Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Z10: Compressible Flows: Instability and Turbulence |
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Chair: G Sidharth, Iowa State University; Nek Sharan, Auburn University Room: 137 |
Tuesday, November 22, 2022 12:50PM - 1:03PM |
Z10.00001: Stationary global instabilities in shock wave/ hypersonic laminar boundary layer interaction G S Sidharth, Anubhav Dwivedi Instabilities in canonical two-dimensional shock-wave boundary layer interactions can provide insight into three-dimensional flow structures and their wall imprints. We identify and report two distinct stationary instabilities in steady two-dimensional shock boundary layer interaction as the free-stream Reynolds number is increased. While both instabilities are associated with three-dimensional flow in the recirculation bubble, the long span instability marks the development of recirculation span cells and the short span instability marks the split of the recirculation core. The appearance of distinct spanwise length scales associated with the two instabilities is confirmed with experimental observations. The physics of perturbation growth is analyzed for both global instabilities and perturbation compressibility is found to play a role in the mechanism. Lastly, we compare and contrast the behavior of the global instabilities with the Gortler instability prevalent in the region of large curvature near boundary layer reattachment. |
Tuesday, November 22, 2022 1:03PM - 1:16PM |
Z10.00002: Prediction of boundary-layer instability in supersonic flow with roughness and shock using machine learning Olaf Marxen, Athanasios T Margaritis, Tim J Flint, Peter J Schmid, Gianluca Iaccarino Numerical simulations of a Ma=4.8 flat-plate boundary layer with discrete roughness element shows an increase of convective disturbance amplification caused by the roughness. Such increase has important implications for the transition location, but cannot be accommodated by current transition prediction methods. We present a new tool to predict the effect of two-dimensional roughness on boundary-layer perturbations, which can account for roughness geometry and location. This tool is based on machine learning techniques such as Gaussian process regression and neural networks. Training data is derived from datasets obtained using O(100) simulations. Prior to training, time-dependent simulation data is postprocessed for data reduction such that for instance only the perturbation's Fourier amplitude is used for training. The high computational cost of generating the simulation dataset is counterbalanced by the speed of prediction, making the tool suitable for applications related to the operation of (e.g. re-entry) vehicles rather than their design. A main benefit of the tool is that it provides transferability, using the existing data to predict cases that have not been simulated (different roughness geometry, location). |
Tuesday, November 22, 2022 1:16PM - 1:29PM |
Z10.00003: Deep learning and variational assimilation of experimental measurements in simulations of hypersonic transition on a cone Pierluigi Morra, Pierluigi Morra, Charles Meneveau, Tamer A Zaki Transition to turbulence in high-speed boundary layers is extremely sensitive to the disturbance environment, which is uncertain. As a result, computational studies often focus on canonical transition scenarios and qualitative comparisons to experiments. In contrast, data assimilation can enable us to estimate the uncertain upstream disturbances whose evolution reproduces the available measurements. We will introduce a data assimilation strategy that combines deep learning and ensemble-variational methods to assimilate experimental measurements in high-fidelity simulations. The measurements are wall-pressure spectra acquired from PCB sensors on a 7-degree cone, at free-stream Mach number M=6. We estimate the upstream instability waves, quantitatively reproduce the wall-pressure spectra, and discover the full spatio-temporal flow field that led to the measured data. |
Tuesday, November 22, 2022 1:29PM - 1:42PM |
Z10.00004: Data assimilation in high-speed boundary layers using deep operator networks Yue Hao, Charles Meneveau, Tamer A Zaki Transition to turbulence in high-altitude, high-speed flight is often caused by the amplification of boundary-layer instability waves. Recently, estimation of the upstream disturbance spectra from wall-pressure data was performed using ensemble variational (EnVar) data assimilation (Buchta & Zaki, J. Fluid Mech., 916, A44, 2021). The computational cost of EnVar is appreciable because it involves the propagation of an ensemble of solutions, thus requiring many direct numerical simulations (DNS) at each iteration. Deep operator networks (DeepONets) bring new opportunities to accelerate EnVar. DeepONets prediction of the observations can either replace or supplement the DNS in the ensemble propagation step. Performance will be assessed in the context of high-speed boundary layer undergoing transition due to subharmonic resonance of planar and oblique instability waves. Using a combined DeepONets-EnVar algorithm, the dominant frequency and phase difference between 2D and 3D perturbations will be estimated, and the accuracy and efficiency of estimation will be assessed. |
Tuesday, November 22, 2022 1:42PM - 1:55PM |
Z10.00005: Thermodynamics induced compressibility in high-pressure shear flows Nek Sharan Turbulence at high pressures, exceeding the species critical value, determines mixing/combustion in various propulsion devices. At such pressures, the thermodynamic conditions induce real gas compressibility, in contrast to the dynamics-based compressibility that is commonly quantified by the convective or the turbulence Mach number. The real gas compressibility modifies the dilatation of a fluid element in response to a pressure fluctuation resulting in pressure-strain correlations and Reynolds stress anisotropy that is different from an ideal gas. The extent of the modification depends on the flow dynamics (the shear rate) as well as the thermodynamics (the pressure/temperature conditions). This study uses compressible homogeneous shear flow simulations to investigate the mechanism by which high-pressure conditions influence flow turbulence. The transport equations for the r.m.s. velocity fluctuations and the pressure variance are assessed to examine turbulent energy growth at various shear rates, turbulence Mach numbers, and pressure conditions. The implications on subgrid-scale modeling of such flows will be discussed.
