Research ^indicates active research areas

Aeroacoustics^

Computational & Experimental Jet Noise^

FIGURE 1: Schematic of twin jet numerical simulations

There are several problems we are currently working in jet noise, spanning modeling, numerical simulations and experiments.

  1. Numerical simulations of merging twin jets: Such configurations are fairly common in space launch vehicles and military aircrafts. We are interested in the mechanistic understanding of the overall noise sources and their impact on radiated sound leading to perhaps more efficient control strategies via reduced-order modeling. In collaboration with Prof. Santosh Hemchandra (IISc), we have developed an explicit-filter based LES (EFLES) and Lighthill's analogy for aeroacoustic computations. We consider a pair of identical subsonic jets (schematic in FIGURE 1) with a Spectral POD (SPOD) technique used to construct the reduced-order models for the sound sources. Our computations will be later extended for supersonic twin jets too.

  2. Effect of nozzle boundary layer on radiated sound: We are exploring the effect of the turbulent boundary layer inside the jet nozzle on the radiated sound via high-resolution EFLES simulations coupled with a new Ffowcs Williams-Hawkings based analogy solver for the sound (see FIGURE 2). Initially this is done for a subsonic, high-Re jet, eventually extended to an imperfectly-expanded supersonic nozzle. The focus is to understand how the nature of turbulent BL is directly correlated to the different radiation mechanisms of such jets.

  3. Experiments on jet noise control: We are initiating experiments with subsonic jets subjected to both passive (chevrons) and active control (microjets) techniques to better understand the role of such methods in mitigating jet noise with the best efficiency. This work is a collaborating with Dr. Arun Perumal (IITK). As a first step an anechoic chamber has been designed, installed and calibrated for testing jet nozzles of various forms.

FIGURE 2: Subsonic high-Re jet with a nozzle & the corresponding FW-H surface

Selected Publications:

  1. Vempati, C., Hemchandra, S. & Samanta, A. 2022 The influence of nozzle-exit boundary-layer state on evolution and radiation of wavepackets in subsonic jets. AIAA Aeroacoustics Conference, Southampton, UK, Jun 2022. AIAA paper no. 2022-3068.

  2. Muthichur, N., Hemchandra, S. & Samanta, A. 2021 Acoustic radiation of coherent structures in turbulent round jets. AIAA Aviation 2021 Forum, Virtual Event, Aug 2021. AIAA paper no. 2021-2245

  3. Muthichur, N., Hemchandra, S., Tummalapalli, H. & Samanta, A. 2020 Sources of sound and its radiation from twin turbulent jets. AIAA Scitech 2020 Forum, Orlando, Florida, USA, Jan 2020. AIAA paper no. 2020-1245

Aeroacoustic Scattering

FIGURE 3: Scattering of vorticity & supersonic instability modes in a cold supersonic jet

We use the Wiener-Hopf method to analytically solve a large class of problems (i.e. PDEs), primarily involving aeroacoustic scattering. Past focus has been on subsonic and supersonic shrouded (ducted) jets with coflow (FIGURE 3) while ongoing effort is on applying these techniques to flow control & related areas. Our past work in this area was the first analytical demonstration of a vortical wave scattering. For supersonic jets, we identified several scattering mechanisms, some unique (see FIGURE 3), which has shown even perfectly-expanded supersonic jets to support strong upstream radiation, resembling the screech tones of under-expanded jets but radiated over broader band of angles.

Selected Publications:

  1. Samanta, A. & Freund, J. B. 2015 A model supersonic buried-nozzle jet: instability and acoustic wave scattering and the far-field sound. J. Fluid Mech., 778, 189–215

  2. Samanta, A. & Freund, J. B. 2008 Finite-wavelength scattering of incident vorticity and acoustic waves at a shrouded jet exit. J. Fluid Mech., 612, 407–438.

Fan Noise^

FIGURE 4: Various types of noise sources in typical open rotor configurations [Courtesy: Rolls Royce]

In fan noise research, we have been studying the open rotor configuration (see FIGURE 4) which has potential due to its improved SFC compared to shrouded configurations. These are not without significant aeroacoustic challenges. Specifically, we are studying the Unsteady Distortion Noise (UDN) and Tip-Vortex Interaction (TVI) Noise, two of the major sources in multi-row rotor configurations that decide on the overall radiated sound spectra using analytical means. Eventually, we would focus on designing improved reduced-order models starting from the classical work of Ffowcs Williams, Amiet and others.

Reacting Mixing Layers

FIGURE 5: Sound radiation from a "fast mode" and "co-layer mode" in a turbulent reacting mixing layer

Linear Parabolized Stability Equations (PSE) are used to model both the near-field dynamics and the radiated sound for supersonic transitional mixing layers. The mean transitional flow is modeled via a composite spreading model – a novel concept. The work demonstrates (see FIGURE 5) increased heat release beyond a certain point to actually increase the instability and radiated sound in mixing layers, owing to the dominant outer modes, with the mixing layer thickness (δ) given via the composite model. This is in stark contrast to expectations from K–H modes, which attenuates/weakens at these higher compressibility levels and hence are expected to yield reduced sound levels.

Selected Publication:

Chary, P. S. & Samanta, A. 2016 Linear models for sound from supersonic reacting mixing layers. Phys. Rev. Fluids, 1, 083801

Acoustic Analogy

FIGURE 6: Effect on radiated sound due to errors introduced into the sound sources of a mixing layer DNS

In the past, we did some work to investigate robustness of several acoustic analogies by introducing finite errors inside the source terms. As the sources are never perfectly realizable, the idea was to find analogies that are less sensitive to errors and hence are preferable. The analysis revealed, quite expectedly, the analogies based on a more complex mean flow description (e.g. Goldstein's analogy) to be more robust to errors in their energetic fluctuations, while the simpler ones (e.g. Lighthill's analogy) can produce large errors in computed sound (see the bottom FIGURE 6).

Selected Publication:

Samanta, A., Freund, J. B., Wei, M. & Lele, S. K. 2006 Robustness of acoustic analogies for predicting mixing-layer noise. AIAA J., 44 (11), 2780–2786