Non-iterative vortex-based smearing correction for the actuator line method

The actuator line method (ALM) is extensively used in wind turbine and rotor simulations. However, its original uncorrected formulation overestimates the forces near the tip of the blades and does not reproduce well forces on translating wings. The recently proposed vortex-based smearing correction for the ALM is a correction based on physical and mathematical properties of the simulation that allows for a more accurate and general ALM. So far, to correct the forces on the blades, the smearing correction depended on an iterative process at every time step, which is usually slower, less stable and less deterministic than direct methods. In this work, a non-iterative process is proposed and validated. First, we propose a formulation of the non-linear lifting line that is equivalent to the ALM with smearing correction, showing that their results are practically identical for a translating wing. Then, by linearizing the lifting line method, the iterative process of the correction is substituted by the direct solution of a small linear system. No significant difference is observed in the results of the iterative and non-iterative corrections, both in wing and rotor simulations. Additional contributions of the present work include the use of a more accurate approximation for the velocity induced by a smeared vortex segment and the implementation of a free-vortex wake model to define the vortex sheet, that contribute to the accuracy and generality of the method. The results present here may motivate the adoption of the ALM by other communities, for example, in fixed-wing applications.Comment: 30 pages, 9 figure

Evaluation of PSE as a Model for Supersonic Jet Using Transfer Functions

Parabolized Stability Equations (PSE) have been shown to model wavepackets and, consequently, the near field of turbulent jets with reasonable accuracy. Because of these capabilities, PSE is a promising reduced-order model to derive control laws that could be employed to reduce the sound generation of a jet. The purpose of this work is to apply PSE to obtain time-domain transfer functions that could estimate both the fluid-dynamic and the acoustic fields of a supersonic jet. The results of this model were compared to results obtained from a database of a well-validated large-eddy simulation of a supersonic jet. Based on the unsteady pressure data at a input position, the time-domain pressure field was estimated using transfer functions obtained using PSE and an empirical method based on the LES data. The prediction scheme employed is a single-input-single-output (SISO), linear model. The unsteady pressure predicted by PSE showed good agreement with the LES results, especially if the input position is outside the mixing layer. For this region, the prediction capabilities of PSE are comparable to those of empirical transfer functions. The agreement is good even for output points taken in the acoustic field, showing that it is possible to estimate the time-domain behaviour of Mach-wave radiation using transfer functions. This indicates that PSE could not only be used to predict the sound generation, but also to open up new potentialities to attenuate noise by means of closed-loop control of the flow. The exploration of the regions where the method displayed good agreement, presented in this work, can guide the positioning of sensors and actuators for experimental implementation of closed-loop control in a jet

On stability of vortices and vorticity generated by actuator lines

Vortices are present in nature and in many flows of industrial importance. The stability of configurations of vortices can have real-world consequences, because vortices play a crucial role in accelerated mixing. In particular, vortices are present in the wake of wind turbines and other rotors. Their blades create a system of multiple helical tip and hub vortices in the wake. The stability of the tip vortices greatly influences the wake recovery behind a turbine and, consequently, can affect the power production and fatigue of a downstream wind turbine in clustered wind farms. Also, concentrated vortices can cause vortex-structure interaction which increases vibration and noise. In this work, the stability of vortices is studied by analytical models and Navier-Stokes simulations. The vorticity generated in these simulations was studied in order to develop improvements to the numerical methods used to simulate blades and wings. Numerical simulations of a moving rotor, representing a floating offshore wind turbine, showed that the wake is dominated by the stability modes predicted by the linear stability theory. Also, the observation that the stability of helical vortices has properties that can be related to the stability of a two-dimensional row of vortices, also noted previously in other works, motivated the development of a new formulation to study the stability of two-dimensional potential flows, based on the bicomplex algebra. Models based on vortex filaments and the Biot-Savart law were developed to study the stability of the system of multiple helical vortices created by turbine blades. The results indicate that the linear stability of the tip vortices is independent of the linear stability of the hub vortices (and vice-versa). For more complex configurations, such as two in-line turbines or blades that create multiple vortices near the tip, the numerical simulations and analytical studies indicate a more complex scenario, with multiple vortices interacting. The Navier-Stokes simulations employ the actuator line method (ALM), which is a method used to model blades that allows coarser grids, reducing computational costs. In this method, the blades are represented by body forces that are calculated from the local flow velocity and airfoil data. However, until recently, the actuator line method misrepresented the forces near the tip of the blades. The recently developed vortex-based smearing correction resolved some of these limitations. In this work, the understanding of the vorticity generated by actuator lines is used to develop more accurate corrections for the velocity induced by a smeared vortex segment and for the magnitude of the vorticity generated in the simulations. Also, a non-iterative procedure for the smearing correction is proposed based on the lifting line method. These modifications improve the agreement of the ALM with a non-linear lifting line method. For the first time, configurations typical of airplane aerodynamics are simulated with the ALM, such as a wing with winglets and a combination of horizontal and vertical tails. The accuracy of these results may motivate other communities to adopt the ALM for a diverse set of applications, beyond rotor aerodynamics.QC 220922</p

