Theoretical and Computational Fluid Dynamics最新文献

Natural convection heat transfer from a porous cylinder put at various positions in a square, cooled enclosure, with air as the working fluid, is investigated in this work. The following setups are taken into account: The hot cylinder is placed in the middle of the enclosure, near the bottom, top, right sides, along diagonal as top-diagonal and bottom-diagonal. The cylinder and the enclosure walls are kept hot and cold, respectively. The lattice Boltzmann method is used to perform a numerical analysis for Rayleigh number (10^{4}le ) Ra (le 10^{6}) and Darcy number (10^{-6}le ) Da (le 10^{-2}). The results are plotted as streamlines, isotherms, and local and mean Nusselt number values. The amount of heat transported from the heated porous cylinder is determined by varying Ra, Da, and the cylinder location. Even at a lower Rayleigh number ((10^{4})), the average Nusselt number grows by nearly 70 % as the cylinder moves from the centre to the bottom and 105% as it moves to bottom-diagonal location when ({Da}=10^{-2}). At Ra (=10^{6}) and Da (=10^{-2}), the heat transfer rate of the cylinder located near the corner of the enclosure at the bottom wall increases by approximately 33% when compared to the case of the cylinder in the centre. Convective effects are more noticeable when the cylinder is positioned towards the enclosure’s bottom wall. This research is applicable to electronic cooling applications in which a collection of electronic components is arranged in a circular pattern inside a cabinet.

A novel surface tracking, and advection algorithm for incompressible fluid flows in two and three dimensions is presented. This method based on the volume-of-fluid (VOF) method, is named VOF-with-center-of-mass-and-Lagrangian-particles (VCLP), and it uses spatially and temporally localized Lagrangian particles (LPs) inside a finite volume framework. The fluid surface is recaptured and reconstructed piecewise using the mean slope and curvature. The fluid mass inside each cell is discretized spatially by LPs and distributed as blue noise. LPs are then advected cell by cell with a choice of two different advection schemes in time using interpolated velocity and approximated acceleration fields. VCLP continuously tracks the center of mass of the fluid parcels in the Lagrangian way and this helps to reduce the errors due to numerical acceleration that results from lack of information to reconstruct the interface accurately. VCLP’s performance is evaluated using standard benchmark tests in 2D and 3D such as translation, single vortex, deformation, and Zalesak’s tests from the literature. VCLP is applied to TSUNAMI2D, a 2D Navier–Stokes model to simulate shoaling and breaking of waves.

Avoiding aliasing in time-resolved flow data obtained through high-fidelity simulations while keeping the computational and storage costs at acceptable levels is often a challenge. Well-established solutions such as increasing the sampling rate or low-pass filtering to reduce aliasing can be prohibitively expensive for large datasets. This paper provides a set of alternative strategies for identifying and mitigating aliasing that are applicable even to large datasets. We show how time-derivative data, which can be obtained directly from the governing equations, can be used to detect aliasing and to turn the ill-posed problem of removing aliasing from data into a well-posed problem, yielding a prediction of the true spectrum. Similarly, we show how spatial filtering can be used to remove aliasing for convective systems. We also propose strategies to prevent aliasing when generating a database, including a method tailored for computing nonlinear forcing terms that arise within the resolvent framework. These methods are demonstrated using a nonlinear Ginzburg–Landau model and large-eddy simulation data for a subsonic turbulent jet.

In the case of microscopic particles, the momentum exchange between the particle and the gas flow starts to deviate from the standard macroscopic particle case, i.e. the no-slip case, with slip flow occurring in the case of low to moderate particle Knudsen numbers. In order to derive new drag force models that are valid also in the slip flow regime for the case of non-spherical particles of arbitrary shapes using computational fluid dynamics, the no-slip conditions at the particle surface have to be modified in order to account for the velocity slip at the surface, mostly in the form of the Maxwell’s slip model. To allow a continuous transition in the boundary condition at the wall from the no-slip case to the slip cases for various Knudsen (Kn) number value flow regimes, a novel specific slip length model for the use with the Maxwell boundary conditions is proposed. The model is derived based on the data from the published experimental studies on spherical microparticle drag force correlations and Cunningham-based slip correction factors at standard conditions and uses a detailed CFD study on microparticle fluid dynamics to determine the correct values of the specific slip length at selected Kn number conditions. The obtained data on specific slip length are correlated using a polynomial function, resulting in the specific slip length model for the no-slip and slip flow regimes that can be applied to arbitrary convex particle shapes.

