This paper describes progress towards the development of an active suppression system for open cavity oscillations. A leading-edge array of piezoelectric zero-net mass-flux (ZNMF) actuators is used to suppress cavity flow oscillations determined from measured pressure fluctuations near the rear wall of cavities with length-to-depth ratios (L/D) of 5 and 6 at Mach 0.3 and 0.4. The waveform parameters of the actuator array excitation signal are systematically investigated using open-loop control. Closed-loop control using the quasi-static downhill simplex and the dynamic ARMARKOV adaptive disturbance rejection and generalized predictive control (GPC) algorithms are applied and compared to open-loop control. Up to 30% broadband reduction in rms pressure fluctuations over a 4 kHz bandwidth and reduction in multiple Rossiter modes are obtained and compare favorably with optimized open-loop sinusoidal control. Through the investigation of different actuator slot geometries, larger slots which generate larger momentum with a forcing frequency near the resonant frequency of the actuator showed the best performance for suppressing the cavity oscillation.
In this research, “Data Assimilation” was applied to a wind tunnel experiment. A square cylinder wind tunnel experiment and the ensemble Kalman filter were used to integrate surface pressures of a square cylinder obtained from computation and experiment. The results indicated that surface pressures of the square cylinder and flow field can be replicated by the present data assimilation method, while they could not be replicated by numerical simulation alone. The results of the ensemble Kalman filter were compared with those of “Measurement-Integrated Simulation,” which is another integrated method between computation and experiment for fluids.
The effect of sinusoidal central gas injection on the in-flight alumina spheroidization process was experimentally clarified by using DC-RF hybrid plasma flow system under constant low input power. Spheroidization ratio was evaluated with correlating to frequency and amplitude of sinusoidal central gas injection, DC arc voltage, and particle size distribution. Spheroidization ratio improves through active mixing of DC-RF hybrid plasma flow with powder by injection of sinusoidal central gas for DC plasma jet forming. The maximum spheroidization ratio of 99 % for processed powder collected at z = 120 mm was obtained by giving the sinusoidal central gas injection at fc = 2 Hz with amplitude of ± 20 %.
The purpose of this study is to improve a two-fluid model applicable to gas-liquid two-phase slug flows in small diameter pipes. Experimental data on void fraction and frictional pressure drop were obtained for vertical upward slug flow in 5 and 9 mm i.d. circular pipes, and those on the interfacial friction force were obtained by substituting the above data into an equation derived from the one-dimensional two-fluids model. The test liquids were a Poly-oxy-ethylene lauryl ether water solution, a 72 wt% glycerin water solution and a tap water at 30 °C, while the test gas was air at near atmospheric pressure. In order to study the effects of liquid properties, surface tension of the test liquids against air was varied from 0.042 to 0.071 N/m, viscosity was 0.797 to 19.6 mPa·s, and density was 996 to 1184 kg/m3. The range of volumetric flux of the liquid was 0.1 to 2.0 m/s, and that of the gas was 0.1 to 22 m/s. In addition, the drag coefficient data of the large bubble was determined from the data on the bubble diameter, the respective void fractions of the liquid slug section and the large bubble one, the interfacial friction force and the length ratio of the large bubble to the total of the liquid slug and the large bubble. These data were used to validate representative two-fluid model codes. Since the prediction by the codes did not fit well the present data, a new drag coefficient correlation for the large bubble was proposed using 5 dimensionless parameters. The predictions by the new correlation and familiar ones in literatures have been tested against the present data, and the applicability of the new one has been demonstrated.
At the initial stage of liquid injection into a gas, the injection liquid has not yet been broken up, and at a low surrounding pressure of less than about 0.1 MPa, the tip of the injection liquid forms the shape of a thin string. This phenomenon and the reason for the shape have not been well studied. At a higher pressure of about 3.0 MPa, the tip of the flow forms a mushroom shape. In this paper, the MARS method for simulating free surfaces is applied to analyze the initial shape of the injection liquid. The above phenomena are reproduced and the reason for them is clarified. Another focus in this paper is the throttling effect due to the nozzle, which causes the formation of small air bubbles near the nozzle wall. These bubbles induce large eddies at the surface of the liquid column and promote the disintegration of the liquid film.
The interaction of a spatially transitional boundary layer flow at freestream Mach number M = 2.0 with an impinging oblique shock wave (β = 35.56) is studied by means of direct numerical simulation (DNS). High amplitude (5%) three-dimensional isotropic disturbances are superimposed on the laminar profile for a Reynolds number based on the boundary layer displacement thickness of Reδ*0 = 1000 at the inlet plane of the computational box. Under the selected flow conditions, the interaction of the boundary layers with the incident shock wave produces three-dimensional separation, thus inducing a shock system (reflected waves and expansion fan). Linear-stability theory and DNS indicate that Λ-shaped vortices are generated in the separated shear layer due to the most unstable oblique mode, which can trigger the breakdown mechanism around the reattachment line. Energy spectral density of the wall-pressure indicates broadband spectra that are observed in the experimental result on shock/turbulent boundary layer interaction.
A three-dimensional lattice Boltzmann method based on the Uehling-Uhlenbeck Boltzmann-BGK equation is presented. The method is directly derived by projecting the kinetic governing equation onto the tensor Hermite polynomials and various hydrodynamic approximation orders can be achieved. The intrinsic discrete nodes of the three-dimensional Gauss-Hermite quadrature provide the natural lattice velocities for the semiclassical lattice Boltzmann method. Simulations of the lid-driven cubic cavity flows based on D3Q19 lattice model for several Reynolds numbers and different particle statistics are shown to illustrate the method. The results indicate distinct characteristics of the effects of quantum statistics.
Electric arc stabilized by a vortex of water (Gerdien arc) is used in a special type of plasma torch for generation of thermal plasma jet with extreme plasma parameters. Extremely high plasma enthalpy and plasma temperature and very low plasma density result in very high efficiency of utilization of plasma enthalpy for heating of material injected into plasma. Plasma torches with Gerdien arc are used in industrial scale for plasma spraying, other plasma chemistry applications like waste treatment or gasification of biomass for syngas production are being investigated. In these applications, special properties of steam plasma produced in this type of arc are beneficial. Experiments with steam plasma gasification of carbon containing materials were made in plasma reactor with Gerdien arc plasma torch. Gasification of polyethylene, plastic waste, wooden pellets and saw dust were studied for arc power 100 - 140 kW. Syngas with high content of hydrogen and carbon monoxide and very low content of carbon dioxide was produced. Very low content of complex hydrocarbons and tar was detected. In optimal conditions the calorific value of produced syngas was substantially higher than electric energy spent for plasma generation. Possibility of usage of the process is discussed for energy storage during excess of electricity available from power plants with fluctuating output.