Stability of a system of multiple helical vortices

The system of vortices created by the hub and tip vortices of rotors and propellers is composed of two subsystems of helical vortices that have different radii and pitches. A system of external and internal vortices can also be created by some blade devices proposed to destabilize the tip vortices of helicopters and a similar system was observed in an idealized simulation of two in-line wind turbines. The steady solution of these systems of vortices was recently described. However, their stability was not studied. The stability of a system of multiple helical vortices is studied in this work using two methods: a complex-step technique combined with a time-stepping algorithm used to linearize the Biot-Savart law and the vorticity transport equations; and an approximate analytical method. It was noted that the hub and tip vortices do not interact and their linear stability can be treated separately if the velocity field induced by one system is considered in the stability analysis of the other. Strong interaction between the vortices was observed for internal vortices closer to the external vortices, with an out-of-phase mechanism appearing as one of the main phenomena.QC 20221024</p

Stability of a system of multiple helical vortices

The system of vortices created by the hub and tip vortices of rotors and propellers is composed of two subsystems of helical vortices that have different radii and pitches. A system of external and internal vortices can also be created by some blade devices proposed to destabilize the tip vortices of helicopters and a similar system was observed in an idealized simulation of two in-line wind turbines. The steady solution of these systems of vortices was recently described. However, their stability was not studied. The stability of a system of multiple helical vortices is studied in this work using two methods: a complex-step technique combined with a time-stepping algorithm used to linearize the Biot-Savart law and the vorticity transport equations; and an approximate analytical method. It was noted that the hub and tip vortices do not interact and their linear stability can be treated separately if the velocity field induced by one system is considered in the stability analysis of the other. Strong interaction between the vortices was observed for internal vortices closer to the external vortices, with an out-of-phase mechanism appearing as one of the main phenomena.QC 20221024</p

Effect of general anaesthesia on functional outcome in patients with anterior circulation ischaemic stroke having endovascular thrombectomy versus standard care: a meta-analysis of individual patient data

Background: General anaesthesia (GA) during endovascular thrombectomy has been associated with worse patient outcomes in observational studies compared with patients treated without GA. We assessed functional outcome in ischaemic stroke patients with large vessel anterior circulation occlusion undergoing endovascular thrombectomy under GA, versus thrombectomy not under GA (with or without sedation) versus standard care (ie, no thrombectomy), stratified by the use of GA versus standard care. Methods: For this meta-analysis, patient-level data were pooled from all patients included in randomised trials in PuMed published between Jan 1, 2010, and May 31, 2017, that compared endovascular thrombectomy predominantly done with stent retrievers with standard care in anterior circulation ischaemic stroke patients (HERMES Collaboration). The primary outcome was functional outcome assessed by ordinal analysis of the modified Rankin scale (mRS) at 90 days in the GA and non-GA subgroups of patients treated with endovascular therapy versus those patients treated with standard care, adjusted for baseline prognostic variables. To account for between-trial variance we used mixed-effects modelling with a random effect for trials incorporated in all models. Bias was assessed using the Cochrane method. The meta-analysis was prospectively designed, but not registered. Findings: Seven trials were identified by our search; of 1764 patients included in these trials, 871 were allocated to endovascular thrombectomy and 893 were assigned standard care. After exclusion of 74 patients (72 did not undergo the procedure and two had missing data on anaesthetic strategy), 236 (30%) of 797 patients who had endovascular procedures were treated under GA. At baseline, patients receiving GA were younger and had a shorter delay between stroke onset and randomisation but they had similar pre-treatment clinical severity compared with patients who did not have GA. Endovascular thrombectomy improved functional outcome at 3 months both in patients who had GA (adjusted common odds ratio (cOR) 1·52, 95% CI 1·09–2·11, p=0·014) and in those who did not have GA (adjusted cOR 2·33, 95% CI 1·75–3·10, p&lt;0·0001) versus standard care. However, outcomes were significantly better for patients who did not receive GA versus those who received GA (covariate-adjusted cOR 1·53, 95% CI 1·14–2·04, p=0·0044). The risk of bias and variability between studies was assessed to be low. Interpretation: Worse outcomes after endovascular thrombectomy were associated with GA, after adjustment for baseline prognostic variables. These data support avoidance of GA whenever possible. The procedure did, however, remain effective versus standard care in patients treated under GA, indicating that treatment should not be withheld in those who require anaesthesia for medical reasons