Showing papers by "Hester Bijl published in 2014"

Kriging regression of PIV data using a local error estimate

Abstract: The objective of the method described in this work is to provide an improved reconstruction of an original flow field from experimental velocity data obtained with particle image velocimetry (PIV) technique, by incorporating the local accuracy of the PIV data. The postprocessing method we propose is Kriging regression using a local error estimate (Kriging LE). In Kriging LE, each velocity vector must be accompanied by an estimated measurement uncertainty. The performance of Kriging LE is first tested on synthetically generated PIV images of a two-dimensional flow of four counter-rotating vortices with various seeding and illumination conditions. Kriging LE is found to increase the accuracy of interpolation to a finer grid dramatically at severe reflection and low seeding conditions. We subsequently apply Kriging LE for spatial regression of stereo-PIV data to reconstruct the three-dimensional wake of a flapping-wing micro air vehicle. By qualitatively comparing the large-scale vortical structures, we show that Kriging LE performs better than cubic spline interpolation. By quantitatively comparing the interpolated vorticity to unused measurement data at intermediate planes, we show that Kriging LE outperforms conventional Kriging as well as cubic spline interpolation.

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

Abstract: 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...

Improvements to gradient-enhanced kriging using a bayesian interpretation

Abstract: Cokriging is a flexible tool for constructing surrogate models on the outputs of computer models. It can readily incorporate gradient information, in which form it is named gradient-enhanced Kriging (GEK), and promises accurate surrogate models in >10 dimensions with a moderate number of sample locations for sufficiently smooth responses. However, GEK suffers from several problems: poor robustness and ill-conditionedness of the surface. Furthermore it is unclear how to account for errors in gradients, which are typically larger than errors in values. In this work we derive GEK using Bayes’ Theorem, which gives an useful interpretation of the method, allowing construction of a gradienterror contribution. The Bayesian interpretation suggests the “observation error” as a proxy for errors in the output of the computer model. From this point we derive analytic estimates of robustness of the method, which can easily be used to compute upper bounds on the correlation range and lower bounds on the observation error. We thus see that by including the observation error, treatment of errors and robustness go hand in hand. The resulting GEK method is applied to uncertainty quantification for two test problems.

Numerical simulation of X-wing type biplane flapping wings in 3D using the immersed boundary method.

Abstract: The numerical simulation of an insect-sized 'X-wing' type biplane flapping wing configuration is performed in 3D using an immersed boundary method solver at Reynolds numbers equal to 1000 (1 k) and 5 k, based on the wing's root chord length This X-wing type flapping configuration draws its inspiration from Delfly, a bio-inspired ornithopter MAV which has two pairs of wings flapping in anti-phase in a biplane configuration The objective of the present investigation is to assess the aerodynamic performance when the original Delfly flapping wing micro-aerial vehicle (FMAV) is reduced to the size of an insect Results show that the X-wing configuration gives more than twice the average thrust compared with only flapping the upper pair of wings of the X-wing However, the X-wing's average thrust is only 40% that of the upper wing flapping at twice the stroke angle Despite this, the increased stability which results from the smaller lift and moment variation of the X-wing configuration makes it more suited for sharp image capture and recognition These advantages make the X-wing configuration an attractive alternative design for insect-sized FMAVS compared to the single wing configuration In the Reynolds number comparison, the vorticity iso-surface plot at a Reynolds number of 5 k revealed smaller, finer vortical structures compared to the simulation at 1 k, due to vortices' breakup In comparison, the force output difference is much smaller between Re = 1 k and 5 k Increasing the body inclination angle generates a uniform leading edge vortex instead of a conical one along the wingspan, giving higher lift Understanding the force variation as the body inclination angle increases will allow FMAV designers to optimize the thrust and lift ratio for higher efficiency under different operational requirements Lastly, increasing the spanwise flexibility of the wings increases the thrust slightly but decreases the efficiency The thrust result is similar to one of the spanwise studies, but the efficiency result contradicts it, indicating that other flapping parameters are involved as well Results from this study provide a deeper understanding of the underlying aerodynamics of the X-wing type, which will help to improve the performance of insect-sized FMAVs using this unique configuration

