Chengwang Lei

Re-examination of the effect of a plane boundary on force and vortex shedding of a circular cylinder

The hydrodynamic forces and vortex shedding of a smooth circular cylinder immersed in different boundary layers were experimentally investigated at Reynolds numbers from 1.30×104 to 1.45×104. The effects of the bed proximity, the thickness of the boundary layer, and the velocity gradient in the boundary layer on the pressure distribution, the hydrodynamic forces and the vortex shedding behavior were examined. The experimental results show that both the drag and lift coefficients strongly depend on the gap ratio, and are affected by the boundary layer. A downward lift is observed at certain gap ratios in rod-generated boundary layers, and an explanation of this downward lift is given. Two different criteria for calculating the Strouhal number in the literature are discussed in this paper. It is found that the variation of the root-mean-square (RMS) lift coefficient reveals the onset or suppression of the vortex shedding. A quantitative method for identifying the vortex shedding suppression point is proposed. The observations show that the vortex shedding is suppressed at a gap ratio of about 0.2–0.3, depending on the thickness of the boundary layer. This critical gap ratio decreases as the thickness of the boundary layer increases.

Unsteady natural convection in a triangular enclosure induced by absorption of radiation

The authors have previously reported a model experiment on the unsteady natural convection in a triangular domain induced by the absorption of solar radiation. This issue is reconsidered here both analytically and numerically. The present study consists of two parts: a scaling analysis and a numerical simulation. The scaling analysis for small bottom slopes reveals that a number of flow regimes are possible depending on the Rayleigh number and the relative value of certain non-dimensional parameters describing the flow. In a typical situation, the flow can be classified broadly into a conductive, a transitional or a convective regime determined merely by the Rayleigh number. Proper scales have been established to quantify the flow properties in each of these flow regimes. The numerical simulation has verified the scaling results.

Vortex shedding suppression for flow over a circular cylinder near a plane boundary

In this study, the Navier-Stokes equations and the pressure Poisson equation for two-dimensional time-dependent viscous flows are solved with a finite difference method in a curvilinear coordinate system. With this numerical procedure, the vortex shedding flow past a circular cylinder near a wall is investigated. The flow is calculated for a broad range of gap ratios for different Reynolds numbers ranging from 80 to 1000. Based on the numerical solutions, the vortex shedding is observed using various methods, and the mechanism for the vortex shedding suppression at small gap ratios is analyzed. The critical gap ratio at which the vortex shedding is suppressed is identified at different Reynolds numbers.

A finite difference solution of the shear flow over a circular cylinder

The incompressible viscous shear flow past a circular cylinder is analyzed by solving two-dimensional Navier-Stokes equations and pressure Poisson equation using a finite difference method. The shear flow is calculated for Reynolds numbers from 80 to 1000, and shear parameters up to 0.25. The numerical results indicate that the vortex shedding persists at the shear parameters up to 0.25 for the present Reynolds number range. The Strouhal number and the drag coefficient decrease as the shear parameter increases. There is a transverse force acting from the high velocity side toward the low velocity side in the shear flow.

Unsteady natural convection in a triangular enclosure induced by absorption of radiation – a revisit by improved scaling analysis

The present study is concerned with radiation-induced natural convection in a water-filled triangular enclosure with a sloping bottom, which is directly relevant to buoyancy-driven flows in littoral regions. An improved scaling analysis is carried out to reveal more detailed features of the flow than a previously reported analysis. Two critical functions of the Rayleigh number with respect to the horizontal position are derived from the scaling for identifying the distinctness and stability of the thermal boundary layer. Four flow scenarios are possible, depending on the bottom slope and the maximum water depth. For each flow scenario, the flow domain may be composed of multiple subregions with distinct thermal and flow features, depending on the Rayleigh number. The dividing points between neighbouring subregions are determined by comparisons of the critical functions of the Rayleigh number with the global Rayleigh number. Position-dependent scales have been established to quantify the flow properties in different subregions. The different flow regimes for the case with relatively large bottom slopes and shallow waters are examined in detail. The present scaling results are verified by numerical simulations.

A direct three-dimensional simulation of radiation-induced natural convection in a shallow wedge

A three-dimensional numerical simulation is carried out to investigate the natural convection in a shallow wedge subject to solar radiation. The study reveals three distinct stages of the flow development from an isothermal and stationary state: an initial stage, a transitional stage and a quasi-steady stage. The heat transfer at the initial stage is dominated by conduction from the sloping bottom. The transitional stage starts with the onset of instabilities, and the quasi-steady state is characterised by steady growth of a spatially averaged temperature. The present results have confirmed the earlier observations in a flow visualisation experiment and a two-dimensional simulation.

A direct stability analysis of a radiation-induced natural convection boundary layer in a shallow wedge

This study considers the convective instability of a water-filled shallow wedge with an absorptive bottom subject to solar radiation. Previous studies have revealed that a thermal boundary layer develops along the sloping bottom due to the absorption of penetrative radiation there, and this boundary layer is potentially unstable to the Rayleigh–Benard instability. The stability properties of the thermal boundary layer are determined in the present study by perturbing the three-dimensional numerical solution. The result of the direct stability analysis has confirmed previous scaling with respect to the convective instability. Additional features of the thermal layer instability have also been revealed from the direct stability analysis.

Spanwise length effects on three-dimensional modelling of flow over a circular cylinder

In this study, both the two-dimensional (2-D) and three-dimensional (3-D) vortex shedding flows around a circular cylinder are solved using a finite difference method. Numerical experiments are carried out to investigate the spanwise length effects on the prediction of global parameters and the near-wake flow field. The calculated Reynolds number is 1000. The results indicate clearly that the 2-D simulation gives inaccurate predictions at the present Reynolds number. The predictions are significantly improved in 3-D simulations, but inaccuracies may also occur if the spanwise length is set too small. Good agreement with experimental data can be achieved with spanwise lengths above twice the cylinder diameters.

Natural convection in a reservoir sidearm subject to solar radiation: A two-dimensional simulation

The natural convection in a reservoir sidearm subject to solar radiation is investigated numerically. It is revealed that the flow development from an isothermal and stationary state passes through three distinct stages, namely, an initial stage dominated by bottom heating due to the absorption of penetrative radiation there, a transitional stage characterized by the onset of convective instability in a form of rising plumes, and a quasi-steady stage evidenced by a quasi-steady large-scale circulation across the sidearm, which is maintained by a distinct horizontal temperature gradient. The numerical results agree qualitatively with those of a model experiment.

Scaling of Natural Convection of an Inclined Flat Plate: Sudden Cooling Condition

A scaling analysis is performed for the transient boundary layer established adjacent to an inclined flat plate following a ramp cooling boundary condition. The imposed wall temperature decreases linearly up to a specific value over a specific time. It is revealed that if the ramp time is sufficiently large then the boundary layer reaches quasi-steady mode before the growth of the temperature is finished. However, if the ramp time is shorter then the steady state of the boundary layer may be reached after the growth of the temperature is completed. In this case, the ultimate steady state is the same as if the start up had been instantaneous. Note that the cold boundary layer adjacent to the plate is potentially unstable to Rayleigh-Benard instability if the Rayleigh number exceeds a certain critical value for this cooling case. The onset of instability may set in at different stages of the boundary layer development. A proper identification of the time when the instability may set in is discussed. A numerical verification of the time for the onset of instability is presented in this study. Different flow regimes based on the stability of the boundary layer have also been discussed with numerical results.