Mean Pressure Distribution Research Articles

A note on the lubricating film in hydrostatic mechanical face seals

A hydrostatic model for analysing the lubrication film in mechanical seals with provision for surface roughness is presented. The model takes into account the wedge angle between the seal faces developed due to thermal gradient. It is useful for predicting the mean pressure distribution, fluid-film thickness, power loss, and leakage rate. An analytical solution for the fluid pressure distribution is obtained by solving the axis-symmetric Reynolds equation, without assuming constant fluid-film thickness. It is demonstrated that surface roughness considerably increases the mean fluid-film thickness. On the basis of predicted film thickness, the leakage rate and dissipated power can be easily predicted. The method and results can be useful as an adjunct analysis for more complicated thermoelastohydrodynamic analysis of mechanical seals.

Large eddy simulation of flow around wavy cylinders at a subcritical Reynolds number

The cross-flow around wavy cylinders of wavelength ratios λ/ D m from 1.136 to 3.333 are investigated at Re = 3000 using large eddy simulation (LES). The mean flow field and the near wake flow structures are presented and compared with those of a circular cylinder at the same Reynolds number. The mean pressure distributions are also calculated. The results show that the mean drag coefficients of the wavy cylinders are less than those of a corresponding circular cylinder due to a longer wake vortex formation length generated by the wavy cylinders. For a subcritical Reynolds number of 3000, a maximum drag coefficient reduction of up to 18% compared with a circular cylinder is obtained corresponding to an optimal wavelength ratio of λ/ D m around 1.9 and an amplitude ratio a/ D m of 0.152. The fluctuating lift coefficients of the wavy cylinders are also greatly reduced or even suppressed. These kinds of wavy surfaces lead to the formation of three-dimensional free shear layers which are more stable than purely two-dimensional free shear layers. Such free shear layers will only roll up into mature vortices at further downstream positions. This significantly modified the near wake structures and the pressure distribution around the cylinder. It was found that the wave amplitude to wavelength ratio a/ λ plays an essential role in determining the 3D vortex structure behind the wavy cylinders which has a significant effect on the reduction of the fluctuating lift and suppression of flow-induced vibration.

Pressure mapping as a complement in clinical sitting analysis in children during activity

The purpose of this study was to investigate whether pressure mapping can be used as a complement in sitting analysis. The purpose was also to study test–retest reliability and to obtain reference values in healthy children. The children performed a structured activity sitting on the pressure mat. The variables studied in children 4–15 years of age were the mean pressure distribution of one half of the body in relation to the whole sitting surface and the transfer of the centre of pressure (COP) in target reaching in relation to the calculated mean COP when sitting still. The standard deviation for the mean COP was calculated and test–retest for reliability was performed. The results showed significant differences between the youngest children and children 8–15 years of age concerning the transfer of COP sideways and in standard deviation of mean COP sideways. The results for weight distribution, COP transfer sideways and deviation from mean COP sideways were repeatable and consistent with studies of post...

Optical analysis of AMPA receptor motility in cultured hippocampal neurons

Turbulent flow around a two-dimensional V-shaped bluff body was experimentally investigated in an air stream. It models a V-gutter used as a flame holder in many jet engines. The mean pressure distribution along the surface and also in the wake was measured. Two-components of the mean and turbulent fluctuating velocities along with the turbulent shear stress were clarified in the wake.

Similarity study on mean pressure distributions of cylindrical and spherical bodies

This paper investigates the similarity existing among the pressure distributions of a circular cylinder and a sphere theoretically and experimentally. The theoretical distributions, in the context of potential flow, have the same expression when re-scaled in terms of coordinates involving quantities at specific points. This expression has been found useful in representing the experimental data (upstream of separation) in a variety of flow situations where the model is either circular or spherical. It is believed that such a rescaling isolates the boundary-layer effect so that the experimental and theoretical distributions are comparable. An application is included to demonstrate how this expression can provide a realistic prediction. The present approach may also be treated as an alternative to representing the experimental data with a finite Fourier series.

Wind tunnel study of a cone-like shaped roof: Reynolds number effects

This paper describes the results of wind tunnel tests performed on a cone-like shaped roof characterised by multiple curved surfaces. Surface pressure measurements were carried out by high-frequency scanners. In particular, Reynolds number and surface roughness effects on the mean pressure distribution and on its unsteady components have been analysed. Methodologies were found in order to validate the wind tunnel wind load measures performed on scale models having the typology of building roofs with curved surfaces.

