Bioinspired fluid-structure interaction problems: gusts, load mitigation and resonance

Mención Internacional en el título de doctorNature often serves as a reference for the design and development of sustainable solutions in numerous different fields. The recent development of small-scale robotic vehicles, asMicro-Air Vehicles (MAVs), is not an exception, and has had an increasingly important impact on society, proposing new alternatives in areas as surveillance or planetary exploration. Trying to mimic the flight of insects and small birds, these devices try to offer more efficient designs and with higher manoeuvrability abilities than the already existing designs. It happens similar with robotic swimmers, with many different existing prototypes. Indeed, it is even possible to find designs of bioinspired small-scale wind turbines based on auto-rotating seeds looking for a more efficient energy harvesting. Besides, in order to develop sustainable designs, increasing their lifetime and reducing the maintenance costs are crucial factors. Depending on the device to design, different methodologies may be followed in order to achieve these two goals while meeting the design requirements. One clear example can be found in the development of wind turbines. Their blades must be designed to withstand not only maximum loads and stresses but also the fatigue caused by the fluctuations around the load required to operate correctly. Reducing fatigue issues by limiting the amplitude of those fluctuations using passive or active control is a viable option to improve their lifetime. The aimof this dissertation is to contribute to the understanding of the underlying physics in biolocomotion. To this end, direct numerical simulations of different examples and problems at low Reynolds number, Re, have been performed using an existing fluid-structure interaction (FSI) solver. This FSI solver relies on the coupling of an incompressible-flow solver with robotic algorithms for the computation of the dynamics of a system of connected rigid bodies. The particularities of this solver are detailed in the thesis. The second part of the thesis includes the analysis of these examples and problems mentioned above.More in detail, the aerodynamic and aeroelastic behaviour of airfoils and wings at Re Æ 1000 in various conditions and environments has been analysed. Natural flyers and swimmers are immersed in turbulent and gusty environments which affect their aerodynamic behaviour. The first problem that has been studied is that of the unsteady response of airfoils impacted by vortical gusts. This first example focuses on how the impact of viscous vortices of different size and intensity on two-dimensional airfoils modify their response. Although in a simplified framework, this analysis allows to gather relevant information about the aerodynamic performance of the airfoils. This aerodynamic response is seen to be self-similar, and the work proposes a semi-empirical model to determine the temporal evolution of the lifting forces based on an integral definition of the vertical velocity induced by the gust, which can be known a priori. The target of the second problem is to analyse the load that can be mitigated in airfoils undergoing oscillations in the angle of attack using passive-pitching trailing edge flaps. This corresponds, for example, to a simplification of the problem of load mitigation in small-scale wind turbines. The use of passive-pitching trailing edge flaps is a strategy that has recently been recently proposed for large-scale wind turbines. Here, we investigate the validity of this strategy on a completely different scenario. Contrary to what happens in experiments at higher Reynolds numbers, whose results match the predictions of a quasi-steady linear model when the kinematics are within the range of applicability of this model, the load mitigation obtained in this work differs from the values of this theory. The load mitigated is larger or smaller than the predicted values depending on the amplitude of the oscillations in the angle of attack. However, the results of this work show that an increase in the length of the flap while the chord of the airfoil is kept constant leads to an equal change in the reduction of load, in line with the predictions of the quasi-steady model. The development of vortical structures is clearly affected by the flap when it is sufficiently large, which also involves changes in the dynamics of the flap and the forces seen by the airfoil. The repercussion that several of the variables defining the parametric space have on the aerodynamic behaviour of the foil and the dynamics of the flap are analysed. This allows to gather more information for an appropriate selection of those variables. Finally, the third and fourth problems involve the study of the effects of spanwise flexibility on both isolated wings and pairs of wings arranged in horizontal tandem undergoing flapping motions. The wings are considered to be rectangular flat plates, and the spanwise flexibility is modelled discretizing these flat plates in a finite number of rigid sub-bodies that are connected using torsional springs. The wings are considered to be rigid in the chordwise direction. Isolated spanwise-flexible wings find an optimal propulsive performance when a fluid-structural resonance occurs. At this flexibility, the time-averaged thrust is maximum and twice the value yielded by the rigid case, and the increment in efficiency is around a 15%. Flexibility and the generation of forces are coupled, such that the structural response modifies the development of the vortical structures generated by the motion of the wing, and vice versa. The optimal performance comes from a combination of larger effective angles of attack, properly timed with the pitching motion such that the projection of the forces is maximum, with a delayed development of the vortical structures. Besides, while aspect ratio effects are important for rigid wings, this effect becomes small when compared to flexibility effects when the wings become flexible enough. In fact, while the increase in thrust coefficient for rigid wings with aspect ratio 4 is 1.2 times larger than that provided by rigid wings with aspect ratio equal to 2, the value of this coefficient for resonant wings is twice the value yielded by rigid wings of aspect ratio 4. While forewings of the tandem systems are found to behave similarly to isolated wings, the aeroelastic response of the hindwings is substantially affected by the interaction with the vortices developed and shed by the forewings. This wake capture effect modifies the flexibility at which an optimal propulsive behaviour is obtained. This wake capture effect is analysed through an estimation of the effective angle of attack seen by both forewings and hindwings, linking the optimal behaviour with the maximisation of the effective angle of attack at the right instants. Based on the obtained results, a proof-of-concept study has been carried out analysing the aerodynamic performance of tandem systems made of wings with different flexibility, which suggests that the latter could outperformsystems of equally flexible wings.This thesis has been carried out in the Aerospace Engineering Department at Universidad Carlos III de Madrid. The financial support has been provided by the Universidad Carlos III de Madrid through a PIPF scholarship awarded on a competitive basis, and by the Spanish Ministry of Economy and Competitiveness through grant DPI2016-76151-C2-2-R (AEI/FEDER, UE).Programa de Doctorado en Mecánica de Fluidos por la Universidad Carlos III de Madrid; la Universidad de Jaén; la Universidad de Zaragoza; la Universidad Nacional de Educación a Distancia; la Universidad Politécnica de Madrid y la Universidad Rovira i VirgiliPresidente: José Ignacio Jiménez González.- Secretaria: Andrea Ianiro.- Vocal: Manuel Moriche Guerrer

