arXiv subjects

Thara Prabhakaran

Publications and source records attributed to Thara Prabhakaran.

3 recordsLinked to original sources

arXiv2025

Drop size distribution from laboratory experiments based on single-drop fragmentation and comparison with aerial in-situ measurements

Laboratory experiments and theoretical modelling are conducted to determine the raindrop size distribution (DSD) resulting from distinct fragmentation processes under various upward airstreams. Since weather radar echoes are proportional to the sixth power of the average droplet diameter, understanding the fragmentation mechanisms that lead to different breakup sizes is crucial for accurate rainfall predictions. We utilize a two-parameter gamma distribution for theoretical modelling and estimate the average droplet diameter from the theoretically obtained characteristic sizes, often treated as assumed input parameters for different rain conditions in rainfall modelling. Our experimental and theoretical findings demonstrate a close agreement with the DSD predicted by the Marshall and Palmer relationship for steady rain conditions. Additionally, in situ DSD measurements at different altitudes were obtained through research flights equipped with advanced sensors, further validating our rainfall model. This study underscores the effectiveness of laboratory-scale experiments and the critical importance of accurately characterizing DSD to enhance rainfall predictions.

physics.flu-dyn↗

arXiv2021

Scaling of mean skin friction in turbulent boundary layers, and fully-developed pipe and channel flows

An asymptotic $-1/2$ power-law scaling and a semi-empirical finite-$Re$ model were recently presented by Dixit et al. (2020) for skin friction in zero-pressure-gradient (ZPG) turbulent boundary layers (TBLs). In this work, a new derivation is presented which shows that these relations (i) fundamentally represent a dynamically-consistent scaling of skin friction for nominally two-dimensional ZPG TBLs and fully-developed pipes and channels, and (ii) apply individually to each of these flows. The new theoretical arguments are based on transfer of kinetic energy from mean flow to large eddies of turbulence and depend neither on flow geometry nor outer boundary condition, both of which distinguish one type of flow from the other. Using skin friction data from the literature, it is demonstrated that the finite-$Re$ model describes, as predicted by the theory, data from individual flows remarkably well; these data cover the complete range of laboratory/simulation Reynolds numbers to date. It is, however, observed that performance of the model degrades while attempting to describe data from all flows in a universal fashion. Differences in outer boundary condition and large-scale structures amongst different types of flows appear to be responsible for this degradation. An empirical correction based on Clauser's shape factor, is proposed to absorb the outer boundary condition effects into the scaling of skin friction. This correction leads to a new universal scaling and a robust, semi-empirical, universal finite-$Re$ model for skin friction in ZPG TBLs, pipes and channels. Remarkable collapse of data from all flows in the new scaling underscores the importance of a dynamically-consistent approach towards revealing universality of skin friction in wall turbulence.

physics.flu-dyn↗

arXiv2020

Mean velocity scaling in plane turbulent wall jets

Studies in the literature on plane turbulent wall jets on flat surfaces, have invariably considered either the nozzle initial conditions or the asymptotic conditions far downstream, as scaling parameters for the streamwise variations of length and velocity scales. These choices, however, do not square with the notion of self similarity which is essentially a "local" concept. We first demonstrate that the streamwise variations of velocity and length scales in wall jets show remarkable scaling with local parameters i.e. there appear to be no imposed length and velocity scales. Next, it is shown that the mean velocity profile data suggest existence of two distinct layers - the wall (inner) layer and the full-free jet (outer) layer. Each of these layers scales on the appropriate length and velocity scales and this scaling is observed to be universal i.e. independent of the local friction Reynolds number. Analysis shows that the overlap of these universal scalings leads to a Reynolds-number-dependent power-law velocity variation in the overlap layer. It is observed that the mean-velocity overlap layer corresponds well to the momentum-balance mesolayer and there appears to be no evidence for an inertial overlap; only the meso-overlap is observed. Introduction of an intermediate variable absorbs the Reynolds-number dependence of the length scale in the overlap layer and this leads to a universal power-law overlap profile for mean velocity in terms of the intermediate variable.

physics.flu-dyn↗