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Jeevanjee, Nadir

Publications and source records attributed to Jeevanjee, Nadir.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

What Controls the Entrainment Rate of Dry Buoyant Thermals with Varying Initial Aspect Ratio?

Here, this study uses theory and numerical simulations to analyze the nondimensional spreading rate a (change in radius with height) of buoyant thermals as they rise and entrain surrounding environmental fluid. A focus is on how a varies with initial thermal aspect ratio A r , defined as height divided by width of the initial buoyancy perturbation. An analytic equation for thermal ascent rate w t that depends on a is derived from the thermal-volume-averaged momentum bud get equation. The thermal top height when w t is maximum, defining a critical height z c , is inversely proportional to a. The height z c also corresponds to the thermal top height when buoyant fluid along the thermal’s vertical axis is fully replaced by entrained nonbuoyant environmental fluid rising from below the thermal. The time scale for this process is controlled by the vertical velocity of parcels rising upward through the thermal’s core. This parcel vertical velocity is approximated from Hill’s analytic spherical vortex, yielding an analytic inverse relation between a and A r . Physically, this a–A r relation is connected to changes in circulation as A r is modified. Numerical simulations of thermals with A r varied from 0.5 to 2 give a values close to the analytic theoretical relation, with a factor of ~3 decrease in a as A r is increased from 0.5 to 2. The theory also explains why a of initially spherical thermals from past laboratory and modeling studies is about 0.15. Overall, this study provides a theoretical underpinning for understanding the entrainment behavior of thermals, relevant to buoyantly driven atmospheric flows.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Dynamic Pressure Drag on Rising Buoyant Thermals in a Neutrally Stable Environment

Abstract This study examines dynamic pressure drag on rising dry buoyant thermals. A theoretical expression for drag coefficient C d as a function of several other nondimensional parameters governing thermal dynamics is derived based on combining the thermal momentum budget with the similarity theory of Scorer. Using values for these nondimensional parameters from previous studies, the theory suggests drag on thermals is small relative to that on solid spheres in laminar or turbulent flow. Two sets of numerical simulations of thermals in an unstratified, neutrally stable environment using an LES configuration of the Cloud Model 1 (CM1) are analyzed. One set has a relatively low effective Reynolds number R e and the other has an order-of-magnitude-higher R e ; these produce laminar and turbulent resolved-scale flows, respectively. Consistent with the theoretical C d , the magnitude of drag is small in all simulations. However, whereas the laminar thermals have C d ≈ 0.01, the turbulent thermals have weakly negative drag ( C d ≈ −0.1). This difference is explained by the laminar thermals having near vertical symmetry but the turbulent thermals exhibiting considerable vertical asymmetry of their azimuthally averaged flows. In the laminar thermals, buoyancy rapidly becomes concentrated around the main centers of rotation located along the horizontal central axis, leading to expansion of thermals via baroclinic vorticity generation but doing little to break vertical symmetry of the flow. Vertical asymmetry of the azimuthally averaged flow of turbulent thermals is attributed mainly to small-scale resolved eddies that are concentrated in the upper part of the thermals.

Meteorology & Atmospheric Sciences↗
DOE OSTI2019

FAT or FiTT: Are Anvil Clouds or the Tropopause Temperature Invariant?

The Fixed Anvil Temperature (FAT) hypothesis proposes that upper tropospheric cloud fraction peaks at a special isotherm that is independent of surface temperature. It has been argued that a FAT should result from simple ingredients: Clausius-Clapeyron, longwave emission from water vapor, and tropospheric energy and mass balance. Here the first cloud-resolving simulations of radiative-convective equilibrium designed to contain only these basic ingredients are presented. This setup does not produce a FAT: the anvil temperature varies by about 40% of the surface temperature range. However, the tropopause temperature varies by only 4% of the surface temperature range, which supports the existence of a Fixed Tropopause Temperature (FiTT). Lastly, in full-complexity radiative-convective equilibrium simulations, the spread in anvil temperature is smaller by about a factor of 2, but the tropopause temperature remains more invariant than the anvil temperature by an order of magnitude. In other words, our simulations have a FiTT, not a FAT.

54 ENVIRONMENTAL SCIENCES↗