Fluid-structure resonance in spanwise-flexible flapping wings

We report direct numerical simulations of the flow around a spanwise-flexible wing in forward flight. The simulations were performed at Re = 1000 for wings of aspect ratio 2 and 4 undergoing a heaving and pitching motion at Strouhal number Stc ≈ 0.5. We have varied the effective stiffness of the wing Π1 while keeping the effective inertia constant, Π0 = 0.1. It has been found that there is an optimal aerodynamic performance of the wing linked to a damped resonance phenomenon, that occurs when the imposed frequency of oscillation approaches the first natural frequency of the structure in the fluid, ωn,f /ω ≈ 1. In that situation, the time-averaged thrust is maximum, increasing by factor 2 with respect to the rigid case with an increase in propulsive efficiency of approximately 15 %. This enhanced aerodynamic performance results from the combination of larger effective angles of attack of the outboard wing sections and a delayed development of the leading-edge vortex. With increasing flexibility beyond the resonant frequency, the aerodynamic performance drops significantly, in terms of both thrust production and propulsive efficiency. The cause of this drop lies in the increasing phase lag between the deflection of the wing and the heaving/pitching motion, which results in weaker leading-edge vortices, negative effective angles of attack in the outboard sections of the wing, and drag generation in the first half of the stroke. Our results also show that flexible wings with the same ωn,f /ω but different aspect ratio have the same aerodynamic performance, emphasizing the importance of the structural properties of the wing for its aerodynamic performance.Funding. This work was partially supported by grant DPI2016-76151-C2-2-R (AEI/FEDER, UE).Publicad

Outcomes from elective colorectal cancer surgery during the SARS-CoV-2 pandemic

This study aimed to describe the change in surgical practice and the impact of SARS-CoV-2 on mortality after surgical resection of colorectal cancer during the initial phases of the SARS-CoV-2 pandemic