Influence of surface roughness on pressure distribution and film thickness in EHL-contacts

To better understand the effect of surface roughness and texture on the formation of a lubricating oil film the mean film thickness and pressure distribution in the Hertzian contact zone were systematically investigated with an FZG twin-disk test rig. The pressure distribution was measured using thin film sensors. The influence of roughness and surface texture was investigated as a function of load and sum velocity. Different types of surfaces were produced by circumferential and transverse grinding as well as structuring by etching.

Study on Flow Induced Vibration of Head Disk Assembly Mechanisms in Actual Hard Disk Drives

It is important to clarify air flow induced bibration characteristics in hard disk drives in order to achieve ultra-high magnetic recording density. We studied flow induced flutter vibration in actual 2.5 inch HDD which has one disk and two magnetic heads. In this study, the disk flutter vibration as well as static pressure distribution between the top cover and disk were measured, changing the track following and seek modes of the magnetic head slider. From the experimental results, it was found that the disk flutter amplitude increases and its frequency decreases, when the magnetic head is operating on a disk. We also found that the mean pressure distribution was changed under various head operating conditions. As a result, it suggests that a disk flutter amplitude increase is due to the change of mean pressure distributions and disk flutter frequency decrease is caused by the effect of head-disk coupling vibration system.

The Effects of Splitter Plates on Turbulent Boundary Layer Developing on a Flat Plate near the Trailing Edge

An experimental investigation has been made for a turbulent boundary layer near the trailing edge on a long flat plate. The flow was controlled by an additional splitter plate fitted to the trailing edge along the wake center line. The length of the splitter plate, l, was varied from a half, to five times the trailing edge thickness, h. Measurements of base pressure behind the trailing edge and of mean velocity and pressure distribution in the turbulent boundary layer on the flat plate were made under the freestream zero-pressure gradient. The absolute value of base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate (bluff body) without splitter plate (l/h=0). A significant increase in the normalized base pressure coefficient (about 50% as same as short one) was achieved with the splitter plate (l/h≥1) fitted to the long flat plate. Within an inner layer in the turbulent boundary layer near the trailing edge, mean velocity increased than that in the upstream position, especially in the case without splitter plate. With splitter plate, however, the base pressure rise made mean velocity distribution come closer to that of fully developed turbulent boundary layer. In the case of l/h≥1, the affected range of acceleration decreased from 10 h upstream of the trailing edge without splitter plate to 5 h upstream.

Wind effects on a large cantilevered flat roof: loading characteristics and strategy of reduction

Mean and extreme pressure distributions on a large cantilevered flat roof model are measured in a boundary layer wind tunnel. The largest peak suction values are observed from pressure taps beneath conical "delta-wing type" corner vortices that occur for oblique winds, then the characteristics and causes of the local peak suctions are discussed in detail. Power spectra of fluctuating wind pressures measured from some typical taps located at the roof edges under different wind directions are presented, and coherence functions of fluctuating pressures are also obtained. Based on these results, it is verified that the peak suctions are highly correlated with the conical vortices. Furthermore, according to the characteristics of wind loads on the roof, an aerodynamic solution to minimize the peak suctions by venting the leading edges and the corners of the roof is recommended. The experimental results show that the suggested strategy can effectively control the generation of the conical vortices and make a reduction of 50% in mean pressures and 25% in extreme local pressures at wind sensitive locations on the roof.

Prediction of wind loads on a large flat roof using fuzzy neural networks

Fuzzy neural networks (FNN) have capability to develop complex, nonlinear functional relationships between input–output patterns based on limited and sometimes inconsistent data, and are therefore suitable to predict wind loads on buildings on the basis of data obtained from model tests in wind tunnels. In this study, simultaneous pressure measurements are made on a large flat roof model in a boundary layer wind tunnel. An FNN approach is developed for prediction of mean pressure distributions on the roof model, and parts of the wind tunnel test results are used as the training sets for the FNN to recognize the pressure distribution patterns. The procedure is further extended to predict the power spectra and cross-power spectra of fluctuating wind pressures for some typical tap locations in the roof corners and leading edge areas under different wind directions. It is found that the developed FNN approach can generalize functional relationships of wind loads varying with incident wind directions and spatial locations on the roof, and can successfully predict the wind loads on the roof which are not fully covered by the wind tunnel measurements. It is demonstrated from this study that the adoption of the FNN approach can lead to a significant reduction of the pressure measurement programs (e.g., incident wind direction configurations and number of required pressure taps) in wind tunnel tests.

