The transformative influence of compressibility on the Kelvin-Helmholtz instability (KHI) at the interface between two fluid streams of different velocities is explicated. When the velocity difference is small (subsonic), shear effects dominate the interface flow dynamics causing monotonic roll-up of vorticity and mixing between the two streams leading to the KHI. We find that at supersonic speed differentials, compressibility forces the dominance of dilatational (acoustic) rather than shear dynamics at the interface. Within this dilatational interface layer, traveling pressure waves cause the velocity perturbations to become oscillatory. We demonstrate that the oscillatory fluid motion reverses vortex roll-up and segregates the two streams leading to KHI suppression. Analysis and illustrations of the compressibility-induced suppression mechanism are presented.

The sensitivity of fog dissipation to the environmental changes in radiation, liquid-water lapse rate, free tropospheric temperature and relative humidity was studied through numerical experiments designed based on the 2007-Paris Fog observations. In particular, we examine how much of the stratocumulus-thinning mechanism can be extended to the near-surface clouds or fog. When the free troposphere is warmed relative to the reference case, fog-top descends and become denser. Reducing the longwave radiative cooling via a more emissive free troposphere favors thickening the physical depth of fog, unlike cloud-thinning in a stratocumulus cloud. Drying the free troposphere allows fog thinning and promotes fog dissipation while sustaining the entrainment rate. The numerical simulation results suggest that the contribution of entrainment drying is more effective than the contribution of entrainment warming yielding the reduction in liquid water path tendency and promoting the onset of fog depletion relative to the reference case studied here. These sensitivity experiments indicate that the fog lifting mechanism can enhance the effect of the inward mixing at the fog top. However, to promote fog dissipation, an inward mixing mechanism only cannot facilitate removing humidity in the fog layer unless a sufficient entrainment rate is simultaneously sustained.Atmosphere 2020, 11, 12 2 of 22 stage. From the modeling standpoint, this challenge is attributed to a lack of knowledge of the physical processes and limitations in the ability to represent these processes in models. Forecasting numerical models predominantly use single-column schemes and also require parameterization of turbulent processes [15][16][17]. The role of turbulence on fog evolution remains controversial. Some argue that turbulence hampers the development of fog [18,19], while others deduce that turbulence favors fog development [20]. Poor predictive capabilities of most fog simulation schemes are attributed to not accurately capturing the local characteristics of turbulent mixing and surface-atmosphere coupling [10,21].The role of entrainment in fog life cycle has been mostly highlighted in marine fog through observational campaigns [22][23][24] and numerical simulations [25][26][27][28]. High entrainment drying is suggested to be not conducive to fog dissipation unless the radiative cooling increases [25]. While enhancement of TKE and Deardroff's convective velocity during the dissipation phase of an advection-radiation fog event has been observed [24], it was shown that due to low entrainment rate, the vertical turbulent fluxes are not strong enough to provide a convective source to prompt fog dissipation [29]. The contribution of the entrainment of drier and warmer air from aloft to fog dissipation has been identified as crucial to the role of the high subsidence rate and the shortwave radiative flux [30]. Some investigators attribute marine fog dissipation to thermal turbulence and dry air entrainment in the fog-top region [23,24,31], and others to the li...

Small perturbation evolution in compressible Poiseuille flow is contrasted against the incompressible case using direct simulations and non-modal linear analysis. The onset of compressibility effects leads to a profound change in the behaviour of pressure and its interaction with the velocity field. Linear analysis shows that the most significant compressibility outcome is the harmonic coupling between pressure and wall-normal velocity perturbations. Oscillations in normal perturbations can lead to periods of negative production causing suppression of perturbation growth. The extent of the influence of compressibility can be characterized in terms of an effective gradient Mach number ($M_{g}^{e}$). Analysis shows that $M_{g}^{e}$ diminishes as the angle of the perturbation increases with respect to the shear plane. Direct numerical simulations show that streamwise perturbations, which would lead to Tollmien–Schlichting instability in the incompressible case, are completely suppressed in the compressible case and experience the highest $M_{g}^{e}$. At the other extreme, computations reveal that spanwise perturbations, which experience negligible $M_{g}^{e}$, are entirely unaltered from the incompressible case. Perturbation behaviour at intermediate obliqueness angles is established. Moreover, the underlying pressure–velocity interactions are explicated.

We investigate the influence of orientation on the evolution of small perturbations in compressible shear layers with inflection points. By using linear analysis, we demonstrate that perturbations along the shear plane are most affected by compressibility. The influence of compressibility gradually diminishes with increasing obliqueness of the perturbations with respect to the shear plane. It is demonstrated that the effective gradient Mach number is an appropriate compressibility parameter. We establish that spanwise perturbations, orthogonal to the shear plane, are impervious to compressibility effects. Direct numerical simulations of compressible mixing layers subject to the perturbations at various obliqueness angles verify the analytical findings.

Altruistic behavior after stress exposure may have important health and psychological benefits, in addition to broader societal consequences. However, so far experimental research on altruism following acute stress has been limited to adult populations. The current study utilized an experimental design to investigate how altruistic donation behavior among children may be influenced by (a) exposure to an acute social stressor, the Trier Social Stress Test modified for use with children (TSST‐M), (b) individual differences in stress physiology, and (c) social support from a parent. The sample consisted of 180 children (54.9% male, 45.1% female; mean age = 9.92 years, SD = 0.56 years) randomly assigned to one of three conditions involving the TSST‐M: (a) prepare for the TSST‐M alone, (b) prepare for the TSST‐M with a parent, and (c) no‐stress control group. Results revealed that children made larger donations post‐stressor if they were alone before the acute stressor, if they had moderate cardiac autonomic balance, reflecting both parasympathetic and sympathetic influence, and if they were older. Children who prepared for the TSST‐M with social support from a parent made comparable donations as children in the no‐stress control group, in accord with stress buffering models. Increased altruism following acute stress among children suggests that a comprehensive understanding of the human stress response needs to incorporate “tend‐and‐befriend” behavior—the tendency for humans to show increased altruistic behavior during times of distress.