Aero-acoustic oscillations inside large deep cavities

This investigation focuses on the pressure amplitude response, within two deep cavities characterized by their length over depth ratios (L/H = 0.2 and 0.41), under varying free stream velocity in a large wind tunnel. Experiments have shown that for deep rectangular cavities at low Mach number, oscillations of discrete frequencies can be produced. These oscillations appear when the free stream velocity becomes higher than a minimum value. In addition, as flow velocity is increased, upward jumps in oscillation frequency are observed in the two cavity configurations. Convection velocity of instabilities along the shear layer was measured using velocity cross-correlations. This study shows that the hydrodynamic modes of the cavity shear layer are correctly predicted by the Rossiter model when the convection velocity is determined and the empirical time delay is neglected. For L/H = 0.2 the first oscillation mode is observed on the spectrogram. For L/H = 0.41, both the first and the second mode have approximately the same amplitude. Time-resolved Particle image velocimetry measurements were performed to obtain the vorticity distribution during the oscillation cycle along the cavity shear layer. It is found that the number of structures in the cavity shear layer depends on the mode order of cavity oscillation

Control of deep cavity tones using a spanwise cylinder at low-subsonic speeds

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Deep cavity configuration at subsonic velocity could be found in many industrial processes, ranging from windows and sunroofs in automobiles and over space between two consecutive train vehicles. These cavities may induce aeroacoustic couplings between the cavity shear layer oscillations and the acoustic modes of the installation. This aero-acoustic coupling can leads to serious damages of vehicles due to resonance of high pressure fluctuation level around the cavity. The study of deep cavity at low velocity presents a great practical interest to suppress acoustic noise. In spite of numerous studies devoted to the cavity and its control, very few of them relate to the deep cavity configuration at low velocity. The focus of the present study is to apply a passive control to the case of the deep cavity flow at relatively low velocities. A Detailed Experimental study of flow over a deep cavity was conducted towards understanding the attenuation of tones using a spanwise cylinder. The cavity length-to-depth aspect ratio is L/H = 0.2. Single hot-wire measurements characterized the incident turbulent boundary. A “no control” cavity was compared with a similar configuration using a cylinder on the leading edge of the cavity. Parametric changes of the spanwise cylinder such as the distance from the wall are studied. Maximum control across the range of studied velocities occurs for a particular position of the spanwise cylinder. Reductions in sound pressure levels (SPL) of up to 36 dB were obtained. Moreover, a shaped cylinder was also studied and shows that the attenuation of tones is not due to high frequency pulsing as suggested in literature, but to an increase of the cavity shear layer thickness due to change in the mean axial velocity profiles.vk201

Scaling of statistics in wall-bounded turbulent flows

High-resolution laser Doppler anemometry (Lda) and hot-wire anemometry (Hwa) measurements are utilized to study a zero-pressure-gradient turbulent boundary layer over the range of momentum thickness Reynolds number of 1170–3720. The primary objective is to investigate the near-wall behavior of this type of flow. We are particularly interested in possible Reynolds- and Kármán-number dependencies. The experimental results are in excellent agreement with most recent direct numerical simulations (Dns), which allow direct comparison of detailed results such as peak value and position of streamwise Reynolds stress, wall values of skewness and flatness factors, and turbulence dissipation rate. Systematic changes of some of these parameters with Kármán number are found when scaled with the inner parameters. A remedy seems to be the alternative mixed scaling that is based on , instead of , which admits direct influence of the outer velocity scale on the wall parameters

Reynolds number dependency of near-wall statistics of zero-pressure-gradient turbulent boundary layer

We report high-resolution LDA and HWA measurements of the streamwise velocity component of a flat-plate turbulent boundary layer (ZPG TBL) over a range of momentum thickness Reynolds number from 1,170 to 3,720. The primary objective of this work is to investigate the near-wall behavior and the scaling of high-order statistics. In particular, we are interested in certain Kármán number dependencies. The obtained data are in excellent agreement with most recent DNS-results, which allows direct comparison of detailed results such as peak value and position of streamwise stress, wall-values of skewness and flatness factors, and turbulence dissipation rate. The experimental data clearly reveal the failure of classical scaling. An alternative mixed scaling based on u?3/2ue1/2 removes these discrepancies

Machine learning control for experimental shear flows targeting the reduction of a recirculation bubble

International audienceThe goal is to experimentally reduce the recirculation zone of a turbulent flow(Re H = 31500). The flow is manipulated by a row of micro-blowers (pulsed jets) that areable to generate unsteady jets proportional to any variable DC. Already, periodic jet injectionat a forcing frequency of St H = 0.226 can effectively reduce the reattachment length andthus the recirculation zone. A model-free machine learning control (MLC) is used to improveperformance. MLC optimizes a control law with respect to a cost function and applies geneticprogramming as regression technique. The cost function is based on the recirculation lengthand penalizes actuation. MLC is shown to outperform periodic forcing. The current studydemonstrates the efficacy of MLC to reduce the recirculation zone in a turbulent flow regime.Given current and past successes, we anticipate numerous experimental MLC applications