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Tuesday, November 22, 2022 1:55PM - 2:08PM |
Z10.00006: A Structured Input-Output Framework for Robust Stability Analysis of Compressible Flows Diganta Bhattacharjee, Talha Mushtaq, Peter Seiler, Maziar S Hemati In this work, we propose a structured input-output (I/O) framework for conducting robust stability analysis of compressible flows. This is based on the notion of structured singular value (SSV) from the robust controls literature. The approach accounts for the structure of the nonlinear terms in the governing equations to improve the stability analysis relative to linear approaches. Here, we make use of an exact quadratic representation of the compressible Navier-Stokes equations to efficiently model the nonlinear terms within the structured I/O framework. Moreover, we propose efficient numerical algorithms that exploit additional structure in this representation and refine upper-bound calculations of the SSV. We demonstrate the structured I/O framework on a compressible plane Couette flow over a range of subsonic and supersonic conditions. Results are compared with those obtained from resolvent analysis and unstructured I/O analysis. Our findings show that accounting for the structure of the nonlinearity reveals different instability mechanisms than these other analysis tools. |
Tuesday, November 22, 2022 2:08PM - 2:21PM |
Z10.00007: Resolvent analysis of self-sustained shock oscillations in transonic airfoil buffet Yuta Iwatani, Hiroyuki Asada, Chi-An Yeh, Kunihiko Taira, Soshi Kawai In this talk, we present the resolvent analysis of transonic airfoil buffet at a high Reynolds number using wall-resolved LES database to reveal the self-sustained mechanisms of shock oscillations. The resolvent analysis specifies the input-output relations in the buffet flowfield as forcing and response modes, and the two mechanisms of the self-sustained shock wave oscillations are identified. One mechanism is closely tied to the periodical change in shock-induced separation height that induces the pressure variations near the trailing edge. The other is related to the phenomenon that pressure variations behind the shock wave go around the shock and reach in front of the shock. Both the mechanisms lead to the changes in the shock-jump condition and thus the shock oscillations. The mechanisms identified by the resolvent analysis are also consistently confirmed by the high-fidelity LES flowfields. |
Tuesday, November 22, 2022 2:21PM - 2:34PM |
Z10.00008: Is transonic buffet a misnomer? Pradeep Moise, Markus Zauner, Neil D Sandham Self-sustained, periodic flow oscillations over wings in the transonic regime are characteristic of a flow phenomenon referred to as transonic buffet which can be detrimental to aircraft performance. Although the physical mechanisms underlying this phenomenon are unclear, it is generally assumed to be shock-induced and unique to the transonic regime. This assumption is shown to be incorrect here by performing large-eddy simulations of infinite wing sections at different freestream Mach numbers, M, and incidence angles, α. Whereas transonic buffet is observed at high M, sustained periodic oscillations that resemble it are observed at lower M even though the entire flow field remains subsonic at all times. A spectral proper orthogonal decomposition shows that the mode shapes associated with these oscillations are essentially the same at all M. By examining higher α, connections are also made between the present results and low-frequency oscillations that have been reported in incompressible flows at α close to stall. These results indicate that the physical mechanisms underlying "transonic" buffet are essentially subsonic in nature. This insight could be useful in reformulating mitigation strategies for "transonic" buffet, by suggesting a shift in focus away from shock waves. |
Tuesday, November 22, 2022 2:34PM - 2:47PM |
Z10.00009: Centrifugal oblique waves in hypersonic separated flows Anubhav Dwivedi, G S Sidharth, Mihailo R Jovanovic Experiments on high-speed separated flows show unsteady fluctuations in the separation zone before the transition to turbulence. We investigate the origin of these unsteady perturbations by evaluating the amplification of external disturbances in hypersonic compression ramp boundary layer using input-output analysis. The dominant unsteady flow response appears in the form of oblique waves in the separated shear layer. We show that the growth of the perturbation energy associated with these oblique waves arises from the streamline curvature of the laminar separated flow, in contrast to the attached boundary layers where no such mechanism exists. This newly identified physical amplification process causes large growth of oblique waves in shock-wave-boundary-layer-interactions (SWBLI). The role of unsteady centrifugal waves in transition to turbulence in compression ramp SWBLI is evaluated using direct numerical simulations (DNS). The dependence of the centrifugal growth on the Reynolds number and the ramp turn angle is also discussed. |
Tuesday, November 22, 2022 2:47PM - 3:00PM |
Z10.00010: Statistics of 2D multi-mode iso-thermally stratified compressible Rayleigh-Taylor instability Denis Aslangil, Man Long Wong The flow compressibility through the strength of the background stratification has been reported to have a stabilization effect in single-mode 2D Rayleigh-Taylor instability (RTI) at Atwood numbers less than 0.05. This work extends the study of such effects on the low Atwood number (A=0.04) 2D RTI to the multi-mode perturbation using direct numerical simulations, DNS. We study the growth of the RTI, mixing, energetics, and the vortical dynamics of the flow with different Mach and Reynolds numbers. In addition, we present a detailed analysis of our DNS to study how compressibility plays a role in the transport of Turbulent Kinetic Energy, TKE, of the multi-mode 2D RTI. |
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