Jfh Jan Willems

Helmholtz-Like Resonator Self-Sustained Oscillations, Part 1: Acoustical Measurements and Analytical Models

We consider self-sustained oscillations of the grazing e ow along the neck of a Helmholtz-like resonator. Such oscillations are driven by a coupling between the intrinsic instability of the shear layer, separating the main e ow from the cavity, and the resonant acoustical e eld in the cavity. Depending on details of the shape of the neck, acoustical velocities through the neck of the resonator of the same order of magnitude as the main e ow velocity can be reached. For particular neck geometries, whistling is suppressed. A nonlinear model, which assumes that the vorticity of the shear layer is concentrated in line vortices traveling at constant velocity, provides insight into the phenomenon. For rounded edges, the model predicts the pulsation amplitude of the e rst hydrodynamic mode surprisingly well but severely overestimates the amplitude of higher hydrodynamic modes. For sharp edges, a modie cation of the original model is proposed, which yields a reasonable prediction of the pulsation amplitude (within a factor of two ) of the e rst hydrodynamic mode and does not overestimate higher hydrodynamic modes.

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.

Simplified models of flue instruments: Influence of mouth geometry on the sound source

Flue instruments such as the recorder flute and the transverse flute have different mouth geometries and acoustical response. The effect of the mouth geometry is studied by considering the aeroacoustical response of a simple whistle. The labium of a transverse flute has a large edge angle (60 degrees) compared to that of a recorder flute (15 degrees). Furthermore, the ratio W/h of the mouth width W to the jet thickness h can be varied in the transverse flute (lips of the musician) while it is fixed to a value W/h approximately 4 in a recorder flute. A systematic experimental study of the steady oscillation behavior has been carried out. Results of acoustical pressure measurements and flow visualization are presented. The sharp edge of the recorder provides a sound source which is rich in harmonics at the cost of stability. The larger angle of the labium of the flute seems to be motivated by a better stability of the oscillations for thick jets but could also be motivated by a reduction of broadband turbulence noise. We propose two simplified sound source models which could be used for sound synthesis: a jet-drive model for W/h>2 and a discrete-vortex model for W/h<2.

A parametric study on the whistling of multiple side branch system as a model for corrugated pipes

Corrugated pipes are widely used in industry due to their inherent character of being globally flexible and locally rigid. Under certain conditions flow through the corrugated pipes causes severe noise and vibration problems. Thus, to understand the phenomenon and parameters that play role is a real asset for industry. This study is a continuation of a research based on multiple side branch system and presented together with results of an investigation performed on corrugated pipes. Many similarities between the corrugated pipes and multiple side branch system have been observed. A Strouhal number which uses as characteristic length the cavity width plus the upstream edge radius yields the best collapse of the data for both corrugated pipes and multiple side branch system. For both systems the upstream edge radius of the cavity has significant effect on pressure fluctuation amplitudes. It can increase the amplitude of the pressure fluctuation by an order of magnitude compared to sharp edges. The radius of the downstream edge has a less pronounced effect on the sound production. Strouhal numbers display two hydrodynamic modes the first with a Strouhal number around 0.1 and the second one varying in the range between 0.4 and 0.6. The variation in critical Strouhal number for the second hydrodynamic mode correlates with the relative corrugation volume compared to the pipe volume. Experiments with corrugated pipes reveal that 1 st hydrodynamic mode is limited to configuration with small relative corrugation volume. The first hydrodynamic mode was not yet observed in the multiple side-branch systems. Copyright

Wave induced growth and evaporation of droplets in a vapour‐gas mixture

Heterogeneous condensation and evaporation induced by an unsteady rarefaction wave and by the passage of a shock wave in a water‐nitrogen mixture is studied. Time resolved measurements of modal droplet radius, droplets radius, droplet number density, pressure, and gas density are presented. Condensation on Cr2O3‐particles of about 10 nm starts at a saturation ratio of 3.4. Typical droplet radii and droplet number densities are 1.5 μm and 1011 m−3. A shock wave with Mach number 1.36 causes full evaporation in 4 ms.

Local Pressure Difference Over a NACA0018 Airfoil with Cavity Using Acoustic Forcing

The influence of a cavity on the steady and unsteady local pressure difference over a NACA0018 airfoil is investigated experimentally and compared to numerical calculations. In the experiments a new experimental method is used, which allows Strouhal numbers at which self-sustained oscillation of cavity flows is expected. The airfoil is fixed to the wind tunnel and the flow is modulated transversally to the main flow by an acoustic standing wave generated by loudspeakers. The objective is to obtain insight into the dynamical behavior of an airfoil with a cavity by comparison of experimental data to the results of a two-dimensional numerical Euler code. In the Strouhal range from 2.5 up to 10, based on the semi-chord, both experiment and numerical simulation do not show a strong effect of the cavity on the dynamical response of the wing. This response is dominated by the added mass of the wing.