Physics

In the present study, we propose a new surrogate model, called common kernel-smoothed proper orthogonal decomposition (CKSPOD), to efficiently emulate the spatiotemporal evolution of fluid flow dynamics. The proposed surrogate model integrates and extends recent developments in Gaussian process learning, high-fidelity simulations, projection-based model reduction, uncertainty quantification, and experimental design, rendering a systematic, multidisciplinary framework. The novelty of the CKSPOD emulation lies in the construction of a common Gram matrix, which results from the Hadamard product of Gram matrices of all observed design settings. The Gram matrix is a spatially averaged temporal correlation matrix and contains the temporal dynamics of the corresponding sampling point. The common Gram matrix synthesizes the temporal dynamics by transferring POD modes into spatial functions at each observed design setting, which remedies the phase-difference issue encountered in the kernel-smoothed POD (KSPOD) emulation, a recent fluid flow emulator proposed in Chang et al. (2020). The CKSPOD methodology is demonstrated through a model study of flow dynamics of swirl injectors with three design parameters. A total of 30 training design settings and 8 validation design settings are included. Both qualitative and quantitative results show that the CKSPOD emulation outperforms the KSPOD emulation for all validation cases, and is capable of capturing small-scale wave structures on the liquid-film surface faithfully. The turbulent kinetic energy prediction using CKSPOD reveals lower predictive uncertainty than KSPOD, thereby allowing for more accurate and precise flow predictions. The turnaround time of the CKSPOD emulation is about 5 orders of magnitude faster than the corresponding high-fidelity simulation, which enables an efficient and scalable framework for design exploration and optimization.

Compressibility effects in a turbulent transport of temperature field are investigated applying the quasi-linear approach for small Péclet numbers and the spectral τ approach for large Péclet numbers. Compressibility of a fluid flow reduces the turbulent diffusivity of the mean temperature field similarly to that for particle number density and magnetic field. However, expressions for the turbulent diffusion coefficient for the mean temperature field in a compressible turbulence are different from those for the mean particle number density and the mean magnetic field. Combined effect of compressibility and inhomogeneity of turbulence causes an increase of the mean temperature in the regions with more intense velocity fluctuations due to a turbulent pumping. Formally, this effect is similar to a phenomenon of compressible turbophoresis found previously [J. Plasma Phys. {\bf 84}, 735840502 (2018)] for non-inertial particles or gaseous admixtures. Gradient of the mean fluid pressure results in an additional turbulent pumping of the mean temperature field. The latter effect is similar to turbulent barodiffusion of particles and gaseous admixtures. Compressibility of a fluid flow also causes a turbulent cooling of the surrounding fluid due to an additional sink term in the equation for the mean temperature field. There is no analog of this effect for particles.

Microfluidics technology offers high efficiency of heat and mass transfer and low safety hazards compared to conventional multiphase processes. The multiphase flow in the microchannels is usually characterized as Taylor flow that includes elongated microbubbles , separated by liquid slugs. In the current study, we employ OpenFOAM CFD package to investigate the effect of the surface tension and the contact angle on the interfacial surface area and transport yield in microscale systems. The results show that contact angle can significantly affect the surface area and the bubble size while the surface tension does not change these parameters in the system. Moreover, in lower contact angle, the flow may turn into bubbly flow, affecting K L , mass transfer coefficient in the liquid phase. Findings of the current study can improve the mass transfer coefficient in microreactors while avoiding thermal runaways, hot spots and other safety issues of conventional reactors.

Direct numerical simulations are performed to contrast turbulent boundary layers over a concave wall without and with free-stream turbulence. Adverse pressure gradient near the onset of curvature leads to sharp decrease in skin friction and intermittent separation. The presence of free-stream turbulence reduces the probability of reverse flow, accelerates the recovery of the boundary layer in the downstream zero-pressure gradient region, and leads to a sustained and appreciable increase in the skin friction. The forcing also promotes the amplification of coherent Görtler structures in the logarithmic layer of the curved-wall boundary layer. Statistically, the spanwise and wall-normal Reynolds stresses intensify and the radial distance between their peaks increases downstream as the Görtler structures expand. The Reynolds shear stress coefficient also increases in the logarithmic layer in contrast to a decrease when a flat-plate boundary layer is exposed to free-stream turbulence. In addition, the more coherent and energetic roll motions in the forced flow promote mixing of free-stream and boundary-layer fluids, where the former is seen more often deep within the buffer layer.

Buoyancy-driven convection is likely the dominant driver of turbulent motions in the universe, and thus, is widely studied by physicists, engineers, geophysicists and astrophysicists. Maybe unsurprisingly, these different communities discuss the gross convective behaviors in different ways, often without significant cross-talk existing between them. Here, we seek to draw connections between these communities. We do so by carrying out a set of basic scale estimations for how heat and fluid momentum transport should behave in non-rotating, slowly rotating and rapidly rotating buoyancy-driven convective environments. We find that slowly and rapidly rotating scalings can be inter-related via one parameter, the so-called convective Rossby number $\RoC$, a dissipation-free parameter measuring the importance of buoyancy driving relative to rotation. Further, we map between non-flux-based and the flux-based, buoyancy-driven scalings used by different groups. In doing so, these scalings show that there are clean connections between the different communities' approaches and that a number of the seemingly different scalings are actually synonymous with one another.

