Inherent and model-form uncertainty analysis for CFD simulation of synthetic jet actuators

A mixed aleatory (inherent) and epistemic (model-form) uncertainty quantification (UQ) analysis method was applied to a computational fluid dynamics (CFD) modeling problem of synthetic jet actuators. A test case, (Case 3, flow over a hump model with synthetic jet actuator control) from the CFDVAL2004 workshop was selected to apply the Second-Order Probability framework implemented with a stochastic response surface obtained from Quadrature-Based Non-Intrusive Polynomial Chaos (NIPC). Three uncertainty sources were considered: (1) epistemic uncertainty in turbulence model, (2) aleatory uncertainty in free stream velocity and (3) aleatory uncertainty in actuation frequency. Uncertainties in both long-time averaged and phase averaged quantities were quantified using a fourth order polynomial chaos expansion (PCE). Results were compared with experimental data available. A global sensitivity analysis with Sobol indices was utilized to rank the importance of each uncertainty source to the overall output uncertainty. The results indicated that for the long-time averaged separation bubble size, the uncertainty in turbulence model had a dominant contribution, which was also observed in the long-time averaged skin friction coefficients at three selected locations. For long-time averaged pressure coefficient, the contributions from free stream velocity and turbulence model are depending on the locations. The mixed UQ results for phase averaged x-velocity distributions at three selected locations showed that the 95% confidence intervals (CI) could generally envelope the experimental data. The Sobol indices showed that near the wall, the turbulence model had a main influence on the x-velocity, while approaching the main stream, the uncertainty in free stream velocity became a larger contributor. The uncertainty in frequency was found to have a very small contribution to both long-time averaged and phase averaged quantities with the range of variance considered --Abstract, page iii

Simulations of Ion Thruster Plume Contamination with a Whole Grid Sputtered Mo Source Model

A particle simulation based source model is developed to calculate the density distribution of the sputtered Mo atoms for a whole ion optics grid. The source model is used in PIC simulation of ion thruster plume contamination for 3-grid and 2-grid ion thrusters. The results show that the commonly used point-source approximation for sputtered Mo atoms is oversimplified and would lead to over-prediction of contamination deposition

Inherent and Model-Form Uncertainty Analysis for CFD Simulation of Synthetic Jet Actuators

A mixed (aleatory and epistemic) uncertainty quantification (UQ) method was applied to computational uid dynamics (CFD) modeling of a synthetic jet actuator. A test case, (ow over a hump model with synthetic jet actuator control) from the CFDVAL2004 work-shop was selected to apply the Second-Order Probability framework implemented with a stochastic response surface obtained from Quadrature-Based Non-Intrusive Polynomial Chaos (NIPC). Three uncertainty sources were considered: (1) epistemic (model-form) uncertainty in turbulence model, (2) aleatory (inherent) uncertainty in free stream veloc-ity and (3) aleatory uncertainty in actuation frequency. Uncertainties in both long-time averaged and phase averaged quantities were quantified using a fourth order polynomial chaos expansion (PCE). A global sensitivity analysis with Sobol indices was utilized to rank the importance of each uncertainty source to the overall output uncertainty. The results indicated that for the long-time averaged separation bubble size, the uncertainty in turbulence model had a dominant contribution, which was also observed in the long-time averaged skin friction coeficients at three selected locations. The mixed uncertainty results for phase averaged x-velocity distributions at three selected locations showed that the 95% confidence interval (CI) could generally envelope the experimental data. The Sobol indices showed that near the wall, the uncertainty in turbulence model had a main inuence on the x-velocity, while approaching the main stream, the uncertainty in free stream velocity be-came a larger contributor. The mixed uncertainty quantification approach demonstrated in this study can also be applied to other computational uid dynamics problems with inherent and model-form input uncertainities

Numerical Simulations of Surface Charging at the Lunar Terminator

A full-particle particle-in-cell (PIC) simulation model is developed to perform fully-kinetic simulations of surface-plasma interactions at the lunar terminator region. This model uses a non-homogeneous immersed-finite-element (IFE) solver to calculate the electric field discontinuity (flux jump) at the plasma-surface interface and surface charging for realistic lunar surface terrain. The simulation includes all plasma species, including the solar wind electrons and ions, as well as photoelectrons with real proton-to-electron mass ratio so the kinetic properties in the mesothermal flow are resolved. We present simulation results of surface charging around a lunar hill at the lunar terminator region

Uncertainty Quantification Integrated to CFD Modeling of Synthetic Jet Actuators

The Point-Collocation Non-intrusive Polynomial Chaos (NIPC) method has been applied to a stochastic synthetic jet actuator problem used as one of the test cases in the CFDVAL2004 workshop to demonstrate the integration of computationally efficient uncertainty quantification to the high-fidelity CFD modeling of synthetic jet actuators. The test case included the simulation of an actuator generating a synthetic jet issued into quiescent air. The Point-Collocation NIPC method is used to quantify the uncertainty in the long-time averaged u and v-velocities at several locations in the flow field due to the uniformly distributed uncertainty introduced in the amplitude and frequency of the oscillation of the piezo-electric membrane. Fifth-order NIPC expansions were used to obtain the uncertainty information, which showed that the variation in the v-velocity is high in the region directly above the jet slot and the variation in the u-velocity is maximum in the region immediately adjacent to the slot. Even with a ±5% variation in the amplitude and frequency, the long-time averaged u and v-velocity profiles could not match the experimental measurements at y=0.1 mm above the slot indicating that the discrepancy may be due to other uncertainty sources in CFD and/or due to the measurement errors

