Numerical simulations have been performed to explore the effects of important design parameters on impingement disturbance properties of multiple hydrazine thruster plumes. The particle behaviour of the far-field plume flows near the impingement surfaces is modelled using the collisionless molecular direct simulation Monte-Carlo method with the inflow boundary condition derived from the velocity and temperature profiles at the nozzle exit. The entering molecular motions and the impact interactions between molecules and solid boundaries are treated on a probabilistic basis, whereas the main flow properties are preserved from the transformation of the microscopic fluid activities of simulated molecules. To validate the present computer code, the predicted transverse Pitot pressure profile was compared with the measured data in the far plume region of an MBB/ERNO 0.5-N conical nozzle. On the basis of the ROCSAT-1 design configuration, the calculated plume disturbance torques are examined in the yaw, roll, and pitch directions for different settings of the canted angle of the thruster mounting and the satellite centre-of-mass (CM) offset. Predictions show that the normalized yaw impingement disturbance torque reaches 6.3 per cent at the cant angle of 30°, indicating that the associated plume collision effect on the attitude control can be important at large cant angles. In addition, the location change of the satellite CM in the axial axis reveals insignificant influence on the impingement torque for the offset value varying from −5.0 to 5.0 cm.
The impinging behaviour of liquid droplets on solid surfaces is studied using a computational approach. The analysis comprises the unsteady three-dimensional conservation equations of mass and momentum, with the surface tension effect treated by the continuous surface force model. Gas-liquid interfacial motions are simulated by the volume-of-fluid method in conjunction with the piecewise linear interface construction technique. In the computer code validation for a water droplet impacting on a polished stainless steel surface, computer-generated images of the time evolution of the droplet impingement dispersal shape are compared with magnified photographs by Pasandideh-Fard et al. The flow and transport phenomena in the impingement flowfield are further examined in detail. In order to respond to the need for its use in practical applications, the study is extended to explore the spreading progression to achieve a better understanding of the interaction of a 30 μm diameter polyethylenedioxy thiophene liquid droplet with a 50 × 50 μm indium tin oxide-coating square cavity at an impact velocity of 6 m/s.
The applications of piezoelectric synthetic jet actuators have shown great potential as active flow control devices. The objective of this study is to investigate the flow phenomenon of a synthetic jet generated by a dual-diaphragm piezo-driven actuator. In this analysis, the computational approach adopted unsteady three-dimensional conservation equations of mass and momentum for examining the development process of synthetic jets. The moving boundary was also treated to represent the motion of the piezo diaphragm. Experimentally, a flow visualization system was employed to acquire the particle-streak images scattered from red fluorescent spheres for observing the synthetic jet flow. The jet velocity along the centre-line was also measured by using a hot-wire anemometer. The system test results demonstrated a satisfactory functioning of the actuator for producing synthetic jets. The predictions were then compared with the visualized particle-streak images and the measured centre-line velocity of the synthetic jet to validate the computer software. In the near-field, both simulation results and experimental observations revealed the time-cyclical formation and advection of a vortex pair in a full sinusoidal actuation cycle at an operating frequency of 4 Hz. When the vortex pair travelled well downstream, the ambient air from the vicinity of the slot was entrained into the cavity of the actuator. However, the overall far-field flow pattern, characterized by longitudinal decay of the centre-line velocity and lateral spreading, resembled a conventional continuous air-jet in essence.
The phenomenon of hydrodynamic focusing in a flow cytometer is investigated using a computational approach. In this work, a three-dimensional two-fluid theoretical model was established to describe the flow transport behaviour and the interaction of two fluids coflowing at different velocities. Treating both sample and sheath fluids as laminar, incompressible, and isothermal flows, the analysis encompasses two sets of three-dimensional unsteady equations for conservation of mass and momentum, with consideration of interfacial momentum exchange. Governing equations are solved numerically through an iterative semi-implicit method for pressure-linked equations consistent algorithm to determine the flow variables. For code validation, both focused width and length in the two-dimensional configuration are predicted at a broad range of u sh /u s ratios and are compared with Lee et al.'s measured data. Subsequently, the work extends to examine the three-dimensional hydrodynamic focusing process and the time required for completion of one focusing event. To explore the feasibility of the proposed flow cytometer in applications, the focused properties are determined by varying the ratio of sheath velocity to sample velocity from 10 to 80. Ten numerical experiments were also conducted to examine the effects of the fluid properties on the length and width of the focused sample stream.
Blowdown and fluid hammer characteristics are studied for a satellite reaction control subsystem. A flow channel network numerical scheme is used to determine the blowdown pressure profile and the steady state pressure drops in the propellant lines. For the transient fluid analysis, a theoretical model based on the method of characteristics (MOC) is solved to simulate the time-dependent transport behaviour of the propellant flow. Predicted results show that the blowdown pressure profile in continuous mode operation ranges from 24.5 to 5.5 bar throughout the lifetime of propellant usage. Under a tank pressure of 24.5 bar, the predicted peak values of the pressure fluctuations caused by the fluid hammer waves are nearly the same and equal to 25.4 bar at the locations of the latching isolation valve inlet and the thruster valve inlet. The Fourier spectral analysis also indicates that the induced excitation from the fluid hammer pressure oscillations will not resonantly couple with the thruster assembly and fuel lines. In considering the fluid hammer effect due to the abrupt closure of valves, the pressure spikes will not lead to any damage to the propulsion components or lines during normal operation.
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