arXiv subjects

Roddam Narasimha

Publications and source records attributed to Roddam Narasimha.

13 recordsLinked to original sources

arXiv2024

Wing Optimisation for a tractor propeller driven Micro Aerial Vehicle

This paper describes an investigation of the possible benefits from wing optimisation in improving the performance of Micro Air Vehicles (MAVs). As an example we study the Avion (3.64 kg mass, 1.60 m span), being designed at the CSIR National Aerospace Laboratories (NAL), Bengaluru. The optimisation is first carried out using the methodology described by Rakshith \emph{et al.} (using an in\textendash house software PROWING), developed for large transport aircraft, with certain modifications to adapt the code to the special features of the MAV. The chief among such features is the use of low Reynolds number aerofoils with significantly different aerodynamic characteristics on a small MAV. These characteristics are taken from test data when available, and/or estimated by the XFOIL code of Drela. A total of 8 optimisation cases are studied for the purpose, leading to 6 different options for new wing planforms (and associated twist distributions along the wing span) with an improved performance. It is found that the improvements in drag coefficient using the PROWING code are about 5%. However, by allowing the operating lift coefficient $C_L$ to float within a specified range, drag bucket characteristics of the Eppler E423 aerofoil used on Avion can be exploited to improve the endurance, which is a major performance parameter for Avion. Thus, compared to the control wing $W_0$ (with operating point at $C_L$ =0.7) used in the preliminary design, permitting a variation of $C_L$ over a range of $\pm$ 10% is shown to enhance the endurance of wing $W_4$ by 18.6%, and of wing $W_{6}$ with a permitted $C_L$ range of $\pm$ 50% by 39.2%. Apart from the philosophy of seeking optimal operating conditions for a given configuration, the advantages of optimising design parameters such as washout of a simple wing proposed in the preliminary design stage, is also demonstrated.

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arXiv2020

The role of the `monopole' instability in the evolution of 2D turbulent free shear layers

The role of instability in the growth of a 2D, temporally evolving, `turbulent' free shear layer is analyzed using vortex-gas simulations that condense all dynamics into the kinematics of the Biot-Savart relation. The initial evolution of perturbations in a constant-vorticity layer is found to be in accurate agreement with the linear stability theory of Rayleigh. There is then a stage of non-universal evolution of coherent structures that is closely approximated not by Rayleigh stability theory, but by the Karman-Rubach-Lamb linear instability of monopoles, until the neighboring coherent structures merge. After several mergers, the layer evolves eventually to a self-preserving reverse cascade, characterized by a universal spread rate found by Suryanarayanan et al. (Phys.Rev.E 89, 013009, 2014) and a universal value of the ratio of dominant spacing of structures ($Λ_f$) to the layer thickness ($δ_ω$). In this universal, self-preserving state, the local amplification of perturbation amplitudes is accurately predicted by Rayleigh theory for the locally existing `base' flow. The model of Morris et al. (Proc.Roy.Soc. A 431, 219-243, 1990.), which computes the growth of the layer by balancing the energy lost by the mean flow with the energy gain of the perturbation modes (computed from an application of Rayleigh theory), is shown, however, to provide a non-universal asymptotic state with initial condition dependent spread-rate and spectra. The reason is that the predictions of the Rayleigh instability, for a flow regime with coherent structures, are valid only at the special value of $Λ_f/δ_ω$ achieved in the universal self-preserving state.

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arXiv2020

Prediction of separation and transition on a low-pressure turbine blade using a RANS grid

Flow past a high-lift low-pressure turbine (LPT) blade in a cascade could be quite complex as phenomena like separation and transition are often involved. For a highly loadedT106A blade at a high incidence and relatively low Reynolds number(25, 000 < Re < 1, 00, 000), separation-induced transition is observed on the suction side of the blade, making it a challenging problem for model-based simulations. In this work, computations for this flow are carried out using RANS and hybrid LES/RANS approaches. The RANS simulations are performed with six popular low- Re turbulence models. While turbulence models by themselves fail to predict any separation on the T106A blade, the four-equation Langtry-Menter transition model predicts a short separation bubble. The characteristic of this bubble, however, is very different from what is observed in experiments and DNS, and therefore transition is not accurately predicted. An embedded hybrid LES/RANS approach, Limited numerical scales(LNS), with an automatic switch to LES in sufficiently resolved grids, is then used for predictions on the sameRANS grid. With the statistical turbulence on fine grids, LES-like behavior of LNS results in an unphysical drop in Reynolds stresses as the turbulent fluctuations are not appropriately represented on the resolved scale. Therefore, the LNS results are very similar to those obtained with turbulence models. However, when synthetic turbulence with correct statistical characteristics is used to stimulate the large eddies in the embedded LES zone, LNS is able to predict separation and recovers a solution very close to DNS and experimental results.

