| 1. |
- Lundell, Fredrik, et al.
(författare)
-
Fluid Mechanics of Papermaking
- 2011
-
Ingår i: Annual Review of Fluid Mechanics. - : Annual Reviews. - 0066-4189 .- 1545-4479. ; 43, s. 195-217
-
Forskningsöversikt (refereegranskat)abstract
- Papermaking is to a large extent a multiphase flow process in which the structure of the material and many of the relevant properties of the final product are determined by the interaction between water and the wood fibers. The dominant feature of a suspension composed of wood fibers and water is its inherent propensity to form bundles of mechanically entangled fibers, known as fiber flocs. However, the phenomena apparent throughout the papermaking process are not unique but in fact have a generic fluid dynamical nature.
|
|
| 2. |
- Ljustell, Pär, 1977-
(författare)
-
Fatigue crack growth experiments and analyses - from small scale to large scale yielding at constant and variable amplitude loading
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is on fatigue crack growth experiments and assessments of fatigue crack growth rates. Both constant and variable amplitude loads in two different materials are considered; a nickel based super-alloy Inconel 718 and a stainless steel 316L. The considered load levels extend from small scale yielding (SSY) to large scale yielding (LSY) for both materials.The effect of different load schemes on the fatigue crack growth rates is investigated on Inconel 718 and compact tension specimens in Paper A. It is concluded that load decreasing schemes give a to high Paris law exponent compared to constant or increasing load amplitude schemes. Inconel 718 is further analyzed in Paper B where growth rates at variable amplitude loading in notched tensile specimens are assessed. The predictions are based on the fatigue crack growth parameters obtained in Paper A. The crack closure levels are taken into consideration and it is concluded that linear elastic fracture mechanics is incapable of predicting the growth rates in notches that experience large plastic cyclic strains. Even if crack closure free fatigue parameters are used and residual stresses due to plasticity are included. It is also concluded that crack closure free and nominal fatigue crack growth data predict the growth rates equally well. However, if the crack closure free parameters are used, then it is possible to make a statement in advance on the prediction in relation to the experimental outcome. This is not possible with nominal fatigue crack growth parameters.The last three papers consider fatigue crack growth in stainless steel 316L. Here the load is defined as the crack tip opening displacement parameter. Paper C constitutes an investigation on the effect of plastic deformation on the potential drop and consequently the measured crack length. It is concluded that the nominal calibration equation obtained in the undeformed geometry can be used at large plastic deformations. However, two conditions must be met: the reference potential must be taken in the deformed geometry and the reference potential needs to be adjusted at every major change of plastic deformation. The potential drop technique is further used in Paper D and Paper E for crack length measurements at monotonic LSY. Constant amplitude loads are considered in Paper D and two different variable amplitude block loads are investigated in Paper E. The crack tip opening displacement is concluded in Paper D to be an objective parameter able to characterize the load state in two different geometries and at the present load levels. Furthermore, if the crack tip opening displacement is controlled in an experiment and the local load ratio set to zero, then only monotonic LSY will appear due to extensive isotropic hardening, i.e. elastic shake-down. This is also the reason why the linear elastic stress-intensity factor successfully could merge all growth rates, extending from SSY to monotonic LSY along a single line in a Paris law type of diagram, even though the generally accepted criteria for SSY is never fulfilled. For the variable amplitude loads investigated in Paper E, the effect of plastic deformation on measured potential drop is more pronounced. However, also here both the crack tip opening displacement parameter and the linear elastic stress-intensity factor successfully characterized the load state.
|
|
| 3. |
- Facciole, L., et al.
(författare)
-
Swirling jets issued from fully developed rotating pipe flow : Experiments and numerics
- 2005
-
Konferensbidrag (refereegranskat)abstract
- Direct Numerical Simulation (DNS) and experimental data are used to study a rotating jet flow. A non-confined swirling jet is generated by a fully developed rotating turbulent pipe flow. Previous experiments have demonstrated the presence of a counter-rotating core appearing approximatively 6 diameters downstream the pipe outlet. The mean azimuthal velocity component changes its sign in the central part of the jet starting to move in the opposite direction with respect to the rotation imposed by the rotation of the pipe. The present paper introduces new investigations intended to analyse the jet flow in the proximity of this phenomenon.
|
|
| 4. |
- Ford, C. L., et al.
