Bas W. van Oudheusden

Effect of Interaction Strength on Unsteadiness in Shock-Wave-Induced Separations

The effect of the interaction strength on the unsteady behavior of a planar shock wave impinging on a low Reynolds turbulent boundary layer is investigated. This is achieved by means of a variation in incident shock angle under otherwise constant flow conditions. In addition, the effect of an order-of-magnitude variation in the Reynolds number is considered. This has been done for equivalent interaction strength, based on a similar probability of occurrence of instantaneous flow separations. The measurement technique employed is two-component planar particle image velocimetry. Common mechanisms for the large-scale reflected-shock unsteadiness are deduced by means of conditional statistics based on the separation bubble height. The results indicate that both upstream and downstream mechanisms are at work, the dominant mechanism depending on the interaction strength. No significant dependence on the Reynolds number was observed for interactions with a similar probability of instantaneous flow separations.

Aerodynamic Experiments on DelFly II: Unsteady Lift Enhancement

Particle image velocimetry measurements and simultaneous force measurements have been performed on the DelFly II flapping-wing MAV, to investigate the flow-field behavior and the aerodynamic forces generated. For flapping wing motion it is expected that both the clap and peel mechanism and the occurrence of a leading edge vortex during the translational phase play an important role in unsteady lift generation. Furthermore, the flexibility of the wing foil is also considered of primary relevance. The PIV analysis shows a strong influx between the wings during the peel but no downward expelling jet during the clap. The force measurements reveal that the peel, oppositely to the clap, contributes significantly to the lift. The PIV visualization suggests the occurrence of a leading edge vortex during the first half of the in- and outstroke, which is supported by a simultaneous augmentation in lift. The early generation of a leading edge vortex during the flex cannot be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift.

Experimental Study of an Incident Shock Wave/Turbulent Boundary Layer Interaction Using PIV

*† ‡ Particle Image Velocimetry is applied to the interaction between an incident shock wave and a flat plate turbulent boundary layer at Mach 2.1. The undisturbed boundary layer is characterized by its mean and turbulence properties. The interaction region is characterized by the mean velocity field, which shows the incident and reflected shock wave pattern, as well as the boundary layer distortion. The unsteady flow properties are inspected by means of instantaneous velocity fields. Patches of reversed-flow are frequently observed at several locations. Although significant reversed-flow is measured instantaneously, on average no reversed-flow is observed. Turbulence properties show the highest turbulence intensity in the region behind the impingement of the incident shock wave. Turbulence anisotropy is found to be present, with the streamwise component dominating. A distinct streamwiseoriented region of relatively large Reynolds shear stress magnitude appears in the redeveloping boundary layer and persists downstream. The recovery of the boundary layer towards its initial equilibrium conditions therefore appears to be a gradual process.

Flow visualization and force measurements on a hovering flapping-wing MAV 'DelFly II'

Particle image velocimetry measurements and simultaneous force measurements have been performed on the DelFly II flapping-wing MAV, to investigate the flow-field behavior and the aerodynamic forces generated. For flapping wing motion it is expected that both the clap and peel mechanism and the occurrence of a leading edge vortex during the translational phase play an important role in unsteady lift generation. Furthermore, the flexibility of the wing foil is also considered of primary relevance. The PIV analysis shows a strong influx between the wings during the peel but no downward expelling jet during the clap. The force measurements reveal that the peel, oppositely to the clap, contributes significantly to the lift. The PIV visualization suggests the occurrence of a leading edge vortex during the first half of the in- and outstroke, which is supported by a simultaneous augmentation in lift. The early generation of a leading edge vortex during the flex cannot be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift.

Experimental Investigation on the Aerodynamics of a Bio-inspired Flexible Flapping Wing Micro Air Vehicle

An experimental investigation on a 10 cm bio-inspired flexible Flapping-Wing Micro Air Vehicle (FWMAV) was conducted in both hovering and forward-flight conditions with the objective to characterize its aerodynamic performance. The measurements in hovering conditions were performed with the particular objective to explore the effect of different wing configurations (i.e. different aspect ratios and wing flexibilities), whereas forward flight tests in a wind tunnel were carried out to assess the aerodynamic performance of the FWMAV as a function of flow speed, flapping frequency and body angle. The cyclic variation of forces (lift and thrust) generated as a result of the wing flapping was captured by means of a high-resolution force sensor, in combination with high-speed imaging to track the wing motion. Results of measurements in hover show that the flapping frequency, aspect ratio and wing flexibility have a crucial impact on the efficiency and the force generation during the flapping cycle. An estimated flight envelop for the MAVs operation is defined from the data obtained in the wind tunnel measurements. Furthermore, additional tests on several brushless DC motors provide a feasible option in future engine selection and design.

