Large Eddy Simulations and modal reconstruction of laminar transonic buffet

Transonic buffet refers to the self-sustained periodic motion of shock waves observed in transonic flows over wings and limits the flight envelope of aircraft. Based on the boundary layer characteristics at the shock foot, buffet has been classified as laminar or turbulent and the mechanisms underlying the two have been proposed to be different (Dandois et al., 2018, J. Fluid Mech., vol. 18, pp. 156-178). The effect of various flow parameters (freestream Mach and Reynolds numbers and sweep and incidence angles) on laminar transonic buffet on an infinite wing (Dassault Aviation's supercritical V2C aerofoil) is reported here by performing Large-Eddy Simulations (LES) for a wide range of parameters. A spectral proper orthogonal decomposition identified the presence of a low-frequency mode associated with buffet and high-frequency wake modes related to vortex shedding. A flow reconstruction based only on the former shows periodic boundary-layer separation and reattachment accompanying shock wave motion. A modal reconstruction based only on the wake mode suggests that the separation bubble breathing phenomenon reported by Dandois et al. is due to this mode. Together, these results indicate that the physical mechanisms governing laminar and turbulent buffet are the same. Buffet was also simulated at zero incidence. Shock waves appear on both aerofoil surfaces and oscillate out of phase with each other indicating the occurrence of a Type I buffet (Giannelis et al., 2018, Aerosp. Sci. Technol., vol. 18, pp. 89-101) on a supercritical aerofoil. These results suggest that the mechanisms underlying different buffet types are the same.Comment: 40 pages, 31 figures, submitted to Journal of Fluid Mechanic

Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils

Self-sustained low-frequency flow unsteadiness over rigid aerofoils in the transonic regime is referred to as transonic buffet. Although the exact physical mechanisms underlying this phenomenon are unclear, it is generally assumed to be unique to the transonic regime. This assumption is shown to be incorrect here by performing large-eddy simulations of flow over a NACA0012 profile for a wide range of flow conditions. At zero incidence and sufficiently high freestream Mach numbers, M, transonic buffet occurs with shock waves present in the flow. However, self-sustained oscillations that occur at similar frequencies are observed at lower M for which shock waves are absent and the entire flow field remains subsonic at all times. At higher incidences, the oscillations are sustained at progressively lower M. Oscillations were observed for M as low as 0.3, where compressibility effects are small. A spectral proper orthogonal decomposition shows that the spatial structure of these oscillations (i.e., mode shapes) are essentially the same for all cases. These results indicate that buffet on aerofoils does not necessarily require the presence of shock waves. Furthermore, the trend seen with increasing incidence angles suggests that transonic buffet on aerofoils and low-frequency oscillations reported in the incompressible regime (Zaman et al., 1989, J. Fluid Mech., vol. 202, pp. 403--442) have similar origins. Thus, models which rely specifically on shock waves to explain transonic buffet are incorrect. These insights could be useful in understanding the origins of ``transonic" buffet and reformulating mitigation strategies by shifting the focus away from shock waves.Comment: 28 pages, 20 figure

On the co-existence of transonic buffet and separation-bubble modes for the OALT25 laminar-flow wing section

Transonic buffet is an unsteady flow phenomenon that limits the safe flight envelope of modern aircraft. Scale-resolving simulations with span-periodic boundary conditions are capable of providing new insights into its flow physics. The present contribution shows the co-existence of multiple modes of flow unsteadiness over an unswept laminar-flow wing section, appearing in the following order of increasing frequency: (a) a low-frequency transonic buffet mode, (b) an intermediate-frequency separation bubble mode, and (c) high-frequency wake modes associated with vortex shedding. Simulations are run over a range of Reynolds and Mach numbers to connect the lower frequency modes from moderate to high Reynolds numbers and from pre-buffet to established buffet conditions. The intermediate frequency mode is found to be more sensitive to Reynolds-number effects compared to those of Mach number, which is the opposite trend to that observed for transonic buffet. Spectral proper orthogonal decomposition is used to extract the spatial structure of the modes. The buffet mode involves coherent oscillations of the suction-side shock structure, consistent with previous studies including global mode analysis. The laminar separation-bubble mode at intermediate frequency is fundamentally different, with a phase relationship between separation and reattachment that does not correspond to a simple `breathing' mode and is not at the same Strouhal number observed for shock-induced separation bubbles. Instead, a Strouhal number based on separation bubble length and reverse flow magnitude is found to be independent of Reynolds number within the range of cases studied

