Base Pressure Measurements Research Articles

Measurements of Fluctuating Pressures on a Circular Cylinder in Subsonic Cross Flow

A circular cylinder is tested in crossflow over the subsonic speed range. Time-resolved pressure distributions give information on surface pressure fluctuations; the corresponding drag and base drag coefficients are provided. Measured base pressure fluctuations at low Mach numbers are in agreement with the findings of other researchers. Flow changes at higher subsonic velocities and into the transonic range are described. Illustrations are drawn from the observations of other researchers, enabling physical explanations to be given to assist toward a more general modeling of the problem. At Mach numbers above 0.6 the changing strength of the vortices reduces the base drag coefficient up to a Mach number of 0.9, at which the onset of sonic flow increases the drag. Strouhal number variation is compared with the measurements of other authors. The paper concentrates on providing reliable measurements of vortex shedding and base pressure over the subsonic speed range rather than attempting to provide universal correlations.

The Effects of Splitter Plates on Turbulent Boundary Layer on a Long Flat Plate Near the Trailing Edge

An experimental investigation has been made on 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 centerline. 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 the base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate without the splitter plate. A significant increase in the base pressure coefficient 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, the mean velocity increased more than that in the upstream position in the case without the splitter plate. With the splitter plate, however, the base pressure rise made the mean velocity distribution more closely approach that of a fully developed turbulent boundary layer.

Base Pressure Measurements under a Scale Model Stockpile: Effects of Offset Reclaim Channels and Predictive Model Developments

It has been shown (McBride, 2006) that under a scale model stockpile a central pressure dip, or ‘M’ pressure, is formed during both the filling and refilling processes of a stockpile containing a central reclaim channel. This article presents the results from further experiments with a noncentral reclaim channel and clearly illustrates that a central pressure dip exists under a conical stockpile with an offset reclaim channel. Additional data were recorded regarding lateral load transfer within the stockpile on initiation of flow, although more experimental work is required to fully understand this aspect. A method for predicting conical stockpile base pressures is presented along with an overpressure theory to account for the vertical load instability induced by the onset of material discharge.

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.

Base Pressure Measurements under a Scale Model Stockpile

ABSTRACT The ability of a granular bulk material to transmit internal shear stress is a fundamental property that is often overlooked or underestimated, yet it is this ability that allows many observed physical phenomena to occur. One such area that continues to be a focus for numerical, theoreticals and experimental researchers is the pressure that a stockpile of granular material places on the supporting surface. While appearing to be a simple problem, the ability of granular materials to transmit shear stress makes this an extremely complex problem. This article presents high-quality data collected under a 2 m high stockpile and full details of the experimental facilities used in the collection of the data. The work presented is of significant value, having a much greater scale than previous studies (Jotaki & Moriyama, 1979; Lee & Herington, 1971; Smid & Novosad, 1981), and the deliberate inclusion of a central reclaim channel offers insight into stress changes during gravity reclaim and refilling. The results of this work have shown that the so-called ‘M’ pressure does exist under larger stockpiles both with and without reclaim hoppers; of more significance is the reemergence of the M pressure upon refilling of an emptied stockpile. This clearly illustrates that the M pressure is a robust and natural pressure distribution for a conical stockpile.

A hot cathode ionization gauge transmitter for industrial vacuum measurement

The measurement of high vacuum pressure with hot cathode ionization gauges has basically been known for a long time. This is also true for the problems related to such measurements, as reproducibility, long-term stability and accuracy. In industrial measurement technology this results in appreciable expenses for maintenance and calibration of such units in systems, e.g. in semiconductor industry.This new sensor works according to the Bayard-Alpert principle, however in the wide range of 10 −10 to 0.1 mbar. In this way both process and base pressure measurements are possible with one sensor. By the design of the sensor and an integrated EEPROM with individual calibration data, high reproducibility and long-term stability of measurement values is achieved. Process capability is secured by electrical and mechanical shielding means. Thus, also under difficult conditions (discharges, spray water, dust, .) stable and reproducible measurement values are produced.Moreover, the microprocessor is also able to correct the pressure indication for different gas compositions depending on the process involved. © 1998 Elsevier Science Ltd. All rights reserved

