Bart Lipkens

Measurements of macrosonic standing waves in oscillating closed cavities

Measurements of macrosonic standing waves in gases in oscillating closed cavities are shown. The strong dependence of the pressure waveform upon cavity shape is demonstrated. This dependence is exploited to provide control of harmonic phase and amplitude, thus avoiding shocks and enabling resonant waveforms to reach macrosonic pressures. The exploitation of this dependence is referred to as resonant macrosonic synthesis (RMS). Power is delivered to the cavity by oscillating it with a linear actuator (entire resonator drive). Standing wave overpressures in excess of 340% of ambient pressure are demonstrated in RMS cavities, compared to maximum overpressures of 17% observed in cylindrical resonators. Ratios of maximum to minimum pressures of 27 were observed in RMS cavities compared to 1.3 for cylinders. Measurements are shown for four axisymmetric cavity shapes: cylinder, cone, horn-cone hybrid, and bulb. Cavities were filled with nitrogen, propane, or refrigerant R-134a (1,1,1,2-tetrafluoroethane). Physic...

Nonlinear standing waves in an acoustical resonator

A one-dimensional model is developed to analyze nonlinear standing waves in an acoustical resonator. The time domain model equation is derived from the fundamental gasdynamics equations for an ideal gas. Attenuation associated with viscosity is included. The resonator is assumed to be of an axisymmetric, but otherwise arbitrary shape. In the model the entire resonator is driven harmonically with an acceleration of constant amplitude. The nonlinear spectral equations are integrated numerically. Results are presented for three geometries: a cylinder, a cone, and a bulb. Theoretical predictions describe the amplitude related resonance frequency shift, hysteresis effects, and waveform distortion. Both resonance hardening and softening behavior are observed and reveal dependence on resonator geometry. Waveform distortion depends on the amplitude of oscillation and the resonator shape. A comparison of measured and calculated wave shapes shows good agreement.

Propagation of finite amplitude sound through turbulence: Modeling with geometrical acoustics and the parabolic approximation

Sonic boom propagation can be affected by atmospheric turbulence. It has been shown that turbulence affects the perceived loudness of sonic booms, mainly by changing its peak pressure and rise time. The models reported here describe the nonlinear propagation of sound through turbulence. Turbulence is modeled as a set of individual realizations of a random temperature or velocity field. In the first model, linear geometrical acoustics is used to trace rays through each realization of the turbulent field. A nonlinear transport equation is then derived along each eigenray connecting the source and receiver. The transport equation is solved by a Pestorius algorithm. In the second model, the KZK equation is modified to account for the effect of a random temperature field and it is then solved numerically. Results from numerical experiments that simulate the propagation of spark-produced N waves through turbulence are presented. It is observed that turbulence decreases, on average, the peak pressure of the N waves and increases the rise time. Nonlinear distortion is less when turbulence is present than without it. The effects of random vector fields are stronger than those of random temperature fields. The location of the caustics and the deformation of the wave front are also presented. These observations confirm the results from the model experiment in which spark-produced N waves are used to simulate sonic boom propagation through a turbulent atmosphere.

Model experiment to study sonic boom propagation through turbulence. Part I: General results

A model experiment to study the effect of atmospheric turbulence on sonic booms is reported. The model sonic booms are N waves produced by electric sparks, and the model turbulence is created by a plane jet. Of particular interest are the changes in waveform, peak pressure, and rise time of the model N waves after they have passed through the model turbulence. A review is first given of previous experiments on the effect of turbulence on both sonic booms and model N waves. This experiment was designed so that the scale factor (approximately 10−4) relating the characteristic length scales of the model turbulence to those of atmospheric turbulence is the same as that relating the model N waves to sonic booms. Most of the results reported are for plane waves. Sets of 100 or 200 pressure waveforms were recorded, for both quiet and turbulent air, and analyzed. Sample waveforms, scatter plots of peak pressure and rise time, histograms, and cumulative probability distributions are given. Results are as follows: ...

The Effect of Frequency Sweeping and Fluid Flow on Particle Trajectories in Ultrasonic Standing Waves

Particle concentration and separation in ultrasonic standing waves through the action of the acoustic radiation force on suspended particles are discussed. The acoustic radiation force is a function of the density and compressibility of the fluid and the suspended particles. A two-dimensional theoretical model is developed for particle trajectory calculations. An electroacoustic model is used to predict the acoustic field in a resonator, driven by a piezoelectric transducer. Second, the results of the linear acoustic model are used to calculate the acoustic radiation force acting on a particle suspended in the resonator. Third, a particle trajectory model is developed that integrates the equation of motion of a particle subjected to a buoyancy force, a fluid drag force, and the acoustic radiation force. Computational fluid dynamics calculations are performed to calculate the velocity field that is subsequently used to calculate fluid drag. For a fixed frequency excitation, the particles are concentrated along the stable node locations of the acoustic radiation force. Through a periodic sweeping of the excitation frequency particle translation is achieved. Two types of frequency sweeps are considered, a ramp approach and a step-change method. Numerical results of particle trajectory calculations are presented for two configurations of flow-through resonators and for two types of frequency sweeping. It is shown that most effective particle separation occurs when the fluid drag force is orthogonal to the acoustic radiation force.

