arXiv subjects

Subrahmanyam Duvvuri

Publications and source records attributed to Subrahmanyam Duvvuri.

7 recordsLinked to original sources

arXiv2026

A model for pulsation in high-speed double cone flow

Periodic large-scale shock-wave unsteadiness over a canonical double cone, termed in literature as "pulsation," is experimentally studied at Mach 6. The general double cone geometry is defined by three non-dimensional geometric parameters: fore- and aft-cone angles ($θ_1$ and $θ_2$), and ratio of the conical slant lengths ($\mathitΛ$). While existing literature on pulsation offers detailed qualitative and phenomenological discussions, it is seen that analytical approaches to obtain insight into the unsteady flow phenomena are missing. The present effort is aimed at addressing this gap. Self-sustained flow pulsations for a particular double cone configuration with $θ_1 = 0^\circ$ and $θ_2 = 90^\circ$, commonly referred to as the spike-cylinder, is investigated in the $\mathitΛ$ parameter space. High-speed schlieren imaging and time-resolved pressure measurements are performed in the unsteady flow. The non-dimensional pulsation frequency (Strouhal number) is observed to increase monotonically with $\mathitΛ$. Schlieren and pressure data suggest that the unsteadiness is driven by a cyclic process involving the formation of high-pressure gas near the aft-cone and its subsequent expansion through the separation region formed over the fore-cone. Building on this understanding, a detailed analytical model for the flow is developed with no empirical parameters. The model successfully predicts the experimentally-measured Strouhal number, and provides an in-depth understanding of the mechanisms that drive flow pulsations.

physics.flu-dyn↗

arXiv2025

Identification of triadic phase coupling in wall-bounded turbulence using the bispectrum

The direction and magnitude of energy transfer between turbulence scale brought about by external forcing on a turbulent boundary layer are uncovered through the bispectrum, bicoherence, and biphase. The bispectrum is a third-order, complex-valued spectrum of the streamwise velocity that preserves the phase information between triadically consistent scales. Normalized bispectrum is the bicoherence, a measure of the relative amount of energy at a higher frequency that results from quadratic phase coupling of two lower frequencies. The phase of the bispectrum, the biphase, measures the phase lag between the high frequency and two lower frequencies that add to it, unveiling whether a triadic interaction produces a forward or reverse cascade of energy. Summing the bispectrum over triadically consistent frequencies allows a spectral decomposition of the velocity skewness and asymmetry, unveiling the triadically active scales in the energy transfer processes. An average sense of energy transfer is inferred from the phase of this skewness spectrum, which shows that scales smaller than the boundary layer thickness contribute to a forward cascade on average, while those larger than the boundary layer thickness have a mix of forward and reverse events. These measures show that the forced scales in the perturbed boundary layer have a mixture of forward and reverse energy transfer processes for different sets of triadic scales and wall-normal locations, providing a method of quantifying the effects of external perturbations on turbulent flows without any need for artificial filtering.

physics.flu-dyn↗

arXiv2025

A network-theoretic approach for characterizing Mack-mode instability in high-speed boundary layers

Here we present a network theory-based approach to investigate the Mack-mode instability signature found in high-speed schlieren data from a Mach 6 laminar boundary layer flow over a $7^\circ$ cone. The data contain instability wave packets in the form of coherent rope-like structures which exhibit intermittency. The intermittency implies that conventional Fourier techniques are not particularly well suited for analysis. Network analysis, which is well known for handling episodic spatio-temporal data in a variety of complex systems, provides an alternate and more suitable framework. Techniques from time-varying spatial proximity networks are applied to the present data. The connected components in the network topology reveal lines of constant phase for coherent wave packets associated with the instability, and localized regions of high schlieren light intensity for intermittent laminar or turbulent flow states. The orientation angle of the connected network components is found to be a suitable metric for identifying components associated with the Mack-mode instability, and that enables detailed characterization of the wavelength and propagation speeds of the instability wave packets. Beyond the characterization exercise, network analysis can provide a powerful framework for understanding the fundamental nature of intermittency and its role in the laminar-to-turbulent flow transition process.