Corner-Type Plasma Actuators for Cavity Flow-Induced Noise Control

Abstract: A novel dielectric barrier discharge plasma actuator configuration for flow control is employed on open cavities to evaluate the potential for aeroacoustic tonal noise reduction. Instead of a planar configuration, the actuator is designed around the cavity opening edges. The investigation focuses on its effectiveness in terms of tonal noise suppression and the associated fluid dynamics. The investigated cavities have a square cross section. A low-Mach-number flow with a thin laminar boundary layer introduces tonal sound emission due to fluid dynamic feedback. Both upstream-mounted and downstream-mounted corner actuators have been tested, where the resulting body force is directed either into or out of the cavity. The upstream-mounted actuators influence cavity tonal feedback. An inward-inducing actuator suppresses the cavity tone up to a freestream velocity of 12.5 m/s. An outward-inducing actuator influenced mode switching. Downstream-mounted actuators did not influence the cavity aeroacoustics. Particl...

Acceleration of Strongly Coupled Fluid-Structure Interaction with Manifold Mapping

Abstract: Strongly coupled partitioned fluid-structure interaction problems require multiple coupling iterations per time step. The fluid domain and the structure domain are solved multiple times in each time step such that the kinematic and dynamic interface conditions on the fluid-structure interface are satisfied. Quasi-Newton methods have been successfully applied in case the fluid and structure solvers are considered as black boxes, i.e. only input and output information of the solvers are used by the coupling techniques. In this contribution a computationally inexpensive low-fidelity model is combined with a high-fidelity model in order to accelerate the convergence of the high-fidelity model. This is achieved by applying the manifold mapping algorithm on the fluid-structure interaction problem in order to minimize the fluid-structure interface residual. Originating from multi-fidelity optimization, the manifold mapping algorithm is applied for the first time in a simulation context, instead of an optimization context. The manifold mapping algorithm is applied on a standard fluid-structure interaction benchmark, namely the cylinder flap FSI3 case. A reduction of 48% in terms of high fidelity iterations is achieved compared with the inverse least squares Quasi-Newton algorithm, resulting in a 42% decrease in computational costs.

Energy-conserving schemes for wind farm aerodynamics

Abstract: Computational demands compel researchers to use coarse grids for the study of wind farm aerodynamics, which necessitates the use of accurate numerical schemes. Energy- conserving (EC) schemes are designed to enforce the conservation of Kinetic Energy (KE), an invariant property of incompressible flows. These schemes are numerically stable and free from artificial dissipation, even on coarse grids and could be used as an alternative to high-order pseudo-spectral schemes. This article details tests on EC schemes in the context of Large Eddy Simulations (LES). Results suggest that the accuracy of EC schemes is influenced by the Subgrid Scale model used for the LES. EC methods use central schemes that lead to dispersion, which is more apparent with a less-dissipative Scale Similarity model. Whereas, the purely dissipative Smagorinsky's model reduces the dispersion and generates a smoother solution but at the cost of accuracy in terms of predicting the KE. Although the impact of EC schemes and SGS models on the LES of wind farms is yet to be assessed, a simple LES of a model wind farm uncovers that EC schemes are quite suitable for wind farm aerodynamics.

Accelerated partitioned fluid-structure interaction using space-mapping

Abstract: The focus of this paper is on acceleration of strong partitioned coupling algorithms for fluid-structure interaction. Strong partitioned coupling requires the solution of a coupled problem at each time step during the simulation. Hereto, an interface residual is defined such that the kinematic and dynamic interface conditions on the fluid-structure interface are satisfied when it amounts to zero. Subsequently, the coupled problem is formulated as a minimization problem of the interface residual which can efficiently be performed using Newton’s method. However, Newton’s method cannot be applied when the fluid and structure solvers are considered black-boxes since the Jacobian of the interface residual is not available. For this reason, Quasi-Newton methods were developed that approximate either the Jacobian or the inverse Jacobian of the interface residual directly from input/output information. In this contribution we present a new algorithm that uses a technique from multifidelity optimization – called space-mapping – to efficiently perform the minimization of the interface residual. The space-mapping technique exploits a computationally inexpensive lowfidelity model in order to accelerate an expensive high-fidelity model using black-box information only. The space-mapping algorithm is applied to the supersonic panel flutter problem in order to demonstrate its effectiveness. The speedup – defined with respect to a Quasi-Newton algorithm – is found to be 1-1.5 for typical time step sizes. It is expected that higher speedups can be obtained when problems are considered that require strong coupling as the time step decreases, e.g. due to the added mass effect when the structure is in interaction with an incompressible fluid.