Bubble and conical forms of vortex breakdown in swirling jets

Experimental investigations of laminar swirling jets had revealed a new form of vortex breakdown, named conical vortex breakdown, in addition to the commonly observed bubble form. The present study explores these breakdown states that develop for the Maxworthy profile (a model of swirling jets) at inflow, from streamwise-invariant initial conditions, with direct numerical simulations. For a constant Reynolds number based on jet radius and a centreline velocity of 200, various flow states were observed as the inflow profile's swirl parameter $S$ (scaled centreline radial derivative of azimuthal velocity) was varied up to 2. At low swirl ( $S=1$ ) a helical mode of azimuthal wavenumber $m=-2$ (co-winding, counter-rotating mode) was observed. A `swelling' appeared at $S=1.38$ , and a steady bubble breakdown at $S=1.4$ . On further increase to $S=1.5$ , a helical, self-excited global mode ( $m=+1$ , counter-winding and co-rotating) was observed, originating in the bubble's wake but with little effect on the bubble itself - a bubble vortex breakdown with a spiral tail. Local and global stability analyses revealed this to arise from a linear instability mechanism, distinct from that for the spiral breakdown which has been studied using Grabowski profile (a model of wing-tip vortices). At still higher swirl ( $S=1.55$ ), a pulsating type of bubble breakdown occurred, followed by conical breakdown at 1.6. The latter consists of a large toroidal vortex confined by a radially expanding conical sheet, and a weaker vortex core downstream. For the highest swirls, the sheet was no longer conical, but curved away from the axis as a wide-open breakdown. The applicability of two classical inviscid theories for vortex breakdown - transition to a conjugate state, and the dominance of negative azimuthal vorticity - was assessed for the conical form. As required by the former, the flow transitioned from a supercritical to subcritical state in the vicinity of the stagnation point. The deviations from the predictions of the latter model were considerable

Large-Eddy Simulations and modal reconstruction of laminar transonic buffet

Transonic buffet refers to the self-sustained periodic motion of shock waves observed in transonic flows over wings and can limit the flight envelope of aircraft. Based on the boundary layer characteristics at the shock foot, buffet has been classified as laminar or turbulent and the mechanisms underlying the two have been proposed to be different (Dandois et al., 2018, J. Fluid Mech., vol. 18, pp. 156–178). The effect of various flow parameters (freestream Mach and Reynolds numbers and sweep and incidence angles) on laminar transonic buffet on an infinite wing (Dassault Aviation’s supercritical V2C aerofoil) is reported here by performing Large-Eddy Simulations (LES) for a wide range of parameters. A spectral proper orthogonal decomposition identified the presence of a low-frequency mode associated with buffet and high-frequency wake modes related to vortex shedding. A flow reconstruction based only on the former shows periodic boundary-layer separation and reattachment accompanying shock wave motion. A modal reconstruction based only on the wake mode suggests that the separation bubble breathing phenomenon reported by Dandois et al. is due to this mode. Together, these results indicate that the physical mechanisms governing laminar and turbulent buffet are the same. Buffet was also simulated at zero incidence. Shock waves appear on both aerofoil surfaces and oscillate out of phase with each other indicating the occurrence of a Type I buffet (Giannelis et al., 2018, Aerosp. Sci. Technol., vol. 18, pp. 89–101) on a supercritical aerofoil. These results suggest that the mechanisms underlying different buffet types are the same

Dataset for Large-Eddy Simulations and modal reconstruction of laminar transonic buffet

This dataset corresponds to several plots presented in the article, &quot;Large-Eddy Simulations and modal reconstruction of laminar transonic buffet&quot; published in the Journal of Fluid Mechanics, 2022. Plots with aerofoil geometry are not provided due to copyright reasons. All files are in ASCII format and named in a &quot;fig[No][subfigure][description].csv&quot; format (e.g. fig30d_X.csv refers to figure 30d in the article with X being the variable stored). Line plots are stored such that columns correspond to x- and y-axis with a header specifying details in a string format. Contour plots contain mesh arrays named based on figure labels (e.g. X and T) and a variable of the same array dimension.</span

