Suction Coefficient Research Articles

Chloride Penetration into Concrete

Reinforced concrete structures may be exposed to an aggressive environment. In this case combined mechanical and environmental actions may act simultaneously and their synergetic influences have to be taken into consideration. In this contribution results of tests on two different types of concrete and mortars are presented and discussed. Capillary absorption of not carbonated and carbonated concrete has been determined. The influence of mechanical load on capillary suction has been studied separately. A moderate compressive load reduces the coefficient of capillary suction, while capillary suction is enhanced by higher mechanical loads as micro-cracks are formed. Chloride diffusion has been measured by means of the diffusion cell test. The diffusion coefficient observed in carbonated concrete is roughly speaking one order of magnitude higher than the value measured on not carbonated concrete.

Measurements of Moisture Suction and Diffusion Coefficient in Hot-Mix Asphalt and Their Relationships to Moisture Damage

A procedure for measuring the total suction in hot-mix asphalt (HMA) mixes was developed in this study. Suction measurements were related to physical and chemical properties of HMA that influence moisture damage. This procedure was achieved in two phases. In the first phase, the total suction was measured in HMA specimens with different types of aggregates and with different air void distributions and aggregate gradations. The results showed that the drying test using a 60° C temperature-controlled room was the most appropriate setup for measuring the total suction in HMA with thermocouple psychrometers. The characteristics of suction-moisture content curves were found to be related to the air void distribution in HMA and to the resistance of the mixtures to moisture damage. In the second phase, total suction was measured in sand asphalt specimens. These specimens had different combinations of aggregates and binders that exhibited different field resistances to moisture damage. The suction measurements we...

Measurements of Moisture Suction and Diffusion Coefficient in Hot-Mix Asphalt and Their Relationships to Moisture Damage

A procedure for measuring the total suction in hot-mix asphalt (HMA) mixes was developed in this study. Suction measurements were related to physical and chemical properties of HMA that influence moisture damage. This procedure was achieved in two phases. In the first phase, the total suction was measured in HMA specimens with different types of aggregates and with different air void distributions and aggregate gradations. The results showed that the drying test using a 60°C temperature-controlled room was the most appropriate setup for measuring the total suction in HMA with thermocouple psychrometers. The characteristics of suction–moisture content curves were found to be related to the air void distribution in HMA and to the resistance of the mixtures to moisture damage. In the second phase, total suction was measured in sand asphalt specimens. These specimens had different combinations of aggregates and binders that exhibited different field resistances to moisture damage. The suction measurements were used to calculate the moisture diffusion coefficient. The results revealed that mixes with a higher diffusion coefficient are those that had poor resistance to moisture damage.

Flow Control of the Vortex Shedding from a Circular Cylinder by Asymmetrically Arranged Deflectors

This paper deals with the flow around a circular cylinder with an element inserted in the wake. A long splitter plate (splitter-plate case) and a circular cylinder of equal diameter (two-cylinder case) were used as the element. Experiments were mainly conducted in a water tank at a Reynolds number of 7.4×103 for the two-cylinder case and 1.1×104 for the splitter-plate case. As the deflectors are horizontally traversed upstream, the base suction coefficient exhibits a critcal tall at a certain position. PIV measurements revealed suppression of regular vortex shedding at the critical positions in both cases and a similar behavior of vortex formation length in the vicinity of the positions.

Flow past a cylinder: shear layer instability and drag crisis

AbstractFlow past a circular cylinder for Re=100 to 107 is studied numerically by solving the unsteady incompressible two‐dimensional Navier–Stokes equations via a stabilized finite element formulation. It is well known that beyond Re ∼ 200 the flow develops significant three‐dimensional features. Therefore, two‐dimensional computations are expected to fall well short of predicting the flow accurately at high Re. It is fairly well accepted that the shear layer instability is primarily a two‐dimensional phenomenon. The frequency of the shear layer vortices, from the present computations, agree quite well with the Re0.67 variation observed by other researchers from experimental measurements. The main objective of this paper is to investigate a possible relationship between the drag crisis (sudden loss of drag at Re ∼ 2 × 105) and the instability of the separated shear layer. As Re is increased the transition point of shear layer, beyond which it is unstable, moves upstream. At the critical Reynolds number the transition point is located very close to the point of flow separation. As a result, the shear layer eddies cause mixing of the flow in the boundary layer. This energizes the boundary layer and leads to its reattachment. The delay in flow separation is associated with narrowing of wake, increase in Reynolds shear stress near the shoulder of the cylinder and a significant reduction in the drag and base suction coefficients. The spatial and temporal power spectra for the kinetic energy of the Re=106 flow are computed. As in two‐dimensional isotropic turbulence, E(k) varies as k−5/3 for wavenumbers higher than energy injection scale and as k−3 for lower wavenumbers. The present computations suggest that the shear layer vortices play a major role in the transition of boundary layer from laminar to turbulent state. Copyright © 2004 John Wiley & Sons, Ltd.

