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Zhenlan Gao

Bio: Zhenlan Gao is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Natural convection & Intermittency. The author has an hindex of 3, co-authored 9 publications receiving 89 citations. Previous affiliations of Zhenlan Gao include Pierre-and-Marie-Curie University & Arts et Métiers ParisTech.

Papers
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Journal ArticleDOI
TL;DR: In this paper, a HLLC-type scheme is presented and implemented in the context of Arbitrary Lagrangian-Eulerian formulation for solving the five-equation models.

47 citations

Journal ArticleDOI
TL;DR: In this paper, high-resolution, unsteady RANS simulations are used to study short episodes of local pollution dispersion in the neighborhood of Bordelongue in Toulouse in the framework of the French ANR project EUREQUA (Haoues-Jouve et al. 2015).
Abstract: Detailed, high resolution, unsteady RANS simulations are used to study short episodes of local pollution dispersion in the neighborhood of Bordelongue in Toulouse in the framework of the French ANR project EUREQUA (Haoues-Jouve et al. 2015). These urban areas consist of various types of buildings and obstacles: small houses, tower blocks, highway, local streets, vegetation areas, etc. The 3D geometry of this urban area was constructed with an in house tool developed around the open-source geometry and mesh generator SALOME, based on the available geophysical data from the French geographical institute (IGN). The open-source computational fluid dynamics (CFD) code Code_Saturne, with the atmospheric option developed at CEREA, was used to carry out the simulations. The vegetation composed of tall trees is considered as a porous volume which induces a drag force to the air flowing through it. The pollutants of the local traffic emissions are considered as passive scalars (no chemical reaction). The global meteorology, including stratification conditions, is taken into account using boundary conditions obtained from mesoscale simulations performed over the region, with a zoom over the city by the Meso-NH code and the TEB urban parameterization. The simulation results of the air flow and pollution dispersion are compared with measurements obtained with fixed stations especially set up in the area during the campaigns. A good agreement is found between the measurements and simulations in terms of wind velocity and air temperature. For the wind direction the agreement is only fair with a Mean Bias of nearly 25° but nevertheless we find a good agreement with the NOx concentration time series at a local measuring station inside the neighborhood. This good agreement is explained partly by the adjustment of the unknown local emission factor but also by the configuration of the ring road surrounding in part the neighborhood, making it less sensitive to wind direction errors. Two urban renewal scenarios proposed by architects and local inhabitants are simulated under the same meteorological conditions. Increasing the height of anti-noise walls (from 3 to 6 m) does not improve the neighborhood air quality (except very locally) and the suppression of a big building block next to the ring road has a mixed effect, displacing the pollution (concentration increased in some area, decreased in some others).

38 citations

Journal ArticleDOI
TL;DR: In this article, a Ginzburg-Landau equation is derived analytically for the flow around a supercritical circle pitchfork bifurcation leading to steady 2D corotating rolls.
Abstract: Natural convection of air between two infinite vertical differentially heated plates is studied analytically in two dimensions (2D) and numerically in two and three dimensions (3D) for Rayleigh numbers $\mathrm{Ra}$ up to 3 times the critical value ${\mathrm{Ra}}_{c}=5708$. The first instability is a supercritical circle pitchfork bifurcation leading to steady 2D corotating rolls. A Ginzburg-Landau equation is derived analytically for the flow around this first bifurcation and compared with results from direct numerical simulation (DNS). In two dimensions, DNS shows that the rolls become unstable via a Hopf bifurcation. As $\mathrm{Ra}$ is further increased, the flow becomes quasiperiodic, and then temporally chaotic for a limited range of Rayleigh numbers, beyond which the flow returns to a steady state through a spatial modulation instability. In three dimensions, the rolls instead undergo another pitchfork bifurcation to 3D structures, which consist of transverse rolls connected by counter-rotating vorticity braids. The flow then becomes time dependent through a Hopf bifurcation, as exchanges of energy occur between the rolls and the braids. Chaotic behavior subsequently occurs through two competing mechanisms: a sequence of period-doubling bifurcations leading to intermittency or a spatial pattern modulation reminiscent of the Eckhaus instability.

23 citations

Journal Article
TL;DR: Natural convection of air between two infinite vertical differentially heated plates is studied analytically and numerically in two and three dimensions for Rayleigh numbers Ra up to 3 times the critical value Ra(c)=5708 and Chaotic behavior subsequently occurs through two competing mechanisms.
Abstract: Natural convection of air between two infinite vertical differentially heated plates is studied analytically in two dimensions (2D) and numerically in two and three dimensions (3D) for Rayleigh numbers Ra up to 3 times the critical value Ra(c)=5708. The first instability is a supercritical circle pitchfork bifurcation leading to steady 2D corotating rolls. A Ginzburg-Landau equation is derived analytically for the flow around this first bifurcation and compared with results from direct numerical simulation (DNS). In two dimensions, DNS shows that the rolls become unstable via a Hopf bifurcation. As Ra is further increased, the flow becomes quasiperiodic, and then temporally chaotic for a limited range of Rayleigh numbers, beyond which the flow returns to a steady state through a spatial modulation instability. In three dimensions, the rolls instead undergo another pitchfork bifurcation to 3D structures, which consist of transverse rolls connected by counter-rotating vorticity braids. The flow then becomes time dependent through a Hopf bifurcation, as exchanges of energy occur between the rolls and the braids. Chaotic behavior subsequently occurs through two competing mechanisms: a sequence of period-doubling bifurcations leading to intermittency or a spatial pattern modulation reminiscent of the Eckhaus instability.