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arXiv2020

Non-Precipitating Shallow Cumulus Clouds: Theory and Direct Numerical Simulation

We model cumulus clouds as transient diabatic plumes, and present a single-fluid formulation for the study of the dynamics of developing shallow cumulus clouds. The fluid, air, carries water vapour and liquid water as scalars that are advected with the flow. We employ the Boussinesq approximation and a simplified form of the Clausius-Clapeyron law for phase changes. We use this formulation to study the ambient conditions required for the formation of cumulus clouds. We show that the temperature lapse rate in the ambient and the relative humidity of the ambient are crucial in deciding when cumulus clouds form. In the phase-space of these two parameters, we determine the boundary that separates the regions where cumulus clouds can form and where they cannot.

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arXiv2020

Turbulent Jet: A DNS Study

The entrainment of ambient fluid into a turbulent shear flow has been a topic of wide interest for several decades. To estimate the entrainment of ambient fluid into turbulent jet, it is essential to define the boundary of the jet. The question arises as to what should be the appropriate criterion to determine the edge of the turbulent jet. From the present DNS simulations, we observe that there is a need to define two boundaries for the turbulent jet, namely, the Rotational/Irrotational (R/IR) interface termed here as "Outer boundary" and the Turbulent/Non-Turbulent (T/NT) interface as "Inner boundary". The main aim of the present paper is to identify the need for defining the two boundaries for the jet. We also present some interesting observations of the jet flow, essentially on self similarity and self preservation.

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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.

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arXiv2019

A DNS Study of entrainment in an axisymmetric turbulent jet as an episodic process

This investigation is based on a DNS of a steady self-preserving incompressible axisymmetric turbulent jet at a Reynolds number of 2400. The DNS data enable accurate maps of the outer irrotational flow field, and also the vorticity field in the turbulent core of the jet. It is found necessary to define two separate boundaries of the jet. The first is an inner boundary (turbulent/nonturbulent, T/NT), from where vorticity rises steeply towards to the core. The second is an outer rotational/irrotational boundary, beyond which the flow may be considered irrotational. The velocity field beyond the outer boundary often has ordered, nearly irrotational circulatory motions. These can be shown, in simpler cases, to be the velocity field induced by one or more vorticity elements in a coherent structure in the turbulent core. A detailed examination of axial and diametral sections indicates that there are periods when there is a large inrush of ambient fluid into parts of the T/NT interface, which gets distorted into a gulf or well that can be both twisted and deep. Sections of these wells often appear as what may be called as lakes of irrotational fluid in diametral sections of the jet flow. Part of the inrushing fluid crosses the T/NT interface within the well and is entrained into the turbulent core, by a process that can legitimately be called nibbling. The duration of such an inrush process can be of the order up to 20 flow units and suggests that entrainment can be an episodic process in which an inrush event accelerates ambient fluid even as it is pushed into a narrowing gulf, where it penetrates the T/NT interface of the gulf by nibbling. In the turbulent round jet, the entrainment burstiness is found to be of order 0.75, comparable to the momentum flux burstiness found in a turbulent boundary layer.

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arXiv2017

An assessment of the two-layer quasi-laminar theory of relaminarization through recent high-Re accelerated TBL experiments

The phenomenon of relaminarization is observed in many flow situations, including that of an initially turbulent boundary layer subjected to strong favourable pressure gradients. Available turbulence models have hitherto been unsuccessful in correctly predicting boundary layer parameters for such flows. Narasimha and Sreenivasan \cite{narasimha1973relaminarization} proposed a quasi-laminar theory (QLT) based on a two-layer model to explain the later stages of relaminarization. This theory showed good agreement with the experimental data available, which at the time was at relatively low $Re$. QLT, therefore, could not be validated at high $Re$. Some of the more recent experiments report for the first time comprehensive studies of a relaminarizing flow at relatively high Reynolds numbers (of order $5\times 10^3$ in momentum thickness), where all the boundary layer quantities of interest are measured. In the present work, the two-layer model is revisited for these relaminarizing flows with an improved code in which the inner-layer equations for quasi-laminar theory have been solved exactly. It is shown that even for high-$Re$ flows with high acceleration, QLT provides a much superior match with the experimental results than the standard turbulent boundary layer codes. This agreement can be seen as strong support for QLT, which therefore has the potential to be used in RANS simulations along with turbulence models.

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arXiv2016

A high-resolution DNS study of compressible flow past an LPT blade in a cascade

Flow past a low pressure turbine blade in a cascade at $Re \approx 52000$ and angle of incidence $α= 45.5^{0}$ is solved using a code developed in-house for solving 3D compressible Navier-Stokes equations. This code, named ANUROOP, has been developed in the finite volume framework using kinetic energy preserving second order central differencing scheme for calculating fluxes, and is compatible with hybrid grids. ANUROOP was verified and validated against several test cases with Mach numbers ranging from 0.1 (Taylor-Green vortex) to 1.5 (compressible turbulent channel flow). The code was found to be robust and stable, and the kinetic energy decay obeys the compressible Navier-Stokes equations. A hybrid grid, with a high resolution hexahedral orthogonal mesh in the boundary layer and unstructured (also hexahedral) elements in the rest of the domain, is used for the turbine blade simulation. Total grid size (160 million) is approximately an order of magnitude higher than in previous simulations for the same flow conditions and using similar numerical methods. The discrepancy in the pressure distribution in earlier studies compared to experimental data has been removed in this simulation. The trailing edge separation bubble has been characterized and a detailed discussion on the effect of surface curvature is presented.

