Active Flow Control at Low Reynolds Numbers by Periodic Airfoil Morphing

This paper investigates the application of a periodically deforming airfoil surface for the purpose of flow control at low Reynolds numbers. A physical model has been fabricated by bonding Macro Fiber Composite (MFC) actuators to the underside of a NACA 4415’s suction surface. The results presented build on work by Jones et al.1 First, the behavior of the surface when actuated at a range frequencies is investigated through a combination of photogrammetric and laser sensor measurements. Second, the aerodynamic performance of this novel flow control technique is presented. It is shown that when the actuation frequency ‘locks-in’ to the surface motion significant improvements in performance are observed in a flight regime notorious for poor airfoil behavior

Buckling of thick cylindrical shells under external pressure: A new analytical expression for the critical load and comparison with elasticity solutions

AbstractIn this paper a set of stability equations for thick cylindrical shells is derived and solved analytically. The set is obtained by integration of the differential stability equations across the thickness of the shell. The effects of transverse shear and the non-linear variation of the stresses and displacements are accounted for with the aid of the higher order shell theory proposed by [Voyiadjis, G.Z. and Shi, G., 1991, A refined two-dimensional theory for thick cylindrical shells, International Journal of Solids and Structures, 27(3), 261–282.]. For a thick shell under external hydrostatic pressure, the stability equations are solved analytically and yield an improved expression for the buckling load. Reference solutions are also obtained by solving numerically the differential stability equations. Both the full set that contains strains and rotations as well as the simplified set that contains rotations only were solved numerically. The relative magnitude of shear strain and rotation was examined and the effect of thickness was quantified. Differences between the benchmark solutions and the analytic expressions based on the refined theory and the classical shell theory are analysed and discussed. It is shown that the new analytic expression provides significantly improved predictions compared to the formula based on thin shell theory

Investigation of the three-dimensional flow past a flatback wind turbine airfoil at high angles of attack

Flatback airfoils are airfoils with a blunt trailing edge. They are currently commonly used in the inboard part of large wind turbine blades, as they offer a number of aerodynamic, structural, and aeroelastic benefits. However, the flow past them at high angles of attack (AoA) has received relatively little attention until now. This is important because they usually operate at high AoA at the inboard part of Wind Turbine blades. The present investigation uses Reynolds averaged Navier–Stokes (RANS) and hybrid RANS + large eddy simulation predictions to analyze the flow in question. The numerical results are validated against previously published wind tunnel experiments. The analysis reveals that to successfully simulate this flow, the spanwise extent of the computational domain is crucial, more so than the selection of the modeling approach. Additionally, a low-drag regime observed at angles of attack before stall is identified and analyzed in detail. Finally, the complex interaction between the three-dimensional separated flow beyond maximum lift (stall cells) with the vortex shedding from the blunt trailing edge is revealed

The flow past a flatback airfoil with flow control devices: benchmarking numerical simulations against wind tunnel data

As wind turbines grow larger, the use of flatback airfoils has become standard practice for the root region of the blades. Flatback profiles provide higher lift and reduced sensitivity to soiling at significantly higher drag values. A number of flow control devices have been proposed to improve the performance of flatback profiles. In the present study, the flow past a flatback airfoil at a chord Reynolds number of 1:5_106 with and without trailing edge flow control devices is considered. Two different numerical approaches are applied, unsteady Reynolds-Averaged Navier Stokes (RANS) simulations and detached eddy simulations (DES). The computational predictions are compared against wind tunnel measurements to assess the suitability of each method. The effect of each flow control device on the flow is examined based on the DES results on the finer mesh. Results agree well with the experimental findings and show that a newly proposed flap device outperforms traditional solutions for flatback airfoils. In terms of numerical modelling, the more expensive DES approach is more suitable if the wake frequencies are of interest, but the simplest 2D RANS simulations can provide acceptable load predictions

Control of low Reynolds number flow around an airfoil using periodic surface morphing: a numerical study

The principal aim of this paper is to use Direct Numerical Simulations (DNS) to explain the mechanisms that allow periodic surface morphing to improve the aerodynamic performance of an airfoil. The work focuses on a NACA-4415 airfoil at Reynolds number Rec=5×104 and 0° angle of attack. At these flow conditions, the boundary layer separates at x∕c=0.42, remains laminar until x∕c≈0.8, and then transitions to turbulence. Vortices are formed in the separating shear layer at a characteristic Kelvin–Helmholtz frequency of Sts=4.9, which compares well with corresponding experiments. These are then shed into the wake. Turbulent reattachment does not occur because the region of high turbulent kinetic energy (and therefore mixing) is located too far downstream and too far away from the airfoil surface to influence the near-wall flow. The effect of three actuation frequencies is examined by performing the simulations on a computational domain that deforms periodically. It is found that by amplifying the Kelvin–Helmholtz instability mechanism, Large Spanwise Coherent structures are forced to form and retain their coherence for a large part of the actuation cycle. Following their formation, these structures entrain high momentum fluid into the near-wall flow, leading to almost complete elimination of the recirculation zone. The instantaneous and phase averaged characteristics of these structures are analyzed and the vortex coherence is related to the phase of actuation. In order to further clarify the process of reduction in the size of recirculation zone, simulations start from the fully-developed uncontrolled flow and continue for 25 actuation cycles. The results indicate that the modification of airfoil characteristics is a gradual process. As the number of cycles increases and the coherent vortices are repeatedly formed and propagate downstream, they entrain momentum, thereby modifying the near wall region. During this transient period, the separated shear layer approaches the airfoil surface and the size of recirculation region decreases. It takes at least 15 cycles for the flow to develop a repeatable, periodic pattern

The earthquake intervent time distribution along the Hellenic subduction Zone.

