arXiv subjects

S. K. Karthick

Publications and source records attributed to S. K. Karthick.

15 recordsLinked to original sources

arXiv2026

Transonic flow past the complex cavity-sub-cavity configurations

The study investigates the physics of unsteady flow in complex cavity geometries operating in the transonic regime. A two-dimensional Detached Eddy Simulation (DES) approach is used for the preliminary analysis. The cavity configuration examined in this work arises from the integration of a scramjet engine with a launch vehicle. In this integrated geometry, the isolator section serves as a deep sub-cavity, while the Single Expansion Ramp Nozzle (SERN) constitutes the primary cavity. The combined arrangement therefore constitutes a complex cavity-sub-cavity system, which is referred to as such throughout the paper. The qualitative analysis revealed a feedback loop within the complex cavity-sub-cavity system, leading to high-pressure oscillations across the geometry. A monotonic increase in pressure loading is observed with increasing Mach number. Varying the cavity topology demonstrated that modifications to the primary cavity geometry strongly alter shear-layer dynamics and significantly affect the pressure distribution within the cavity-sub-cavity system. To mitigate adverse pressure oscillations, passive control strategies, including trailing-edge wall chamfering and a ventilated (slotted) sub-cavity, are investigated. Among the configurations studied, the slotted sub-cavity case exhibits the most pronounced suppression of pressure loads, particularly on the sub-cavity end wall. Spectral Proper Orthogonal Decomposition (SPOD) analysis also revealed the restructuring of dominant coherent modes in response to topological variations and to the implementation of passive control, providing insight into the underlying governing mechanism.

physics.flu-dyn↗

arXiv2026

Axisymmetric cavities in hypersonic flow

A detailed experimental campaign is conducted to investigate the shear layer characteristics of an axisymmetric open cavity exposed to a Mach $6$ freestream. Experiments are performed in a Ludwieg tunnel for varying Reynolds numbers ($23000\leq Re_D \leq 74000$) based on cavity depth ($D$). The effects of geometry are examined through length-to-depth ratios ($[L/D]=[2,4,6]$) and non-dimensional rear-face height differences ($[Δh/D]=[-0.5,-0.25,0,0.25,0.5]$). Shear layer evolution is interpreted using qualitative schlieren and Planar Laser Rayleigh Scattering (PLRS) along with quantitative unsteady pressure measurements. For all $[L/D]$, the shear layer remains laminar at low $Re_D$ and develops Kelvin-Helmholtz (K-H) vortices as $Re_D$ increases. For the longest cavity ($[L/D]=6$), transition to turbulence occurs at the highest $Re_D$ due to a longer K-H growth length. Spectral analysis of pressure signals and PLRS intensity shows a shift in dominant frequency from the first Rossiter mode to higher modes for $[L/D]=6$. Except for $[L/D]=6, [Δh/D]=0$, dominant frequencies agree with Rossiter predictions and remain largely Reynolds-number independent. Variation of $[Δh/D]$ leads to mode switching identified using POD of PLRS snapshots. Negative $[Δh/D]$ favors K-H modes (5th-6th Rossiter), whereas positive values promote a strong flapping mode (1st Rossiter) due to pressure build-up inside the cavity. At $[Δh/D]=0$, both modes may coexist depending on $Re_D$. Azimuthal measurements indicate dominant axisymmetric behavior in flapping cases and weaker correlation for K-H dominated shear layers.

physics.flu-dyn↗

arXiv2023

On the flow unsteadiness and operational characteristics of a novel supersonic fluidic oscillator

A novel supersonic jet oscillating method is investigated both experimentally and numerically. A rectangular primary supersonic jet is issued into a confined chamber with sudden enlargement. Secondary control jets are issued from the top, and bottom backwards-facing step regions formed due to sudden enlargement. The secondary jet also expands in the confined chamber shrouding the primary jet from the top and bottom sides. The primary jet is oscillated in the transverse direction by blowing the secondary jets in the streamwise direction in a pulsating manner with a phase shift. The out-of-phase secondary jet blowing causes the primary jet to periodically adhere to the upper and lower part of the confined chamber, causing flapping of the primary jet and acting as a supersonic fluidic oscillator. The supersonic jet oscillation characteristics are experimentally investigated using shadowgraph type flow visualization technique and steady and unsteady pressure measurements. Quantitative analysis of the shadowgraph images using the construction of $y-t$ and $y-f$ plots reveals the presence of periodic jet oscillation with a discrete dominant frequency similar to the secondary jet excitation frequency. The existence of linearity between the excitation frequency and the flapping jet frequency on the low-frequency ($0.66-6.6$ Hz) side is first proven experimentally. Later, the high-frequency ($16.67-5000$ Hz) operation extent of the supersonic fluidic oscillator is further demonstrated using unsteady computational studies owing to the existing experimental facility's limitations. It is found from the computational studies that there exists a limiting oscillation frequency for the present fluidic oscillator (nearly $4.486$ kHz with the particular geometric size and the injection momentum considered in the present study).