Classic Papers in Shock Compression Science

A Paper on the Theory of Sound.- On a Difficulty in the Theory of Sound.- On the Mathematical Theory of Sound.- The Propagation of Planar Air Waves of Finite Amplitude.- On the Thermodynamic Theory of Waves of Finite Longitudinal Disturbance.- The Life and Work of Pierre Henri Hugoniot.- On the Propagation of Motion in Bodies and in Perfect Gases in Particular -I.- On the Propagation of Motion in Bodies and in Perfect Gases in Particular - II.- Aerial Plane Waves of Finite Amplitude.- The Conditions Necessary for Discontinuous Motion in Gases.- On the Theory of Shock Waves for an Arbitrary Equation of State.- Shock Waves in Arbitrary Fluids.

Energy losses in an acoustical resonator

A one-dimensional model has recently been developed for the analysis of nonlinear standing waves in an acoustical resonator. This model is modified to include energy losses in the boundary layer along the resonator wall. An investigation of the influence of the boundary layer on the acoustical field in the resonator and on the energy dissipation in the resonator is conducted. The effect of the boundary layer is taken into account by introducing an additional term into the continuity equation to describe the flow from the boundary layer to the volume. A linear approximation is used in the development of the boundary layer model. In addition to the viscous attenuation in the boundary layer, the effect of acoustically generated turbulence is modeled by an eddy viscosity formulation. Calculatons of energy losses and a quality factor of a resonator are included into the numerical code. Results are presented for resonators of three different geometries: a cylinder, a horn cone, and a bulb-type resonator. A comparison of measured and predicted dissipation shows good agreement.

Prediction and measurement of particle velocities in ultrasonic standing waves.

A numerical model has been developed to predict particle trajectories in ultrasonic standing waves. The model includes an electroacoustic model that calculates the characteristics of the one‐dimensional standing wave as a function of the input voltage to the piezoelectric transducer driving the cavity. Next, the acoustic radiation force is calculated for particles residing within the water filled cavity. Finally, the particle trajectories are calculated through integration of the equations of motion of the particles. Particle translation is achieved through a periodic sweeping of the excitation frequency. Translational velocities of 6‐μm‐diameter polystyrene spheres are calculated for a 2‐MHz standing wave driven by a PZT‐4 transducer. In the experiment a cavity is filled with water and polystyrene particles. A PZT‐4 transducer operates near its resonance frequency of 2 MHz. Through a periodic sweeping of the frequency the particles are translated away from the transducer face and ultimately clump togethe...

Macro-scale acoustophoretic separation of lipid particles from red blood cells.

Autologous blood salvage is frequently used in cardiac surgery. However, shed mediastinal blood contains lipid particles ranging in size from 10 to 60 μm. Lipid emboli flow and subsequently lodge in the brain capillaries resulting in strokes, leading to neurocognitive dysfunction and death. A novel acoustophoretic filtration system has been developed to separate the lipids from the red blood cells (RBCs). The system works at the macro-scale, supporting flow rates in excess of 2 L/hr. The system is designed such that the acoustic radiation force is able to overcome the combined effects of fluid drag and buoyancy forces. Both RBCs and lipid particles are therefore trapped in the ultrasonic standing wave. Due to the opposite contrast factors of lipids and RBCs, the two components separate at opposite nodes within the standing wave, with lipids concentrating at pressure anti-nodes and RBCs at pressure nodes. Subsequent gravitational separation is used to separate the lipids and RBCs. Preliminary results were ...

Model experiment to study sonic boom propagation through turbulence. Part II. Effect of turbulence intensity and propagation distance through turbulence.

A model experiment was reported to be successful in simulating the propagation of sonic booms through a turbulent atmosphere [B. Lipkens and D. T. Blackstock, J. Acoust. Soc. Am. 103, 148-158 (1998)]. In this study the effect on N wave characteristics of turbulence intensity and propagation distance through turbulence are investigated. The main parameters of interest are the rise time and the peak pressure. The effect of turbulence intensity and propagation distance is to flatten the rise time and peak pressure distributions. Rise time and peak pressure distributions always have positive skewness after propagation through turbulence. Average rise time grows with turbulence intensity and propagation distance. The scattering of rise time data is one-sided, i.e., rise times are almost always increased by turbulence. Average peak pressure decreases slowly with turbulence intensity and propagation distance. For the reported data a threefold increase in average rise time is observed and a maximum decrease of about 20% in average peak pressure. Rise times more than ten times that of the no turbulence value are observed. At most, the maximum peak pressure doubles after propagation through turbulence, and the minimum peak pressure values are about one-half the no-turbulence values. Rounded waveforms are always more common than peaked waveforms.