Direct numerical simulations (DNS) are used to study the evolution of hairpin vortices to understand the late-stage turbulent breakdown of vortex rings. Several hairpin initial configurations with Reynolds number italic Re equals 1500 Re = 1500 $\textit{Re} = 1500$ were considered, both isolated and multiple. The isolated hairpins evolve in a quiescent flow, demonstrating reconnection events in stages (bridging, cut and reconnect, rapid separation of two parts, residual threads), similar to prior observations in antiparallel vortices. Next, multiple configurations of six hairpin vortices in a circular array are simulated, with the background flow extracted from the DNS of the near wake of a vortex ring undergoing azimuthal instability. Here, the alignment of vorticity at the tips with that of the azimuthal component of background flow vorticity is found to be an important parameter. When the vorticity components are opposite, hairpin tips deflect inward so that neighbouring hairpins also participate in reconnection events, yielding a complex set of reconnections, further amplified for staggered hairpins. Otherwise, hairpin tips deflect outwards and evolve much as an isolated hairpin. While higher italic Re Re $\textit{Re}$ makes the onset and reconnections faster, the extent of vortex stretching in the specific background flow relative to the viscous dissipation decides the temporal evolution of total enstrophy. It rises sharply just before reconnection events, falling rapidly afterwards, while for the DNS-extracted ring wake flow, the maximum enstrophy is higher compared with the simpler shear flows. Reconnection events distribute the energy over a broader spectrum, especially to higher wavenumbers.

We experimentally investigate the effects of fluidic injection via min.ets on a Mach 0.5 jet by focusing on the radiated sound while also exploring the near-field hydrodynamics. In this work, two types of injection systems are reported. One of them is located upstream of the nozzle exit with two fluidic injectors located diametrically opposite, operated steadily for different mass flow ratios. The second system is a newly designed fluidic injection system that can do both steady and unsteady injections at jet downstream locations. Our second reported case uses this latter system with two active injectors similar to the upstream setup. For the cases reported, fluidic injection alters the centreline velocity decay and its turbulent fluctuations via reducing the corresponding Reynolds stresses. For the downstream injection, this is translated to reduced peak sound at all directivity angles, but especially at the downstream shallow angles. For the upstream injected jet, the radiated sound at all angles mostly remains unaltered.

The nature of the wavepackets in the nozzle boundary layer and their influence on the wavepackets downstream of the nozzle exit is studied in a $M=0.9$, $Re_{D}=10^{6}$ jet issued from a cylindrical nozzle with a transitional boundary layer. Coherent wavepackets are educed from LES data by performing Spectral Proper Orthogonal Decomposition (SPOD) in a global sense with the nozzle and jet flowfields considered together. We find that dynamics in the near-nozzle exit region is modulated by three types of coherent structures: wavepackets in the nozzle boundary layer, KH-type wavepackets in the initial shear layer region and the trapped acoustic modes inside the potential core of the jet. Further, the wavepackets in the nozzle boundary layer are directly linked to the KH-type wavepackets in the initial shear layer of the jet. We find that the wavepackets in the nozzle boundary layer are characterised by a change of phase in the wall-normal direction consistent with observations in previous investigations in the literature. Further, these structures become stretched and perturbed as they approach the nozzle exit possibly due to disturbances from the nozzle exit and scattering from the nozzle lip.