physics.flu-dyn↗

arXiv2022

Supersonic flow unsteadiness induced by control surface deflections

Control surface deployment in a supersonic flow has many applications, including flow control, mixing, and body-force regulation. The extent of control surface deflections introduces varying flow unsteadiness. The resulting fluid dynamics influence the downstream flow characteristics and fluid-structure interactions severely. In order to understand the gas dynamics, an axisymmetric cylindrical body with a sharp-tip cone at zero angles of attack ($α=0^\circ$) is examined in a free stream Mach number of $M_\infty=2.0$ and Reynolds number of $Re_{D}=2.16 \times 10^6$ ($D=50$ mm). Four static control surface deflection angles ($θ= π/36,π/6,π/3,π/2$, rad) are considered around the base body. The cases are computationally investigated through a commercial flow solver adopting a two-dimensional detached eddy simulation (DES) strategy. Recirculation bubble length, drag coefficient's variation, wall-static pressure statistics, acoustic loading on the model and the surroundings, $x-t$ trajectory and $x-f$ spectral analysis, pressure fluctuation's correlation coefficient on the model, and modal analysis are obtained to understand the flow unsteadiness. At $θ= [π/36]$, the wall-static pressure fluctuations behind the control surface are minimal and periodic, with a mere acoustic load of about 50 dB. At $θ= [π/2]$, a violent periodic fluctuation erupted everywhere around the control surface, leading to a higher acoustic load of about 150 dB (3 times higher than the previous). For $θ= [π/6]$ and $[π/3]$, high-frequency fluctuations with small and large-scale structures continuously shed along the reattaching shear layer, thereby causing a broadened spectra in the control surface wake.

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arXiv2022

A review of diaphragmless shock tubes for interdisciplinary applications

Shock tubes have emerged as an effective tool for applications in various fields of research and technology. The conventional mode of shock tube operation employs a frangible diaphragm to generate shockwaves. The last half-century has witnessed significant efforts to replace this diaphragm-bursting method with fast-acting valves. These diaphragmless methods have good repeatability, quick turnaround time between experiments, and produce a clean flow, free of diaphragm fragments in contrast to the conventional diaphragm-type operation. The constantly evolving valve designs are targeting shorter opening times for improved performance and efficiency. The present review is a compilation of the different diaphragmless shock tubes that have been conceptualized, developed, and implemented for various research endeavors. The discussions focus on essential factors, including the type of actuation mechanism, driver-driven configurations, valve opening time, shock formation distance, and operating pressure range, that ultimately influence the shockwave parameters obtained in the shock tube. A generalized mathematical model to study the behavior of these valves is developed. The advantages, limitations, and challenges in improving the performance of the valves are described. Finally, the present-day applications of diaphragmless shock tubes have been discussed, and their potential scope in expanding the frontiers of shockwave research and technology are presented.

physics.flu-dyn↗

arXiv2022

Unsteadiness in hypersonic leading-edge separation

Hypersonic leading-edge separation is studied towards understanding the varying shock-related unsteadiness with freestream Reynolds number ($1.66 \times 10^5 \leq Re_D \leq 5.85 \times 10^5$) in the newly constructed hypersonic Ludwieg tunnel (HLT) at a freestream design Mach number of $M_\infty=6.0$. An axisymmetric flat-face cylinder of base body diameter $D=35$ mm is fitted with protrusions of different fineness ($d/D=0.1,0.2,0.26,0.34$ at $L/D=1.4$) and slenderness ($L/D=0.7,1,1.4,1.9$ at $d/D=0.2$) ratio to induce a wide range of leading-edge separation intensities. Qualitative and quantitative assessments are made using schlieren imaging, planar laser Rayleigh scattering, and unsteady pressure measurements. A well-known to-and-fro shock motion called pulsation and a flapping shock-shear layer oscillation is observed as $Re_D$ changes. A shorter protrusion length ($L/D=0.7$) produces a pressure loading that is four orders higher than the cases with longer protrusion lengths. There exists a critical separation length ($L/D \geq 1.4$) beyond which the separated shear layer trips to turbulence and introduces fluctuations in the recirculation region as $Re_D$ increases. The effect of the separated turbulent shear layer is dampened by an order provided the reattachment angle is shallow by increasing the fineness ratio ($d/D=0.4$). There also exists a critical geometrical parameter ($L/D=1, d/D=0.2$) for which the unsteady modes switch between successive runs based on the upstream fluctuations. From the modal analysis of the Rayleigh scattering images, the first four dominant modes that drive flapping are identified as translatory flapping, sinuous flapping, large and small-scale shedding.

