Browsing by Author "Harish Choudhary"

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A new universal model for friction factor in smooth pipes
(American Institute of Physics Inc., 2021) Shivsai Ajit Dixit; Abhishek Gupta; Harish Choudhary; A.K. Singh; Thara Prabhakaran
Friction factor models for turbulent flow in smooth pipes express friction factor λ as a function of the bulk Reynolds number ReD and may be broadly grouped into two categories: power-law models and log-law models. While the former stem from the spectral scaling arguments applied to eddy momentum transfer close to the wall, the latter are derived from the mean velocity log law and are known to be consistent with the attached eddy model of wall turbulence structure. Interestingly, none of these models individually describes the entire range of Reynolds numbers (Re) accessed to date, without requiring adjustment of coefficients and/or exponents, i.e., these models are not universal. In this work, we present a new semi-empirical universal model that combines, without introducing any additional empirical coefficients, the essence of both power-law and log-law models. Due to this, our model successfully describes the variation of friction factor over the entire range of Reynolds numbers (more than four decades in ReD) at once. The physical basis for our model is the observation that at finite Reynolds numbers, the flow appears to be a small perturbation of the so-called ultimate regime of smooth-pipe turbulence, as far as friction is concerned; in the ultimate regime, λ → 0 asymptotically as R e D → ∞. The new model has significant potential toward accurate estimation of friction factor or flow rate in smooth pipe flows. © 2021 Author(s).
Asymptotic scaling of drag in flat-plate turbulent boundary layers
(American Institute of Physics Inc., 2020) Shivsai Ajit Dixit; Abhishek Gupta; Harish Choudhary; A.K. Singh; Thara Prabhakaran
A new asymptotic -1/2 power-law scaling is derived from the momentum integral equation for the drag in flat-plate turbulent boundary layers. In the limit of infinite Reynolds number, the appropriate velocity scale for drag is found to be M/ν, where M is the boundary layer kinematic momentum rate and ν is the fluid kinematic viscosity. Data covering a wide range of Reynolds numbers remarkably collapse to a universal drag curve in the new variables. Two models, discrete and continuous, are proposed for this universal drag curve, and a robust drag estimation method, based on these models, is also presented. © 2020 Author(s).
Experimental investigation of the structure of plane turbulent wall jets. Part 1. Spectral analysis
(Cambridge University Press, 2024) Harish Choudhary; Abhishek Gupta; Shibani Bhatt; Thara Prabhakaran; A.K. Singh; Anandakumar Karipot; Shivsai Ajit Dixit
Plane turbulent wall jets are traditionally considered to be composed of a turbulent boundary layer (TBL) topped by a half-free jet. However, certain peculiar features, such as counter-gradient momentum flux occurring below velocity maximum in experiments and numerical simulations, suggest a different structure of turbulence therein. Here, we hypothesize that turbulence in wall jets has two distinct structural modes, wall mode scaling on wall variables and free-jet mode scaling on jet variables. To investigate this hypothesis, experimental data from our wall jet facility are acquired using single hot-wire anemometry and two-dimensional particle image velocimetry at three nozzle Reynolds numbers 10 244, 15 742 and 21 228. Particle image velocimetry measurements with four side-by-side cameras capture the longest field of view studied so far in wall jets. Direct spatial spectra of these fields reveal modal spectral contributions to variances of velocity fluctuations, Reynolds shear stress, shear force, turbulence production, velocity fluctuation triple products and turbulent transport. The free-jet mode has wavelengths scaling on the jet length scale, and contains two dominant submodes with wavelengths and. The region of flow above the velocity maximum shows the presence of the outer jet mode whereas the region below it shows robust bimodal behaviour attributed to both wall and inner jet modes. Counter-gradient momentum flux is effected by the outer jet mode intruding into the region below velocity maximum. These findings support the hypothesis of wall and free-jet structural modes, and indicate that the region below velocity maximum could be much complex than a conventional TBL. © The Author(s), 2024.
Scale-Aware Overlap in Turbulent Wall Jets
(Springer Science and Business Media Deutschland GmbH, 2021) Abhishek Gupta; Harish Choudhary; A.K. Singh; Thara Prabhakaran; S.A. Dixit
Data from experiments on two-dimensional turbulent wall jets suggest existence of two distinct layers, the wall (inner) layer, and the jet (outer) layer, each having its own universal scaling independent of the local Reynolds number Re τ. This view is distinct from most earlier approaches that are either not clear about what the outer flow is, or consider the wall jet to be comprising of two regions—the region below the velocity maximum is a turbulent boundary layer having its own two-layer structure with a logarithmic overlap, and the region above the velocity maximum is a half free jet. These regions are smoothly patched at the velocity maximum. The present view considers the outer flow to be a universal full jet (rather than a half jet), centered at the velocity maximum, which overlaps with the universal inner wall flow. The hypothesis of a scale-aware overlap of these universal scaling regions leads to the prediction of an Re τ -dependent power-law velocity profile in the overlap layer. Further, an intermediate variable approach is shown to effectively absorb this Re τ dependence leading to a universal power-law profile for mean velocity in the overlap layer. Experiments show strong support for this description. © 2021, Springer Nature Singapore Pte Ltd.
Scaling mean velocity in two-dimensional turbulent wall jets
(Cambridge University Press, 2020) Abhishek Gupta; Harish Choudhary; A.K. Singh; Thara Prabhakaran; Shivsai Ajit Dixit
Studies in the literature on two-dimensional, fully developed, turbulent wall jets on flat surfaces, have invariably reckoned on 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 the 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. © The Author(s), 2020. Published by Cambridge University Press.