We construct reduced-order models of aeroacoustic sources for single and twin subsonic jets ( $M_j=0.9$ , $Re=3600$ ), with the goal of accurately recovering the far-field sound over a wide band of frequencies $St=[0.07,1.0]$ and directivity angles $\phi = [30^{\circ },120^{\circ }]$ within a subdecibel level accuracy. These models are realized via combining spatio-temporally coherent spectral proper orthogonal decomposition (SPOD) modes extracted directly from Lighthill's stress tensor, itself calculated using large-eddy simulation (LES). We consider two sets of twin subsonic jets of diameter $D$ each, with spacings of $0.1D$ and $1D$ , where the jets merge upstream and downstream of breakdown, respectively. The closely spaced twin jet decays the slowest due to reduced turbulent stresses which are, however, more broadband due to early merging. Such jets show strong shielding in the plane of jets, especially at shallow directivity angles where sound levels may drop below that of the single jet. The farther spaced twin jets have dynamics more akin to the constituent single jet with turbulent fluctuations peaking here at $St=0.34$ , but showing very little shielding, with their overall sound pressure level (OASPL) mostly linked to the nature of extra flow structures created during merging. Three-dimensional, energy-ranked, coherent structures for twin jets exhibit rather poor low-rank behaviour, especially at the far-field spectral peak $St=0.14$ . At $St \gtrsim 0.3$ , the SPOD wavepackets show strong visual coherence, resembling Kelvin–Helmholtz instability modes upstream of breakdown, while at the lower frequencies, there is very little spatial coherence with wavepackets peaking downstream of breakdown. Although the leading SPOD modes radiate poorly, reduced-order models using a subset of them, up to $45$ SPOD modes per frequency, show a remarkable match (within $1$ dB) against the LES-predicted sound over $0.1 \lesssim St \lesssim 0.5$ , at all angles investigated. At other frequencies, the closely spaced twin jet shows more error, due to its greater hierarchy of spatio-temporal structures, showing slower convergence at the shallower angles.

The influence of the nozzle lip on the potentially-radiating turbulent structures in its vicinity is investigated in addition to investigating the radiating characteristics of the small-scale turbulent structures in the nozzle boundary layer. Studies are conducted by considering two sets of permeable Ffowcs Williams and Hawkings (FW-H) surfaces which instead of encapsulating the nozzle are inside it and their farfield spectra is compared against the ``true'' farfield spectra of the configuration. In the first set of FW-H surfaces, the nozzle lip is outside the FW-H surface while in second set, turbulent structures in the nozzle boundary layer are left outside it. We find that the nozzle lip has no role in the farfield spectra for $St <\approx 2$ and thus for peak frequencies. At high frequencies, significant deviations from the true spectra of the configuration are found due to not including the nozzle lip in the FW-H surface. We also observe that turbulent structures inside the nozzle radiate to the far-field at mid to high frequencies ($St >\approx 0.7$). Comparisons against previous studies suggest that the ability of the turbulent structures inside the nozzle to radiate is a function of Mach number of the flow. Further, both the small-scale structures in the nozzle and near-nozzle lip sources do not contribute to low frequency noise.

Within the Lab Directed R&D Program at Fermilab, and in partnership with National Magnetics, we have recently begun to study and attempt to improve the loss parameter in garnet material. This could be used for fast tuner applications such as in rapid cycling synchrotrons.

The acoustic radiation emanating from the coherent structures present in a $Re=3600$, $M=0.9$ free turbulent round jet is investigated by obtaining a modal decomposition of the acoustic sources modelled by the Lighthill stress tensor. The acoustic sources in the jet are computed by performing a Large Eddy Simulation which employs explicit filtering, while the acoustic far-field is predicted using the standard Lighthill's acoustic analogy. For analyzing the nature of the acoustic sources, the Lighthill stress tensor is decomposed azimuthally and each of these components is further subjected to a Spectral Proper Orthogonal Decomposition (SPOD) . The acoustic radiation from each of these SPOD modes is then computed to determine which of these modes have significant contribution to the far-field radiation. Analysis of the SPOD modes show the presence of energetic wavepacket structures in the flow which do not radiate to the far-field. The radiating wavepacket structures are isolated from the non-radiating ones by using the fact that only wavepacket structures with a supersonic phase speed radiate to the far-field. From this analysis, it is also found that $T_{11}$ is the dominant component of the Lighthill stress tensor for acoustic radiations at shallower angles and lower Strouhal numbers. The above analysis is extended to a twin jet with a spacing of $0.1D$ between their inner shear layers. Time-frequency analysis of the twin jet configuration is done using Wavelet Proper Orthogonal Decomposition (WPOD) to show the time dependent flow oscillatory behaviour of the wavepackets generated from these jets.

