Reliable Force Predictions for a Flapping-wing Micro Air Vehicle:A "Vortex-lift" Approach

Vertical and horizontal force of a flapping-wing micro air vehicle (MAV) has been measured in slow-speed forward flight using a force balance. Detailed information on kinematics was used to estimate forces using a blade-element analysis. Input variables for this analysis are lift and drag coefficients. These coefficients are usually derived from steady-state measurements of a wing in translational flow. Previous studies on insect flight have shown that this method underestimates forces in flapping flight, mainly because it cannot account for additional lift created by unsteady phenomena. We therefore derived lift and drag coefficients using a concept for delta-wings with stably attached leading-edge vortices. Resulting lift coefficients appeared to be a factor of 2.5 higher than steady-flow coefficients, and match the results from previous (numerical) studies on instantaneous lift coefficients in flapping flight. The present study confirms that a blade-element analysis using force coefficients derived from steady-state wind tunnel measurements underestimates vertical force by a factor of approximately two. The equivalent analysis, using "vortex-lift" enhanced coefficients from a delta-wing analogue, yields very good agreement with force balance measurements, and hence seems to be a good approximation for lift-enhancing flow phenomena when modelling flapping flight

Rotational superradiant scattering in a vortex flow

When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. In theory, if the obstacle is rotating, waves can be amplified in the process, extracting energy from the scatterer. Here we describe in detail the first laboratory detection of this phenomenon, known as superradiance 1, 2, 3, 4. We observed that waves propagating on the surface of water can be amplified after being scattered by a draining vortex. The maximum amplification measured was 14% ± 8%, obtained for 3.70 Hz waves, in a 6.25-cm-deep fluid, consistent with the superradiant scattering caused by rapid rotation. We expect our experimental findings to be relevant to black-hole physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole 5, 6, 7, 8, 9, 10, as well as to hydrodynamics, due to the close relation to over-reflection instabilities 11, 12, 13

Evaluation of a Desktop 3D Printed Rigid Refractive-Indexed-Matched Flow Phantom for PIV Measurements on Cerebral Aneurysms

Purpose Fabrication of a suitable flow model or phantom is critical to the study of biomedical fluid dynamics using optical flow visualization and measurement methods. The main difficulties arise from the optical properties of the model material, accuracy of the geometry and ease of fabrication. Methods Conventionally an investment casting method has been used, but recently advancements in additive manufacturing techniques such as 3D printing have allowed the flow model to be printed directly with minimal post-processing steps. This study presents results of an investigation into the feasibility of fabrication of such models suitable for particle image velocimetry (PIV) using a common 3D printing Stereolithography process and photopolymer resin. Results An idealised geometry of a cerebral aneurysm was printed to demonstrate its applicability for PIV experimentation. The material was shown to have a refractive index of 1.51, which can be refractive matched with a mixture of de-ionised water with ammonium thiocyanate (NH4SCN). The images were of a quality that after applying common PIV pre-processing techniques and a PIV cross-correlation algorithm, the results produced were consistent within the aneurysm when compared to previous studies. Conclusions This study presents an alternative low-cost option for 3D printing of a flow phantom suitable for flow visualization simulations. The use of 3D printed flow phantoms reduces the complexity, time and effort required compared to conventional investment casting methods by removing the necessity of a multi-part process required with investment casting techniques

A laboratory-numerical approach for modelling scale effects in dry granular slides

Granular slides are omnipresent in both natural and industrial contexts. Scale effects are changes in physical behaviour of a phenomenon at different geometric scales, such as between a laboratory experiment and a corresponding larger event observed in nature. These scale effects can be significant and can render models of small size inaccurate by underpredicting key characteristics such as ow velocity or runout distance. Although scale effects are highly relevant to granular slides due to the multiplicity of length and time scales in the flow, they are currently not well understood. A laboratory setup under Froude similarity has been developed, allowing dry granular slides to be investigated at a variety of scales, with a channel width configurable between 0.25-1.00 m. Maximum estimated grain Reynolds numbers, which quantify whether the drag force between a particle and the surrounding air act in a turbulent or viscous manner, are found in the range 102-103. A discrete element method (DEM) simulation has also been developed, validated against an axisymmetric column collapse and a granular slide experiment of Hutter and Koch (1995), before being used to model the present laboratory experiments and to examine a granular slide of significantly larger scale. This article discusses the details of this laboratory-numerical approach, with the main aim of examining scale effects related to the grain Reynolds number. Increasing dust formation with increasing scale may also exert influence on laboratory experiments. Overall, significant scale effects have been identified for characteristics such as ow velocity and runout distance in the physical experiments. While the numerical modelling shows good general agreement at the medium scale, it does not capture differences in behaviour seen at the smaller scale, highlighting the importance of physical models in capturing these scale effects

