic conditions was calculated and compared in one-dimensional (1D) and two-dimensional (2D) conditions. By analyzing collected data, it was observed that, results of 1D were underestimated in comparison to 2D calculations. Concrete slab thickness could be observed that fluctuations have significant effect on thicknesses. However, such calculations can provide designers with general ideas on how to better understand the conditions.

Transfer function and working principle of a pressure/temperature sensor based on carbon black/silicone rubber composites

ABSTRACTPressure/temperature sensitive silicon rubber (SR) filled with carbon black (CB) was prepared by a liquid mixing method. The transfer function of a pressure/temperature sensor based on CB/SR was derived by general effective media theory. The results show that the transfer functions coincided well with the experimental data, and the negative pressure coefficient of the resistance/positive temperature coefficient of resistance are shown. The working principles of these two kinds of sensors are different. The working principle of the pressure sensor based on CB/SR was related to the volume fraction of CB. With increasing volume fraction of CB, the working principle of this kind of pressure sensor varied from a piezo‐resistive effect to a strain effect. In addition, the working principle of the temperature sensor based on CB/SR was that the resistivity changed with temperature; this was not related to the volume fraction of CB. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2016,133, 42979.

Carrier mediated reduction of stiffness in nanoindented crystalline Si(100)

We report the observation of carrier mediated decrease in the stiffness of crystalline (c)-Si(100) under nanoindentation. The apparent elastic moduli of heavily doped (∼1 × 1021 cm−3) p- and n-type c-Si are observed to be lower by 5.3%–7.5% than the estimated value for intrinsic (∼1 × 1014 cm−3) c-Si. The deviation observed with respect to elastic modulus remarkably matches with the estimated value while considering the electronic elastic strain effect on carrier concentration as an influence of negative pressure coefficient of band gap for Si (Γ-X). The value is predominantly higher than the reported value of a decrease of 1%–3% in stiffness as an effect of impurity in c-Si.

Impedance characteristics of surface pressure-sensitive carbon black/silicone rubber composites

The surface pressure-sensitive characteristics of carbon black/silicon rubber (CB/SR) composites with positive pressure coefficient of impedance (PPCI) effect were researched. The linearity is very good when the impedance’s logarithm changes with the applied load at optimal operating frequency (1KHz). The pressure sensor based on the flexible composites exhibit fast response when pressures are applied and canceled with a response time of 3s. The sensor appears a higher sensitivity in AC electrical field than that in DC electrical field. Dielectric polarization theory was used to explain the sensitive behavior of the pressure sensor. Internal structure diagrams of CB/SR composites were used to explain the positive pressure coefficient of resistive (PPCR) effect and negative pressure coefficient of capacitive (NPCC) effect. AC electric field is applicable for the flexible pressure sensor based on conductive silicon rubber composites.

Pressure-Induced Phase Transformations of Zircon-Type LaVO4 Nanorods

A combination of synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy has been used to study high-pressure behavior of the zircon-type LaVO4 nanorods. In situ high-pressure XRD results identified an irreversible zircon-to-monazite phase transition at similar to 5 GPa and a reversible transition to an undetermined second high-pressure phase (phase III) at similar to 12.9 GPa. Through Le Bail refinements of the XRD patterns with zircon-type structure, we show that the zircon-type LaVO4 nanorods possess the smallest bulk modulus among zircon-type rare-earth orthovanadates. Furthermore, negative pressure coefficients of external translational T(Eg) and internal upsilon(2)(B-2g) bending modes have been observed in Raman measurements. The Raman spectra of phase III with distinctive features have been fully recorded for the first time, and a related structure associated with a coordination increase for V is suggested in terms of the postmonazite phase in LaVO4 nanorods. Finally, analysis of the transmission electron microscopy both before and after compression indicates that a large number of nanorods can be recovered in the quenched samples, allowing us to verify the orientation relationship for zircon-to-monazite phase transformation.