(författare)
-
Development of a pressure based vortex-shedding meter : measuring unsteady mass-flow in variable density gases
- 2016
-
Ingår i: Measurement science and technology. - : Institute of Physics Publishing (IOPP). - 0957-0233 .- 1361-6501. ; 27:8
-
Tidskriftsartikel (refereegranskat)abstract
- An entirely pressure-based vortex-shedding meter has been designed for use in practical time-dependent flows. The meter is capable of measuring mass-flow rate in variable density gases in spite of the fact that fluid temperature is not directly measured. Unlike other vortex meters, a pressure based meter is incredibly robust and may be used in industrial type flows; an environment wholly unsuitable for hot-wires for example. The meter has been tested in a number of static and dynamic flow cases, across a range of mass-flow rates and pressures. The accuracy of the meter is typically better than about 3% in a static flow and resolves the fluctuating mass-flow with an accuracy that is better than or equivalent to a hot-wire method.
|
|
| 5. |
- Ford, C. L., et al.
(författare)
-
Vortex-meter design : The influence of shedding-body geometry on shedding characteristics
- 2018
-
Ingår i: Flow Measurement and Instrumentation. - : Elsevier Ltd. - 0955-5986 .- 1873-6998. ; 59, s. 88-102
-
Tidskriftsartikel (refereegranskat)abstract
- The periodic vortex shedding from bluff bodies may be used in flow metering applications. However, because the bluff-body is highly confined (typically in a pipe) the shed vortices may interact with the pipe wall; causing an undesirable non-linear behaviour. An experimental investigation has been conducted; examining the vortex-shedding characteristics of highly confined bluff-bodies in pipe flow, at high Reynolds number (ReD=4.4×104 to 4.4×105). The bluff-bodies were comprised of a forebody and tail; both of which affected the primary-shedding characteristics. The shedders typically produced two unsteady modes: Mode-I was associated with the vortex shedding and mode-II resulted from a separation of the pipe-wall boundary layer. The mode-I behaviour allowed two classes of shedder to be defined: long-tails and short-tails. Modes I and II interacted, particularly for long-tailed geometries. When the length-scale of mode-II exceeded 0.8κ (where κ is the physical scale of the primary shedding vortex), mode-II disrupted mode-I, as the mode-frequency ratio (fII/fI) approached an integer value. The coupling of modes I and II caused mode-I to deviate from its preferred Strouhal number. When the deviation exceeded 25–30%, mode-I locked on to the mode-II frequency. This did not happen for short-tailed geometries, as the length-scale of mode-I was always dominant. Mode-coupling for short-tails occurred only when the mode frequencies were equal.
|
|
| 6. |
- Alfredsson, K. Svante, et al.
(författare)
-
Flexural analysis of discontinuous tile core sandwich structure
- 2012
-
Ingår i: Composite structures. - : Elsevier. - 0263-8223 .- 1879-1085. ; 94:5, s. 1524-1532
-
Tidskriftsartikel (refereegranskat)abstract
- Three-point flexure loading of sandwich beams with a core consisting of discrete ceramic tiles (DTSS) is considered. The tile gaps may be bonded or unbonded (open gaps). The analysis utilizes a layer-wise beam theory approach. The general formulation for the displacements and stresses in the face sheets, face/core adhesive layer, and core is derived. Solutions for stresses and displacements of the beam constituents are obtained from finite element formulation based on analytical solution of the face sheet/tile unit cell. The approach is verified by comparison to stress results obtained from ordinary finite element analysis where each layer is modeled discretely. Effects of load introduction and support conditions on the effective flexural stiffness are examined. It is demonstrated that the face sheets experience substantial stress concentrations at the tile joint locations, especially if the gaps are unfilled. Analysis of beam compliance reveals sensitivity to details of load introduction and support conditions, especially when the span length becomes comparable to the tile length.
|
|
| 7. |
- Alfredsson, K. Svante, et al.
