Arnab Samanta

Robustness of Acoustic Analogies for Predicting Mixing-Layer Noise

Acoustic analogies for the prediction of flow noise are exact rearrangements of the flow equations N(q) = 0 into a nominal sound source S(q) and sound propagation operator C such that L q = S(q). In practice, the sound source is typically modeled and the propagation operator inverted to make predictions. Because the rearrangement is exact, any sufficiently accurate model of the source will yield the correct sound, and so other factors must determine the merits of any particular formulation. Using data from a two-dimensional mixing-layer direct numerical simulation, we evaluate the robustness of several formulations to different errors intentionally introduced into the source. The motivation is that because S cannot be perfectly modeled, analogies that are less sensitive to errors in S are preferable. Our assessment is made within the framework of Goldsteins generalized acoustic analogy. A uniform base flow yields a Lighthill-like analogy, which we evaluate against a formulation in which the base flow is the actual mean flow of the direct numerical simulation and also against a globally parallel base flow that gives a Lilley-like analogy. The more complex mean-flow formulations are found to be significantly more robust to errors in the energetic turbulent fluctuations, but the advantage is less clear when errors are introduced at smaller scales.

Computational Modeling and Experiments of Natural Convection for a Titan Montgolfiere

Computational models are developed to predict the natural convection heat transfer and buoyancy for a Montgolfiere under conditions relevant to the Titan atmosphere. Idealized single and double-walled balloon geometries are simulated using algorithms suitable for both laminar and (averaged) turbulent convection. Steady-state performance results are compared to existing heat transfer coefficient correlations. The laminar results, in particular, are used to test the validity of the correlations in the absence of uncertainties associated with turbulence modeling. Some discrepancies are observed, especially for convection in the gap, and appear to be primarily associated with temperature nonuniformity on the balloon surface. The predicted buoyancy for the single-walled balloon in the turbulent convection regime, predicted with a standard k ǫ turbulence model, was within 10% of predictions based on the empirical correlations. There was also good agreement with recently conducted experiments in a cryogenic facility designed to simulate the Titan atmosphere.

Parabolized stability equation models for predicting large-scale mixing noise of turbulent round jets

Parabolized stability equation (PSE) models are being developed to predict the evolution of low-frequency, large-scale wavepacket structures and their radiated sound in highspeed turbulent round jets. Linear PSE wavepacket models were previously shown to be in reasonably good agreement with the amplitude envelope and phase measured using a microphone array placed just outside the jet shear layer. 1,2 Here we show they also in very good agreement with hot-wire measurements at the jet centerline in the potential core, for a different set of experiments. 3 When used as a model source for acoustic analogy, the predicted far field noise radiation is in reasonably good agreement with microphone measurements for aft angles where contributions from large-scale structures dominate the acoustic field. Nonlinear PSE is then employed in order to determine the relative importance of the mode interactions on the wavepackets. A series of nonlinear computations with randomized initial conditions are use in order to obtain bounds for the evolution of the modes in the natural turbulent jet flow. It was found that nonlinearity has a very limited impact on the evolution of the wavepackets for St � 0.3. Finally, the nonlinear mechanism for the generation of a low-frequency mode as the difference-frequency mode 4,5 of two forced frequencies is investigated in the scope of the high Reynolds number jets considered in this paper.

The robustness of acoustic analogies

Acoustic analogies for the prediction of flow noise are exact rearrangements of the flow equations N(right arrow q) = 0 into a nominal sound source S(right arrow q) and sound propagation operator L such that L(right arrow q) = S(right arrow q). In practice, the sound source is typically modeled and the propagation operator inverted to make predictions. Since the rearrangement is exact, any sufficiently accurate model of the source will yield the correct sound, so other factors must determine the merits of any particular formulation. Using data from a two-dimensional mixing layer direct numerical simulation (DNS), we evaluate the robustness of two analogy formulations to different errors intentionally introduced into the source. The motivation is that since S can not be perfectly modeled, analogies that are less sensitive to errors in S are preferable. Our assessment is made within the framework of Goldsteins generalized acoustic analogy, in which different choices of a base flow used in constructing L give different sources S and thus different analogies. A uniform base flow yields a Lighthill-like analogy, which we evaluate against a formulation in which the base flow is the actual mean flow of the DNS. The more complex mean flow formulation is found to be significantly more robust to errors in the energetic turbulent fluctuations, but its advantage is less pronounced when errors are made in the smaller scales.

Super-Resonances in AEDC Altitude Test Cells

We report on simulation and analysis efforts to understand and predict the high amplitude aeroacoustic resonances (& 160dB) that have been observed in Arnold Engineering Development Center (AEDC) jet engine test cells. The basic configuration of these cells is a jet exhausting through a cylindrical duct. We summarize progress in developing analytical tools to diagnose the energetics of the resonating acoustic modes, direct numerical simulations to investigate the phenomenology of the resonance in model geometries, and full-scale simulation tools to anticipate resonances.

