A double-bypass variable cycle compression system (fan, core driven fan stage, and high-pressure compressor, with a mode selector valve on the outer bypass and a forward variable area bypass injector on the inner bypass) changes its operating mode in flight by opening the mode selector valve. Three-dimensional steady simulations showed that opening this valve alone drives a reverse flow (recirculation) in the outer bypass that distorts the core driven fan stage inflow; separately, lowering the bypass backpressure weakens but does not remove the recirculation and pushes the core driven fan stage deeper into choke, while closing the forward variable area bypass injector restores stator performance but leaves the recirculation untouched. The recommended combined strategy of applying both control actions simultaneously has never been simulated or tested: whether it can fully eliminate the outer-bypass recirculation while maintaining acceptable core driven fan stage stator performance and avoiding deep choking, and how recirculation, distortion, and component matching evolve during a real-time continuous (unsteady) transition executed under this combined action remain open.

Background

A variable cycle engine (VCE) is a turbofan whose flow path can be reconfigured in flight so the engine operates as a low-bypass-ratio, turbojet-like engine with high specific thrust for supersonic flight, or as a high-bypass-ratio, turbofan-like engine with low fuel consumption at subsonic cruise. In the double-bypass architecture considered here, the compression system consists of a fan, a core driven fan stage (CDFS, a fan stage driven by the high-pressure spool upstream of the high-pressure compressor), and a high-pressure compressor (HPC). Downstream of the fan the flow splits into an outer and an inner bypass duct: the mode selector valve (MSV), a variable-area valve at the outer-bypass entrance, opens to admit fan air into the outer bypass when the engine transitions from single-bypass to double-bypass mode, and the forward variable area bypass injector (FVABI), a variable-area device at the inner-bypass outlet, controls the inner-bypass outflow area and its mixing with the outer stream. The static pressure at the bypass outlet (the backpressure p2p_2), set by the downstream nozzle state, is a controllable boundary condition.

Mode transition is an operability-critical maneuver. Liu, Wang and Yu (2020) performed three-dimensional steady Reynolds-averaged Navier-Stokes (RANS) simulations of this compression system - the time-averaged form of the Navier-Stokes equations with a turbulence model that is the standard workhorse for such flows - and showed that opening the MSV alone at fixed boundaries raises the fan operating point toward surge, pushes the CDFS into deep choke, and, because the static pressure at the inner/outer bypass junction exceeds the outer-bypass inlet pressure, drives a reverse flow (recirculation) in the outer bypass that imposes radial inflow distortion on the CDFS. Their separate one-parameter sweeps established complementary individual effects: reducing p2p_2 from 1.15p2ref1.15p_{2\mathrm{ref}} to 0.83p2ref0.83p_{2\mathrm{ref}} (MSV at 8∘8^\circ) weakens but does not eliminate the recirculation, lowers the fan total pressure ratio by about 10%, and deepens CDFS choking with more severe CDFS stator separation; closing the FVABI (axial length ratio l/lrefl/l_{\mathrm{ref}} from 1.0 to 7.0) raises the CDFS matching point, increases the CDFS total pressure ratio by up to 5%, improves the CDFS stator total-pressure recovery coefficient from 0.965 to 0.984, and removes stator separation, but leaves the outer-bypass ratio and hence the recirculation essentially unchanged. On this basis they recommended decreasing the backpressure and closing the FVABI simultaneously during the real transition - a combination they did not simulate, since all their cases were steady states at frozen transition positions.

Later work built bypass-matching theory and operating-range maps for this system (Wang et al. 2021, 2023, 2024), engine-level (zero-dimensional, component-characteristic-based) mode-switching control schedules (Chen et al. 2022, 2026; Wang et al. 2023; Chen et al. 2025), and dynamic-simulation methods for continuous mode transition with coupled valve-and-stator adjustment (Song et al. 2023; Wang et al. 2022). However, no published study reports a three-dimensional simulation or experiment of the combined backpressure-plus-FVABI control action, its optimization for a given MSV schedule, or the transient evolution of recirculation and CDFS distortion under such an action.

Problem Statement

For a double-bypass variable cycle compression system of the configuration studied by Liu, Wang and Yu (2020) - a fan, a core driven fan stage (CDFS), and a high-pressure compressor (HPC), with the outer bypass admitted by the mode selector valve (MSV), the inner bypass terminated by the forward variable area bypass injector (FVABI), and a controllable bypass outlet backpressure p2p_2 - the engine transitions from single-bypass to double-bypass mode by opening the MSV, coordinated with the other two control variables. The objective is a recirculation-free, aerodynamically acceptable mode transition of the whole compression system under coordinated control. Specifically, determine by three-dimensional flow simulation and/or experiment:

  1. Whether - and for which combinations of backpressure reduction and FVABI closure - simultaneously applying both control actions during the transition can fully eliminate the outer-bypass flow recirculation while maintaining acceptable CDFS stator performance (in the sense of the source paper's stator total-pressure-recovery and separation-free criteria) and avoiding deep CDFS choking, noting that neither action alone achieves all three, since backpressure reduction alone does not remove the recirculation and FVABI closure alone leaves it untouched. The question is whether such a combination exists, not which combination is optimal.
  2. How, during a real-time continuous (unsteady) transition executed under this combined control action, the outer-bypass recirculation, the radial inflow distortion imposed on the CDFS, and the matching of the fan, CDFS, and HPC evolve in time, and how they deviate from quasi-steady predictions.

Both questions serve the single objective stated above: the first concerns whether the combined control action attains the objective, and the second whether the quasi-steady conclusions survive the real unsteady transition.

References

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