Sj Steven Hulshoff

Self-sustained oscillations in a closed side branch system

Self-sustained oscillations of the flow in a closed side branch system due to a coupling of vortex shedding with acoustical resonances are considered. The configuration consists of two closed side branches of same length placed opposite to each other along a main pipe. This is called a cross-junction. Numerical simulations, based on the Euler equations for two-dimensional inviscid and compressible flows, are performed. As the radiation into the main pipe is negligible at the resonance frequency, this acoustically closed system is a good test-case of such Euler numerical calculations. The numerical results are compared to acoustical measurements and flow visualization obtained in a previous study. Depending on the flow conditions, the predicted pulsation amplitudes are about 30–40% higher than the measured amplitudes. This is partially due to the absence of visco-thermal dissipation in the numerical model but also to the effect of wall vibrations in experiments. A simple analytical model is proposed for the prediction of the pulsation amplitudes. This model is based on Nelsons representation of the shear layer as a row of discrete vortices convected at constant velocity from the upstream edge towards the downstream edge. When the downstream edge is sharp, this results in a spurious interaction between the singularity of the vortices and of the edge flow. This artefact is partially compensated by suppressing the singularity of the acoustical flow at the edge, or when a junction with rounded edges, as found in engineering practice, is considered. In spite of its crudeness, the analytical model provides a fair prediction (within 30%) which makes it useful for engineering applications.

The relevance of conservation for stability and accuracy of numerical methods for fluid–structure interaction

Numerical simulation of fluid–structure interactions has typically been done using partitioned solution methods. However, partitioned methods are inherently non-conservative and generally numerically unstable. The deficiencies of partitioned methods have motivated the investigation of monolithic solution methods. Conservation is possible for monolithic methods, the conditions have recently been presented [E.H. van Brummelen, S.J. Hulshoff, R. de Borst, Energy conservation under incompatibility for fluid–structure interaction problems, Comput. Methods Appl. Mech. Engrg. 192 (2003) 2727–2748]. In the present paper we investigate the relevance of maintaining conservation for a model fluid–structure interaction problem, viz., the piston problem. To distinguish the effect of the error induced by the interface coupling from the fluid and structure discretization errors, we use fluid subcycling and an exact time-integration method for the structure. A comparison between conservative and non-conservative monolithic methods as well as partitioned methods is made. We show that maintaining conservation has considerable impact on the stability and accuracy of the numerical method. These results also indicate that only for a conservative monolithic scheme the improvement in accuracy over partitioned methods warrants the computational cost associated with a monolithic solution. Moreover, we illustrate the implications that particular combinations of fluid and structure discretizations can have on the conservation properties of the fluid–structure interaction problem.

Helmholtz-like resonator self-sustained oscillations, part 2 : detailed flow measurements and numerical simulations

A global description of the effect of the neck geometry on self-sustained oscillations of a grazing e ow along a Helmholtz-like resonator has been given in a companion paper. Detailed e ow measurements taken by means of hot-wire anemometry and numerical simulations based on the Euler equations for inviscid and two-dimensional compressible e ows are now given. Vortex shedding is obtained in an inviscid e ow simulation by considering a neck geometry with sharp edgesat which the code predictse ow separation. Although two-dimensional e owcalculations are attractive because of their computational efe ciency, they are not able to represent the three-dimensional acoustical radiation from the resonator into free space without special frequency-dependent boundary condition treatments. Frequency-independent time-domain boundary conditions are considered. In view of the crudeness of this approximation, the agreement between theory and experiments is quite fair. The effects of changes in the geometry of theneck are qualitatively predicted by the model. The detailed e owinformation provides someinsight into the ine uence of the shape of the upstream edge of the neck that could not be obtained from analytical models proposed in the companion paper.

Flow Separation Control on Airfoil With Pulsed Nanosecond Discharge Actuator

An experimental study of flow separation control with a nanosecond pulse plasma actuator was performed in wind-tunnel experiments. The discharge used had a pulse width of 12 ns and rising time of 3 ns with voltage up to 12 kV. Repetition frequency was adjustable up to 10 kHz. The first series of experiments was to measure integral effects of the actuator on lift and drag. Three different airfoil models were used, NACA-0015 with the chord of 20 cm, NLF-MOD22A with the chord of 60 cm and NACA 63-618 with the chord of 20 cm. Different geometries of the actuator were tested at flow speeds up to 80 m/s. In stall conditions the significant lift increase up to 20% accompanied by drag reduction (up to 3 times) was observed. The critical angle of attack shifted up to 5–7 degrees. The relation of the optimal discharge frequency to the chord length and flow velocity was proven. The dependence of the effect on the position of the actuator on the wing was studied, showing that the most effective position of the actuator is on the leading edge in case of leading edge separation. In order to study the mechanism of the nanosecond plasma actuation experiments using schlieren imaging were carried out. It shown the shock wave propagation and formation of large-scale vortex structure in the separation zone, which led to separation elimination. PIV diagnostics technique was used to investigate velocity field and quantitative properties of vortex formation. In flat-plate still air experiments small scale actuator effects were investigated. Measured speed of flow generated by actuator was found to be of order of 0.1 m/s and a span-wise nonuniformity was observed. The experimental work is supported by numerical simulations of the phenomena. The formation of vortex similar to that observed in experiments was simulated in the case of laminar leading edge separation. Model simulations of free shear layer shown intensification of shear layer instabilities due to shock wave to shear layer interaction.