A model of probability density function of non-Gaussian wind pressure with multiple samples

The extreme value of non-Gaussian wind pressure coefficients is usually estimated by fitting the probability density function (PDF) of maximum or minimum values while a large number of observations except the peak values in the measured samples are discarded. The implicit or explicit translation between non-Gaussian and Gaussian histories can also be utilized to estimate the extreme value of non-Gaussian wind pressure coefficients while the randomness of aerodynamic effect is not taken into account. The Hermite moment models are summarized and applied to formulate the PDF of non-Gaussian peak factors which is expressed as the joint PDF (JPDF) of the shape parameters of the Hermite moment model and the PDF of the translated Gaussian peak factors. After the variations of mean, standard deviation and non-Gaussian peak factor are considered, an innovative and analytical PDF formula of extreme wind pressure coefficients for multiple samples, which is expressed as a function of the JPDF of mean, standard deviation and the PDF of non-Gaussian peak factor, is presented in this paper. The theoretical developments are applied to establish the PDF and cumulative density function (CDF) of the negative peak wind pressure coefficients with multiple samples. It is verified that the analytical probability distribution model is a reasonable model to estimate the peak pressure coefficients.

Piezoresistive and compression resistance relaxation behavior of water blown carbon nanotube/polyurethane composite foam

The in-situ bulk polycondensation process in combination with a ball milling dispersion process was used to prepare the water blown multiwall carbon nanotubes (CNT)/polyurethane (PU) composite foam. The mechanical properties, piezoresistive properties, strain sensitivity, stress and resistance relaxation behaviors of the composite foams were investigated. The results show that the CNT/PU composite foam has a better compression strength than the unfilled polyurethane foams and a negative pressure coefficient behavior under uniaxial compression. The resistance response of CNT/PU nanocomposites foam under cyclic compressive loading was quite stable. The nanocomposite foam containing a weight fraction of carbon nanotubes close to the percolation threshold presents the largest strain sensitivity for the resistance. The characteristic of resistance relaxation of CNT/PU composite foam is different from the stress relaxation due to the different relaxation mechanism. During compressive stress relaxation, the CNT/PU foam composites have excellent resistance recoverability while poor stress recoverability.

Dilatometric Studies on Single Crystalline Barlowite – A Structurally Perfect Spin-1/2 Kagome System

We present results of high-resolution thermal expansion measurements on single crystalline barlowite – a structurally perfect spin-1/2 kagome system. The data reveal strongly pronounced and anisotropic second-order phase transition anomalies at the Néel transition at TN = 16K. From these data, together with literature results on the specific heat, the uniaxial-pressure dependences of TN are derived. We find a rather large positive pressure coefficient for uniaxial pressure along the hexagonal c axis of ∂TN/∂pc = (2.3 ± 0.2) K/GPa and smaller negative in-plane pressure coefficient of ∂TN/∂pin-plane = -(0.6 ± 0.03) K/GPa. These effects result in a small positive pressure coefficient under hydrostatic-pressure conditions of ∂TN/∂phydr = (1.1 ± 0.2) K/GPa. Bond-lengths considerations indicate that inter-layer Cu-O bonds, being larger than those typically found in stable Cu-O complexes, are responsible for this behavior.

Prediction of negative peak wind pressures on roofs of low-rise building

In this paper, a probability distribution which is consistent with the observed phenomenon at the roof corner and, also on other portions of the roof, of a low-rise building is proposed. The model is consistent with the choice of probability density function suggested by the statistical thermodynamics of open systems and turbulence modelling in fluid mechanics. After presenting the justification based on physical phenomenon and based on statistical arguments, the fit of alpha-stable distribution for prediction of extreme negative wind pressure coefficients is explored. The predictions are compared with those actually observed during wind tunnel experiments (using wind tunnel experimental data obtained from the aerodynamic database of Tokyo Polytechnic University), and those predicted by using Gumbel minimum and Hermite polynomial model. The predictions are also compared with those estimated using a recently proposed non-parametric model in regions where stability criterion (in skewness-kurtosis space) is satisfied. From the comparisons, it is noted that the proposed model can be used to estimate the extreme peak negative wind pressure coefficients. The model has an advantage that it is consistent with the physical processes proposed in the literature for explaining large fluctuations at the roof corners.

