Coherent vorticity dynamics and dissipation in a utility-scale wind turbine wake with uniform inflow

The vorticity dynamics and its relationship to dissipation in the wake of a utility-scale wind turbine are investigated through large-eddy simulation.The vorticity dynamics is assessed through the enstrophy,which is related to the turbulent dissipation.The averaged enstrophy and turbulent dissipation are shown to be quantitatively similar in the wake.Using temporal phase averaging,the vorticity fluctuations are decomposed into coherent and random fluctuations with respect to the frequency of the tip vortices.The enstrophy in the tip vortices is dominated by coherent fluctuations,while the coherent fluctuations of root vortices are immediately saturated by the random vorticity fluctuations of the unstable hub vortex.The coherent strain rate has significant differences com pared to the coherent enstrophy within one diameter downwind of blade tip,but the random enstrophy and strain rate are relatively similar.Differences in coherent enstrophy and strain rate decrease further from the rotor.

Interplay of surface geometry and vorticity dynamics in incompressible flows on curved surfaces

Incompressible viscous flows on curved surfaces are considered with respect to the interplay of surface geometry, curvature, and vorticity dynamics. Free flows and cylindrical wakes over a Gaussian bump are numerically solved using a surface vorticity- stream function formulation. Numerical simulations show that the Gaussian curvature can generate vorticity, and non-uniformity of the Gaussian curvature is the main cause. In the cylindrical wake, the bump dominated by the positive Gaussian curvature can significantly affect the vortex street by forming velocity depression and changing vorticity transport. The results may provide possibilities for manipulating surface flows through local change in the surface geometry.

A STUDY OF OSCILLATING FLOWS OVER PROPAGATING RIPPLES:PART Ⅱ. VORTICITY DYNAMICS

Vortex shedding for an oscillating flow over ripples is numerically investigated by solving 2 D time dependent incompressible Navier Stokes equations in curvilinear coordinates. The dynamics of vortex structures generated by flow separation is studied in order to understand some phenomena which take place near the bottom and in particular to get insight into the mechanism through which sediment grains are lifted up and maintained in suspension. Based on our calculated results, some important phenomena, such as vortex shedding, vortices pairing, vortices coalescence, vortex interaction with the free shear layer and so on, can be found. The behaviors of energy dissipation in the flow field and of shear stress along the ripple surface are also discussed for studying the dynamics of sediment grains in suspension and to obtain new information on the suspended load over the ripples.

Extremely fast vortex dynamics in Bi_(2)Sr_(2)Ca_(2)Cu_(3)O_(10+δ) crystalline nanostrip

The maximum velocity of a mobile vortex in movement is generally limited by the phenomenon of flux-flow instability(FFI),which necessitates weak vortex pinning and fast heat removal from non-equilibrium electrons.We here demonstrate exfoliations and nano-fabrications of Bi_(2)Sr_(2)Ca_(2)Cu_(3)O_(10+δ) crystalline nanostrips,which possess a rather weak pinning volume of vortices,relatively low resistivity,and large normal electron diffusion coefficient.The deduced vortex velocity in Bi_(2)Sr_(2)Ca_(2)Cu_(3)O_(10+δ) crystalline nanostrips can be up to 300 km/s near the superconducting transition temperature,well above the speed of sound.The observed vortex velocity is an order of magnitude faster than that of conventional superconducting systems,representing a perfect platform for exploration of ultra-fast vortex matter and a good candidate for fabrications of superconducting nanowire single photon detectors or superconducting THz modulator.