Der Druck-Scherversuch zur Ermittlung der interlaminaren Scherfestigkeit (ILSF) von faserverstärkten Keramiken bei unterschiedlichen Beanspruchungsbedingungen

Es wird ein neues Prüfverfahren zur Ermittlung der Interlaminaren Scherfestigkeit (ILSF) von laminierten Verbundwerkstoffen vorgestellt, bei dem unsymmetrisch gekerbte Proben unter axialer Druckbelastung getestet werden. Mit dieser Vorgehensweise werden wesentliche Nachteile der bisher am häufigsten angewendeten Prüfverfahren vermieden, die aus deren komplexer Beanspruchung bzw. aus komplizierten Spannungsüberlagerungen resultieren. Dagegen können in den Druck-Scherproben entlang der künftigen Versagensfläche relativ homogene Schubspannungsverteilungen ohne die Überlagerung kritischer Normal-Zugspannungen erzielt werden, die zu einem eindeutigen interlaminaren Versagen führen. Wegen der einfachen Art der Lasteinleitung ist diese Methode außerdem auch für kompliziertere Beanspruchungen geeignet, wie z.B. zur Messung der ILSF unter zyklischer oder dynamischer Belastung sowie bei extrem hohen Temperaturen. Mit Hilfe modifizierter Druck-Scherproben und Belastungsvorrichtungen wird darüber hinaus der Einfluß überlagerter Normal-Druckspannungen im Hochtemperaturbereich untersucht und dargestellt

Die elektrische Emission beim Versagen von Faserverbundwerkstoffen und ihren Komponenten

Deformation and failure of fiber-reinforced materials (FRM) can cause electric charge displacements. This, consequently, leads to variations in the external electric field. These can be observed and recorded during the loading process without any contact to the sample. Analyzing, these signals named electric emission (EE) can be done individually and also statistically when an acoustic emission equipment is used. Fracture of carbon and glass fibers yields EE signals of large amplitudes, whereas the polycarbonate matrix material exhibits smaller ones. The signals obtained in a tensile test with the composite materials exceed the ones of the matrix material but do not attain those of the fiber material. From the shape of the EE signals conclusions can be made on the elementary fracture process. From these experiments it can be concluded that the EE method is a valuable tool with respect to the detection of failure occurence of composite materials as is the acousti emission technique. The EE Technique is a field method and does, therefore, not require any sample preparation. This makes it a low cost technique which can be possibly applied in the field as well as in the laboratory

The Interlaminar Shear Strength of C/C-SiC

Ceramic matrix composites (CMC) are interesting materials for an increasing number of applications, not only in the aerospace industry. One major manufacturing process of these materials is the Liquid Silicon Infiltration (LSI). In this process a porous carbon/carbon (C/C) preform is infiltrated with liquid silicon to form C/C-SiC-composites. The LSI process sig-nificantly lowers component fabrication time and therefore, reduces component costs compared to other CMC manufacturing processes [1]. In the present paper a thermal fibre pre-treatment (TFP) at different temperatures was applied before manufacturing the initial carbon fibre reinforced plastic (CFRP) green body. The goal was to reduce the fibre/matrix adhesion in the final C/C-SiC. One suitable parameter for the assessment of the mechanical properties of laminated CMC is the interlaminar shear strength (ILSS). The new Double Notched Compression Shear (DNCS) test was used to evaluate the influence of the TFP on the ILSS at different stages during the manufacturing of C/C-SiC