The Experts below are selected from a list of 267 Experts worldwide ranked by ideXlab platform
Hyung Jin Sung - One of the best experts on this subject based on the ideXlab platform.
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Assessment of the organization of a turbulent separated and reattaching flow by measuring wall pressure fluctuations
Experiments in Fluids, 2005Co-Authors: Yingzheng Liu, Woong Kang, Hyung Jin SungAbstract:The effect of local forcing on the organization of a turbulent separated and reattaching flow was assessed by measuring wall pressure fluctuations. Multi-arrayed microphones were installed on the surface to measure the simultaneous spatial and temporal wall pressure fluctuations. Local forcing at the separation edge was applied to the separated flow over a backward-facing step through a thin slit. The organization of the separated and reattaching flow was found to be greatest at the effective forcing frequency. The flow structure was diagnosed by analyzing several characteristics of the wall pressure fluctuations: the wall pressure fluctuation coefficients, wall pressure spectrum, wavenumber-frequency spectrum, coherence, cross-correlation, and multi-resolution autocorrelations of pressure fluctuations using the maximum overlap discrete wavelet transform and continuous wavelet transform. Features indicative of the amalgamation of vortices under the local forcing were observed; this amalgamation process accounted for the observed reduction of the Reattachment Length. Examination of the wall pressure fluctuations revealed that introduction of local forcing enhanced flapping motion as well as the streamwise and spanwise dispersions of vortical structures.
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multiple arrayed pressure measurement for investigation of the unsteady flow structure of a reattaching shear layer
Journal of Fluid Mechanics, 2002Co-Authors: Inwon Lee, Hyung Jin SungAbstract:Spatio-temporal characteristics of wall pressure fluctuations in separated and reattaching flows over a backward-facing step were investigated through an extensive pressure-velocity joint measurement with an array of microphones. The experiment was performed in a wind tunnel with a Reynolds number of 33 000 based on the step height and the free-stream velocity. Synchronized wavelet maps showed the evolutionary behaviour of pressure fluctuations and gave further insight into the modulated nature of large-scale vortical structures. To see the relationship between the flow eld and the relevant spatial mode of the pressure eld, a new kind of wavenumber ltering, termed ‘spatial box ltering’ (SBF), was introduced and examined. The vortical flow eld was reconstructed using every single-point velocity measurement by means of the conditional average based on the SBF second mode of pressure fluctuations. The flow eld showed a well-organized spanwise vortical structure convected with a speed of 0:6U0 and a characteristic ‘sawtooth’ pattern of the unsteady trace of Reattachment Length. In addition to the coherent vortical structures, the periodic enlargement/shrinkage process of the recirculation region owing to flapping motion was analysed. The recirculation region was found to undergo an enlargement/shrinkage cycle in accordance with the lowpass-ltered component of pressure fluctuations. In addition, such modulatory behaviour of the vortical structure as the global oscillation phase was discussed in connection with the conditionally averaged flow eld.
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visualization of a locally forced separated flow over a backward facing step
Experiments in Fluids, 1998Co-Authors: K B Chun, Hyung Jin SungAbstract:A laboratory water channel experiment was made of the separated flow over a backward-facing step. The flow was excited by a sinusoidally oscillating jet issuing from a separ- ation line. The slit was connected to a cavity in which water was forced through a rigid pipe by a scotch-yoke system. The Reynolds number based on the step height (H) was fixed at Re H \1200. The forcing frequency was varied in the range 0.305OSt H O0.955 at the forcing amplitude A 0 \0.3. Time- averaged flow measurements were made by a LDV system, especially in the recirculating region behind the backward- facing step. To characterize the large-scale vortex evolution due to the local forcing, flow visualizations were performed by a dye tracer method with fluorescent ink. The vortex amalgamation process was captured at the effective forcing frequency (St H \0.477) for laminar separation. This vortex merging process enhances flow mixing, which leads to the shortening of the Reattachment Length.
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control of turbulent separated flow over a backward facing step by local forcing
Experiments in Fluids, 1996Co-Authors: K B Chun, Hyung Jin SungAbstract:An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 ⩽ Re H ⩽ 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A 0 ⩽ 0.07) and forcing frequency (0 ⩽ St H ⩽ 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in Reattachment Length as compared to the unforced flow. The normalized minimum Reattachment Length (x r )min/x x0 was obtained at St θ ≅ 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.
Pascal Thorigny - One of the best experts on this subject based on the ideXlab platform.