非対称に仕切板を用いた円柱後流渦のPIV解析(S22-3 物体周り流れの制御-1,S22 流れ制御の技術)

The wake behind a circular cylinder (diameter d) with a long splitter plate inserted is investigated experimentally in a water tank at Re=1.12×10^4. The splitter plate is traversed at three levels (Z/d=0, 0.5, 1.0). As the splitter plate is shifted upstream horizontally, the base suction coefficient increased gradually, but it falls critically at a certain position depending on Z/d. To make clear the mechanism of the critical change, PIV measurements are conducted for pre- and post-critical regime. It is found that the stream pattern changes depending on level Z.

Drag Reduction on n Circular Cylinder using a Detached Splitter Plate

Control of drag farce on a circular cylinder using a detached splitter plate is numerically studied for laminar flow. A splitter plate with the same length as the cylinder diameter(d) is placed horizontally in the wake region. Its position is described by the gap ratio(G/d), where G represents the gap between the cylinder base point and the leading edge of the plate. The drag varies with the gap ratio; it has the minimum value at a certain gap ratio for each Reynolds number. The drag sharply increases past the optimum gap ratio; this seems to be related to the sudden change in bubble size in the wake region. This trend is consistent with the experimental observation currently available in case of turbulent flow. It is also found that the net drag coefficient significantly depends on the variation of base suction coefficient.

Flow control of vortex shedding by a short splitter plate asymmetrically arranged downstream of a cylinder

The flow around a circular cylinder (diameter d) was investigated, behind which a short thin splitter plate with a chord of 1.0d was inserted horizontally as an interference element. The plate was traversed upstream along the wake; hence its relative position is defined by gap G from the cylinder base to the plate tip and by level Z from the wake centerline. The variation in both base suction coefficient and Strouhal number with G/d significantly depends on Z/d. In the circular cylinder case with Z/d=0.5–1.3, as the plate approaches the cylinder, the base suction coefficient exhibits a critical fall in a similar fashion to Roshko’s experiment where Z/d=0. Interestingly, however, unlike his experiment, the Strouhal number exceeds the natural one for some range of G/d beyond the critical gap. To further examine the flow mechanism, a similar situation was investigated by using a rectangular cylinder (height h, depth 0.1h) in place of the circular cylinder. A rise in the Strouhal number is observed for Z/h=1.1. These rises in Strouhal number are explained by the flow mode in which the shear layer separated from the gap side is forced to flow into the gap and to interact in the nearer wake by the approach of the splitter plate.

B-Spline Method and Zonal Grids for Simulations of Complex Turbulent Flows

A numerical technique for computations of turbulent flows is described. The technique is based on B-splines and allows grid embedding in physically significant flow regions. Numerical tests, which include solutions of nonlinear advection-diffusion equations and computations of flow over a circular cylinder at Reynolds numbers up to 300, indicate that the method is accurate and efficient. In computations of flow over a cylinder, the lift, drag, and base suction coefficients agree well with existing experimental data and previous numerical simulations.

Liquid suction by porous ceramic materials 4: influence of firing conditions

A study was undertaken of the liquid suction rate and properties characterising the porous structure of 10 specimens prepared from a typical wall tile composition and fired using two different firing cycles. The experimental results satisfactorily fit a previously derived equation relating the suction coefficient to certain physical properties of the liquid (density, viscosity, and surface tension) and to the properties characterising the solid's porous structure (effective suction porosity, average pore radius, and average tortuosity factor).

