| 1. |
- Alfredsson, P. Henrik, et al.
(författare)
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Instability, transition and turbulence in plane Couette flow with system rotation
- 2005
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Ingår i: IUTAM Symposium on Laminar-Turbulent Transition and Finite Amplitude Solutions. - Berlin/Heidelberg : Springer Netherlands. - 1402040482 ; , s. 173-193
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Konferensbidrag (refereegranskat)abstract
- System rotation may have either stabilizing or destabilizing effects on shear flows depending on the direction of rotation vector as compared to the vorticity vector of mean flow. This study describes experimental results of laminar, transitional and turbulent plane Couette flow with both stabilizing and destabilizing system rotation. For laminar flow with destabilizing rotation roll cells appear in the flow which may undergo several different types of secondary instabilities, especially interesting is a repeating pattern of wavy structures followed by breakdown, thereafter roll cells reappear in a cyclic pattern. For higher Reynolds number roll cells appear also in a turbulent environment. It is also shown how stabilizing rotation may quench the turbulence completely.
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| 2. |
- Apazidis, Nicholas, et al.
(författare)
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An experimental and theoretical study of converging polygonal shock waves
- 2002
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 12:1, s. 39-58
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Tidskriftsartikel (refereegranskat)abstract
- An experimental investigation was carried out to explore the possibility of producing converging polygonal shocks in an essentially two-dimensional cavity. Previous calculations by Apazidis and Lesser (1996) suggested that such configurations could be produced by reflecting a cylindrical outgoing shock from a smoothly altered circular boundary, the alteration having n-gonal symmetry. In the experiments the outgoing shock was produced by a spark discharge which yielded shocks in the Mach number range from 1.1 to 1.7 at a radius just prior to the reflection. Polygonal shocks were observed as predicted by using a modified form of geometrical shock dynamics, derived in the above paper. In addition, the modified theory was used to calculate the results of an experiment carried out by Sturtevant and Kulkarny (1976). The results of the numerical calculations were found to be in substantial agreement with both experiments, suggesting that the modifications in geometrical shock dynamics for non-uniform flow ahead of an advancing shock are useful in the case of shock focusing. The experiment. also showed that the polygonal shapes were stable in the examined range of shock Mach numbers. a result that may be of importance for a number of practical situations in which shock focusing is present.
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| 3. |
- Apazidis, Nicholas, et al.
(författare)
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High energy concentration by symmetric shock focusing
- 2013
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 23:4, s. 361-368
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Tidskriftsartikel (refereegranskat)abstract
- High-energy concentrations in gas are achieved experimentally in a specially constructed shock tube facility at KTH Mechanics. The high-energy concentration is manifested by a formation of a hot, light-emitting gas core. Experimental, numerical and theoretical investigations show that the shape of the imploding shock is of pivotal importance for the final energy concentration. Cylindrical shocks are unstable. Symmetric polygonal shocks are shown to be dynamically stable and are produced by various methods, e.g. thin wing profiles placed radially in the test section. Such symmetric polygonal shocks are able to produce extremely high energy levels at the focal point. Spectral data from 60 nanosecond short intervals of 8 microsecond light pulse give temperatures in the range of 6,000 K.
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| 4. |
- Caraghiaur, Diana, 1978-, et al.
(författare)
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Detailed pressure drop measurements in single- and two-phase adiabatic air-water turbulent flows in realistic BWR fuel assembly geometry with spacer grids
- 2004
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Ingår i: The 6<sup>th</sup> International Conference on Nuclear Thermal Hydraulics, Operations and Safety (NUTHOS-6) Nara, Japan, October 4-8, 2004.
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Konferensbidrag (refereegranskat)abstract
- In recent years, advanced numerical simulation tools based on CFD methods have been increasingly used in various multi-phase flow applications. One of these is two-phase flow in fuel assemblies of Boiling Water Reactors. The important and often missing aspect of this development is the validation of CFD codes against proper experimental data. The purpose of the current paper is to present detailed pressure measurements over a spacer grid in adiabatic single- and two-phase flow, which will be used to validate and further develop a CFD code for BWR fuel bundle analysis. The experiments have been carried out in an asymmetric 24-rod sub-bundle, representing ¼ of Westinghouse SVEA-96 nuclear reactor fuel assembly. Single-phase measurements have been performed at superficial velocities comprised between jliq: 0.90 – 4.50 m/s and in the two-phase, which was simulated by air-water mixture, measurements have been performed at void fractions ranging from 4 to 12% and liquid superficial velocity jliq : 4.50 m/s. In order to increase the number of the measured points, five pressure taps were drilled in one of the rods, which was easily moved vertically by a traverse system, covering most of the points in axial direction. The possibility to substitute any of the rods in the fuel bundle by the pressure sensing rod and the possibility to change the pressure taps facing-angle provides more measuring points inside the subchannels. A detailed pressure distribution comparison between single- and two-phase flows for different subchannel positions and different flow conditions was performed over one of the spacers. In addition, single-phase pressure drop measurements on the upper part of the test section comprising two spacer grids has been carried out.
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| 5. |
- Eliasson, Veronica, et al.
(författare)
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Controlling the form of strong converging shocks by means of disturbances.
