The offshore wind industry is rapidly expanding to meet clean and secure energy needs. New developments are now increasingly constrained to deeper waters, where the water column is seasonally stratified. Here flows past offshore wind infrastructure will increase water column mixing, although such processes and their extent are poorly understood. Studies have so far been limited to field-scale simulations, which make sweeping assumptions regarding flow-structure interactions and fine-scale stratified turbulence, and field observations, which are limited by the sparsity of measurement campaigns and data captured. To isolate and quantify the key processes governing water column mixing by infrastructure, we present fully structure-resolved direct numerical simulations of two-layer stratified flow past a vertical cylinder. We identify two wake regimes by systematically varying the flow Reynolds and Richardson numbers: (1) a weakly stratified regime, characterized by a narrow but highly energetic wake dominated by horizontal shear and (2) a strongly stratified wake, characterized by the emergence of a thermocline-spanning recirculation cell attached to the cylinder. Here strong vertical motions develop from the recirculation cell which are responsible for the formation of large-scale stationary internal waves. These waves account for up to 10% of the total energy budget and provide a mechanism for far-field energy propagation. The weakly stratified wake regime is characteristic of existing offshore wind sites where temperature gradients are relatively weak; the identified strongly stratified regime describes the dynamics to be expected in future deep water offshore wind sites. This difference between the two regimes offers an explanation for the previously enigmatic discrepancies in field observations regarding wake persistence and detectability. Future work must focus on narrowing the gap between idealized simulations and field-scale flows, for which the datasets herein will provide a critical benchmark for validation.
This study investigates scalar (smoke) dispersion in the turbulent near-wake of an Ahmed body. The effects of rear slant angles (Phi = 15(degrees), 25(degrees), 35(degrees), and 40(degrees)) on the flow structures, scalar concentration, and dispersion are investigated. Smoke was essentially a passive scalar, with a typical Stokes number of 3.62 & times; 10(-5). Smoke particle concentration fields were measured (at Re-l = 1.9 & times; 10(5)) using a nonintrusive technique, quantitative smoke visualization, based on laser Mie-scattering. Smoke dispersion was quantified using nondimensional dispersion (D) and dispersion length scales (L-y, L-z). The predominant turbulent wake structures were obtained by particle image velocimetry. The connection between wake structures and the dispersion phenomenon was explored. It is found that the existence, strength, and size of wake structures (spanwise and trailing vortices) depend on , and these structures play a key role in the dispersion phenomenon. Two distinct flow regimes and their implications on dispersion are explored. For Phi = 15(degrees), 25(degrees), there is no separation over the rear slant, and the trailing vortices are generated, which dominate and determine the dispersion scales beyond the recirculation region (x(& lowast;) > 1). The entrainment of flow due to trailing vortices increases the lateral dispersion (L-z) of smoke rapidly. For Phi = 35(degrees), 40(degrees), a massive separation on the rear slant is observed, and the trailing vortices do not exist. The spanwise vortices dominate the dispersion phenomenon in the recirculation region (x* <= 1) and determine the vertical dispersion (L-y) of the smoke. Based on the observed smoke dispersion for both the flow regimes, the characteristics of the near-wake are proposed.
Near-critical fluids are known to exhibit anomalous behavior in their thermophysical properties, such as diverging compressibility and vanishing surface tension on approaching the critical point. These fluids undergo high-density stratification very close to the critical point. The Navier-Stokes equations coupled to a phase field model are solved with a van der Waals equation of state. The pressure is expanded around the critical point and shows a cubic variation with the reduced density. Density-stratified base states are computed numerically. The supercritical case is treated through the linear stability analysis. The spectrum, the eigenmodes/eigenfunctions, and sound waves are then computed and plotted for temperatures close to the critical point (CP). Very close to CP, the speed of sound significantly changes as a function of the vertical direction, and the frequency bands strongly differ from the situation of an ideal gas. The same process of linearization has been performed for the subcritical state. Here, the Rayleigh-Taylor instability (RTI) is considered. Finally, numerical simulations confirm the propagation of acoustic waves in the supercritical case. In the subcritical case, spinodal decomposition is found for a randomly distributed initial density close to the critical one. Time series in the Rayleigh-Taylor unstable domain show interesting features. The time evolution of the total kinetic energy exhibits three distinct phases: high-frequency sound waves appear at early times, then RTI manifests, and finally, at longer times, surface waves appear with a much smaller frequency.
