Datta V. Gaitonde

Analysis of Airfoil Stall Control Using Dynamic Mode Decomposition

Airfoil stall is a major inhibitor of aircraft performance. Many methods have been successfully shown to inhibit or eliminate stall. However, the underlying dynamics of control remains relatively o...

Mean Flowfield Structure of a Supersonic Three-Dimensional Base Flow

The mean flowfield structure in the wake region of a circular cylinder at 10 o angle of attack in Mach 2.5 flow is described. Results from the full three-dimensional Reynolds averaged Navier-Stokes equations, supplemented with a compressibility-corrected two-equation turbulence model, are compared with extensive available experimental data. The main elements of the shock pattern, surface oil flow on the afterbody and the base surfaces, velocity vector plots in the symmetry and lateral planes and end-view flow images are reproduced to good accuracy. Based on this success, the computed solution is employed to generate a model for the mean flow. The main feature is a pair of longitudinal vortices, separated by high-speed fluid entrained primarily from the leeward and lateral afterbody boundary layers, and bounded from below by the windward afterbody boundary layer. The three-dimensional wave structure is elucidated and correlated with the streamline structure. Vorticity distribution analysis indicates that in the mean flow description, the vortex cores are two legs of a horseshoe-like structure. The predictive capability of the present RANS approach is baselined for future detached eddy simulations by comparing mesh resolved surface pressures on the afterbody and the base region: these are found to differ by between 10% to 15%.

Multi-scale interactions in a compressible boundary layer

ABSTRACT The properties of spectral subranges of scales in a boundary layer at Mach=2.3 and friction Reynolds number Reτ = 570 are investigated by analysing DNS data. One major aim is to examine whether footprinting and modulation of small-scale near-wall motions by outer large structures, observed at high Reynolds numbers, also pertain to this low-Reynolds-number case, or whether the logarithmic layer simply contains a continuous hierarchy of motions without specific outer scales playing a distinctive role. To this end, the spectrum of scales is decomposed into modes by application of the “Empirical Mode Decomposition”. The properties of different scales are then investigated by means of spectra, maps of isotropy/anisotropy parameters, the premultiplied derivative of the second-order structure function, correlation coefficients and joint probability density function (PDF), the last constructed from conditionally sampled data for the small-scale motions within the large-scale footprints. A clear commonality is identified between interactions in high-Reynolds-number channel flow and the present low-Reynolds-number boundary layer.

Directivity and intermittency in the nearfield of a Mach 1.3 jet

Local fluctuations in a Mach 1.3 cold jet are tracked to understand the genesis of nearfield directivity and intermittency. A newly developed approach leveraging two synchronized large-eddy simulations is employed to solve the forced Navier–Stokes equations, linearized about the evolving unsteady base flow. The results are summarized by exposing the effect of two acoustically significant turbulent regions: the lip-line and core collapse location. The near-acoustic field displays the clear signature of the two regions. However, for both regions, the nearfield evolution of the perturbation field is characterized by generation of intermittent wavepackets, which propagate into the near-acoustic field and gradually acquire their expected broadband and narrowband characteristics at sideline and downstream angles respectively. The simulations elucidate how higher frequencies are obtained in the sideline directions as lower frequencies are filtered out of the forcing fluctuations. Likewise, shallow-angle acoustic signals arise through filtering of high frequency content in that direction. The directivity and intermittency are connected to the filtering of scales by jet turbulence with empirical mode decomposition. The observations highlight the gradual evolution of seemingly random core turbulence into well-defined intermittent wavepackets in the nearfield of the jet. The manner in which centerline fluctuations are segregated into upstream, sideline, and downstream components is examined through narrowband correlations. A similar analysis for the lipline contribution shows primarily upstream and downstream patterns because of the larger structures in the shear layer.