Analysis of vortical gust impact on airfoils at low Reynolds number

The response of a NACA0012 airfoil impacted by vortical gusts is investigated perform-ing Direct Numerical Simulations of the two-dimensional incompressible flow. Taylorvortices of different diameter and intensity located at different vertical separationswith respect to the airfoil are deployed in the free stream. These vortices, which arecharacterized by its compact distribution of vorticity, are advected downstream tointeract with the airfoil, set at a fixed angle of attack. For the low Reynolds numberused in these simulations (Re=1000), the effect of the different parameters definingthe vortical gust and the impact is characterized. It is found that the change in thetime evolution of the variation of the lift coefficient with respect to the steady state,∆Cl(t), is fairly independent on the angle of attack, at least in the range ofαconsideredin this study. Furthermore, it is found that the time at which the peak in∆Clisproduced scales with the diameter of the viscous core of the vortex and the free-streamvelocity,D/U∞. On the other hand, the maximum value of∆Clis roughly proportionalto the non-dimensional vortex circulation, but varies non-linearly with the verticaldistance between the vortex and the airfoil. This dependency can be captured by scaling∆Clwith the relative intensity of the vertical velocity induced over the airfoil andthe free-stream velocity (wh/U∞), where the former is defined as an integral of thevortex velocity profile over the chord of the airfoil. Using this scaling, the profiles of∆Cl(tU∞/D)/(wh/U∞) collapse over a single curve for the different vortex intensities,sizesandverticalseparationsconsideredinthepresentstudy,speciallyduringtheinitialevolution of the vortical gust impact. The self-similar profile of∆Cl(tU∞/D)/(wh/U∞) isfound to depend on the velocity profile of the vortex (i.e., Taylor vortices versus Lamb–Oseenvortices).However,thepeakaerodynamicforceandthetimetopeakaerodynamicforce seem to scale withD/U∞andwh/U∞irrespective of the velocity profile of thevortex, suggesting that our definition ofwhis sufficiently robust

Association of mechanical bowel preparation with oral antibiotics and anastomotic leak following left sided colorectal resection: an international, multi-centre, prospective audit.

This is the peer reviewed version of the following article: , (2018), Association of mechanical bowel preparation with oral antibiotics and anastomotic leak following left sided colorectal resection: an international, multi‐centre, prospective audit. Colorectal Dis, 20: 15-32. doi:10.1111/codi.14362, which has been published in final form at https://doi.org/10.1111/codi.14362. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived VersionsINTRODUCTION: The optimal bowel preparation strategy to minimise the risk of anastomotic leak is yet to be determined. This study aimed to determine whether oral antibiotics combined with mechanical bowel preparation (MBP+Abx) was associated with a reduced risk of anastomotic leak when compared to mechanical bowel preparation alone (MBP) or no bowel preparation (NBP). METHODS: A pre-planned analysis of the European Society of Coloproctology (ESCP) 2017 Left Sided Colorectal Resection audit was performed. Patients undergoing elective left sided colonic or rectal resection with primary anastomosis between 1 January 2017 and 15 March 2017 by any operative approach were included. The primary outcome measure was anastomotic leak. RESULTS: Of 3676 patients across 343 centres in 47 countries, 618 (16.8%) received MBP+ABx, 1945 MBP (52.9%) and 1099 patients NBP (29.9%). Patients undergoing MBP+ABx had the lowest overall rate of anastomotic leak (6.1%, 9.2%, 8.7% respectively) in unadjusted analysis. After case-mix adjustment using a mixed-effects multivariable regression model, MBP+Abx was associated with a lower risk of anastomotic leak (OR 0.52, 0.30-0.92, P = 0.02) but MBP was not (OR 0.92, 0.63-1.36, P = 0.69) compared to NBP. CONCLUSION: This non-randomised study adds 'real-world', contemporaneous, and prospective evidence of the beneficial effects of combined mechanical bowel preparation and oral antibiotics in the prevention of anastomotic leak following left sided colorectal resection across diverse settings. We have also demonstrated limited uptake of this strategy in current international colorectal practice

Evolution over Time of Ventilatory Management and Outcome of Patients with Neurologic Disease∗

