Unsteady Effects of Blade Row Interaction on Flow Field and Aerodynamic Performance of a Transonic Centrifugal Compressor Impeller

This paper discusses the unsteady effects associated with the impeller/diffuser interaction on the internal flow field and aerodynamic performance of a centrifugal compressor. In centrifugal compressors with a vaned diffuser, the flow field is inherently unsteady due to the influence of interaction between the impeller and the diffuser, and the unsteadiness of the flow field can often have a great influence on the aerodynamic performance of the compressor. Especially in high-load compressors, it is considered that large unsteady effects are produced on the compressor performance with a strong flow unsteadiness. The unsteady effect on aerodynamic performance of the compressor has not been fully revealed yet, and sometimes the steady-state RANS simulation finds it difficult to predict the compressor performance. In this study, numerical simulations have been conducted for a transonic centrifugal compressor with a vaned diffuser. The unsteady effects were clarified by comparing the numerical results between a single-passage steady-state RANS analysis and a full-annulus unsteady RANS analysis. The comparison of simulation results showed the difference in entropy generation in the impeller. The impingement of diffuser shock wave with the impeller pressure surface brought about a cyclic increase in the blade loading near the impeller trailing edge. Accordingly, with increasing tip leakage flow rate, a second tip leakage vortex was newly generated in the aft part of the impeller, which resulted in additional unsteady loss generation inside the impeller.

Stall mitigation using wing spoilers

The objective of this thesis was to conduct a two dimensional Compuational Fluid Dyanmic analysis on wing-spoiler lift effectiveness in delaying stall effects using the NACA 2412 airfoil section. The project was divided into three sub-areas; grid-independent study, the baseline study and spoiler performance study. The grid independent study was carried out for the purpose of mesh optimization, i.e. to determine the point at which computed solutions had little or no change in value with increasing number of mesh nodes. This study was conducted at an angle of attack of 16 degrees as it served as both a high pitch angle value as well as a pre-stall point in which unsteady effects were not a determining influence that may have served as a deterrent in resolving potential grid error. The baseline study was conducted to establish a data foundation to be used as a comparison to the spoiler study to effectively determine its effects on the lift performance. Results for the baseline study were shown to match experimental values most closely using the Transition SST Turbulence Model at a Mach number of 0.17. Therefore to remain consistent the spoiler study was carried out for the same Mach value with the viscosity of 1.84E-5 [kg/ms] and Temperature of 300 [K], which produced a Reynolds number of approximately 3.79E6. Therefore, using the same flight conditions employed in the baseline study, the spoiler grids were generated in ANSYS ICEM CFD and imported into Fluent and solved using the Transition SST turbulence model. Results for spoiler deflections of 4, and 10 degrees were carried out across spoiler locations of 60%, 65 and 70% leading edge chord-wise locations. Results were shown to be optimal for 4 degree spoiler deflections across all locations tested, with highest values for lift effectiveness recorded at 70% leading edge. The findings of this thesis provides much potential for utilizing spoilers as lift enhancing devices and adds an alternate perspective in improving aircraft performance.

Stall mitigation using wing spoilers

The objective of this thesis was to conduct a two dimensional Compuational Fluid Dyanmic analysis on wing-spoiler lift effectiveness in delaying stall effects using the NACA 2412 airfoil section. The project was divided into three sub-areas; grid-independent study, the baseline study and spoiler performance study. The grid independent study was carried out for the purpose of mesh optimization, i.e. to determine the point at which computed solutions had little or no change in value with increasing number of mesh nodes. This study was conducted at an angle of attack of 16 degrees as it served as both a high pitch angle value as well as a pre-stall point in which unsteady effects were not a determining influence that may have served as a deterrent in resolving potential grid error. The baseline study was conducted to establish a data foundation to be used as a comparison to the spoiler study to effectively determine its effects on the lift performance. Results for the baseline study were shown to match experimental values most closely using the Transition SST Turbulence Model at a Mach number of 0.17. Therefore to remain consistent the spoiler study was carried out for the same Mach value with the viscosity of 1.84E-5 [kg/ms] and Temperature of 300 [K], which produced a Reynolds number of approximately 3.79E6. Therefore, using the same flight conditions employed in the baseline study, the spoiler grids were generated in ANSYS ICEM CFD and imported into Fluent and solved using the Transition SST turbulence model. Results for spoiler deflections of 4, and 10 degrees were carried out across spoiler locations of 60%, 65 and 70% leading edge chord-wise locations. Results were shown to be optimal for 4 degree spoiler deflections across all locations tested, with highest values for lift effectiveness recorded at 70% leading edge. The findings of this thesis provides much potential for utilizing spoilers as lift enhancing devices and adds an alternate perspective in improving aircraft performance.

Noncertainty Equivalent Adaptive Backstepping Control for Advanced Fighter Subject to Unsteady Effects and Input Constraints

This paper presents a noncertainty equivalent adaptive backstepping control scheme for advanced fighter attitude tracking, in which unsteady effects, parameter uncertainties, and input constraints are all considered which increase the design difficulty to a large extent. Based on unsteady attitude dynamics and the noncertainty equivalent principle, a new observer is first developed to reconstruct the immeasurable and time-varying unsteady states. Afterwards, the unsteady aerodynamics is compensated in the backstepping controller where the command filter is introduced to impose physical constraints on actuators. In order to further enhance the robustness, the noncertainty equivalent adaptive approach is again used to estimate the uncertain constant parameters. Moreover, stability of the closed-loop system that includes the state observer, parameter estimator, and backstepping controller is proven by the Lyapunov theorem in a unified architecture. Finally, simulation results show that performance of the deterministic control system can be captured when attractive manifolds are achieved. The effectiveness and robustness of the proposed control scheme are verified by the Herbst maneuver.

Unsteady Effects on NOx Measurements in Pulse Detonation Combustion

Emission measurements from unsteady combustion systems such as Pulse Detonation Combustion (PDC) are challenging due to the inherently large variations in pressure, temperature, composition, and flow velocity of the exhaust gas. Comparison of experimental data is additionally complicated by differences in operating conditions and gas sampling setup between different facilities. Qualitative considerations with regard to the sampling process from PDC, based on one-dimensional simulations, indicate a systematic influence of the sampling setup and extraction process on the resulting concentration measurements. Therefore, operating frequency, sample time, fill time, as well as PDC outlet and probe geometry were varied experimentally in order to assess the degree to which each of these parameters impact the resulting measured $${\rm NO}_{\rm x}$$ NO x in order to better inform researchers of these effects when making measurements. It was shown that measured $${\rm NO}_{\rm x}$$ NO x emissions can vary significantly depending on the choice of these parameters and therefore care must be exercised in order to reduce the influence of the sampling technique when aiming for comparable results.