The effect of particle image blur on the correlation map and velocity measurement in PIV

In PIV particle image blur is usually observed near fluid optical interfaces, i.e. shock waves, and thin flow structure with large density variations, e.g. shear layers and boundary layers. In such an environment the particle image is not only subject to blur, but is also displaced from its actual position due to refraction, which is denoted as optical displacement. In this study particle image blur near a shock wave is investigated in relation to the auto- and cross-correlation map, measurement accuracy and confidence level. The results from a numerical study are supported by PIV measurements of a shock wave in a supersonic wind tunnel. It is demonstrated that particle images are blurred in the direction of lower refractive index (directional blurring). The particle images are also skewed. Therefore particle image blur not only causes correlation peak broadening due to the fact that the particle images increase in size, but more importantly can introduce an asymmetry in the correlation peak and in turn introduce a small bias error in the measured velocity. However, experimental results indicate that particle image blur itself is not the main cause for the increase in measurement uncertainty near shock waves, but that the reduced accuracy can be attributed to the optical displacement. The observation of particle image blur can be used as a detection criterion for a qualitative assessment of the optical displacement. Certain combinations of experimental parameters (viewing angle, f/# and interrogation window size) yield significant errors in the measured velocity. Under certain circumstances optical distortion can become so strong to introduce an unphysical acceleration within the shock wave, visualized as an inflection point with positive slope in the velocity profile across the shock. The study provides some practical suggestions to limit the effect of aero-optical distortion on the velocity measurement.

Experimental Investigation of Aerodynamics of Flapping-Wing Micro-Air-Vehicle by Force and Flow-Field Measurements

This study explores the aerodynamic characteristics of a flapping-wing micro aerial vehicle (MAV) in hovering configuration by means of force and flowfield measurements. The effects of flapping frequency and wing geometry on force generation were examined using a miniature six-component force sensor. Additional high-speed imaging allowed identification of the notable different deformation characteristics of the flexible wings under vacuum condition in comparison to their behavior in air, illustrating the relevance of aeroelastic effects. Flow visualization around the flapping wing by means of planar particle image velocimetry (PIV) measurements revealed the formation, development, and shedding of the vortical structures by the wings during flapping motion, with particular emphasis on the clap-and-fling phase. Further stereoscopic PIV measurements performed in the wake showed a momentum surplus wake induced by the clap-and-fling, indicative of thrust generation. The vortical structures in the wake formed d...

Experiments on Hypersonic Boundary Layer Separation and Reattachment on a Blunted Cone-Flare using Quantitative InfraRed Thermography.

Transient heat transfer measurements have been carried out on a blunted coneflare model in a short duration hypersonic facility at Mach 9 using quantitative infrared thermography (QIRT). The surface temperature transient was measured using an infrared camera and the temperature data were successfully correlated to convective heat transfer using two distinctly different data reduction techniques. Measurements were performed at different Reynolds numbers to investigate the phenomenon of boundary layer separation and reattachment in the hypersonic flow regime. A smooth nose and a roughened one were used to investigate the effect of roughness induced boundary layer turbulence on flow separation and reattachment. For both configurations the length of the separated region was found to decrease with increasing Reynolds number. Along the flare a monotonic increase in heat transfer is measured. Both QIRT and schlieren visualizations suggest that flow transition occurs along the flare.

Numerical and Experimental Investigations of the Supersonic Microramp Wake

The flow past a microramp immersed in a supersonic turbulent boundary layer is studied by means of numerical simulations with the implicit large-eddy simulation technique and experiments conducted with tomographic particle image velocimetry. The experimental data are mostly used to verify the validity of the numerical results by ample comparisons on the time-averaged velocity, turbulent statistics, and vortex intensity. Although some discrepancies are observed on the intensity of the upwash motion generated by the streamwise vortex pair, the rates of the recovery of momentum deficit and the decay of streamwise vortex pair intensity are found in good agreement. The instantaneous flow organization is inspected, making use of the flow realizations available from implicit large-eddy simulation. The flow behind the microramp exhibits significant large-scale unsteady fluctuations. Notably, the quasi-conical shear layer enclosing the wake is strongly undulated under the action of Kelvin–Helmholtz (K–H) vortices....

Modeling of particle imaging through shock waves

In compressible flows particle imaging, as done in Particle Image Velocimetry (PIV), is far from trivial. The inhomogeneous refractive index field can cause aero-optical aberrations including blurring of the image, especially near optical interfaces such as shock waves. The understanding of the process causing particle image blur (or blurring of the point spread function of the imaging system) is important in order to assess the measurement accuracy of optical measurement systems, such as PIV. A model for imaging through a shock wave is presented to determine the characteristic shape of blurred particle images when imaged across shock waves. The conjectured model is validated through a PIV experiment, where particle image recordings of the flow across a steady oblique shock wave are obtained in a supersonic wind tunnel. The parametric study focuses on two dominating parameters: 1) the angle between the viewing axis and the shock wave; 2) the numerical aperture of the imaging optics.