High-frequency Pressure Measurements Research Articles

Axisymmetric Jet Shear-Layer Excitation Induced by Laser Energy and Electric Arc Discharges

Two energy deposition methods (electric arcing and laser-induced optical breakdown) were used to force and control compressible mixing layers of axisymmetric jets. The effects of energy-deposition forcing methods have been experimentally investigated with schlieren imaging, particle image velocimetry, product formation flow visualizations, and high-frequency pressure measurements. Large-scale structures were forced in perfectly expanded jets with nozzle-exit Mach numbers of 1,38,1.5, and 2.0, utilizing single pulse-laser energy deposition focused at the nozzle exit. Structures were successfully forced over a range of convective Mach numbers from 0.63 to 0.85 using laser pulse energies from 5 to 40 mJ. The large-scale structure forced by laser perturbation in the Mach 1.38 jet was characterized with detailed measurements of the velocity and vorticity fields and the fluctuating pressure history

Monitoring slurry-loop reactors for early detection of hydrodynamic instabilities

Various regimes can be encountered in solid–liquid flows. In slurry-loop systems, such as an industrial polyethylene reactor, instabilities can occur at high solids concentration. An early warning method based on high-frequency pressure measurements is presented that can detect these instabilities in an early stage, using a monitoring method based on attractor comparison. With this method, deviations from the preferred process conditions are detected using a statistical quantifier of the changes of the system's attractor. In this way, process control actions can be taken to prevent unwanted instabilities to occur, which offers opportunities to operate closer to an undesirable flow regime. An early warning procedure is developed and validated, using both a scaled version of an industrial polyethylene reactor, and full-scale operational data.

Time-Series Analysis of Supersonic Base-Pressure Fluctuations

An investigation into the dynamic nature of supersonic base-pressure fluctuations was conducted. Two axisymmetric models, a blunt and a boattailed afterbody, were studied at a freestream Mach number of 2.46 and zero angle of attack. High-frequency pressure measurements were recorded with both radially and circumferentially positioned transducers. Blunt afterbody measurements showed rms pressure fluctuations on the order of 5% of the average base pressure. Power spectral density (PSD) estimates at the two outermost radial locations for the blunt afterbody revealed that a significant portion of the pressure fluctuation energy is contained in a peak centered near 850 Hz (Sr = 0.094). RMS pressure fluctuations on the boattailed afterbody were on the order of 4.3% of the average base pressure. The PSD estimates at the two outermost locations contained a more defined and larger peak centered near 800 Hz (Sr = 0.089) as compared to the blunt base. The two outermost radial locations on both afterbodies are subjected to similar pressure histories and, hence, display large values of coherence at low frequencies. The pressure history at the center position is not well correlated to those at the two outermost locations, resulting in low values of coherence. Possible mechanisms for the substantial pressure fluctuation energy contained at low frequencies are pulsing or flapping of the recirculation region, whereas higher-frequency energy could be attributed to large-scale turbulent structures in the shear layer.

Particle densities, velocities and size distributions in large avalanches from impact-sensor measurements

AbstractIn winter 1998/99, high-frequency pressure measurements with 10 cm sensors mounted 1–19 m above ground were carried out in the upper run-out zone of the avalanche test site at Vallée de la Sionne, Switzerland. Two large dry-snow avalanches clearly revealed a three-layered structure, with surprisingly low pressures in the suspension (or powder-snow) layer. The height of the saltation layer varied between 1 and > 3 m. From the duration, impulse and frequency of single-particle impacts (observed in the saltation layer and intermittently in the dense flow),particle-size and velocity distribution functions as well as strongly varying saltation-layer densities were found. With improved methods for peak detection and correction for grazing impacts, pressure measurements will become premier tool for testing granular flow models.

Surface Heat Transfer Fluctuations on a Turbine Rotor Blade Due to Upstream Shock Wave Passing

A theoretical model to explain observed rapid large-scale surface heat transfer rate fluctuations associated with the impingement of nozzle guide vane trailing edge shock waves on a transonic turbine rotor blade is described. Experiments were carried out in the Oxford Isentropic Light Piston Cascade Tunnel using an upstream rotating bar system to simulate the shock wave passing. High-frequency surface heat transfer and pressure measurements gave rapidly varying, large, transient signals, which schlieren photography showed to be associated with the impingement of passing shock waves on the surface. Heat transfer rates varying from three times the mean value to negative quantities were measured. A simple first-order perturbation analysis of the boundary layer equations shows that the transient adiabatic heating and cooling of the boundary layer by passing shock waves and rarefactions can give rise to high-temperature gradients near the surface. This in turn leads to large conductive heat transfer rate fluctuations. The application of this theory to measured fluctuating pressure signals gave predictions of fluctuating heat transfer rates that are in good agreement with those measured. It is felt that the underlying physical mechanisms for shock-induced heat transfer fluctuations have been identified. Further work will be necessary to confirm them in rotating experiments.

Determination of the reflection coefficient and the cross-sectional area function from high-frequency pressure measurements in the ear canal

At moderate to high frequencies, incoming and outgoing pressure waves combine to form standing waves in the ear canal. The exact form of the pressure distribution depends on the geometry of the ear canal and on the magnitude, and phase, of the reflected wave relative to the incoming wave. An asymptotic theory describing this interaction is combined with pressure measurements in the ear canal in order to determine the reflection coefficient at the eardrum, the cross-sectional area function of the ear canal, and the relative phase of the reflected wave. The theory is based on a high-frequency multiscale solution of the one-dimensional horn equation and is shown to agree well with experimental measurements of standing wave patterns and with the distribution of phase in the ear canal. Since the absorption of energy at the eardrum is determined, this method is particularly well suited to determine the input to the ear at high frequencies. [Work supported, in part, by the Whitaker Foundation.]