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Neil D Sandham - One of the best experts on this subject based on the ideXlab platform.
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two dimensional Unsteadiness map of oblique shock wave boundary layer interaction with sidewalls
Journal of Fluid Mechanics, 2019Co-Authors: P K Rabey, Satya P Jammy, Paul J K Bruce, Neil D SandhamAbstract:The low-frequency Unsteadiness of oblique shock wave/boundary layer interactions (SBLIs) has been investigated using large-eddy simulation (LES) and high-frequency pressure measurements from experiments. Particular attention has been paid to off-centreline behaviour: the LES dataset was generated including sidewalls, and experimental pressure measurements were acquired across the entire span of the reflected shock foot. The datasets constitute the first maps of low-frequency Unsteadiness in both streamwise and spanwise directions. The results reveal that significant low-frequency shock motion (with $St\approx 0.03$ ) occurs away from the centreline, along most of the central separation shock and in the corner regions. The most powerful low-frequency Unsteadiness occurs off-centre, likely due to the separation shock being strengthened by shocks arising from the swept interactions on the sidewalls. Both simulation and experimental results exhibit asymmetry about the spanwise centre. In simulations, this may be attributed to a lack of statistical convergence; however, the fact that this is also seen in experiments is indicative that some SBLIs may exhibit some inherent asymmetry across the two spanwise halves of the separation bubble. There is also significant low-frequency power in the corner separations. The relation of the Unsteadiness in the corner regions to that in the centre is investigated by means of two-point correlations: a key observation is that significant correlation does not extend across the attached flow channel between the central and corner separations.
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instability and low frequency Unsteadiness in a shock induced laminar separation bubble
Journal of Fluid Mechanics, 2016Co-Authors: Andrea Sansica, Neil D Sandham, Zhiwei HuAbstract:Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low frequency Unsteadiness. A laminar boundary-layer interacting with an oblique shock-wave at M = 1:5 is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary-layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency Unsteadiness is found near the separation point with a peak at a Strouhal number of 0:04, based on the separation bubble length. The appearance of the low-frequency Unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary-layer, with a strong response near the separation point. The existence of the Unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.
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Forced response of a laminar shock-induced separation bubble
Physics of Fluids, 2014Co-Authors: Andrea Sansica, Neil D SandhamAbstract:The source of Unsteadiness in shock-wave/boundary-layer interactions is currently disputed. This paper considers a two-dimensional separation bubble induced by an oblique shock wave interacting with a laminar boundary layer at a free-stream Mach number of 1.5. The global response of the separated region to white noise forcing is analyzed for different interaction strengths, which generate small and large separation bubbles. Forcing location and amplitude effects have been examined. For both interaction strengths and for forcing both upstream and inside the bubble, the wall-pressure spectra downstream of the separation show a high-frequency peak that is demonstrated to be a Kelvin-Helmholtz instability. A low-frequency response at the separation point is also found when the separation bubble is only forced internally, therefore with a disturbance-free upstream boundary layer. For low-amplitude internal forcing, the low-frequency response at the separation point and downstream of the bubble is linear. However, when forced upstream the low-frequency Unsteadiness of the large separation bubble is found to be driven by nonlinearities coming from the downstream shedding. The same nonlinear behavior is found when the separation bubble is internally forced over a narrow band around the shedding frequency, without low-frequency disturbances. This analysis for a laminar interaction is used to interpret the low-frequency Unsteadiness found at the foot of the shock of turbulent interactions. Here, the low-frequency Unsteadiness occurs in the absence of upstream disturbances and a linear relationship is found between the internal forcing and the response near the separation point. When low-frequencies are not present in the forcing they are generated from weak nonlinearities of the shear-layer instability modes.
Dulal Pal - One of the best experts on this subject based on the ideXlab platform.
