| 1. |
- Kurian, Thomas, 1981-
(författare)
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An experimental investigation of disturbance growth in boundary layer flows
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the early stages of transition to turbulence in two different baseflows, namely the Falkner-Skan-Cooke boundary layer (FSC) and the asymptotic suction boundary layer (ASBL). Grid-generated turbulence is studied in order to characterise the isotropy levels, free-stream turbulence levels and characteristic length scales that will be present in the receptivity study. By varying the grids and their location it is possible to control the turbulence intensity level, Tu, and the integral length scale independently. Comparisons with other studies show that for increasing Re_M the isotropy levels and the rate of kinetic energy decay asymptotically approach the theoretical values. The FSC describes a a 3D boundary layer subjected to a pressure gradient. The FSC is stable to TS-waves, but becomes susceptible to both travelling and stationary crossflow disturbances. In the experiments the travelling modes were triggered using free-stream turbulence (FST) and the stationary modes were triggered using an array of cylindrical roughness elements. The receptivity phase to FST was linear as well as the initial growth. For high enough $Tu$ inside the boundary layer, nonlinear behaviour was observed further downstream. The stationary mode could only be triggered using tall roughness elements, with low heights resulting in no noticeable disturbances. The receptivity is found to be nonlinear for the roughness heights tested and the growth of the disturbances is exponential. For low levels of FST, Tu < 0.25%, the travelling mode as well as the stationary mode grew. The ASBL is formed when uniform suction is applied to the surface of a porous plate with a flow over it. This baseflow is very stable to TS-waves, and was used to study the transient growth. For the ASBL, stationary disturbances were triggered using a spanwise array or cylindrical roughness elements. The velocity signals were decomposed using a spatial Fourier transform to study the growth of individual modes. The fundamental mode as well as some harmonics were seen to undergo transient growth, before finally decaying exponentially. Comparisons were made to the experimental data using optimal perturbation theory. The global optimals did not describe the transient growth effects well. The calculations were redone for suboptimal times and showed agreement with the experimental data, showing that optimal perturbation theory can describe transient growth if the initial disturbance state is known.
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| 2. |
- Kurian, Thomas, et al.
(författare)
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Boundary layer receptivity to free-stream turbulence and surface roughness over a swept flat plate
- 2011
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 23:3, s. 034107-
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Tidskriftsartikel (refereegranskat)abstract
- An experimental study of the receptivity of disturbances and their subsequent development into a three-dimensional boundary layer has been carried out. The three-dimensional boundary layer was set up using a flat plate with a swept leading edge and a pressure gradient using a displacement body at the ceiling of the test section. Low level free-stream turbulence was generated with five different screens and was shown to generate traveling crossflow modes for all but the lowest turbulence level, i.e., for Tu > 0.2%, where instead a stationary crossflow disturbance dominated. Stationary crossflow disturbances were triggered by small cylindrical roughness elements arranged in an array. For high enough roughness Reynolds number (Re-k) stationary disturbances growing exponentially were seen and their amplitude seems to scale with Re-k(2.3).
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| 3. |
- Kurian, Thomas, et al.
(författare)
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Transient growth in the asymptotic suction boundary layer
- 2011
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 51:3, s. 771-784
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Tidskriftsartikel (refereegranskat)abstract
- In the present experimental setup, the transient disturbance growth in a spatially invariant boundary layer flow, i.e., the asymptotic suction boundary layer (ASBL), has been investigated. The choice of the ASBL brings along several advantages compared with an ordinary spatially growing boundary layer. A unique feature of the ASBL is that the Reynolds number (Re) can be varied without changing the boundary layer thickness, which in turn allows for parameter variations not possible to carry out in traditional boundary layer flows. A spanwise array of discrete surface roughness elements was mounted on the surface to trigger modes with different spanwise wavenumbers (beta). It is concluded that for each mode there exists a threshold roughness Reynolds number (Re (k) ), below which no significant transient growth is present. The experimental data suggests that this threshold Re (k) is both a function of beta and Re. An interesting result is that the energy growth curves respond differently to a change in Re (k) when caused by a change in roughness height k, implying that Re remains constant, compared with a change in the free-stream velocity U(infinity), which also affects the Re. The scaling of the energy rowth curves both in level and the downstream direction is treated and appropriate scalings are found. The result shows a complex non-linear receptivity mechanism. Optimal perturbation theory, which has failed to predict the energy evolution in growing boundary layers, is tested for the ASBL and shows that it may satisfactorily predict the evolution of all transiently growing modes that are triggered by the roughness elements.
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| 4. |
- Mark, Andreas, 1980, et al.
(författare)
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Simulation of a Non-Newtonian Dense Granular Suspension in a Microfluidic Contraction
- 2013
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Ingår i: 8th International Conference on Multiphase Flow.
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Konferensbidrag (refereegranskat)abstract
- The success of a solder paste jet printer is based on an uninterrupted flow of fluid, specifically dense fluid suspensions, through a series of ducts inside the printing head. It is well known that the flow of dense suspensions is prone to jamming and sedimentation effects, both of which could entail detrimental failure modes in the printing heads. A thorough understanding of the fluid dynamics of suspensions as they flow through ducts and connections is of utmost importance. The purpose of this study is to propose a novel simulation framework and to show that it captures the main effects such as mass flow and partial jamming in a cylindrical duct test configuration. The granular suspension is a generic solder paste with solder particlesimmersed in a flux. The simulations are performed in the multi-phase flow solver IBOFlow. A two fluid model is used for the granular suspension and the discretization is done an Euler-Euler framework. The averaged momentum equations from Enwald et al. (1996) are solved together with the common continuity equation generating a shared pressure field. Explicit constitutive equations for the interfacial momentum transfer and particle pressure are employed. To capture the shear thinning effects of the non-Newtonian suspensions the standard Carreau rheology model is used. To study how the fluid flow affects the local volume fraction and partial jamming in the duct, simulations are performed for different applied pressure drops ranging from one to five bars. For both particle pressure models, the resulting mean bulk velocities are compared with experiments with good agreement, and partial jamming is observed. Hence, it is concluded that the proposed framework is suitable to model and simulate the granular suspension in a micro fluid contraction.
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