DSpaceCRIS@UNITENhttp://dspace.uniten.edu.my/jspuiThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Fri, 07 Aug 2020 12:20:29 GMT2020-08-07T12:20:29Z50141- A systematic investigation of roughness height and wavelength in turbulent pipe flow in the transitionally rough regimehttp://dspace.uniten.edu.my/jspui/handle/123456789/9432Title: A systematic investigation of roughness height and wavelength in turbulent pipe flow in the transitionally rough regime
Authors: Chan, L.; Macdonald, M.; Chung, D.; Hutchins, N.; Ooi, A.
Abstract: Direct numerical simulations (DNS) are conducted for turbulent flow through pipes with three-dimensional sinusoidal roughnesses explicitly represented by body-conforming grids. The same viscous-scaled roughness geometry is first simulated at a range of different Reynolds numbers to investigate the effects of low Reynolds numbers and low R<inf>0</inf>/h, where R<inf>0</inf> is the pipe radius and h is the roughness height. Results for the present class of surfaces show that the Hama roughness function ΔU+ is only marginally affected by low Reynolds numbers (or low R<inf>0</inf>/h), and observations of outer-layer similarity (or lack thereof) show no signs of sensitivity to Reynolds number. Then, building on this, a systematic approach is taken to isolate the effects of roughness height h+ and wavelength λ+ in a turbulent wall-bounded flow in both transitionally rough and fully rough regimes. Current findings show that while the effective slope ES (which for the present sinusoidal surfaces is proportional to h+/λ+) is an important roughness parameter, the roughness function ΔU+ must also depend on some measure of the viscous roughness height. A simplistic linear'log fit clearly illustrates the strong correlation between ΔU+ and both the roughness average height k+<inf>a</inf> (which is related to h+) and ES for the surfaces simulated here, consistent with published literature. Various definitions of the virtual origin for rough-wall turbulent pipe flow are investigated and, for the surfaces simulated here, the hydraulic radius of the pipe appears to be the most suitable parameter, and indeed is the only virtual origin that can ever lead to collapse in the total stress. First- and second-order statistics are also analysed and collapses in the outer layer are observed for all cases, including those where the largest roughness height is a substantial proportion of the reference radius (low R<inf>0</inf>/h). These results provide evidence that turbulent pipe flow over the present sinusoidal surfaces adheres to Townsend's notion of outer-layer similarity, which pertains to statistics of relative motion. © © 2015 Cambridge University Press.
Thu, 01 Jan 2015 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94322015-01-01T00:00:00Z
- The minimal channel: A fast and direct method for characterising roughnesshttp://dspace.uniten.edu.my/jspui/handle/123456789/9431Title: The minimal channel: A fast and direct method for characterising roughness
Authors: Macdonald, M.; Chung, D.; Hutchins, N.; Chan, L.; Ooi, A.; García-Mayoral, R.
Abstract: Roughness only alters the near-wall region of turbulent flow and leaves the outer-layer unaffected, making it a prime candidate for the minimal-span channel framework which only captures the near-wall flow. Recently, Chung et al. (J. Fluid Mech., vol. 773, 2015, pp. 418-431) showed that the minimal-span channel can accurately characterise the hydraulic behaviour of roughness. Following on from this, we aim to further optimise the minimal-span channel framework by primarily noting that the outer layer it produces is inherently incorrect, and as such modifications to this region can be made to improve performance. Firstly, a half-height channel with slip wall is shown to reproduce the near-wall behaviour seen in a standard channel, but with half the number of grid points. Next, a forcing model is introduced into the outer layer of a half-height channel. This reduces the high streamwise velocity associated with the minimal channel and allows for a larger computational time step. The streamwise length of the channel is also investigated independent of the previous improvements, and suggests the minimum length should be at least 3 times the spanwise width and also 1000 viscous-units long, whichever is longer. Finally, an investigation is conducted to see if varying the roughness Reynolds number with time is a feasible method for obtaining the full hydraulic behaviour of a rough surface, instead of running multiple simulations at fixed roughness Reynolds numbers.
Fri, 01 Jan 2016 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94312016-01-01T00:00:00Z
- Turbulent flow over transitionally rough surfaces with varying roughness densitieshttp://dspace.uniten.edu.my/jspui/handle/123456789/9430Title: Turbulent flow over transitionally rough surfaces with varying roughness densities
Authors: Macdonald, M.; Chan, L.; Chung, D.; Hutchins, N.; Ooi, A.