Η ελληνική ζώνη υποβύθισης είναι η πιο σεισμική περιοχή της Ευρώπης. Η κατανομή Weibullέχει χρησιμοποιηθεί σαν μοντέλο για την περιγραφή των χρονικών διαστημάτων μεταξύ διαδοχικών σεισμών σε συγκεκριμένες ρηξιγενείς ζώνες (Rikitake, 1976; Rikitake, 1991), καθώς και για την περιγραφή της σεισμικότητας μίας ευρύτερης περιοχής (Hasumietal., 2009). Taδεδομένα που αναλύουμε σε αυτή την εργασία αφορούν την ελληνική ζώνη υποβύθισης και καλύπτουν την περίοδο 1976-2009. Χρησιμοποιούμε τις σεισμικές πηγές επιφανειακών σεισμών όπως αυτές ορίζονται από τους PapaioannouandPapazachos(2000), καθώς και τονκατάλογο σεισμών των Makropoulosetal. (2012) για την Ελλάδα και τις παρακείμενες περιοχές. Η εφαρμογή της κατανομής Weibullστα χρονικά διαστήματα μεταξύ διαδοχικών σεισμών αναλύεται και περιγράφεται σε αυτή την εργασία.The Hellenic Subduction Zone (HSZ) is the most seismically active region in Europe (Becker and Meier, 2010). The evolution of such an active region is characterized by complex phenomenology and is expressed through seismicity. Seismicity temporal patterns remain as one of the most important topics in earth sciences. The Weibull distribution has been used as a recurrence time model for large earthquakes (Rikitake, 1976; Rikitake, 1991). Moreover,Hasumi et al. (2009) used the Weibulllog Weibull distribution for the study of the interoccurrence times of earthquakes in Japan.The dataset formed in this study concerns the seismic belt of the HSZ during the period 1976-2009. We use the external seismic sources of shallow earthquakes in the Aegean and the surrounding area (Papaioannou and Papazachos, 2000) along with the updated and extended earthquake catalogue for Greece and adjacent areas (Makropoulos et al., 2012).The application of the Weibull distribution to the interevent times of the formed dataset is analyzed and discussed

Acute Partial Sleep Deprivation and High-Intensity Exercise Effects on Cardiovascular Autonomic Regulation and Lipemia Network

Autonomic nervous system imbalance demonstrated by decreased heart rate variability (HRV) is linked to acute partial sleep deprivation (ASPD) and cardiovascular disease (CVD). Cardiometabolic lipemia has been linked to changes in HRV. Habitually active individuals exercising in the morning hours under APSD and consuming afterwards a high-fat breakfast, may disrupt the network coordination of both the cardiovascular autonomic regulation and cardiometabolic lipemic systems jeopardizing their health. The human organism is comprised by an integrated network of interconnected organ systems and functions, therefore, a disruption/failure of one system can trigger a cascade of failures manifested as disease state. PURPOSE: To investigate the postprandial network interactions of autonomic regulation assessed by heart rate variability (HRV) and cardiometabolic lipemia assessed by low-density lipoprotein (LDL) cholesterol under APSD and after a high-intensity interval exercise (HIIE). METHODS: Fifteen healthy males (age 31 ± 5 years) participated in: (a) reference sleep (RS) (~ 9.5 h) and HIIE (RSX) and (b) APSD and HIIE (SSX). HIIE was performed in 3:2 min intervals at 90% and 40% of VO2reserve. HRV selected time and frequency domain indices were recorded the night before (D1), the morning of the next day (D2), 1 hr post-HIEE (1hrPE), 2 hr (2hrPE)-, 4hr (4hrPE), and 6-hr post-HIIE (6hrPE). Postprandial LDL was assessed at D1, D2, 1hrPE and 4hrPE. Pearson correlation coefficients and correlation matrices were used to investigate the physiologic network during RSX and SSX. Interactions within each network were computed by the number of links (i.e. number of significant Pearson correlations) and presented as positive and negative links. RESULTS: The total number of links increased by 90% under SSX compared to RSX due to: (i) manifestation of weak and intermediate negative links between the HRV and the LDL sub-networks and (ii) a 100% increase of positive links within the LDL sub-network. CONCLUSION: This study shows a complex network of interactions between autonomic regulation and cardiometabolic lipemia. Our results uncover how this physiological network reorganizes in response to APSD confirming the inverse relationship between HRV and LDL. HRV can be used as an alternative non-invasive marker of CVD

Experimental Investigation of the Influence of the Foreploughshare and the Disk Coulter on the Tillage Quality and the Tractor Fuel Consumption

Rosana G. Moreira, Editor-in-Chief; Texas A&M UniversityThis is a Technical Paper from International Commission of Agricultural Engineering (CIGR, Commission Internationale du Genie Rural) E-Journal Volume 4 (2002): A. Natsis, G. Papadakis, and I. Pitsilis. Experimental Investigation of the Influence of the Foreploughshare and the Disk Coulter on the Tillage Quality and the Tractor Fuel Consumption. Vol. IV. December 2002