Effects of a splitter plate on the near wake of a circular cylinder in 2 and 3-dimensional flow configurations

Results of flow visualization, hot wire, and base pressure measurements were conducted for an investigation of the near wake of a circular cylinder at subcritical Reynolds numbers between 2700 to 46000. A base mounted splitter plate allowed for the modification of the formation region characteristics without disrupting the normal Karman shedding. The results provide an explanation for the non-linearity in the relationship between shedding frequency and splitter plate length and extend the previous investigations of Roshko (1954), Gerrard (1966) and Apelt et al. (1973). In addition to the nominal 2-D configurations, a sinuous trailing edge splitter plate, cylinder taper, and shear flow were incorporated to study the effects of mild 3-dimensionality. A strong spanwise coherence was found to exist in the formation region. A superposition principle was discovered which showed that certain 3-D geometry and flow configurations could be combined to produce a nominal 2-D wake.

Some effects of ground clearance and ground plane boundary layer thickness on the mean base pressure of a bluff vehicle type body

Abstract The use of a moving ground plane is an established wind tunnel technique for ground simulation in surface vehicle tests. However, the data available relating to the influence of variations in ground plane boundary layer thickness on bluff body flowfields is limited. Results from a short series of wind tunnel tests, using a moving ground facility, are presented, in which mean base pressure measurements are made on three simple bluff body models while; (i) the moving ground speed, relative to the freestream speed, and (ii) the model ground clearance, are varied. The results show that reducing the moving ground speed has a similar effect on the base pressure to a reduction in the model ground clearance. This effect on base pressure is limited to moving ground speeds less than those which result in a significant increase in the ground boundary layer displacement thickness, except in the case of a more complex tandem module body. The data for this model, chosen to simulate a typical cab-container commercial vehicle, suggest that the presence of a gap between the front and rear modules increases significantly the sensitivity of the base pressure to the moving ground speed. Data from this limited study may be useful when comparing measurements made on vehicles with particularly small ground clearances above fixed and/or moving ground planes. They may also serve as a guide to the assessment of the tolerance of a moving ground system to belt speed variations in terms of the ground boundary layer displacement thickness.

Obtaining extremely high vacuum using sintered fine copper powder as a cryosorbent

Fine copper powder (FCP), sintered to the cryopanels of cryopumps, was evaluated as a cryosorbent for H2. A Gifford–McMahon refrigerator cooled the tested cryopanel of a two‐stage cryopump. The third stage of a three‐stage cryopump that was tested was cooled by Joule–Thomson expansion of He. 30 g of FCP were sintered to the second stage panel (20 K) of the two‐stage bakeable cryopump, whereas 10 g of FCP were sintered to the third stage panel (4.3 K) of the three‐stage bakeable cryopump. Hydrogen pumping speed, capacity, and base pressure measurements are given as a function of temperature. Cryopanel preparation and system bakeout procedures, reliably leading to pressures <10−9 Pa, are given. Hydrogen thermal desorption peaks for the FCP sintered cryopanel are also reported.

Note on a Discontinuous Change in Wake Pattern for a Rectangular Cylinder

Base pressure measurements on rectangular cylinders with different side ratios, d/h = 0·4-0·6, where d is the depth and h is the height, are presented. A cylinder with d/h = 0·5 shows characteristic time variations in base pressure, with alternatively high and low values, almost equal to those for d/h = 0·4 and 0·6 (the critical depth), respectively. This is caused by a sudden change in the flow pattern around the cylinder, associated with the vortex formation: for cylinders with depths below the critical, the flow around a cylinder initially changes gradually with increasing d/h, but, d/h reaches a transitional range around d/h = 0·5, there is an abrupt change in the flow pattern.