Kinetic Particle Simulations Of Plasma Charging At Lunar Craters Under Severe Conditions

This paper presents fully kinetic particle simulations of plasma charging at lunar craters with the presence of lunar lander modules using the recently developed Parallel Immersed-Finite-Element Particle-in-Cell (PIFE-PIC) code. The computation model explicitly includes the lunar regolith layer on top of the lunar bedrock, taking into account the regolith layer thickness and permittivity as well as the lunar lander module in the simulation domain, resolving a nontrivial surface terrain or lunar lander configuration. Simulations were carried out to study the lunar surface and lunar lander module charging near craters at the lunar terminator region under mean and severe plasma environments. The lunar module\u27s position is also investigated to see its effect on the plasma charging relative to the craters. Differential surface charging was clearly resolved by the simulations. For the charging of a lunar lander module made of conducting materials, the results show a near-uniform potential close to that of its surrounding environment and moderate levels of local electric fields. Additionally, the risks associated with charging and discharging increase significantly under a more severe plasma charging environment as shown in the severe plasma environment cases

Numerical Modeling of Dust Dynamics around Small Asteroids

Dynamics of dust transport around an airless body has been a focused area of research in recent years, however, various challenging aspects still remain to be addressed. This paper presents an investigation of charged dust transport and distribution around small asteroids utilizing a full particle Particle-in-Cell (PIC) model to simulate plasma flow around an asteroid and calculate surface charging self-consistently from charge deposition on asteroid. Material properties of asteroid are also explicitly included in the simulation. PIC simulation results are fed into a 3D dust dynamics model to simulate charged dust levitation, transport and distribution. In addition to electrostatic and gravitational forces, the dynamics of dust surface impacts and asteroid body rotation are also included in the model. We discuss the effects on dust levitation and transport by comparing dust grain charge-mass ratio, local electrostatic field and dust grain size. We present simulation results of dust distribution around small spherical asteroids. The study highlights the sensitivity to electrostatic field and grain characteristics while following the general trend that gravity dominates in the far field, where as local electric field prevails at low altitude

Uncertainty Quantification Integrated to the CFD Modeling of Synthetic Jet Actuators

The Point Collocation Non-Intrusive Polynomial Chaos (NIPC) method has been applied to two stochastic synthetic jet actuator problems used as test cases in the CFDVAL2004 workshop to demonstrate the integration of computationally efficient uncertainty quantification to the high-fidelity CFD modeling of synthetic jet actuators. In Case1 where the synthetic jet is issued into quiescent air, the NIPC method is used to quantify the uncertainty in the long-time averaged u and v-velocities at several locations in the flow field, due to the uniformly distributed uncertainty introduced in the amplitude and frequency of the oscillation of the piezo-electric membrane. Fifth order NIPC expansions were used to obtain the uncertainty information, which showed that the variation in the v-velocity is high in the region directly above the jet slot and the variation in the u-velocity is maximum in the region immediately adjacent to the slot. Even with a ten percent variation in the amplitude and frequency, the long-time averaged u and v velocity profiles could not match the experimental measurements at y=0.1mm above the slot indicating that the discrepancy may be due to other uncertainty sources in CFD or measurement errors. In Case 2 which includes a cross flow, the free stream velocity is treated as an uncertain input variable. Fifth degree NIPC expansions were employed to quantify the uncertainty in phase averaged velocity profiles as well as long-time averaged wall pressure and skin friction coefficient distributions. The results of Case 2 show that the uncertainty in phase averaged velocity profiles gets larger when approaching the main stream. The size of a separation bubble observed in this case remains relatively insensitive to the uncertain free stream velocity within the tolerance range considered

Kinetic Simulations of Plasma Plume Potential in a Vacuum Chamber

Direct Simulation Monte Carlo and Particle-in-Cell simulations are carried out to study the potential of a mesothermal plasma plume in a vacuum chamber. The results show that the beam potential with respect to the ambient in a vacuum chamber is different from that in space because the facility plasma can prematurely terminate the plume expansion process. As a result, the plume potential measured in a vacuum chamber may be significantly lower than that under the in-space condition. This can lead to under estimation of the backflow of CEX ions and ionized contaminants in plasma thruster plume modeling

Experimental and Numerical Investigations of Dust Effects on Surface Charging in Plasma

A series of 3-D, fully kinetic particle-in-cell (PIC) simulations were performed to simulate mesothermal plasma flow self-consistently with surface charging. Simulation results of plasma charging of a conducting surface covered by a thin dust layer in a plasma of cold ions and thermal electrons are presented. The surface potentials and potentials inside the dust layer are compared with experimental results. Results show that a layer of dust over a conducting surface creates a capacitance, which drives the surface more negative with respect to the ambient plasma