Abstract: We investigate rough-wall turbulent flows through direct numerical simulations of flow over three-dimensional transitionally rough sinusoidal surfaces. The roughness Reynolds number is fixed at , where is the sinusoidal semi-amplitude, and the sinusoidal wavelength is varied, resulting in the roughness solidity (frontal area divided by plan area) ranging from 0.05 to 0.54. The high cost of resolving both the flow around the dense roughness elements and the bulk flow is circumvented by the use of the minimal-span channel technique, recently demonstrated by Chung eta al. (J. Fluid Mech., vol. 773, 2015, pp. 418-431) to accurately determine the Hama roughness function, . Good agreement of the second-order statistics in the near-wall roughness-affected region between minimal- and full-span rough-wall channels is observed. In the sparse regime of roughness the roughness function increases with increasing solidity, while in the dense regime the roughness function decreases with increasing solidity. It was found that the dense regime begins when , in agreement with the literature. A model is proposed for the limit of , which is a smooth wall located at the crest of the roughness elements. This model assists with interpreting the asymptotic behaviour of the roughness, and the rough-wall data presented in this paper show that the near-wall flow is tending towards this modelled limit. The peak streamwise turbulence intensity, which is associated with the turbulent near-wall cycle, is seen to move further away from the wall with increasing solidity. In the sparse regime, increasing reduces the streamwise turbulent energy associated with the near-wall cycle, while increasing in the dense regime increases turbulent energy. An analysis of the difference of the integrated mean momentum balance between smooth- and rough-wall flows reveals that the roughness function decreases in the dense regime due to a reduction in the Reynolds shear stress. This is predominantly due to the near-wall cycle being pushed away from the roughness elements, which leads to a reduction in turbulent energy in the region previously occupied by these events. © 2016 Cambridge University Press.
Fri, 01 Jan 2016 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94302016-01-01T00:00:00Z
- Large eddy simulation and Reynolds-averaged Navier-Stokes calculations of supersonic impinging jets at varying nozzle-to-wall distances and impinging angleshttp://dspace.uniten.edu.my/jspui/handle/123456789/9434Title: Large eddy simulation and Reynolds-averaged Navier-Stokes calculations of supersonic impinging jets at varying nozzle-to-wall distances and impinging angles
Authors: Chan, L.; Chin, C.; Soria, J.; Ooi, A.
Abstract: This paper utilises two different computational methods to investigate the characteristics of a supersonic impinging jet at non-dimensionalised nozzle-to-wall distances (Z n/D) of 1.5 and 2.5 with the impinging angles from 0° to 45°. The static Smagorinsky subgrid-scale model was chosen for the LES and the two equation k - {small element of} turbulence model for the RANS. Computational parameters applied in the simulations emulated the experimental setup conducted by Risborg (2008). From the results obtained, both methodologies were able to predict the location of the first shock cell fairly accurately when compared to the steady-state shadowgraph images of Risborg (2008). However, the intensities of the shocks were significantly different between the two numerical methods, with the RANS underestimating the value of the density gradients at the shocks. The pressure distribution near the impinging plate have been investigated and found to differ between the RANS and the LES for small impinging angles (0° and 10°) when Z n/D = 1.5. In addition, the RANS data was not able to capture the recirculation zone for Z n/D = 1.5 and 0 °. The instantaneous velocity fluctuations and temperature contours of the LES were also plotted to visualise the shear layer instability and also the chaotic nature of the supersonic jet. For Z n/D = 2.5 and 0 °, the jet experiences high velocity fluctuations as the configuration causes the axially flapping instability. Overall, there are discrepancies between the RANS and LES but both are able to capture the key averaged flow features of the supersonic impinging jets. © 2014 Elsevier Inc.
Wed, 01 Jan 2014 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94342014-01-01T00:00:00Z
- A fast direct numerical simulation method for characterising hydraulic roughnesshttp://dspace.uniten.edu.my/jspui/handle/123456789/9433Title: A fast direct numerical simulation method for characterising hydraulic roughness
Authors: Chung, D.; Chan, L.; Macdonald, M.; Hutchins, N.; Ooi, A.
Abstract: We describe a fast direct numerical simulation (DNS) method that promises to di- rectly characterise the hydraulic roughness of any given rough surface, from the hy- draulically smooth to the fully rough regime. The method circumvents the unfavourable computational cost associated with simulating high-Reynolds-number flows by employ- ing minimal-span channels (Jiḿenez & Moin 1991). Proof-of-concept simulations demon- strate that flows in minimal-span channels are sufficient for capturing the downward velocity shift, that is, the Hama roughness function, predicted by flows in full-span chan- nels. We consider two sets of simulations, first with modelled roughness imposed by body forces, and second with explicit roughness described by roughness-conforming grids. Ow- ing to the minimal cost, we are able to conduct DNSs with increasing roughness Reynolds numbers while maintaining a fixed blockage ratio, as is typical in full-scale applications. The present method promises a practical, fast and accurate tool for characterising hy- draulic resistance directly from profilometry data of rough surfaces.