Pressure dependence of the refractive index in wurtzite and rocksalt indium nitride

We have performed high-pressure Fourier transform infrared reflectance measurements on a freestanding InN thin film to determine the refractive index of wurtzite InN and its high-pressure rocksalt phase as a function of hydrostatic pressure. From a fit to the experimental refractive-index curves including the effect of the high-energy optical gaps, phonons, free carriers, and the direct (fundamental) band-gap in the case of wurtzite InN, we obtain pressure coefficients for the low-frequency (electronic) dielectric constant ε∞. Negative pressure coefficients of −8.8 × 10−2 GPa−1 and −14.8 × 10−2 GPa−1 are obtained for the wurtzite and rocksalt phases, respectively. The results are discussed in terms of the electronic band structure and the compressibility of both phases.

A flexible pressure-sensitive array based on soft substrate

A 4×4 flexible pressure-sensitive array based on PET/ITO (polyethylene terephthalate/indium tin oxides) was designed and fabricated, with a size of 2.5cm×2.5cm×0.75mm. The design effectively reduces the complexity of device structure. The array shows giant negative pressure coefficient of resistivity (NPCR) effect under 0–4N (0–0.32MPa). The pressure-sensitive array exhibits high sensitivity and fast response-recovery to loads. And the response and recovery times are less than 2s. The sensitive size and azimuth of the applied loading on the pressure-sensitivity array can be accurately recognized easily. The pressure-sensitivity array is stable, low-cost and easy for fabricating large-area sensors. The designed micro array may be used for in robot skin.

Wind Effects of Architectural Details on Gable-Roofed Low-Rise Buildings in Southeastern Coast of China

Gable-roofed low-rise buildings with ridges, protruding gable walls and eaves gutters are usually found on the southeastern coast of China, which are often invaded by typhoons. The wind effects due to ridges, protruding gable walls and eaves gutters on low-rise buildings as well as the wind loads on the architectural details themselves have been studied through a series of wind tunnel tests. The mean block pressure coefficient and the negative peak block pressure coefficient on different roof regions were compared, and architectural details' single tap pressure coefficients and drag coefficients were investigated. The study shows that each of the architectural details reduces roof wind load impact to various extents, and their coexistence has a most significant effect on decreasing roof wind loads, up to the reduction of 56% on the ‘Ru’ windward region for the mean block pressure coefficient, and 47% and 45% on the ‘C1’ and ‘C2’ corner regions for the worst negative peak block pressure coefficient, respectively. The worst negative peak pressure at the interior surface of the architectural details occurs when the wind blows obliquely on the exterior surface of the architectural details. This condition leads to very large peak net pressure coefficients. The worst net pressure coefficients of the protruding gable wall, ridge and eaves gutter are about 18.0, 10.5 and 6.7, respectively. Finally, the drag coefficients on the architectural details were also examined, and the results are suitable for engineering application.

Wind Tunnel Study on the Pressure Coefficient Performance of Different Venturi Shaped Roof Configurations for Wind Induced Natural Ventilation

Wind catchers are structures used for natural ventilation using wind induced into buildings. Recently this has attracted attention for green buolding features There is limited studies on the different venturi shapes and their effects on inducing wind into buildingss.. This study considered three configurations ie. the shallow ellipse, the ellipse and the hemisphere in a wind tunnel with different speeds ranging from 8 m/s to 20 m/s. The negative pressure coefficient at the lower center of the roof is considered as the criteria for higher ventilation rate. The shallow ellipse performed the best but due to construction limitations other alternatives are recommended.

