arXiv subjects

Sourabh S. Diwan

Publications and source records attributed to Sourabh S. Diwan.

8 recordsLinked to original sources

arXiv2022

A wavelet-based detector function for characterizing intermittent velocity signals

In this work, we propose a new detector function based on wavelet transform to discriminate between turbulent and non-turbulent regions in an intermittent velocity signal. The derivative-based detector function, which is commonly used in intermittency calculation schemes, shows large fluctuations within turbulent parts of the signal and requires averaging over a certain ``smoothing period'' to remove the fake drop-outs, introducing subjectivity in calculating intermittency. The new detector function proposed here is obtained by averaging the ``pre-multiplied wavelet energy'' over the entire frequency range, resulting in a function that is much smoother than the derivative-based detector and at the same time has a good discriminatory property. This makes the choice of the smoothing period unnecessary and removes the subjectivity associated with it. We demonstrate the effectiveness of the wavelet detector within the framework of the widely-used method by Hedley and Keffer (1974, J. Fluid Mech., V64, pp625), for a range of velocity signals representing the different stages of roughness-induced transition. The wavelet detector function works well in detecting the edge intermittency of a canonical turbulent boundary layer as well, thereby highlighting its generality. Our detector can, in principle, be used with any method of specifying threshold for obtaining an indicator function.

physics.flu-dyn↗

arXiv2022

Absolute and "upstream" convective instabilities in plane Couette-Poiseuille flow

Here we report some interesting new features of the spatio-temporal instability of the incompressible plane Couette-Poiseuille flow (CPF). First of all, this flow represents the first instance of a "non-inflectional" absolute instability, within constant-viscosity formulation, which is triggered when one of the plates moves opposite to the bulk motion. More strikingly, with further increase in the negative plate motion, the absolute instability ($\textrm{AI}$) transitions to an "upstream" convective instability ($\textrm{CI}^-$), wherein an unstable wave packet moves opposite to the direction of the bulk flow. Thus, the CPF exhibits a unique $\textrm{CI}^+ \to \textrm{AI} \to \textrm{CI}^-$ transition, for a given Reynolds number ($Re$), where $\textrm{CI}^+$ denotes the commonly-observed case of a "downstream" convective instability. This type of transition has not been reported for other known examples of absolutely unstable flows. We compute the leading and trailing edge velocities for an amplifying wave packet and find that, for the plane Poiseuille flow, both these velocities approach zero as $Re \to \infty$. As a result, at high $Re$, even the slightest of negative plate motions is sufficient to trigger $\textrm{AI}$ and subsequently $\textrm{CI}^-$, as observed for the CPF. The wave-packet dispersion first increases with $Re$, followed by a decrease, which points to a peculiar "dual" role of viscosity in sustaining $\textrm{AI}$ in the CPF, namely, viscosity promotes sustenance of $\textrm{AI}$ at moderate Reynolds numbers but suppresses it at low and high Reynolds numbers. These results can be well understood within the Ginzburg-Landau framework, and therefore can be expected to have a wider applicability.

physics.flu-dyn↗

arXiv2021

Direct numerical simulation of a moist cough flow using Eulerian approximation for liquid droplets

The COVID-19 pandemic has inspired several studies on the fluid dynamics of respiratory events. Here, we propose a computational approach in which respiratory droplets are coarse-grained into an Eulerian liquid field advected by the fluid streamlines. A direct numerical simulation is carried out for a moist cough using a closure model for space-time dependence of the evaporation time scale. Estimates of the Stokes number are provided, for the initial droplet size of $10 μ$m, which are found to be <<1 thereby justifying the neglect of droplet inertia. Several of the important features of the moist-cough flow reported in the literature using Lagrangian tracking methods have been accurately captured using our scheme. Some new results are presented, including the evaporation time for a "mild" cough, a saturation-temperature diagram and a favourable correlation between the vorticity and liquid fields. The present approach is particularly useful for studying the long-range transmission of virus-laden droplets.

physics.flu-dyn↗

arXiv2021

Virus transmission by aerosol transport during short conversations

Pathogens like the SARS-CoV-2 are transmitted not only through violent expiratory events like coughing, but also through routine activities like breathing/speaking/singing. We perform direct numerical simulations of the turbulent transport of potentially infectious aerosols in short conversations. It is shown that a two-way conversation significantly reduces the aerosol exposure compared to a relative monologue by one person and relative silence of the other. This is because the interaction of the jets ejected from the mouth of each speaker produce a "canceling" effect. Unequal conversation is shown to significantly increase the risk of infection to the person who talks less. Interestingly, a small height difference is worse for infection spread, due to reduced interference between the two speech jets, than two faces at the same level! For small axial separation, speech jets show large oscillations and reach the other person intermittently. We suggest a range of lateral separations between two people to minimize transmission risk. A realistic estimate of the infection probability is provided by including exposure through eyes and mouth, in addition to the more common method of using inhaled virions alone. We expect that our results will provide useful inputs to epidemiological models and to disease management.