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combined effects of non uniform heat source sink and thermal radiation on heat transfer over an unsteady stretching permeable surface
Communications in Nonlinear Science and Numerical Simulation, 2011Co-Authors: Dulal PalAbstract:Abstract The present paper is concerned with the study of flow and heat transfer characteristics in the unsteady laminar boundary layer flow of an incompressible viscous fluid over continuously stretching permeable surface in the presence of a non-uniform heat source/sink and thermal radiation. The Unsteadiness in the flow and temperature fields is because of the time-dependent stretching velocity and surface temperature. Similarity transformations are used to convert the governing time-dependent nonlinear boundary layer equations for momentum and thermal energy are reduced to a system of nonlinear ordinary differential equations containing Prandtl number, non-uniform heat source/sink parameter, thermal radiation and Unsteadiness parameter with appropriate boundary conditions. These equations are solved numerically by applying shooting method using Runge–Kutta–Fehlberg method. Comparison of numerical results is made with the earlier published results under limiting cases. The effects of the Unsteadiness parameter, thermal radiation, suction/injection parameter, non-uniform heat source/sink parameter on flow and heat transfer characteristics as well as on the local Nusselt number are shown graphically.
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Combined effects of non-uniform heat source/sink and thermal radiation on heat transfer over an unsteady stretching permeable surface
Communications in Nonlinear Science and Numerical Simulation, 2011Co-Authors: Dulal PalAbstract:Abstract The present paper is concerned with the study of flow and heat transfer characteristics in the unsteady laminar boundary layer flow of an incompressible viscous fluid over continuously stretching permeable surface in the presence of a non-uniform heat source/sink and thermal radiation. The Unsteadiness in the flow and temperature fields is because of the time-dependent stretching velocity and surface temperature. Similarity transformations are used to convert the governing time-dependent nonlinear boundary layer equations for momentum and thermal energy are reduced to a system of nonlinear ordinary differential equations containing Prandtl number, non-uniform heat source/sink parameter, thermal radiation and Unsteadiness parameter with appropriate boundary conditions. These equations are solved numerically by applying shooting method using Runge–Kutta–Fehlberg method. Comparison of numerical results is made with the earlier published results under limiting cases. The effects of the Unsteadiness parameter, thermal radiation, suction/injection parameter, non-uniform heat source/sink parameter on flow and heat transfer characteristics as well as on the local Nusselt number are shown graphically.
Tienan Zhang - One of the best experts on this subject based on the ideXlab platform.
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evaluation of Unsteadiness in effervescent sprays by analysis of droplet arrival statistics the influence of fluids properties and atomizer internal design
Experimental Thermal and Fluid Science, 2011Co-Authors: Meng Liu, Yufeng Duan, Tienan ZhangAbstract:Abstract The ideal spray theory of Edwards and Marx was used to investigate the dependence of effervescent spray Unsteadiness on fluid properties and atomizer internal design. Results demonstrate that fluid properties and internal design of atomizer directly affect the two-phase flow pattern inside the atomizer which consequently affects the spray Unsteadiness of the atomizer. Water sprays are more unsteady when the air to liquid ratio ( ALR ) increases, whereas, more unsteady is observed for using glycerol/water mixture (high-viscosity Newtonian fluid) or glycerol/water/xanthan (non-Newtonian fluid) mixture as ALR reduces. In addition, sprays using low-viscosity or strong non-Newtonian fluids usually are more unsteady, regardless of ALR . A short mixing chamber results in less unsteady for water but has no effect on spray Unsteadiness for high-viscosity or non-Newtonian fluids at ALR of 0.15. Otherwise, the influence of mixing chamber distance on the spray quality is weak at ALR of 0.15. Large diameter of inclined aeration holes shows the low spray Unsteadiness and good spray quality for water but causes more unsteady for glycerol/water/xanthan mixture at ALR of 0.15. Furthermore, the diameter of the inclined aeration holes has little influence on spray Unsteadiness for glycerol/water mixture. Spray Unsteadiness and quality are not affected by the angle of aeration holes for three fluids at ALR of 0.15.
