ORCID Profile
0000-0002-5966-9927
Current Organisations
University of Sydney
,
Dalian University of Technology
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Publisher: AIP Publishing
Date: 04-2019
DOI: 10.1063/1.5087907
Abstract: The convective flow in a differentially heated cavity, with linear temperature profiles at two sidewalls, is investigated in the present study by a scaling analysis and direct numerical simulations (DNS). Scales for the thermal boundary layer and the subsequent intrusion are obtained through the scaling analysis. The velocity scale reveals that the characteristic velocity of the thermal boundary layer depends on both the streamwise position and the time after the initiation of the flow, which suggests a two-dimensional growth at the start-up stage, rather than the well-known one-dimensional growth of the thermal boundary layer induced by a constant temperature boundary condition. Furthermore, unlike the typical transition of the thermal boundary layer to a two-dimensional and steady stage that is characterized by the dying out of a “temperature overshoot” phenomenon, the thermal boundary layer under consideration enters a two-dimensional and steady stage smoothly, without the occurrence of the temperature overshoot. It is also found that, with the passage of time, whilst the characteristic velocity of the thermal boundary layer depends on the streamwise position, the thickness of the thermal boundary layer is streamwise position independent due to infinitesimally small time. Four possible flow regimes and corresponding scales for the unsteady intrusion flow underneath the cavity ceiling are finally obtained, which are two types of viscous-buoyancy dominated regimes and two types of inertial-buoyancy dominated ones. The important scales obtained in the present study are validated by corresponding DNS results.
Publisher: American Physical Society (APS)
Date: 02-05-2022
Publisher: Elsevier BV
Date: 12-2020
Publisher: Elsevier BV
Date: 05-2023
Publisher: Springer Science and Business Media LLC
Date: 16-05-2020
Publisher: Elsevier BV
Date: 10-2019
Publisher: Elsevier BV
Date: 05-2014
Publisher: Elsevier BV
Date: 04-2019
Publisher: Elsevier BV
Date: 12-2022
Publisher: Emerald
Date: 15-07-2022
Abstract: Hydrodynamic forces and efficiency of bare propeller and ducted propellers with a wide range of advance ratio ( J ) and attack angle ( θ ) are examined. The thrust and torque coefficients and the efficiency are presented and discussed in detail. The present results give a reliable guidance to the improvement of the hydrodynamic characteristics of ducted propellers. The effect of a duct on the hydrodynamic performance of the KP458 propeller is numerically investigated in this study. Finite volume method (FVM)-based simulations are performed for a wide range of advance ratio J (0 ≤ J ≤ 0.75) and attack angle θ of the duct (15° ≤ θ ≤ 45°). A cubic computational domain is employed in this study, and the moving reference frame (MRF) approach is adopted to handle the rotation of the propeller. Turbulence is accounted for with the RNG k - ε model. The present numerical results are first compared against available experimental data and a good agreement is achieved. The simulation results demonstrate that the hydrodynamic forces and efficiency increases and decreases with J, respectively, at the same attack angle. In addition, it is demonstrated that the hydrodynamic forces and efficiency are both improved due to the presence of the duct, which eventually leads a better hydrodynamic performance at high advance ratios. It is further revealed that as the attack angle increases, the pressure difference between the suction- and pressure-surfaces of the propeller is also augmented, which results in a larger thrust. The wake field is more uniform at θ = 30°, suggesting that a higher efficiency can be obtained. The present study aims to investigate the effect of a duct on the KP458 propeller subjected to uniform inbound flow. The relationship between the uniform incoming flow and the attack angle of the duct is mainly focused, and the design of the ducted propellers for any ship hull can be improved according to this relationship.
Publisher: Elsevier BV
Date: 10-2019
Publisher: Elsevier BV
Date: 08-2020
Publisher: American Physical Society (APS)
Date: 21-10-2019
Publisher: AIP Publishing
Date: 03-2021
DOI: 10.1063/5.0044638
Abstract: In this study, the convective instability of incompressible convective boundary layers induced by linearly heating condition is investigated by its receptivity to controlled perturbations and the subsequent streamwise evolution of the disturbance waves. The unstable disturbance waves are triggered by two modes of perturbations. The random mode calculations demonstrate the boundary layer could be distinctly ided into a low frequency band (LFB), a mixed frequency band (MFB) and a high frequency band (HFB), which are essentially the same as the extensively studied homogenously heated problems. It is, however, found that the dominant frequency shifts lower in the HFB segment when Ra is higher than 1 × 109 at s = −2, where Ra = gβΔTH3/νκ is Rayleigh number and s = dθw(y)/dy is stratification factor. By examining temperature profiles, it is known that this behavior is associated with the negative buoyant effect which fundamentally differentiates the present flow from the classic s = 0 ones. The sinuous mode calculations reveal that the maximum perturbation litude is achieved at fc. From the spectral power of the perturbations in the convective boundary layer, the characteristic frequency fc is determined and appropriate scale laws are proposed for fc in various scenarios. In addition, the propagation speed of the instability waves in the boundary layer is obtained from the present calculations. It is revealed that the disturbance wave always travels faster than the base flow and the speed ratio ξ is larger at s = −2 than at s = 0. It is further found that, similar to the perturbation litude, the heat transfer also maximizes at fc.