Evolution of Vortical Structure over Delta-Wing with Transient Along-Core Blowing

Over delta-wing with a 75-deg sweep angle and 40-deg of the angle of attack, the vortex core subject to transient along-core blowing is investigated experimentally. In response to the transient along-core blowing,the vortex core, before breakdown, serves as a transmitting vessel that steadily delivers the supplied momentum to overcome the strong adverse along-core pressure gradient, especially near the onset location of the vortex breakdown. The central region of the vortex core is found to be more sensitive to the external perturbation than the outer region. This key characteristic of the vortex core is the reason for transformation of the along-core velocity distribution from the velocity excess (or jet-like) profile to the velocity deficit (or wake-like) profile as the onset of vortex breakdown is encountered. The magnitude of suction peak coefficient measured on the wing surface along the vortex core direction increases in response to the transient along-core blowing. Then it recovers to the unperturbed value as the vortex breakdown recovers to its long-time averaged location. Variation of pressure gradient along the vortex center differs from the gradient of the suction peak coefficient measured on the wing surface along the vortex core direction.

Suction effect on an impulsively started circular cylinder: Vortex structure and drag reduction

Drag reduction on an impulsively started circular cylinder by surface suction has been investigated with a joint numerical and experimental study. Two suction slots entraining mass with various rates were located symmetrically in a range of angles downstream of the points of zero shear stress. The Reynolds number Re of this study was between 500 and 2000, while the suction coefficient Ṁ varied from 0 to 0.08. Analysis based on the force contributed by fluid elements with nonvanishing vorticity [Chang, Proc. R. Soc. London Ser. A 437, 517 (1992)] has provided a clear view of the physical mechanism causing the drag reduction. For the impulsive flow with surface suction, the initial drag reduction is due to the strengthening of the front boundary layer, while the slender vortices in the near wake due to suction then become the dominant factor in reduction of the drag force in later stages. Close agreements were found between the experimental and numerical results, including streakline flow visualization and surface vorticity measurements on the top of the cylinder. In addition, a parametric study on the slot location, the suction angle, and the width of the suction slot has also been carried out in detail.

An Evaluation of Several Wind Turbine Trailing-Edge Aerodynamic Brakes

An investigation was undertaken to identify the aerodynamic performance of five separate trailing-edge control devices and to evaluate their potential for wind turbine overspeed control applications. A modular two-dimensional wind tunnel model was constructed and aerodynamic performance data were acquired. To further interpret their potential, the controls were evaluated using a performance analysis computer program and a generic wind turbine geometry. On the basis of the investigation results, the Spoiler-Flap control configuration was deemed best suited for turbine braking applications. This particular control exhibited a good suction coefficient behavior over a broad angle-of-attack range and good turbine braking capabilities, especially at low tip-speed ratios.

Multipoint inverse airfoil design method for slot-suction airfoils

Owing to a renewed interest in advanced-concept transport aircraft with active boundary-layer control (suction and/or blowing), a generalized multipoint method for the inverse design of airfoils with slot suction in incompressible potential flow is presented. The results predicted by theory show good agreement with experiment. Such a design tool could be used interactively to perform rapid trade studies to examine the potential payoff for boundary-layer control as applied to the advanced-concept wings. The method, which is based on conformal mapping, draws on the theory first published by Eppler (Airfoil Design and Data, Springer-Verlag, New York, 1990) and extended by Selig and Maughmer (A Multipoint Inverse Airfoil Design Method Based on Conformal Mapping, AIAA Journal, Vol. 30, No. 5,1992, pp. 1162-1170). The theory predicts an increased lift coefficient and a nonzero value for the inviscid drag coefficient as a result of the slot suction, a price to be paid for the increased lift. The theory also shows that this nonzero value of the inviscid drag coefficient is exactly equal to twice the suction coefficient. Two examples are discussed to illustrate the method and the corresponding change in the aerodynamic characteristics of the airfoils as a result of slot suction.

Control of streamwise vortices using selective suction

The breakdown of streamwise vortices was controlled using selective suction. A Gortler flowfield was used to produce streamwise vortices emulating turbulent boundary-layer eddies in a steady laminar-like environment. Suction was applied at single locations under the low-speed region between counter-rotating vortex pairs. A suction coefficient two orders of magnitude less than that required for an asymptotic profile was sufficient to significantly delay the breakdown of the vortices. At high-suction rate, however, premature breakdown resulted due to the creation of an additional instability in the spanwise direction. Results suggest that an ideal control method should produce fuller profiles in the normal direction and eliminate the difference between low- and high-speed regions in the spanwise direction.