OBJECTIVES: To describe the changes in ventilator management over time in patients with neurologic disease at ICU admission and to estimate factors associated with 28-day hospital mortality. DESIGN: Secondary analysis of three prospective, observational, multicenter studies. SETTING: Cohort studies conducted in 2004, 2010, and 2016. PATIENTS: Adult patients who received mechanical ventilation for more than 12 hours. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Among the 20,929 patients enrolled, we included 4,152 (20%) mechanically ventilated patients due to different neurologic diseases. Hemorrhagic stroke and brain trauma were the most common pathologies associated with the need for mechanical ventilation. Although volume-cycled ventilation remained the preferred ventilation mode, there was a significant (p < 0.001) increment in the use of pressure support ventilation. The proportion of patients receiving a protective lung ventilation strategy was increased over time: 47% in 2004, 63% in 2010, and 65% in 2016 (p < 0.001), as well as the duration of protective ventilation strategies: 406 days per 1,000 mechanical ventilation days in 2004, 523 days per 1,000 mechanical ventilation days in 2010, and 585 days per 1,000 mechanical ventilation days in 2016 (p < 0.001). There were no differences in the length of stay in the ICU, mortality in the ICU, and mortality in hospital from 2004 to 2016. Independent risk factors for 28-day mortality were age greater than 75 years, Simplified Acute Physiology Score II greater than 50, the occurrence of organ dysfunction within first 48 hours after brain injury, and specific neurologic diseases such as hemorrhagic stroke, ischemic stroke, and brain trauma. CONCLUSIONS: More lung-protective ventilatory strategies have been implemented over years in neurologic patients with no effect on pulmonary complications or on survival. We found several prognostic factors on mortality such as advanced age, the severity of the disease, organ dysfunctions, and the etiology of neurologic disease

Safety of primary anastomosis following emergency left sided colorectal resection: an international, multi-centre prospective audit.

This is the peer reviewed version of the following article: group, T. E. S. o. C. c. (2018). "Safety of primary anastomosis following emergency left sided colorectal resection: an international, multi-centre prospective audit." Colorectal Disease 20(S6): 47-57., which has been published in final form at https://doi.org/10.1111/codi.1437. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived VersionsINTRODUCTION: Some evidence suggests that primary anastomosis following left sided colorectal resection in the emergency setting may be safe in selected patients, and confer favourable outcomes to permanent enterostomy. The aim of this study was to compare the major postoperative complication rate in patients undergoing end stoma vs primary anastomosis following emergency left sided colorectal resection. METHODS: A pre-planned analysis of the European Society of Coloproctology 2017 audit. Adult patients (> 16 years) who underwent emergency (unplanned, within 24 h of hospital admission) left sided colonic or rectal resection were included. The primary endpoint was the 30-day major complication rate (Clavien-Dindo grade 3 to 5). RESULTS: From 591 patients, 455 (77%) received an end stoma, 103 a primary anastomosis (17%) and 33 primary anastomosis with defunctioning stoma (6%). In multivariable models, anastomosis was associated with a similar major complication rate to end stoma (adjusted odds ratio for end stoma 1.52, 95%CI 0.83-2.79, P = 0.173). Although a defunctioning stoma was not associated with reduced anastomotic leak (12% defunctioned [4/33] vs 13% not defunctioned [13/97], adjusted odds ratio 2.19, 95%CI 0.43-11.02, P = 0.343), it was associated with less severe complications (75% [3/4] with defunctioning stoma, 86.7% anastomosis only [13/15]), a lower mortality rate (0% [0/4] vs 20% [3/15]), and fewer reoperations (50% [2/4] vs 73% [11/15]) when a leak did occur. CONCLUSIONS: Primary anastomosis in selected patients appears safe after left sided emergency colorectal resection. A defunctioning stoma might mitigate against risk of subsequent complications

The impact of conversion on the risk of major complication following laparoscopic colonic surgery: an international, multicentre prospective audit.

This is the peer reviewed version of the following article: The and E. S. o. C. c. groups (2018). "The impact of conversion on the risk of major complication following laparoscopic colonic surgery: an international, multicentre prospective audit." Colorectal Disease 20(S6): 69-89., which has been published in final form at https://doi.org/10.1111/codi.14371. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.BACKGROUND: Laparoscopy has now been implemented as a standard of care for elective colonic resection around the world. During the adoption period, studies showed that conversion may be detrimental to patients, with poorer outcomes than both laparoscopic completed or planned open surgery. The primary aim of this study was to determine whether laparoscopic conversion was associated with a higher major complication rate than planned open surgery in contemporary, international practice. METHODS: Combined analysis of the European Society of Coloproctology 2017 and 2015 audits. Patients were included if they underwent elective resection of a colonic segment from the caecum to the rectosigmoid junction with primary anastomosis. The primary outcome measure was the 30-day major complication rate, defined as Clavien-Dindo grade III-V. RESULTS: Of 3980 patients, 64% (2561/3980) underwent laparoscopic surgery and a laparoscopic conversion rate of 14% (359/2561). The major complication rate was highest after open surgery (laparoscopic 7.4%, converted 9.7%, open 11.6%, P < 0.001). After case mix adjustment in a multilevel model, only planned open (and not laparoscopic converted) surgery was associated with increased major complications in comparison to laparoscopic surgery (OR 1.64, 1.27-2.11, P < 0.001). CONCLUSIONS: Appropriate laparoscopic conversion should not be considered a treatment failure in modern practice. Conversion does not appear to place patients at increased risk of complications vs planned open surgery, supporting broadening of selection criteria for attempted laparoscopy in elective colonic resection