On the axisymmetric stability of heated supersonic round jets

We perform an inviscid, spatial stability analysis of supersonic, heated round jets with the mean properties assumed uniform on either side of the jet shear layer, modelled here via a cylindrical vortex sheet. Apart from the hydrodynamic Kelvin–Helmholtz (K–H) wave, the spatial growth rates of the acoustically coupled supersonic and subsonic instability waves are computed for axisymmetric conditions (m=0) to analyse their role on the jet stability, under increased heating and compressibility. With the ambient stationary, supersonic instability waves may exist for any jet Mach number Mj≥2, whereas the subsonic instability waves, in addition, require the core-to-ambient flow temperature ratio Tj/To>1. We show, for moderately heated jets at Tj/To>2, the acoustically coupled instability modes, once cut on, to govern the overall jet stability with the K–H wave having disappeared into the cluster of acoustic modes. Sufficiently high heating makes the subsonic modes dominate the jet near-field dynamics, whereas the supersonic instability modes form the primary Mach radiation at far field.

Upstream radiation from supersonic buried-nozzle jets via scattering at the shroud edge

We investigate a scattering mechanism of upstream and sideline radiation from supersonic buried-nozzle jets, which for some conditions is comparable to the well-known, dominant downstream radiation from the Mach waves of such jets. In our buried-nozzle configuration, where the inner nozzle is buried within an outer nozzle-like shroud, the core inner nozzle jet is supersonic while the coflows are subsonic, with co-axial shear layers modeled as vortex sheets separating the respective jets. The mechanisms include incident perturbations in the form of tonal and multimodal shroud acoustic modes to scatter at the shroud edge and radiate sound waves. Depending upon the mix of acoustic modes in the incident perturbation, this radiation is shown to have the potential to generate a strong upstream component, comparable to the aft-angled Mach radiation.

MODELING AND SIMULATION OF HYDRODYNAMIC INTERACTION OF DNA IN A MICRO-FLUIDIC CHANNEL

The motion of DNA (in the bulk solution) and the non-Newtonian effective fluid behavior are considered separately and self-consistently with the fluid motion satisfying the no-slip boundary condition on the surface of the confining geometry in the presence of channel pressure gradients. A different approach has been developed to model DNA in the micro-channel. In this study the DNA is assumed as an elastic chain with its characteristic Youngs modulus, Poissons ratio and density. The force which results from the fluid dynamic pressure, viscous forces and electromotive forces is applied to the elastic chain in a coupled manner. The velocity fields in the micro-channel are influenced by the transport properties. Simulations are carried out for the DNAs attached to the micro-fluidic wall. Numerical solutions based on a coupled multiphysics finite element scheme are presented. The modeling scheme is derived based on mass conservation including biomolecular mass, momentum balance including stress due to Coulomb force field and DNA-fluid interaction, and charge transport associated to DNA and other ionic complexes in the fluid. Variation in the velocity field for the non-Newtonian flow and the deformation of the DNA strand which results from the fluid-structure interaction are first studied considering a single DNA strand. Motion of the effective center of mass is analyzed considering various straight and coil geometries. Effects of DNA statistical parameters (geometry and spatial distribution of DNAs along the channel) on the effective flow behavior are analyzed. In particular, the dynamics of different DNA physical properties such as radius of gyration, end-to-end length etc. which are obtained from various different models (Kratky-Porod, Gaussian bead-spring etc.) are correlated to the nature of interaction and physical properties under the same background fluid environment.

Numerical and Experimental Modeling of Natural Convection for a Cryogenic Prototype of a Titan Montgolfiere

Natural convection in a spherical geometry is considered for prediction of the buoyancy characteristics of one meter diameter single- and double-walled balloons in a cryogenic environment. The steady-state flow characteristics obtained by solving the ReynoldsAveraged Navier Stokes equations (RANS) with a standard k-e e e e model are used to determine the balloon performance in terms of net buoyancy as a function of heat input. Thermal radiation effects on the overall balloon performance are also investigated. The results obtained compared favorably with the corresponding cryogenic experiments conducted at the same scale in a cryogenic facility. In addition, both numerical and experimental results were compared with engineering heat transfer correlations used in system-level models of the Titan Montgolfiere. Finally, we examine scaling issues for the full-scale Titan Montgolfieres.

Acoustic reflection of vorticity waves at a shrouded-jet exit in "howling" resonances

At some operating conditions shrouded jets are observed to undergo an intense howling resonance. The coupling of vortical modes in the jet with acoustic modes at the shroud exit, which potentially close the resonance loop, are investigated with a vortex sheet model, which is solved using scalar Wiener-Hopf techniques. Peculiar sensitivities to flow parameters are observed that might explain the sporadic character of the observed howling resonance. So-called howling resonances have been observed in several shrouded jets, 1–4 all sharing the basic configuration shown in figure 1. Shocks cells in the jet are thought to sometimes play a role, 5 but for subsonic jets the mechanism is expected to involve interaction with the jet and its shroud. 1 To close a resonance, vortical jet structures must couple with the acoustic modes of the shroud. The most likely place for this to occur is at the exit, which is the focus of our investigation. We consider a semi-infinite cylindrical shroud with a co-flowing inner jet issuing out of it. The uniform inner, outer and ambient flows are separated by vortex sheets, with the outer vortex sheet originating at the shroud exit. A scalar Wiener–Hopf method is used to calculate how vorticity waves on the inner shear layer scatter at the shroud exit into reflected upstream propagating acoustic waves. The interaction of interest is fundamentally finite wavelength and apparently asymmetric, so high- or low-frequency approximations can not be made to simplify the analysis. This finite-wavelengths analysis builds upon an existing theoretical framework for edge scattering problems. 6–11