arXiv subjects

R. Vinuesa

Publications and source records attributed to R. Vinuesa.

24 records · Page 2Linked to original sources

arXiv2019

Turbulent boundary layers around wing sections up to Rec = 1,000,000

Reynolds-number effects in the adverse-pressure-gradient (APG) turbulent boundary layer (TBL) developing on the suction side of a NACA4412 wing section are assessed in the present work. To this end, we analyze four cases at Reynolds numbers based on freestream velocity and chord length ranging from Rec = 100, 000 to 1,000,000, all of them with 5 degree angle of attack. The results of four well-resolved large-eddy simulations (LESs) are used to characterize the effect of Reynolds number on APG TBLs subjected to approximately the same pressure-gradient distribution (defined by the Clauser pressure-gradient parameter beta). Comparisons of the wing profiles with zero-pressure-gradient (ZPG) data at matched friction Reynolds numbers reveal that, for approximately the same beta distribution, the lower-Reynolds-number boundary layers are more sensitive to pressure-gradient effects. This is reflected in the values of the inner-scaled edge velocity Ue+ , the shape factor H, the components of the Reynolds-stress tensor in the outer region and the outer-region production of turbulent kinetic energy. This conclusion is supported by the larger wall-normal velocities and outer-scaled fluctuations observed in the lower-Rec cases.Thus, our results suggest that two complementing mechanisms contribute to the development of the outer region in TBLs and the formation of large-scale energetic structures: one mechanism associated with the increase in Reynolds number, and another one connected to the APG. Future extensions of the present work will be aimed at studying the differences in the outer-region energizing mechanisms due to APGs and increasing Reynolds number.

physics.flu-dyn↗

arXiv2018

Assessment of turbulent boundary layers on a NACA4412 wing section at moderate Re

The results of a DNS of the flow around a wing section represented by a NACA4412 profile, with Rec = 400, 000 and 5 degree angle of attack, are presented in this study. The high-order spectral element code Nek5000 was used for the computations. The Clauser pressure-gradient parameter ? ranges from 0 and 85 on the suction side, and the maximum Re_theta and Re_tau values are around 2,800 and 373, respectively. Comparisons between the suction side with ZPG TBL data show a more prominent wake, a steeper logarithmic region and lower velocities in the buffer region. The APG also leads to a progressively increasing value of the inner peak in the tangential velocity fluctuations, as well as the development of an outer peak, which is also observed in the other components of the Reynolds stress tensor. Other effects of strong APGs are increased production and dissipation profiles across the boundary layer, together with enhanced viscous diffusion and velocity-pressure-gradient correlation values near the wall. All these effects are connected to the fact that the large-scale motions of the flow become energized due to the APG, as apparent from spanwise premultiplied power spectral density plots.

physics.flu-dyn↗

arXiv2018

History effects and near-equilibrium in turbulent boundary layers with pressure gradient

Turbulent boundary layers under adverse pressure gradients are studied using well-resolved large-eddy simulations (LES) with the goal of assessing the influence of the streamwise pressure development. Near-equilibrium boundary layers were identified with the Clauser parameter $β$. The pressure gradient is imposed by prescribing the free-stream velocity. In order to fulfill the near-equilibrium conditions, the free-stream velocity has to follow a power-law distribution. The turbulence statistics pertaining to cases with a constant Clauser pressure-gradient parameter ? were compared with cases with a non-constant pressure distribution at matched ? and friction Reynolds number Re_tau . It was noticed that the non-constant cases appear to approach far downstream a certain state of the boundary layer, which is uniquely characterised by beta? and Re_tau. The investigations on the flat plate were extended to the flow around a wing section. Comparisons with the flat-plate cases at matched Re_tau and ? revealed some interesting features: In turbulent boundary layers with strong pressure gradients in the development history the energy-carrying structures in the outer region are strongly enhanced, which can be detected by the pronounced wake in the mean velocity as well as the large second peak in the Reynolds stresses. Furthermore, a scaling law suggested by Kitsios et al. (2015), proposing the edge velocity and displacement thickness as scaling parameters, was tested on a constant pressure gradient case. The mean velocity and Reynolds stress profiles were found to be dependent on the downstream development, indicating that their conclusion might be the result of a too short constant pressure gradient region.

physics.flu-dyn↗

arXiv2018

Revisiting the amplitude modulation in wall-bounded turbulence: towards a robust definition

The present study revisits the amplitude modulation phenomenon, specifically for the robustness in its quantification. To achieve this, a well-resolved large-eddy simulation (LES) data set at Re_theta= 8200 is used. First, the fluctuating streamwise velocity signal is decomposed into its small- and large-scale components using both Fourier filters and empirical mode decomposition (EMD), allowing the comparison among different separation filters. The effects of these filters on various definitions for quantifying the amplitude modulation have been discussed. False positive identification of the amplitude modulation has also been tested using a randomised signal. Finally, the impact of the inclination angle of the large-scale structures on the modulation quantification has been assessed.

physics.flu-dyn↗

arXiv2018

History effects for cambered and symmetric wing profiles

The characteristics of complex turbulent boundary layers under adverse pressure gradients are assessed through well-resolved large-eddy simulations (LES) with the spectral-element code Nek5000. Two wing sections are analysed: a NACA0012 at 0 degree angle of attack which presents a mild adverse-pressure gradient (APG) along the chord and a NACA4412 at 5 degree angle of attack with a strong adverse pressure gradient on the suction side, both profiles at Rec = 400,000. The turbulent statistics show that the mild-APG turbulent boundary layer (TBL) of the NACA0012 presents a slight deviation of the velocity fluctuations in the outer region while the strong-APG TBL of the NACA4412 shows significantly larger fluctuations throughout the wall-normal direction with respect to the zero-pressure-gradient (ZPG) TBL. These differences are more substantial in the outer region of the boundary layer. Spectral analyses show that the APG has a significant impact on the largest scales in the boundary layer. Our results indicate that the APG increases the turbulent kinetic energy (TKE) of the TBL, more prominently in the outer layer, and suggest a different mechanism than the one related to high Re in ZPGs.

physics.flu-dyn↗

arXiv2018

Assessment of skin-friction-reduction techniques on a turbulent wing section

The scope of the present project is to quantify the effects of uniform blowing and body-force damping on turbulent boundary layers subjected to a non-uniform adverse-pressure-gradient distribution. To this end, well-resolved large-eddy simulations are employed to describe the flow around the NACA4412 airfoil at moderate Reynolds number 200, 000 based on freestream velocity and chord length. In the present paper we focus on uniform blowing and the conference presentation will include a comparison with body-force damping applied in the same region. The inner-scaled profiles of the mean velocity and of selected components of the Reynolds-stress tensor are examined and compared with the uncontrolled cases. It is known that uniform blowing and adverse-pressure gradients share some similarities in their effect on the boundary layers, and our results will show that these effects are not independent. The behaviour of the skin-friction coefficient is analyzed through the FIK decomposition, and the impact of this control strategy on the aerodynamic efficiency of the airfoil is discussed.

physics.flu-dyn↗