arXiv subjects

Dan Henningson

Publications and source records attributed to Dan Henningson.

3 recordsLinked to original sources

arXiv2026

Numerical simulations of transition and long-term response of a wind turbine airfoil

Numerical simulations are performed for an FFA-W3 wind-turbine airfoil corresponding to a section of the DTU 10-MW Reference Wind Turbine. Wall-resolved large-eddy simulations (LES) are carried out with Nek5000 and EllipSys at chord Reynolds number $Re_c=1\times10^5$ and effective angle of attack $AoA=3.1^\circ-3.3^\circ$. A spanwise domain width of 10 percent of the chord is sufficient to reproduce the time-averaged flow and the evolution of the main disturbances. EllipSys is validated against Nek5000 for LES, showing close agreement for the mean flow and most amplified perturbations. EllipSys underpredicts the amplitude of Tollmien-Schlichting waves in the attached boundary layer, owing to higher numerical dissipation, but closely predicts the evolution of the Kelvin-Helmholtz (KH) mode in the laminar separation bubble, in agreement with parabolized stability equation (PSE) results. The mode shape is extracted using spectral proper orthogonal decomposition (SPOD), revealing the KH wavepacket forming in the separation bubble. Long-time EllipSys simulations show a slow modulation of the normal-force coefficient, with amplitude 10.5 percent and period 48 flow-through times, corresponding to $f=f^*c/U_\infty=0.021$ and $St=f\sin(AoA)=0.0012$. This frequency is associated with low-frequency oscillations reported in airfoil studies, although the Strouhal number is lower than previously observed and occurs at smaller angle of attack. For the DTU 10-MW turbine, the oscillation period corresponds to 7.7 blade rotations. Periodic stalling and reattachment may trigger the oscillation, while sufficiently high reverse flow on both sides of the airfoil may permit absolute instability and periodic bubble bursting.

physics.flu-dyn↗

arXiv2023

Numerical investigation of the boundary layer stability on a section of a rotating wind turbine blade

Laminar-turbulent transition on a rotating wind turbine blade at a chord Reynolds number of $1 \times 10^5$ and varying angles of attack ($AoA$) is studied with direct numerical simulations and linear stability theory. The rotation effects depend on the streamwise pressure gradient and direct/reverse flow state. In the $AoA=12.8^\circ$ case, rotation retards the flow in the laminar separation bubble (LSB) and renders the mixed Tollmien-Schlichting/Kelvin-Helmholtz (TS/KH) instability more unstable. Rotation fosters an oblique secondary instability mechanism, rapidly breaking the KH rolls into small-scale turbulence. A sub-harmonic mechanism is dominant in the non-rotating case, retarding transition. However, rotation accelerates the boundary layer upstream of separation, subject to a strong adverse pressure gradient (APG), stabilizing TS waves and delaying transition in $3\%$ and reattachment in $4\%$. In the $AoA=4.2^\circ$ and $AoA=1.2^\circ$ cases, rotation decelerates the flow upstream of separation, subject to a favorable pressure gradient (FPG), which makes TS waves more unstable. Nonetheless, rotation accelerates the separated flow, partially stabilizing the KH mechanism. Rotation further promotes the appearance of inflectional crossflow velocity profiles that triggers stationary and traveling crossflow modes. These modes are less unstable than the TS/KH mechanism of the separated shear layer but lead to the formation of coherent spanwise-velocity structures. For $AoA=4.2^\circ$, rotation strengthens the sub-harmonic secondary instability mechanism in the rotating case over the oblique one under no rotation. For $AoA=1.2^\circ$, the sub-harmonic mechanism is dominant regardless of rotation. Notice that the transition location is not changed by rotation in the $AoA=4.2^\circ$ and $AoA=1.2^\circ$ cases. Finally, rotation stabilizes the absolute instability found in these cases.

physics.flu-dyn↗

arXiv2020

SPOD and resolvent analysis of near-wall coherent structures in turbulent pipe flows

Direct numerical simulations, performed with a high-order spectral-element method, are used to study coherent structures in turbulent pipe flow at friction Reynolds numbers $Re_τ = 180$ and $550$. The database was analysed using spectral proper orthogonal decomposition (SPOD) to identify energetically dominant coherent structures, most of which turn out to be streaks and quasi-streamwise vortices. To understand how such structures can be modelled, the linear flow responses to harmonic forcing were computed using the singular value decomposition of the resolvent operator, using the mean field as a base flow. The SPOD and resolvent analysis were calculated for several combinations of frequencies and wavenumbers, allowing to map out the similarities between SPOD modes and optimal responses for a wide range of relevant scales in turbulent pipe flows. In order to explore physical reasons behind the agreement between both methods, an indicator of lift-up mechanism in the resolvent analysis was introduced, activated when optimal forcing represents quasi-streamwise vortices and associated response corresponds to streaks. Good agreement between leading SPOD and resolvent modes is observed in a large region of parameter space. In this region, a significant gain separation is found in resolvent analysis, which may be attributed to the strong amplification associated with the lift-up mechanism. For both Reynolds numbers, the observed concordances were generally for structures with large energy in the buffer layer. The results highlight resolvent analysis as a pertinent reduced-order model for coherent structures in wall-bounded turbulence, particularly for streamwise elongated structures corresponding to near-wall streamwise vortices and streaks.

physics.flu-dyn↗