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Evaluation of effervescent atomizer internal design on the spray Unsteadiness using a phase/Doppler particle analyzer
Experimental Thermal and Fluid Science, 2010Co-Authors: Yufeng Duan, Tienan ZhangAbstract:The purpose of this research was to investigate the dependence of effervescent spray Unsteadiness on operational conditions and atomizer internal design by the ideal spray theory of Edwards and Marx. The convergent-divergent effervescent atomizer spraying water with air as atomizing medium in the ''outside-in'' gas injection was used in this study. Results demonstrated that droplet formation process at various air to liquid ratio (ALR) led to the spray Unsteadiness and all droplet size classes exhibited Unsteadiness behavior in spray. The spray Unsteadiness reduced quickly at ALR of 3% and decreased moderately at ALR of other values as the axial distance increased. When the axial distance was 200 mm, the spray Unsteadiness reduced dramatically with the increase in radial distance, but lower spray Unsteadiness at the center of spray and higher spray Unsteadiness at the edge of spray were shown as the axial distance increased. The spray Unsteadiness at the center region of spray increased with the injection pressure. Low spray Unsteadiness and good atomization performance can be obtained when the diameter of incline aeration holes increased at ALR of 10%. Although short mixing chamber with large discharge orifice diameter for convergent-divergent effervescent atomizer produced good atomization, the center region ofmore » spay showed high spray Unsteadiness and maybe formed the droplet clustering. (author)« less
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evaluation of effervescent atomizer internal design on the spray Unsteadiness using a phase doppler particle analyzer
Experimental Thermal and Fluid Science, 2010Co-Authors: Meng Liu, Yufeng Duan, Tienan ZhangAbstract:The purpose of this research was to investigate the dependence of effervescent spray Unsteadiness on operational conditions and atomizer internal design by the ideal spray theory of Edwards and Marx. The convergent-divergent effervescent atomizer spraying water with air as atomizing medium in the ''outside-in'' gas injection was used in this study. Results demonstrated that droplet formation process at various air to liquid ratio (ALR) led to the spray Unsteadiness and all droplet size classes exhibited Unsteadiness behavior in spray. The spray Unsteadiness reduced quickly at ALR of 3% and decreased moderately at ALR of other values as the axial distance increased. When the axial distance was 200 mm, the spray Unsteadiness reduced dramatically with the increase in radial distance, but lower spray Unsteadiness at the center of spray and higher spray Unsteadiness at the edge of spray were shown as the axial distance increased. The spray Unsteadiness at the center region of spray increased with the injection pressure. Low spray Unsteadiness and good atomization performance can be obtained when the diameter of incline aeration holes increased at ALR of 10%. Although short mixing chamber with large discharge orifice diameter for convergent-divergent effervescent atomizer produced good atomization, the center region ofmore » spay showed high spray Unsteadiness and maybe formed the droplet clustering. (author)« less
Pierre Dupont - One of the best experts on this subject based on the ideXlab platform.
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mechanism of shock Unsteadiness in separated shock boundary layer interactions
Physics of Fluids, 2015Co-Authors: Lionel Agostini, Lionel Larchevêque, Pierre DupontAbstract:A large-eddy simulation-based study is presented and focuses on different Unsteadiness-source features in a Mach 2.3 shock reflection with separation. The sources of Unsteadiness are localized and the path taken by disturbance as it spreads out to the whole field is defined. It is shown that the phenomena arising inside the recirculation bubble govern the whole interaction, at both low and intermediate frequencies. Indeed, the shock motion appears to mirror phenomena found in the separated zone. Moreover, features of separated-flow Unsteadiness bear some resemblance to those occurring in incompressible flows. An equivalent inviscid scheme of the unsteady interaction is established in order to describe the whole shock-system Unsteadiness at low and intermediate frequencies and the downstream unsteady-pressure field.