Mesoscale Dynamics and Its Application in Torrential Rainfall Systems in China

Progress over the past decade in understanding moisture-driven dynamics and torrential rain storms in China is reviewed in this paper. First, advances in incorporating moisture effects more realistically into theory are described, including the development of a new parameter, generalized moist potential vorticity（GMPV） and an improved moist ageostrophic Q vector（Qum）. Advances in vorticity dynamics are also described, including the adoption of a ＂parcel dynamic＂ approach to investigate the development of the vertical vorticity of an air parcel; a novel theory of slantwise vorticity development, proposed because vorticity develops easily near steep isentropic surfaces; and the development of the convective vorticity vector（CVV）as an effective new tool. The significant progress in both frontal dynamics and wave dynamics is also summarized, including the geostrophic adjustment of initial unbalanced flow and the dual role of boundary layer friction in frontogenesis, as well as the interaction between topography and fronts, which indicate that topographic perturbations alter both frontogenesis and frontal structure. For atmospheric vortices, mixed wave/vortex dynamics has been extended to explain the propagation of spiral rainbands and the development of dynamical instability in tropical cyclones. Finally, we review wave and basic flow interaction in torrential rainfall, for which it was necessary to extend existing theory from large-scale flows to mesoscale fields, enriching our knowledge of mesoscale atmospheric dynamics.

Generalized boundary dilatation flux on a flexible wall

In this paper,by applying theoretical method to the governing equations of compressible viscous flow,we derive the theoretical formula of the boundary dilatation flux(BDF)on a flexible wall,which generalizes the most recent work of Mao et al.(Acta Mechanica Sinica 38(2022)321583)for a stationary wall.Different boundary sources of dilatation are explicitly identified,revealing not only the boundary generation mechanisms of vortex sound and entropy sound,but also some additional sources due to the surface vorticity,surface angular velocity,surface acceleration and surface curvature.In particular,the generation mechanism of dilatation at boundary due to the coupled divergence terms is highlighted,namely,the product of the surface velocity divergence(▽_(■B)·U)and the vorticity-induced skin friction divergence(V_(■B)·τ_(ω)).The former is attributed to the surface flexibility while the latter characterizes the footprints of near-wall coherent structures.Therefore,by properly designing the surface velocity distribution,the dilatation generation at the boundary could be controlled for practical purpose in near-wall compressible viscous flows.

Causal mechanism behind the stall delay by airfoil's pitching-up motion

Why the stall of an airfoil can be significantly delayed by its pitching-up motion？ Various attempts have been proposed to answer this question over the past half century, but none is satisfactory. In this letter we prove that a chain of vorticity-dynamics processes at accelerating boundary is fully responsible for the causal mechanism underlying this peculiar phenomenon. The local flow behavior is explained by a simple potential-flow model.

A numerical study of fluid injection and mixing under near-critical conditions

Nitrogen injection under conditions close vicinity of the liquid-gas critical point is studied numerically. The fluid thermodynamic and transport properties vary drasti- cally and exhibit anomalies in the near-critical regime. These anomalies can cause distinctive effects on heat-transfer and fluid-flow characteristics. To focus on the influence of ther- modynamics on the flow field, a relatively low injection Reynolds number of 1 750 is adopted. For comparisons, a reference case with the same configuration and Reynolds number is simulated in the ideal gas regime. The model accommodates full conservation laws, real-fluid thermody- namic and transport phenomena. Results reveal that the flow features of the near-critical fluid jet are significantly differ- ent from their counterpart. The near-critical fluid jet spreads faster and mixes more efficiently with the ambient fluid along with a more rapidly development of the vortex pairing pro- cess. Detailed analysis at different streamwise locations in- cluding both the flat shear-layer region and fully developed vortex region reveals the important effect of volume dilata- tion and baroclinic torque in the near-critical fluid case. The former disturbs the shear layer and makes it more unstable. The volume dilatation and baroclinic effects strengthen the vorticity and stimulate the vortex rolling up and pairing pro- cess

On the refracted patterns produced by liquid vortices

A theoretical analysis of the refracted shadows produced by steady and time-decaying liquid vortices under uniform illumination from above is given in this article. An expression for the induced shadow intensity is derived and found to be a function of the vortex＇s free surface profile, i.e., function of the static pressure distribution. The patterns for different focusing depth are given and compared with previous visualization results from the literature. The phenomenon is examined and illustrated as a bench mark case by using both steady and time-decaying algebraic vortex models. However, this study can be extended to check the feasibility of recovering the main flow properties by analyzing the luminous image intensity of the refracted patterns. The present analysis is valid only when the swirl velocity is order of magnitude higher than the meridional flow components and the vorticity is concentrated within the core region and of intense conditions.