Abstract Understanding the thermal conditions inside a burning cigarette is a top priority for controlling chemical emissions and cigarette design. Since experimental methods are difficult to observe in depth, this paper starts from the perspective of numerical simulation and models the structure of the tobacco distribution of the cigarette, integrating the end surface ignition model, puffing model, chemical reaction model, heat and mass transfer and diffusion model have established a three-dimensional comprehensive model that can represent the changes in combustion cone morphology during cigarette combustion. The model covers chemical reaction and mass transfer as well as generation, flow and reaction mechanism. The simulation results show that the model can better predict the temperature distribution, component distribution and combustion cone morphology changes during cigarette smoking and combustion. It provides an effective means for in-depth research on cigarette combustion.

Abstract In this research, a three-dimensional turbulent wall jet was modeled using an Improved Delayed Detached Eddy Simulation to examine its flow and thermal properties. The accuracy of the simulation was confirmed by comparing key flow characteristics with experimental data. The study involved introducing an oscillating wall and a hot wall within the computational domain to observe their effects on thermal behavior and turbulence structure. OpenFOAM v2012 was utilized for the simulations based on a 3D channel design. The turbulent structure exhibited distinct separated, small-scale, and large-scale turbulence within the domain. The findings indicated that reducing the computational domain height increased the Nusselt number, and positioning the hot wall near the core of the jet also increased the Nusselt number. Additionally, increasing the frequency and amplitude of the oscillating wall resulted in a higher Nusselt number. These results contribute to a deeper understanding of the fluid physics in this specific scenario and can enhance knowledge in the fields of solid and turbulence fluid structure interaction. The analysis of the turbulence structure revealed that a lower domain height created elongated turbulence structures, and placing the hot wall at the end of the computational domain had less impact on smoothing the turbulence structures due to the presence of very strong, large turbulence structures.

Abstract Analysis methods based on mode decomposition have been proposed to describe the characteristics of flow phenomena. Among them, proper orthogonal decomposition (POD), which decomposes modes into eigenvalues and basis vectors, has long been used. Many studies have shown that POD is a useful method for capturing the characteristics of unsteady flow. In particular, Snapshot POD has attracted much recent attention and has been used to solve unsteady flow problems. However, the basis vectors of the mode obtained by conventional POD is different for each condition. Therefore, whether the basis vectors of each mode are switching in the direction of parameters (e.g. different shapes or different Reynolds numbers) or whether they develop or decay is difficult to discuss. As a result, discussions on conventional POD tend to be qualitative. To address this issue, the present study uses Parametric Global POD, a method that perfectly matches basis vectors in results with different parameters (in this study, different Reynolds numbers). Parametric Global POD method was applied to the analysis of the flow field in a curved pipe and found to capture the development or decay of modes with major basis vectors in the direction of parameters, which is difficult to achieve with conventional POD methods.

The drag reduction of the single circular cylinder is achieved by changing the position of an additional control rod in the present work. In order to look for where to place the control rod will help the drag force exerted on the main cylinder surface as little as possible, the additional control rod is arranged at various positions in the downstream region, the upstream region, and the middle region of the single circular cylinder, respectively. The numerical results show that the maximum drag reduction rate of the single circular cylinder can be up to 21.68%. While the additional control rod is located at some specific positions in the flow field, the drag of the single circular cylinder will increase. However, if two bluff bodies (the main circular cylinder and the control rod) are considered as a whole system, due to the extra drag contribution of the control rod, the maximum drag reduction effect of the system is 8.65%. Additionally, the lift exerted on the main cylinder and the control rod has changed a lot due to the interaction between two bluff bodies. Furthermore, the Dynamic Mode Decomposition analysis method is employed to probe the mechanism of drag variation employing the dominant flow modes.

In this study, we used a three-dimensional lattice Boltzmann method to simulate the settling motion of a spherical squirmer in a square tube under the effect of gravity. A spherical squirmer model with chirality was chosen to simulate the motion of a real microswimmer in a three-dimensional space and to systematically analyze its kinematic properties. According to the results of this study, we identified seven different motion modes: diagonal plane large-amplitude oscillation, central stable sedimentation, bidirectional spiral motion, rebound motion, unidirectional spiral motion, corner stable motion, and near-wall attraction oscillation. It was shown that the formation of different motion modes is caused by the effects of squirmer-type factor and chirality. squirmer-type factor determines the stable motion position of the squirmer in the channel. Chirality makes the head direction of the squirmer more susceptible to change, thus changing the motion trajectory of the squirmer. In addition, it was found that the self-propelling strength determines the speed of squirmer’s motion, which affects the motion frequency of squirmer’s periodic oscillations.