The impact of surgical delay on resectability of colorectal cancer: An international prospective cohort study

AimThe SARS-CoV-2 pandemic has provided a unique opportunity to explore the impact of surgical delays on cancer resectability. This study aimed to compare resectability for colorectal cancer patients undergoing delayed versus non-delayed surgery.MethodsThis was an international prospective cohort study of consecutive colorectal cancer patients with a decision for curative surgery (January-April 2020). Surgical delay was defined as an operation taking place more than 4 weeks after treatment decision, in a patient who did not receive neoadjuvant therapy. A subgroup analysis explored the effects of delay in elective patients only. The impact of longer delays was explored in a sensitivity analysis. The primary outcome was complete resection, defined as curative resection with an R0 margin.ResultsOverall, 5453 patients from 304 hospitals in 47 countries were included, of whom 6.6% (358/5453) did not receive their planned operation. Of the 4304 operated patients without neoadjuvant therapy, 40.5% (1744/4304) were delayed beyond 4 weeks. Delayed patients were more likely to be older, men, more comorbid, have higher body mass index and have rectal cancer and early stage disease. Delayed patients had higher unadjusted rates of complete resection (93.7% vs. 91.9%, P = 0.032) and lower rates of emergency surgery (4.5% vs. 22.5%, P ConclusionOne in 15 colorectal cancer patients did not receive their planned operation during the first wave of COVID-19. Surgical delay did not appear to compromise resectability, raising the hypothesis that any reduction in long-term survival attributable to delays is likely to be due to micro-metastatic disease

An international multicentre prospective audit of elective rectal cancer surgery; operative approach versus outcome, including transanal total mesorectal excision (TaTME)

IntroductionTransanal total mesorectal excision (TaTME) has rapidly emerged as a novel approach for rectal cancer surgery. Safety profiles are still emerging and more comparative data is urgently needed. This study aimed to compare indications and short-term outcomes of TaTME, open, laparoscopic, and robotic TME internationally.MethodsA pre-planned analysis of the European Society of Coloproctology (ESCP) 2017 audit was performed. Patients undergoing elective total mesorectal excision (TME) for malignancy between 1 January 2017 and 15 March 2017 by any operative approach were included. The primary outcome measure was anastomotic leak.ResultsOf 2579 included patients, 76.2% (1966/2579) underwent TME with restorative anastomosis of which 19.9% (312/1966) had a minimally invasive approach (laparoscopic or robotic) which included a transanal component (TaTME). Overall, 9.0% (175/1951, 15 missing outcome data) of patients suffered an anastomotic leak. On univariate analysis both laparoscopic TaTME (OR 1.61, 1.02-2.48, P=0.04) and robotic TaTME (OR 3.05, 1.10-7.34, P=0.02) were associated with a higher risk of anastomotic leak than non-transanal laparoscopic TME. However this association was lost in the mixed-effects model controlling for patient and disease factors (OR 1.23, 0.77-1.97, P=0.39 and OR 2.11, 0.79-5.62, P=0.14 respectively), whilst low rectal anastomosis (OR 2.72, 1.55-4.77, P<0.001) and male gender (OR 2.29, 1.52-3.44, P<0.001) remained strongly associated. The overall positive circumferential margin resection rate was 4.0%, which varied between operative approaches: laparoscopic 3.2%, transanal 3.8%, open 4.7%, robotic 1%.ConclusionThis contemporaneous international snapshot shows that uptake of the TaTME approach is widespread and is associated with surgically and pathologically acceptable results