physics.flu-dyn↗

arXiv2022

Unsteady dynamics in a subsonic duct flow with a bluff body

A series of reduced-order numerical simulations on a specific bluff body type (v-gutters) in a subsonic duct flow is done to assess the unsteady wake dynamics. Two of the v-gutter's geometrical parameters are varied: the v-gutter's base angle ($θ$) and the size of a slit ($ξ$) at the leading-edge of the v-gutter. Turbulent flow kinematics and pressure field are analyzed to evaluate the unsteadiness at a freestream Mach number of $M_\infty = 0.25$ and a freestream Reynolds number based on bluff body's transverse length (L) of $Re_L=0.1 \times 10^6$. Five v-gutter angles are considered ($θ$, $^\circ = π/6, π/4, π/3, 5π/12, π/2$) and three slit sizes ($ξ$, mm =0,0.25,0.5) are considered only for a particular $θ= [π/6]$. In general, high fluctuations in velocity and pressure are seen for the bluffest body in consideration ($θ= π/2$) with higher drag ($c_d$) and total pressure loss ($Δp_0$). On the other hand, the presence of a slit on a streamlined body ($θ= π/6$) tends to efficiently stabilize the wake and thus, producing almost a periodic shedding structure with lower $c_d$ and $Δp_0$. For $θ= [π/6]$, broadened spectra in vortex shedding is seen with a peak at $[fL/u_\infty] \sim 0.08$. For $θ\geq [π/4]$, a dominant discrete shedding frequency is seen with a gradual spectral decay. Similarly, the effects of $ξ$ on the $θ= [π/6]$ case produce a discrete shedding frequency instead of a broadened one, as told before. The shedding frequency increases to a maximum of $[fL/u_\infty] \sim 0.26$ for the maximum slit size of $ξ= 0.5$. More insights on the shedding vortices, momentum deficit in the wake, varying energy contents in the flow field, and the dominant spatiotemporal structures are also provided.

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arXiv2021

Unsteady pulsating flowfield over spiked axisymmetric forebody at hypersonic flows

The paper gives experimental observations on the hypersonic flow past an axisymmetric flat-face cylinder with a protruding sharp-tip spike at a freestream Mach number of $M_\infty = 8.16$ at two different freestream Reynolds numbers based on the base body diameter ($Re_D = 0.76 \times 10^6$, and $3.05 \times 10^6$). Furthermore, modal analysis is done on schlieren images to understand the flow dynamics parallel with the unsteady pressure measurements. The protruding spike of length to base body diameter ratio of $[l/D]=1$ creates a familiar unsteady flowfield called 'pulsation.' Pressure loading and fluctuation intensity at two different $Re_D$ cases are calculated. A maximum drop of 98.24\% is observed in both parameters between the high and low ReD cases. Based on the analysis, a difference in the pulsation characteristics are noticed, which arise from two vortical zones, each from a system of two `$λ$' shocks formed during the `collapse' phase ahead of the base body. The interaction of shedding vortices from the $λ$-shocks' triple-points, along with the rotating stationary waves, contributes to the asymmetric high-pressure loading and the observation of shock pulsation on the flat-face cylinder. The vortical interactions form the second dominant spatial mode with a temporal mode carrying a dimensionless frequency ($f_2D/u_\infty \approx 0.34$) almost twice that of the fundamental frequency ($f_1D/u_\infty \approx 0.17$). The observed frequencies are invariant irrespective of the ReD cases. However, for the high-frequency range, the spectral pressure decay is observed to follow an inverse and -7/3 law for the low and high $Re_D$ cases, respectively.

physics.flu-dyn↗

arXiv2021

On the unsteady dynamics of partially shrouded compressible jets

We experimentally investigate a partially shrouded sonic jet (a sonic free-jet shielded by a solid wall-extension on one side) exiting from a planar nozzle at two different nozzle pressure ratio ($ζ=4$ and $5$). We experimentally show that the inherent jet unsteadiness from the shock-induced flow separation on the wall and the emitted noise in the far-field is strongly coupled through a series of experiments like high-speed schlieren, wall-static pressure, unsteady pressure spectra, and microphone measurements. The partially shrouded jet's lateral free expansion is also identified to be complicated, three-dimensional, and the produced noise is directional. The emitted acoustic pulses from the flapping-jet, the radiated noise from the shock-induced separation on the wall, and the shock-shear layer interaction on the other side of the wall are responsible for the generated acoustic disturbances. The non-uniform aeroacoustic forcing on the top and bottom portion of the partially wall-bounded jet shear layer leads to a self-sustained jet oscillation and a discrete sound emission. The vital features are identified through the proper orthogonal decomposition of high-speed schlieren images and supplemented by other measurements.