The aeroacoustics of interacting identical twin subsonic cold jets is studied, with an aim to explore the influence of jet shear layer interaction and breakdown of the resulting coherent structures into turbulence on the spectral characteristics of the radiated far-field sound. In this work, the point of first interaction between the pair of jets is varied by changing their centerline spacing. The cases studied reflect two scenarios where the jets interact upstream and downstream respectively of their individual potential core breakdown locations. The turbulent near-field of the jets is computed using a large eddy simulation (LES), while the acoustic far-field is obtained by using the standard Lighthill's acoustic analogy. Our solvers are validated against prior direct numerical simulations (DNS) and experiments of a single jet configuration. The results for the twin jets show that the far-field acoustic power spectral density for the $1D$ jet to be comparable to or lower than that for the $0.1D$ jet for both shallow and sideline angles, while the single jet is much quieter than either of them at these angles. A decomposition of the Lighthill sources determined from LES into shear and self noise source components shows that the former dominates the source term in all cases studied, while some differences in the self noise between the twin jets do not seem to yield much difference in the corresponding radiated sound, even at sideline angles.

We perform inviscid and viscous, global, linear stability analyses of vortex rings which are compared with asymptotic theories and numerical simulations. We find growth rates of rings to be very sensitive to the details of vorticity distribution, in a way not accounted for in asymptotic theories, clearly demonstrated in our analyses of equilibrated rings-ring base flows initially obtained from Gaussian rings evolved to a quasi-steady state before any instabilities set in. Such equilibrated rings with the same epsilon = a/R, the ratio of core radius a to ring radius R, but evolved with different viscosities, have inviscid growth rates differing by up to 9%, though the differences in vorticity at any point are small. In contrast, the growth rates of rings with a Gaussian vorticity distribution are found to be up to 33% smaller than the inviscid asymptotic theories over 0.4 > epsilon > 0.05. We attribute these differences to the nature of velocity fields at O(epsilon(2)), between equilibrated and Gaussian rings, where the former shows a good quantitative match with the asymptotic theories. Additionally, there are some differences with previous direct numerical simulations (DNS), but in very close quantitative agreement with our DNS results. Our calculations provide a new relation capturing the near-linear dependence of growth rates on the reciprocal of a strain rate-based Reynolds number (R) over cape. Importantly, our equilibrated ring calculations do tend to the inviscid limit of asymptotic theories, once corrections for ring radius evolution and equilibrated distribution are imposed, unlike for Gaussian rings.

We obtain linearized, BiGlobal thermoacoustic solutions in a pulse tube driven via an imposed mean temperature gradient. Here, the pulse tube is treated as a key unit of a thermoacoustic heat engine, in which the conversion of thermal energy to useful acoustic fluctuations occurs. A primary goal of this work is to understand the hydrodynamic efficiency of the energy conversion process and how it depends upon some of the important operating parameters, including the geometry of the device which in the limit of long length-to-diameter ratio approaches the so-called narrow tube approximation. As this limit is frequently imposed in the wave propagation analyses of thermoacoustic devices, it is critical to investigate the physical connections of such a model to more realistic finite-length pulse tube configurations, which we do here. The mean flow is quiescent with an analytic mean temperature profile that still models the necessary physical details of the hot heat exchanger and regenerator. The computed thermoacoustic oscillations are found to be globally stable, approaching neutral stability conditions at the narrow tube limit. In finite-length tubes, three distinct types of modes are identified and analyzed. Here, within a linear framework, radial modes do appear to act as key enablers for longitudinal modes to be the primary carriers of acoustic energy from the pulse tube section, while the identified boundary modes, essentially numerical constructs, are ignored in the analysis. Further, a disturbance energy-based efficiency metric is constructed that provides mechanistic understanding of some of the key parameters in pulse tube operation. For finite-length tubes, it shows oscillations of the first asymmetric mode to be the most efficient, while the axisymmetric perturbations dominate for longer tubes that eventually lead to the idealized plane wave propagation.