High-pressure effects in anti-post-perovskite superconductorsV3PnNx(Pn= P, As)

Pressure effects on the superconductors ${\mathrm{V}}_{3}$Pn${\mathrm{N}}_{x}$ (Pn = P and As) with the two-dimensional anti-post-perovskite structure were investigated. It was found that with increasing pressure, the superconducting transition temperature (${T}_{c}$) decreases with a negative differential pressure ($P$) coefficient $d{T}_{c}$/dP = \ensuremath{-}0.19 and \ensuremath{-}0.013 K/GPa for Pn = P and As, respectively. This is in marked contrast to the positive differential pressure coefficients reported for other vanadium-based superconductors with a three-dimensional crystal structure. The observed change in ${T}_{c}$ under pressure is revealed to be closely correlated with the evolution of the normal-state resistivity. The results support the phonon-mediated superconductivity in ${\mathrm{V}}_{3}$Pn${\mathrm{N}}_{x}$.

Monitoring of wind effects on a low-rise building during typhoon landfalls and comparison to wind tunnel test results

SUMMARY A full-scale instrumented experimental low-rise building was built and implemented to monitor wind effects on the building during typhoons. Field measurements of the near-surface wind velocity field and wind-induced pressures on the low-rise building were carried out during the passages of 10 typhoons in 2008–2011. This paper presents a combined study of field measurements and wind tunnel testing. Detailed analysis of the measured data has been conducted to investigate the typhoon-generated wind characteristics and wind-induced pressures on the roof of the experimental building under extreme wind conditions during typhoon landfalls. On the other hand, wind tunnel experiments have been performed to study the wind loads on the low-rise building. The model test results were compared with the full-scale measurements to evaluate the accuracy of the model test results and the adequacy of the techniques used in the wind tunnel tests. It was found that the wind tunnel tests underestimated the real fluctuating and negative peak pressure coefficients on a roof corner zone for quartering winds. Copyright © 2014 John Wiley & Sons, Ltd.

Experimental investigation to study flow and heat transfer characteristics over a NACA0018 aerofoil for different height ratios

Experiments are carried out to study flow and heat transfer characteristics over NACA0018 aerofoil when the body approaches the wall of a wind tunnel. Investigations have been done to study the effect of wall proximity due to flow separation around the body at Reynolds number 2.5 × 105, different height ratios and various angles of attack. The static pressure distribution has been measured on upper and lower surfaces of the aerofoil. The results have been presented in the form of pressure coefficient, drag coefficient for different height ratios. Pressure coefficient values are decreased and then increased on the lower surface of the aerofoil and decreased on the upper surface of the aerofoil at all angles of attack. The negative pressure coefficient and drag coefficient decreases as the body approaches the upper wall of wind tunnel. The maximum value of drag coefficient has been observed at an angle of attack 30° for the aerofoil at all height ratios. The Heat transfer experiments have been carried out under constant heat flux condition. Heat transfer coefficients are determined from the measured wall temperature and ambient temperature and presented in the form of Nusselt number. The variation of local as well as average Nusselt number has been shown with non dimensional distance for different angles of attack and for various height ratios. The local as well as average Nusselt number decreases as the height ratio decreases for all non-dimensional distance and angles of attack respectively. Maximum value of average Nusselt number has been observed at an angle of attack 40°.

풍동실험을 통한 능동위상배열레이더에서 다층레이돔에 작용하는 공기력과 풍압의 실험적 연구

In this paper, we investigated the sensitivity of aerostatic force coefficients of multi-layer radom in the various wind speeds. The test was conducted in KOCED Wind Tunnel Center in Chonbuk National University, and wind speeds were in the range from 5 m/s to 26 m/s in order to determine the Reynolds number independence. The test results of present multi-layer radom were not affected by the Reynolds number, The maximum positive pressure coefficient was found to be 1.08 at the center of the front of the plane in angle of attack of 0 degree, the maximum negative pressure coefficient was -2.03 at the upper right corner in angle of attack of 120 degree, while maximum drag coefficient was 1.11 in angle of attack of 180 degree.