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unsteadiness of an axisymmetric separating reattaching flow numerical investigation
Physics of Fluids, 2007Co-Authors: Sebastien Deck, Pascal ThorignyAbstract:The separated flow over a cylinder elongated by another cylinder of a smaller diameter is investigated numerically at the high subsonic regime using zonal detached eddy simulation (ZDES) and compared with the experimental data of Depres, Reijasse, and Dussauge [AIAA J. 42, 2541 (2004)]. First, it is shown that this axisymmetric step flow has much in common with the two-dimensional facing step flows as regards the shear layer instability process. Second, the statistical and spectral properties of the pressure fluctuations are scrutinized. Close to the step, the surface pressure signature is characterized by low frequencies f.Lr∕U∞=O(0.08) (where Lr and U∞ denote, respectively, the mean Reattachment Length and free-stream velocity) and an upstream velocity of 0.26U∞ while in the second half-part of the recirculation higher frequencies fluctuations at f.Lr∕U∞≈0.6 and a downstream convection velocity 0.6U∞ are the dominant features. The current calculation shows that the separated bubble dynamics depends on v...
John Kim - One of the best experts on this subject based on the ideXlab platform.
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direct numerical simulation of turbulent flow over a backward facing step
Journal of Fluid Mechanics, 1997Co-Authors: Parviz Moin, John KimAbstract:Turbulent flow over a backward-facing step is studied by direct numerical solution of the Navier–Stokes equations. The simulation was conducted at a Reynolds number of 5100 based on the step height h and inlet free-stream velocity, and an expansion ratio of 1.20. Temporal behaviour of spanwise-averaged pressure fluctuation contours and Reattachment Length show evidence of an approximate periodic behaviour of the free shear layer with a Strouhal number of 0.06. The instantaneous velocity fields indicate that the Reattachment location varies in the spanwise direction, and oscillates about a mean value of 6.28h. Statistical results show excellent agreement with experimental data by Jovic & Driver (1994). Of interest are two observations not previously reported for the backward-facing step flow: (a) at the relatively low Reynolds number considered, large negative skin friction is seen in the recirculation region; the peak |Cf| is about 2.5 times the value measured in experiments at high Reynolds numbers; (b) the velocity profiles in the recovery region fall below the universal log-law. The deviation of the velocity profile from the log-law indicates that the turbulent boundary layer is not fully recovered at 20 step heights behind the separation.The budgets of all Reynolds stress components have been computed. The turbulent kinetic energy budget in the recirculation region is similar to that of a turbulent mixing layer. The turbulent transport term makes a significant contribution to the budget and the peak dissipation is about 60% of the peak production. The velocity–pressure gradient correlation and viscous diffusion are negligible in the shear layer, but both are significant in the near-wall region. This trend is seen throughout the recirculation and Reattachment region. In the recovery region, the budgets show that effects of the free shear layer are still present.
R Saidur - One of the best experts on this subject based on the ideXlab platform.
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convective heat transfer and fluid flow study over a step using nanofluids a review
Renewable & Sustainable Energy Reviews, 2011Co-Authors: H A Mohammed, A A Alaswadi, N H Shuaib, R SaidurAbstract:Research in convective heat transfer on internal separated flows has been extensively conducted in the past decades. This review summarizes numerous researches on two topics. The first section focuses on studying the fluid flow and heat transfer behavior of different types of single-phase fluid flows over backward facing step (BFS) at different orientations. The second section concentrates on everything related to nanofluids; its preparation, properties, behavior, applications, and many others. The purpose of this article is to get a clear view and detailed summary of the influence of several parameters such as the geometrical specifications, boundary conditions, type of fluids, and inclination angle on the hydrodynamic and thermal characteristics using (BFS). The Reattachment Length and maximum Nusselt number are the main target of such research where correlation equations were developed and reported in experimental and numerical studies. The heat transfer enhancement of nanofluids along with the nanofluids preparation technique, types and shapes of nanoparticles, base fluids and additives, transport mechanisms, and stability of the suspension are also discussed.
Yujia Chen - One of the best experts on this subject based on the ideXlab platform.
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end wall heat transfer of a rectangular bluff body at different heights temperature sensitive paint measurement and computational fluid dynamics
Applied Thermal Engineering, 2017Co-Authors: Zahra Ghorbanitari, Yujia ChenAbstract:Abstract Complementary methods — temperature sensitive paint measurement and computational fluid dynamics — are used to elucidate the end-wall heat transfer of a rectangular body in a turbulent channel flow. Four systems with bluff body-height-to-channel-height ratio Hb/H = 0.25, 0.5, 0.75, and 1.0 are compared. For Hb/H = 1.0, an extremely high heat transfer rate is observed in the upstream area, attributing to the highly unstable corner vortex; the absence of pair vortices entails a distinctly different flow pattern and heat transfer in the downstream region. For Hb/H = 0.75, the presence of the paired vortices together with Karman vortex shedding leads to the arch-shaped vortex and heat transfer augmentation. Ongoing from Hb/H = 0.75 to 0.25, the growth of the arch vortex is highly weakened. At Hb/H = 0.25, the heat transfer measurement reflects the absence of the Karman vortex in the wake region. Augmentation of the convective heat transfer pertinent to the shear layer’s Reattachment is shown to be sensitive to Hb/H. As Hb/H decreases, the reduced Reattachment Length results in attenuated heat transfer in the wake region; the strength of the horseshoe vortex structure, characterized by the heat transfer rate, is also progressively weakened.
