Direct Numerical Simulation of Turbulence Structure at a Wavy-Sheared Air-water Interface

Turbulence structure and heat and mass transfer mechanism across a wavy sheared gasliquid interface are fluid-mechanically investigated in a wind-wave tunnel. Heat and mass transfer velocities are reported and the relationship between the scalar transfer velocities and the turbulence structure is discussed. In addition, three-dimensional direct numerical simulation is carried out to investigate the flow structure over a rigid-wavy wall similar to that over the wave. The results show that the organized motion in the air flow intermittently appears on the front side of the wave crest, and its structure is rather similar to the flow structure over the rigid-wavy wall. The organized motion in the air flow induces the organized motion in the water flow and the organized motion renews the air-water interface. The scalar transfer across a wavy sheared gas-liquid interface is controlled by the organized surface-renewal motion in the water flow

DNS of scalar transfer across an air-water interface during inception and growth of Langmuir circulation

Analysis of wavy surface effects on the characteristics of wireless optical communication downlinks

We investigate oxygen transfer across an air-water interface using an integrated Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF) system. This setup allows for the simultaneous measurement of velocity and dissolved oxygen (DO) concentration fields, alongside the visualization of the air-water interface. The imaging process begins with zero DO concentration and continues until oxygen saturation in the water is achieved. Calibration of the oxygen LIF intensity field is performed using these images, benchmarked against measurements from an optical oxygen point probe, to ensure accurate conversion to a physically relevant unit. The air channel generates an air flow with a center-line mean velocity of 8.84 m/s, corresponding to a Taylor Reynolds number of 100. This high-velocity air flow results in the formation of wavy structures on the water interface creating situations similar to those found in nature (such as rivers and oceans). The wavy structures create reflections from the air-water interface; these reflections are mitigated during post-processing using a Wavelet-Fast Fourier Transform (W-FFT) method to remove high intensity noise. Additionally, the wind shear generated by the air flow induces high turbulence intensity and vortices near the air-water interface, significantly enhancing the dissolution of oxygen into the water. This turbulence and the associated vortical structures play a crucial role in the mixing process and the overall oxygen transfer efficiency. The study presents instantaneous dissolved oxygen structures overlapped with velocity structures in a flow with deformable interface, providing some insights into the oxygen transfer and mixing processes.

Analysis of flow field around a ribbed helix lip seal

Local oxygen transfer velocities measured in a linear wind-wave tunnel with respect to wind speed and fetch are presented in this thesis. For this, a non-intrusive laser-induced fluorescence (LIF) method was developed to measure vertical oxygen concentration profiles in the water-sided mass boundary layer. The fluorophore used is a water soluble ruthenium complex, which is quenched according to the Stern-Volmer equation. This equation, which originally describes the quenching only for a weak excitation, was generalized for arbitrary laser irradiance. Measurements confirm this generalization and yield a new value for the Stern-Volmer constant.
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\nThe LIF method was applied with high spatial and temporal resolution of 6.2 µm and 1.2 kHz, respectively, in order to resolve the mass boundary layer and fast processes. To obtain mean oxygen concentration profiles with high precision, an algorithm was developed to detect the water surface in the recorded images. The measured mean concentration profiles show a transition in the self-similar shape with the onset of waves. The results for a flat water surface are in agreement with the surface renewal model. For a wavy water surface, the small eddy model and the surface renewal model both describe the data equally well. Vanishing oxygen concentration fluctuations at the flat water surface were measured, which is in agreement with existing models for a rigid interface. The local transfer velocities obtained from mean concentration profiles are best parametrized with the friction velocity. In this work, the great potential of LIF measurements to probe transfer velocities locally is demonstrated.

Cooling of process streams is a standard operation in many industries, and where possible water cooling is the most cost effective solution. However in areas where water supply is limited Air Cooled Heat Exchangers (ACHE’s) are often the only alternative. The overall coefficient maybe limited by the air side heat transfer. To improve this coefficient the air flow should be evenly distributed and at as high velocity as possible over the whole bundle. The limits are set by the fan power consumption and the generated noise level. The required fan power forms a significant part of running costs. In order to keep these costs as low as possible knowledge concerning the flow distributions within the bundle can be important. Fluid bypass within the tube-side can be of similar importance. In design calculations an equal fluid flow velocity per tube is typically assumed from the total inlet mass flow. However if some of the fluid bypasses the tube bundle due to poor header design lower than expected fluid velocities in the tubes will be found, leading to underperforming ACHE’s, since heat transfer and fouling behaviour are influenced by the fluid velocity. Computational Fluid Dynamics (CFD) has been found to be a powerful tool to investigate how fluid flows through a defined geometry. In this paper CFD techniques will be used to investigate the tube side and air side flow of ACHE’s. The main focus will be on finding areas of fluid maldistribution within the air side and the effect of bypass on the tube-side of ACHE’s. As a benchmark the pressure drop results of CFD simulations were compared to correlations available in the public domain and agreed well, giving confidence in the fluid flow patterns produced by the CFD simulations. The effect of the tube-side bypass was found to reduce the duty of the ACHE by 27 % in the worst case scenario that was tested. Maldistribution of the air-side flow was found in the ACHE, with the number of fans and depth of the plenum influencing the amount seen.