A parametric, experimental analysis of conical vortices on curved roofs of low-rise buildings

Different methods to reduce the high suction caused by conical vortices have been reported in the literature: vertical parapets, either solid or porous, placed at the roof edges being the most analysed configuration. Another method for alleviating the high suction peaks due to conical vortices is the use of some non-standard parapet configuration like cantilever parapets. In this paper the influence of roof curvature on the conical vortex pattern appearing on a curved roof (Fig. 1) when subject to oblique winds is experimentally analysed by testing the mean pressure distribution on the curved roofs of low-rise building models in a wind tunnel. Also, the efficiency of cantilever parapets to reduce mean suction loads on curved roofs is experimentally checked. Very high suction loads have been measured on curved roofs, the magnitude of these high suction loads being significantly decreased when cantilever parapets are used. Thus, the suitability of these parapets to reduce wind pressure loads on curved roofs is demonstrated.

Control of Vortical Flow over a Rounded Leading-Edge Delta Wing

An attempt is made for fluidic control of the vortical flow and its related vortex breakdown phenomenon over a delta wing with a particular geometry. The model has a sweep angle of A = 60 deg and a rounded leading edge. The control system is based on four rectangular slots along the portside leading edge, and it provides continuous or pulsed jets normal to the leading-edge surface and parallel to the leeward side plane. The mass flow rate and the frequency of injection can be varied independently. The results are compared to a reference case defined by a microflap fixed along the leading edge and set perpendicular to it. Qualitative measurements are obtained with surface flow visualizations and boundary-layer transition detections with acenaphten. Quantitative data consist in mean and instantaneous surface pressure distributions measurements under the primary vortex axis and in mean three-dimensional velocity held measurements. The control effect is analyzed by means of laser sheet visualizations to detect the vortex breakdown position and by normal force and rolling moment measurements with a five-component balance.

Similarities of pressure induced by separation bubble in grid-generated turbulent flow

Some existing wind-tunnel data on mean pressure and fluctuation pressure distributions, induced by a separation bubble near the leading edge of a blunt plate, in the presence of grid-generated freestream turbulence, are systematically analyzed in this paper. The mean pressure distributions, when expressed in reduced coordinates, are self-similar and may be approximated by the pressure variation induced by an inviscid rotational vortex hovering above a flat surface in uniform smooth flow. These coordinates are expressed in terms of characteristic parameters of the mean flow field and each parameter has been correlated with the longitudinal component of the turbulence intensity and scale non-dimensioned by the plate thickness. In analyzing the pressure fluctuation, a similar approach is used. The results from the rotational vortex are consistently useful when considering the leading portion of the bubble characterized by the location of maximum fluctuation. The remaining portion may be represented by an exponential function characterized by the turbulence intensity only. The outcome of this study may be used as a prediction method whose validity has been checked with experimental data.

Effect of the number of grooves on flow characteristics around a circular cylinder with triangular grooves

In the flow around a circular cylinder, a sudden decrease in the mean drag coefficient occurs at a high Reynolds number, but the same phenomenon occurs at a lower Reynolds number in the case where there exist grooves or roughness on the cylinder surface. In this paper, in order to make clear the flow characteristics around a cylinder with 20, 26 and 32 triangular grooves, the mean drag coefficient, pressure distribution, velocity distribution and turbulence intensity distribution were measured. Moreover, the flow around the cylinder was analyzed by applying the RNG k - e turbulent model, and the surface flow pattern was investigated using the oil-film technique. From these results, it was found that a sudden decrease in the mean drag coefficient of a cylinder with 32 triangular grooves occurs at a lower Reynolds number compared with 20 and 26 triangular grooves.

A world without Greenland: impacts on the Northern Hemisphere winter circulation in low- and high-resolution models

To investigate the effect of Greenland’s orography on the northern hemisphere winter circulation experiments with an atmospheric GCM are conducted: a perturbed integration where standard orography is reduced to sea level in the Greenland area is compared to a standard orography control integration. The outcome of these experiments suggests that the existence of high mountains at Greenland causes a reinforcement of the stationary wave field in the Atlantic sector, colder temperatures to the west of Greenland and warmer temperatures to the east and south, over the North Atlantic. The impact on the flow field cannot be understood in the framework of standing Rossby waves, but it indicates a resonance between remotely forced stationary waves and local (thermo-) dynamics. The pattern of the North Atlantic Oscillation (NAO), in particular the northern centre, lies further to the east in the flat-Greenland experiment compared to the control run and the observations. Together with the fact that the climatological low-pressure system around Iceland hardly shifts, this suggests that the location of the NAO is not necessarily tied to the time mean pressure distributions. Considering the model resolution as a parameter, experiments with a high resolution (T106) suggest that the near-field changes are represented sufficiently by a T42 resolution, a standard resolution used in state-of-the-art coupled climate models. In contrast, far-field changes depend critically on model resolution. Hemispheric circulation and temperature changes differ substantially from low to high resolution, and generalized statements about the impact of Greenland’s orography cannot be made.

