S. Unnikrishnan

Florida State University | FSU ·  Aero-Propulsion, Mechatronics and Energy (AME)

A computational analysis is performed to study the three-dimensional response of rectangular shear layers to plasma actuator-based control, in the context of sound mitigation of supersonic non-axisymmetric jets. A Mach 1.5 rectangular jet with an aspect ratio 2 : 1 is controlled using experimentally informed actuation patterns, referred to as M0, M...

The effect of LAFPA based control on the flow-field and acoustics of an over-expanded jet exiting a 2:1 aspect ratio rectangular nozzle is studied through high-fidelity simulations. Using eight actuators around the inner nozzle surface, we test two forcing sequences, M0 (all actuators firing in sequence) and M1 (phase difference of /4 between firin...

Insights into high-speed transition over canonical tangent-ogive forebodies (TOF) representative of hypersonic vehicles is obtained with a generalized formulation of the nonlinear spatial eigenvalue problem. The linear stability predictions identify amplification characteristics of first, second, and entropy-layer modes. Mode synchronization and ph...

Ogive cylinders are representative designs of hypersonic forebodies and harbor several flow complexities. This work evaluates the effects of nose bluntness of ogive-cylinder forebodies on their laminar, transitional, and turbulent boundary layers (BLs). Laminar simulations indicate that the thickness of BL and entropy layer (EL) increases with nose...

We perform a computational study on the effects of localized arc filament plasma actuator (LAFPA) based control on the flow-field and acoustics of supersonic 2:1 aspect ratio rectangular jet. Post validation of the baseline jet, effects of control in the context of noise reduction are studied at experimentally-guided forcing parameters, including...

Aspects of transition mechanisms on a 14-degree sharp-nosed ogive-cylinder at Mach 6 are elucidated by considering linear and nonlinear disturbance evolution of freestream stochastic and wave packet forcing at different locations downstream of the ogive-cylinder junction. The spectral response of the stochastic forcing displays favorable agreement...

Boundary layer instabilities and shock/boundary-layer interactions (SBLIs) critically affect the performance and safe operation of mixed- compression air intakes. We present a computational study anchored in companion experiments, to evaluate the multi-modal mechanisms driving the dynamics of the external compression ramp flow and the cowl SBLI in...

Aspects of transition mechanisms on a 14 degree sharp-nosed ogive-cylinder at Mach 6 are elucidated by considering linear and nonlinear disturbance evolution of freestream stochastic and wave packet forcing at different locations downstream of the ogive-cylinder junction. The spectral response of the stochastic forcing displays favorable agreement...

View Video Presentation: https://doi.org/10.2514/6.2022-3712.vid Prediction and control of unstart phenomena are critical to ensure safe operation of supersonic intakes. A computational study of a mixed-compression supersonic intake is performed to first validate a Reynolds-Averaged Navier Stokes (RANS) solver using complementary experiments. RANS...

A joint experimental and computational study is performed to quantify the effects of various expansion ratios on the flowfields and acoustic properties of a jet, issuing from a diamond-shaped nozzle. Three expansion ratios are studied corresponding to perfectly expanded, overexpanded, and underexpanded regimes, which progressively increase the far-...

The near-field dynamics of a rectangular heated overexpanded jet of aspect ratio two is examined with an implicit large-eddy simulation using experimental data for validation purposes. The conical nozzle, representative of practical configurations, results in multiple shock trains from the throat region as well as the overexpanded operating conditi...

View Video Presentation: https://doi.org/10.2514/6.2022-1152.vid A joint computational and experimental study is performed to quantify the effects of heating on the flow-field of an overexpanded jet, issuing from a diamond-shaped nozzle. At the current aspect ratio, AR = 2 , the heated and unheated jets exhibit asymmetric far-field acoustic signatu...

View Video Presentation: https://doi.org/10.2514/6.2022-2403.vid Rectangular propulsion nozzles have seen widespread recent interest owing to thrust-vectoring and air-frame-integration advantages over their more commonly studied circular counterparts. However, their more complex near-field dynamics violate simplifying properties like azimuthal homo...

Linear theory continues to play a critical role in extracting the underlying primary instability dynamics of boundary layer transition. However, such methods have difficulty in isolating the dynamics of secondary instabilities, which dominate the later stages of transition. A Floquet analysis can bridge this gap; however, typical matrix-based appro...

Rectangular propulsion nozzles offer thrust-vectoring and air-frame-integration advantages over their more commonly studied circular counterparts. However, they display many distinguishing features which violate assumptions, such as azimuthal homogeneity, typically used in prediction tools for circular jets. In the present paper, we examine the uti...

Boundary layer development in a supersonic inlet is studied, focusing on the flow over the compression ramp and the ramp-isolator junction. Two designs of the junction, including a backward-facing step (BFS) and a backward-facing notch (BFN), are first evaluated to identify three linear frequency-bands (high, medium, and low) of the shear layer ins...