Rapidly rotating convection-driven dynamos are investigated under different kinematic and magnetic boundary conditions using direct numerical simulations. At a fixed rotation rate, represented by the Ekman number$Ek=5\times 10^{-7}$, the thermal forcing is varied from$2$to$20$times its value at the onset of convection ($\mathcal {R}=Ra/Ra_c=2\unicode{x2013}20$, where$Ra$is the Rayleigh number), keeping the fluid properties constant ($Pr=Pr_m=1$, where$Pr$and$Pr_m$are the thermal and magnetic Prandtl numbers). The statistical behaviour of the dynamos, including the force balance, energetics, and heat transport, depends on the boundary conditions that dictate both the boundary layer and the interior dynamics. At a fixed thermal forcing ($\mathcal {R}=3$), the structure and strength of the magnetic field produced by the dynamos, especially near the walls, depend on both velocity and magnetic boundary conditions. Though the leading-order force balance in the bulk remains geostrophic, the Lorentz force becomes comparable to the Coriolis force inside the thermal boundary layer with no-slip, perfectly conducting conditions. In this case, a term signifying the work done by the Lorentz force in the turbulent kinetic energy (TKE) equation is found to have some components that extract energy from the velocity field to produce the magnetic field, while some other components extract energy from the magnetic field to produce TKE. However, with no-slip, pseudo-vacuum conditions, all the components of the work done by the Lorentz force perform unidirectional energy transfer to produce magnetic energy from the kinetic energy of the fluid to sustain dynamo action. We find heat transfer enhancement in the rotating dynamo convection, as compared with non-magnetic rotating convection, with the peak enhancement lying in the range$\mathcal {R}=3\unicode{x2013}4$. For free-slip conditions, in the absence of an Ekman layer, the dynamo action may alter the heat transport significantly by suppressing the formation of large-scale vortices. However, the highest heat transfer enhancement is found at$\mathcal {R}=3$with no-slip, perfectly conducting walls, which can be attributed to a local magnetorelaxation of the rotational constraint due to enhanced Lorentz force inside the thermal boundary layer.

Direct numerical simulation of free and forced square jets

SummaryDisplacement‐based seismic assessment of buildings containing unreinforced masonry (URM) walls requires as input, among others, estimates of the in‐plane drift capacity at the considered limit states. Current codes assess the drift capacity of URM walls by means of empirical models with most codes relating the drift capacity to the failure mode and wall slenderness. Comparisons with experimental results show that such relationships result in large scatter and usually do not provide satisfactory predictions. The objective of this paper is to determine trends in drift capacities of modern URM walls from 61 experimental tests and to investigate whether analytical models could lead to more reliable estimates of the displacement capacity than the currently used empirical models. A recently developed analytical model for the prediction of the ultimate drift capacity for both shear and flexure controlled URM walls is introduced and simplified into an equation that is suitable for code implementation. The approach follows the idea of plastic hinge models for reinforced concrete or steel structures. It explicitly considers the influence of crushing due to flexural or shear failure in URM walls and takes into account the effect of kinematic and static boundary conditions on the drift capacity. Finally, the performance of the analytical model is benchmarked against the test data and other empirical formulations. It shows that it yields significantly better estimates than empirical models in current codes. The paper concludes with an investigation of the sensitivity of the ultimate drift capacity to the wall geometry, static, and kinematic boundary conditions.

It is of great interest to investigate the effects of rain on mass transfer across the wavey-sheared air-water interface in precisely estimating the mass transfer rate across the air-sea interface. The mass transfer rate across the air-water interface changes depending on the conditions of the free surface and water flow near the free surface. The purpose of this study is, therefore, to investigate the effects of rain on wind-driven turbulence through laboratory experiments in a wind-wave tank with a rain simulator. The values of the wave height and the velocity in the liquid side were measured for both no rain case and rain case (R=133mm/h), and we compared the values in no rain case with that in the rain case. Figure 1 shows the significant wave height (H s ) against the free stream wind speed (U ~). It is found that Hs decreases due to rain at low wind speeds, because of the damping effect noted by Tsimplis and Thorpe[1]. However there is no rain effect on Hs at high wind speeds. Figure 2 shows the vertical distributions of the streamwize and vertical mean velocities (U w , V w ), the streamwize and vertical turbulence intensities (\( u_w^\prime ,v_w^\prime \)) and the Reynolds stress (\( \overline {u_w v_w } \)) in the liquid side. These values are increased by rain near the free surface. This suggests that the turbulence near the free surface in the liquid side is promoted by the impingement of raindrops.