Base pressure measurements on a cone at Mach numbers from M∞ = 5 to 7

Base pressure measurements were performed on a blunt cone in the Ludwieg-Tube facility at the DLR in Gottingen at Mach numbers from M∞ = 4.99 to 6.83. The angle of incidence was varied between α = 0° and 15°. The results show the influence of Mach number and angle of incidence on the base pressure.

Flight tests of external modifications used to reduce blunt base drag

The effectiveness of a trailing disk (the trapped vortex concept) in reducing the blunt base drag of an 8-in diameter body of revolution was studied from measurements made both in flight and in full-scale wind-tunnel tests. The experiment demonstrated the significant base drag reduction capability of the trailing disk to Mach 0.93. The maximum base drag reduction obtained from a cavity tested on the flight body of revolution was not significant. The effectiveness of a splitter plate and a vented-wall cavity in reducing the base drag of a quasi-two-dimensional fuselage closure was studied from base pressure measurements made in flight. The fuselage closure was between the two engines of the F-111 airplane; therefore, the base pressures were in the presence of jet engine exhaust. For Mach numbers from 1.10 to 1.51, significant base drag reduction was provided by the vented-wall cavity configuration. The splitter plate was not considered effective in reducing base drag at any Mach number tested.

Interference-free measurements of the subsonic aerodynamics of slanted-base ogive cylinders

Drag, lift, pitching moment, and base-pressure measurements have been made, free of support interference, on a range of slanted-base ogive cylinders, using the NASA Langley Research Center 13-in magnetic suspension and balance system. Test Mach numbers were in the range 0.04-0.2. Two types of wake flow were observed, a quasi-symmetric turbulent closure or a longitudinal vortex flow. Aerodynamic characteristics differ dramatically between the two wake types. Drag measurements are shown to be in agreement with previous tests. A hysteretic behavior of the wake with varying Reynold's number has been discovered for the 45-deg base. An interaction between forebody boundary-layer state and wake flow and base pressures has been detected for higher slant angles.

Factors affecting cryopump base pressure

Increased device sizes and the demand for higher yields in the semiconductor fabrication industry has led to a requirement for improved base pressure in the vacuum systems used to process devices. This has led to the need to improve the ability of the cryopumps used on these systems to achieve a lower base pressure. Specifically, cryopumps used on production sputtering systems purposely limit pumping speeds for all gases except water vapor. This ensured a reduction in throughput for the sputtering gases used at high pressures (>10−1 Pa) but potentially limits the ultimate pressure that can be reached. Base pressures reached by cryopumps used on production sputtering systems will be described. Furthermore, the base pressure attained by a standard pump will be compared to that of a specially designed all‐metal pump. Finally, the effect of bake‐out on these pumps will be presented. Cryopumps are capture pumps. It is important to know how the base pressure is affected as gases collect on the pumping surfaces. Base pressure measurements made after accumulating gases by either cryocondensation or by cryosorption will be reported. The evolution of gases from the pumping surfaces as a result of short‐term pumping interruptions (power failure, etc.) will be shown and any effect on the subsequent base pressure achieved by the pump will be reported.

Wall-interference corrections for parachutes in a closed wind tunnel

An extensive test program was conducted to gather information on wall-interference effects for parachutes in closed wind tunnels. Drag area and base pressure measurements were made for a set of ribbon parachutes of 7%, 15% and 30% geometric porosity in six different wind tunnels, covering a range of geometric blockages from two to thirty-five percent. The resulting data have been used to formulate and validate approximate blockage correction equations based on the theory of Maskell. The corrections are applicable to single parachutes and clusters of two and three parachutes. 8 refs., 7 figs., 1 tab.