Thu, 01 Jan 2015 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94332015-01-01T00:00:00Z
- Investigation of the flow structures in supersonic free and impinging jet flowshttp://dspace.uniten.edu.my/jspui/handle/123456789/9437Title: Investigation of the flow structures in supersonic free and impinging jet flows
Authors: Chin, C.; Li, M.; Harkin, C.; Rochwerger, T.; Chan, L.; Ooi, A.; Risborg, A.; Soria, J.
Abstract: A numerical study of compressible jet flows is carried out using Reynolds averaged Navier-Stokes (RANS) turbulence models such as k-É and k-ω-SST. An experimental investigation is performed concurrently using high-speed optical methods such as Schlieren photography and shadowgraphy. Numerical and experimental studies are carried out for the compressible impinging at various impinging angles and nozzle-to-wall distances. The results from both investigations converge remarkably well and agree with experimental data from the open literature. From the flow visualizations of the velocity fields, the RANS simulations accurately model the shock structures within the core jet region. The first shock cell is found to be constraint due to the interaction with the bow-shock structure for nozzle-to-wall distance less than 1.5 nozzle diameter. The results from the current study show that the RANS models utilized are suitable to simulate compressible free jets and impinging jet flows with varying impinging angles. © 2013 by ASME.
Tue, 01 Jan 2013 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94372013-01-01T00:00:00Z
- Direct numerical simulation of pulsatile flow in pipeshttp://dspace.uniten.edu.my/jspui/handle/123456789/9435Title: Direct numerical simulation of pulsatile flow in pipes
Authors: Chen, W.X.; Chan, L.; Hutchins, N.; Poon, E.K.W.; Ooi, A.
Abstract: This study presents data from direct numerical simulation (DNS) of pulsatile flow in a rigid smooth pipe approximating the blood flow condition in the human aorta. Since blood flow behaves in a laminar fashion in certain regions in the human aorta but turbulent in other regions, pulsatile flows are numerically studied in both laminar and turbulent flow regimes. Pure oscillatory simulations are carried out over a range of Womersley numbers (α = 1,5,10,15) in the laminar regime. Numerical velocity profiles and pressure-flow relationship from these results in the laminar regime are validated with the analytical solution. Blood flow inside an aorta is simulated by superimposing a mean pressure gradient component to an oscillatory pressure gradient. The mean Reynolds number based on bulk velocity is Re0 ≈ 5300 (equivalent to Reynolds number based on uτ, Reτ = 180) and the oscillatory-flow Reynolds number Rew is determined by the oscillatory component. The simulated flow driven by the total pressure gradient falls in the turbulent regime. An instantaneous flow field visualisation and mean statistics are presented and analysed. The pressure-flow relationship of turbulent flow is also investigated. Turbulent pipe flow with and without pulsation are compared and the effects of the oscillatory pressure gradient on the mean velocity and wall shear stress are demonstrated in this paper.
Wed, 01 Jan 2014 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94352014-01-01T00:00:00Z
- Numerical simulation of supersonic impinging jet flows using Reynolds averaged Navier-Stokes and Large Eddy Simulationhttp://dspace.uniten.edu.my/jspui/handle/123456789/9438Title: Numerical simulation of supersonic impinging jet flows using Reynolds averaged Navier-Stokes and Large Eddy Simulation
Authors: Chan, L.; Chin, C.; Soria, J.; Ooi, A.
Abstract: The flow structures of a supersonic impinging jet are investigated via numerical simulation. A converging inlet nozzle is used to accelerate the flow to sonic velocity and the flow impinges on a wall located at a normalized stand-off distance (z/D) of 2.5. The Reynolds averaged Navier-Stokes (RANS) and the Large Eddy Simulation (LES) methodologies are used to simulate the shock structures of the impinged jet. The characteristics of the underexpanded supersonic impinging jet is then visually analysed in the LES model. Next, the results of the numerical simulations are compared to experimental data obtained by [7], which uses Schlieren photography and shadowgraphy technique to visualize the shock structures. Results from the RANS and LES numerical models which are consistent with the experimental data demonstrate the capability of both methodologies in simulating supersonic impinging jet flows.