Publisher: Elsevier BV
Date: 05-2020
Publisher: Elsevier BV
Date: 05-2022
Publisher: AIP Publishing
Date: 07-2021
DOI: 10.1063/5.0060202
Abstract: The convective boundary layer flow on the external surface of an isothermally heated horizontal cylinder is investigated in this study. Numerical simulations are first carried out for a wide range of flow parameters, i.e., Rayleigh and Prandtl numbers, and scale relations quantifying the boundary layer flow are then determined from the simulation data. The numerical results suggest that the curved boundary layer experiences an initial growth state, a transitional state, and a developed state, which are essentially identical to the extensively studied flat boundary layers. Scale relations quantifying the local flow variables are obtained, and the proposed scale laws indicate that during the initial growth, the present curved boundary layer flow follows a two-dimensional growth rather than the well-known one-dimensional startup of flat boundary layers. It is further demonstrated that the characteristic velocity of the boundary layer flow maximizes at π/2, but its thickness is circumferential location independent. In the steady state, however, the maximum streamwise velocity of the boundary layer shifts to approximately 7π/9 and the thickness consistently increases with the circumferential location. It is also shown that the thickness of the inner viscous boundary layer could be obtained by properly considering the Prandtl number effect, i.e., by the term (1 + Pr−1/2)−1. The proposed scale relations could reasonably describe the curved boundary layer flow, and all regression constants are above 0.99.
Publisher: Elsevier BV
Date: 02-2020
Publisher: Elsevier BV
Date: 05-2020
Publisher: Elsevier BV
Date: 06-2019
Publisher: Elsevier BV
Date: 06-2018
Publisher: Elsevier BV
Date: 02-2015
Publisher: Elsevier BV
Date: 06-2020
Publisher: ASME International
Date: 06-02-2023
DOI: 10.1115/1.4056485
Abstract: The transient convective flow adjacent to an inclined semi-infinite plate which is heated by a linear temperature gradient is investigated with scaling analysis and direct numerical simulation (DNS) in this study. Both Pr & 1 and Pr & 1 fluids are considered. The initial ambient fluid is quiescent and thermally homogeneous. Important parameters characterizing the thermal boundary layer flow are thickness, characteristic velocity, and time to reach the steady stage. Scaling analysis is carried out to obtain scales for these flow parameters. Compared to previous similar studies, the obtained scale relations are more generalized and they can be utilized for different inclination angles. The derived scales are compared against the DNS results for a variety of flow parameters, e.g., Rayleigh number Ra, Prandtl number Pr, stratification factor s (s = dθw(y)/dy, where θw(y) is the local temperature at a streamwise location of y), inclination angle of the heated plate α, evolutionary time τ, and streamwise location y. The scale relations and the DNS results compare well suggesting the proposed scale laws can provide a sound description for the dynamics of the convective flow subjected to a tilted surface and a linear heating condition.
Publisher: Elsevier BV
Date: 02-2020
Publisher: AIP Publishing
Date: 10-2019
DOI: 10.1063/1.5115073
Abstract: The convective instability of the natural convection boundary layers of air (Pr = 0.7) in the laminar-to-turbulent transition regime (Ra = 8.7 × 107–1.1 × 109) is investigated by stability analysis in the framework of direct numerical simulations. To understand the spatial and temporal evolution of the convective instability of the thermal boundary layers, small- litude random-mode numerical perturbations are first introduced into the boundary condition of the boundary layer flow. The prescribed full spectral perturbations (i.e., white noise) are mostly d ed out immediately by a limited upstream boundary layer. A low-frequency band is initially distinct in the upstream near the leading edge but decays spatially as the instability propagates downstream. In contrast, a high-frequency band emerges to finally become the most dominant frequency band in the thermal boundary layer transition regime. To obtain further insights into the nature of the established high-frequency band, single-mode perturbations of various frequencies are then introduced into the boundary layer near the leading edge. It is found that a single-mode perturbation at the peak frequency within the high-frequency band excites the maximum response of the thermal boundary layer, suggesting that the peak frequency is in fact the characteristic frequency or resonance frequency of the thermal boundary layer. The dimensionless form of the dependence of the characteristic frequency on Ra is then found to be fc = 0.07Ra2/3. The single-mode perturbation numerical experiments also revealed the propagation speed of convective instability waves, which was significantly greater than the convection speed of the thermal boundary layer. The smaller the Ra, the larger the difference between the two propagation speeds. A semi-analytical scaling of the wave propagation speed in the form csc ∼ Ra1/2y1/2Pr was derived (y denoting the streamwise location of the boundary layer), providing a predictive correlation that can be used for thermal boundary layer control.
Publisher: American Physical Society (APS)
Date: 17-08-2023
No related grants have been discovered for Yang LIU.