Direct numerical simulation and theoretical analysis of acoustic receptivity are performed for the boundary layer on a flat plate in Mach 6 flow at various angles of attack (AoA). Slow or fast acoustic wave passes through: a bow shock at AoA (=-5^{circ }), a weak shock induced by the viscous–inviscid interaction at AoA (=0^{circ }) or an expansion fan emanating from the plate leading edge at AoA (=5^{circ }). The study is focused on cases where the integral amplification of unstable mode S (or Mack second mode) is sufficiently large ((Napprox 8.4)) to be relevant to transition in low-disturbance environments. It is shown that excitation of dominant modes F and S occurs in a small vicinity of the plate leading edge. The initial disturbance propagates further downstream in accord with the two-mode approximation model accounting for the mean-flow nonparallel effects and the intermodal exchange mechanism. This computationally economical model can be useful for predictions of the second mode dominated transition onset using the physics-based amplitude method.

The use of multitaper estimates for spectral proper orthogonal decomposition (SPOD) is explored. Multitaper and multitaper-Welch estimators that use discrete prolate spheroidal sequences (DPSS) as orthogonal data windows are compared to the standard SPOD algorithm that exclusively relies on weighted overlapped segment averaging, or Welch’s method, to estimate the cross-spectral density matrix. Two sets of turbulent flow data, one experimental and the other numerical, are used to discuss the choice of resolution bandwidth and the bias-variance tradeoff. Multitaper-Welch estimators that combine both approaches by applying orthogonal tapers to overlapping segments allow for flexible control of resolution, variance, and bias. At additional computational cost but for the same data, multitaper-Welch estimators provide lower variance estimates at fixed frequency resolution or higher frequency resolution at similar variance compared to the standard algorithm.

The influence of turbulence inflow generation on direct numerical simulations (DNS) of high-speed turbulent boundary layers at Mach numbers of 2 and 5.84 is investigated. Two main classes of inflow conditions are considered, based on the recycling/rescaling (RR) and the digital filtering (DF) approach, along with suitably modified versions. A series of DNS using very long streamwise domains is first carried out to provide reliable data for the subsequent investigation. A set of diagnostic parameters is then selected to verify achievement of an equilibrium state, and correlation laws for those quantities are obtained based on benchmark cases. Simulations using shorter domains, with extent comparable with that used in the current literature, are then carried out and compared with the benchmark data. Significant deviations from equilibrium conditions are found, to a different extent for the various flow properties, and depending on the inflow turbulence seeding. We find that the RR method yields superior performance in the evaluation of the inner-scaled wall pressure fluctuations and the turbulent shear stress. DF methods instead yield quicker adjustment and better accuracy in the prediction of wall friction and of the streamwise Reynolds stress in supersonic cases. Unrealistically high values of the wall pressure variance are obtained by the baseline DF method, while the proposed DF alternatives recover a closer agreement with respect to the benchmark. The hypersonic test case highlights that similar distribution of wall friction and heat transfer are obtained by both RR and DF baseline methods.

Determining the behavior of the leading-edge suction force, represented non-dimensionally by the leading-edge suction parameter (LESP), can reliably help indicate the state of flow over the airfoil and therefore the force and moment characteristics. The current work aims at studying the variations in the LESP, forces, and pitching moment with freestream Reynolds number and airfoil thickness in unsteady flows. Computational data for the NACA 0012, 0015, and 0018 airfoils undergoing a baseline pitching motion over a range of freestream Reynolds number conditions are analyzed. The critical LESP, which is the instantaneous value of LESP at leading-edge vortex initiation, is observed to first decrease and subsequently increase with Reynolds number. This behavior can be correlated to the rate at which leading-edge flow curvature increases with Reynolds number. Thicker airfoils are observed to sustain a larger amount of suction force prior to breakdown and ensuing leading-edge vortex (LEV) shedding. Lift, drag, and moment are found to be dependent on thickness and Reynolds number prior to LEV shedding due to differences in the boundary layer characteristics, but independent after suction breakdown due to the similarity in LEV dynamics. These findings serve to support the development of a more generalized definition of a suction-force parameter that is independent of flow conditions and airfoil geometry.

Motivated by recent advances in the development of the numerical calculation of fine flow in liquid film, the thermocapillary convection in thin liquid film (1mm) due to temperature difference is studied in this paper. To describe the formation of the thermocapillary convection on gas-liquid interface, a two-phase system was designed, in which the momentum and energy interact directly through the free surface. The finite volume method is used to solve the N-S equation in gas phase and liquid phase, respectively, and the velocity and temperature information are exchanged on the free surface in each time step. The results show that a thermocapillary wave appears in the liquid film when the temperature difference exceeds a certain value. The temperature and velocity fluctuations on the free surface show a radiation shape. The flow field structure is completely symmetrical in the basic state, but it is axisymmetric in the case of oscillation state. The propagation direction of thermocapillary wave is affected by many factors (ambient temperature or inner wall rotation). The wave propagation direction is consistent with the rotation direction when the inner wall rotates. When the angular velocity of inner wall rotation is 8 rad/s, the wave number of thermocapillary wave will be reduced to 3, which is independent of the rotation direction.