@article{li_choudhari_paredes_scholten_2024, title={Nonlinear Evolution of Instabilities in a Laminar Separation Bubble at Hypersonic Speeds}, ISSN={["1533-385X"]}, DOI={10.2514/1.J063284}, abstractNote={ The development of both convective stationary perturbation and global instabilities in the vicinity of a laminar separation bubble above an axisymmetric compression corner in hypersonic flow was investigated using numerical simulations. The flow configuration of interest corresponded to the cone–cylinder–flare model used in experimental measurements in the Boeing/U.S. Air Force Office of Scientific Research Mach-6 Quiet Tunnel (BAMQT) at Purdue University. For a flare angle of 10 deg and unit Reynolds number of [Formula: see text], their surface flow visualizations identified the presence of streamwise elongated thermal streaks near the reattachment location that had a dominant azimuthal wave number [Formula: see text]. Previous linear stability analyses predicted that the amplification characteristics of small-amplitude, unsteady, and convective instabilities within this flow were consistent with the surface pressure fluctuations measured in the experiment. However, their accompanying investigation of global instabilities showed that the separation bubble was weakly unstable at the 10 deg flare angle, with the most unstable global mode corresponding to a stationary disturbance with [Formula: see text] (i.e., well below the measured wave number of [Formula: see text]. Besides characterizing the global instability for selected flare angles, the present numerical simulations quantify the details of the stationary equilibrium state associated with supercritical bifurcation resulting from the nonlinear saturation of the unstable global mode. Although velocity perturbations associated with the saturated global mode are dominated by the fundamental spanwise wavelength associated with the linear global instability, the surface heat flux downstream of the reattachment is dominated by [Formula: see text], in agreement with the experimental measurements. }, journal={AIAA JOURNAL}, author={Li, Fei and Choudhari, Meelan and Paredes, Pedro and Scholten, Anton}, year={2024}, month={Mar} }
 @article{benitez_borg_scholten_paredes_mcdaniel_jewell_2023, title={Instability and transition onset downstream of a laminar separation bubble at Mach 6}, volume={969}, ISSN={["1469-7645"]}, DOI={10.1017/jfm.2023.533}, abstractNote={Instability measurements of an axisymmetric, laminar separation bubble were made over a sharp cone-cylinder-flare with a$12^{\circ }$flare angle under hypersonic quiet flow. Two distinct instabilities were identified: Mack's second mode (which peaked between 190 and 290 kHz) and the shear-layer instability in the same frequency band as Mack's first mode (observed between 50 and 150 kHz). Both instabilities were measured with surface pressure sensors and were captured with high-speed schlieren. Linear stability analysis results agreed well with these measured instabilities in terms of both peak frequencies and amplification rates. Lower-frequency fluctuations were also noted in the schlieren data. Bicoherence analysis revealed nonlinear phase-locking between the shear-layer and second-mode instabilities. For the first time in axisymmetric, low-disturbance flow, naturally generated intermittent turbulent spots were observed in the reattached boundary layer. These spots appeared to evolve from shear-layer-instability wave packets convecting downstream. This work presents novel experimental evidence of the hypersonic shear-layer instability contributing directly to transition onset for an axisymmetric model.}, journal={JOURNAL OF FLUID MECHANICS}, author={Benitez, Elizabeth K. and Borg, Matthew P. and Scholten, Anton and Paredes, Pedro and McDaniel, Zachary and Jewell, Joseph S.}, year={2023}, month={Aug} }
 @article{paredes_scholten_choudhari_li_2023, title={Modal Instabilities over Blunted Cones at Angle of Attack in Hypersonic Flow}, ISSN={["1533-6794"]}, DOI={10.2514/1.A35590}, abstractNote={ The effects of nose radius and angle of attack on the linear modal amplification over blunt circular cones at hypersonic speeds are computationally investigated. The three-dimensional laminar flow solutions over a 1.5-m-long, 7°-half-angle cone with 5.080, 9.525, and 25.40 mm nosetip radii are computed for selected angle-of-attack values and freestream conditions that match the Mach 10 experiments at a freestream unit Reynolds number of 17.1 million per meter conducted within the Hypervelocity Wind Tunnel 9 at the Air Force Arnold Engineering Development Complex (AEDC). Results indicate that the linear amplification of stationary and traveling crossflow waves along inflection lines increases with the angle of attack and decreases with the nosetip radius. The trend in transition front