physics.flu-dyn↗

arXiv2024

An aeroacoustic mechanism to explain universal behavior in hypersonic wake flow oscillations

Recent experimental studies reveal that the near-wake region of a circular cylinder at hypersonic Mach numbers exhibits self-sustained flow oscillations. The oscillation frequency was found to have a universal behavior. Experimental observations suggest an aeroacoustic feedback loop to be the driving mechanism of oscillations. An analytical aeroacoustic model which predicts the experimentally observed frequencies and explains the universal behavior is presented here. The model provides physical insights and informs of flow regimes where deviations from universal behavior are to be expected.

physics.flu-dyn↗

arXiv2024

A model for frequency scaling of flow oscillations in high-speed double cones

Coherent small-amplitude unsteadiness of the shock wave and the separation region over a canonical double cone flow, termed in literature as oscillation-type unsteadiness, is experimentally studied at Mach 6. The double cone model is defined by three non-dimensional geometric parameters: fore- and aft-cone angles ($θ_1$ and $θ_2$), and ratio of the conical slant lengths ($Λ$). Previous studies of oscillations have been qualitative in nature, and mostly restricted to a special case of the cone model with fixed $θ_1 = 0^\circ$ and $θ_2 = 90^\circ$ (referred to as the spike-cylinder model), where $Λ$ becomes the sole governing parameter. In the present effort we investigate the self-sustained flow oscillations in the $θ_1$-$Λ$ parameter space for fixed $θ_2 = 90^\circ$ using time-resolved schlieren visualization. The experiments reveal two distinct sub-types of oscillations, characterized by the motion (or lack thereof) of the separation point on the fore-cone surface. The global time scale associated with flow oscillation is extracted using spectral proper orthogonal decomposition. The non-dimensional frequency (Strouhal number) of oscillation is seen to exhibit distinct scaling for the two oscillation sub-types. The relationship observed between the local flow properties, instability of the shear layer, and geometric constraints on the flow suggests that an aeroacoustic feedback mechanism sustains the oscillations. Based on this insight, a simple model with no empiricism is developed for the Strouhal number. The model predictions are found to match well with experimental measurements. The model provides helpful physical insight into the nature of the self-sustained flow oscillations over a double cone at high-speeds.

physics.flu-dyn↗

arXiv2022

Strouhal number universality in high-speed cylinder wake flows

Flow oscillations in the near-wake region of a 2D circular cylinder are experimentally investigated at Mach 6 over the Reynolds number range $2.3\times10^5$ to $5\times10^5$. The oscillation frequency is obtained by spectral proper orthogonal decomposition of high-speed schlieren data. The Strouhal number based on the length of the near-wake shear layers is found to exhibit universal behavior. This corroborates experimental findings at Mach 4 from recent literature, and further, the universal behavior is also seen to hold with respect to Mach number. Time-resolved pressure measurements at the flow separation points on the cylinder aft surface show that coherent oscillatory activity occurs with a phase difference of $π$ radians between the two statistically-symmetric halves of the flow. This aspect of the flow dynamics at high speeds is in common with its low-speed counterpart, i.e. the canonical problem of cylinder wake in an incompressible flow.

physics.flu-dyn↗

arXiv2020

Large- and small-amplitude shock wave oscillations over axisymmetric bodies in high-speed flow

The phenomena of self-sustained shock wave oscillations over conical bodies with a blunt axisymmetric base subject to uniform high-speed flow are investigated in a hypersonic wind tunnel at Mach number $M = 6$. The flow and shock wave dynamics are dictated by two non-dimensional geometric parameters presented by the three length scales of the body, two of which are associated with the conical forebody and one with the base. Time-resolved schlieren imagery from these experiments reveals the presence of two disparate states of shock wave oscillations in the flow, and allows for the mapping of unsteadiness boundaries in the two-parameter space. Physical mechanisms are proposed to explain the oscillations and the transitions of the shock wave system from steady to oscillatory states. In comparison to the canonical single-parameter problem of shock wave oscillations over spiked-blunt bodies reported in literature, the two-parameter nature of the present problem introduces distinct elements to the flow dynamics.

physics.flu-dyn↗