physics.flu-dyn↗

arXiv2021

Shock and shear layer interaction in a confined supersonic cavity flow

The impinging shock of varying strengths on the free shear layer in a confined supersonic cavity flow is studied numerically using the detached-eddy simulation. The resulting spatiotemporal variations are analyzed between the different cases using unsteady statistics, $x-t$ diagrams, spectral analysis, and modal decomposition. A cavity of length to depth ratio $[L/D]=2$ at a freestream Mach number of $M_\infty = 1.71$ is considered to be in a confined passage. Impinging shock strength is controlled by changing the ramp angle ($θ$) on the top-wall. The static pressure ratio across the impinging shock ($p_2/p_1$) is used to quantify the impinging shock strength. Five different impinging shock strengths are studied by changing the pressure ratio: $1.0,1.2,1.5,1.7$ and $2.0$. As the pressure ratio increases from 1.0 to 2.0, the cavity wall experiences a maximum pressure of 25% due to shock loading. At [$p_2/p_1]=1.5$, fundamental fluidic mode or Rossiter's frequency corresponding to $n=1$ mode vanishes whereas frequencies correspond to higher modes ($n=2$ and $4$) resonate. Wavefronts interaction from the longitudinal reflections inside the cavity with the transverse disturbances from the shock-shear layer interactions is identified to drive the strong resonant behavior. Due to Mach-reflections inside the confined passage at $[p_2/p_1]=2.0$, shock-cavity resonance is lost. Based on the present findings, an idea to use a shock-laden confined cavity flow in an enclosed supersonic wall-jet configuration as passive flow control or a fluidic device is also demonstrated.

physics.flu-dyn↗

arXiv2020

On the unsteady throttling dynamics and scaling analysis in a typical hypersonic inlet-isolator flow

The flow field in a two-dimensional three-ramp hypersonic mixed-compression inlet in a freestream Mach number of $M_\infty=5$ is numerically solved to understand the unsteady throttling dynamics. Throttling conditions are simulated by varying the exit area of the isolator in the form of plug insets. Different throttling ratios between $0\leq ζ\leq 0.7$ in steps of 0.1 are considered. No unsteadiness is observed for $ζ\leq 0.2$ and severe unsteadiness is found for $0.3 \leq ζ\leq 0.7$. The frequency of unsteadiness ($f$) increases rapidly with $ζ$. As $ζ$ increases, the amount of reversed mass inside the isolator scales with the frequency and the exit mass flow rate. A general framework is attempted to scale the unsteady events based on the gathered knowledge from the numerical study. The inlet-isolator flow is modeled as an oscillating flow through a duct with known upstream design conditions like the freestream Mach number ($M_\infty$) and the isolator inlet Mach number ($M_i$). Factors like the mass occupied by the duct volume, the characteristic unsteady frequency, throttling ratio, and the exit mass flow rate through the duct are used to form a non-dimensional parameter $β$, which scales with the upstream design parameter $ξ=M_i/M_\infty$. The scaling parameters are further exploited to formulate a semi-empirical relation using the existing experimental results at different throttling ratios from the open literature. The unsteady frequencies from the present two-dimensional numerical exercise are also shown to agree with the proposed scaling and the resulting semi-empirical relation.

physics.flu-dyn↗

arXiv2020

Shock related unsteadiness of axisymmetric spiked bodies in the supersonic flow

Shock related unsteadiness over axisymmetric spiked body configurations is experimentally investigated at a freestream supersonic Mach number of 2.0 at 0$^\circ$ angle of attack. Three different forebody configurations mounted with a sharp spike-tip ranging from blunt to streamlined (flat-face, hemispherical, and elliptical) are considered. Steady and unsteady pressure measurements, short-exposure high-speed shadowgraphy, shock footprint analysis from $x-t$ plots, and identification of dominant spatiotemporal modes through modal analysis are carried out to explain the unsteady flow physics. The present investigation tools are validated against the well-known events of `pulsation' associated with the flat-face case. The hemispherical case is characterized by the formation of a separated free shear layer and associated localized shock oscillations. The cycle of charging and ejection of fluid mass from the recirculation zone, confined between the separated shear layer and the spiked body, is identified to drive the flow unsteadiness. Such an event triggers the out-of-phase motion between the separated and reattachment shocks. In the elliptical case, the overall flow field resembles that of the hemispherical case, except with dampened unsteadiness. The value of the cone angle ($λ$) associated with the recirculation region is found to be responsible for the fluctuations from the charging and ejection of fluid mass. Thereby it controls the extent of out-of-shock phase motion. In the elliptical case, $λ$ is observed to be smaller and exhibits a reduction in shock unsteadiness. \hlt{Based on the gathered results and understanding, the reduction in unsteadiness associated with the aerodisk mounted on the hemispherical forebody is explained via the almost complete elimination of the out-of-phase shock motion.