Flow past two-dimensional ribbon parachute models

Aerodynamic characteristics of two-dimensional, slotted bluff bodies are experimentally investigated. The models consist of a series of thin flat plates arranged to form flat and curved geometries. Flow visualizations, base pressure measurements, mean velocity vector measurements, arid drag force measurements are conducted to analyze the effects of spacing ratio (i.e., porosity), curvature, and a center opening (i.e., vent). Low-porosity model configurations produced stable near-wake patterns with enhanced vortex sheddings downstream. Model curvature reduced drag forces and weakened the vortex sheddings. Stabilizing effect of curvature on the nearwake patterns was also found. A vent combined with large model curvature was found to reduce drag force significantly as well as suppress the vortex sheddings among models with equivalent porosity.

Effects of External Compression on an Axisymmetric Turbulent Near Wake

An experimental study of base flows for a 5.715-cm-diam projectile at Mach 3 with and without disturbances simulating external burning is reported. Base pressure and wake structure measurements from systematic variations in axisymmetric external compression fields show that substantial base thrust can be produced and that the pressure and length scales of the external disturbances are imposed on the near wake. Measurements with comparable axisymmetric and asymmetric compression contours show that peripherial gradients reduce the base pressure rise. Furthermore, relatively large changes in the near wake length scales and a slight reduction in the base pressure occur when the solid-body blockage effects of discrete radial fuel jets are modeled with pegs mounted near the base of the test projectile.

Influence of rear body eccentricity on supersonic base pressure

Base pressure measurements for a blunt cone in the presence of a following body of different eccentricities are presented. The measurements were carried out for sting-supported models on a blowdown-type Mach 2 wind tunnel. The results were obtained for a range of Reynold's number in which its influence was suspected to be appreciable, on the basis of earlier investigations by other authors. The variation of Reynold's number was obtained by changing the stagnation pressure of the tunnel. The influence of following bodies on the base pressure was found to be appreciable, whereas the dependence of the base pressure on the Reynold's number in the range of investigation was not found to be marked. Also, it was found that the asymmetric body affected the symmetry of base pressure, setting a cross force on the blunt cone under test. The study could be of relevance to the prediction of base pressure of multistage rockets in flight when the lower stage after separation is in proximity to the upper stages. Nomenclature d • = diameter of rear body D = diameter of base of blunt cone x = axial distance of the front face of the rear body from the base e = eccentricity (i.e., distance between axes of the test body and the rear body), mm PB = base pressure Pco =freestream static pressure NRg = Reynold's number based on base diameter NRL = Reynold's number based on body length

Wind-tunnel-wall constraint on two-dimensional rectangular-section prisms

The effect of wall constraint on rectangular-section prisms is investigated experimentally in a low-turbulence wind tunnel up to a blockage ratio of 0.25. The depth to width ratio ( d h ) of the sections varies from 0.5 to 5.0. It is shown that the effect of confinement on the mean surface pressure on the models is not uniform in the streamwise direction and therefore cannot be regarded as an increase in the effective dynamic pressure estimated from drag or base pressure measurements as assumed by some investigators. The current bluff-body blockage correction formulae are found to be inadequate for correcting the drag and base pressure for these sections, particularly at large blockage ratios. A new equation, which is a modification of Maskell's equation and includes a shape factor to allow for the depth of the rectangular section, yields satisfactorily corrected drag and base pressure. Due to the large depth of the sections in the stream direction, corrections to the pressure distribution around them for blockage using this equation, are not very satisfactory. The correction for the Strouhal number using the new equation is valid up to d h = 3.0 .

Projectile Thrust-Drag Optimization with External Burning

Theme A analysis using the method of characteristics with heat addition and the Crocco-Lees base flow model is described for calculating the external supersonic flow with heat addition about the wake of a 5-in. projectile. Various amounts and locations of heat addition are found to generate a net thrust, and an optimum location for the combustion zone is predicted. Good agreement is observed between calculated and experimental base pressure measurements using a free-jet combustion simulator. The results from computing base pressure ratio over a wide range of Reynolds numbers are also presented.