Fluctuating wind loads across gable-end buildings with planar and curved roofs

Wind tunnel model studies were carried out to determine the wind load distribution on tributary areas near the gable-end of large, low-rise buildings with high pitch planar and curved roof shapes. Background pressure fluctuations on each tributary area are described by a series of uncorrelated modes given by the eigenvectors of the force covariance matrix. Analysis of eigenvalues shows that the dominant first mode contributes around 40% to the fluctuating pressures, and the eigenvector mode-shape generally follows the mean pressure distribution. The first mode contributes significantly to the fluctuating load effect, when its influence line is similar to the mode-shape. For such cases, the effective static pressure distribution closely follows the mean pressure distribution on the tributary area, and the quasi-static method would provide a good estimate of peak load effects.

An experimental and numerical investigation on a confined impinging air jet at high Reynolds numbers

An experimental and numerical study is carried out to investigate flow field of a confined jet issuing from the lower surface and impinging normally on the upper surface. The mean velocity, turbulence intensity and pressure distributions in the impingement region were obtained for Reynolds numbers ranging from 30,000 to 50,000 and a nozzle-to-plate spacing range of 0.2–6. The effects of Reynolds number and nozzle-to-plate spacing on the flow structure are examined. A subatmospheric region occurs on the impingement plate at nozzle-to-plate spacings up to 2 for Reynolds numbers studied and it moves radially outward from the stagnation point with increasing nozzle-to-plate spacing. It is concluded that there exists a linkage among the subatmospheric region, turbulence intensity and heat transfer coefficients. The numerical results obtained using the standard k– ε turbulence model are in agreement with the experimental results except for the nozzle-to-plate spacings less than one.

Numerical investigations of flow over a sphere in the subcritical and supercritical regimes

The flow field around a sphere in an uniform flow has been analyzed numerically for conditions corresponding to the subcritical (laminar separation) and supercritical (turbulent separation) regimes spanning a wide range of Reynolds numbers (104–106). Particular attention has been devoted to assessing predictions of the pressure distribution, skin friction, and drag as well as to understanding the changes in the wake organization and vortex dynamics with the Reynolds number. The unsteady turbulent flow is computed using detached-eddy simulation, a hybrid approach that has Reynolds-averaged Navier–Stokes behavior near the wall and becomes a large eddy simulation in the regions away from solid surfaces. For both the subcritical and supercritical solutions, the agreement with experimental measurements for the mean drag and pressure distribution over the sphere is adequate; differences in skin friction exist due to the simplistic treatment of the attached boundary layers in the computations. Improved agreement in the skin-friction distribution is obtained for the supercritical flows in which boundary layer transition is fixed at the position observed in experiments conducted at the same Reynolds numbers. For the subcritical flows the Strouhal number, St, associated with the large-scale shedding is predicted at St∼0.195 along with a higher frequency component associated with the development of the Kelvin–Helmholtz instabilities in the detached shear layers. If in the subcritical regime the wake assumes a helical-like form due to the shedding of hairpin-like vortices at different azimuthal angles, in the supercritical regime the wake structure is characterized by “regular” shedding of hairpin-like vortices at approximately the same azimuthal angle and at a much higher frequency (St∼1.3) that is practically independent of the Reynolds number and not sensitive to the position of laminar-to-turbulent transition.

Experimental investigation on large amplitude standing wave induced in closed tubes with varying cross section

When a gas-filled closed tube is driven near the resonant frequency of the gas-column, large amplitude standing wave is induced in the tube. The use of large amplitude standing wave in practical field is based on the exploitation of high amplitude pressure fluctuation together with the associated phenomena like acoustic streaming, thermoacoustic effect and mean pressure distribution. It is already known that shock wave appears in cylindrical tube and thereby the wave motion is limited in the peak pressure that can be generated. In shaped tubes, it is possible to avoid the problem of shock formation and as a result, large amplitude standing wave can be achieved. And also the mode of standing wave induced in shaped tubes depends on the tube geometry. For practical application of large amplitude standing wave, the selection of optimum tube shape must be required. In this report, experimental results are described for three types of tube geometry, such as conical, exponential and half cosine. HFC134a was selected as the working gas for the reason that the effects of tube geometry in the wide range of pressure amplitude can be investigated than air. The frequency responses and pressure fluctuation at closed end of each tube were monitored. The result revealed that the half cosine geometry is suitable in the case of design of the acoustic compressor.