We investigate the existence of short-time, local transient growth in the helical modes of a rapidly swirling, high-speed jet that has transitioned into an axisymmetric bubble breakdown state. The time-averaged flow consisting of the bubble and its wake downstream constitute the base state, which we show to exhibit strong transient amplification owing to the non-modal behaviour of the continuous eigenspectrum. A pseudospectrum analysis mathematically identifies the so-called potential modes within this continuous spectrum and the resultant non-orthogonality between these modes and the existing discrete stable modes is shown to be the main contributor to such growth. As the swirling flow develops post the collapsed bubble, the potential spectrum moves further toward the unstable half-plane, which along with the concurrent weakening of exponential growth from the discrete unstable modes, increases the dynamic importance of transient growth inside the wake region. The transient amplifications calculated at several locations inside the bubble and wake confirm this, where strong growths inside the wake far outstrip the corresponding modal growths (if available) at shorter times, but especially at the higher helical orders and smaller streamwise wavenumbers. The corresponding optimal perturbations at initial times consist of streamwise streaks of azimuthal velocity, which if concentrated inside the core vortical region, unfold via the classical Orr mechanism to yield structures resembling core (or viscous) Kelvin waves of the corresponding Lamb–Oseen vortex. However, in contrast to that in Lamb–Oseen vortex flow, where critical-layer waves are associated with higher transient gains, here, such core Kelvin modes with the more compact spiral structure at the vortex core are seen to yield the maximum transient amplifications.

We investigate the spatial stability of compressible, viscous pipe flows with radius-dependent mean density profiles, subjected to solid body rotations. For a fixed Rossby number $\unicode[STIX]{x1D716}$ (inverse of the rotational speed), as the Reynolds number $Re$ is increased, the flow transitions from being stable to convectively unstable, usually leading to absolute instability. If flow compressibility is unimportant and $Re$ is held constant, there appears to be a maximum $Re$ below which the flow remains stable irrespective of any rotational speed, or a minimum azimuthal Reynolds number $Re_{\unicode[STIX]{x1D703}}$ $(=Re/\unicode[STIX]{x1D716})$ is required for any occurrence of absolute instabilities. Once compressible forces are significant, the effect of pressure–density coupling is found to be more severe below a critical $Re$, where as rotational speeds are raised, a stable flow almost directly transitions to an absolutely unstable state. This happens at a critical $Re_{\unicode[STIX]{x1D703}}$ which reduces with increased flow Mach number, pointing to compressibility aiding in the instability at these lower Reynolds numbers. However, at higher $Re$, above the critical value, the traditional stabilizing role of compressibility is recovered if mean density stratification exists, where the gradients of density play an equally important role, more so at the higher azimuthal modes. A total disturbance energy-based formulation is used to obtain mechanistic understanding at these stability states, where we find the entropic energy perturbations to dominate as the primary instability mechanism, in sharp contrast to the energy due to axial shear, known to play a leading role in incompressible swirling flows.

The quality of working environment impacts employee morale, attitude productivity and engagement both positively and negatively. Poorly designed workstations are prone to occupational disease and it impacts on employee’s performance. How well they engage with the organization, especially with their immediate environment, influences to a great extent their error rate, level of innovation and collaboration with other employees, absenteeism and ultimately, how long they stay in the job. To meet the standards of organization employees need a working environment that allows them to work freely without problems that may restrain them from performing upto the level of their full potential. This paper tries to identify the factors of the working environment and its impact on the employees working in the IT sector in Kolkata.

Received 6 February 2017DOI:https://doi.org/10.1103/PhysRevFluids.2.029901©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAerodynamic noiseFlow instabilityReacting flowsShear flowsFluid Dynamics