with respect to increasing angle of attack and bluntness is found to be consistent with the predicted increase in the amplification factors for Mack’s second mode (MM) disturbances along the streamline trajectories for the small and medium bluntness cases. The increase in the MM amplification along the windward ray for higher angles of attack is shown to be the result of a progressively earlier entropy-layer swallowing. Computations also indicate that the transition amplification factor along the windward ray increases with the angle of attack and decreases with the nosetip radius. Furthermore, the transition [Formula: see text]-factors along the leeward ray are below 2 and, therefore, rather small to lead to transition. }, journal={JOURNAL OF SPACECRAFT AND ROCKETS}, author={Paredes, Pedro and Scholten, Anton and Choudhari, Meelan M. and Li, Fei}, year={2023}, month={Mar} }
 @article{paredes_scholten_choudhari_li_benitez_jewell_2022, title={Boundary-Layer Instabilities over a Cone-Cylinder-Flare Model at Mach 6}, ISSN={["1533-385X"]}, DOI={10.2514/1.J061829}, abstractNote={Computations are performed to investigate the convective and global boundary-layer instabilities over a sharp cone–cylinder–flare model at zero-degree angle of attack. The model geometry and the flow conditions are selected to match the experiments conducted in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. The geometry consists of a nominally sharp 5 deg half-angle cone, followed by a cylindrical segment, and then a 10 deg flare. Additionally, flare half angles equal to 8 and 12 deg and nosetip radii equal to 0.1, 1, and 5 mm are studied. An axisymmetric separation bubble is generated as a result of the laminar shock–boundary-layer interaction in the cylinder–flare region. The comparison of the laminar flow solution and the schlieren images shows a remarkable agreement between the respective locations of both the boundary-layer edge and the reattachment shock. The linear amplification of first and second Mack mode instabilities that begin to amplify in the cone region are computed with a combination of the parabolized stability equations and the harmonic linearized Navier–Stokes equations. The predicted frequency spectra of the surface pressure fluctuations associated with both planar and oblique instability waves capture the distinct lobes within the disturbance amplification spectra measured with PCB® and Kulite sensors. To our knowledge, this represents the first successful comparison between convective instability analysis and measured surface pressure fluctuations for a hypersonic configuration with a separation bubble. Finally, the global instability analysis shows that the laminar flow becomes supercritical for flare half angles larger than 8 deg.}, journal={AIAA JOURNAL}, author={Paredes, Pedro and Scholten, Anton and Choudhari, Meelan M. and Li, Fei and Benitez, Elizabeth K. and Jewell, Joseph S.}, year={2022}, month={Jul} }
 @article{scholten_paredes_hill_borg_jewell_choudhari_2022, title={Linear Instabilities over Ogive-Cylinder Models at Mach 6}, volume={60}, ISSN={["1533-385X"]}, DOI={10.2514/1.J061611}, abstractNote={Computational investigations of a tangent ogive-cylinder geometry with varying forebodies at zero degrees angle of attack are presented. The model geometry and conditions are selected to match experiments conducted in the Air Force Research Laboratory (AFRL) Mach 6 Ludwieg Tube. Specifically, five forebodies of interest consisting of sharp and blunt ogives of 4 and 2 caliber and a hemispherical shape are studied at a freestream unit Reynolds number of . Boundary-layer-edge properties and wall-normal profiles of velocity and temperature are compared across streamwise locations past the ogive-cylinder junction. Modal stability analysis is used to characterize the most amplified frequencies corresponding to Mack’s first and second modes and those are found to agree with experimental results for the sharp forebodies. The entropy layer induced by blunt forebodies envelopes the boundary layer and stabilizes modal disturbances. Nonmodal analysis revealed a broadband set of disturbances present for the blunter forebodies, in agreement with experimental observations. Flow perturbation contours of most amplified planar and oblique disturbances are shown to qualitatively match wind tunnel schlieren images, with a switch from rope-like to elongated structures, i.e., from high-frequency Mack’s second modes to low-frequency Mack’s first modes, as the forebody angle for the sharp tip is increased, and from boundary-layer to entropy-layer disturbances as the bluntness is increased.}, number={8}, journal={AIAA JOURNAL}, author={Scholten, Anton and Paredes, Pedro and Hill, J. Luke and Borg, Matthew and Jewell, Joseph S. and Choudhari, Meelan M.}, year={2022}, month={Aug}, pages={4478–4491} }