Jiahuan Cui

Numerical Investigation of Contrasting Flow Physics in Different Zones of a High-Lift Low-Pressure Turbine Blade

Using a range of high-fidelity large eddy simulations (LES), the contrasting flow physics on the suction surface, pressure surface, and endwalls of a low-pressure turbine (LPT) blade (T106A) was studied. The current paper attempts to provide an improved understanding of the flow physics over these three zones under the influence of different inflow boundary conditions. These include: (a) the effect of wakes at low and high turbulence intensity on the flow at midspan and (b) the impact of the state of the incoming boundary layer on endwall flow features. On the suction surface, the pressure fluctuations on the aft portion significantly reduced at high freestream turbulence (FST). The instantaneous flow features revealed that this reduction at high FST (HF) is due to the dominance of “streak-based” transition over the “Kelvin–Helmholtz” (KH) based transition. Also, the transition mechanisms observed over the turbine blade were largely similar to those on a flat plate subjected to pressure gradients. On pressure surface, elongated vortices were observed at low FST (LF). The possibility of the coexistence of both the Gortler instability and the severe straining of the wakes in the formation of these elongated vortices was suggested. While this was true for the cases under low turbulence levels, the elongated vortices vanished at higher levels of background turbulence. At endwalls, the effect of the state of the incoming boundary layer on flow features has been demonstrated. The loss cores corresponding to the passage vortex and trailing shed vortex were moved farther from the endwall with a turbulent boundary layer (TBL) when compared to an incoming laminar boundary layer (LBL). Multiple horse-shoe vortices, which constantly moved toward the leading edge due to a low-frequency unstable mechanism, were captured.

Numerical Study of Purge and Secondary Flows in a Low-Pressure Turbine

The secondary flow increases the loss and changes the flow incidence in the downstream blade row. To prevent hot gases from entering disk cavities, purge flows are injected into the mainstream in a real aero-engine. The interaction between purge flows and the mainstream usually induces aerodynamic losses. The endwall loss is also affected by shedding wakes and secondary flow from upstream rows. Using a series of eddy-resolving simulations, this paper aims to improve the understanding of the interaction between purge flows, incoming secondary flows along with shedding wakes and mainstream flows on the endwall within a stator passage. It is found that for a blade with an aspect ratio of 2.2, a purge flow with a 1% leakage rate increases loss generation within the blade passage by around 10%. The incoming wakes and secondary flows increase the loss generation further by around 20%. The purge flow pushes the passage vortex further away from the endwall and increases the exit flow angle deviation. However, the maximum exit flow angle deviation is reduced after introducing incoming wakes and secondary flows. The loss generation rate is calculated using the mean flow kinetic energy equation. Two regions with high loss generation rate are identified within the blade passage: the corner region and the region where passage vortex interacts with the boundary layer on the suction surface. Loss generation rate increases dramatically after the separated boundary layer transitions. Since the endwall flow energizes the boundary layer and triggers

Effect of Fan on Inlet Distortion: Mixed-Fidelity Approach

Yunfei Ma∗ Department of Engineering, University of Cambridge, United Kingdom, CB2 1PZ Jiahuan Cui† School of Aeronautics and Astronautics, and ZJU-UIUC institute, Zhejiang University, People’s Republic of China, 310007 Nagabhushana Rao Vadlamani‡ Department of Engineering, University of Cambridge, United Kingdom, CB2 1PZ Paul Tucker§ Department of Engineering, University of Cambridge, United Kingdom, CB2 1PZ

A Mixed-Fidelity Numerical Study for Fan–Distortion Interaction

Inlet distortion often occurs at off-design points when a flow separates within an intake and this unsteady phenomenon can seriously impact fan performance. Fan-distortion interaction is a highly unsteady aerodynamic phenomenon into which high-fidelity simulations can provide detailed insights. However, due to computational resource limitations, the use of an eddy resolving method for a fully resolved fan calculation is currently infeasible within industry. To solve this problem, a mixed-fidelity CFD method is proposed. This method uses the Large Eddy Simulation (LES) approach to resolve the turbulence associated with separation, and the Immersed Boundary Method with Smeared Geometry (IBMSG) to model the fan. The method is validated by an experiment on the Darmstadt Rotor, which shows a good agreement in terms of total pressure distributions. A detailed investigation is then conducted for a subsonic rotor with an annular beam-generating inlet distortion. A number of studies are performed in order to investigate the fans influence on the distortions. A comparison to the case without a fan shows that the fan has a significant effect in reducing distortions. Three fan locations are examined which reveal that the fan nearer to the inlet tends to have a higher pressure recovery. Three beams with different heights are also tested to generate various degrees of distortion. The results indicate that the fan can suppress the distortions and that the recovery effect is proportional to the degree of inlet distortion.