- 2007
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 17:1-2, s. 29-42
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Tidskriftsartikel (refereegranskat)abstract
- The influence of artificial disturbances on the behavior of strong converging cylindrical shocks is investigated experimentally and numerically. Ring-shaped shocks, generated in an annular cross sectional shock tube are transformed to converging cylindrical shocks in a thin cylindrical test section, mounted at the rear end of the shock tube. The converging cylindrical shocks are perturbed by small cylinders placed at different locations and in various patterns in the test section. Their influence on the shock convergence and reflection process is investigated. It is found that disturbances arranged in a symmetrical pattern will produce a symmetrical deformation of the converging shockfront. For example, a square formation produces a square-like shock and an octagon formation a shock with an octagonal front. This introduces an alternative way of tailoring the form of a converging shock, instead of using a specific form of a reflector boundary. The influence of disturbances arranged in non-symmetric patterns on the shape of the shockfront is also investigated.
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| 6. |
- Eliasson, Veronica, et al.
(författare)
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Focusing of strong shocks in an annular shock tube
- 2007
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 15:3-4, s. 205-217
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Tidskriftsartikel (refereegranskat)abstract
- Focusing of strong shock waves in a gas-filled thin convergence chamber with various forms of the reflector boundary is investigated experimentally and numerically. The convergence chamber is mounted at the end of the horizontal co-axial shock tube. The construction of the convergence chamber allows the assembly of the outer chamber boundaries of various shapes. Boundaries with three different shapes have been used in the present investigation-a circle, an octagon and a smooth pentagon. The shock tube in the current study was able to produce annular shocks with the initial Mach number in the range M-s = 2.3-3.6. The influence of the shape of the boundary on the shape and properties of the converging and reflected shock waves in the chamber has then been investigated both experimentally and numerically. It was found that the form of the converging shock is initially governed by the shape of the reflector and the nonlinear interaction between the shape of the shock and velocity of shock propagation. Very close to the center of convergence the shock obtains a square-like form in case of a circular and octagonal reflector boundary. This is believed to stem from the instability of the converging shock front triggered by the disturbances in the flow field. The outgoing, reflected shocks were also observed to be influenced by the shape of the boundary through the flow ahead as created by the converging shocks.
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| 7. |
- Eliasson, Veronica, et al.
(författare)
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Light emission during shock wave focusing in air and argon
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:10, s. 106106-1-106106-17
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Tidskriftsartikel (refereegranskat)abstract
- The light emission from a converging shock wave was investigated experimentally. Results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The light emission appears as the shock wave collapses. The full width at half maximum of the light pulse is about 200 ns for all geometrical shapes. It was also found that argon as a test gas produces more light than air. Numerical simulations showed good agreement with experimental results regarding the shape of the shock and the flow field behind the shock. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there.
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| 8. |
- Facciolo, Luca, et al.
(författare)
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A study of swirling turbulent pipe and jet flows
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:3
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Tidskriftsartikel (refereegranskat)abstract
- Axially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress.
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| 9. |
- Hiwatashi, Kazuaki, et al.
(författare)
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Experimental observations of instabilities in rotating plane Couette flow
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:4
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Tidskriftsartikel (refereegranskat)abstract
- The transition from the two-dimensional (2D) longitudinal roll cell state to 3D flows in the rotating plane Couette system, predicted by the theoretical investigation [M. Nagata, J. Fluid Mech. 358, 357 (1998)], is examined experimentally. The streamwise and spanwise wave numbers of observed steady 3D flows seem to agree with those predicted by the theory when the rotation rate is relatively large. However, we observe unsteady 3D states in the region where the theory predicts stable steady 3D flows when the rotation rate is small.
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| 10. |
- Kalpakli, Athanasia, et al.
(författare)
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Experimental investigation on the effect of pulsations on exhaust manifold-related flows aiming at improved efficiency
- 2012
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Ingår i: Institution of Mechanical Engineers - 10th International Conference on Turbochargers and Turbocharging. - : Elsevier. - 9780857092090 ; , s. 377-387
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Konferensbidrag (refereegranskat)abstract
- The gas flowing through the exhaust manifold of the internal combustion engine to the inlet of the turbine side of a turbocharger is highly pulsating and turbulent. The gas enters the turbine after travelling through a complex curved and branched pipe system where the effect of centrifugal (from the acute curvature), inertia and viscous forces result in a three-dimensional, non-symmetric flow field. Additionally, vortical structures are being formed and dissolved due to the co-existence of these forces that change in magnitude under a pulse period. This complex flow field, typical for the inflow condition to the turbine, is the focus of the present study. Instantaneous mass flow rate and pressure measurements that provide information on changes in the turbine map when a sharp bend is mounted at the inlet of the turbine are performed and complemented with time-resolved stereoscopic particle image velocimetry measurements of the pulsating turbulent flow downstream a 90° pipe bend. The results indicate, that the time-averaged operation point in a turbine map is only marginally affected by the inflow conditions and the pulsation frequency. The hysteresis loops, on the other hand, exhibit differences not only for different pulsation frequencies, but also for different inflow conditions as a comparison between a straight and a curved pipe section upstream the turbine shows.
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