R. de Kat

Aspect-ratio effects on aeromechanics of membrane wings at moderate reynolds numbers

The aspect ratio of a membrane wing at moderate Reynolds number Re=67,500 affects aerodynamic performance as well as membrane deformations. Wind-tunnel experiments are conducted using three different low-aspect-ratio wings (aspect ratios of 1, 1.5, and 2). Aerodynamic performance is determined from force and moment measurements, which are performed using a six-component force transducer. Membrane deformations are obtained using photogrammetry. Mean values and unsteady effects are examined for both aerodynamic performance and membrane deformations. Mean deflection results indicate that lower-aspect-ratio membrane wings show defined U-shape deflections along the span, whereas higher aspect ratios display a progressive rise in deformation to the wing tip. Dominant chordwise vibration modes of the membrane and their spectral content show that lower aspect ratios exhibit higher mode shapes and frequencies, likely caused by increased downwash, which delays the influence of vortex shedding into higher incidences...

Pressure from particle image velocimetry for convective flows: a Taylor’s hypothesis approach

Taylors hypothesis is often applied in turbulent flow analysis to map temporal information into spatial information. Recent efforts in deriving pressure from particle image velocimetry (PIV) have proposed multiple approaches, each with its own weakness and strength. Application of Taylors hypothesis allows us to counter the weakness of an Eulerian approach that is described by de Kat and van Oudheusden (2012 Exp. Fluids 52 1089–106). Two different approaches of using Taylors hypothesis in determining planar pressure are investigated: one where pressure is determined from volumetric PIV data and one where pressure is determined from time-resolved stereoscopic PIV data. A performance assessment on synthetic data shows that application of Taylors hypothesis can improve determination of pressure from PIV data significantly compared with a time-resolved volumetric approach. The technique is then applied to time-resolved PIV data taken in a cross-flow plane of a turbulent jet (Ganapathisubramani et al 2007 Exp. Fluids 42 923–39). Results appear to indicate that pressure can indeed be obtained from PIV data in turbulent convective flows using the Taylors hypothesis approach, where there are no other methods to determine pressure. The role of convection velocity in determination of pressure is also discussed.

The convection of large and intermediate scale fluctuations in a turbulent mixing layer

The convection velocity of large and intermediate scale velocity fluctuations in a nominally two-dimensional planar mixing layer, and its dependence upon the length scale, is explored by carrying out particle image velocimetry (PIV) experiments. A “global” convection velocity, containing the convection of all the length scales present in the flow, is produced by examining the autocorrelation functions between velocity fluctuations in successive PIV records across the mixing layer. This “global” convection velocity is found to be similar to the mean flow, although fluctuations on the low speed side of the mixing layer on average convect at speeds greater than the mean and fluctuations on the high speed side of the mixing layer are observed to convect at speeds less than the mean. Scale specific convection velocity profiles are then produced by examining the phase difference between the spectral content specific to one wavenumber in streamwise velocity fluctuation traces in successive PIV records, offset by...