Improved Hierarchical Modelling for Aerodynamically Coupled Systems

© 2017 ASME. Strong aerodynamic coupling can make the high fidelity simulation of a number of critical aero-engine components prohibitively expensive - particularly within the timeframes of industrial design cycles. This paper develops a body force based hierarchy of approaches to modelling the effects of blade rows. These are envisaged as allowing the computationally expensive parts of coupled systems to be resolved much more cheaply, rendering the cost of the overall simulation as more manageable. Simulation of the coupling that exists between the flow around an aero-engine intake and its fan is particularly emphasised, as this is becoming stronger and more performance critical with the modern trends towards the reduction of the relative diffuser length. The use of the viscous smeared geometry level of fidelity is initially shown to be an effective model over a number of cases a simple compressor blade row, a modern high bypass fan, and the Darmstadt rotor. After this, it is shown working as part of a coupled system in an intake experiencing crossflow. Higher fidelity geometry representations are then considered, which mimic the effect of struts. Finally, a mix of various fidelity geometry representations and turbulence modelling approaches is shown to bring otherwise hugely expensive calculations within the realm of practical computation, in the form of a full fan-to-flap calculation.

A critical examination of a correlation-based transition model for low pressure turbines

© 2017 by Mr Demetrios Lefas, Dr Jiahuan Cui and Prof. Paul Tucker. Two of the main requirements identified for a fully compatible CFD transition model demand that: first it does not affect the underlying turbulence model in fully turbulent regimes and second, that it is applicable to three dimensional flows. The purpose of this paper is to determine whether the γ - Reθ model satisfies these requirements for Low Pressure Turbines (LPTs). The γ - Reθ model’s performance for LPTs is positive overall. A significant improvement on the underlying SST model is observed. The total pressure loss coefficient (Yp) predicted at the exit plane by the γ - Reθ model is only 4% greater than that determined by quasi-DNS, compared to 16% greater for the SST model. In the midspan region, the γ - Reθ model successfully predicts a laminar boundary layer on the suction surface. This laminar boundary layer separates near the trailing edge and then reattaches after transition, resulting in a fully turbulent boundary layer downstream. The location of separation and subsequent separation-induced transition from the γ - Reθ model agrees almost in full with the quasi-DNS results. However, a number of significant limitations are identified, especially in the endwall region. First, γ close to the endwall tends towards zero in the γ - Reθ model, resulting in a zero production of turbulent kinetic energy. Second, even though the laminar region close to the pressure surface is readily identified up to z+ = 14.3, for the higher z-planes examined this is not the case. This contradicts the quasi-DNS results. Finally, the production of turbulence along the suction surface by the corner vortex is significantly underestimated by the γ - Reθ model.

Numerical Study of Purge and Secondary Flows in a Low Pressure Turbine

© 2016 by ASME. The secondary flow increases the loss and changes the flow incidence in the downstream blade row. To prevent hot gases from entering disk cavities, purge flows are injected into the mainstream in a real aero-engine. The interaction between purge flows and the mainstream usually induces aerodynamic losses. The endwall loss is also affected by shedding wakes and secondary flow from upstream rows. Using a series of eddy-resolving simulations, this paper aims to improve the understanding of the interaction between purge flows, incoming secondary flows along with shedding wakes and mainstream flows on the endwall within a stator passage. It is found that for a blade with an aspect ratio of 2.2, a purge flow with a 1% leakage rate increases loss generation within the blade passage by around 10%. The incoming wakes and secondary flows increase the loss generation further by around 20%. The purge flow pushes the passage vortex further away from the endwall and increases the exit flow angle deviation. However, the maximum exit flow angle deviation is reduced after introducing incoming wakes and secondary flows. The loss generation rate is calculated using the mean flow kinetic energy equation. Two regions with high loss generation rate are identified within the blade passage: The corner region and the region where passage vortex interacts with the boundary layer on the suction surface. Loss generation rate increases dramatically after the separated boundary layer transitions. Since the endwall flow energizes the boundary layer and triggers earlier transition on the suction surface, the loss generation rate close to the endwall at the trailing edge is suppressed.

Flow disturbance environment in low pressure turbines

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA, All rights reserved. Flows within the low pressure turbine are under constant disturbance. These disturbances include free stream turbulence, incoming wakes, surface roughness and so on. To achieve higher blade loading, modern blade design features a separation bubble on the suction surface under steady ow conditions. This separation bubble has to be suppressed by the flow disturbance within the real engine. Due to this advanced blade design philosophy, understanding of the disturbance environment within the low pressure turbine becomes increasingly important. Through several eddy resolving simulations, the current paper provides a broad picture of the effect of different disturbances on the ow at the midspan. The free stream turbulence, incoming wakes and surface roughness are found to be effective in suppressing the separation bubble. These disturbances generate streaky structures within the boundary layer. As these streaks travel to the separated region, they trigger earlier transition and then suppress the separation bubble.

Numerical investigation of flow separation from aircraft tail surfaces

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA . All rights reserved. The study of the flow around a vertical tail plane (VTP) in design conditions is challenging. The flow is unsteady and detaches massively from the tail surfaces. Current industrial design techniques are based on steady RANS approaches, and, for this reason, they might fail in computing the flow when separation occurs. Moreover, in design conditions, the VTP is characterized by the defection of the rudder, which allows control and balance of the aircraft. Therefore, flow separation might occur also on the rudder. However, in this case, the flow detaches because the presence of a mild pressure gradient, and it is even more difficult to compute accurately this kind oh physical conditions with current design methods. This paper addresses to the study of flow separation with hybrid RANS/LES techniques, as a follow-up of a precursor study which had shown that steady and unsteady RANS are not satisfactory for this kind offlows. Delayed Detached-Eddy Simulations (DDES) are performed on a generic aircraft tail geometry, showing a good improvement of the computedflow solution with respect to precursor studies on the same test case.