Shielding Effects of Surrounding Buildings on Wind Loads on Low-Rise Building Roofs

Low-rise buildings are often built in groups, and their wind loads differ from those on isolated buildings. In this study, the wind pressures on flat roofs of low-rise buildings surrounded by similar buildings were measured with a series of wind tunnel tests. The general properties of the shielding effects of surrounding buildings were discussed. The effects of area density, relative height, and arrangement patterns of the surrounding buildings on the shielding factors of the smallest minimum negative extreme wind pressure coefficients and of the largest peak uplift force coefficients on the whole roof of the shielded building for all wind directions were analyzed. For practical purposes, both shielding factors were expressed as equations of the building area density and the relative height of the surrounding buildings based on the wind tunnel test data, which can cover most of the test results.

Residual stress dependant anisotropic band gap of various (hkl) oriented BaI2 films

The thermally evaporated layer structured BaI2 grows in various completely preferred (hkl) film orientations with different growth parameters like film thickness, deposition rate, substrate temperature, etc. which were characterized by structural, morphological, and optical absorption measurements. Structural analysis reveals the strain in the films and the optical absorption shows a direct type band gap. The varying band gaps of these films were found to scale linearly with their strain. The elastic moduli and other constants were also calculated using Density Functional Theory (DFT) formalism implemented in WIEN2K code for converting the strain into residual stress. Films of different six (hkl) orientations show stress free anisotropic band gaps (2.48–3.43 eV) and both positive and negative pressure coefficients. The negative and positive pressure coefficients of band gap are attributed to the strain in I-I (or Ba-Ba or both) and Ba-I distances along [hkl], respectively. The calculated band gaps are also compared with those experimentally determined. The average pressure coefficient of band gap of all six orientations (−0.071 eV/GPa) found to be significantly higher than that calculated (−0.047 eV/GPa) by volumetric pressure dependence. Various these issues have been discussed with consistent arguments. The electron effective mass me*=0.66m0 and the hole effective mass mh*=0.53m0 have been determined from the calculated band structure.

Wind Loads on Single-span Plastic Greenhouses and Solar Greenhouses

Wind tunnel tests were conducted in an NH-2-type wind tunnel to investigate the wind pressure coefficients and their distribution on the surfaces of a single-span plastic greenhouse and a solar greenhouse. Wind pressures at numerous points on the surfaces of the greenhouse models were simultaneously measured for various wind directions. The critical wind speeds, at which damage occurred on the surfaces of single-span plastic greenhouses and solar greenhouses, were derived. To clearly describe the wind pressure distribution on various surface zones of the greenhouses, the end surface and top surface of the plastic greenhouse and the transparent surface of the solar greenhouse were divided into nine zones, which were denoted as Zone I to Zone IX. The results were as follows: 1) At wind direction angles of 0° and 45°, the end surface of the single-span plastic greenhouse was on the windward side, and the maximum positive wind pressure coefficient was near 1. At wind direction angles of 90° and 180°, the entire end surface of the single-span plastic greenhouse was on the leeward side, and the maximum negative wind pressure coefficient was near −1. The maximum positive wind pressure on the end surface of the single-span plastic greenhouse appeared in Zone IV at a wind direction angle of 15°, whereas the maximum negative pressure appeared in Zone VIII at a wind direction angle of 105°. 2) Most of the wind pressure coefficients on the top surface of the plastic greenhouse were negative. The maximum positive and negative wind pressure coefficient on the top surface of the plastic greenhouse occurred in Zones I and II, respectively, at a wind direction angle of 60°. 3) At a wind direction angle of 0°, the distribution of wind pressure coefficient contours was steady in the middle and lower zones of the transparent surface of the solar greenhouse, and the wind pressure coefficients were positive. At a wind direction angle of 90°, the wind pressure coefficients were negative on the transparent surface of the solar greenhouse. A maximum positive wind pressure coefficient was attained at a wind direction angle of 30° in Zone IX, whereas the maximum suction force occurred in Zone VII at a wind direction angle of 135°. 4) The minimum critical wind speeds required to impair the single-span plastic greenhouse and solar greenhouse were 14.5 and 18.9 m·s−1, respectively.