- Title
- Can turbulence be maintained in a low Re channel flow?: A CFD analysis
- Creator
- Anika, Nisat Nowroz
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Mixing is difficult to achieve in low Reynolds number channel or pipe flows. It can be achieved by generating turbulent motion, which is also important in the context of increasing momentum transport as well as heat and mass transfer phenomenon. A direct numerical simulation based on lattice Boltzmann method (LBM) was carried out in a laminar channel flow, aiming to investigate whether local ‘synthetic’ turbulence can be achieved and sustained for a period long enough at critical and subcritical Reynolds number. The range of Reynolds number (Re), based on half height of the channel, was varied between 0.5 to 2963. In the present study to generate turbulent ‘puff’ in viscous dominated flows, three different methods were considered in smooth wall channel flow namely, (a) active, (b) passive, and (c) active+passive (combination). Pulsed jets and externally applied electrical field, and two-dimensional (2D) square bars were used in laminar channel flows as an active and passive methods, respectively. The investigation of generation of localized turbulence was performed in smooth wall Poiseuille flow through low energy wall jets at Re = 999, 700, 500 and 5, based on bulk velocity and half height of the channel. Two wall jets were issued only at the bottom wall of the channel with only one pulse along the wall normal direction. It is observed that local hair-pin structures (or puffs) were generated by the jets. The individual symmetry of each structure was broken at Re > 500 when the generated puffs interact with each other and the upper wall. Within a few turnover time, the symmetry broken and the puffs yielded many new small quasi-longitudinal structures through interactions. Jet interaction in the channel flow and the evolution of coherent structures were visualised. No mechanism of jet interaction was observed for Re < 5 in a smooth wall channel flow. The turbulent intensities, root-mean-square (rms) of the velocity fluctuations, Reynolds shear stress and correlation coefficients were calculated with and without jets and compared. In the case of rough wall channel flow, 2D roughness elements were mounted on both walls of the channel to inject extra energy to help trigger turbulence. The transverse square bars were equally spaced and spanned the width (z direction) of both walls. The Reynolds number was fixed Re = 2100. It was observed that the flow consisted of 2D turbulence (ϖ = 0 i.e. spanwise velocity component is zero). The flow mainly consisted of steady secondary motions in the space between bars and laminar in the central part of the channel. In order to assess whether or not turbulence can be generated purely through the roughness elements, simulations were carried out with the bars spanning only half of width of the channel on both walls in a staggered way. The channel Reynolds number was fixed at Re = 2963 and 880. Without any background instability or initial noise, turbulence was generated due to the breaking the spanwise homogeneity of the flow. Rough wall channel flow under the action of the wall pulsed jet(s) is investigated. The roughness elements mounted on both walls consisted of transverse bars and spanned the width of the channel. The jets are pulsed only once with the second jet activated at the end of the cycle of the first jet. The channel flow Reynolds number was Re ≃ 600. Without activating the jets, the flow consists mainly in steady secondary motions in the canopies (spaces between bars) and a skimming two-dimensional laminar flow with some oscillations above the canopies. When the jets are activated, a localised three-dimensional turbulence develops and grows in the channel. Not only it is found that this localised turbulence bears strong similarity to that of a fully rough wall turbulent channel flow, but it appears to be in a pseudo fully rough regime, as observed by the relatively negligible (averaged) viscous drag as compared to the form drag generated by the roughness elements. The physical mechanism which allows this to occur is discussed. An externally applied electric field is also used in low Reynolds number channel flow, (0.5 < Re < 50). As an active method, electro-osmotic flow is performed in both the smooth and rough wall channel flows with a symmetric charged surface. For the rough wall electroosmotic case, the ratio of height (k) of roughness elements to channel half height (h) is below k/h ≤ 0.2 to avoid the blockage effect. The simulation was carried out for a two-dimensional configuration. The governing equations for the electrokinetic transport were solved by Poisson-Boltzmann method for monovalent ion. Also, simulations were performed for the combined effects of blowing and suction with and without the presence of an electric field at Re ≃ 50. The simultaneous use of active and passive methods in electro-osmotic flow was also carried out to study whether swirling motion at such low Re number can be generated. In conclusion, generating and maintaining the turbulence is possible in a laminar channel flow at low Reynolds numbers. The present numerical investigation of initiating and maintaining turbulence with suitable strategies: active and passive, is significant for enhancing mixing between several streams of fluids in microdevices.
- Subject
- laminar channel flow; lattice Boltzmann method; mixing; low Reynolds number; localized turbulence; rough wall; electrokinetic; pulsed jets; CFD; LBM
- Identifier
- http://hdl.handle.net/1959.13/1390972
- Identifier
- uon:33149
- Rights
- Copyright 2018 Nisat Nowroz Anika
- Language
- eng
- Full Text
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