physics.flu-dyn↗

arXiv2020

On the fluidic behavior of an over-expanded planar plug nozzle under lateral confinement

The present work aims to study the fluidic behavior on lateral confinement by placing side-walls on the planar plug nozzle through experiments. The study involves two cases of nozzle pressure ratio (NPR=3, 6), which correspond to over-expanded nozzle operating conditions. Steady-state pressure measurements, together with schlieren and surface oil flow visualization, reveal the presence of over-expansion shock and subsequent interaction and modification of the flow field on the plug surface. The flow remains attached to the plug surface for NPR=3; whereas, for NPR=6, a separated flow field with a recirculation bubble is observed. Spectral analysis of the unsteady pressure signals illustrates a clear difference between the attached and the separated flow. Besides, other flow features with a distinct temporal mode associated with and without lateral confinement are observed. The absence of lateral confinement reduces the intensity of low-frequency unsteadiness; however, on the contrary, the interaction region is relatively reduced under lateral confinement.

physics.flu-dyn↗

arXiv2020

Elliptic supersonic jet morphology manipulation using sharp-tipped lobes

Elliptic nozzle geometry is attractive for mixing enhancement of supersonic jets. However, jet dynamics, such as flapping, gives rise to high-intensity tonal sound. We experimentally manipulate the supersonic elliptic jet morphology by using two sharp-tipped lobes. The lobes are placed on either end of the minor axis in an elliptic nozzle. The design Mach number and the aspect ratio of the elliptic nozzle and the lobed nozzle are 2.0 and 1.65. The supersonic jet is exhausted into ambient at almost perfectly expanded conditions. Time-resolved schlieren imaging, longitudinal and cross-sectional planar laser Mie-scattering imaging, planar Particle Image Velocimetry, and near-field microphone measurements are performed to assess the fluidic behavior of the two nozzles. Dynamic Mode and Proper Orthogonal Decomposition (DMD and POD) analysis are carried out on the schlieren and the Mie-scattering images. Mixing characteristics are extracted from the Mie-scattering images through the image processing routines. The flapping elliptic jet consists of two dominant DMD modes, while the lobed nozzle has only one dominant mode, and the flapping is suppressed. Microphone measurements show the associated noise reduction. The jet column bifurcates in the lobed nozzle enabling a larger surface contact area with the ambient fluid and higher mixing rates in the near-field of the nozzle exit. The jet width growth rate of the two-lobed nozzle is about twice as that of the elliptic jet in the near-field, and there is a 40\% reduction in the potential core length. Particle Image Velocimetry (PIV) contours substantiate the results.

physics.flu-dyn↗

arXiv2020

Parametric experimental studies on the shock related unsteadiness in a hemispherical spiked body at supersonic flow

Experimental studies are carried out to investigate the effects of the geometrical parameters with a drag reducing spike on a hemispherical forebody in a supersonic freestream of $M_\infty=2.0$ at $0^{\circ}$ angle of attack. The spike length $(l/D=0.5,1.0,1.5,2.0)$, spike stem diameter $(d/D=0.06,0.12,0.18)$, and spike tip shapes are varied and their influence on the time-averaged, and time-resolved flow field are examined. When $l/D$ increases, a significant reduction in drag ($c_d$) is achieved at $l/D=1.5$, whereas the variation in $d/D$ has only a minor effect. The intensity of the shock-related unsteadiness is reduced with an increase in {$d/D$ from $0.06$ to $0.18$}, whereas changes in $l/D$ have a negligible effect. The effects of spike tip geometry are studied by replacing the sharp spike tip with a hemispherical one having three different base shapes (vertical base, circular base, and elliptical base). Hemispherical spike tip with a vertical base is performing better by reducing $c_d$ and flow unsteadiness. The dominant Spatio-temporal mode arising due to the shock-related unsteadiness is represented through modal analysis of time-resolved shadowgraph images and the findings are consistent with the other measurements.

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