Water wave breaking at the ocean’s surface is a turbulent phenomenon and encompasses a wide range of physical processes such as generation of bubbles/drops with different spatio-temporal scales. In the present study, breaking of periodic waves in deep water regime has been simulated using phase-field lattice Boltzmann method (LBM). Compared to direct Navier–Stokes equation based numerical models, the present LB model is Poisson-free and explicit in time integration. Both the flow field and interface are solved using LBM with two sets of distribution functions. The normalized pressure-velocity formulation is used for the flow field, whereas the conservative Allen–Cahn equation is used to capture the interface. An impulsively started wave approach with Stokes third order wave profile has been used to initiate the breaking process. Through systematic variation of Weber number and initial steepness, the effect of surface tension on the kinematic, geometric, and energy dissipation characteristics has been studied for three breaker types, viz., spilling, weak plunging, and plunging breakers. The results are in good agreement with the past experimental observations and direct numerical simulation. In particular, the obtained breaking strength for different Weber numbers agrees well with the semi-empirical relation used in a wind-wave forecasting models.

Polymers inhibit the breakup of a liquid filament thinning under surface tension. The coupling of elasticity, capillarity and inertia leads to the well-known beads-on-a-string (BOAS) formation. Additionally, under different conditions, smaller satellite drops can form along the liquid bridge between the main beads. The development of BOAS and satellite drops is controlled by the rheology of the polymer solution. In this study, we consider the roles played by finite extensibility and anisotropic drag on the formation of satellite beads. In particular, we show that the more stretching a polymer chain can undergo, satellite beads are suppressed. The latter stages of capillary thinning has been shown to result in a phase separation resulting in what is referred to as a blistering pattern. We thus also conduct simulations of an inhomogeneous dilute polymer model that considers the competing effects of diffusion and stress gradients. We show that polymer is pulled axially towards the region connecting string and bead. This simple model does not predict a phase separation, but does reveal that pinchoff could be inhibited by the buildup of polymer concentration.

The hydrodynamic characteristics of underwater vehicles are significantly affected by the ventilated cavity covered by the vehicle surface. In this paper, the unsteady flow characteristics of this ventilated cavity are studied using experimental and numerical methods, and the unsteady entrainment behaviour of the ventilated air mass is emphasised. The flow pattern of the ventilated air mass is recorded using a high-speed camera. The large eddy simulation turbulence model is employed for the numerical simulations, and a good agreement is observed between the experimental and numerical results. In the early stage of the formation of the ventilated air mass, the internal structure exhibits a symmetric kidney vortex system, while the ventilated cavity below the vent hole has a continuous hairpin vortex structure. The ventilated air mass experiences a growth stage, an entrainment stage, and a shedding stage. The entrainment behaviour enables the ventilated air mass to quickly fill the ventilated cavity and modifies the surface pressure distribution of the vehicle. As the cavitation number decreases, the radial size of the ventilated cavity increases, and the contact area between the cavity and the water body increases, thus enhancing the vertical drag coefficient of the vehicle.

At the ocean surface, the wind shear stress not only drives the ocean currents, but also generates waves and breaks waves. The turbulence in the ocean surface layer (OSL) is much enhanced due to wave breaking. A typical field measurement of this enhanced turbulence is the dissipation of turbulent kinetic energy (TKE) ɛ at a depth y below the ocean surface, which scales as ε∝y−a . However, there is significant uncertainty as to the value of this power law exponent a in different field measurements. Here we consider the dissipation of TKE in the OSL by adding the wave breaking influence into the theoretical framework of the spectral link, originally proposed in sheared flows with no waves (Gioia et al 2010 Phys. Rev. Lett. 105 184501). The expanded theory recovers the power law of ɛ and sheds light on the uncertainty of the power law exponent a by relating it to the variation of the turbulent Prandtl number. In addition, we also apply this theory to oscillating-grid turbulence.

The sedimentation of particles with various shapes and orientations in a closed channel using smoothed particle hydrodynamics is investigated in this paper. The continuity and momentum equations of both fluid and solid are discretized using kernel approximation in the Lagrangian frame. The sedimentation behavior of different general shapes, including circle, pentagon, square, ellipse, rectangle, and triangle, at various initial orientations in the suspending fluid is simulated. The stable equilibrium orientation (SEO) of these shapes is examined, excluding the circle which serves as a validation case. Specifically, the major axis of the ellipse and rectangle tends to align horizontally, whereas the orientations of the pentagon and square seem to be random due to the lack of a major axis and the finite channel height. The settling behavior of the three types of triangles is also discussed, and the von Mises stress of these shapes during their settling is presented. This study offers valuable insights into fluid-particle interactions, specifically regarding the SEO and internal stress of settling particles with varying shapes and orientations.