The dynamics of a fully three-dimensional lid-driven cubical cavity (3D-LDC) flow at several post-critical conditions, i.e., beyond the first bifurcation, are elucidated using both linear and nonlinear analyses. When the Reynolds number is increased beyond the critical value, symmetry breaks down intermittently with subsequent gradual growth in spa...

Wall cooling has substantial qualitative and quantitative effects on the development of instabilities and subsequent transition processes in hypersonic boundary layers (HBLs). A sequence of linear stability theory, nonlinear two-dimensional and three-dimensional direct numerical simulations is used to analyse Mach 6 boundary layers, with wall tempe...

Despite advances in Direct Numerical Simulation (DNS) capability, linear theory continues to play a critical role in extracting the underlying instability dynamics of boundary layer transition, especially in hypersonic flows which display a richer variety of modes than low-speed flows. The application of linear theory to steady basic-states has pro...

High-speed laminar-to-turbulent transition over blunt bodies is relevant to a variety of aerodynamic applications. Experiments have observed that beyond a critical value of the nose radius the initially downstream movement of transition location is reversed. Linear stability and receptivity analyses have been unsuccessful at predicting this reversa...

Acoustic emissions from supersonic jets are of interest due to their deleterious effects on nearby personnel and structures. As axisymmetric jets are highly prevalent, their aeroacoustic characteristics have been well-documented, whereas relatively few studies have examined non-axisymmetric jets. The current study focuses on the aeroacoustics of su...

Backward Facing Step (BFS) flows are prevalent in aerial vehicles and most of the applications involve modifications to the conventional step geometry. The present study is concerned with the use of BFS geometry around the ramp-isolator junction of supersonic mixed-compression intakes. The dynamics of laminar supersonic flows around a conventional...

The evolution of the potent second-mode instability in hypersonic boundary layers (HBLs) is examined holistically, by tracking its linear and nonlinear evolution, followed by its role in initiating transition and eventual breakdown of the HBL into a fully turbulent state. Linear stability theory (LST) is utilized to first identify the features of t...

Fluid Dynamics Reviews - Seminar at University of Maryland 25 September, 2020

Wall cooling has substantial effects on the development of instabilities and transition processes in hypersonic boundary layers (HBLs). A sequence of linear stability theory, two-dimensional and non-linear three-dimensional DNSs is used to analyze Mach~6 boundary layers, with wall temperatures ranging from near-adiabatic to highly cooled conditions...

The 2D second-mode is a potent instability in hypersonic boundary layers (HBLs). We study its linear and nonlinear evolution, followed by its role in transition and eventual breakdown of the HBL into a fully turbulent state. Linear stability theory (LST) is utilized to identify the second-mode wave through FS-synchronization, which is then recreate...

Aeroacoustic analyses of jet flows have benefited greatly from a decomposition of turbulent pressure fluctuations into hydrodynamic and acoustic components. This is typically accomplished using signal processing techniques based on phase speeds, coherence properties or spectral analyses. We present an approach, building on the Momentum Potential Th...

Mack mode or second mode instability plays a critical role in hypersonic boundary layer (HBL) transition. The sensitivity of Mack modes to wall thermal conditions, specifically cooling, is of great interest, since it can destabilize this mode. In this work, we utilize linear stability theory (LST) and direct numerical simulations (DNS), to characte...

Rectangular nozzles are receiving increased attention because of, among other reasons, the ease with which they may be integrated into the airframe. However, their dynamics have not been examined in as much detail as circular configurations, particularly at operating conditions relevant to practical applications. The present work investigates the n...

Laminar-to-turbulent transition over blunt bodies is relevant to a variety of aerodynamic applications. For flows over flat plates/cones of small nose radii, it has been experimentally observed, and theoretically explained, that the nose bluntness effect leads to a delay of boundary-layer transition. Experiments have, however, also shown that this...

Global stability modes of flows provide significant insight into their dynamics. Direct methods to obtain these modes are restricted by the daunting sizes and complexity of Jacobians encountered in general three-dimensional flows.Jacobian-free iterative approaches such as those based on the Arnoldi algorithm have greatly alleviated the required com...

The mechanisms underlying transition of a Mach 6 flat plate boundary layer with adiabatic wall conditions are explored with Direct Numerical Simulations (DNS) using a high-order hybrid upwind-centered method. The forcing employed is obtained from linear stability theory (LST), which is used to identify Tollmien-Schlichting (T-S) or first-mode insta...

Direct methods to obtain global stability modes are restricted by the daunting sizes and complexity of Jacobians encountered in general three-dimensional flows. Jacobian-free iterative approaches such as Arnoldi methods have greatly alleviated the required computational burden. However, operations such as orthonormalization and shift-and-invert tra...