Energy conservation under incompatibility for fluid-structure interaction problems

Concurrent numerical methods for fluid–structure interaction problems are typically based on partitioned solution procedures. However, such partitioned methods are inherently non-conservative. In the present work, we investigate the conservation properties of monolithic discretisations for fluid–structure interaction problems. We consider a prototypical fluid–structure interaction problem, viz., the piston problem. A variational formulation allows us to establish precisely the conservation properties of the continuum problem and its discretisation by the finite-element method. We show that the conservation of energy by monolithic discretisations is only trivially maintained under restrictive compatibility conditions on the approximation spaces in the fluid and the structure. Moreover, we introduce a new discretisation based on coincidence conditions which ensures energy conservation under incompatibility. Numerical results which illustrate the effectiveness of the new discretisation are presented.

Uncertainty and reliability analysis of fluid-structure stability boundaries

Fluid–structure interactions provide design constraints in many fields, yet methods available for their analysis normally assume that the structural properties are exactly known. In this contribution, these properties are more realistically modeled using random fields. Stochastic finite element methods are applied to perform uncertainty and reliability analysis on fluid–structure interaction problems with random input parameters. As an example we consider panel divergence and panel flutter. Numerical simulations demonstrate the appropriateness of sensitivitybased methods for characterization of the statistical moments of the critical points as well as for the determination of the probability of occurrence of undesired phenomena

A modal-based multiscale method for large eddy simulation

We introduce a modal spectral element method which employs the variational multiscale approach for large eddy simulation. The method is sufficiently general for application to complex flow geometries, but requires relatively few degrees of freedom for a given mesh and polynomial order. We investigate the performance of the method for fully-developed turbulent channel flow. It is shown that large increases in accuracy can be obtained when the basic dynamics of near-wall coherent structures can be represented using the portion of the modal basis which is free of the subgrid-scale model. This implies that in spite of its lack of orthogonality, the modal basis provides sufficient scale separation to exploit the advantages of the variational multiscale approach.

Effectiveness of Side Force Models for Flow Simulations Downstream of Vortex Generators

Vortex generators are a widely used means of flow control, and predictions of their influence are vital for efficient designs. However, accurate computational fluid dynamics simulations of their effect on the flowfield by means of a body-fitted mesh are computationally expensive. Therefore, the Bender–Anderson–Yagle and jBAY models, which represent the effect of vortex generators on the flow using source terms in the momentum equations, are popular in industry. In this contribution, the ability of the Bender–Anderson–Yagle and jBAY models to provide accurate flowfield results is examined by looking at boundary-layer properties close behind vortex generators. The results are compared with both body-fitted mesh and other source term model Reynolds-averaged Navier–Stokes simulations of three-dimensional incompressible flows over flat-plate and airfoil geometries. The influence of mesh resolution and the domain of application on the accuracy of the models is shown, and the influence of the source term on the ...

Experimental Study and Numerical Simulation of Flow Separation Control with Pulsed Nanosecond Discharge Actuator

Active flow separation control with a nanosecond pulse plasma actuator, which is essentially a simple electrode system on the surface of an airfoil, introducing lowenergy gas discharge into the boundary layer, with little extra weight and no mechanical parts, was performed in wind-tunnel experiments on various airfoil models. In stall conditions the significant lift increase up to 30% accompanied by drag reduction (up to 3 times) was observed. The critical angle of attack shifted up to 5–7 degrees. Schlieren imaging show the shock wave propagation and formation of large-scale vortex structure in the separation zone, which led to separation elimination. The experimental work is supported by numerical simulations of the phenomena. The formation of vortex similar to that observed in experiments was simulated in the case of laminar leading edge separation. Model simulations of free shear layer show intensification of shear layer instabilities due to shock wave to shear layer interaction. The mechanism of flow control by nanosecond plasma discharge is based on extra vorticity created by the shock wave, which is produced from the layer of the hot gas. This hot gas in generated during the fast thermalisation process, in which up to 60% of the discharge energy is converted to heat in less than 1 µs [1]. This phenomenon gives an opportunity for nanosecond discharge actuator to be effective at high velocities [2, 3]. The current work continues studying the performance of nanosecond plasma actuator. A series of wind tunnel experiments was carried out with different actuator layouts at flow velocities up 80 m/s at various airfoils with chords up to 1.5 m and spans up to 5 m. A numerical model was developed to prove the shock wave mechanism of actuator operation. 2. Experiment In the present work, a linear actuator was used [4]. The actuator consisted of a base layer of insulator attached onto the surface of the airfoil, a covered electrode, an interelectrode layer of insulation and an exposed electrode. In the majority of the cases, exposed electrode was ground, and the high-voltage electrode was covered one. High-voltage nanosecond pulses were provided by three different nanosecond pulsers, which were capable of producing pulses of up to 50 kV with rising time of 3-15 ns and duration from 10 to 50 ns at repetition frequencies up to 10 kHz. Low-speed experiments was carried out in open jet wind tunnel using the NACA0015 airfoil with the chord of 20 cm and span about 75 cm. The tunnel was equipped with an

A Comparison of Space-Time Variational-Multiscale Discretizations

Three variational-multiscale space-time finite-element discretizations are compared. The first is based on the modal spectral-element method, the second on the partition-of-unity concept, and the last on the discontinuous-Galerkin method. Their performance is evaluated using one-dimensional viscous Burgers computations, which allows subgrid-scale modelling parameters to be clearly identified. The effects of mesh refinement, number of scales and scale partitioning on solution accuracy are illustrated and discussed.