Fernando Zigunov

Bio: Fernando Zigunov is an academic researcher from Florida State University. The author has contributed to research in topics: Wake & Mechanics. The author has an hindex of 3, co-authored 8 publications receiving 23 citations. Previous affiliations of Fernando Zigunov include Florida A&M University – Florida State University College of Engineering.

Reynolds number and slant angle effects on the flow over a slanted cylinder afterbody

Abstract: The cylinder with a slanted base is a simplified, canonical bluff body geometry that shares similarities to aircraft fuselages, which are known to produce a strong vortex pair due to their upswept afterbody. This work will examine in detail the surface flow and near-field characteristics of the flow over the slanted cylinder for slant angles of , where the formation of either a separated wake flow state or a vortex-dominated flow state is dependent upon initial conditions, i.e. the presence of overshoot of the free-stream velocity during wind tunnel start-up.

Instability modes of millimeter-scale supersonic jets

Abstract: The appearance of instability modes on subsonic and supersonic impinging jets is a phenomenon of great interest that has been observed in jets produced by nozzles of different sizes. The present study describes these instabilities in millimeter-sized supersonic jets produced by convergent and convergent–divergent nozzles ( $$D_\mathrm{throat} = 2 \hbox { mm}$$ ). The smaller nozzle sizes allowed for a comprehensive parametric study involving 378 impingement distance values between $$H{/}D=0.5$$ and $$H{/}D=9.9$$ , as well as 30 nozzle pressure ratio values for both nozzles, totaling a set of more than 22,000 experiments. The detailed resolution on the impingement distance parameter allowed for the observation of acoustic resonant tones that alternate between two or three distinct frequency bands. It was observed that very small changes in the impingement distance ( $$\Delta H{/}D \le 0.5$$ ) are sufficient for resonant mode switching, which is related to the instability mode switching phenomenon. The complementary high-speed schlieren images, captured for a set of 36 cases, allowed for the effective observation and classification of the jet resonant modes via feature tracking. A correlation between the resonant mode shape (helical or axisymmetric) and the resonant tone was clearly observed and quantified. Further experiments with high-resolution schlieren images showed the shape of the acoustic waves produced, connecting the acoustic wave packets with the production of shear layer instabilities. In the helical instability mode, it was possible to observe what appears to be a helix-shaped acoustic wave.

Beyond actuator line arrays in active flow control studies: Lessons from a genetic algorithm approach

Abstract: Active flow control with microjets in crossflow is a promising technology to improve the performance of many engineering flows. Predicting effective placement for the microjets at the surface of the aerodynamic model remains an unsolved challenge due to the complex interactions between the jets and the main flow. We propose and demonstrate a fully experimental, model-free approach using a solenoid array and a genetic algorithm to find a highly effective actuator pattern for drag reduction in the flow over a simplified fuselage afterbody, deploying thousands of actuator configurations in a single experiment and reaching a configuration that achieves a 10% reduction in drag.

Time-resolved particle image velocimetry and pressure sensitive paint measurements of afterbody flow dynamics

Abstract: A pair of counter-rotating vortices is produced in the wake of typical cargo aircraft fuselages due to their ramp shape. With a fast polymer-ceramic pressure sensitive paint (PC-PSP), we measure the global pressure fluctuations at very low velocities (15 to 75 m/s) in a surrogate cylinder with a slanted edge model. Through spectral proper orthogonal decomposition, we observe a family of convective pressure waves with peak energy at frequencies matching the vortex wandering frequencies in this flow.

Resolvent-analysis-based design of airfoil separation control

Abstract: We use resolvent analysis to design active control techniques for separated flows over a NACA 0012 airfoil. Spanwise-periodic flows over the airfoil at a chord-based Reynolds number of . Near the leading edge, localized unsteady thermal actuation is introduced in an open-loop manner with two tunable parameters of actuation frequency and spanwise wavelength. To provide physics-based guidance for the effective choice of these control input parameters, we conduct global resolvent analysis on the baseline turbulent mean flows to identify the actuation frequency and wavenumber that provide large perturbation energy amplification. The present analysis also considers the use of a temporal filter to limit the time horizon for assessing the energy amplification to extend resolvent analysis to unstable base flows. We incorporate the amplification and response mode from resolvent analysis to provide a metric that quantifies momentum mixing associated with the modal structure. This metric is compared to the results from a large number of three-dimensional large-eddy simulations of open-loop controlled flows. With the agreement between the resolvent-based metric and the enhancement of aerodynamic performance found through large-eddy simulations, we demonstrate that resolvent analysis can predict the effective range of actuation frequency as well as the global response to the actuation input. We believe that the present resolvent-based approach provides a promising path towards mean flow modification by capitalizing on the dominant modal mixing.

On state instability of the bi-stable flow past a notchback bluff body

Abstract: The wake of a notchback Ahmed body presenting a bi-stable nature is investigated by performing wind tunnel experiments and large-eddy simulations. Attention is confined to the Reynolds number (. This leads to a higher possibility to break the asymmetric state, resulting in highly frequent switches showing symmetry.

Experimental Characterization of Supersonic Single- and Dual-Impinging Jets

Abstract: Supersonic single-impinging jet flows are known to exhibit strong unsteadiness levels and acoustic signatures due to large-scale resonant motions. However, the flowfield characteristics of two such...

Dynamics of Three-Dimensional Shock-Wave/Boundary-Layer Interactions

Abstract: Advances in measuring and understanding separated, nominally two-dimensional (2D) shock-wave/turbulent-boundary-layer interactions (STBLI) have triggered recent campaigns focused on three-dimensional (3D) STBLI, which display far greater configuration diversity. Nonetheless, unifying properties emerge for semi-infinite interactions, taking the form of conical asymptotic behavior where shock-generator specifics become insignificant. The contrast between 2D and 3D separation is substantial; the skewed vortical structure of 3D STBLI reflects the essentially 2D influence of the boundary layer on the 3D character of the swept shock. As with 2D STBLI, conical interactions engender prominent spectral content below that of the turbulent boundary layer. However, the uniform separation length scale, which is crucial to normalizing the lowest-frequency dynamics in 2D STBLI, is absent. Comparatively, the spectra of 3D STBLI are more representative of the mid-frequency, convective, shear-layer dynamics in 2D, while phenomena associated with 2D separation-shock breathing are muted. Asymptotic behavior breaks down in many regions important to 3D-STBLI dynamics, occurring in a configuration-dependent manner. Aspects of inceptive regions near shock generators and symmetry planes are reviewed. Focused efforts toward 3D modal and nonmodal analyses, moving-shock/boundary-layer interactions, fluid/structure interactions, and flow control are suggested as directions for future work. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 55 is January 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.