arXiv subjects

Benjamin S. Savino

Publications and source records attributed to Benjamin S. Savino.

6 recordsLinked to original sources

arXiv2026

Improving boundary-layer separation prediction by an IDDES turbulence model using a pressure-gradient sensor

This work extends a pressure-gradient sensor for boundary-layer separation originally developed for the $k-ω$ shear-stress transport Reynolds-averaged Navier-Stokes (RANS) model (Griffin et al., 2025, J. Turb.) to the Improved Delayed Detached Eddy Simulation (IDDES) turbulence model of Gritskevich et al. (2012, Flow Turbul. Combust.). The pressure-gradient sensor identifies local regions of strong adverse pressure-gradient where the eddy-viscosity is reduced, as in the original RANS model. Additionally, to promote separation in the IDDES model, the elevation term in the IDDES length scale, designed to augment the RANS-mode Reynolds stress in attached flow regions, is turned off where the pressure-gradient sensor is active. The model is applied on various airfoils representative of both wind energy and aerospace applications, and is used in fully turbulent and transitional IDDES model variants. The proposed model improves the prediction of stall onset and post-stall regimes relative to the baseline IDDES model without significant degradation of attached-flow regimes relative to state-of-the-art RANS models or deep-stall regimes relative to state-of-the-art IDDES models. Significant overall improvements are observed in predictions of lift and drag polars across 90 degrees of angle of attack, yielding a unified model able to predict various (two- and three-dimensional) flow regimes. Shortcomings are identified to be related to the underlying RANS pressure-gradient sensor rather than the extension of the sensor to the IDDES model, which is the focus of this work.

physics.flu-dyn↗

arXiv2025

Attached Decelerating Turbulent Boundary Layers over Riblets

Turbulent boundary layers over riblets subjected to adverse pressure gradients (APGs) are investigated by direct numerical simulation. Multiple APG strengths and riblet sizes are examined, permitting evaluation of drag modification by riblets, and associated physical mechanisms, in various regimes established for zero-pressure-gradient (ZPG) riblet flows. The APG strengths are selected such that the flow remains attached. It is found that during APGs, riblets reduce drag beyond what has been achieved in ZPG flows. In extreme cases, an upstream force (i.e., negative drag) is attained. The significant drag reduction is found to be a product of Kelvin-Helmholtz roller vortices forming near the riblet crest, which are augmented in size, strength, and frequency during the APG. The preliminary results reported here indicate the need to modify existing metrics to predict drag reduction and the onset of KH rollers by riblets when the pressure gradient is non-negligible. Further analysis will be documented in the final paper.

physics.flu-dyn↗

arXiv2024

Separation of a Laminar Boundary Layer Subjected to Pressure Gradients with Spanwise Variations

The characteristics of three-dimensional laminar separation bubbles (LSBs), compared with their quasi-two-dimensional counterparts, are studied using direct numerical simulation. A three-dimensional suction-blowing distribution is applied on the top boundary to induce flow separation. Three levels of suction strength (i.e., adverse pressure gradient (APG) strength) are tested. For each suction level, the spanwise extent of the suction is varied. Two non-uniform suction distributions are compared with uniformly-applied suction across the full span (implying a two-dimensional LSB), totaling nine cases. All cases are performed at $Re_δ= U_{\infty}δ/ν= 1000$ based on the inflow boundary layer thickness ($δ$) and free-stream velocity ($U_{\infty}$). Initial results indicate stark contrast between three-dimensional and two-dimensional LSBs. Most notably, the role of suction width is determining the topology of the LSB and its reattachment mechanism. Meanwhile, APG is responsible for determining the size of the LSB and level of turbulence at reattachment. Streamwise-oriented vortices are identified along the crest of the three-dimensional LSBs.

physics.flu-dyn↗

arXiv2024

Pressure-Induced Separation of a Laminar Boundary Layer over a Partially-Slip Wall

The characteristics of pressure-induced laminar separation bubbles (LSBs) over a partially-slip wall, compared with that over a canonical no-slip wall, are studied using direct numerical simulation. Three cases, two utilizing linear Robin-type slip boundary conditions of differing slip length ($Λ$), and one non-slip are compared. For the partial-slip cases, a streamwise distribution of slip profile is employed ensuring smooth transition between no-slip and partial-slip (transition from no-slip to a constant slip length takes 5$δ$, where $δ$ is the inflow boundary layer thickness). The constant target slip length is maintained for $20δ$ upstream and during the onset of flow separation. The separation is induced by a wall-normal velocity profile applied at the top boundary. All cases are performed at $Re_δ= U_{\infty}δ/ν= 455$. Initial results indicate that as slip length increases, separation and reattachment are delayed. Most notably, the formation and shedding of roller vortices is mitigated as slip length increases, resulting in a less turbulent wake, despite that self-similarity of the plane shear layer is maintained.

physics.flu-dyn↗

arXiv2024

Thrust Generation by Shark Denticles

DNS is performed for flow separation over a bump in a turbulent channel. Comparisons are made between a smooth bump configuration and one where the lee side is covered with replicas of complete shark denticles. As flow over the bump is under an adverse pressure gradient (APG), a reversed pore flow (RPF) is formed in the porous cavities underneath the crowns of the denticle array. Remarkable thrust is generated by the RPF as denticle necks accelerate the fluid passing between them in the upstream streamwise direction. Several geometrical features of shark denticle, including some that had not previously been considered hydrodynamically functional, are identified to form an anisotropic permeable porous media that enables and sustains the RPF and thrust generation. The RPF is activated by the APG before massive flow reversal. The results indicate a proactive, on-demand drag reduction mechanism that leverages and transforms the APG into a favorable outcome.

physics.flu-dyn↗

arXiv2024

Impact of Spanwise Rotation on Flow Separation and Recovery Behind a Bulge in Channel Flows

Direct numerical simulations of spanwise-rotating turbulent channel flow with a parabolic bump on the bottom wall are employed to investigate the effects of rotation on flow separation. Four rotation rates of $Ro_b := 2ΩH/U_b = \pm 0.42, \; \pm 1.0$ are compared with the non-rotating scenario. The mild adverse pressure gradient induced by the lee side of the bump allows for a variable pressure-induced separation. The separation region is reduced (increased) when the bump is on the anti-cyclonic (cyclonic) side of the channel, compared with the non-rotating separation. The total drag is reduced in all rotating cases. Through several mechanisms, rotation alters the onset of separation, reattachment, and wake recovery. The mean momentum deficit is found to be the key. A physical interpretation of the ratio between the system rotation and mean shear vorticity, $S:=Ω/Ω_s$, provides the mechanisms regarding stability thresholds of $S=-0.5$ and $-1$. The rotation effects are explained accordingly with reference to the dynamics of several flow structures. For anti-cyclonic separation, particularly, the interaction between the Taylor-Görtler vortices and hairpin vortices of wall-bounded turbulence is proven to be responsible for the breakdown of the separating shear layer. A generalized argument is made regarding the essential role of near-wall deceleration and resultant ejection of enhanced hairpin vortices in destabilizing an anti-cyclonic flow. This mechanism is anticipated to have broad impacts on other applications in analogy to rotating shear flows, such as thermal convection and boundary layers over concave walls.

physics.flu-dyn↗