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Mechanism of shock Unsteadiness in separated shock/boundary-layer interactions
Physics of Fluids, 2015Co-Authors: Lionel Agostini, Lionel Larchevêque, Pierre DupontAbstract:An LES-based study is presented and focuses on different Unsteadiness-source features in a Mach 2.3 shock reflection with separation. Sources of Unsteadiness are localized and the path taken by disturbance as it spreads out to the whole field is defined. It is shown that phenomena arising inside the recirculation bubble govern the whole interaction, at both low and intermediate frequencies. Indeed the shock motion appears to mirror phenomena found in the separated zone. Moreover, features of separated–flow Unsteadiness bear some resemblance to those occurring in incompressible flows. An equivalent inviscid scheme of the unsteady interaction is established in order to describe the whole shock-system Unsteadiness at low and intermediate frequencies and the downstream unsteady-pressure field.
Ioan Pop - One of the best experts on this subject based on the ideXlab platform.
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Unsteady flow due to a contracting cylinder in a nanofluid using Buongiorno’s model
International Journal of Heat and Mass Transfer, 2014Co-Authors: Khairy Zaimi, Anuar Mohd Ishak, Ioan PopAbstract:Abstract The unsteady flow and heat transfer of a nanofluid over a contracting cylinder is studied. Using a similarity transformation, the unsteady Navier–Stokes equations are transformed into a system of ordinary differential equations, which are then solved numerically using a shooting method. The effects of the Unsteadiness parameter and the Brownian motion parameter on the flow field and heat transfer characteristics are analyzed and graphically presented. Dual solutions are found to exist for a certain range of the Unsteadiness parameter. It is observed that the skin friction coefficient, the Nusselt number and the Sherwood number decrease with increasing values of the Unsteadiness parameter. The magnitude of the Nusselt number and the Sherwood number decrease as the Brownian motion parameter increases.
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unsteady three dimensional boundary layer flow due to a permeable shrinking sheet
Applied Mathematics and Mechanics-english Edition, 2010Co-Authors: Norfifah Bachok, Anuar Mohd Ishak, Ioan PopAbstract:The unsteady viscous flow over a continuously permeable shrinking surface is studied. Similarity equations are obtained through the application of similar transformation techniques. Numerical techniques are used to solve the similarity equations for different values of the Unsteadiness parameter, the mass suction parameter, the shrinking parameter and the Prandtl number on the velocity and temperature profiles as well as the skin friction coefficient and the Nusselt number. It is found that, different from an unsteady stretching sheet, dual solutions exist in a certain range of mass suction and Unsteadiness parameters.
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Heat transfer over an unsteady stretching permeable surface with prescribed wall temperature
Nonlinear Analysis: Real World Applications, 2009Co-Authors: Anuar Mohd Ishak, Roslinda Mohd. Nazar, Ioan Pop, Ioan PopAbstract:The unsteady laminar boundary layer flow over a continuously stretching permeable surface is investigated. The Unsteadiness in the flow and temperature fields is caused by the time-dependence of the stretching velocity and the surface temperature. Effects of the Unsteadiness parameter, suction/injection parameter and Prandtl number on the heat transfer characteristics are thoroughly examined.
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analysis of the flow of a power law fluid film on an unsteady stretching surface by means of homotopy analysis method
Journal of Non-newtonian Fluid Mechanics, 2006Co-Authors: Chun Wang, Ioan PopAbstract:Abstract Flow of a power-law fluid film on an unsteady stretching surface is analyzed by means of homotopy analysis method (HAM [1] ). For real power-law index and the Unsteadiness parameter in wide ranges, analytic series solutions are given and compared with the numerical results. The good agreement between them shows the effectiveness of HAM to this problem. Additionally, unlike previous studies, the value of the critical Unsteadiness parameter S 0 , above which no solution exists, is determined analytically in this paper.