8 citations

Journal ArticleDOI
TL;DR: The transition to the chaos of the air flow between two vertical plates maintained at different temperatures is studied in the Boussinesq approximation, highlighting the complementary role of linear stability analysis and nonlinear simulation.
Abstract: The transition to the chaos of the air flow between two vertical plates maintained at different temperatures is studied in the Boussinesq approximation. After the first bifurcation at critical Rayleigh number Ra_{c}, the flow consists of two-dimensional (2D) corotating rolls. The stability of the 2D rolls is examined, confronting linear predictions with nonlinear integration. In all cases the 2D rolls are destabilized in the spanwise direction. Efficient linear stability analysis based on an Arnoldi method shows competition between two eigenmodes, corresponding to different spanwise wavelengths and different types of roll distortion. Nonlinear integration shows that the lower-wave-number mode is always dominant. A partial route to chaos is established through the nonlinear simulations. The flow becomes temporally chaotic for Ra=1.05Ra_{c}, but remains characterized by the spatial patterns identified by linear stability analysis. This highlights the complementary role of linear stability analysis and nonlinear simulation.

3 citations


Cited by
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Journal ArticleDOI
TL;DR: Why RANS is still frequently used and whether this is justified or not is illustrated by examples for five application areas in building simulation: pedestrian-level wind comfort, near-field pollutant dispersion, urban thermal environment, natural ventilation of buildings and indoor airflow.
Abstract: Large Eddy Simulation (LES) undeniably has the potential to provide more accurate and more reliable results than simulations based on the Reynolds-averaged Navier-Stokes (RANS) approach. However, LES entails a higher simulation complexity and a much higher computational cost. In spite of some claims made in the past decades that LES would render RANS obsolete, RANS remains widely used in both research and engineering practice. This paper attempts to answer the questions why this is the case and whether this is justified, from the viewpoint of building simulation, both for outdoor and indoor applications. First, the governing equations and a brief overview of the history of LES and RANS are presented. Next, relevant highlights from some previous position papers on LES versus RANS are provided. Given their importance, the availability or unavailability of best practice guidelines is outlined. Subsequently, why RANS is still frequently used and whether this is justified or not is illustrated by examples for five application areas in building simulation: pedestrian-level wind comfort, near-field pollutant dispersion, urban thermal environment, natural ventilation of buildings and indoor airflow. It is shown that the answers vary depending on the application area but also depending on other—less obvious—parameters such as the building configuration under study. Finally, a discussion and conclusions including perspectives on the future of LES and RANS in building simulation are provided.

278 citations

Journal ArticleDOI
TL;DR: In this study, CFD simulations of urban microclimate are performed for a dense highly heterogeneous district in Nicosia, Cyprus and validated using a high-resolution dataset of on-site measurements of air temperature, wind speed and surface temperature conducted for the same district area.

98 citations

Journal ArticleDOI
TL;DR: A new reconstruction scheme, so-called MUSCL-THINC-BVD scheme, to solve the five-equation model for interfacial two phase flows and is able to capture the material interface as a well-defined sharp jump in volume fraction, and obtain numerical solutions of superior quality in comparison to other existing methods.

84 citations

Journal ArticleDOI
TL;DR: In this paper, the weakly nonlinear stability of electrohydrodynamic (EHD) flow of insulating fluids subject to strong unipolar injection, with and without cross-flow, was analyzed.
Abstract: We analyse in detail the weakly nonlinear stability of electrohydrodynamic (EHD) flow of insulating fluids subject to strong unipolar injection, with and without cross-flow. We first consider the hydrostatic electroconvetion induced by a Coulomb force confined between two infinite flat electrodes, taking into account the charge diffusion effect. The effects of various non-dimensionalized parameters are examined in order to depict in detail and to understand better the subcritical bifurcation of hydrostatic electroconvetion. In addition, electrohydrodynamics with low- or high- cross-flow is also considered for investigating the combined effect of inertia and the electric field. It is found that the base cross-flow is modified by the electric effect and that, even when the inertia is dominating, the electric field can still strengthen effectively the subcritical characteristics of canonical channel flow. In this process, however, the electric field does not contribute directly to the subcriticality of the resultant flow and the intensified subcritical feature of such flow is thus entirely due to the modified hydrodynamic field as a result of the imposed electric field. This finding might be important for flow control strategies involving an electric field. Theoretically, the above results are obtained from a multiple-scale expansion method, which gives rise to the Ginzburg–Landau equation governing the amplitude of the first-order perturbation. The conclusions are deduced by probing the changes of value of the coefficients in this equation. In particular, the sign of the first Landau coefficient indicates the type of bifurcation, being subcritical or supercritical. Moreover, as a quintic-order Ginzburg–Landau equation is derived, the effects of higher-order nonlinear terms in EHD flow are also discussed.

53 citations

Journal ArticleDOI
TL;DR: In this article, a table look-up method with bicubic interpolation based on the IAPWS-IF97 EoS formulation is proposed to calculate water properties when the independent variables of the EoS are the density and specific internal energy.

52 citations