An experimental investigation of the flow around a circular cylinder: influence of aspect ratio

The investigation is concentrated on two important quantities – the Strouhal number and the mean base suction coefficient, both measured at the mid-span position. Reynolds numbers from about 50 to 4 × 104 were investigated. Different aspect ratios, at low blockage ratios, were achieved by varying the distance between circular end plates (end plate diameter ratios between 10 and 30). It was not possible, by using these end plates in uniform flow and at very large aspect ratios, to produce parallel shedding all over the laminar shedding regime. However, parallel shedding at around mid-span was observed throughout this regime in cases when there was a slight but symmetrical increase in the free-stream velocity towards both ends of the cylinder. At higher Re, the results at different aspect ratios were compared with those of a ‘quasi-infinite cylinder’ and the required aspect ratio to reach conditions independent of this parameter, within the experimental uncertainties, are given. For instance, aspect ratios as large as L/D = 60–70 were needed in the range Re ≈ 4 × 103–104. With the smallest relative end plate diameter and for aspect ratios smaller than 7, a bi-stable flow switching between regular vortex shedding and ‘irregular flow’ was found at intermediate Reynolds number ranges in the subcritical regime (Re ≈ 2 × 103).

Flow past a wind-assisted ship propulsion device

An experimental investigation has been carried out on a wind-assisted ship propulsion device (WASP), consisting of a cylinder-flap wing with boundary layer suction. Measurements of the pressure distribution, lift and drag have been presented for various angular locations of the suction area and flap. It was found that a lift coefficient of 4.2 is obtained, with suction coefficient of 0.09, when the suction area and flap are symmetrically placed at angular locations of 110° from the relative wind axis; the corresponding lift-drag ratio is 6. Also presented is a polar plot which ascertained that the propulsion force is not significantly affected by small variations (±10°) in the angular position of the flap. However, a corresponding variation in the suction location will significantly affect the propulsion force.

Control of low-speed airfoil aerodynamics

Nomenclature CD = drag coefficient ( = 2D/pUlc) Cf = local skin-friction coefficient ( = 2rw/pUl) CL = lift coefficient ( = 2L/pU^c) Cq = suction coefficient ( = I vw I / UQ) c = airfoil's chord D = drag force per unit span L = lift force per unit span P = instantaneous hydrostatic pressure PO = pressure outside boundary layer P = mean pressure R = wall's radius of curvature /?6* = displacement thickness Reynolds number (= Uod*/v) RO = momentum thickness Reynolds number ( = U0de/v) Re = Reynolds number based on distance from leading edge (or chord) and freestream velocity T = instantaneous temperature T = mean temperature J7, = instantaneous velocity component Uj = mean velocity component U0 = velocity outside the boundary layer t/oo = freestream velocity HI = fluctuating velocity component u* = friction velocity ( = Vr^/p) vw = normal velocity of fluid injected or withdrawn through the wall X-, = Cartesian coordinates x = streamwise distance from leading edge y = normal distance from the wall y = normal distance in wall units ( = yu*/v) Z = spanwise coordinate a. = angle of attack 6 = boundary-layer thickness 60 = momentum thickness 6* = displacement thickness /x = dynamic coefficient of viscosity v = kinematic viscosity v/u * = viscous length scale (wall unit) p = density — p~uv = tangential Reynolds stress T.W = shear stress at the wall ( = pu *) [AJo = instantaneous spanwise vorticity at the wall _ [fijo = rnean spanwise vorticity at the wall (= — [dU/dy]Q)

Selective suction for controlling bursting events in a boundary layer

Continuous and intermittent suction from a single streamwise slot was used to eliminate either a single burstlike event or a periodic train of artificially generated bursts in laminar and turbulent boundary layers. The experiments were conducted using a flat plate towed in an 18-m water channel. Flow visualization and hot-film probe measurements were used together with a burst detection algorithm to demonstrate the feasibility of controlling bursting events in a boundary layer. When the suction parameters were optimized, equivalent values of the suction coefficient as low as 0.0006-0.0015 were sufficient to eliminate the artificially generated hairpin vortices.

Effect of a round airfoil nose on leading-edge suction

Kulfan (1979) assumed that the angle of attack for initial vortex separation on a slender wing with rounded leading edges could be obtained by equating the leading-edge suction (LES) and nose drag coefficients. In the present study, this assumption is examined and is shown to predict reasonably well the initial angle of attack at which laminar separation occurs near the airfoil nose. However, the assumption is shown to be slightly less accurate for thick or cambered airfoils. Attainable LES estimated by Kulfan's method seemed to agree well with that obtained from an airfoil aerodynamics code and experimental data on a NACA 64A009 airfoil at M = 0.4 and Re = 0.86 x 10 to the 6th.