Sun, 01 Jan 2012 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94382012-01-01T00:00:00Z
- Numerical simulation of a rough-wall pipe from the transitionally rough regime to the fully rough regimehttp://dspace.uniten.edu.my/jspui/handle/123456789/9436Title: Numerical simulation of a rough-wall pipe from the transitionally rough regime to the fully rough regime
Authors: Chan, L.; MacDonald, M.; Chung, D.; Hutchins, N.; Ooi, A.
Abstract: Turbulent flow of a rough-wall pipe is simulated using Direct Numerical Simulations (DNS) at low and medium Reynolds number from the transitionally rough regime to the fully rough regime. The rough surfaces simulated consist of three-dimensional sinusoidal roughness elements. The size of the roughness (roughness semi-amplitude height h+ and wavelength λ+) is increased geometrically while maintaining the height-to-wavelength ratio of the sinusoidal roughness element. A method is developed to accurately calculate the roughness function ΔU+ for the simulations conducted at low Reynolds number. For this surface, the flow is fully rough when h+ ≈ 60 (simulated at Reτ = 540). This corresponds to k+s ≈ 4.1h+ where k+s is Nikuradse's equivalent sandgrain roughness. A linear trend is observed when the ratio of the apparent wall shear stress due to form drag on the roughness elements to the total wall shear stress Rτ = τR/τT is plotted against the log of the roughness height h+. This linear trend is also observed in the transitionally rough regime. For all the rough-wall pipe simulations conducted, Townsend's [15] outer layer similarity hypothesis holds true.
Wed, 01 Jan 2014 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94362014-01-01T00:00:00Z
- Effect of averaging period on wind resource assessment for wind turbine installation project at UNITENhttp://dspace.uniten.edu.my/jspui/handle/123456789/11468Title: Effect of averaging period on wind resource assessment for wind turbine installation project at UNITEN
Authors: Roslan, E.; Mohamed, H.; Chan, L.; Isa, M.R.
Abstract: Assessing the wind resource available at a particular site is the initial step of determining the feasibility of a wind turbine installation project. Several methods are frequently used to assess wind resource at a particular site, such as using wind map, local site measurement and measure-correlate-predict. A certain degree of measurement at site is needed for the final two methods. Wind speed data is usually measured and collected at a certain frequency or sampling rate, based on the resolution of the measurement device used. This data is then averaged at a certain period, depending on the standards used or its application. This research investigates the effect of having different averaging period when processing the raw wind speed data on the estimated power and energy potential of a wind turbine installation project at Universiti Tenaga Nasional (UNITEN), Malaysia. Results have shown that different averaging periods 2-minute and 10-seconds have 27.15% and 54.3% difference respectively to hourly averaged data in terms of energy yield estimation. These differences can affect a feasibility study of a wind turbine project. © 2018 Author(s).
Mon, 01 Jan 2018 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/114682018-01-01T00:00:00Z
- The minimal-span channel for rough-wall turbulent flowshttp://dspace.uniten.edu.my/jspui/handle/123456789/9429Title: The minimal-span channel for rough-wall turbulent flows
Authors: MacDonald, M.; Chung, D.; Hutchins, N.; Chan, L.; Ooi, A.; Garciá-Mayoral, R.
Abstract: Roughness predominantly alters the near-wall region of turbulent flow while the outer layer remains similar with respect to the wall shear stress. This makes it a prime candidate for the minimal-span channel, which only captures the near-wall flow by restricting the spanwise channel width to be of the order of a few hundred viscous units. Recently, Chung et al. (J. Fluid Mech., vol. 773, 2015, pp. 418-431) showed that a minimal-span channel can accurately characterise the hydraulic behaviour of roughness. Following this, we aim to investigate the fundamental dynamics of the minimal-span channel framework with an eye towards further improving performance. The streamwise domain length of the channel is investigated with the minimum length found to be three times the spanwise width or 1000 viscous units, whichever is longer. The outer layer of the minimal channel is inherently unphysical and as such alterations to it can be performed so long as the near-wall flow, which is the same as in a full-span channel, remains unchanged. Firstly, a half-height (open) channel with slip wall is shown to reproduce the near-wall behaviour seen in a standard channel, but with half the number of grid points. Next, a forcing model is introduced into the outer layer of a half-height channel. This reduces the high streamwise velocity associated with the minimal channel and allows for a larger computational time step. Finally, an investigation is conducted to see if varying the roughness Reynolds number with time is a feasible method for obtaining the full hydraulic behaviour of a rough surface. Currently, multiple steady simulations at fixed roughness Reynolds numbers are needed to obtain this behaviour. The results indicate that the non-dimensional pressure gradient parameter must be kept below 0.03-0.07 to ensure that pressure gradient effects do not lead to an inaccurate roughness function. An empirical costing argument is developed to determine the cost in terms of CPU hours of minimal-span channel simulations a priori. This argument involves counting the number of eddy lifespans in the channel, which is then related to the statistical uncertainty of the streamwise velocity. For a given statistical uncertainty in the roughness function, this can then be used to determine the simulation run time. Following this, a finite-volume code with a body-fitted grid is used to determine the roughness function for square-based pyramids using the above insights. Comparisons to experimental studies for the same roughness geometry are made and good agreement is observed. © 2017 Cambridge University Press.