CFD modeling of MPS coal mill with moisture evaporation

When small particles like, flour, pollen, etc., contact an air-liquid interface, they disperse rapidly in the lateral direction. Using direct numerical simulation (DNS) we showed that the rapid dispersion is due to the fact that the capillary force pulls particles into the interface causing them to accelerate to a large velocity. The vertical motion of a particle during its adsorption causes a radially-outward lateral flow on the interface that causes nearby particles to move away. The goal of this study is to experimentally analyze the reasons for the rapid dispersion of particles when they are simultaneously adsorbed at a two-fluid interface. Specifically, we will analyze the effect of particle size on the oscillatory behavior of a single particle, as it is being trapped at an air-water interface. The diameter of a particle will be varied between 300–850 μm. Our experimental setup consists of a high speed camera with a resolution of 512×512 pixel and the recording speed up to 3000 frames per second which is connected to a 12X microscope. The camera outputs are analyzed to determine the frequency and amplitude of oscillation during adsorption. The measured amplitudes and frequencies for the micro glass spheres used in the experiment were found to be in qualitative match with the DNS results.

The primary breakup under high-altitude relight conditions is investigated in this study where ambient pressure is as low as 0.4 bar and air, fuel and engine parts are as cold as 265 K. The primary breakup is crucial for the fuel atomization. As of today, the phenomena dictating the primary breakup are not fully understood. Direct Numerical Simulations (DNS) of liquid breakup under realistic conditions and geometries are hardly possible. The embedded DNS (eDNS) approach represents a reliable numerical tool to fill this gap. The concept consists of three steps: a geometry simplification, the generation of realistic boundary conditions for the DNS and the DNS of the breakup region. The realistic annular airblast atomizer geometry is simplified to a Y-shaped channel representing a planar geometry. Inside this domain the eDNS is located. The eDNS domain requires the generation of boundary conditions. A Large Eddy Simulation (LES) of the entire Y-shaped channel and a Reynolds-Averaged Navier-Stokes Simulation (RANS) of the liquid wall film are performed prior to the DNS. All parameters are stored transiently on all virtual DNS planes. These variables are then mapped to the DNS. Thus, high-quality boundary conditions are generated. The Volume-of-Fluid (VOF) method is used to solve for the two-phase flow. The results provide a qualitative insight into the primary breakup under realistic high-altitude relight conditions. Instantaneous snapshots in time illustrate the behavior of the liquid wall film along the prefilmer lip and illustrate the breakup process. It is seen that a slight variation of the surface tension force has a strong impact on the appearance of the primary breakup. Case 1 with the surface tension corresponding to kerosene at 293 K indicates large flow structures that are separated from the liquid sheet. By lowering the surface tension related to kerosene at 363 K, the breakup is dominated by numerous small structures and droplets. This study proves the applicability of the eDNS concept for investigating breakup processes as the transient nature of the phase interface behavior can be captured. At this time, the authors only present a qualitative insight which can be explained by the lack of quantitative data. The approach offers the potential of simulating realistic annular highly-swirled airblast atomizer geometries under realistic conditions.

Miles (1997) recently presented a short review of the progress in theoretical modelling of wave generation by wind during the period since his own pioneering work and that of Benjamin in the late fifties. With minor analytical elaborations and the major advances in computational capabilities, the original Miles-Benjamin model is still widely applied in linear instability analysis of this important practical phenomenon with very useful results. It appears likely, however, that further significant advances in predictive modelling will require improved understanding of the highly complex physics of the interaction process between the turbulent wind flow at the interface and the deforming water surface. At present, this knowledge can only come from direct numerical simulation and advanced physical experimentation, paralleling similar studies of solid wall layer turbulence. On the other hand, a laminar flow is a simple and fixed model. The classic experiment of Schubauer and Skramstad and the later numerical calculation of Kaplan have proved that, on a solid plate, the initial growth rate of some selected frequencies of artificial two-dimensional air waves inside the laminar boundary layer can be explained by linear instability theory. It is, therefore, interesting to know, for the coupled flow at an air-water interface, if the physical processes of the initial formation of water waves under the action of laminar air flow will be dominated by linear instability theory. Following this reasoning, the present wind tunnel study has been designed to provide information on the basic mechanics and physics of initial formation and growth of waves on a flat water surface under the action of laminar air flow. Primary objectives of the study therefore include measurement of the wave growth rate and phase velocity as a function of wave frequency and space under the fundamentally driving laminar flow conditions. A numerical calculation to solve the coupled Orr-Sommerfeld equations in the space-amplified growth rate was also developed for comparison with observed results.