In low Mach number aeroacoustics, the well known disparity of scales makes it possible to apply efficient hybrid simulation models using different meshes for flow and acoustics, which leads to a powerful computational procedure. Our study applies the hybrid workflow to the computationally efficient perturbed convective wave equation with only one scalar unknown, the acoustic velocity potential. The workflow of this aeroacoustic approach is based on three steps: 1. perform unsteady incompressible flow computations on a sub-domain; 2. compute the acoustic sources; 3. simulate the acoustic field using a mesh specifically suited for computational aeroacoustics. In general, hybrid aeroacoustic methods seek for robust and conservative mesh-to-mesh transformation of the aeroacoustic sources while high computational efficiency is ensured. In this paper, we investigate the accuracy of a cell-centroid based conservative interpolation scheme compared to the more accurate cut-volume cell approach and their application to the computation of rotating systems, namely an axial fan. The capability and robustness of the cut-volume cell interpolation in a hybrid workflow on different meshes are investigated by a grid convergence study. The results of the acoustic simulation are in good agreement with measurements thus demonstrating the applicability of the conservative cut-volume cell interpolation to rotating systems.

Air curtains are commonly used as separation barriers to reduce exchange flows through an open-door of a building.Here, we investigated the effectiveness of an air curtain to prevent the transport of contaminants by a person walking along a corridor from a dirty zone into a clean zone. We conducted small-scale waterbath experiments with freshwater, brine and sugar solutions, with the brine as a passive tracer for the contaminant in the wake of person. A cylinder representing human walking was pulled between two fixed points in the channel across the air curtain. We observed that the air curtain can prevent up to 40% of the contaminant transport due to the wake of a moving person.We proposed a new way to evaluate the performance of an air curtain in terms of the deflection modulus and the effectiveness defined for this iso-density situation, similar to quantities typically used for the case where the fluid densities in the two zones are different. We observed that the air curtain has an optimal operating condition to achieve a maximum effectiveness. Dye visualisations and time-resolved particle image velocimetry of the air curtain and the cylinder wake were used to examine the re-establishment process of the planar jet after its disruption by the cylinder and we observed that some part of the wake is separated by the re-establishing curtain. We observed that the exchange flux peaks after the cylinder passes the air curtain and reduces to a typical value after the re-establishment of the curtain.

Air curtains are installed in open doorways of a building to reduce buoyancy-driven exchange flows across the doorway. Although an air curtain allows an unhampered passage of humans and vehicles, the interaction of this traffic with an air curtain is not well understood. We study this problem by conducting small-scale waterbath experiments with fresh water and salt water solutions. As a model of human passage, a vertical cylinder is pulled through a planar jet representing an air curtain and separating two zones at different densities. For a fixed travel distance of the cylinder before and after the air curtain, the average infiltration flux of dense fluid in light fluid side increases with increasing cylinder velocity. However, we find that the infiltration flux is independent of density difference across the doorway and the travel direction of the cylinder. As a consequence, the sealing effectiveness of an air curtain reduces with an increasing cylinder speed and this reduction is independent of the direction of the buoyancy-driven flow. Dye visualisations of the air curtain and the cylinder wake are used to examine the re-establishment process of the air curtain after its disruption by the cylinder. We observe that the re-establishment time of the air curtain and the infiltration in the cylinder wake increases with an increasing cylinder speed.

In this paper, the effectiveness of electromagnetic forces on controlling the motion of a sedimenting elliptical particle is investigated using the immersed interface-lattice Boltzmann method (II-LBM), in which a signed distance function is adopted to apply the jump conditions for the II-LBM and to add external electromagnetic forces. First, mechanisms of electromagnetic control on suppressing vorticity generation based on the vorticity equation and vortex shedding based on the streamwise momentum equation are discussed. Then, systematical investigations are performed to quantify and qualify the effects of the electromagnetic control by changing the electromagnetic strength, the initial orientation angle of the elliptical particle, and the density ratio of the particle to the fluid. To demonstrate the control effect of different cases, comparisons of vorticity fields, particle trajectories, orientation angles, and energy transfers of the particles are presented. Results show that the rotational motion of the particle can be well controlled by appropriate magnitudes of electromagnetic forces. In a relatively high solid to fluid density ratio case where vortex shedding appears, the sedimentation speed can increase nearly 40\% and the motion of the particle turns into a steady descent motion once an appropriate magnitude of the electromagnetic force is applied. When the magnitude of the electromagnetic force is excessive, the particle will deviate from the center of the side walls. In addition, the controlling approach is shown to be robust for various initial orientation angles and solid to fluid density ratios.

We investigate the evaporation of a two-dimensional droplet on a solid surface. The solid is flat but with smooth chemical variations that lead to a space-dependent local contact angle. We perform a detailed bifurcation analysis of the equilibrium properties of the droplet as its size is changed, observing the emergence of a hierarchy of bifurcations that strongly depends on the particular underlying chemical pattern. Symmetric and periodic patterns lead to a sequence of pitchfork and saddle-node bifurcations that make stable solutions to become saddle nodes. Under dynamic conditions, this change instability suggests that any perturbation in the system can make the droplet to shift laterally while relaxing to the nearest stable point, as is confirmed by numerical computations of the Cahn-Hilliard and Navier-Stokes system of equations. We also consider patterns with an amplitude gradient that creates a set of disconnected stable branches in the solution space, leading to a continuous change of the droplet's location upon evaporation.