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arXiv2015

Spatially evolving vortex-gas turbulent free shear layers: Part 2. Coherent structure dynamics in vorticity and concentration fields

This paper examines the mechanisms of coherent structure interactions in spatially evolving turbulent free shear layers at different values of the velocity ratio parameter λ=$(U_1-U_2)/(U_1+U_2)$, where $U_1$ and $U_2 (\leq U_1)$ are the free stream velocities on either side of the layer. The study employs the point-vortex (or vortex-gas) model presented in part I (arXiv:1509.00603) which predicts spreading rates that are in the close neighborhood of results from most high Reynolds number experiments and 3D simulations. The present (2D) simulations show that the well-known steep-growth merger events among neighboring structures of nearly equal size (Brown & Roshko 1974) account for more than 70% of the overall growth at λ< 0.63. However the relative contribution of such 'hard merger' events decreases gradually with increasing λ, and accounts for only 27% of the total growth at the single-stream limit (λ = 1). It is shown that the rest of the contribution to layer growth is largely due to the increasing differential in size between neighboring structures as λ increases from 0.6 to 1.0, and takes place through two different routes. The first occurs via what may be called 'soft' mergers, involving extraction of little patches or filaments of vorticity over time from appreciably smaller (usually upstream) structures. This is consistent with earlier computational work on asymmetric two-vortex mergers (Yasuda & Flierl, 1995). The second route involves assimilation of disorganized 'vortex dust', which contains remnants of disrupted earlier structures not yet transferred to surviving ones. A combination of these processes is shown to cause an apparently nearly continuous increase in structure size with downstream distance at λ = 1, of the kind reported by D'Ovidio & Coats (2013)and attributed to mixing transition ...

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arXiv2015

Spatially evolving vortex-gas turbulent free shear layers: Part 1. Effect of velocity ratio, and upstream and downstream conditions on spread-rate

The relevance of the vortex-gas model to the large scale dynamics of temporally evolving turbulent free shear layers in an inviscid incompressible fluid has recently been established by extensive numerical simulations (Suryanarayanan et al, Phys. Rev. E 89, 013009, 2014). Here, the effects of the velocity ratio across a spatially evolving 2D free shear layer are investigated by vortex-gas simulations, using a computational model based on Basu et al (1992, 1995), but with a crucial improvement that ensures conservation of global circulation. These are carried out for a range of values of the velocity ratio parameter $λ= (U_1-U_2)/(U_1+U_2)$, where $U_1$ and $U_2$ ($< U_1$) are respective velocities across the layer. The simulations show that the conditions imposed at the beginning of the free shear layer and at the exit to the domain can affect the flow evolution in their respective neighborhoods, the latter being particularly strong as $λ\rightarrow 1$. In between the two neighborhoods is a regime of universal self-preserving growth rate given by a universal function of $λ$ The computed growth rates are located within the scatter of experimental data on plane mixing layers, and in close agreement with recent high Reynolds number experiments and 3D LES studies, past the mixing transition, and support the view that free shear layer growth can be largely explained by the 2D vortex dynamics of the quasi-two-dimensional large scale structures.

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arXiv2012

The Turbulent 'Mixing' Layer as a Problem in the Non-equilibrium Statistical Mechanics of a Vortex Gas

The objective of this paper is to unravel any relations that may exist between turbulent shear flows and statistical mechanics, through a detailed numerical investigation in the simplest case where both can be well defined. The shear flow considered for the purpose is the 2D temporal mixing layer, which is a time-dependent flow that is statistically homogeneous in the streamwise direction (x) and evolves from a plane vortex sheet in the direction normal to it (y) in a periodic-in-x domain with period L. The connections to statistical mechanics are explored by revisiting, via extensive computer simulations, an appropriate initial value problem for a finite but large collection of (N) point vortices of same strength (γ) and sign constituting a 'vortex gas'. Such connections may be expected to be meaningful as hydrodynamics, since the flow associated with the vortex gas is known to provide weak solutions of the Euler equation. Over ten different initial conditions classes are investigated using simulations involving up to 10^4 vortices, with ensemble averages evaluated over up to 10^3 realizations and integration over 10^4 L/(ΔU) (where ΔU is the velocity differential across the layer, given by Nγ/L). (see PDF for complete abstract)

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arXiv2010

Point-Vortex Simulations Reveal Universality Class in Growth of 2D Turbulent Mixing Layers

A central but controversial issue in free turbulent shear flows has been the universality (or otherwise) of their growth rates. We resolve this issue here in the special case of a temporal 2D mixing layer in a point vortex gas by extensive high-precision numerical simulations, utilizing for the first time a powerful ensemble-averaging strategy. The simulations show that the momentum thickness of such a mixing layer grows at the universal asymptotic rate of 0.0167 +/- 0.00017 times the velocity differential across the layer over a wide range of initial conditions, often after very long transients.

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