Dataset for the journal article &#39;Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils&#39;

This dataset corresponds to figures presented in the article, &quot;Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils&quot; published in the Journal of Fluid Mechanics, 2024. Plots with aerofoil geometry are not provided due to copyright reasons. All files are in a &#39;comma-separated variable&#39; format and named in a &quot;fig[No][subfigure]_[description].csv&quot; format (e.g. fig2a_M0p60.csv refers to figure 2a in the article with M0p6 representing data for M=0.6). Line plots are stored such that columns correspond to x- and y-axis with a header specifying details in a string format. Figures included: 2a,3a,5a,b,6a,b,8a,b,10a,b,11a,b,14a,b,19a,b,22a,b</span

Dataset in support of the publication: On the co-existence of transonic buffet and separation-bubble modes for the OALT25 laminar-flow wing section

Data-set corresponding to the publication: &quot;On the co-existence of transonic buffet and separation-bubble modes for the OALT25 laminar-flow wing section&quot; Markus Zauner, Pradeep Moise, Neil D. Sandham Journal of Flow Turbulence and Combustion (2023) 10.1007/s10494-023-00415-4 https://link.springer.com/article/10.1007/s10494-023-00415-4</span

Connecting transonic buffet with incompressible low-frequency oscillations on aerofoils

Self-sustained, low-frequency, coherent flow unsteadiness over rigid, stationary aerofoils in the transonic regime is referred to as transonic buffet. This study examines the role of shock waves in sustaining this transonic phenomenon and its relation to low-frequency oscillations that occur in flow over aerofoils in the incompressible regime (Zaman et al., 1989, J. Fluid Mech., vol. 202, pp. 403–442). This is investigated by performing large-eddy simulations of the flow over a NACA0012 profile for a wide range of flow conditions under free-transition conditions. At low Reynolds numbers, zero incidence angle and sufficiently high freestream Mach numbers, &#x1d440;, transonic buffet occurs with shock waves present in the flow. However, when &#x1d440; alone is lowered, self-sustained, periodic oscillations at a low frequency are observed even though shock waves are absent and the entire flow field remains subsonic at all times. At higher incidence angles, the oscillations are sustained at progressively lower &#x1d440; and are present even at &#x1d440; = 0.3, where compressibility effects are low. A spectral proper orthogonal decomposition (SPOD) shows that the spatial structure of these oscillations is consistent for all cases. The SPOD modes are topologically similar, suggesting a connection between transonic buffet and low-frequency oscillations in the incompressible regime. Comparisons with other studies examining transonic buffet on various aerofoils, under forced transition and fully-turbulent conditions support this hypothesis. Future studies using tools of global linear stability analysis, especially at high freestream Reynolds numbers are required to examine whether the underlying mechanisms of transonic buffet and incompressible low-frequency oscillations are the same

Experimental study and optimization of Plasma Actuators for Flow control in subsonic regime

Experimental study and optimization of Plasma Ac- tuators for Flow control in subsonic regime PRADEEP MOISE, JOSEPH MATHEW, KARTIK VENKATRAMAN, JOY THOMAS, Indian Institute of Science, FLOW CONTROL TEAM | The induced jet produced by a dielectric barrier discharge (DBD) setup is capable of preventing °ow separation on airfoils at high angles of attack. The ef-fect of various parameters on the velocity of this induced jet was studied experimentally. The glow discharge was created at atmospheric con-ditions by using a high voltage RF power supply. Flow visualization,photographic studies of the plasma, and hot-wire measurements on the induced jet were performed. The parametric investigation of the charac- teristics of the plasma show that the width of the plasma in the uniform glow discharge regime was an indication of the velocity induced. It was observed that the spanwise and streamwise overlap of the two electrodes,dielectric thickness, voltage and frequency of the applied voltage are the major parameters that govern the velocity and the extent of plasma.e®ect of the optimized con¯guration on the performance characteristics of an airfoil was studied experimentally