Sun, 01 Jan 2017 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94292017-01-01T00:00:00Z
- Investigation of the optimal position of wind sensors and wind turbines on a building: A computational fluid dynamics studyhttp://dspace.uniten.edu.my/jspui/handle/123456789/11306Title: Investigation of the optimal position of wind sensors and wind turbines on a building: A computational fluid dynamics study
Authors: Chan, L.; Mohd Roslan, M.E.B.; Mohamed, H.B.
Abstract: Most numerical studies on flow over buildings simplify the geometry of the roof and assume that it is flat. This may lead to misrepresentation of the flow as the roof of actual buildings contains some sort of roughness. In this study, the flow over the administrative building of Universiti Tenaga Nasional is investigated for multidirectional flow conditions. The actual topology of the building is gridded and simulated using the steady-state Reynolds-averaged Navier-Stokes equation. Four points at the top of the building are identified and the wind statistics at these designated locations at three different heights are investigated. The optimal location with the highest average wind speed and consistent wind speeds for all wind angles is identified and is earmarked as a potential location to install the wind turbine. © 2017 Institute of Advanced Engineering and Science. All rights reserved.
Sun, 01 Jan 2017 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/113062017-01-01T00:00:00Z
- Effect of averaging period on wind resource assessment for wind turbine installation project at UNITENhttp://dspace.uniten.edu.my/jspui/handle/123456789/11063Title: Effect of averaging period on wind resource assessment for wind turbine installation project at UNITEN
Authors: Roslan, E.; Mohamed, H.; Chan, L.; Isa, M.R.
Abstract: Assessing the wind resource available at a particular site is the initial step of determining the feasibility of a wind turbine installation project. Several methods are frequently used to assess wind resource at a particular site, such as using wind map, local site measurement and measure-correlate-predict. A certain degree of measurement at site is needed for the final two methods. Wind speed data is usually measured and collected at a certain frequency or sampling rate, based on the resolution of the measurement device used. This data is then averaged at a certain period, depending on the standards used or its application. This research investigates the effect of having different averaging period when processing the raw wind speed data on the estimated power and energy potential of a wind turbine installation project at Universiti Tenaga Nasional (UNITEN), Malaysia. Results have shown that different averaging periods 2-minute and 10-seconds have 27.15% and 54.3% difference respectively to hourly averaged data in terms of energy yield estimation. These differences can affect a feasibility study of a wind turbine project. © 2018 Author(s).
Mon, 01 Jan 2018 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/110632018-01-01T00:00:00Z
- Analysis of the coherent and turbulent stresses of a numerically simulated rough wall pipehttp://dspace.uniten.edu.my/jspui/handle/123456789/9428Title: Analysis of the coherent and turbulent stresses of a numerically simulated rough wall pipe
Authors: Chan, L.; MacDonald, M.; Chung, D.; Hutchins, N.; Ooi, A.
Abstract: A turbulent rough wall flow in a pipe is simulated using direct numerical simulation (DNS) where the roughness elements consist of explicitly gridded three-dimensional sinusoids. Two groups of simulations were conducted where the roughness semi-amplitude h+ and the roughness wavelength λ+ are systematically varied. The triple decomposition is applied to the velocity to separate the coherent and turbulent components. The coherent or dispersive component arises due to the roughness and depends on the topological features of the surface. The turbulent stress on the other hand, scales with the friction Reynolds number. For the case with the largest roughness wavelength, large secondary flows are observed which are similar to that of duct flows. The occurrence of these large secondary flows is due to the spanwise heterogeneity of the roughness which has a spacing approximately equal to the boundary layer thickness δ. © Published under licence by IOP Publishing Ltd.
Sun, 01 Jan 2017 00:00:00 GMThttp://dspace.uniten.edu.my/jspui/handle/123456789/94282017-01-01T00:00:00Z