Global stability modes of flows provide significant insight into their dynamics. Direct methods to obtain these modes are restricted by the daunting sizes and complexity of Jacobians encountered in general three-dimensional flows. Jacobian-free approaches have greatly alleviated the required computational burden. Particularly, the most common Arnol...

Discrete unstable modes of hypersonic laminar boundary layers, obtained from an eigenvalue analysis, provide insight into key transition scenarios. The character of such modes near the leading edge is often identified with the corresponding asymptotic freestream behavior of acoustic, vortical or entropic (thermal) content, which we designate fluid...

The dominant acoustic radiation from turbulent jets has been associated with coherent wavepacket structures in the plume. Jet noise models are therefore often designed using the statistics of decomposed coherent fluctuations, which display wavepacket attributes. In the absence of a universal definition for wavepacket fluctuation components, sever...

Instability modes in hypersonic boundary layers (HBL) are often identified through their phase speeds. Here, we adopt a technique to quantify the physical components in various instability modes in an HBL, namely vortical, acoustic and thermal, which we designate fluid-thermodynamic (FT) components. A linear stability analysis is first performed to...

To aid in jet noise source identification, several studies have attempted to isolate the propagating component of pressure from that which convects with structures. While the former contributes to farfield sound of the jet (acoustic pressure), the latter decays rapidly in the nearfield (aerodynamic or hydrodynamic pressure). Fourier- and wavelet-an...

The stability properties of basic states are often elucidated by examining the evolution of small disturbances. Such studies have recently been successfully applied to mean turbulent states, obtained through averaging of experimental measurements or Large-Eddy Simulations (LES), for both wall-bounded as well as free shear flows. Typically, the equa...

The effectiveness of a Navier-Stokes based Mean Flow Perturbation (NS-MFP) technique in obtaining linear response of basic states upon application of small perturbation is demonstrated. The method solves the full non-linear Navier-Stokes equations in the time domain, using forcing to maintain the invariance of the basic state. When subjected to a r...

Underexpanded jets exhibit interactions between turbulent shear layers and shock-cell trains that yield complex phenomena that are absent in the more commonly studied perfectly expanded jets. We quantitatively analyze these mechanisms by considering the interplay between hydrodynamic (turbulence) and acoustic modes, using a validated large-eddy sim...

We examine the three-dimensional energy-exchange mechanisms between stochastic turbulence and an organized radiating wavepacket, and subsequent scattering and transmission of acoustic energy. A validated round-jet large-eddy simulation (LES) is subjected to Kovásznay-type decomposition into hydrodynamic, acoustic and thermal modes, as encapsulated...

Local fluctuations in a Mach 1.3 cold jet are tracked to understand the genesis of nearfield directivity and intermittency. A newly developed approach leveraging two synchronized large-eddy simulations is employed to solve the forced Navier–Stokes equations, linearized about the evolving unsteady base flow. The results are summarized by exposing th...

General compressible solver

The three-dimensional spatio-temporal evolution of the acoustic mode in a supersonic jet is analyzed using Doak's Momentum Potential Theory on an LES database. The acoustic mode exhibits a well-defined wavepacket nature in the core and convects at sonic speed. Its spatial coherence is significantly higher than the hydrodynamic component, resulting...

Large-eddy simulation data for a Mach 1.3 round jet are decomposed into acoustic, hydrodynamic and thermal components using Doak’s momentum potential theory. The decomposed fields are then analysed to examine the properties of each mode and their dynamics based on the transport equation for the total fluctuating enthalpy. The solenoidal fluctuation...

Small perturbation propagation in fluid flows is usually examined by linearizing the governing equations about a steady basic state. It is often useful, however, to study perturbation evolution in the unsteady evolving turbulent environment. Such analyses can elucidate the role of perturbations in the generation of coherent structures or the produc...

Nearfield intermittency and spatio-temporal correlations of a Mach 1.3 cold jet are analyzed to highlight the filtering and directivity of turbulence scales. The intermittent wavepackets at the end of the potential core are found to occur primarily between St = 0.5 and St = 1 from the Hilbert Spectrum and the spec-trogram. The component scales of c...

The effects of turbulent fluctuations on the axis and lip-line of a Mach 1.3 cold jet are analyzed with a synchronized pair of Large Eddy Simulations. The method uses two LESs to effectively boost naturally existing perturbations in one or more selected regions (" sources ") of the jet and track their evolution through the jet turbulence. The metho...

Jet noise prediction and control are of great interest to civil and military aviation because of health hazards, associated higher medical expenses and stricter noise regulations at airports. Decades of research has greatly improved understanding of noise generation dynamics, but significant gaps remain. In this work, a novel method is proposed to...

The spatial stability properties of a mixed convection boundary layer developed over a heated horizontal flat plate is investigated under the linear and quasi-parallel assumptions by solving the Orr-Sommerfeld equations. The main aim of this work is to find a critical buoyancy parameter above which a qualitative change in the stability characterist...