(författare)
-
Stress analysis of axially and thermally loaded discontinuous tile core sandwich with and without adhesive filled core gaps
- 2011
-
Ingår i: Composite structures. - : Elsevier. - 0263-8223 .- 1879-1085. ; 93:7, s. 1621-1630
-
Tidskriftsartikel (refereegranskat)abstract
- An analytical study is performed to investigate the stress states in an axially and thermally loaded sandwich structure with a discontinuous ceramic tile core. General and simplified models are developed to determine stresses in the constituents of the sandwich structure with and without adhesive in the gaps between adjacent tiles. A general model that allows local bending of the face sheet and a simplified model which assumes uniform through-thickness stress distribution in the face sheets are developed. It is shown that the normal stress in the face sheet decreases when the gap is filled by adhesive, although the tile stress increases. The analytical model shows that normal and shear stresses at the face/core interface can be reduced by filling the gaps between tiles. Filled gaps also elevate the axial stiffness of the structure. Model results are verified by comparison to a previously developed analytical model and finite element analysis. (C) 2011 Elsevier Ltd. All rights reserved.
|
|
| 8. |
- Fallenius, Bengt E. G., et al.
(författare)
-
Assessment of wall vibrations in the long pipe facility at CICLoPE
- 2019
-
Ingår i: Springer Proceedings in Physics. - Cham : Springer Nature. ; , s. 203-208
-
Konferensbidrag (refereegranskat)abstract
- The present investigation aims at finding out whether there are pipe vibrations in the higher Reynolds number range at the Long Pipe Facility at the CICLoPE facility and to quantify their amplitude and frequency. Since vibrations are natural to any wind-tunnel facility, similar vibration measurements have also been performed in an established high-quality wind-tunnel facility, viz. the Minimum Turbulence Level (MTL) wind tunnel at KTH Royal Institute of Technology, in order to provide reference data. Results affirm that the amplitudes observed in Willert et al. (J Fluid Mech 826, 2017) are most likely the result of an amplification due to the optical set-up that is attached to the window plug, rather than vibrations of the pipe structure.
|
|
| 9. |
- Medici, Davide, 1973-
(författare)
-
Experimental studies of wind turbine wakes : power optimisation and meandering
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wind tunnel studies of the wake behind model wind turbines with one, two and three blades have been made in order to get a better understanding of wake development as well as the possibility to predict the power output from downstream turbines working in the wake of an upstream one. Both two-component hot-wire anemometry and particle image velocimetry (PIV) have been used to map the flow field downstream as well as upstream the turbine. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the free stream direction (the yaw angle was varied from 0 to 30 degrees). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side showing the potential of controlling the wake position by yawing the turbine. The power output of a yawed turbine was found to depend strongly on the rotor. The possibility to use active wake control by yawing an upstream turbine was evaluated and was shown to have a potential to increase the power output significantly for certain configurations. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low frequency fluctuation both in the wake and in the flow outside. This fluctuation was found both with and without free stream turbulence and also with a yawed turbine. The non-dimensional frequency (Strouhal number) was independent of the freestream velocity and turbulence level but increases with the yaw angle. However the low frequency fluctuations were only observed when the tip speed ratio was high. Porous discs have been used to compare the meandering frequencies and the cause in wind turbines seems to be related to the blade rotational frequency. It is hypothesized that the observed meandering of wakes in field measurements is due to this shedding.
|
|
| 10. |
- Winroth, Marcus, et al.
(författare)
-
On discharge from poppet valves : effects of pressure and system dynamics
- 2018
-
Ingår i: Experiments in Fluids. - : Springer. - 0723-4864 .- 1432-1114. ; 59:2
-
Tidskriftsartikel (refereegranskat)abstract
- Simplified flow models are commonly used to design and optimize internal combustion engine systems. The exhaust valves and ports are modelled as straight pipe flows with a corresponding discharge coefficient. The discharge coefficient is usually determined from steady-flow experiments at low pressure ratios and at fixed valve lifts. The inherent assumptions are that the flow through the valve is insensitive to the pressure ratio and may be considered as quasi-steady. The present study challenges these two assumptions through experiments at varying pressure ratios and by comparing measurements of the discharge coefficient obtained under steady and dynamic conditions. Steady flow experiments were performed in a flow bench, whereas the dynamic measurements were performed on a pressurized, 2 l, fixed volume cylinder with one or two moving valves. In the latter experiments an initial pressure (in the range 300–500 kPa) was established whereafter the valve(s) was opened with a lift profile corresponding to different equivalent engine speeds (in the range 800–1350 rpm). The experiments were only concerned with the blowdown phase, i.e. the initial part of the exhaustion process since no piston was simulated. The results show that the process is neither pressure-ratio independent nor quasi-steady. A measure of the “steadiness” has been defined, relating the relative change in the open flow area of the valve to the relative change of flow conditions in the cylinder, a measure that indicates if the process can be regarded as quasi-steady or not.
|
|