physics.flu-dyn↗

arXiv2021

Instability Mechanisms and Intermittency Distribution in Adverse Pressure Gradient Attached and Separated Boundary Layers

Direct numerical simulation has been carried out on one attached and two separated boundary layer flows (involving small and large separation) under the influence of an adverse pressure gradient. A unified picture of the pre-transitional boundary layer for the three cases has been provided that reveals a "mixed-mode" instability, involving contribution from instability waves and streamwise streaks. A time-frequency analysis of the transitional velocity signals has been performed which shows that as the Reynolds number decreases, the character of the time traces evolves continuously from a "spotty" behaviour (exhibiting distinct turbulent spots) for the attached case to a "non-spotty" behaviour (involving more "uniform" distribution of turbulent fluctuations in time) for the large separation case, encompassing the entire spectrum of transition scenarios. The variation of the intermittency factor within the transition zone is seen to compare well with the Narasimha universal intermittency distribution. We find that although the time variation of velocity for large separation is non-spotty (or more "uniform"), the spanwise variation of velocity is spotty, showing a clear clustering of high-wavenumber fluctuations separated by quasi-laminar regions. Thus, all the cases exhibit spottiness in the transition zone with different manifestations. We present a physical cartoon for the transition scenarios for the attached and separated cases, using the ideas of vortex-wall interaction and instability of spanwise vortical structures. We find that concentrated breakdown is exhibited by all the three cases near the transition onset and the spot breakdown processes are broadly consistent with the postulates underlying the universal intermittency distribution.

physics.flu-dyn↗

arXiv2021

Predominance of pressure transport in spatial energy budget for a mixing layer approaching absolute instability

In this letter, we report the outcome of a spatial energy budget performed for the linear convective instability of the plane incompressible mixing layer within the inviscid framework. We find that, as the critical condition for the onset of absolute instability is approached, the integrated pressure-transport term becomes increasingly more prominent as compared to the integrated production term and it dominates the energy budget completely at the critical condition. This implies that, near the threshold of absolute instability, the growth of disturbances is almost entirely due to the pressure transport mechanism (rather than the production mechanism), which is a striking result. The part of the pressure-transport term that represents the work done by the fluctuating pressure forces is seen to be primarily responsible for the observed shift in the energy balance. These results can help us better understand the physical processes causing absolute instability in a mixing layer. In particular, the redistribution of disturbance energy in streamwise direction by fluctuating pressure, which is "non-local" in character for incompressible flows, seems to play a key role in this respect.

physics.flu-dyn↗

arXiv2019

Laboratory Investigations of the Dynamics of Anomalous Entrainment in Cumulus-cloud flows

Anomalous entrainment in cumulus clouds has been a topic of investigation over many decades in the past. Its importance stems from the fact that entrainment rate is one of the major inputs to several cumulus-parameterization schemes. Recently Narasimha et al. (PNAS; 2011) have successfully simulated the large-scale dynamics of cumulus-cloud flows in the laboratory and provided a mechanistic explanation for the observed cumulus-entrainment anomalies. They showed a favourable comparison of a dilution-related quantity (called 'purity') between the laboratory measurements and cloud-resolving-model computations, and discussed the important problem of homogeneous vs. inhomogeneous mixing in cumulus clouds. The main purpose of the present report is to provide additional supporting information and a more detailed account of the entrainment-related issues not included in the PNAS paper due to space constraints. We believe this report, in conjunction with the paper, will present to the reader a comprehensive and more-or-less complete documentation on the issues mentioned above.

physics.flu-dyn↗

arXiv2019

Intermediate Scaling and Logarithmic Invariance in Turbulent Pipe Flow

A three-layer asymptotic structure for turbulent pipe flow is proposed, revealing in terms of intermediate variables, the existence of a Reynolds-number invariant logarithmic region. It provides a theoretical foundation for addressing important questions in the scaling of the streamwise mean velocity and variance. The key insight emerging from the analysis is that the scale separation between two adjacent layers is proportional to $\sqrt{Re_τ}$, rather than $Re_τ$. This suggests that, in order to realise Reynolds-number asymptotic invariance, much higher Reynolds numbers may be necessary to achieve sufficient scale separation. The formulation provides a theoretical basis for explaining the presence of a power law for the mean velocity in pipe flow at low Reynolds numbers and the co-existence of power and log laws at higher Reynolds numbers. Furthermore, the Townsend-Perry `constant' for the variance is shown to exhibit a systematic Reynolds-number dependence.

physics.flu-dyn↗